astrobites: Daily Paper Summaries 2020

Find out the latest thinking about our universe.
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Investigating Early Populations of Galaxies

Post by bystander » Wed Feb 05, 2020 7:53 pm

Investigating Early Populations of Galaxies
with the Best Telescopes in the Universe

astrobites | Daily Paper Summaries | 2020 Feb 05
Lukas Zalesky wrote:
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
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Where the Solar System Ends

Post by bystander » Wed Feb 12, 2020 6:49 pm

Where the Solar System Ends
astrobites | Daily Paper Summaries | 2020 Feb 06
Briley Lewis wrote:
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
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Where Are All the Baryons?

Post by bystander » Wed Feb 12, 2020 7:06 pm

Where Are All the Baryons?
astrobites | Daily Paper Summaries | 2020 Feb 06
Jason Hinkle wrote:
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
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Polarime-trying to Map Magnetic Fields in the Orion Nebula

Post by bystander » Wed Feb 12, 2020 7:26 pm

Polarime-trying to Map Magnetic Fields in the Orion Nebula
astrobites | Daily Paper Summaries | 2020 Feb 08
Ashley Piccone wrote:
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
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Extreme star-forming galaxy reveals all…!?

Post by bystander » Wed Feb 12, 2020 7:37 pm

Extreme star-forming galaxy reveals all…!?
astrobites | Daily Paper Summaries | 2020 Feb 10
Joanna Ramasawmy wrote:
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
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Unlocking the secrets of chaotic planetary systems

Post by bystander » Wed Feb 12, 2020 7:45 pm

Unlocking the secrets of chaotic planetary systems
astrobites | Daily Paper Summaries | 2020 Feb 11
Spencer Wallace wrote:
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
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More Clues to the Environment in Which FRBs Originate?

Post by bystander » Wed Feb 19, 2020 4:36 pm

More Clues to the Environment in Which FRBs Originate?
astrobites | Daily Paper Summaries | 2020 Feb 12
Haley Wahl wrote:
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
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What happens when you throw a satellite at the Sun?

Post by bystander » Wed Feb 19, 2020 4:48 pm

What happens when you throw a satellite at the Sun?
astrobites | Daily Paper Summaries | 2020 Feb 13
Briley Lewis wrote:
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
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The Beating Heart(-Shaped Region) of Pluto

Post by bystander » Wed Feb 19, 2020 5:00 pm

The Beating Heart(-Shaped Region) of Pluto
astrobites | Daily Paper Summaries | 2020 Feb 14
Kaitlyn Shin wrote:
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
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Signs of Dead Galaxies at Cosmic Dawn

Post by bystander » Wed Feb 19, 2020 5:13 pm

Deciphering Spitzer’s Legacy: Signs of Dead Galaxies at Cosmic Dawn
astrobites | Daily Paper Summaries | 2020 Feb 15
John Weaver wrote:
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
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Making a “Mega-Telescope” for Exoplanets

Post by bystander » Wed Feb 19, 2020 5:22 pm

Making a “Mega-Telescope” for Exoplanets
astrobites | Daily Paper Summaries | 2020 Feb 17
Briley Lewis wrote:
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
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Red Galaxies at Night, Astronomers’ Delight!

Post by bystander » Wed Feb 19, 2020 5:36 pm

Red Galaxies at Night, Astronomers’ Delight!
astrobites | Daily Paper Summaries | 2020 Feb 18
Mitchell Cavanagh wrote:
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
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Joint Survey Constraints for Cheap!

Post by bystander » Tue Feb 25, 2020 9:28 pm

Joint Survey Constraints for Cheap!
astrobites | Daily Paper Summaries | 2020 Feb 20
Paul Rogozenski wrote:
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
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Baby Stars, X-rays, and Planets: How are they Related?

Post by bystander » Tue Feb 25, 2020 9:42 pm

Baby Stars, X-rays, and Planets: How are they Related?
astrobites | Daily Paper Summaries | 2020 Feb 21
Abygail Waggoner wrote:
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
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Milky Matter Magnifies Magellanic Motion

Post by bystander » Tue Feb 25, 2020 10:35 pm

Through the Lens: Milky Matter Magnifies Magellanic Motion
astrobites | Daily Paper Summaries | 2020 Feb 22
Luna Zagorac wrote:
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 θt, angular scale βt, and effective lens velocity direction vt 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
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Table Salt detected on Europa!

Post by bystander » Wed Feb 26, 2020 3:04 am

Table Salt detected on Europa!
astrobites | Daily Paper Summaries | 2020 Feb 24
Ishan Mishra wrote:
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
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