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black hole

The black holes we’ve observed in the universe typically fall into two categories: small star-sized black holes, and gargantuan black holes lurking at the centers of galaxies. Now, a new black-hole discovery sheds some light on the gray area between these extremes.

Growing Together

two types of accreting black holes

Illustrations of two types of accreting black holes: a stellar-mass black hole accreting from a binary companion (top) and a supermassive black hole accreting gas in a galaxy’s center (bottom). [Top: ESA/NASA/Felix Mirabel; Bottom: ESO/M. Kornmesser]

Stellar-mass black holes of up to 100 solar masses are scattered by the millions throughout galaxies. At the opposite end of the spectrum, most galaxies are thought to contain just one massive black hole: a black hole of millions to tens of billions of solar masses that lies in the galaxy’s core.

Intriguingly, the mass of these central black holes seems to be inherently tied to that of their host. An empirical relationship known as the M-σ relation shows a correlation between a central black hole’s mass and the spread of star velocities in its host galaxy’s bulge, which acts as a proxy for the bulge mass. The M-σ relation and other, similar relationships show that black holes seem to grow in tandem with their host galaxies throughout the universe.

If the M-σ relation holds across a broad range of masses, then we would expect to find smaller massive black holes at the hearts of especially low-mass galaxies. So far, evidence for these low-mass central black holes has been scarce. But a new study led by Ingyin Zaw (New York University Abu Dhabi, UAE) has now delivered a low-mass massive black hole for us to contemplate.

IC 750

A 4’ x 4’ view of IC 750, a low-mass galaxy that hosts a massive (though less so than expected!) black hole at its center. [Sloan Digital Sky Survey]

Mass Measurement from Masers

Zaw and collaborators used the Very Long Baseline Array to obtain radio observations of the low-mass galaxy IC 750.

At the galaxy’s heart, the authors found emission from water masers, clumps of water molecules that emit light naturally in a process similar to laser emission. Light from the masers shows that they are orbiting in a disk around a compact central mass — a massive black hole — and Zaw and collaborators used their motion to measure the mass enclosed in their orbit, providing an upper limit on the black hole’s mass.

The authors then reduced and analyzed publicly available multiwavelength data to understand the location of the black hole and measure the properties of its host galaxy.

A Decidedly Low-Mass Monster

M-sigma relation

This plot of black hole mass vs. bulge stellar velocity dispersion (the M-σ relation) shows IC 750 marked with a red star, with upper mass limits indicated by downward-pointing arrows. It falls two orders of magnitude below where we’d expect it to lie on the relation. Click to enlarge. [Zaw et al. 2020]

The result? IC 750’s central massive black hole is a definite lightweight, with an upper limit of 140,000 solar masses — and it may actually be less than a third of that weight. Not only is this remarkably small for a central massive black hole, it’s also unusually light even relative to the mass of its host galaxy: IC 750’s black hole lies two orders of magnitude below where it should sit on the M-σ relation!

What’s going on with this unusual object? There are two possible explanations: either there’s more scatter at the low-mass end of the M-σ relation, or the scaling relationship is simply different for low-mass galaxies. The latter option is supported by some simulations that suggest that black holes don’t grow efficiently in low-mass galaxies.

Though we don’t yet know which explanation is more likely, more observations like those presented here will eventually fill in our picture of these low-mass massive monsters.

Citation

“An Accreting, Anomalously Low-mass Black Hole at the Center of Low-mass Galaxy IC 750,” Ingyin Zaw et al 2020 ApJ 897 111. doi:10.3847/1538-4357/ab9944

UX Tauri A disk

The gaseous, dusty disks surrounding newly born stars can reveal a wealth of information about how distant stellar systems form and evolve. In a new study, scientists have now watched the interaction of two such disks in a stellar flyby.

Spotting Spirals

In the past decade, new instrumentation has led to a dramatic improvement in our views of circumstellar environments. We’ve spotted remarkable structure in the dusty disks that surround newborn stars — including, in many cases, pronounced spiral arms.

MWC 758

An example of spiral arms detected in a protoplanetary disk, MWC 758. [NASA/ESA/ESO/M. Benisty et al]

The presence of these spiral arms has provoked much discussion and debate. Are they caused by gravitational instabilities in the gas and dust? Or are they produced by perturbations from unseen, newborn planets orbiting within the disks? While both of these explanations could be at play in different systems, there’s an additional possibility to consider: the arms could be excited by tidal interactions with another star.

In a new study led by Luis Zapata (UNAM Radio Astronomy and Astrophysics Institute, Mexico), a team of scientists has used the sensitive and high-angular-resolution observations of the Atacama Large Millimeter/submillimeter Array (ALMA), located in Chile, to understand how tidal interactions with an orbiting star might be responsible for spiral arms observed in UX Tauri.

A New Disk Found

Located ~450 light-years away, the UX Tauri system consists of four stars: UX Tau A (the main star), UX Tau B (a binary star), and UX Tau C (a close companion that lies just to UX Tau A’s south). Past observations have revealed a disk of gas and dust around UX Tau A exhibiting distinct spiral arms.

ALMA obs of UX Tau

The intensity (top) and radial velocities (bottom) of the molecular gas observed in UX Tauri reveals a disk around both UX Tau A (top star in both images) and UX Tau C (bottom star), as well as a stream of gas connecting the two. Curves tracing the spiral arms in the disk surrounding UX Tau A are overlaid in the top image. [Adapted from Zapata et al. 2020]

Zapata and collaborators have now followed up with detailed ALMA observations to explore the structure of the molecular gas and dust in UX Tau. In addition to further resolving the disk around UX Tau A, the team was also able to detect — for the first time — molecular gas swirling in a disk around UX Tau C. What’s more, the observations reveal tidal interactions between the two disks that surround these stars.

Drama in UX Tau

What do these findings mean? Zapata and collaborators suggest that we’re witnessing a close flyby of UX Tau C as it progresses on a wide, evolving, and eccentric orbit around the disk of UX Tau A. As UX Tau C plowed through UX Tau A’s circumstellar disk, it captured some of the gas, forming its own disk. Through its motion and this tidal interaction, UX Tau C also excited the observed spiral arms in UX Tau A’s disk.

The drama spotted in UX Tauri represents one of the few cases of binary disk interactions that have been mapped out in molecular gas — but this is likely a common occurrence, since stars often occur in multiple-star systems. Sensitive observations like the ALMA detections presented here will likely reveal more such interactions in the future, shining additional light on the process of star and planet formation.

Citation

“Tidal Interaction between the UX Tauri A/C Disk System Revealed by ALMA,” Luis A. Zapata et al 2020 ApJ 896 132. doi:10.3847/1538-4357/ab8fac

STS-41C Nelson and SolarMax

In 1984, a rescue mission took place in space — ultimately saving a spacecraft that went on to make some of the most detailed observations we have of solar flares during a highly active solar cycle. Now, more than three decades later and thanks to some clever recalibration, we’re still reaping the rewards.

A Dramatic Rescue

STS-41-C Nelson and van Hoften

STS-41-C astronauts George Nelson (right) and James van Hoften (left) repair the SolarMax satellite in the Challenger shuttle’s open payload bay. [NASA]

In the first-ever attempted repair of an orbiting satellite — almost a decade before the first Hubble servicing mission — the crew of space shuttle mission STS-41-C were tasked with capturing the Solar Maximum Mission (SolarMax) spacecraft in orbit so that they could repair its fine-pointing system and several instruments.

The attempted retrieval process was anything but smooth. The astronauts’ jetpack-powered space-walk rendezvous with SolarMax (pictured above) — during which they intended to dock with the satellite and slow its gentle rotation — failed, with the spacecraft entering an uncontrolled tumble after an astronaut tried to slow it by grasping a solar panel. Unable to point its panels at the Sun, SolarMax’s batteries started to drain, and the astronauts were forced to retreat to the shuttle to regroup.

Ultimately, the team managed a last-minute rescue, retrieving the satellite using the shuttle’s robotic grappling arm and maneuvering it into the payload bay. After performing a multi-hour repair, the victorious crew returned SolarMax to orbit — and their rescue extended the mission’s lifetime by another five years.

Curved Crystals to Gather X-rays

What was SolarMax, and why was it worth repairing? The satellite carried an array of instruments all designed to observe the flaring Sun at ultraviolet, X-ray, and gamma-ray wavelengths.

BCS

Schematic of the Bent Crystal Spectrometer shows how a curved crystal was used to produce X-ray spectra. [Sylwester et al. 2020]

Launched at the peak of solar cycle 21, SolarMax gathered observations from February to November 1980, and then again from 1984 to 1989 after its repair. The combination of Solar Max’s long operation time and the especially high activity levels of the solar cycles it observed renders much of its data unparalleled for flare studies.

One of the valuable instruments on SolarMax was the Bent Crystal Spectrometer (BCS), an instrument that pioneered the concept of using curved crystals in space instrumentation to produce X-ray spectra. The spectral resolution of the BCS remains the highest of any long-operating solar X-ray spectrometer, and the instrument’s observations provide valuable insight into the properties of the hot plasma in solar flares.

Now, in a new study led by Janusz Sylwester (Space Research Center, Polish Academy of Sciences), a team of scientists has conducted a valuable recalibration of BCS spectra.

Looking Beyond the Sun

By examining data produced during the scan of a solar flare in November 1980, Sylwester and collaborators are able to identify and quantify the effect of small deformations in the crystal curvature of one of the channels. Accounting for these deformations resolves a long-standing mystery of certain anomalies in the ratios of emission lines in BCS data.

EV Lacertae

Artist’s impression of a record-setting stellar flare from the nearby star EV Lacertae. [Casey Reed/NASA]

The authors additionally improve other calibration aspects, ultimately producing high-resolution line spectra that they suggest could now be used as templates for the analysis and interpretation of future observed X-ray spectra — in particular, spectra gathered from other active, flaring stars in our galaxy.

With revitalization efforts like this, observations from BCS thus continue to be valuable many years after the mission end — well justifying the daring in-orbit rescue of SolarMax.

Citation

“A Unique Resource for Solar Flare Diagnostic Studies: the SMM Bent Crystal Spectrometer,” J. Sylwester et al 2020 ApJ 894 137. doi:10.3847/1538-4357/ab86ba

Ancient Mars oceans

Was Mars once partially covered in ancient seas? A recent study has found new evidence to support the Mars ocean hypothesis.

Signs Point to Seas

Mars topography

A map of Mars’s topography created by the Mars Orbiter Laser Altimeter shows two distinct southern and northern hemispheres. Click to enlarge. [NASA/JPL/GSFC/MOLA. Map by Emily Lakdawalla]

The debate about large bodies of water on Mars has raged for decades.

The discovery of the Martian dichotomy — a ~30 km difference in height between the lower, smooth northern hemisphere and higher, heavily cratered southern hemisphere — was an early indication that the northern hemisphere may once have been underwater.

Long, unbroken features identified as potential ancient shorelines were spotted on Mars as early as the 1970s by the Viking orbiters, further supporting a picture of vast past oceans.

And dozens of further clues have piled up: signs of stream channels and river deltas, indications of past rainfall, hints of historical tsunamis from asteroid impacts, atmospheric signatures that point to a past abundant water supply — the list goes on.

Mars sea level

In this proposed map of an ancient ocean on Mars, underwater regions are indicated in white, and the above- and below-water regions are separated by a proposed sea level (solid line). [Saberi 2020]

A Liquid Controversy

Yet the Mars ocean hypothesis remains highly controversial.

The Martian dichotomy could have been caused by a megaimpact, or by thinning of the northern-hemisphere crust. Observed “stream channels” might instead have been created by wind erosion. And those possible ancient shorelines? They’re not actually level — they vary in elevation by a couple of kilometers along their length, which is not something a sea level should do.

On this other hand, the shorelines might have started level and been deformed more recently. Clearly, more evidence is needed in the debate over Martian oceans — and now we’ve got it, in a new study by Abbas Ali Saberi (University of Tehran, Iran; University of Cologne, Germany).

Moving the Level

Saberi argues that a giant ancient ocean covering part of Mars should have left distinct signatures in the elevation profile of the planet — signatures that we can identify in comparison with Earth’s topography. Saberi combines detailed maps of Mars’s surface altitudes from the Mars Orbiter Laser Altimeter with statistical modeling called percolation theory to search for these signatures.

phase transition

As sea level height is changed in the authors’ models, the relative surface area of the largest island to the total area of Mars (red) and Earth (blue) shows a jump for both planets that corresponds to Earth’s current sea level and a sea level height of 1,400 m on Mars. [Saberi 2020]

By modeling an ocean covering Earth’s surface and varying the height of the sea level, Saberi shows that the planet undergoes a “phase transition” at a characteristic sea level when its land masses connect. For Earth, this transition occurs right around our present mean sea level, corresponding to oceans covering 71% of Earth’s surface (as we observe today).

When Saberi applies this model to Mars’s topography, a phase transition occurs in the exact same way, showing a distinct change at a sea level of 1,400 meters (measured relative to Mars’s equatorial radius of 3,396.2 km) and indicating that Mars may indeed have had an ancient sea of this height. This elevation also displays the longest contour lines in Mars’s topography, further strengthening the argument that it corresponds to an ancient sea level.

So is the question of past oceans on Mars settled? Far from it! But we can expect clues to continue to roll in with upcoming missions to the Red Planet, like the Mars 2020 mission launching later this month.

Citation

“Evidence for an Ancient Sea Level on Mars,” Abbas Ali Saberi 2020 ApJL 896 L25. doi:10.3847/2041-8213/ab982d

SN 1994D

Accurate distance measurements are critical to astronomy. A Type Ia supernova is one of the few objects that we can trust for making distance measurements since they have a fixed peak brightness. But can the brightness of such a supernova change significantly based on the properties of its host galaxy? And what does this mean for our understanding of dark energy?

Measurements of the Hubble constant via different methods over time. Type Ia supernovae are used in conjunction with Cepheid variable stars for the Cepheid method. CMB stands for Cosmic Microwave Background. TRGB stands for “tip of the red giant branch”, which refers to a certain set of stars. The discrepancy between measurements of the Hubble constant has grown with time despite increasing precision. Click to enlarge. [Freedman et al. 2019]

Lighthouses in the Distant Universe

A Type Ia supernovae is what’s known as a “standard candle” — we know what its brightness is at a particular distance, and when we observe these supernovae in distant galaxies, we can extrapolate to determine how far away those galaxies are. Like lighthouses, the fainter a Type Ia supernova is, the further away it is.

Accurate distance measurements form the backbone of astronomy, and Type Ia supernovae are especially valuable because they allow us to measure extremely large distances beyond the reach of other standard candles. However, the properties of Type Ia supernovae were and are determined empirically, so a mistaken assumption about a supernova can trickle down and cause miscalculations further down the road.

Type Ia supernova distances are in play in a rather contentious part of astronomy: measurements of the Hubble constant. The value of the Hubble constant is measured in one of two ways: using the cosmic microwave background (CMB), and using Type Ia supernovae and variable stars called Cepheids to measure the distances and velocities of far-off galaxies. The value measured with supernovae is significantly larger than the value measured with the CMB, however — and it suggests the presence of “dark energy”, a mysterious energy that’s accelerating the expansion of the universe.

Recently, it’s been hypothesized that the supernova-based measurement is biased by an overlooked relation between peak Type Ia supernova brightnesses and the age of their host galaxies. Accounting for this brightness–age relation, if it holds up, could eliminate the need for dark energy and relieve the discrepancy between the two measurements of the Hubble constant. But a new study led by Benjamin Rose (Space Telescope Science Institute) now refutes this proposed relation.

Light curve of the supernova SN2003ic in three different bands. SN2003ic was one of the 10 supernovae that didn’t pass cosmological quality cuts. Excluding this supernova alone causes the significance of the brightness–age relation to drop below the necessary threshold. [Rose et al. 2020]

Sample Adjustments

Rose and collaborators started their analysis by examining the sample of 34 Type Ia supernovae that was used to claim the possible brightness–age relation. The authors found that 10 supernovae in the sample fail at least one of the quality cuts typically used for cosmological studies. These include the supernova not being observed prior to its peak brightness, and an overall lack of observations.

Rose and collaborators also argue that the prior study didn’t correctly account for the error on Type Ia supernova distances. Once these errors are accounted for and quality cuts are made to the supernova sample, the brightness–age relation appears negligible to measurements of the Hubble constant.

Hubble–Lemaître residuals versus host galaxy age for Type Ia supernovae in the Pantheon sample. The residuals denote the difference between predicted and measured values of the supernova brightness. The black points are individual supernovae and the blue points are bins of 25 supernovae. The red dashed line is the claimed brightness–age relation based on the sample of 34 Type Ia supernovae, and the dark line is the relation as determined by the Pantheon sample. [Rose et al. 2020]

Rose and collaborators also attempted to determine a brightness–age relation using a larger, robust sample of 254 Type Ia supernovae. They found no relation significant enough to suggest that the supernova distances had been misestimated — so there should be no changes to the supernova-based measurement of the Hubble constant.

While this particular relation may not have borne out, Rose and collaborators agree that the properties of Type Ia supernovae must be constrained as much as possible for reliable distance measurements to be made. For now, however, it looks like dark energy may be here to stay!

Citation

“Evidence for Cosmic Acceleration Is Robust to Observed Correlations between Type Ia Supernova Luminosity and Stellar Age,” B. M. Rose et al 2020 ApJL 896 L4. doi:10.3847/2041-8213/ab94ad

FRB

Earlier this year, we gained new insight into the origins of fast radio bursts (FRBs) when FRB 180916 became the first of these strange sources observed to exhibit repeated bursts in a periodic pattern.

Now, we’re taking a look at four recent studies detailing some of the latest observations and theories of FRB 180916 — and what this tells us about the population of FRBs as a whole.

What Do We Already Know About FRB 180916?

FRBs are incredibly energetic bursts of radio waves that last just a few milliseconds and originate from extragalactic sources. We’ve detected more than 100 of them, and this number is growing rapidly. Yet despite these observations, we still don’t know what causes FRBs — though we have dozens of theories.

FRB 180916

The location of FRB 180916 is in a galaxy’s spiral arm, marked with a green circle in this image from the Gemini-North telescope. [NSF’s Optical-Infrared Astronomy Research Laboratory/Gemini Observatory/AURA]

The majority of the FRBs we’ve detected are one-off events, but a growing number have been observed to flash repeatedly — usually in a wildly unpredictable fashion. Recently, however, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope announced the first detection of a periodic pattern in one repeating FRB’s bursts.

The intriguing source is FRB 180916, a repeating FRB that’s under 500 million light-years away. CHIME’s recent observations of FRB 180916 indicate that its activity is modulated by a 16.35-day period: its bursts fall within an active window of about 5 days, and then the source is quiet for about 11 days before restarting activity.

This new information is helping scientists narrow the possible explanations of what causes FRBs — but other recent observations have provided additional insight.

New Clues from Multiple Telescopes

In two new studies led by Maura Pilia (INAF Cagliari Observatory, Italy) and Pragya Chawla (McGill University, Canada), independent teams have now detected the lowest-frequency observations of radio bursts from FRB 180916 — or, indeed, of any FRB — yet.

FRB 180916 burst profile

A profile of one of the bursts from FRB 180916 detected by the Sardinia Radio Telescope. [Adapted from Pilia et al. 2020]

Pilia and collaborators observed three bursts at 328 MHz using the Sardinia Radio Telescope in Italy, and Chawla and collaborators observed seven bursts in the 300–400 MHz frequency band using the Green Bank Telescope in Virginia.

These observations tell us two main things:

  1. The environment surrounding the source of the bursts doesn’t significantly affect the emission we see.
  2. The lower-energy cutoff for the emission from FRB 180916’s bursting mechanism is lower than we thought it was — if there is even a cutoff at all!

These clues work to constrain the possible models of FRB sources — for instance, rendering the models that predict dense environments around the source, such as young supernova remnants, unlikely. Some models still seem highly plausible, however — and highly magnetized neutron stars are looking especially appealing.

Analyzing a Plausible Explanation

Two independent theoretical studies — one by J.J. Zanazzi (CITA, University of Toronto) and Dong Lai (Cornell University and UC Berkeley), and the other by a team of researchers led by Yuri Levin (Columbia University, Flatiron Institute; Monash University, Australia) — recently focused on the picture of a lone, spinning neutron star threaded with a strong magnetic field.

precessing magnetar schematic

Schematic showing how a precession-driven wobble of a magnetar can lead to magnetic flares that move in and out of our line of sight. [Levin et al. 2020]

Both studies demonstrate that such a magnetar can become deformed by its internal magnetic field, causing it to wobble as it rotates, precessing around its rotational axis. As this hyperactive star releases magnetic flares, we then see those bursts only periodically as the precession of the star brings them into and out of our view.

This picture is consistent with the periodic radio emission we’ve detected thus far. What’s more, we may be able to test this theory soon, since magnetized neutron stars are expected to gradually spin slower over their lifetimes. If such an object is the source of FRB 180916, then we’d expect to see a detectable increase in the periodic modulation of the bursts within a year. 

There’s still work to do, but with growing observations — and the possibility now of exploring at even lower frequencies — it’s exciting to watch our progress toward an explanation for these mysterious bursts!

Citation

“Detection of Repeating FRB 180916.J0158+65 Down to Frequencies of 300 MHz,” P. Chawla et al 2020 ApJL 896 L41. doi:10.3847/2041-8213/ab96bf
“The Lowest-frequency Fast Radio Bursts: Sardinia Radio Telescope Detection of the Periodic FRB 180916 at 328 MHz,” M. Pilia et al 2020 ApJL 896 L40. doi:10.3847/2041-8213/ab96c0
“Periodic Fast Radio Bursts with Neutron Star Free Precession,” J. J. Zanazzi and Dong Lai 2020 ApJL 892 L15. doi:10.3847/2041-8213/ab7cdd
“Precessing Flaring Magnetar as a Source of Repeating FRB 180916.J0158+65,” Yuri Levin et al 2020 ApJL 895 L30. doi:10.3847/2041-8213/ab8c4c

AGN outflow star formation

Looking for a fireworks show this 4th of July? Try checking out the distant universe, where powerful jets flung from supermassive black holes slam into their surroundings, lighting up the sky. Though these jets are hidden behind shrouds of gas and dust, a new study has now revealed some of these young powerhouses.

A Galaxy–Black-Hole Connection

Centaurus A

This composite image of Centaurus A shows an example of large-scale jets  launched from an AGN, which can eventually extend far beyond the galaxy, as seen here. [ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray)]

In the turbulent centers of active galaxies (active galactic nuclei, or AGN), gas and dust rains onto supermassive black holes of millions to billions of solar masses, triggering dramatic jets that plow into the surrounding matter and light up across the electromagnetic spectrum.

The growth of a supermassive black hole is thought to be closely tied to the evolution of its host galaxy, and feedback like these jets may provide that link. As the jets collide with the gas and dust surrounding the galaxy’s nucleus, they can trigger a range of effects — from shock waves that drive star formation, to gas removal that quenches star formation.

To better understand the connections between supermassive black holes and their host galaxies, we’d especially like to observe AGN at a time known as Cosmic Noon. This period occurred around 10 billion years ago and marks a time when star formation and supermassive black hole growth was at its strongest.

The Hidden World of Cosmic Noon

But there’s a catch: around Cosmic Noon, galaxies were heavily shrouded in thick gas and dust. This obscuring material makes it difficult for us to observe these systems in short wavelengths like optical and X-ray. Instead, we have to get creative by searching for our targets at other wavelengths.

redshift distribution

The redshift distribution of the authors’ sample, based on spectroscopic redshifts of 71 sources. The sources span the period of peak star formation and black hole fueling around Cosmic Noon. [Patil et al. 2020]

Since AGN emission is absorbed by the surrounding dust and re-radiated in infrared, we can use infrared brightness to find obscured but luminous sources. To differentiate between hidden clumps of star formation and hidden AGN, we also look for a compact radio source — a signature that points to a jet emitted from a central black hole.

A team of scientists led by Pallavi Patil (University of Virginia and the National Radio Astronomy Observatory) has now gone on the hunt for these hidden sources at Cosmic Noon.

Newly-Triggered Jets Caught in the Act

Patil and collaborators observed a sample of 155 infrared-selected sources, following up with high-resolution imaging from the Jansky Very Large Array to identify compact radio sources. From their observations and modeling of the jets, the authors estimate these sources’ properties.

10 GHz continuum images

The JVLA 10 GHz radio continuum observations for four sources in the authors’ sample. The cyan plus symbol marks the infrared-obtained source position. The color bars indicate flux in mJy/beam. [Adapted from Patil et al. 2020]

The authors find bright luminosities, small sizes, and high jet pressures — all of which suggest that we’ve caught newly-triggered jets in a short-lived, unique phase of AGN evolution where the jets are still embedded in the dense gas reservoirs of their hosts. The jets are expanding slowly because they have to work hard to push through the thick clouds of surrounding material. Over time, the jets will likely expand to larger scales and clear out the surrounding matter, causing the sources to evolve into more classical looking radio galaxies.

What’s next? The authors are currently working on a companion study to further explore the shapes of the jets and their immediate environments. These young, hidden sources will provide valuable insight into how supermassive black holes evolve alongside their host galaxies.

Citation

“High-resolution VLA Imaging of Obscured Quasars: Young Radio Jets Caught in a Dense ISM,” Pallavi Patil et al 2020 ApJ 896 18. doi:10.3847/1538-4357/ab9011

beta pic

The planets in our solar system all orbit in roughly the same direction as the Sun spins — but this isn’t true for all planetary systems! Recent measurements of the spin angle of a nearby, planet-hosting star provide new insight into how solar systems form.

The Birth of a Solar System

Protoplanetary disk

If planets form from the same rotating material that created their host star, why don’t all planets have orbits that are aligned with their stars’ spins? [NASA/JPL-Caltech]

In a widely accepted theory for solar system formation, a star and its planets are born from the same swirling nebula of gas and dust. As the nebula collapses, it forms a spinning star at its center, with the remaining matter flattening into a rotating disk around the newborn star. Planets later form within this disk.

In this picture, conservation of angular momentum suggests that the spin axis of a star should be aligned with the orbital angular momentum vectors of its planets — a state known as spin–orbit alignment.

This is true in our solar system: our planets’ orbits are aligned to within 7° of the Sun’s spin. But roughly a third of the planets we’ve measured in other systems have significant misalignments — ranging from slightly tilted orbits to orbits that are fully the opposite direction of their star’s spin.

Explaining Crooked Paths

There are two possible explanations for these misaligned orbits:

  1. A star’s spin and its planet-forming disk might be misaligned from the start — perhaps due to effects like turbulence or dynamical interactions in the star’s birth environment.
  2. Planet orbits and stellar spin start out aligned, but some planets get scattered or nudged onto misaligned orbits after they form.
Beta Pictoris image

β Pictoris b is a directly imaged planet that orbits at ~10.6 AU from its host star, as seen at the center of this composite infrared image of the system. [ESO/A.-M. Lagrange et al.]

So far, we’ve only been able to measure spin–orbit alignment angles for short-period hot Jupiters, planets that we already suspect may have formed elsewhere and been driven inward onto their current close orbits. These planets could easily have had their orbits misaligned during this migration — which lends credence to the second explanation above.

But to confirm that this explanation fits, we’d also need to show that the spin–orbit alignments of long–period planets — which are less likely to have been perturbed over their lifetimes — are typically aligned.

In a new study, a team of scientists led by Stefan Kraus (University of Exeter, UK) have now made the first measurement of the spin–orbit alignment of a directly imaged, wide-orbit exoplanet: β Pictoris b.

Confirmation from a Wide Orbiter

Located just ~60 light-years away, the recently formed star β Pictoris hosts a roughly 13-Jupiter-mass exoplanet — β Pictoris b — that orbits with a semimajor axis of 10.6 AU within a young debris disk that surrounds the star.

beta pictoris schematic

Schematic illustrating the components of the β Pictoris system. Click to enlarge. [Adapted from Kraus et al. 2020]

Kraus and collaborators conduct high spectral resolution observations of β Pictoris with the GRAVITY instrument on the Very Large Telescope Interferometer in Chile, identifying the angle of the star’s spin from subtle signatures in its spectrum. From these measurements, the authors establish that β Pictoris’s stellar spin, its debris disk rotation, and its planet’s orbit are all aligned to within roughly 3°.

Kraus and collaborators’ results support the idea that solar systems initially form with aligned stellar spin and planet orbits; misalignments are introduced only later as planets migrate. Additional observations of wide-orbit planets will be needed to confirm this picture — but we can hope to gather more in the future using the techniques demonstrated in this study!

Citation

“Spin–Orbit Alignment of the β Pictoris Planetary System,” Stefan Kraus et al 2020 ApJL 897 L8. doi:10.3847/2041-8213/ab9d27

early solar system

The solar system extends well beyond Pluto, encompassing small objects on their own unusual orbits around the Sun. How did they get there? A new study attempts to answer this question with simulations.

Models and Moving Objects

The largest objects in the solar system wield the most influence. Models that account for the Sun and the outer planets — Jupiter, Saturn, Uranus, and Neptune — can produce realistic approximations of the solar system’s overall gravitational influence.

So if you have a model of the major gravitational forces at play, you can drop in orbiting objects and see what they do over time. This sounds simple, but it’s a powerful tool when it comes to understanding the current structure of our solar system.

Apsidal Alignment

The evolution of the surface density of the disk with time (starting from the upper-left) as seen face-on (top) and edge-on (bottom). Click to enlarge. Yellow regions have a higher density than blue regions. The timescale P represents 1,000 years. The authors note the “cone” of orbits present prior to t = 4,300 P, as well as the coherent ring of orbits most prominent at t = 9,900 P, which corresponds to an “m = 1 mode”. [Zderic et al. 2020]

A new study led by Alexander Zderic (University of Colorado Boulder) looks at what would happen to a large disk of small objects orbiting in the outskirts of our solar system. This study is the latest in a line of similar studies attempting to understand large scale structure in the solar system.

A Disk on the Outskirts

The disk being examined by Zderic and collaborators consists of objects orbiting at roughly 100 to 1,000 astronomical units (au) from the Sun. For context, Pluto’s farthest distance from the Sun is just 50 au, so these distances definitely qualify as the outer solar system. The orbits of the disk objects all start off in the same plane (which is also the plane in which the solar system’s planets orbit), but they have a higher than average eccentricity (as conditions in the outer solar system require).

In previous studies with higher mass disks, the disk conditions have been shown to reach a consistent state within 660 million years of simulated time. Zderic and collaborators were interested in this consistent state, which reflects the long-term behavior of the disk. To reach this state more quickly, the authors used an equivalent setup: they started with a less massive disk, and they ran their simulation for just under 10 million years.

Evolution of orbital parameters

Evolution of two orbital parameters for a particular object, specifically eccentricity (y-axis) and the longitude of perihelion (x-axis; the sum of two other orbital properties that sets the orientation of the orbit relative to a plane). Between 5,000 P and 9,000 P, the object under consideration is in the m = 1 mode. [Zderic et al. 2020]

Modes in Models

As the disk of objects evolves, the authors show that the collective gravity of the small bodies can induce an instability. As a result, the final state of the disk has a significant feature: orbits appear to cluster in a particular region. This is called being in a “mode”, which is shorthand for a group of orbital parameters having specific values. Zderic and collaborators note that later in the simulation, objects tend to settle into the m = 1 mode, though objects also fall in and out of the mode. Additionally, adding more particles to the simulation shows that objects stay in the mode longer. Extrapolating to the solar system, the mode may be stable for as long as the solar system is around.

Why is this interesting? These simulations show that the collective gravity of small bodies in a disk can naturally reproduce many of the observed behaviors of objects in our outer solar system — including extreme trans-Neptunian objects (TNOs), small bodies beyond the orbit of Neptune that are on very unusual orbits.

eTNO orbit clustering

Schematic showing the observed alignment of the orbits of detached extreme TNOs and the proposed orbit of a hypothetical super-Earth-mass planet (in green). But is Planet Nine actually necessary to explain the extreme TNO orbits? [Sheppard et al. 2019]

In particular, extreme TNOs have been observed to have clustered orbital properties — a fact that has been used to argue for the presence of an additional, hypothesized giant planet in our outer solar system, Planet Nine. But if Zderic and collaborators are correct, there’s no need for a hidden planet to explain extreme TNO alignments.

Further work will require the simulation of high mass disks that are more similar to the early solar system. Keep an eye out for future studies exploring the cause of our solar system’s structure!

Citation

“Apsidal Clustering following the Inclination Instability,” Alexander Zderic et al 2020 ApJL 895 L27. doi:10.3847/2041-8213/ab91a0

neutron star or black hole

Neutron star or black hole? That’s the question scientists are asking about the latest gravitational-wave detection announced from the Laser Interferometer Gravitational-wave Observatory (LIGO) and its sister observatory, Virgo. In a new publication, scientists detail this newest addition to the list of confirmed collisions — and explain why it’s rather unexpected.

neutron-star merger

Artist’s impression of two merging neutron stars. [National Science Foundation/LIGO/Sonoma State University/A. Simonnet]

Still More from O3

Things have been decidedly quiet on the LIGO-Virgo front lately. The gravitational-wave detectors’ third observing run, O3, wrapped up in March (cut unfortunately short due to COVID-19). Since then, the collaboration has announced only two discoveries from this run: another binary neutron star merger, and the collision of two black holes of very unequal masses.

Yet there remain many dozens of potential candidates recorded during O3 that are still undergoing analysis to confirm whether they’re “true” detections — and, if they are, to identify the properties and astrophysical implications of the mergers.

GW190814 localization

Despite GW190814’s relatively small sky localization (shown here), no associated electromagnetic signature was found — but we also wouldn’t expect one from such an unequal-mass binary, as the secondary would likely have been swallowed whole. [Abbott et al. 2020]

Today, another one of these is officially confirmed: GW190814, an especially unusual collision of a large, ~23-solar-mass black hole with a smaller, ~2.6-solar-mass mystery object.

Thwarting Expectations

GW190814 was detected in August of last year by all three of the LIGO-Virgo detectors. The signal was localized to a region of just 18.5 square degrees, but follow-up observations didn’t detect any corresponding electromagnetic signatures from the area. Despite the lack of fireworks, however, GW190814 is anything but mundane — in fact, it’s a source unlike any other known compact binary merger.

 What makes GW190814 so weird? There are three main factors:

  1. Very unequal masses
    The two objects that collided to produce GW190814 had a mass ratio of q = 0.112. This is the most unequal mass ratio yet measured in a merger; most mergers consist of two objects that are nearly the same mass.
  2. black hole mass gap

    The mass gap between the heaviest neutron stars and the lightest black holes is clearly visible in this diagram of the masses of known compact binary components, compiled at the end of 2018. [LIGO-Virgo/Frank Elavsky/Northwestern]

    Low primary spin
    The primary object in the binary, the 23-solar-mass black hole, has an extremely low measured spin of χ1 ≤ 0.07. This is the tightest constraint we’ve ever placed on the spin of the primary component of a gravitational-wave source.
  3. Mass-gap secondary
    The secondary object in the binary, the 2.6-solar-mass body, falls right in the middle of what’s known as the mass gap: its mass is heavier than the heaviest confirmed neutron star (~2.5 solar masses), but lighter than the lightest confirmed black hole (~5 solar masses). So which is it: a neutron star or a black hole?
secondary mass

The marginalized posterior distribution describing the likely mass of the secondary object, for different waveform models. The 90% credible range is 2.50–2.67 solar masses, which lies firmly in the mass gap. [Abbott et al. 2020]

A Challenge to the Paradigm

Regardless of whether this unusual merger occurred between two black holes or between a black hole and a neutron star, GW190814 challenges our current models of compact binaries and their components.

How could a binary containing such a heavy black-hole primary and such a light neutron-star or black-hole secondary form? And what about the perceived mass gap between neutron stars and black holes — is it real? Or is it just an observing bias?

We have a lot of questions, and a lot of work to do to account for GW190814 in our theoretical models. But if GW190814 does, indeed, represent a whole new kind of compact binary merger, then we also have an exciting road ahead as LIGO-Virgo inevitably discovers more of them, allowing us to gradually piece together the puzzle of these unexpected collisions.

Citation

“GW190814: Gravitational Waves from the Coalescence of a 23 Solar Mass Black Hole with a 2.6 Solar Mass Compact Object,” R. Abbott et al 2020 ApJL 896 L44. doi:10.3847/2041-8213/ab960f

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