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binary black hole merger

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: First measurement of the Hubble constant from a dark standard siren using the Dark Energy Survey galaxies and the LIGO/Virgo binary–black–hole merger GW170814
Author: Marcelle Soares-Santos, Antonella Palmese, et al. (DES, LIGO, and Virgo collaborations)
First Author’s Institution: Brandeis University (M. S.-S.), Fermi National Accelerator Laboratory (A. P.)
Status: Published in ApJL

Disclaimer: The author of this Astrobites post is a member of the Dark Energy Survey but researches a different topic and did not take part in this analysis.

Nearly all of the galaxies we observe in the night sky are rushing away from us. Only the Andromeda galaxy is moving toward us — we are trapped in a gravitational dance that will end in a major collision about 4.6 billion years from now. The remainder of galaxies are receding due to the expansion of the universe. But how fast are the rest of the galaxies flying away from us? This is actually a difficult question to answer, partly because it is difficult to accurately measure distances across the universe. Today’s paper details a new method to measure how quickly the universe is expanding using the gravitational-wave (GW) signals from binary black hole collisions.

The Sounds of the Universe

Gravitational waves are ripples of spacetime itself, analogous to sound waves traveling through the air. They are generated in violent collisions between compact objects like neutron stars and black holes. The LIGO and Virgo collaborations have detected 11 such collisions, ten of which have been the collisions of two black holes (see Figure 1). The frequency and amplitude of the GWs, or the pitch and volume of the “sound,” encode information about the mass of the merging system and how far away it is. Exactly how the signal evolves tells us everything we need to know about the gravitational “brightness,” or luminosity, of the event. By comparing the measured amplitude to the calculated amplitude, we get a precise distance to the source. The ability to do this with GW signals has earned their sources the name “standard sirens.”

stellar graveyard

Figure 1. Our current knowledge of the end states of massive stars, namely black holes and neutron stars. Because the GW signals are so precisely tied to the properties of the system, we can determine the masses of the initial objects before merger in addition to the mass of the final object after merger. [LIGO-Virgo/Frank Elavsky/Northwestern U.]

What does this have to do with measuring the expansion rate of the universe? Hubble’s Law tells us that the velocity v at which an object at redshift z recedes away from us depends on its distance from us: v(z) = H0d, where H0 is the expansion rate of the universe. Previous measurements of this parameter, called the Hubble constant, have used electromagnetic radiation from either the cosmic microwave background (CMB) or Type Ia supernovae. These measurements currently conflict with one another, suggesting there might be some missing physics in our understanding of the universe. Further, measuring distances in the universe is tricky. CMB and Type Ia supernovae measurements rely on the cosmic distance ladder, so errors from one rung will propagate to the next.

On the other hand, standard sirens with electromagnetic counterparts don’t rely on the cosmic distance ladder and so offer an independent way to measure H0. In this case, the electromagnetic signal pins down the host galaxy’s location, which identifies the redshift of the signal and thus its velocity. At the same time, the GW signal gives the precise distance to the source. In fact, this has already been done using the binary neutron star merger GW170817. But we need many more than one binary neutron star event to truly pin down H0 (Figure 3), and ten of the 11 LIGO/Virgo events have not had accompanying EM signals. Today’s paper shows that there is still a way to calculate H0 from these events!

Listening in the Dark

The authors of today’s paper report the first measurement of H0 using the “dark” siren GW170814, a GW signal from two colliding black holes with no accompanying electromagnetic radiation. Recall from Hubble’s Law that we need a distance and a redshift to calculate H0 — but to determine a redshift, we need to know what galaxy the source was located in. That’s a hard thing to determine with no electromagnetic counterpart signal. The probability maps produced by LIGO/Virgo for the on-sky location of the GW signal can encompass a large area containing tens of thousands of galaxies at as many different redshifts!

GW170814 happened to fall smack in the middle of the Dark Energy Survey (DES, see Figure 2). DES has produced exquisite galaxy maps of a quarter of the Southern Hemisphere sky, complete with estimated redshifts calculated from the coarse “spectrum” of DES’s five wavelength filters. Soares-Santos, Palmese, et al. devised a statistical analysis that selected potential host galaxies from DES’s galaxy maps using the LIGO/Virgo maps and calculated what H0 would be for each in turn.

Figure 2. The LIGO/Virgo highest probability region for where GW170814 originated from, overlaid on the DES survey area. [Dark Energy Survey Collaboration]

After analyzing 77,000 galaxies, the authors calculate that H0 = 75.2 +(-) 39.5(32.4) km s-1 Mpc-1. Figure 3 shows how this value compares to previous measurements using the CMB and Type-Ia supernovae. While the uncertainties are quite large using only one GW event, the authors estimate that uncertainties comparable to the CMB and supernovae measurements are possible with ~100 GW events. Improvements to the LIGO detectors were recently completed and the observatory’s third run (O3) started on April 1st. There could potentially be a dark siren event every week, meaning we might only have to wait a couple of years to measure H0 to sufficient precision using GW events!

Figure 3. Comparison of values of H0 calculated from the CMB (Planck, dark blue), supernovae (ShoES, light blue), the binary neutron star event (GW170817, grey), and the dark siren (DES GW170814, red). With ~100 GW events, we will approach the sensitivity of the traditional electromagnetic measurements, giving an independent measurement of H0. [Soares-Santos et al. 2019]

Having a new way to measure H0 is a big deal for potentially resolving the tension between the CMB and supernovae measurements. If the dark/standard siren methods, which probe the late-time universe, end up being consistent with the early-universe CMB results, that might imply that something is wrong with our cosmic distance ladder and the late-universe supernovae measurements. On the other hand, if the GW measurements are consistent with the supernovae results, we might need to add new physics to our current understanding of the universe to explain why H0 would evolve with time. Either way, the next few years will be a very exciting time in precision cosmology!

About the author, Stephanie Hamilton:

Stephanie is a physics graduate student and NSF graduate fellow at the University of Michigan. For her research, she studies the orbits of the small bodies beyond Neptune in order learn more about the Solar System’s formation and evolution. As an additional perk, she gets to discover many more of these small bodies using a fancy new camera developed by the Dark Energy Survey Collaboration. When she gets a spare minute in the midst of hectic grad school life, she likes to read sci-fi books, binge TV shows, write about her travels or new science results, or force her cat to cuddle with her.

protoplanetary disks

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Protoplanetary Disk Rings and Gaps Across Ages and Luminosities
Author: Nienke van der Marel, Ruobing Dong, James di Francesco, Jonathan P. Williams, John Tobin
First Author’s Institution: Herzberg Astronomy & Astrophysics Programs, National Research Council of Canada
Status: Published in ApJ

If you’re well-versed in exoplanets (or even the formation of your own planet), you may be familiar with the term protoplanetary disk. These objects are disks of gas and dust surrounding a fairly newborn star, although newborn here means up to several million years old. Interestingly, images of protoplanetary disks captured by astronomers reveal gaps in the disks — or rather, separate rings of material, depending on your perspective. These types of disks were once expected to be completely smooth, so why are we seeing gap-like features in essentially all resolved images of them?

disk images

Figure 1: Left: Continuum emission for the disk sample. The beam size is shown in the lower left corner with the spectral type of each star in the right. HL Tau, TW Hya, and V1247 Ori, which had higher resolution, have been reduced to reflect the 20-AU beam size of the other images in order to make them comparable. Right: Enhanced image representations for each disk, which were used in the analyses. Gaps have been made easier to see using a variety of unsharp masking techniques. Transition disks are RXJ 1615, AA Tau, DM Tau, V 1247 Ori, HD 97048, HD 100546, TW Hya, HD 169142, and HD 135344B. [Adapted from van der Marel et al. 2019]

The authors of today’s paper use ALMA data to explore what could be causing these gaps. They examine images of 16 different protoplanetary and transition disks (disks where the material closest to the star has been cleared out; see Figure 1). The disks in the sample surround stars of various spectral types, and each exhibits multiple gap features. Since these gaps are present throughout the sample, some correlation between them should reveal the responsible mechanism, right? After all, we would expect that these features evolve in similar ways for most disk systems.

Before they can answer this question, the authors first determine each star + disk’s luminosity and use this information to age each star with model evolutionary tracks. The resulting age range gave them a way to classify their disks as older or younger. They also determined approximate gap locations and sizes via a type of intensity profile fitting, which essentially models the light coming from the star + disk in each image (where there is a gap, there is less light detected, etc.).

Searching for Answers

gap properties

Figure 2: Gap properties for each disk. Each red dot represents a gap in the corresponding disk, with the disks sorted by increasing age. The gap center radius is raised to the 3/2 power, to mimic orbital resonance ratios. [Adapted from van der Marel et al. 2019]

With this information, the authors searched for trends in
the data — any correlations between the stars’ properties and the disk properties that may point to an origin story. Figures 2 and 3 show these chosen parameters … as well as the obvious dearth of trends. The only visible trend seems to be the decrease in outermost ring radius for the four oldest stars when compared to the rest of the sample (see Figure 3, bottom). This sure does make it hard to imagine a common mechanism responsible for the gaps.

Two of the most common theories for gaps within disks are 1) planets and 2) stuff freezing. Certain compounds are present in the disk, and there should be a point away from the star where the temperature becomes just right for those compounds to freeze into ice crystals. This is called the snow (or frost) line. At that snow line, it is thought that as these crystals all form together, they may stick to each other and the disk’s dust, which could clear out some space and turn into the gaps we see.

The authors looked to 5 different molecules in order to test this hypothesis: N2, CO, CH4, CO2, and NH3. This is the “stuff” we were talking about freezing. They determine the location of each respective snow line for each star and find that these locations don’t seem to have anything to do with the gaps — there are a few instances where the gap and snow line overlap, but it does not seem to be a systematic trend. So it seems the snow line scenario is a no-go.

Planetary formation is another story. It is thought that as planetesimals accrete surrounding disk material, a gap forms as it carves out its orbit. The authors state that they simply don’t have enough information to disprove or support this theory. Neptune-mass planets are currently undetectable and only one disk (HD 163296) in the sample has been suspected of having a planet. They do note however that one model of planet formation (cold-start model) would allow for giant-planet formation at the location of many of the observed gaps. So, planet formation is still a possible culprit.

gap properties

Figure 3: Additional gap properties across ages and luminosities, with each red dot representing a gap. [Adapted from van der Marel et al. 2019]

In case you were wondering, authors also ponder the case of graviational instability. Gravitational instabilities in the disks could potentially cause the gaps, but this depends on the gas-to-dust mass ratio. Unfortunately this is very much uncertain and hard to measure, so the trail stops there.

So What Do We Know?

The only solid correlation present in the sample is that the outer disk ring is much closer in for the four oldest stars — i.e., the older disks are smaller in diameter. In that case, it seems we are missing some outer rings. So maybe the outer rings dissipate faster than the inner rings (due to drag forces or radiation pressure). Either that, or these outer dust grains accrete to become planetesimal sized and are therefore undetectable at the observed wavelengths.

Although the authors didn’t find the origin story they were looking for, they can say a few things with certainty. The lack of trends in their data show that disk gaps are diverse and their presence is largely independent of stellar properties, like spectral type or age.  They also found that snow lines don’t have anything to do with the gaps we observe, but planets very well might. And last but not least, transition disks seem to host these features in the same manner as the truly protoplanetary disks, implying that they evolve in the same way, even if we don’t know what that way is. This is actually quite a big step in the right direction. These clues get astronomers one step closer to to closing the gap on … gaps.

About the author, Lauren Sgro:

I am a PhD student at the University of Georgia and, as boring as it may sound, I study dust. This includes debris disk stars and other types of strange, dusty star systems. Despite the all-consuming nature of graduate school, I enjoy doing yoga and occasionally hiking up a mountain.

M-dwarf flare

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The Surface UV Environment on Planets Orbiting M-Dwarfs: Implications for Prebiotic Chemistry & Need for Experimental Follow-up
Author: Sukrit Ranjan, Robin Wordsworth, & Dimitar D. Sasselov
First Author’s Institution: Harvard-Smithsonian Center for Astrophysics
Status: Published in ApJ

Note: The reference to “life” in this post refers to “life as we know it here on Earth.”

M dwarfs, the smallest and coolest (in temperature and by reputation) of the main sequence stars, are also the most abundant type in our galaxy. As this Astrobite post notes, Earth-sized and super-Earth sized planets are also quite common around these stars, with 0.86 of these types of planets per M dwarf. Also, as M dwarfs are cooler, their habitable zone is located much closer in — and the consequent shorter periods of M dwarfs’ habitable-zone planets makes it easier to observe multiple transits. Therefore, if we want to search for a habitable planet, then we should be looking for it around an M-dwarf. Several habitable-zone planets have already been discovered around M-dwarfs; Trappist-1 has 3 of them!

Unfortunately, just because a planet is in the habitable zone doesn’t necessarily mean that it can support life. M dwarfs produce a lot of ultraviolet (UV) light, especially at the shorter UV wavelengths (extreme UV or EUV). Not only does UV light give you a sunburn, it can wreak havoc on life, from destroying cells to stripping a planet of its water and atmosphere. Most studies agree UV is bad for business and there is a high probability that M-dwarf planets can’t support life as we know it.

The authors of today’s paper shine a new light (pun intended) on the effect of UV on life. They note that previous laboratory studies have discovered that UV light is actually necessary to create the building blocks required for life, such as RNA, amino acids, and sugars. The creation of these building blocks is also known as prebiotic photochemistry. Could M-dwarf UV radiation actually be fueling life instead of destroying it?

Early Earth Around an M Dwarf

Figure 1: Spectra of multiple types of M dwarfs and the approximate spectra of the 3.9-billion-year-old young Sun in black. Top panel is the total amount of flux the stars create at varying wavelengths. At shorter ultraviolet wavelengths, both M dwarfs and the young Sun radiate a comparable amount of flux. Bottom panel is the amount of light that reaches the surface of the early Earth. The cutoff at 200 nm is due to absorption by carbon dioxide in the atmosphere. A significant amount of UV light from the young Sun reaches the planet surface compared to any of the M dwarfs. [Ranjan et al. 2017]

To address this question, the authors first determined what sort of UV environment a pre-life Earth would experience around different types of M dwarfs. They assumed their planet had a simple 1-bar atmosphere composed of 90% nitrogen and 10% carbon dioxide. This pre-life Earth was then hit with a variety of UV radiation levels based on known M-dwarf spectra. The authors found that the carbon dioxide in the atmosphere protected the surface of the planet from the harmful EUV radiation — a plus for life. However, because M dwarfs are smaller, they only produce 1–10% as much near-UV radiation as our young Sun (Figure 1). Using the amount of light that reaches the planet surface, known as surface radiance, the authors determined that the reaction rates for prebiotic photochemistry would take 2–4 times longer on an M-dwarf planet than on early Earth.

Can Life Develop or Not?

This doesn’t necessarily mean that life as we know it couldn’t have developed on a M-dwarf planet, just that the pathways to achieving prebiotic chemistry would take substantially longer (on order of 1010 years or almost the current age of the universe). If this is the case, then there hasn’t been enough time for life on most M-dwarf planets to develop! However, if the reactions take too long, the paper points out that other chemistry might dominate that which is needed for life, though it is unclear what the timescale for its development would be.

But there may be a way around this. M dwarfs are fairly active and are known to flare often. The authors repeat their experiment using the spectrum of a known flare from the M dwarf AD Leo. They find that for a short period of time, the planet experiences UV flux that is 10x that of the Earth for some wavelengths, leading to reaction rates which are up to 10x as fast. A comparison of reaction rates before and after a flare is shown in Figure 2. The Relative Dose Rate corresponds to reaction rates. The different colored boxes in Figure 2 represent different chemical reactions. Before a flare, AD Leo, a fairly active M dwarf, drives reaction rates that are slower than those driven by an early-Sun host star. But during a high-energy flare, its reaction rates jump orders of magnitude!

Figure 2: Reaction rates (Relative Dose Rate) of the young Sun, AD Leo when it is not flaring (i.e., it is quiescent), and AD Leo when it does flare. Each colored box represents different prebiotic photochemical reactions. [Adapted from Ranjan et al. 2017]

With enough flares hitting the planet, it may be possible for prebiotic photochemistry to occur on similar timescales to that of early Earth. Though how many flares are needed, with how much energy, and what this could do to the atmosphere of the planet, are all still questions that need answering both through experiments in the lab and continued M-dwarf observations. The authors argue that we should consider targeting more active or flaring M dwarfs in our search for life.

As to whether UV radiation is good or bad, we keep coming up with the same answer: “we don’t know.” It turns out we continue to prove a very obvious conclusion: life and the development of life is really complicated.

About the author, Jessica Roberts:

I am a graduate student at the University of Colorado, Boulder, where I study extra-solar planets. My research is currently focused on understanding the atmospheres of the extremely low-mass low-density super-puffs. Out of the office, you will probably find me running, cross-stitching, or playing with my dog.

protoplanetary disk

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: A Survey of CH3CN and HC3N in Protoplanetary Disks
Author: Jennifer B. Bergner, Viviana G. Guzman, Karin I. Öberg, Ryan A. Loomis, Jamila Pegues
First Author’s Institution: Smithsonian Astrophysical Observatory and Sternberg Astronomical Institute
Status: Published in ApJ

The Prebiotic Importance of Nitrogen-Bearing Molecules

Did you know that you are made out of primordial dust and gas? It’s true! Our solar system formed from a gravitationally collapsing cloud of interstellar dust and hydrogen gas, birthing a proto-Sun in the center of this hot dense material. The various planets are then thought to have formed out of the material in the solar nebula, the disk-shaped cloud of gas and dust left over from the Sun’s formation, which ultimately takes the shape of a rotating protoplanetary disk. As discussed in a previous astrobite, these disks exhibit an intriguing variety of chemistry that is only beginning to be probed with the higher sensitivity of telescopes such as ALMA. Understanding how the early inventory of organic molecules present in this stage of planet formation developed into the vast complexity of biochemistry we see today is key to the study of the origins of life.

Chemists and astronomers alike have been especially interested in nitrile-bearing molecules, which contain a carbon-nitrogen triple bond. These molecules likely play a crucial role in prebiotic chemistry, as recent studies have shown that this particular bond is involved in prebiotic synthesis of RNA and protein precursors. In today’s astrobite, we take a look at these important types of molecules in a recent survey of protoplanetary disks, and we explore the implications for nitrogen-based chemistry in our early solar system.

Surveying CH3CN and HC3N in Planet-Forming Disks

Up to this point, few disks had well-characterized nitrile abundances. More importantly, it was unknown if other disks contain similar nitrile abundances as the solar nebula. Also, it was unclear how robust the nitrile chemistry is in different circumstellar environments. For instance, do differences in the density and temperature around young stars lead to significantly different amounts of nitrogen-bearing molecules, or can they survive in a wide range of physical conditions? The authors of today’s bite sought to obtain a larger sample of observations to answer these questions.

The authors used ALMA to survey six nearby protoplanetary disks at distances ranging from 235 to 466 light-years from Earth. Cyanoacetylene (HC3N) is detected in all but one of the disks, while methyl cyanide (CH3CN) is firmly detected in three of the six disks with additional tentative detections in another two disks. A face-on view of the HC3N and CH3CN gas, along with the observed spectra in half of the observed disks, is shown in Figure 1.

Figure 1: Face-on view of total radio emission from cold dust (left column) and CH3CN and HC3N (middle columns) around the IM Lup, V4046 Sgr, and MWC 480 disks. Images are centered on the peak dust emission (“+”). Darker colors and additional contours represent increasing strength of the signal. The x- and y-axes are in equatorial coordinates in arcseconds and, depending on source distance, they show a total range of 360–805 AU, or nearly 7–16 times the size of our solar system. The light gray circle in the lower left indicates the resolution of the radio observations. The disk-integrated spectra from rotational transitions of each molecule (right column) are shown in terms of velocity and radio intensity. The double-peaked structures indicate that the disks are undergoing Keplerian rotation. [Bergner et al. 2018]

Discovery of a Robust Nitrile Chemistry

In protoplanetary disks, the underlying chemistry is determined by both the presence of gas-phase molecules and dust grains. In the case of these two molecules, astrochemists have discerned that HC3N is only able to form efficiently in the gas phase, while CH3CN forms either in the gas phase or on the surfaces of dust grains. To investigate not only these formation mechanisms, but also to test the chemical dependence on various physical conditions, the disks selected for this study were carefully chosen to span a range of physical conditions, including their age, mass, and luminosity. However, after careful modeling, the authors find no strong trends in nitrile emission strength or abundances with these environmental differences. This result implies that nitrile chemistry is chemically robust and is likely to be found with similar outcomes even in substantially different physical conditions.

Protostellar and Cometary Comparisons

To further investigate this unexpected chemical resilience, the authors compare their results in protoplanetary disks with previous observations of protostellar envelopes, as well as with solar-system comets, which are thought to be relatively pristine records of our own solar nebula. To do so, the authors compare abundance ratios among HC3N, CH3CN, and HCN with those measured in protostars and comets in Figure 2. For each of these disks, the molecule HCN, which is a precursor molecule to HC3N, had previously been observed and thus is also included in their comparisons.

Figure 2: Abundance comparisons between (a) CH3CN/HCN, (b) HC3N/HCN, and (c) CH3CN/HC3N. Due to uncertainties in the determined gas temperatures, the ratios are shown for a range of temperatures from 30–70 K. The top two panels show comparisons against typical cometary fractions, while the bottom panel is compared with the measured fraction in protostellar envelopes from a previous study by the same authors. The authors find that the nitrile chemistry exhibits similar abundance ratios in protoplanetary disks, comets, and protostellar envelopes. [Bergner et al. 2018]

Nitrile abundance ratios are surprisingly consistent across the surveyed disks as well as with comets and protostellar envelopes, despite the inherent evolutionary differences of these objects. This consistency again demonstrates that complex nitrile species should be reliably produced in a variety of different star- and planet-forming environments. These results also suggest that the solar system is not unique in its nitrile chemistry, which means that the raw materials needed for the biochemical beginnings of life may be common around newly forming planets in other solar systems!

About the author, Charles Law:

Hi! I’m a first-year graduate student at Harvard/CfA. I’m interested in observationally studying the chemical complexity found in space, including throughout high-mass star-forming regions and in protoplanetary disks. In my free time, I enjoy hiking, bicycling, and traveling.

accreting black hole

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: A Population of Bona Fide Intermediate Mass Black Holes Identified as Low Luminosity Active Galactic Nuclei
Author: Igor V. Chilingarian et al.
First Author’s Institution: Smithsonian Astrophysical Observatory and Sternberg Astronomical Institute
Status: Published in ApJ

Most, if not all, galaxies contain a supermassive black hole (SMBH) at their center. But where did these giants come from? Astronomers know that less massive black holes can form when a star collapses in on itself. These stellar-mass black holes can have the mass of a few dozen Suns, but they do not come close to the millions or billions of solar masses SMBHs have.

There are a few ideas for where SMBHs may have come from. One scenario is that the earliest stars left behind small baby black holes, which over time merged together to form more and more massive grown-up black holes that now reside at the center of galaxies. If this were the case, we’d expect to see some teenage, intermediate-mass black holes (IMBHs) — black holes larger than those born from stars but smaller than those we see in the center of galaxies.

Another possibility is that large gas clouds in the early universe collapsed to form massive black-hole “seeds” the size of hundreds of thousands of stellar masses. We then wouldn’t expect to see the adolescent phase of IMBHs, as SMBHs would start growing from these large seeds instead of small ones left behind by stars.

The authors of today’s paper wanted to see if they could find IMBHs, and thus evidence for the first scenario for SMBH formation.

Finding AGNsty Adolescents

While previous searches for IMBHs looked at only a few, pre-selected galaxies, Chilingarian et al. set up an automated search through 1 million catalogued galaxies to look for signs of SMBHs at their centers that are actively gaining mass — objects known as active galactic nuclei (AGN). AGN show distinct spectral lines, as seen in the bottom left of Figure 1. The authors used the width and strength of some of these lines to calculate the mass of the black hole for each galaxy.

Fig 1: The top row shows a diagram breaking down the components of an AGN. The black hole at the center (far right) has an accretion disk emitting light, while the clouds in the broad- and narrow-line regions (center) absorb and reemit the light, causing the peaks seen in the spectra at the lower left. The lower right plots show the curves used to fit the emission lines. [Chilingarian et al. 2019]

Of the million objects analyzed, 305 had black holes with masses less than that of 200,000 times that of the Sun, categorizing these as potential IMBHs.

By selecting a subset of these candidates for follow-up observations, Chilingarian et al. confirmed the AGN nature of ten of these candidates by detecting them in X-rays. This provided the authors with a sample of ten bona fide AGN with black-hole masses measured between 43,000 and 202,000 solar masses, five of which were previously known. Images of the ten galaxies that host the proposed IMBHs are shown in Figure 2.

Fig 2: Optical images from the Sloan Digital Sky Survey of the ten IMBH host galaxies. The white line shows the scale; five kiloparsecs (kpc) is approximately 1017 kilometers. The red circle is the location of their observations in X-ray wavelengths. The number at the top is their catalogue designation and the number at the bottom is their mass estimate in solar-mass units. The bottom row contains the galaxies with proposed IMBHs found in previous studies. [Chilingarian et al. 2019]

Results

These results are promising for the scenario where supermassive black holes are grown from stellar-mass seeds. If SMBHs grew from seeds larger than these observed intermediate-mass black holes, then we would not expect to see any IMBHs.

From their original sample of 305 possible IMBH host galaxies, 14 currently have enough observational evidence to test the most stringent observing criteria for IMBHs. Only six of those 14 (or 43%) successfully pass all the tests, suggesting that they host a real IMBH. Thus, the authors estimate a lower limit of 131 galaxies in their sample will have real IMBHs when further follow-up observations are performed.

The 305 possible IMBHs the authors explore in this paper are only those that are actively collecting more material. There may be many more non-accreting IMBHs out there that cannot be detected because they are too far away for our current instruments.

The accretion of matter from a host galaxy is not enough to explain the growth of stellar-mass black holes to IMBHs, so IMBHs must form from the merger of smaller black holes. The authors conclude that at least some of the SMBHs we observe must therefore ultimately be built from mergers of small, stellar-mass seed black holes.

About the author, Bryanne McDonough:

First year graduate student at Boston University where I am currently studying the distribution of dark matter and satellites around galaxies using data from the Illustris simulations. My primary research interests are galactic and extragalactic astrophysics using computational methods. Outside of grad school I enjoy reading and crafting.

WASP-12b

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Obliquity Tides May Drive WASP-12b’s Rapid Orbital Decay
Author: Sarah Millholland, Gregory Laughlin
First Author’s Institution: Yale University
Status: Published in ApJ

It’s not an easy life being a hot Jupiter. Besides their eternal loss of privacy due to being on the hit list of astronomers since the very moment they are detected, theirs is a tale of intense drama. The extreme radiation and tidal flexing they experience due to their proximity to their host stars make them ideal targets for studying planetary science in extreme physical conditions, particularly since there are no hot-Jupiter analogs in our solar system. One of these weirdos, and also one of the most studied hot Jupiters in recent years, is WASP-12b. In addition to being one of the hottest hot Jupiters around (with equilibrium temperature of ~2500 K) and losing mass at an exceptionally high rate, it is also the only known hot Jupiter inspiralling so rapidly towards its star that we can observe this decay in real time. It has been speculated that sustained tidal interactions between the star and the planet could be responsible for the observed orbital decay of the planet. The authors of today’s paper investigate the case of WASP-12b in this context to understand what might be driving its orbital decay.

Turning with the Tide

Tidal interactions between a star and its planet act together to dissipate the orbital energy of the system in the interiors of the planet and the star. Think of the planet and the star acting like resistors that dissipate the gravitational energy of their orbits into heat due to tidal flexing in their interiors. This tidal dissipation is most effective when a planet has large orbital eccentricity and obliquity (the angle between the orbital plane and spin axes of the planet). However, for a lonely hot Jupiter with no other planetary companion in the system, tidal torques from the host star over time would eventually force the planet to reach an equilibrium state with low orbital eccentricity and obliquity, which would reduce the tidal dissipation the planet experiences. The fact that WASP-12b’s orbit is decaying rapidly indicates that there may be some other force at work helping to drive its sudden inspiral. The authors of today’s paper propose that the presence of another planet in the system (see Figure 1) could sustain WASP-12b’s obliquity, causing the planet to continue to experience high tidal dissipation and preventing its inspiral from stabilizing.

spin-orbit interactions

Figure 1: Schematic of the spin-orbit interactions between WASP-12b and another planet in the system. In the case of spin-orbit resonance (called a Cassini state), the spin angular momentum vector S1 and orbital angular momentum vector L1 precess about a common axis at the same rate. [Millholland & Laughlin 2018]

If there is another planet in the system, it’s likely that WASP-12b could have gotten locked in a state of spin–orbit resonance (known as a Cassini state, also observed in the case of Earth–moon system) which means that the rate of precession of its orbital axis is the same as the spin axis precession rate. This implies that as the orbital precession rate of WASP-12b changes due to dynamical interactions with the perturbing planet in the system, this will drive a change in its spin precession rate as well, increasing WASP-12b’s obliquity.

The authors simulate the evolution of the WASP-12 system using an obliquity tide model that couples two things: the relations governing the secular dynamical interactions between WASP-12b and another planet, and the tidal interaction between the star and WASP-12b. These interactions then combine to cause the inspiral. With very little fine-tuning of the properties of the perturbing planet and the efficiency of tidal dissipation (which is dependent on the unknown interior structure of WASP-12b), they are able to reproduce the inspiral rate that we have observed for the system (Figure 2). An interesting result of their analyses using the obliquity tide model is that even if WASP-12b starts with a very low obliquity (< 1°), it can easily get locked into a Cassini state and attain a high obliquity within the lifetime of the planetary system.

WASP-12b's obliquity evolution

Figure 2: Simulated evolution of WASP-12b’s obliquity (gray) and semi-major axis (purple) with respect to time from the obliquity tide model. Note that tidal dissipation due to obliquity tides is a runaway process: orbital decay (represented by the decreasing semi-major axis) due to tidal dissipation leads to increasing obliquity as the other planet forces WASP-12b to maintain the spin-orbit resonance. This further increases the efficiency of tidal dissipation. [Millholland & Laughlin 2018]

So obliquity tides can cause the observed orbital decay — but how do we confirm this? The good news is that, based on the predictions of their obliquity tide model, the authors conclude that the perturber planet is very likely within the limits of detection for extreme-precision radial velocity instruments coming up in the near future. Tighter constraints on transit duration variations or transit timing variations (a signature of orbital precession) from more precise long-term photometric monitoring of the system by TESS could also help in strengthening the obliquity tide hypothesis. If confirmed, obliquity tides might additionally be able to explain WASP-12b’s extreme radius inflation and unusual features in its thermal phase curves and will provide compelling evidence for in situ formation of hot Jupiters. Since tidal interactions are intimately tied to the interiors of the planets, they could also be an unprecedented tool for X-raying the interior structure, formation histories, and demographics of exoplanets.

About the author, Vatsal Panwar:

I am a PhD student at the Anton Pannekoek Institute for Astronomy, University of Amsterdam. I work on characterization of exoplanet atmospheres to understand the diversity and origins of planetary systems. I also enjoy yoga, Lindyhop, and pushing my culinary boundaries every weekend.

IGR J17062–6143

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: NICER Discovers the Ultracompact Orbit of the Accreting Millisecond Pulsar IGR J17062–6143
Author: T. E. Strohmayer et al.
First Author’s Institution: NASA Goddard Space Flight Center
Status: Published in ApJL

Dancing with the Stars

pulsar

Figure 1: Artist’s impression of a pulsar in a binary system, being fed by an accretion disk. [NASA/Dana Berry]

What does a ballerina doing a pirouette and a millisecond pulsar (MSP) have in common? Just as the ballerina will spin faster and faster as she brings her arms in toward her body, a MSP will also rotate more rapidly as the star’s radius is reduced by gravitational compression. These swiftly spinning stars are neutron stars — one of the possible end stages of stellar life. Neutron stars are born during a core-collapse supernova, which occurs once a massive star can no longer withstand its own gravity after fusion has ceased. These exotic compact objects are among the densest stars in the universe. They have 1.4 times the mass of our Sun crammed into a sphere that is roughly 20 km in diameter (the size of a large city!). As these neutron balls rotate, they can produce various types of electromagnetic radiation near their magnetic poles. If the magnetic axis and rotation axis of the neutron star are misaligned, we typically observe periodic pulses of radiation as the star rotates (if the radiation is beamed towards our telescopes) — hence the nickname “pulsar”! MSPs are particularly special compared to ordinary pulsars. They are typically spun-up by accretion of material from a nearby orbiting star via angular momentum transfer (see Figure 1), and they rotate hundreds of times per second (in some cases, even faster than your kitchen blender).

IGR J17062–6143: A Record-Breaking Orbit

In today’s astrobite, we cover the discovery of the orbit of an ultracompact X-ray binary, IGR J17062–6143 (depicted in the cover image above), which harbors an accreting MSP. Using the recently commissioned Neutron Star Interior Composition Explorer (NICER) X-ray instrument on board the International Space Station, Strohmayer et al. showed that the system consists of a very low-mass white dwarf in a record-setting 38-minute orbit with a pulsar that rotates at about 9,800 revolutions per minute.

The authors’ findings are based on an analysis of recurring X-ray pulses produced from hot spots near the pulsar’s magnetic poles (see Figure 2). In the case of IGR J17062–6143, these hot regions are formed from the white dwarf’s infalling material, which collects into an accretion disk around the pulsar. By calculating Fast Fourier Transform (FFT) power spectra of the NICER light curve, the authors found that the MSP’s orbital motion produced sidebands near the rotational frequency of the pulsar and its harmonics, which is a discerning feature of pulsars in binary systems.

Figure 2: Average power spectrum from NICER observations of IGR J17062–6143 obtained in 2017 August. The presence of sidebands indicates that the X-ray photons are affected by the orbital motion of the pulsar. [Strohmayer et al. 2018]

As the MSP whirls around its companion, the X-ray pulses are Doppler shifted by the orbital motion of the pulsar (see Figure 3). In other words, some pulses will arrive sooner when the MSP is moving toward us, and others will arrive later when the pulsar is moving away from us. This important property allowed the authors to show that IGR J17062–6143 has a nearly circular orbit, and the MSP and white dwarf companion are separated by only roughly 186,000 miles (300,000 km) — less than the distance between the Earth and the Moon! The authors also provide constraints on the viewing angle of the binary and the mass of the white dwarf, which they find is ~57–65 times less massive than the Sun assuming that the donor star is hydrogen-deficient. However, the precise chemical composition of the white dwarf is still under debate. This is interesting because it is thought that accretion of hydrogen-deficient material on short timescales is a feature unique to ultracompact binaries. Ultracompact binaries like IGR J17062–6143 therefore serve as laboratories for studying the physics of accretion in a unique environment. Further investigation is also needed to understand the nature of the X-ray outbursts observed from this object, which are believed to be powered by thermonuclear reactions on the surface of the neutron star.

Figure 3: Best-fit orbital solution (blue) and pulse phase residuals (black) as a function of mean pulsar longitude, derived from NICER observations of IGR J17062–6143. The orbit-predicted phase delay is added to the residuals to show the orbital variability. [Strohmayer et al. 2018]

A NICER Look Going Forward

Ultracompact X-ray binaries like IGR J17062–6143 are interesting for a variety of reasons. Since the donor star in these systems are sometimes helium rich, when one sees signatures of carbon and/or oxygen in their stellar spectra, we can learn about how the core of the star has evolved through helium burning. This is one of the topics Strohmayer et al. plan to investigate in their next work. Future NICER observations of IGR J17062–6143 will also allow astronomers to watch for enormous helium-powered thermonuclear explosions on the surface of the neutron star. Additionally, observations of X-ray pulsations from these systems will help astronomers probe the fundamental physics associated with neutron stars and their surrounding environments. With such an exciting discovery from the first set of NICER data, we can expect that there will be more remarkable things ahead!

About the author, Aaron Pearlman:

I am a Ph.D. candidate in Physics at Caltech. My research focuses on searching for new pulsars near the center of the Galaxy using JPL’s Deep Space Network radio dishes in the southern hemisphere. I am also interested in studies of magnetars, FRBs, gravitational-wave searches, and high-energy observations of compact objects. When I’m not hunting for pulsars, I can usually be found hanging out with dogs or trying the latest vegetarian cuisine Los Angeles has to offer!

reionization

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Constraining the neutral fraction of hydrogen in the IGM at redshift 7.5
Author: Austin Hoag, Maruša Bradač, Kuang-Han Huang, et al.
First Author’s Institution: University of California, Los Angeles
Status: Submitted to ApJ

The epoch of reionization (EoR) refers to a period in the universe’s history in which the element hydrogen, which to this day constitutes the majority of the baryonic matter in the universe, transitioned from being mostly neutral to mostly ionized (Figure 1). This transition occurred by a fairly intuitive process. Early on in the universe’s history, since there were no stars or galaxies to produce light, there were no energetic photons present to dislodge the electrons orbiting the nuclei of hydrogen atoms. However, once stars and galaxies began to form, there was an increase in the availability of such photons — and once the universe was roughly 1 billion years old, nearly 100% of hydrogen atoms had been ionized.

Figure 1: Timeline of the history of the universe, showing the EoR (click to enlarge). The term redshift in the x-axis label refers to the reddening of light from its rest color as a result of relative motion between the emitter and the observer. It is also written as “z“. [NAOJ]

Studying reionization presents an interesting puzzle for astronomers because we know the starting point (almost all hydrogen is neutral) and the ending point (almost all hydrogen is ionized), but have little idea of what path the universe took on its journey between the two. Did reionization start early or late in the universe’s history? Did it progress quickly or slowly? When did the various sources of ionizing photons (quasars, galaxy clusters, Population III stars, etc.) become abundant enough to have a significant effect on the transition?

One reason why these seemingly basic questions are still open is that astronomers mainly study light, and neutral hydrogen is notorious for absorbing lots of it. In fact, any photons with wavelengths of less than 912 Angstroms, the Lyman limit, will be absorbed by neutral hydrogen. Effectively, the neutral hydrogen that defines the EoR prevents it from being studied. This has forced astronomers to devise creative ways of studying this period of time.

Today’s paper covers one team’s strategy to measure the neutral hydrogen fraction, a value that is used to characterize the amount of remaining neutral hydrogen compared to the total. If this fraction equals one, then all the hydrogen in the universe is neutral. If it equals 0, then all the hydrogen in the universe is ionized. The larger the neutral hydrogen fraction, the deeper into the EoR measurements are probing.

Methods

The investigation detailed in today’s paper relied on a combination of observations and simulations of reionization. First, the authors observe the brightnesses of faint galaxies situated well within the EoR. Second, the authors use a simulation of reionization to model how the neutral fraction affects the observed brightness of galaxies. For the third and final step, they perform a statistical analysis comparing the observed and modeled brightnesses to find the neutral fraction at the redshift of their observed galaxies.

To obtain their sample of faint galaxies, the authors used MOSFIRE to observe several large galaxy clusters to target distant lensed background galaxies (Figure 2). A byproduct of general relativity, gravitational lenses are caused by massive foreground objects like galaxy clusters bending space. This bending effect focuses and amplifies the light of objects behind the lens, magnifying dim background galaxies and making them easier to detect. Singling out the lensed galaxy population in their data, the authors map out the distribution of brightnesses of these galaxies at a particular wavelength of light, called Lyman alpha (Lyα). Using Lyα is important both because it can be incredibly bright over large distances, and because the transmission of Lyα photons through space is strongly affected by the presence of neutral hydrogen.

Figure 2: One of the observations used in today’s paper, showing the contour of a modeled gravitational lens (orange line) and the locations of the authors’ targeted galaxies. [Hoag et al. 2019]

Using their observations and the modeled Lyα transmission from simulations, the authors use a Bayesian framework to infer the neutral hydrogen fraction contemporary with their observed Lyα-emitting galaxies. Inputting their observed Lyα measurements and corresponding galaxy ultraviolet luminosities into the model, they find that it is most likely that the neutral hydrogen fraction at a redshift of z = 7.6 is about 0.88. In other words, when the universe was about 700 million years old, its hydrogen was about 88% neutral. This result is suggestive of a late and rapid reionization scenario, compared with some theories and simulations that have reionization approaching this neutral fraction at much higher redshifts.

Figure 3: The average hydrogen neutral fraction obtained in different studies; the legend indicates the method used to find the values. The results from today’s paper are indicated by the red star, located markedly above most of the other measurements. [Hoag et al. 2019]

The Epoch Conclusion

This finding is an especially important result given that the neutral fraction is strongly biased towards the environment that sources the observations, and most observations from the EoR are of unusually luminous objects. For example, previous observations of quasar spectra at similar redshifts have been suggestive of much lower neutral fractions (see e.g. “QSO damping wings” in Figure 3). Discrepancies like these could arise because the quasar itself is contributing ionizing photons towards reionization, decreasing the local neutral fraction. The authors’ choice to target faint galaxies, which are much more common than quasars, may be preferentially targeting a more representative region of space, thus giving a better estimate of the global neutral fraction.

There are a multitude of methods being used to measure the neutral hydrogen fraction during reionization, and every last one of them is crucial to understanding the early history of the universe. Today’s paper demonstrates the application of a powerful tool that can be added to the high-redshift astronomer’s arsenal.

About the author, Caitlin Doughty:

I am a fourth year graduate student at New Mexico State University. I use cosmological simulations to study galaxy evolution during the epoch of reionization, with a focus on metal absorption in the circumgalactic medium.

trans-Neptunian object

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Discovery and Dynamical Analysis of an Extreme Trans-Neptunian Object with a High Orbital Inclination
Author: J. C. Becker, T. Khain, S. J. Hamilton, et al. (DES Collaboration)
First Author’s Institution: University of Michigan
Status: Published in AJ

Three years ago, Mike Brown and Konstantin Batygin published their seminal paper predicting a massive, undiscovered Planet Nine. The publication was quickly picked up by the media and reinvigorated the study of objects beyond the orbit of Neptune, appropriately called trans-Neptunian objects (TNOs). Among the TNOs are Kuiper Belt objects, which have well-behaved orbits, and “extreme” TNOs (ETNOs), which have highly eccentric and inclined orbits.

There are many open questions about ETNOs. Were they born extreme or scattered into extreme orbits later in life? How long will their orbits be stable? How many ETNOs even exist? Does the existence of Planet Nine explain or complicate the population of ETNOs? Few researchers were asking or trying to answer these questions until recently, when the search for Planet Nine uncovered a batch of ETNOs that are more exciting than previously thought.

The Search for Planet Nine Continues

Many telescope surveys have tried without success to find Planet Nine in the three years since the theory was published, leading some astronomers and groups to question the hypothesis. Searching for Planet Nine requires special imaging techniques to identify slowly moving, cold, small objects from a trove of astronomical images, which has had the side effect of producing more discoveries of other types of TNOs.

This is where the Dark Energy Survey (DES) comes in. DES’s five years of high-resolution images have been put through a special pipeline designed to pick out only TNOs through a method called difference imaging. This technique is combined with machine learning to find the faintest TNOs possible and then “connect the dots” of each detection into complete orbits.

Extreme … In a Good Way

This paper reports on DES’s discovery of 2015 BP519, the “most extreme” TNO yet, because it has a highly elliptical orbit with an eccentricity of 0.92 and is inclined a whopping 54° out of the plane of the planets.

The high inclination of this ETNO is puzzling because the solar system formed from a disk, so something would have had to severely disturb the orbit of 2015 BP519. To test that hypothesis, the authors ran a simulation of 2015 BP519 forward and backward in time, showing how it would change orbit by interacting with the current solar-system objects.

Figure 1 shows many individual simulations (red lines) of how orbital inclination (i), eccentricity (e), semi-major axis (a), and perihelion (q) vary over billions of years. Looking backwards in time from today, the simulated inclination and eccentricity do not vary significantly. To the authors, this suggests whatever perturbed this ETNO is missing from the model.

Figure 1. Numerical simulations of 2015 BP519 forward and backward in time interacting with the gas-giant planets. Only the gas giants are simulated because the terrestrial planets contribute negligibly to the overall angular momentum of the solar system. Each red line represents one simulation. Inclination (i), which is the tilt of the orbit, and eccentricity (e), which is the measure of how elliptical the orbit is, do not vary much over billions of years. [Becker et al. 2018]

The authors then added Planet Nine into their simulation and ran it forward in time; results are shown in Figure 2. Inclination, eccentricity, semi-major axis and perihelion all smear out over time, which means that in some simulations, interactions with Planet Nine could bring 2015 BP519 back into the plane of the planets and into a more circular orbit. By the same logic, when 2015 BP519 was born in the plane of the solar system, interactions with Planet Nine over billions of years could be one potential method to scatter it into an extreme orbit. The authors only ran the simulations for Figure 2 forward because the history of Planet Nine is entirely uncertain. It is also possible, given the wide range of simulation outcomes, that Planet Nine had nothing to do with making 2015 BP519 so extreme; these results are merely consistent with the Planet Nine hypothesis.

Figure 2. Simulations of 2015 BP519 as Figure 1, but this time including Planet Nine. Inclination and eccentricity vary far more wildly here, showing how the presence of Planet Nine might explain how 2015 BP519 became so extreme. [Becker et al. 2018]

A Natural Fit

Another analysis of interest from this paper is provided in Figure 3, which plots all known TNOs, where bluer color indicates more “extremeness.” Semi-major axis is plotted on the x-axis versus the orbital elements on the y-axis. These quantities describe the orientation of the orbit in physical space. Figure 3 is quite detailed, but the important point is the higher density of dots in the shaded regions, which was the original impetus for the Planet Nine theory. 2015 BP519, marked by a star, adds another data point to support this theory.

Figure 3. Visualization of orbital elements on the y-axis versus semi-major axis on the x-axis for all known TNOs. Bluer dots represent more extreme orbits. BP519 coincides with the clustering of TNOs in the shaded regions that inspired the Planet Nine hypothesis. [Becker et al. 2018]

The bottom line is that these simulations and results are consistent with a massive ninth planet — but that is a far cry from requiring the existence of Planet Nine. Considering the difficulties in simulating the formation of planetary systems, the authors conclude that finding more TNOs and ETNOs would aid considerably in determining the conditions under which our solar system (and other planetary systems) formed. Regardless, 2015 BP519 is another in a growing set of bizarre objects that occupy the outer reaches of our solar system.

About the author, Will Saunders:

I am a first year Ph.D. student at Boston University, where I am trying to decide what to study. I received my Bachelors in Physics from the University of Pennsylvania. In my free time I enjoy listening to podcasts, visiting museums, and tasting new wines.

voids

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The Local Perspective on the Hubble Tension: Local Structure Does Not Impact Measurement of the Hubble Constant
Author: W. D’Arcy Kenworthy,  Dan Scolnic, and Adam Riess
First Author’s Institution: Johns Hopkins University
Status: Submitted to ApJ

Why so Tense?

Feeling a bit tense these days? So is the value of the Hubble constant. This parameter, written as H0, governs the rate of expansion of the universe, caused by some unknown dark energy. Despite cosmologists’ best efforts at massaging out the knots, H0 has long suffered from a tension between competing measurements. Today’s paper uses more precise data to reevaluate one possible cause of this tension.

The Hubble constant is the proportionality factor between the distance of a galaxy and the speed with which it moves away from us; galaxies that are further away move away increasingly faster, as expected from the expansion of space. We measure H0 in units of km/s/Mpc, which I like to think of as the clogs of the unit world: clunky-looking but really quite sensible. It means that for every megaparsec (Mpc) further away you look, the galaxies there appear to speed away faster by H0 kilometers per second.

There are two main independent ways H0 has been measured: from the cosmic microwave background (CMB) and from the cosmic distance ladder. The former measurement was most recently made by the Planck satellite; for details, check out this astrobite. This gives a value of H0 = 67.4 ± 0.5 km/s/Mpc.

Cosmic distance ladder

Figure 1: The cosmic distance ladder. We use stellar parallax, Cepheid variable stars and Type Ia supernovae (among others) as rungs along the ladder to measure distances to sources. [NASA/ESA]

For the latter measurement, we can just plot the speed of each galaxy as a function of its distance away from us. Fitting a line to this relation gives a value of H0 = 73.52 ± 1.62 km/s/Mpc. At first glance this isn’t too shabby, but looking closer, it disagrees with the CMB measurement by a quite statistically significant 3σ. Either there is some unknown physics at play, or at least one of our two methods is wrong.

A Void in the Distance Ladder

One place an issue could be hiding is in how we measure the distances to the galaxies, with what is known as the cosmic distance ladder, shown in Figure 1. The latest measurements are based on on Type Ia supernovae, which are fairly reliable standard candles. But to calibrate these distances, we need other more close-by candles, often Cepheid variable stars in local galaxies. You can guess where this is going: to calibrate the Cepheids, we have to keep stepping down the ladder. If any of these ladder rungs are “broken,” the more distant measurements will have a systematic offset.

Today’s paper considers what would happen if our distance measurements to nearby galaxies were off. This could be the case if the Milky Way were at the center of a local void (an under-density of galaxies), also known as a Hubble Bubble. This would cause the surrounding galaxies to be more strongly drawn towards higher-density regions, away from us. The extra pull would make the value for H0 that we measure locally higher than the true value; fixing this would bring it closer to the CMB measurement.

A Drop-Off in the Hubble Constant?

The authors use a sample of 1,295 supernovae covering a range of distances to probe the local structure with higher precision than has been done previously. They plot a modified Hubble diagram, known as a magnitude-redshift diagram (Figure 2), from which they calculate the value of H0.

supernova Hubble diagram

Figure 2: A Hubble diagram of the supernovae in today’s paper. Rather than the classic plot of galaxy velocity as a function of distance, with the slope of the points giving the value of the Hubble constant, this uses magnitude as a proxy for distance, and the x-axis is related to the velocity. The Hubble constant can be calculated from the x-intercept. [Kenworthy et al. 2019]

To investigate if there could be a void altering their measurement, they check how the Hubble constant changes with redshift. If there is a sharp drop-off in the value of H0, that could suggest a void with an edge at that redshift. The authors measure this by splitting the supernovae into two bins, above and below a given redshift zsplit, and calculating the difference in H0 between these samples.

Devoid of Voids

The results of this are shown in Figure 3. The authors find that the biggest change is at z = 0.023, but only to a significance of <2σ. This means that any inhomogeneities in the local density would have only a small effect on their measurement of the Hubble constant. All of the differences are much weaker than the offset needed to resolve the tension with CMB data, so voids clearly can’t explain the entire discrepancy.

Hubble constant as a function of void redshift

Figure 3: The change in the value of the Hubble constant as a function of the redshift of a potential void edge. The red crosses are voids predicted in previous works. The small changes disfavor the idea that a local void at any redshift has a significant effect on the Hubble constant. [Kenworthy et al. 2019]

To see if voids could still make some difference, the paper revisits two void models that have been predicted in previous works. They are at redshifts of z = 0.05 (KBC) and z = 0.07 (WS14), plotted as red crosses on Figure 3. They find that with their updated analysis, evidence for the voids evaporates: the changes in H0 at both of these redshifts disfavor the void models by 6.2σ and 4.5σ respectively.

While the paper only investigates a simple, sharp-edged void model, it makes a strong case that local voids aren’t significant to the measurement of the Hubble constant. Future supernova observations will allow us to probe other systematics that may be just the masseuse we need to relieve the Hubble tension.

About the author, Kate Storey-Fisher:

Kate is a PhD student in the Center for Cosmology and Particle Physics at New York University. She studies the large-scale structure of the universe using cosmological simulations and galaxy surveys. She is still waiting for the galaxies to respond to the SurveyMonkey she beamed to them.

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