astrobites: Daily Paper Summaries 2020

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Investigating Early Populations of Galaxies
with the Best Telescopes in the Universe
astrobites | Daily Paper Summaries | 2020 Feb 05

In the coming years, we will see the launch of one of the most powerful space-based telescopes ever built, the James Webb Space Telescope (JWST), and we will see a new class of colossal ground-based observatories built with primary mirrors exceeding 30 meters in diameter. However, despite all of our technical ingenuity, the most powerful telescopes in the universe are in fact galaxy clusters. As the most massive gravitationally bound structures, galaxy clusters severely distort their local spacetimes and can magnify substantial areas of the sky through the phenomenon of gravitational lensing. Cluster lenses allow astronomers to observe many distant sources in unprecedented detail that would otherwise be too faint to study (e.g., Fig. 1). Indeed, the possibility of discovering and characterizing some of the earliest and most distant galaxies observable was a primary motivation for conducting a deep survey of six galaxy clusters known to be powerful lenses. This project, dubbed The Hubble Frontier Fields, involved hundreds of hours of observations with the Hubble Space Telescope and the Spitzer Space Telescope. By combining our best telescopes with those that nature provides, astronomers uncovered hundreds of distant galaxies from times as early as one billion years after the Big Bang. In this astrobite, we cover a work that uses this rich sample of galaxies to trace the growth of stellar mass across the first few billion years of the universe.

In this paper, the authors exploit the power of gravitational lensing to magnify and reveal intrinsically faint sources at great distances, sources that would otherwise be impossible to study. The team begins by identifying high-redshift galaxies through the Lyman break method, (a.k.a., the “dropout” technique). UV radiation from distant galaxies is absorbed by neutral intervening gas, causing high redshift sources to appear faint in blue filters – thus, high redshift galaxies can be identified quickly by their colors. Combining all available imaging of the Hubble Frontier Fields, the team uses the Lyman break method to find a total of 357 magnified galaxies at 6 < z < 9, when the universe was less than a billion years old. ...

Early Low-Mass Galaxies and Star-Cluster Candidates at z~6-9 Identified by the
Gravitational Lensing Technique and Deep Optical/Near-Infrared Imaging ~ Shotaro Kikuchihara et al

• arXiv.org > astro-ph > arXiv:1905.06927 > 16 May 2019

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Where the Solar System Ends
astrobites | Daily Paper Summaries | 2020 Feb 06

Where does the solar system actually end? We could say it’s where the Sun’s gravity stops being strong enough to hold onto things. This would make it the edge of the Oort Cloud, the loosely bound sphere of rocky and icy bits left over from the solar system’s formation, extending almost 3 light-years from the Sun. Or, we could say it’s where the energetic particles from the Sun (the solar wind) stop flowing away from us, blocked by the pressure of all the other gas that’s between stars, the interstellar medium

Today, we’ll focus on the latter: the heliopause, the boundary where the solar wind meets the interstellar medium (ISM), which marks the edge of the heliosphere, the bubble of gas surrounding the Sun. Both the solar wind and the ISM are made of plasma, the 4th state of matter. In a plasma, some of the electrons have been stripped off the atoms, leaving charged particles (ions) to move around. There are a few different parts of the heliosphere, and the Voyager missions, launched in the 1970s, have traveled through all of them. ...

Voyager 2 plasma observations of the heliopause and interstellar medium ~ John D. Richardson et al

• Nature Astronomy 3(11):1019 (Nov 2019) DOI: 10.1038/s41550-019-0929-2

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Where Are All the Baryons?
astrobites | Daily Paper Summaries | 2020 Feb 06

All of the material we see around us is made up of atoms, also known as baryonic matter. From studies of Big Bang Nucleosynthesis and the Cosmic Microwave Background (CMB), we know that baryons make up only 5% of the Universe. The rest is made of still largely unknown forms of matter and energy we call dark matter and dark energy. We have a problem though with the 5% of the Universe we know about: we don’t know where all the baryons are.

Baryons in galaxies and galaxy clusters make up only ~20% of all baryons in the Universe. The existence of another ~30% of the baryons can be inferred from the Lyman-alpha forest. Cosmological simulations suggest that the rest of the baryons (roughly 50%) reside in the WHIM (warm-hot intergalactic medium). The WHIM is composed of filaments and sheets of warm-hot gas that connect galaxies and galaxy clusters. Because emission from this gas is faint, finding baryons in the WHIM is difficult. Previous work has relied on studies of absorption lines of distant quasars and X-ray absorption. Using these methods, we have found just 20% of the total baryon budget in the WHIM, far less than the theoretically expected 50%. Thus, summing up all the known baryons gives us only 70% of all the baryons in the Universe.

An alternative technique to measure baryons in the WHIM is known as the thermal Sunyaev-Zeldovich (tSZ) effect. The tSZ effect measures the change in energy of photons from the CMB caused by interactions (inverse Compton scattering) with hot particles along our line of sight. In order to apply this method, this paper utilizes maps of galaxies from the Sloan Digital Sky Survey to create both a sample of galaxy pairs that are likely to be connected by a filament and a control sample of pairs of galaxies that are physically unrelated, but closely separated on the sky. The authors also use the Planck map of the Compton y-parameter, which quantifies the strength of the tSZ effect.

Because the tSZ signal from the WHIM is weak, the authors add together the signals of over a million pairs of galaxies, rotating and scaling each particular pair as needed. In order to measure the tSZ signal from the filament, rather than the galaxies themselves, a model is used to subtract the contribution of hot gas in the galaxy halos. Figure 1 shows the summed signal, the galaxy halo models, and the residual signal from the filament. ...

Probing the missing baryons with the Sunyaev-Zel’dovich effect from filaments ~ Anna de Graaff et al

• Astronomy & Astrophysics 624:A48 (Apr 2019) DOI: 10.1051/0004-6361/201935159
• arXiv.org > astro-ph > arXiv:1709.10378 > 29 Sep 2017 (v1), 01 Mar 2019 (v3)

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Polarime-trying to Map Magnetic Fields in the Orion Nebula
astrobites | Daily Paper Summaries | 2020 Feb 08

The Orion Molecular Cloud 1 (OMC-1) is part of the Orion Nebula, and one of the most massive star-forming regions in the solar neighborhood. The gas and dust within OMC-1 act as a nursery for young stars, providing them with the necessary materials to develop. As such a close and large stellar nursery, OMC-1 is an easily accessible and important laboratory for studying the still-mysterious conditions that surround and encourage star formation. Today’s paper contributes to our understanding of star formation by determining OMC-1’s magnetic field and dust properties using polarimetry (more on this technique later!).

OMC-1 is a particularly interesting target for magnetic field and dust measurements because of the variation in structure across the cloud, which is shown in Figure 1. In front of OMC-1, there is an HII region ionized by a relatively young group of stars, the Trapezium cluster. The west side of OMC-1 hosts the Kleinman-Low (KL) Nebula and the Becklin-Neugebauer (BN) object. The KL Nebula is a clump of molecular gas and dust with a bunch of massive stars inside, of which the BN object is the brightest. In the infrared, the KL Nebula appears to be exploding because stellar winds from the massive stars heat up the surrounding gas. The southeast region of OMC-1 contains the Orion Bar, a photodissociation region that is cold, neutral, and creates the divide between HII and molecular gas. These features contribute to a complex magnetic field structure within OMC-1 that today’s authors map with polarimetry measurements. ...

HAWC+/SOFIA Multiwavelength Polarimetric Observations of OMC-1 ~ David T. Chuss et al

• Astrophysical Journal 872(2):187 (2019 Feb 20) DOI: 10.3847/1538-4357/aafd37
• arXiv.org > astro-ph > arXiv:1810.08233 > 18 Oct 2018 (v1), 08 Jan 2019 (v2)

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Extreme star-forming galaxy reveals all…!?
astrobites | Daily Paper Summaries | 2020 Feb 10

Surveys of the infrared sky have led to the discovery of thousands of dust obscured, highly star-forming galaxies — often referred to as submillimetre galaxies (SMGs) due to the submillimetre-wave emission that characterises their cool, dusty nature. Unfortunately, this long wavelength emission presents an observational challenge: the resolution of a telescope has a physical limit, directly proportional to the wavelength of light divided by the diameter of the telescope. For longer wavelengths such as submillimetre, a telescope must be much larger than an optical telescope to achieve comparable resolution. As such, submillimetre telescopes are limited by the feasible sizes of single mirrors, and for decades infrared astronomy was stuck with low resolution images. Out of which has grown a science of fuzzy blobs (see Fig 1).

Since the advent of ALMA, the Atacama Large Millimetre Array, these infrared-bright sources have been under scrutiny at much higher resolution made possible by the power of interferometry. Upon high res inspection, many of these very bright sources turn out to be the combined light of several galaxies, merged together in a lower resolution telescope observation. This paper investigates one such source, and finds some very curious things indeed about the nature of this particular fuzzy blob. ...

Hyperluminous starburst gives up its secrets ~ R. J. Ivison et al

• Monthly Notices of the RAS 489(1):427 (Oct 2019) DOI: 10.1093/mnras/stz2180
• arXiv.org > astro-ph > arXiv:1908.03199 > 08 Aug 2019

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Unlocking the secrets of chaotic planetary systems
astrobites | Daily Paper Summaries | 2020 Feb 11

It shows up in nearly every field of study – from weather forecasting, to physics, to economics – even sociology – and of course, astronomy. Chaos theory is the study of systems whose seemingly random behavior is the result of an extreme sensitivity to initial conditions. (For an excellent, more in-depth explanation of chaos, check out this astrobite). Chaos is a subject that commonly comes up when trying to understand the long-term stability of planetary systems.

It turns out that certain arrangements of planets are inherently unstable – that is – if you place them in a certain configuration and let them orbit their star for long enough, the gravitational interactions between the planets will fling some (or sometimes all) of the bodies clear out of the system. Unfortunately, determining how and when this will happen is not possible to work out on paper. Or at least, no one has been clever enough to figure it out yet.

Fortunately, computers make this problem somewhat tractable. By gradually evolving a collection of massive bodies over many tiny time steps, it is possible to get an incredibly accurate estimate of where and how these bodies will be moving sometime in the future (or the past, for that matter). Given enough computing power, you can simply take a planetary system and evolve it forward in time and see what happens. Does it stay stable? Do any planets get ejected? Using this technique, astronomers can try placing extra bodies in known planetary systems and see if things remain stable. If not, this sometimes can rule out the presence of additional, undetected planets.

As mentioned above, these types of systems are sometimes chaotic. If so, this means, by definition, that the outcome of whether the system is stable not, and how long it takes to become unstable, is highly sensitive to the initial conditions. The authors of today’s paper want to examine how reliable the estimates of instability timescales from these simulations actually are. If the initial conditions are tweaked just slightly, does this timescale change? And if so, is there an underlying pattern? ...

Fundamental limits from chaos on instability time predictions
in compact planetary systems ~ Naireen Hussain, Daniel Tamayo

• Monthly Notices of the RAS 491(4):5258 (Feb 2020) DOI: 10.1093/mnras/stz3402
• arXiv.org > astro-ph > arXiv:2001.04606 > 14 Jan 2020

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More Clues to the Environment in Which FRBs Originate?
astrobites | Daily Paper Summaries | 2020 Feb 12

Fast radio bursts (FRBs) are one of the hottest topics in astronomy right now. First discovered by Dr. Duncan Lorimer in 2007, these intense millisecond-long bursts of radio emission have continued to captivate scientists across the planet because they keep defying our expectations with discoveries like the repeater. Now, with the discovery of an interesting property of a new FRB just outside a major galaxy, we may be getting one step closer to finally solving one of the many puzzles of FRBs.

Our questions about FRBs seem to fall into two categories: What causes the bursts? And how can they be put to use? Each time the community moves toward an answer on one of these questions, a new discovery throws a wrench in it. For example, astronomers thought FRBs were single events but a discovery in 2016 showed that they can actually repeat. This opens new questions, like whether the repeaters and non-repeaters come from the same mechanism. In another case, we thought FRBs only came from dwarf galaxies until one was localized to a massive spiral galaxy. This finding opened more questions about the types of environments that could produce FRBs in very different galaxies. The authors of today’s article present a newly discovered FRB with a very high rotation measure that may give clues to the kind of environment FRBs originate from. ...

A bright, high rotation-measure FRB that skewers the M33 halo ~ Liam Connor et al

• arXiv.org > astro-ph > arXiv:2002.01399 > 04 Feb 2020

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What happens when you throw a satellite at the Sun?
astrobites | Daily Paper Summaries | 2020 Feb 13

The Sun isn’t exactly a hospitable place for a satellite. It’s extremely hot, surrounded by a strong flow of charged particles called the solar wind. But recently, NASA launched a new mission called the Parker Solar Probe, designed to dive closer to the Sun than ever before. Its goal is to understand the plasma, magnetic fields, and charged particles near the surface of the Sun, specifically in the solar wind and the tenuous outer layer known as the corona. By learning about these energetic flows of particles around the Sun, we can better understand how the Sun gives off energy, and why (possibly hazardous to Earth) events like solar flares and coronal mass ejections occur.

Now that the Parker Solar Probe (PSP) has completed its first two close passages of the Sun (out of 24 total planned!), the mission team has released their first results. We’ll take a look at some of these findings in today’s paper. ...

Alfvénic velocity spikes and rotational flows in the near-Sun solar wind ~ J. C. Kasper et al

• Nature 576(7786):228 (12 Dec 2019) DOI: 10.1038/s41586-019-1813-z

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The Beating Heart(-Shaped Region) of Pluto
astrobites | Daily Paper Summaries | 2020 Feb 14

In 2015, hours before the New Horizons spacecraft made its closest approach to Pluto, the probe took a high resolution photo (Figure 1), sent it back to Earth, and forever changed our view of the mysterious (dwarf) planet.

The stunning heart-shaped region in the image has been officially named “Tombaugh Regio” after Clyde Tombaugh, who discovered Pluto in 1930 (ninety years ago!). Further images showed that the left side of the heart, named “Sputnik Planitia” after the first artificial satellite, is much younger than the other side of the heart. In fact, geological studies suggested that the absence of craters (Figure 2) implied an age of less than 10 million years. Pluto, once believed to be geologically dead, actually has a surprising amount of geological activity, especially in Tombaugh Regio. ...

Using the New Horizons topography data and an updated high-resolution version of a 3D global climate model developed at the Laboratoire de Météorologie Dynamique, the authors of today’s paper simulate the cycle of nitrogen and methane over different timescales, as well as Pluto’s weather and winds. These essentially cutting-edge weather forecast simulations help understand how the observed distribution of ices on Pluto’s surface came to be. ...

Pluto’s Beating Heart Regulates the Atmospheric Circulation: Results
from High Resolution and Multi-Year Numerical Climate Simulations ~ T. Bertrand et al

• Journal of Geophysical Research: Planets (online 04 Feb 2020) DOI: 10.1029/2019JE006120 (pdf)

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Deciphering Spitzer’s Legacy: Signs of Dead Galaxies at Cosmic Dawn
astrobites | Daily Paper Summaries | 2020 Feb 15

After a prodigious career, the Spitzer Space Telescope was shut off on 30th January 2020.

Originally named the Space Infrared Facility when it was launched in 2003, Spitzer peered out into the dark universe of dust and gas to reveal entirely new phenomena that are inaccessible from under the Earth’s infrared-absorbing atmosphere. Equipped with three science instruments, the Infrared Camera (IRAC), Infrared Spectrograph (IRS), and Multiband Imager (MIPS), Spitzer provided key clues to the nature of star-formation, the formation of exoplanets, and the dusty structures within the galaxies, among others.

Following the loss of its remaining liquid helium coolant in 2009, Spitzer transitioned into its post-cryogenic mission. Despite operating with only two channels (i.e. bandpasses) of its IRAC infrared camera, Spitzer continued to live up to its reputation by discovering a planet 13,000 light-years away as well as the most distant galaxy to date, seen as it was 13.4 billion years ago.

Today’s astrobite focuses on a lasting mystery precipitated by a series of observations made possible by Spitzer’s unique capabilities. ...

Interpreting the Spitzer/IRAC Colours of 7<z<9 Galaxies: Distinguishing Between Line
Emission and Starlight Using ALMA ~ Guido Roberts-Borsani, Richard S. Ellis, Nicolas Laporte

• arXiv.org > astro-ph > arXiv:2002.02968 > 07 Feb 2020

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Making a “Mega-Telescope” for Exoplanets
astrobites | Daily Paper Summaries | 2020 Feb 17

TRAPPIST-1 might be the best known system with multiple planets, but HR 8799 has quite a few cool things going for it, too. It’s one of the first systems discovered with direct imaging (actually taking pictures of the planets themselves), and since then people have been observing its four planets moving around in their orbits. The kinds of planets we see around HR 8799 are also very different than those around TRAPPIST-1. The transit method, used to discover the 7 terrestrial TRAPPIST-1 planets, is better suited to find planets very close to their host stars. Direct imaging, on the other hand, is best for the biggest, furthest out planets – young super-Jupiters and brown dwarfs, orbiting 10s to 100s of AU from their stars.

Direct imaging is also able to provide useful information about planetary orbits – it’s really clear to see where each of the HR 8799 planets is (as in Figure 1), whereas with the transit or radial velocity methods it takes a bit more untangling to sort through the overlapping signals of multiple planets. The goal is to determine the orbits, masses, and compositions of these kinds of giant planets, so that we can understand what they’re like and how they formed. For example, looking at the composition of the atmosphere, we can observe how much carbon there is compared to oxygen (the C/O ratio) to figure out where it formed in the protoplanetary disk. The D/H ratio (deuterium to hydrogen) can tell us about how many icy bodies (like Kuiper Belt Objects) a planet must have accreted in its past.

This all sounds great, having a way to trace the formation of big planets – so what’s the catch? Because of the immense challenges of directly imaging a faint exoplanet around a bright star and the limited sizes of our telescopes, we don’t have the spatial resolution needed to really precisely constrain the orbits of these planets or see fine details in their spectra. That’s where today’s paper comes in, describing the first observations of an exoplanet using optical interferometry, a technique that allows for higher spatial resolution by combining multiple telescopes in clever ways. ...

First Direct Detection of an Exoplanet by Optical Interferometry:
Astrometry and K-band Spectroscopy of HR 8799 e ~ GRAVITY Collaboration, S. Lacour et al

• Astronomy & Astrophysics 623:L11 (Mar 2019) DOI: 10.1051/0004-6361/201935253
• arXiv.org > astro-ph > arXiv:1903.11903 > 28 Mar 2019

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Red Galaxies at Night, Astronomers’ Delight!
astrobites | Daily Paper Summaries | 2020 Feb 18

Galaxies are a true wonder of the universe. Unimaginably vast, they can contain up hundreds of billions of stars. The location of a galaxy is an important factor in its overall evolution, as this process can be influenced by its surroundings. A key quantity that can measure the effect of a galaxy’s environment is the star formation rate (SFR). Among other things, the SFR gives an insight into how active the galaxy is. Curiously, the overall star formation rate of galaxies in the universe has decreased over time, with peak star formation having already occurred in the early universe. Even more perplexing is how this general reduction has been shown to apply to galaxies across (almost) all stellar masses. Today’s work is tasked with determining whether this general reduction applies across all environments. ...

The Dawn of the Red: Star Formation Histories of Group
Galaxies over the Past 5 Billion Years ~ Sean L. McGee et al

• Monthly Notices of the RAS 413(2):996 (May 2011) DOI: 10.1111/j.1365-2966.2010.18189.x
• arXiv.org > astro-ph > arXiv:1012.2388 > 10 Dec 2010

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Joint Survey Constraints for Cheap!
astrobites | Daily Paper Summaries | 2020 Feb 20

Today’s astrobite takes a detour from typical observational astronomy to talk about the statistics of large sky surveys. Surveys often go through a grueling phase of theory before observations begin, where Monte Carlo Markov Chains (MCMCs) are usually used to infer model parameters from predictive data. The MCMC does not actually simulate data, but uses Bayesian statistics and what is known a priori about a physical model of interest (e.g. the standard cosmological model) to find best-fit values and their errors from inputted data. This is done by ‘sampling’ the probability distribution, or evaluating a probability distribution at a certain point in your model. The next sample is found by proposing a small change to the current sample, evaluating the probability distribution at the proposed point, and observing whether the proposed point’s values are more probable within the given model. Methods to effectively sample a probability distribution is an active topic of research and many frameworks exist to run MCMCs for cosmological surveys, like Polychord and CosmoMC.

Combining survey data is an excellent way to find better constraints on a model. Using MCMCs on independent surveys is computationally expensive, and using them on a joint survey is even more expensive. Separate surveys often measure separate model parameters, making comparisons between surveys and joint-data analyses difficult without running a computationally costly joint MCMC. The authors of today’s paper offer a simple method to infer joint distributions using MCMC runs from independent surveys. ...

Reconstructing Probability Distributions with Gaussian Processes ~ Thomas McClintock, Eduardo Rozo

• Monthly Notices of the RAS 489(3):4155 (Nov 2019) DOI: 10.1093/mnras/stz2426
• arXiv.org > astro-ph > arXiv:1905.09299 > 22 May 2019

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Baby Stars, X-rays, and Planets: How are they Related?
astrobites | Daily Paper Summaries | 2020 Feb 21

So, how are baby stars, X-rays, and planets related?

Surprise, the answer is CHEMISTRY! Planet formation occurs in protoplanetary disks surrounded young baby stars, known as T-Tauri stars. These baby stars, quite like baby humans, are loud and tend to throw temper tantrums in the form of elevated levels of high energy radiation, such as X-rays. Stellar radiation directly impacts the physical disk structure and triggers chemical reactions in the disk through ionization, which causes the destruction of old molecules and formation of new molecules. This process directly impacts the materials available in the formation of planets.

X-rays are able to penetrate deep into the inner disk layers (Figure 1) where ices exist and planet formation occurs. Ices are complex systems (Figure 2) essential in planet formation, as ice provides a sticky coating on dust grains. This coating allows for inelastic collisions that eventually leads to planet formation. However, the desorption of water ice by X-rays had not been studied before this paper. Today’s paper uses experimental techniques to determine if X-rays can desorb water ice via a process known as X-ray photodesorption. Water is a high interest molecule when studying ice chemistry, as water is the main constituent of interstellar ices and is essential for the possible formation of life. However, the desorption of water ice by X-rays has not been studied before. ...

X-ray photodesorption from water ice in protoplanetary disks and X-ray dominated regions ~ Rémi Dupuy et al

• Nature Astronomy 2(10):796 (Oct 2018) DOI: 10.1038/s41550-018-0532-y
• arXiv.org > astro-ph > arXiv:1807.03725 > 10 Jul 2018

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Through the Lens: Milky Matter Magnifies Magellanic Motion
astrobites | Daily Paper Summaries | 2020 Feb 22

There is about five times more invisible “dark matter” than its luminous counterpart in the universe—but how do we go about detecting something that can’t be directly imaged? One way is to look for the gravitational effects of dark matter clumps on images of normal matter along the same line of sight. This type of effect is called gravitational lensing. In today’s paper, the authors specifically look for the effects of weak lensing from low-mass structures consisting entirely of dark matter. The foreground dark matter structure creates a lens that bends the light coming towards an observer from some background luminous source. Unlike strong lensing, weak lensing doesn’t impact a single background source, but instead serves to preferentially align several background sources along some field. For more information on different types of lensing and how they work, check out this bite.

Alignments of foreground and background sources that lead to weak lensing are much more common than those leading to strong lensing. Because low-mass dark matter structures are predicted to exist in the Milky Way, they should be both common in observational data sets and detectable through microlensing signatures. Furthermore, because such structures are completely devoid of normal matter, they pose a “pristine testing ground” for probing the microphysics of dark matter without the interference of normal, luminous matter.

The authors use a template approach, which is similar to the one used when detecting astrophysical signals with LIGO. Figure 1 shows the dipole pattern of velocity corrections of background stars which stems from weak lensing. The exact shape and size of the template depend on the angular position θ_t, angular scale β_t, and effective lens velocity direction v_t of the dark matter lens. The details of the matched filter to the lens-induced velocity vector profile also include information about the density profile of the dark matter lens. This means that finding the correct shape of velocity corrections in the data and comparing its magnitude with the theoretical template model can inform the size, position, and density profile (and subsequently, mass) of the dark matter lens. ...

First Results on Dark Matter Substructure from Astrometric Weak Lensing ~ Cristina Mondino et al

• arXiv.org > astro-ph > arXiv:2002.01938 > 05 Feb 2020

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Table Salt detected on Europa!
astrobites | Daily Paper Summaries | 2020 Feb 24

When the Voyager 1 mission first saw the criss-crossed and crater-deficient landscape on Europa (Fig. 1), back in 1979, it was speculated that both of these features indicate a surface that is tectonically active and regularly resurfacing, just like Earth. What did we think was facilitating these processes? A global, subsurface ocean of liquid water underneath the water-ice shell. This was confirmed by the Galileo mission in the 1990s, which detected an induced magnetic field signature close to Europa, consistent with a conducting layer beneath Europa’s surface — like a salty subsurface ocean. This discovery propelled Europa to solar system stardom, and for many scientists (including yours truly) it’s the most exciting place to go to look for life outside of Earth.

The potential habitability of Europa’s subsurface ocean depends heavily on its composition, which remains largely unknown. Currently, our best window to understanding the composition of the subsurface ocean is to study the chemistry of its geologically young and active surface, especially the disrupted chaos terrains, which are believed to have formed from direct contact with the warm ocean water. The spectra provided by the Galileo Near-Infrared Mapping Spectrometer (NIMS) suggest a surface dominated by water-ice, sulfuric acid, and sulfate salts. The sulfur-based species are not surprising, since Europa is constantly getting bombarded with sulfur ions from Io’s volcanic eruptions. These sulfur ions then undergo reactions with water-ice on Europa’s surface, in the heavy ionizing-radiation environment of Jupiter. The regions experiencing the heaviest bombardment, like the trailing hemisphere of Europa (see Fig. 2), show these sulfur chemistry signatures. On the other hand, recent ground-based infrared observations have suggested that the more pristine material (possibly originating from the subsurface ocean), which are shielded from the sulfur bombardment, might have a chlorine dominated composition. Now, pure chlorides don’t have distinctly identifiable features in the infrared (~ 1000-1500 nm). However, under particle irradiation like what Europa’s surface experiences, they develop distinct features in the visible wavelengths (see Figs. 1-3 here & Figs. 1-2 here). Using the Hubble Space Telescope (HST), the authors of today’s paper have detected one such feature, the first definitive spectral signature of a chloride on Europa. And not just any chloride, but our favorite salt sodium chloride (NaCl)! ...

Sodium chloride on the surface of Europa ~ Samantha K. Trumbo1, Michael E. Brown, Kevin P. Hand

• Science Advances 5(6):aaw7123 (12 Jun 2019) DOI: 10.1126/sciadv.aaw7123

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The Habitability of Our Closest Neighbor
astrobites | Daily Paper Summaries | 2020 Feb 25

Since its discovery in 2016, the planet orbiting Proxima Centauri, known as Proxima Centauri b, has fascinated astrophysicists and astronomy enthusiasts alike. Proxima Centauri is our closest neighbor at only 4.22 light-years away, which makes Proxima Centauri b the most favorable exoplanet (a planet that orbits a star other than the Sun) for exploration through concept missions like Breakthrough Starshot.

Proxima Centauri is an M-dwarf – the smallest, coolest, faintest type of star. Because these stars are so cool, their habitable zones, the regions around a star where a planet can support liquid water, are much closer to the star (see this astrobite for more information regarding the habitable zone). Planets that orbit in the habitable zones of M-dwarfs, like Proxima Centauri b, are thus much more susceptible to stellar winds. ...

Because an enduring atmosphere is so crucial to planet habitability and the detectability of biosignatures, the authors of this paper chose to investigate whether our neighboring exoplanet is capable of maintaining an atmosphere long enough for life to form and evolve.

The authors of this paper examined two cases in their study: Case 1, in which the stellar wind pressure is at a maximum, and Case 2, in which the stellar wind pressure is at a minimum. In both instances, they considered a magnetized and unmagnetized case for Proxima Centauri b. The authors then utilized a sophisticated model originally developed for Venus and Mars, which takes into account the various chemical reactions that can take place in a planetary atmosphere. ...

Is Proxima Centauri b Habitable? A Study of Atmospheric Loss ~ Chuanfei Dong et al

• Astrophysical Journal Letters 837(2):L26 (2017 Mar 10) DOI: 10.3847/2041-8213/aa6438
• arXiv.org > astro-ph > arXiv:1702.04089 > 14 Feb 2017 (v1), 02 Mar 2017 (v2)

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A New Approach to Tilting Uranus
astrobites | Daily Paper Summaries | 2020 Feb 26

Songs about the solar system were a significant part of my childhood. These songs taught one fact about each of the 9 planets (these were the pre-Pluto’s-demotion days) and yes, I find myself singing them in the shower every now and then. The fact about Uranus is always the same: “Uranus spins on its side.” Not only is this bizarre and memorable to a child, it happens to be true. Uranus has an obliquity (tilt) of 98º, making its axis of rotation closer to the ecliptic plane than any other planet. And yet, nobody knows how it got that way.

The conventional wisdom for many years has been that one or more giant impacts must have turned Uranus onto its side when it was very young and giant impacts were common. The authors of today’s paper outline four potential problems with this theory. ...

The authors propose a different way to tilt Uranus: spin-orbit resonance caused by a massive circumplanetary disk. To determine if this is possible, they simulated a young Uranus and Neptune evolving, each with a large orbiting disk of dust and gas. ...

Tilting Ice Giants with a Spin–Orbit Resonance ~ Zeeve Rogoszinski, Douglas P. Hamilton

• Astrophysical Journal 888(2):60 (2020 Jan 10) DOI: 10.3847/1538-4357/ab5d35
• arXiv.org > astro-ph > arXiv:1908.10969 > 28 Aug 2019 (v1), 09 Jan 2020 (v3)

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You were cool, Betelgeuse
astrobites | Daily Paper Summaries | 2020 Feb 27

Veteran star Betelgeuse has been getting a lot of attention in recent months. This is because this nearby red supergiant (RSG) has been looking fainter than usual. Given that Betelgeuse is (typically) the brightest RSG in the night sky, professional and amateur astronomers alike have been able to track changes in its brightness over time, termed its ‘light curve.’ ...

Many in the scientific community and general public have interpreted Betelgeuse’s erratic behaviour as an indication of an imminent supernova, which would be visible with the naked eye and likely to last for many days. While this is an exciting interpretation, it is also the least likely explanation. It is instead argued by many — including today’s authors — that the dimming could be due to the composition of Betelgeuse itself. Specifically, variations on the surface of Betelgeuse could lower its apparent temperature temporarily, which, according to laws of blackbody emission, would push some of its emitted light into longer wavelengths, which wouldn’t be observed in the V band.

The authors of today’s paper investigate how cool Betelgeuse actually is, and whether it is enough to explain its dimming in the night sky. They start by performing optical spectrophotometry on the RSG on February 15th 2020 using the optical spectrograph on the 4.3-meter Lowell Discovery Telescope (LDT). The term ‘spectrophotometry’ simply means measuring the spectrum of the star, where its flux is scaled according to its wavelength. ...

Betelgeuse Just Isn't That Cool: Effective Temperature Alone Cannot
Explain the Recent Dimming of Betelgeuse ~ Emily M. Levesque, Philip Massey

• arXiv.org > astro-ph > arXiv:2002.10463 > 24 Feb 2020

viewtopic.php?t=40345

[bystander]

A Gravitationally Unstable Protoplanetary Disk
astrobites | Daily Paper Summaries | 2020 Feb 29

The total gas mass of a protoplanetary disk sets a limit on the material available for forming planetary systems. Although fundamentally important for understanding the planet formation process, measuring robust disk masses remains challenging. Most studies rely on observations of CO gas, which is the brightest and easiest gas tracer to detect in disks, and then extrapolate to a total gas mass by assuming a constant CO/H^₂ abundance ratio. However, if this CO line emission is optically thick, it only allows us to probe emission from the disk surfaces, which results in underestimated disk masses. To alleviate this, astronomers instead use line emission from rarer CO isotopologues, such as ^₁₃C^₁₆O and ^₁₂C^₁₈O, which differ only in their isotopic composition, to estimate gas masses. Due to their relative intrinsic rarity, these isotopologues are optically thin and trace gas deeper down into the mid-plane of disks. This allows for a more complete accounting of total gas mass, not just that from the disk surface layers. In today’s astrobite, we take a look at new observations of the rarest stable CO isotopologue ^₁₃C^₁₇O in the nearby protoplanetary disk HL Tau. These observations reveal a significantly more massive disk than previously estimated and imply that large regions of HL Tau may be gravitationally unstable.

Today’s authors detected line emission from ^₁₃C^₁₇O gas using ALMA observations of the protoplanetary disk around HL Tau, which is at distance of 460 light years away from Earth. HL Tau has long been known to be a young (~1 Myr), embedded disk-hosting star. In fact, it was the first circumstellar disk observed with ALMA’s long baselines, which revealed an ordered series of concentric rings and gaps in the disk’s dust distribution, as shown in Figure 1a. The presence of these prominent substructural features has lead to numerous theories, including the formation and growth of planets within the disk.

However, today’s study focuses instead on the gas surrounding HL Tau and represents only the second detection of ^₁₃C^₁₇O in a protoplanetary disk. The distribution and velocity structure of the ^₁₃C^₁₇O gas are shown in Figure 1b and c, respectively. ^₁₃C^₁₇O is detected out to approximately 140 AU from the central star, with a velocity pattern consistent with that of other gas tracers and the expected rotation patterns of protoplanetary disks. As shown in Figure 1d, there is a deficit of emission in both the integrated intensity map and radial emission profile at approximately 50 AU, but the authors believe that this is due to obscuration from high continuum opacities from dust, rather than a true physical gas feature. ...

^₁₃C^₁₇O Suggests Gravitational Instability in the HL Tau Disc ~ Alice S. Booth, John D. Ilee

• Monthly Notices of the RAS: Letters 493(1):L108 (Mar 2020) DOI: 10.1093/mnrasl/slaa014
• arXiv.org > astro-ph > arXiv:2001.07550 > 21 Jan 2020

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Using Gaia as a Gravitational-Wave Detector
astrobites | Daily Paper Summaries | 2020 Mar 02

Gravitational waves (GWs) are disturbances in spacetime produced by any massive object moving asymmetrically. However, only the most massive and most relativistic objects produce large enough GWs to be detectable (though they are still super small). The LIGO and Virgo detectors are using laser interferometry to detect these tiny ripples in the fabric of spacetime. They have already detected dozens of GWs from binaries of black holes and neutron stars. Another method of detecting GWs is creating a so called pulsar timing array (PTA), where dozens of millisecond pulsars are monitored to look for the signatures of nanohertz GWs, that is waves with much lower frequencies than those seen by LIGO and Virgo.

Yet another way of detecting GWs is by using astrometry, that is by measuring the apparent positions of stars. This method was first introduced in 1990, and nowadays it’s getting more and more relevant due to the Gaia mission, which makes astrometric observations at unprecedented precision. Today’s astrobite describes how one can detect GWs from individual supermassive black-hole binaries using data from Gaia.

Astrometric Search Method for Individually Resolvable Gravitational Wave Sources with Gaia ~ Christopher J. Moore et al

• Physical Review Letters 119(26):1102 (29 Dec 2017) DOI: 10.1103/PhysRevLett.119.261102
• arXiv.org > astro-ph > arXiv:1707.06239 > 19 Jul 2017

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Hiding in Obscured Sight
astrobites | Daily Paper Summaries | 2020 Mar 03

Light emanating from within a host galaxy travels a huge distance to reach an observer. Along its path, the photons can encounter obstacles which change their wavelength or diminish the total intensity. Depending on the host galaxy’s orientation relative to Earth, said emission could even be impeded by material contained within the galaxy. This can make the identification and classification of the photon’s source much harder. Models predict that there are a huge number of active galactic nuclei (AGN) that grow behind dense screens of gas and dust that surround its host galaxy. Deep X-ray surveys are thought to produce the most complete and unbiased surveys of the AGN population but we are yet to observationally confirm the high predicted fraction of these obscured AGN. Today’s paper re-evaluated such a deep X-ray survey containing a large sample of AGN in the Chandra Deep Field South (CDFS) and investigated whether the lower luminosity sources are, in fact, bright sources hidden behind larger amounts of obscuring material than previously thought.

The original study contains approximately 500 X-ray selected AGN in the CDFS at redshifts above 0.5. For each detection, the original study calculated an estimate of the column density – a quantity describing the likely density of gas and dust around the galaxy that impedes photons travelling between source and observer. Using the column density of each AGN, today’s authors corrected the original observations and produced an intrinsic X-ray luminosity. Assuming the original study calculated the column density correctly then this intrinsic X-ray luminosity should make all the AGN in the sample appear completely un-obscured. Alongside this, they gathered mid-IR (6 and 24µm) and optical emission line data which can be used to describe other aspects of the AGN’s behaviour. ...

A Large Population of Obscured AGN in Disguise as Low Luminosity AGN in Chandra Deep Field South ~ Erini Lambrides et al

• arXiv.org > astro-ph > arXiv:2002.00955 > 03 Feb 2020

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Galactic Outflows: A Stellar Matter?
astrobites | Daily Paper Summaries | 2020 Mar 04

Galactic images captured by spaced based telescopes, such as Hubble, can be breathtaking. Vibrant hues of stellar light infuse with warm gas and dust emission to illuminate the composition of these stunning structures.

However, despite their ethereal nature, some of the Universe’s most extreme interactions occur near their centers. Here, intense feedback – the mechanism by which outflowing matter and radiation impact their environments – rages on.

The two primary modes of galactic feedback are Active Galactic Nuclei (AGN) feedback and supernova feedback. AGN feedback is fueled by the rapid accretion of material onto a supermassive black hole, which is converted into radiation, jets, and winds. Comparatively, supernova feedback results from the explosion of a massive dying star. ...

The authors of today’s paper explored the properties of galactic stellar feedback. They utilized the Mapping Galaxies at Apache Point Observatory (MaNGA) survey to spatially resolve galactic-scale outflows in a sample of 405 high mass (log M_✶/M_☉ ≥ 10) nearby galaxies (z ~ 0). ...

Outflows in Star-forming Galaxies: Stacking Analyses of Resolved Winds
and the Relation to Their Hosts' Properties ~ Guido W Roberts-Borsani et al

• Monthly Notices of the RAS (online 17 Feb 2020) DOI: 10.1093/mnras/staa464
• arXiv.org > astro-ph > arXiv:2002.05724 > 13 Feb 2020

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The Times They Are A-Changin’ (But the Stellar
Mass Function of Massive Galaxies is Not!)
astrobites | Daily Paper Summaries | 2020 Mar 05

Before the great Shapley-Curtis debate in 1920, astronomers knew virtually nothing about galaxies outside our own. At this time, it was unclear whether there even were galaxies beyond the Milky Way. Today, the study of galaxy evolution is nearly a century old, and our understanding of the diversity of galaxies has grown exponentially. Our current cosmological model (LCDM) predicts nearly all that we observe on the largest scales, including the distribution of galaxies across the sky. However, a complete theory of the evolution of galaxies that explains all the complexity we observe (e.g., Figure 1!) has evaded us. Basic questions such as, “How do galaxies grow over time?” remain unanswered. We explore this very question in today’s Astrobite, featuring exciting new results regarding the evolution of galaxies in the past 9.5 billion years. ...

On the (Lack of) Evolution of the Stellar Mass Function
of Massive Galaxies from z=1.5 to 0.4 ~ Lalitwadee Kawinwanichakij et al

• arXiv.org > astro-ph > arXiv:2002.07189 > 17 Feb 2020

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Good Luck Sorting This Hat
astrobites | Daily Paper Summaries | 2020 Mar 07

Is this galaxy an elliptical (Gryffindor) or ordinary spiral (Slytherin)?

The Sombrero galaxy, famous for its hat-like shape, has been observed many times. However, it maintains a certain level of mystery: much like the sorting hat struggled to sort Harry Potter into a Hogwarts house, the Sombrero galaxy is difficult to sort into a galaxy classification. According to Hubble’s galaxy classification system, galaxies fit into four main categories: ellipticals, ordinary spirals, barred spirals, and irregulars. We have a fantastic edge-on view of the Sombrero galaxy, which allows us to image both its disk and hazy bulge, as seen in Figure 1. Because of its disk structure and lack of developed spiral arms, many astronomers classify the Sombrero galaxy as an early-type spiral. However, there is evidence that the size of the Sombrero’s halo (its extended sphere of stars) and its number of globular clusters are more similar to values found in elliptical galaxies. This leads us to believe that the Sombrero galaxy may have two parent components that merged: a spiral disk galaxy and an elliptical galaxy, and therefore simultaneously belongs to two different Hogwarts houses.

In order to determine how the Sombrero galaxy formed, the authors of today’s paper used Hubble Space Telescope (HST) images to analyze the halo of the galaxy. Their fields of view for the two images were 16 and 33 kiloparsecs (kpc) above the center of the galaxy, which is pretty far from the brightest component that we usually recognize as the Sombrero (see Figure 1). ...

The Strikingly Metal-Rich Halo of the Sombrero Galaxy ~ Roger E. Cohen et al

• Astrophysical Journal 890(1):52 (2020 Feb 10) DOI: 10.3847/1538-4357/ab64e9
• arXiv.org > astro-ph > arXiv:2001.01670 > 06 Jan 2020 (v1), 07 Jan 2020 (v2)