AAS 233: Day 3


Editor’s Note: This week we’re at the 233rd AAS Meeting in Seattle, WA. Along with a team of authors from Astrobites, we will be writing updates on selected events at the meeting and posting each day. Follow along here or at astrobites.com. The usual posting schedule for AAS Nova will resume next week.

Plenary Talk: The Energetic Universe in Focus: Twenty Years of Science with the Chandra X-ray Observatory (by Mia de los Reyes)

The Chandra X-ray Observatory is now almost 20 years old! Ryan Hickox from Dartmouth College started off the third day of #AAS233 by reviewing some of the remarkable discoveries that Chandra has made in the last two decades.

First, Hickox reminded everyone why exactly X-rays are so useful: they probe the bulk of the baryonic matter in the universe, especially in extreme environments. He also discussed the basic capabilities and structure of Chandra before turning to some of the major science questions that Chandra has helped answer.

Part 1: The lives and deaths of stars

Chandra has been used to study star-forming regions like Orion, monitoring how young stars produce intense X-ray flares.

Hickox then turned to the deaths of stars, highlighting beautiful X-ray observations of the supernova remnant Cas A. Chandra has observed the X-ray spectrum of Cas A and mapped out the spatial locations of different elements in the remnant, helping astronomers understand the explosion mechanism and how different elements were produced in the supernova. Chandra has even directly observed the neutron star at the center of the Cas A remnant!

Part 2: The growth and evolution of galaxies and black holes

“Chandra’s done lots of other work trying to understand black holes,” Hickox noted. For example, Chandra obtained a spectrum of the black hole binary GRO J1655; the absorption lines in the spectrum were strongly blueshifted, suggesting that large amounts of material are outflowing from the binary system. Such a large wind of material may be magnetically driven.

Cen A

Chandra image of Centaurus A, a galaxy hosting a gargantuan jet. [NASA/CXC/U.Birmingham/M.Burke et al.]

Chandra has also studied black holes much closer to home. Sagittarius A* is the supermassive (over millions of times the mass of the Sun) black hole in the center of our own Milky Way. Since X-ray luminosity is roughly proportional to black hole accretion rate, the observed X-ray outbursts from Sgr A* provide a handle on how it actually accretes. In other nearby supermassive black holes, like the active galactic nucleus Centaurus A, Chandra has observed relativistic jets and lobes — evidence for active galactic nuclei injecting energy into the interstellar medium of galaxies.

Hickox’s own research aims to understand these active galactic nuclei (AGN). In particular, what drives the growth of supermassive black holes over cosmic time? What’s the difference between AGN and other passive galaxies? X-ray surveys are uniquely suited to study these questions, because they can identify weak or obscured AGN that are undetectable in other wavelengths. Indeed, X-ray surveys with Chandra have found AGN in all types of galaxies; these data have shown that although black holes grow along with host galaxies over cosmic time, they “flicker” on short (less than million-year) timescales! These observations have opened more exciting questions about how exactly these black holes grow over time.

Part 3: Large-scale structures and their cosmic history

On even larger scales, Chandra has been used to probe the most massive structures in the universe. Chandra observations of the Virgo cluster have identified huge bubbles of hot gas around the galaxy at the center of the cluster, showing that the central galaxy is an AGN injecting energy fast enough to keep gas from cooling and forming stars. This impressive finding demonstrates that black holes regulate baryons on large scales in the universe!

Chandra observations have also been used to directly probe dark matter! The Bullet Cluster is one of the most famous examples of the direct observation of dark matter. The hot gas in the cluster, observable in X-rays, comprises most of the baryonic mass in the cluster. However, this gas is spatially offset from the total mass observed with gravitational lensing, suggesting that there is another source of matter — dark matter — in the cluster. Through this and other measurements, Chandra has helped obtain key constraints on cosmological parameters.

These won’t be the last of Chandra’s discoveries. As Hickox says, “Chandra is good to go!” The Chandra team expects at least ten more years of observations! They’re also starting to think about what comes next. Hickox says that deeper X-ray observations with greater instrument sensitivity and angular resolution will be needed to probe signatures of the first black holes, and we’re looking forward to see what future X-ray missions may bring.

Press Conference: Things That Go Bump in the Night Sky (by Susanna Kohler)

This morning’s conference was presided over by Astrobites’ own Kerry Hensley, in her capacity as the AAS Media Fellow. Welcoming a packed room, Kerry kicked off a session full of exciting results that stand to significantly advance our understanding of — for lack of a better word — weird astronomical phenomena.


The CHIME telescope incorporates four 100-meter long U-shaped cylinders of metal mesh. [CHIME]

First up, Deborah Good (University of British Columbia) and Vicky Kaspi (McGill University) presented some intriguing news about fast radio bursts from the Canadian Hydrogen Intensity Mapping Experiment (CHIME). Fast radio bursts (FRBs) are very brief, powerful bursts of radio emission that come from distant sources beyond our galaxy — but we haven’t yet figured out what these sources are! Good provided an overview of CHIME, a brand new radio telescope in British Columbia that has only just begun its search of the skies. Kaspi then announced the results: in just three weeks of observing this year, during its pre-commissioning phase, CHIME has already detected 13 new fast radio bursts. These include — drumroll, please — the second FRB ever observed to repeat.

Why is this a big deal? Of the ~60 FRBs detected so far, only one had previously been known to repeat. This source led both to questions — is this repeating burst rare or unusual? If not, why don’t the other bursts repeat? — and to answers: the repeating burst allowed us to finally hunt down an FRB source location. CHIME’s discovery of a second repeating burst suggests that repeating FRBs perhaps aren’t so rare after all, and it provides us with further opportunity to look for the origin of these weird signals and, hopefully, figure out what they are! Press release


Artist’s illustration of a magnetar, a rotating neutron star with incredibly powerful magnetic fields. [NASA/CXC/M.Weiss]

Next up, Aaron Pearlman (California Institute of Technology) presented observations of a different weird radio source: a magnetar that lies at the center of our galaxy. Magnetars are neutron stars — dense remnants of dead stars — with emission powered by the decay of strong magnetic fields. Radio magnetars are extremely rare: they comprise just 0.1% of the pulsar population. The magnetar at our galactic center therefore provides us with a unique opportunity to study these odd beasts — and new observations have revealed bizarre radio behavior, including some intriguing similarities with fast radio bursts. So far, our observations raise more questions than answers, so we can be sure that this source will be a target of future study. Press release

Rounding out the session, Erin Kara (University of Maryland) presented exciting new results in the world of X-ray binaries, or black holes accreting matter from a binary stellar companion. One of the big mysteries of X-ray binaries is what causes their emission to change over time: sometimes the X-ray emission is dominated by light from the disk of accreting material, but sometimes it’s dominated by light from the hot cloud of gas that lies above the disk, known as the corona. New observations from NASA’s Neutron star Interior Composition Explorer (NICER), an X-ray detector mounted on the International Space Station, have shown one X-ray binary’s corona contracting over the span of several weeks, shrinking from hundreds of miles to just ~10 miles in vertical extent. This observed change in structure may provide insight into how material funnels onto stellar-mass black holes and releases energy in the process. Press release

Plenary Talk: The Climates of Other Worlds: Exoplanet Climatology as a Pathway to Accurate Assessments of Planetary Habitability (by Vatsal Panwar)

Aomawa Shields (University of California Irvine) gave the next plenary talk of the day on the ongoing hunt for the holy grail of exoplanet science — an Earth-like habitable planet. Shields started off by referring to the first image of Earth taken by Apollo 17 (The Blue Marble) and recalling that until about two decades ago, before the discovery of first exoplanets, we believed that Earth was possibly the only habitable planet. With 3,885 confirmed exoplanet detections to date, we have come a long way since then — largely due to the Kepler Space Telescope, which detected transiting exoplanets for the last 9 years.

With the baton now passed to the Transiting Exoplanet Survey Satellite (TESS) — the first all-sky survey of its kind in space — we will soon be discovering many more Earth-analog planets (in terms of size and mass), which will be quite interesting to follow up for characterisation. In particular, systems similar to TRAPPIST-1 around M dwarfs in the solar neighbourhood will be the most ideal targets for atmospheric characterisation by infrared instruments on the upcoming James Webb Space Telescope (JWST) and Extremely Large Telescopes, and next generation concept missions like LUVOIR, HabEx, and Origins Space Telescope.

Shields emphasised that some of these newly detected planetary systems could really push the definitions of a habitable planet. Prioritising the ones for follow-up studies in this context would require a good understanding of a planet’s climate and the observable biosignatures associated with it.

From our understanding of life as we know on Earth, some of the essential ingredients to sustain biological metabolism are liquid water; availability of bioessential elements like carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulphur; and a source of energy. The current search of habitable worlds primarily hinges on detection of liquid water. However, the dense web of dependencies between the factors governing a planet’s ability to sustain liquid water on its surface suggests how difficult it is to theoretically model a planet’s habitability. Shields has been developing a hierarchy of planetary climate models centred around the conventional general circulation models that take into account physical aspects governing a planet’s climate, like radiative–convective heat redistribution, overall energy balance, and planetary orbit and rotation. There are several other factors like the obliquity of a planet, spectral type of its host star, and variations in stellar irradiation levels, surface pressure, and synchronous rotation that control the climate and the habitability of a terrestrial planet.

Artist’s impression of Earth in a “snowball state”. [NASA Astrobiology]

In addition to these, Shields is specifically interested in studying the various feedback mechanisms (like the ice albedo feedback) and how they can lead to snowball-Earth scenarios on a terrestrial planet. The studies on widening of the habitable zone around a star as a result of suppression of water ice and snow albedo effect on planets around red dwarfs is what Shields describes as the “light bulb” that inspired her research interests during her PhD. Evidently a coherent consideration of all the subtle factors controlling a planet’s climate leads to a much more sophisticated definition of the habitable zone around a star.

For instance the wavelength band in which water ice is most reflective (blue end of the optical spectrum) overlaps with the band in which a G-type star like the Sun is most bright. In comparison, the ice albedo is much less in bands where M dwarfs are most bright (redder end of the spectrum), which means that it is much harder for a terrestrial planet around an M-dwarf star to go into a snowball state due to ice albedo feedback. However other effects like cloud formation, extent of atmospheric circulation, and salinity of surface ice are also responsible for deciding how easily a planet can get into a snowball state. In case of a multiple-planet system, the dynamical evolution of the system also affects a planet’s climate over geological timescales.

Shields and her group are working on tackling the subtle climatic and geological phenomena that ultimately decide a planet’s habitability. She concludes that going from “an artist’s impression to an actual photograph” of a terrestrial exoplanet and being able to assess its habitability correctly would need careful co-development of both observations and theory that comprehensively consider all of these factors.

Press Conference: Black Holes and Galaxies Near & Far (by Caitlin Doughty)

To start off the press conference, Xiaohui Fan (University of Arizona), announced the discovery of a quasar at redshift z=6.51, corresponding to a distance of 12.8 billion light years away, seemingly with an inherent brightness of 600 trillion solar luminosities, which would make it the brightest known quasar. Noting some apparent contaminants in the quasar’s spectrum and a slight stretching in the images, Fan and collaborators obtained deep HST images of the system and were able to observe what was really going on: the quasar is in fact being gravitationally lensed by a foreground galaxy located at z=0.6. It’s split into three images by the complex arrangement of matter in the foreground lens, and it’s magnified by a factor of 50. This also indicates that the quasar’s true luminosity is on the order of 12 trillion solar luminosities, relatively faint. Spectra from the Very Large Telescope will let Fan and collaborators study the intervening absorption and learn about late-stage cosmic reionization, while upcoming observations by the Atacama Large Millimeter Array (ALMA) will allow study of the region within 150 light years of the black hole, i.e., within its gravitational influence. Future work will also hopefully address whether there is an unknown population of lensed quasars. Press release

Next, Dheeraj Pasham (MIT Kavli Institute for Astrophysics and Space Research) reported study of an astrophysical transient called a tidal disruption flare, which led to the calculation of the spin of a black hole. A tidal disruption flare occurs when a star ventures too close to a black hole and is gradually shredded, the raw material settling into an accretion disk around the black hole. The infalling material heats up, reaching millions of degrees Celsius and emitting abundant X-rays as a result. The flare that spurred this announcement was observed in the All-Sky Automated Survey for Supernovae (ASAS-SN), and it sprang from the center of a galaxy roughly 290 million miles away from Earth. Careful analysis of the light curve of the X-ray signal showed a flicker in the brightness that occurred every 130 seconds. This flicker is related to the spin rate of a clump of material orbiting the black hole, and thus to the black hole spin rate. Pasham found that the material is orbiting at about 50% the speed of light, about 334 million miles an hour. This high rotation speed indicates that the black hole probably grew by pulling material off an accretion disk rather than by merging with other black holes. Further study of these tidal disruption flares may allow study of how black holes grew as the universe aged. Press release

The radio galaxy Cygnus A showing locations of jets, hotspots, and the hole surrounding hotspot E. Credit: X-ray: NASA/CXC/Columbia Univ./A. Johnson et al.; Optical: NASA/STScI

The third story was reported by Amalya Johnson (Columbia University), and gives evidence of a jet “ricocheting” off of a hotspot in the radio galaxy Cygnus A. Based on a Chandra X-ray observatory study of Cygnus A, located 600 million light years from Earth. The galaxy hosts an Active Galactic Nucleus (AGN) and jets creating lobes on the eastern and western fronts of the galaxy that glow brightly in the radio. The ricocheting effect was discovered upon noticing that so-called hotspot E in the galaxy, one of several small, radio-bright clumps created where the jet material encounters the intergalactic medium, was surrounded by an apparent lack of X-ray emission. The region lacking X-ray emission is a hole, deeper than it is wide, which indicates that the jet material is ricocheting off the material in hotspot E and being redirected towards another region, called hotspot D. There is also some indication that a similar ricocheting effect may be happening between the hotspots on the western side of the galaxy, between hotspots B and A. Press release

For the last story, Erik Rosolowsky (University of Alberta) reported on some preliminary from the Physics at High Angular Resolution in Nearby Galaxies with ALMA (PHANGS-ALMA). This current survey, with nearly 90% of observations already complete, is designed to observe 74 nearby galaxies and 100,000 molecular clouds (MCs) with the ultimate goal of discovering whether MCs are more efficient at forming stars in low-mass galaxies or in high-mass galaxies. The survey covers a wide range of galaxy masses and star formation rates to improve the robustness of their results. After mapping emission from the carbon monoxide molecule and counting the youngest stars in close proximity to the clouds, Rosolowsky has preliminary results from 12 of the eventual 74-galaxy sample that suggest that low-mass galaxies may actually be more efficient at transforming molecular gas into stars. The end goal of this work is to determine the effects on the MCs by the conditions in the host galaxy. Press release

Plenary Talk: Annie Jump Cannon Award: Tracing the Astrochemical Origins of Familiar and Exotic Planets (by Kerry Hensley)

It’s a really exciting era to study planets and planet formation! Dr. Ilse Cleeves (University of Virginia), winner of this year’s Annie Jump Cannon Award for “groundbreaking work on planet formation and protoplanetary disks,” explained our current understanding of the chemistry of protoplanetary disks, the questions left unanswered, and what we can hope to learn in the future.

We’ve reached the point where we’re able study not only the atmospheres of distant stars, but also the atmospheres of the planets that orbit them. Despite our ability to detect whiffs of gases in far-off atmospheres, there’s still plenty we don’t know about our own planet: Where did Earth get its water? Why does it have less carbon and nitrogen than we expect? What’s the core made of? These are just a few of the lingering questions we have about the planet we know the most about!

Even if we could explain every facet of Earth’s formation, though, we would still be far from explaining the wide diversity of planetary and solar system bodies — from Jupiter with its nearly solar composition (but curiously high carbon and low oxygen abundance) to comets, the “icy time capsules” of the early solar system. And when we consider all the truly wild forms that exoplanets can take (lava worlds and diamond planets, I’m looking at you), we have to ask the question — where does all this diversity come from? The answer, of course, is the chemistry happening in protoplanetary disks!

Snow lines and planets

The presence of snow lines has a huge effect on the chemical environment from which planets form.

Thanks to changing radiation levels, dust grain sizes, and gas temperature, it’s possible for planets formed from the same stellar nebula to have vastly different core and envelope (aka atmosphere) compositions. For example, as we move farther away from the star, the gas temperature drops and chemical species like water, carbon dioxide, and carbon monoxide gradually freeze out as solids. This transition changes the ratio of carbon to oxygen in both the gaseous and solid material, which drives different chemistry; high C/O ratios tend to yield lots of hydrocarbons, whereas abundant oxygen leads to lots of CO. Plus, at a more fundamental level, the gradual freezing out of molecules as you move away from the star changes the material available to be accreted onto a planet: If CO2 is a solid where the planet is forming, there won’t be any CO2 in its atmosphere.

We’ve been able to make huge advances in studying disk composition and structure with observatories like Herschel and the Atacama Large Millimeter/submillimeter Array (ALMA). In order to understand fully what’s going on in protoplanetary disks, we need to combine observations with both modeling and laboratory studies. Plus, protoplanetary disk astrophysicists — like so many members of our community — are eagerly awaiting the launch of the James Webb Space Telescope (JWST), which should greatly enhance our understanding of disk chemistry and help us learn how planets form. Keep your fingers crossed for successful testing and a smooth launch!

Plenary Talk: Henry Norris Russell Lecture: The Limits of Cosmology (by Stephanie Hamilton)

There is a lot we don’t know about our universe. In fact, we call 96% of it “dark” because we can’t see or measure it directly with any of the methods we’ve developed thus far, all of which rely on light. Worse, we’re starting to reach the limits of what our current methods can do in terms of probing the nature of dark matter and dark energy. Dr. Joseph Silk, professor at Johns Hopkins and the 2019 Henry Norris Russell lecturer, recapped the status of both fields in the final plenary of the day and suggested some possible next steps (spoiler: it may involve telescopes on the Moon!)

Dr. Silk’s hand-drawn, colorful plots received a laugh as he simultaneously advised researchers not to forget the pretty colors.

Dr. Silk kicked off his plenary talk with some advice for younger researchers. He noted that sometimes success happens by being in the right place at the right time, so take advantage of opportunities that arise! He also advised that intuition isn’t always enough to “come to grips with the universe” — often, the data reveal completely unexpected results. Dr. Silk’s next piece of advice was to make sure you do at least one analytical project, but don’t forget about the data or the pretty pictures. Finally, he encouraged young researchers to choose an interdisciplinary field where there is a gap to be bridged.

Dark matter is a mysterious substance whose existence we know of only through its gravitational effects. Attempts to explain it away with theory have been unsuccessful, and astronomers have now accepted that it really must exist. Leading theories predict that it is some type of weakly interacting massive particle (WIMP), forcing direct detection experiments underground so as to escape the bombardment of cosmic radiation that would otherwise overwhelm detectors with noise. Unfortunately, nothing has been discovered yet, and our experiments keep getting bigger and bigger. We are rapidly approaching the dreaded “neutrino floor,” below which even neutrinos (which would interact with one atom on average passing through a light-year of lead) become a significant source of noise.

Not all is lost, though. When dark matter particles collide, astronomers think the collision would result in a measurable gamma-ray signature. Further, collisions of protons at the LHC could produce dark-matter particles that would appear in analyses as missing energy. In a different proposal altogether, a prevalence of sub-Earth-mass black holes remains a viable, albeit unlikely, explanation of dark matter that doesn’t require any new physics to explain. Scientists are already studying these phenomena, though none have yet explained dark matter.

Dr. Silk then shifted gears to dark energy. So far, experiments suggest that dark energy is explained by a cosmological constant, or a constant vacuum energy density. But we have not converged upon a single model, and there are still open questions. Measured values of the universe’s expansion, H0, differ between early- and late-time experiments, suggesting a possible need for new physics. Further, we do not yet have the sensitivity to distinguish between various models of inflation. Precision cosmology using either the cosmic microwave background (CMB) or galaxies is limited simply by the number of features we can measure. Thus, we need a method that gives us a larger number of objects to measure.

The solution? The Moon! Specifically, Dr. Silk proposed studying the gas clouds in the universe’s dark ages via 21-cm astronomy (enabled by the hyperfine atomic transition of hydrogen). The problem here is that not only will features so far away (redshift z~50) be incredibly faint, but the redshifted 21cm signal falls into the noisy radio bands of Earth. The far side of the Moon happens to be the most radio-silent place in the Solar System — perhaps the next step for understanding our early universe is the construction of a radio telescope on the Moon!

Special Session: Implementing the Next Decadal Survey: Status Report of Astro2020 from the Committee on Astronomy and Astrophysics at the National Academy of Sciences (by Susanna Kohler)

What’s planned for the next decade of astronomy? We went to the Astro 2020 Decadal Survey Town Hall to find out.

The Decadal Survey is a process implemented in astronomy every ten years, during which the astronomy community determines — hopefully by consensus — what to prioritize for government investment over the next ten years. These priorities can include missions large and small, facilities, and programs.

In the town hall tonight, the co-chairs of the Decadal, Fiona Harrison and Rob Kennicutt, outlined the process ahead. Though it’d be awesome if we could simply fund all our favorite missions and projects, the reality of the Decadal process is much more complex than this, and it requires careful research and difficult decisions.

The first step is for members of the astronomy community to submit what are known as “white papers” — brief summaries of what’s important in the scientific future of a specific subfield, discussions of the state of the profession, etc. These white papers can be submitted by a lone individual with a great idea, or by groups of scientists who all converge on support for a single project or goal.

Harrison and Kennicutt repeatedly emphasized the importance of community involvement via white papers, attending town halls and public discussions, etc. — the goal is not for the Decadal survey committee of 18 people to make decisions in a vacuum about what astronomy should be pursued in the next decade, but rather for the whole community to participate in this process. In particular, early-career scientists are encouraged to get involved: Harrison commended grad-student journal clubs, for instance, that meet to plan white-paper submissions.

To learn more about the Decadal, and to submit white papers or nominate committee members, you can go here. Keep an eye out for the white-paper deadline on 19 February 2019, and the deadline for nominations to the survey committee, 22 January 2019. And keep an ear to the ground as we continue to develop our priorities as a community for the next ten years of astronomy!