astrobites: Daily Paper Summaries 2019

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Are AGN Quiescent Adolescents?

Post by bystander » Tue Aug 13, 2019 5:36 pm

Are AGN Quiescent Adolescents?
astrobites | Daily Paper Summaries | 2019 Aug 07
Keir Birchall wrote:
It has long been understood that a connection exists between galaxies and the supermassive black holes (SMBHs) that exist at their centres. Gas from the host galaxy finds its way into the centre where it is consumed by the black hole. The sated black hole then bathes the host galaxy in huge amounts of radiation that can be detected across the electromagnetic spectrum, a phenomenon known as an active galactic nucleus (AGN). The difficult question is: how does the gas find its way into the centre and what effects might this have on the host galaxy? The well-cited Alexander & Hickox (2012) paper suggests that these AGN are fuelled mainly by mergers between galaxies and is outlined in figure 1. ...

Today’s authors studied a sample of obscured quasars in this so-called ‘adolescent’ phase (figure 1; stage 3) to investigate whether AGN and their host galaxies will always follow this evolutionary path. To accomplish this the authors identified and imaged a sample of 10 FeLoBAL quasars at the lowest possible redshifts (0.6 < z < 1.1) using the Hubble Space Telescope (HST). FeLoBAL quasars are active galaxies described by broad absorption lines in their spectrum, particularly iron, indicating powerful outflows of material. A sample of 20 blue, unobscured quasars from the author’s previous work were used as a control sample. The aim was to characterise and compare the shapes of both samples of galaxies. Assuming this merger-driven theory is correct, the authors would expect an enhancement of merger signatures in the FeLoBAL quasar sample when compared to their unobscured counterparts as FeLoBAL quasars more recently experienced the initial merger event. ...

The host galaxies of FeLoBAL quasars at z ∼ 0.9
are not dominated by recent major mergers
~ C. Villforth et al
viewtopic.php?t=39684
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Moonetesimals likely form relatively quickly

Post by bystander » Tue Aug 13, 2019 5:51 pm

Moonetesimals likely form relatively quickly
astrobites | Daily Paper Summaries | 2019 Aug 08
Samuel Factor wrote:
Disks of gas and dust around young stars are fairly common; they are the birthplaces of planetary systems, hence the term protoplanetary disk. In the past few years, astronomers have used the Atacama Large Millimeter/Submillimeter Array (ALMA) to image these circumstellar disks and even catch planets forming within them. But the giant planets in our solar system also have systems of moons. How do they form? Most likely in a similar process by which the planets themselves formed, though in a circumplanetary disk (or protolunar) rather than a circumstellar disk (or protoplanetary). Today’s paper combines new and old observations of young giant planets that looked for, but failed to detect, these circumplanetary disks in order to constrain the timescale of moon formation. ...

Upper limits on protolunar disc masses using ALMA
observations of directly imaged exoplanets
~ Sebastián Pérez et al
viewtopic.php?t=39492#p293669
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A Proposed Moon Formation Theory: The Multiple-Impact Hypothesis

Post by bystander » Tue Aug 13, 2019 6:26 pm

A Proposed Moon Formation Theory: The Multiple-Impact Hypothesis
astrobites | Daily Paper Summaries | 2019 Aug 12
Jacob Azoulay wrote:
It’s been with us since before humans laid foot on Earth. Any person you have ever known or heard of has seen it shine bright in the night sky. Yet, researchers can only speculate about its creation.

The Moon is estimated to have formed 4.53 billion years ago, when a Mars-sized celestial body collided with Earth, spewing out debris that eventually clumped together. But what if this “giant-impact hypothesis” is not, in fact, how the Moon formed? Luckily, by using computer simulations, Robert I. Citron, Hagai B. Perets, and Oded Aharonson — the authors of today’s paper — can help us better understand our Moon’s origins. ...

The Role of Multiple Giant Impacts in the Formation of the Earth–Moon System ~ Robert I. Citron, Hagai B. Perets, Oded Aharonson
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When stretching stars with black holes gets unstable

Post by bystander » Tue Aug 13, 2019 6:40 pm

When stretching stars with black holes gets unstable
astrobites | Daily Paper Summaries | 2019 Aug 13
Elen Golightly wrote:
When a star wanders too close to a supermassive black hole at the centre of a galaxy, it can get stretched and pulled apart in what astronomers refer to as a ‘tidal disruption event’ or ‘TDE’. For this to occur the star must pass within tidal radius, the distance at which the tidal force from the black hole can overcome the self-gravity that keeps the star intact and spherical. Passing stars are not always disrupted however—those that plunge too deep within the tidal radius may be swallowed whole, while others may only be partially disrupted or miss the tidal field altogether, avoiding disruption completely.

A star that finds itself within the tidal radius begins to stretch into a stream, getting longer as it continues its orbit around the black hole. Approximately half of the elongated stream of stellar debris is then gravitationally bound to the black hole, while the other half remains unbound and escapes out into the galaxy at high velocities. The bound debris begins to fall back towards the black hole and forms an accretion disc that feeds it (see Figure 1). This accretion process can power a highly luminous and detectable flare from the disc.

The authors of this paper have modelled these TDEs computationally and were the first to simulate the full evolution of the events with realistic parameters. They simulated these events using PHANTOM, a 3D hydrodynamical code. Their system used a solar mass (1M) star, modelled as an adiabatic sphere of gas made up of one million particles, and a 106M black hole (a black hole mass between 105−108M is a reasonable choice). The authors started the star outside of the black hole’s tidal influence and ran the simulation until 90% of the disrupted material had returned to the disc, which was about 10 years after disruption started. ...

Variability in Tidal Disruption Events: Gravitationally Unstable Streams ~ Eric R. Coughlin, Chris Nixon
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The Imprint of an Invisible Giant

Post by bystander » Fri Aug 16, 2019 11:28 pm

The Imprint of an Invisible Giant
astrobites | Daily Paper Summaries | 2019 Aug 14
Spencer Wallace wrote:
Without Jupiter, the solar system might be a vastly different place. Our largest planetary companion is thought to be responsible for everything from clearing out planet-building material near the sun to throwing Neptune and Uranus to the outer solar system to delivering water to the Earth. Given the crucial role that this behemoth played in shaping our solar system, a natural question to ask is how important gas giants are for building planets around other stars.

Unfortunately, we can’t just look at stars with terrestrial planets and then determine which of those also have something resembling Jupiter. With the exception of a rather exotic and unexpected subclass of close-in giant planets known as hot Jupiters, most gas giants take far too long to orbit their host stars to be easily detectable. To illustrate this point, consider how an observer from outside the solar system would detect Jupiter. There are two ways to do this. The first involves gradually watching the sun redshift and blueshift as Jupiter’s gravity tugs on it over the course of an orbit. Alternatively, if you are extremely lucky, Jupiter might pass between you and the sun, producing a momentary dimming of the sun’s light. Both of these methods require you to potentially wait for Jupiter to complete nearly an entire orbit around the sun. This would take 12 years! Unless you’re continuously watching the sun for over a decade, you probably wouldn’t ever detect this giant.

Although extrasolar terrestrial planets are found using these same methods, the problem mentioned above is not quite so severe because these types of worlds tend to lie close to their star. This means that they complete an orbit fairly quickly and so you don’t have to wait long for them to alter the star’s light. For this reason, any large-scale exoplanet survey such as Kepler or TESS will tend to detect mostly close-in planets, while missing most of the longer period ones. This brings us to the goal of today’s paper, which is to determine how the presence of a gas giant, many of which lie on wide, long period orbits, affects the distribution of inner terrestrial worlds during the planet formation process. With a better understanding of this connection, it might be possible to use the measured properties of the more easily detectable terrestrial planets to figure out where the gas giants actually are (or aren’t). ...

Giant Planet Effects on Terrestrial Planet Formation and System Architecture ~ Anna C. Childs et al
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A Historical Nova and the First Radioactive Molecule

Post by bystander » Fri Aug 16, 2019 11:44 pm

A Historical Nova and the First Radioactive Molecule in Space
astrobites | Daily Paper Summaries | 2019 Aug 15
Charles Law wrote:
Nearly 350 years ago, two Sun-like stars collided in a spectacular explosion and formed a new type of star. The source, known as CK Vulpeculae (or more simply, CK Vul), was first seen in 1670 when observers reported the appearance of a new bright, red star. Initially visible with the naked eye, Nova CK Vul quickly faded and astronomers now need large telescopes to study the remnants left behind: a dim central star and surrounding hot, glowing gas (Figure 1). This gaseous debris, cast out into space during the violent stellar merger that created CK Vul, affords astronomers a unique opportunity to study the dense inner layers of a star, where heavy elements and radioactive isotopes are produced. In today’s astrobite, we take a look at the detection of a radioactive version of aluminum in this gaseous debris around CK Vul, which represents the first definitive detection of an unstable radioactive molecule outside of our Solar System. ...

Astronomical Detection of a Radioactive Molecule 26AlF in a Remnant of an Ancient Explosion - Tomasz Kamiński et al
viewtopic.php?t=38561
viewtopic.php?t=38744#p286404
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Subgiant in the Spotlight

Post by bystander » Fri Aug 16, 2019 11:57 pm

Subgiant in the Spotlight: Characterising a New Benchmark Star
astrobites | Daily Paper Summaries | 2019 Aug 16
Oliver Hall wrote:
We are currently living in the era of big data astronomy; and especially so in the field of asteroseismology – the study of stellar properties through measuring their pulsations. Between the recent Kepler, K2 and TESS missions, as well as supplementary data from Gaia, ensemble studies are often the norm, studying population effects of many stars in tandem. However in this era of large data sets it is important to continue to calibrate our methods, and this is where detailed studies of single, ‘benchmark’ stars are still very important.

The authors of today’s paper set out to study such a benchmark star, HR 7322 in great detail, using multiple complementary methods, with the intention of using the measured parameters of this star to identify problems with asteroseismic theory. The star they picked was a subgiant — a star that has burned through all the hydrogen in its core and is on its way to become a red giant. Recent results have shown a disagreement between radii of subgiant stars calculated from Gaia mission data and those calculated using asteroseismology. The authors study this subgiant to great precision in order to identify the source of this disagreement. ...

The Subgiant HR 7322 as an Asteroseismic Benchmark Star ~ Amalie Stokholm et al
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Guide to finding planets around ultracool dwarfs

Post by bystander » Thu Aug 22, 2019 3:32 pm

A step by step guide to finding planets around ultracool dwarfs
astrobites | Daily Paper Summaries | 2019 Aug 19
Emma Foxell wrote:
Over the last 20 or so years, over 3000 exoplanets have been detected, most around stars like our Sun. Recently, many exoplanet hunters have been focusing on M dwarfs, as Earth-sized planets at habitable temperatures are easier to detect. But could we go even cooler? L and T dwarfs (LTs) are a mixed bunch. While all have radii about 1 RJup and temperatures 500-2200 K, their masses vary wildly from stellar main sequence (0.08MSun) through brown dwarfs to planetary mass objects. Earth around a L0 star would produce transits blocking 1% of their host star’s light and receive the same amount of flux orbiting in just 1.65 days, making the habitable zone even more accessible. The habitable zone may actually be further out due to tidal heating, as happens to Jupiter’s moon Io.

But can these planets exist? Theory and observations suggest so. Circumstellar disks, from which planets form, have been observed around M-type brown dwarfs and 3 MJup objects have been detected around brown dwarfs at large separation by direct imaging. While slightly higher mass, TRAPPIST-1 is the prime example of an ultracool M8 dwarf with 7 Earth-sized planets. Theory suggests that brown dwarfs may be able to form planets up to 5 Earth masses, and considering planets are very common around M dwarfs, perhaps it is reasonable to expect planets around LTs.

Discovering planets around LTs would inform our general theory on satellite formation and help us understand how planet occurrence rates vary compared to higher mass stars and within the wide range of LTs. Astronomers are also interested whether planets around LTs are predominately terrestrial. So what is the best way to search for them? ...

Design Considerations for a Ground-Based Search for Transiting
Planets around L and T Dwarfs
~ Patrick Tamburo, Philip S. Muirhead
  • arXiv.org > astro-ph > arXiv:1908.03593 > 09 Aug 2019 (v1), 21 Aug 2019 (v2)
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UV Won’t Stop Exoplanets from Becoming Habitable

Post by bystander » Thu Aug 22, 2019 3:44 pm

Let’s Not Make Assumptions: UV Radiation
Won’t Stop Exoplanets from Becoming Habitable

astrobites | Daily Paper Summaries | 2019 Aug 20
Yesenia Ruano wrote:
Astrophysicists have been investigating the habitability of exoplanets—planets that orbit around stars other than the Sun—as well as the factors which contribute to a planet’s habitability, like UV radiation.

What’s UV radiation? It’s the part of the electromagnetic spectrum that we fight off while using sunblock at the beach. UV radiation emitted from stars is harmful to the surfaces of planets, including the Earth’s. Radiation of this kind can cause the erosion of a planet’s atmosphere and hinder the formation of organic life, making the planet less habitable. As organic molecules absorb UV radiation, the efficiency of nucleic acids (a substance that makes up all of our DNA) can be damaged. By now, you might think that there is no way in which exoplanets near a radiating star can sustain life, but surprisingly they still have a chance.

In this paper, the authors argue that exoplanets that are exposed to high levels of UV radiation can still harbor life. They use M star systems—systems that consist of exoplanets orbiting a dim long-lasting star called a red dwarf—to test this. ...

Lessons from early Earth: UV surface radiation should not limit the
habitability of active M star systems
~ Jack T. O'Malley-James, Lisa Kaltenegger
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Why are Jupiter and Saturn Spinning so Slowly??

Post by bystander » Thu Aug 22, 2019 4:00 pm

Why are Jupiter and Saturn Spinning so Slowly??
astrobites | Daily Paper Summaries | 2019 Aug 21
Jenny Calahan wrote:
The rotation periods of Jupiter and Saturn are 9.93 hours and 10.7 hours, respectively. Now, compared to our tiny Earth that lazes around on a 24-hour rotational period, you might think, “wow, those are some zoomy-bois.” However, our best theories of planet formation tell us that, based on how massive they were when they formed, they should really be doin’ a faster spin. ...

Today’s paper attempts to lay the groundwork for solving this angular momentum problem in Jovian-planet formation using magnetohydrodynamics. Big (scary) word, yes, but put more simply, this paper creates a semi-analytic model of a newly forming Jovian planet with a strong magnetic field, and explores how this might slow the planet down. ...

On the Terminal Rotation Rates of Giant Planets ~ Konstantin Batygin
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Is the Fault in their Stars?

Post by bystander » Thu Aug 22, 2019 4:10 pm

Is the Fault in their Stars?
astrobites | Daily Paper Summaries | 2019 Aug 22
Keir Birchall wrote:
At the centre of almost every massive galaxy lies a supermassive black hole (SMBH) into which gas and dust occasionally fall. When this happens, the central region is bathed in huge amounts of light and an active galactic nucleus (AGN) is born. They are a diverse phenomenon, emitting light in different wavelengths, with different intensities and for different lengths of time. Such variation in observed properties makes AGN very difficult to study. What factors or processes can trigger an AGN? What role does the host galaxy play? And, more specifically, how does star formation rate affect likelihood of triggering?

Previous observations of AGN have shown that galaxies with similar properties (stellar mass, star formation rate, etc.) are observed to have a wide range of luminosities. Thus, trying to understand the underlying connections between AGN activity and the properties of its host galaxy using only the observed AGN luminosity is extremely challenging. For example, two galaxies with the same mass could host AGN with hugely different observed luminosities, from which you could then conclude that the host galaxy mass played very little in role in triggering AGN. However, this approach fails to take into account that the varying AGN luminosities could be explained by the central SMBH accreting material at different rates. Today’s paper attempts to take into account these varying rates of accretion of material, and connect a galaxy’s star formation rate (SFR) to the likelihood that it will host an AGN. ...

X-rays across the galaxy population - III. The incidence of AGN as a
function of star formation rate
~ James Aird, Alison L. Coil, Antonis Georgakakis
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Fluorescent Worlds: Searching for Life’s Glow

Post by bystander » Fri Aug 30, 2019 5:55 pm

Fluorescent Worlds: Searching for Life’s Glow
astrobites | Daily Paper Summaries | 2019 Aug 27
Jamie Wilson wrote:
Are we alone? It is perhaps one of the most profound questions we humans have ever thought to ask ourselves. An even more astonishing aspect of this question is that we may well be living in the first period of history where it might be possible to obtain the answer. Recent research has shown that our galaxy appears to be teeming with new worlds to discover and a growing number of the already known worlds orbit within their host star’s habitable zone, the region where water can exist in liquid form on a planet’s surface. The nearest potentially habitable planet orbits the star Proxima Centauri at just over four light years away. However, whilst the presence of water might indicate that a planet could be habitable, it’s not the same as saying that a planet is inhabited. Since we have little hope of visiting these new worlds in the near future, the best we can do is search remotely for atmospheric biosignatures – chemicals present in a planet’s atmosphere that suggest the existence of past or present life, for example oxygen, which exists in large proportions in the Earth’s atmosphere because it is continually produced by vegetation. Now a team of researchers, led by Jack O’Malley-James at Cornell’s Carl Sagan Institute, have discovered a previously unknown way to search for life in the Universe – observing the protective glow of biofluorescent organisms triggered by ultraviolet flares from red dwarf stars. ...

Biofluorescent Worlds – II. Biological Fluorescence Induced by Stellar UV
Flares, a New Temporal Biosignature
~ Jack T. O'Malley-James, Lisa Kaltenegger
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Looking at Stars but Seeing the Kuiper Belt

Post by bystander » Fri Aug 30, 2019 6:07 pm

Looking at Stars but Seeing the Kuiper Belt
astrobites | Daily Paper Summaries | 2019 Aug 29
Will Saunders wrote:
Detecting small Kuiper Belt Objects (KBOs) is challenging even with state-of-the-art telescopes and massive collaboration surveys. A tiny dot on a CCD could be a KBO, cosmic ray, asteroid, or anything else. Distinguishing a KBO from the alternatives requires many nights of successive imaging of the same objects. And astronomical instruments are expensive, costing upwards of tens of millions of dollars.

However, the paper in this Astrobite details how two amateur telescopes costing only $32,000 discovered the first KBO roughly 2 km in diameter. The goal of the work is not to directly compete with massive astronomical surveys, but rather to demonstrate the groundbreaking research can be done at significantly lower cost. ...

A kilometre-sized Kuiper belt object discovered by stellar occultation using amateur telescopes ~ K. Arimatsu et al
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Explaining Ancient Stellar Populations

Post by bystander » Fri Aug 30, 2019 6:24 pm

Explaining Ancient Stellar Populations
astrobites | Daily Paper Summaries | 2019 Aug 30
Caitlin Doughty wrote:
Of the multitude of galaxies in the universe, none are so close to us and so numerous as the dwarf galaxies. With 59 little galaxies whirling around the Milky Way, one might think that astronomers would understand them perfectly by now; they are, after all, relatively accessible. However, mysteries abound surrounding their origins and evolutionary history, and they have introduced all kinds of problems into our understanding of astronomy. There doesn’t appear to be enough of them, occasionally they appear to be missing their dark matter and, most pertinent to today’s paper (although perhaps less exciting-sounding), some of them have peculiarly old stellar populations. A few of these galaxies contain stars that, the youngest of which appear to have formed when the universe was less than one billion years old, a mere 7% of its current age. What could cause these particular dwarf galaxies to host only incredibly ancient stars?

One explanation that has been put forward is that an event from early in the universe’s history, called reionization, could have played a hand in this. During this time period, the amount of radiation coming from stars and galaxies ramped up and ionized almost all of the hydrogen in the universe, deconstructing hydrogen atoms into their component nuclei and electrons (this is where the name “reionization” comes from). But even as this radiation was splitting apart hydrogen atoms, it was simultaneously heating up all of the gaseous elements, including the hydrogen, as well as helium and the other less abundant elements. It is theorized that this heating effect on the gas may have somehow either (1) prevented dwarf galaxies from collecting new gas or (2) caused them to lose what gas they already had, as it photoevaporated away. It is also believed to be more significant for smaller galaxies.

The investigators behind today’s paper have used cosmological simulations—which model dark matter and baryonic matter (i.e. the kind that both stars and your coffee cup are made of) from very early times to form galaxies, the cosmic web, and any other large-scale object in the universe that you can think of—to examine how including a simulated reionization event may have affected dwarf galaxies’ ability to accrete and retain the gas necessary for forming stars. These simulations in particular, called SPHINX, incorporate radiative transfer, allowing the researchers to create a more realistic representation of reionization than a simulation that excludes these calculations. ...

How to Quench a Dwarf Galaxy: The Impact of Inhomogeneous
Reionization on Dwarf Galaxies and Cosmic Filaments
~ Harley Katz et al
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With Age Comes Wisdom

Post by bystander » Wed Sep 04, 2019 5:19 pm

With Age Comes Wisdom
astrobites | Daily Paper Summaries | 2019 Sep 02
Jessica Roberts wrote:
The age of something seems like such a simple, easy to obtain, and fundamental property in science. But it is a critical measurement for helping us better understand the history and evolution of most everything in the Universe. For instance, you might want to know how old a tree is so you can trace back its history and understand why it grew a certain way. We know that our Solar System is about 4.5 billion years (Gyrs) old. From this knowledge we can look back in history and explain how the Sun and its planets must have evolved into what we see today. We believe the Universe is around 13.7 billion years old, and based on the ages of other galaxies, we conclude that galaxies must have formed shortly after the Big Bang (in cosmological terms). But determining the age of something in astronomy is extremely difficult. Stars in our own galaxy, the Milky Way, pose an interesting challenge. It might surprise you that we know the age of the Universe more precisely than we do most stars nearby. Why this discrepancy? You would think that stars closer to us should be easier to date than the entire Universe! However to know the age of a star, we need to know how bright or luminous it actually is. With this information, stellar evolution models can trace back the star’s origin and age. Sounds easy enough, but in order to know how bright a star truly is we need to know how far away the star is. After all, a flashlight can outshine a spotlight if we move close enough to it. The same principle applies to stars. However, accurate distance measurements to these stars have for decades been difficult to obtain. Then Gaia came along and provided precise distance measurements to millions of stars in the Milky Way. Equipped with this new powerful information, the authors of today’s paper determine the ages of many of these stars and begin to piece together the most detailed history of our Milky Way. ...

Uncovering the Birth of the Milky Way through Accurate Stellar Ages with Gaia ~ Carme Gallart et al
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Cloudy with a chance of wind

Post by bystander » Tue Sep 10, 2019 3:58 pm

Cloudy with a chance of wind
astrobites | Daily Paper Summaries | 2019 Sep 04
Stephanie Hamilton wrote:
Jupiter’s horizontal banding is arguably one of the most iconic features in our Solar System, alongside Saturn’s rings. But did you know that Saturn also has similar horizontal banding? The two largest planets in our Solar System appear quite different, but they also share similarities. Today’s paper looks at data from two different spacecraft, Juno and Cassini, to compare and contrast the winds of both planets. ...

Comparison of the deep atmospheric dynamics of Jupiter and Saturn
in light of the Juno and Cassini gravity measurements
~ Yohai Kaspi et al
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What do galaxy clusters and Russian dolls have in common?

Post by bystander » Tue Sep 10, 2019 4:12 pm

What do galaxy clusters and Russian dolls have in common?
astrobites | Daily Paper Summaries | 2019 Sep 05
Sunayana Bhargava wrote:
Astronomers have observed that clusters of galaxies have a peculiar property: they all look roughly the same regardless of their mass or distance. Unlike individual galaxies, which are distinguishable by their distinct shapes or “morphologies,” galaxy clusters are intriguingly similar looking.

This peculiarity of galaxy clusters is referred to as self-similarity. When we say galaxy clusters are self-similar, we mean they have an identical appearance at different masses or distances from us. Strong self-similarity states that a smaller cluster is an identical, scaled version of a massive one.

Weak self-similarity is a bit subtler. When we look at distant clusters, we are looking back in time at a younger universe which had a higher overall density. However, if we consider the changing density in the universe, a distant cluster is identical to a nearby cluster of the same mass. ...

In a recent paper, a team of authors showed that 25 massive galaxy clusters generated from a “dark-matter only” simulation are astonishingly self-similar at redshifts (z) greater than 1. While this is an interesting result, the authors of today’s paper take self-similarity to task with a different approach to measuring the evolution of cluster halos. ...

The Three Hundred Project: The evolution of galaxy cluster density profiles ~ Robert Mostoghiu et al
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Dust Accretion & Double Suns

Post by bystander » Tue Sep 10, 2019 4:25 pm

Dust Accretion & Double Suns
astrobites | Daily Paper Summaries | 2019 Sep 09
Lauren Sgro wrote:
Imagine walking out your front door tomorrow morning and gazing up at the sky. What if, instead being lit up by the light of one sun, your world was illuminated by two suns.

This isn’t as crazy as it sounds. Astronomers have found plenty of planets whose potential alien life forms would witness such a double sunrise. However, there are many unanswered questions about these planets that form around binary stars, such as how they end up living so close to their host stars, just outside the dynamical stability limit (the point past which planets/satellites can survive in a stable orbit).

Today’s authors seek to find out if these planets are more likely to form where they are observed (in situ formation), or if they form farther out and migrate inward (ex situ formation). Previous studies have shown that the dynamics of the circumbinary disk may have a lot to do with which of these two formation options is most viable. Using Smoothed Particle Hydrodynamics (SPH), the authors model their binary systems with various eccentricities (a measure of how circular or extended an orbit is), mass ratios (q = the ratio of the primary mass to the secondary mass), and dust properties to determine how dust and gas accrete onto the companion stars and their individual (circumstellar) or shared (circumbinary) disks. Their results shed light on where planets form most easily in binary systems, and whether or not in situ formation can actually take place where astronomers have observed such planets. ...

Dust accretion in binary systems: Implications for planets and transition discs ~ Yayaati Chachan et al
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Using FRBs to Constrain the Diffuse Gas Fraction

Post by bystander » Tue Sep 17, 2019 6:04 pm

New Cosmological Detectives: Using FRBs
to Constrain the Diffuse Gas Fraction

astrobites | Daily Paper Summaries | 2019 Sep 11
Kaitlyn Shin wrote:
Although Fast Radio Bursts (FRBs), brief millisecond flashes of extremely energetic radio emission, were first discovered over a decade ago in 2007, only in the past few years has the community seen a revolutionary increase in the number of detected FRBs. In early March, the known total number of FRBs was at least 65, two of which were repeating sources, and one of which had been localized to a host galaxy. The number is now at least 85 known FRBs, with nine repeaters announced this past August (eight new sources and one previously single-burst source), and two more localized FRBs announced this past summer. (See this astrobite for more on one of them!)

Beyond their mysterious origins, FRBs have also captured the attention of some scientists because of their possible applications to cosmology. FRBs have large observed dispersion measures (DMs), which means that the lowest energy radio photons from the burst are observed some time after the most energetic photons, and the time depends on the number of free electrons the photons travel through. Free electrons can be found in the intergalactic medium (IGM), circumgalactic medium (CGM), Milky Way, and the host galaxies of FRBs. FRBs that are very far away –– at cosmological distances –– may have a “cosmic DM” with contributions to the DM from the IGM and the CGM.

If the redshifts (z) of the FRBs are measured, possible only if they are localized to a host galaxy, then a relation between cosmological DM and redshift, DM(z), can probe cosmological parameters. The DM(z) relation could also shed light on the “missing baryon problem” by constraining the fraction of diffuse, ionized gas –– baryons –– in the IGM. These problems can be explored with DM(z) because DM as a function of z can provide an estimate for the baryonic matter in the IGM and CGM between us and the source as a function of redshift (and the density of baryonic matter is a cosmological parameter). We don’t have a lot of DM(z) data from FRBs yet, but it’s always useful to think ahead and characterize what kind of science can be done if the data were there. That’s where simulations, and today’s paper, come into play. ...

Probing Diffuse Gas with Fast Radio Bursts ~ Anthony Walters et al
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Galaxy collisions and star-formation

Post by bystander » Tue Sep 17, 2019 6:40 pm

Galaxy collisions and star-formation:
a surprising causal examination

astrobites | Daily Paper Summaries | 2019 Sep 12
John Weaver wrote:
The fundamental constituents of galaxies are stars, gas, dust, black holes, and dark matter. The extent to which we understand each of them and their interplay varies greatly. Yet current understanding drives us to acknowledge that the growth of the mass of galaxies must be driven by either active star-formation, galaxy-galaxy mergers, or both. But which of these two effects dominates is an ongoing mystery nearly as old as the discovery of galaxies themselves. ...

Today’s astrobite explores the causal connection between star-formation and galaxy mergers. ...

Effect of galaxy mergers on star formation rates ~ W. J. Pearson et al
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Hide and Seek with Satellite Galaxies

Post by bystander » Tue Sep 17, 2019 7:12 pm

Hide and Seek with Satellite Galaxies
astrobites | Daily Paper Summaries | 2019 Sep 13
Bryanne McDonough wrote:
Simulations of galaxy formation are powerful laboratories for testing astronomer’s ideas about how the universe works. If those ideas are wrong, the simulations will not produce galaxies that look like the ones we observe. Satellite galaxies, which are small galaxies gravitationally bound to larger hosts, are especially good tests of the accuracy of simulations, since their properties are sensitive to many of the processes that affect their host.

Encompassing a host and its satellites is a dark matter halo, and the most massive of these have been shown to lie on spots of over-density in the early universe. In a previous paper, today’s authors predicted a significant over-density in the number of satellite galaxies around massive halos (greater than 10^12 times the mass of the sun). However, observations have not come back with concrete results confirming this idea. So where are these extra satellites? ...

The authors of today’s paper wanted to understand the effect that stellar feedback has on satellite galaxies, so they left out AGN feedback from their simulations. They then ran two versions of the same simulation of a massive galaxy at a redshift of z=6 (about 12.8 Gigayears ago, roughly 1 Gy since the universe began), one with radiation (light) from stars and one without. ...

The hidden satellites of massive galaxies and quasars at high-redshift ~ Tiago Costa, Joakim Rosdahl, Taysun Kimm
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A Bare Hot Rock with No Atmosphere

Post by bystander » Tue Sep 17, 2019 10:18 pm

A Bare Hot Rock with No Atmosphere
astrobites | Daily Paper Summaries | 2019 Sep 16
Vatsal Panwar wrote:
Exoplanets come in a wide range of flavours and sizes, ranging from puffy gas giants to small rocky worlds. Characterizing the atmospheres of this diverse population gives us an insight into their formation processes and potential habitability. On this front the most attractive candidates are terrestrial exoplanets around M dwarfs, which form a significant fraction of the known terrestrial exoplanets. The smaller size of M dwarfs as compared to Sun-like stars means that the signal due to a transiting exoplanet is relatively larger making it easier to detect and characterize even the small rocky planets orbiting them. But what makes rocky worlds around M dwarfs so intriguing? Since M dwarfs have a fraction of the Sun’s luminosity, their habitable zones are much closer in — about a tenth of the Earth-Sun distance.

However, M dwarfs are also infamous for frequently spewing high energy flares which over time can strip the atmospheres of close-in rocky exoplanets around them. Observing more such systems with a range of configurations and physical properties can help us in understanding the survival of atmospheres on exoplanets orbiting M dwarfs. The authors of today’s paper present an investigation of one such exoplanet recently detected by TESS. Meet LHS 3844 — a nearby (~ 14 parsecs) M dwarf hosting a rocky exoplanet 1.3 times the size of Earth and with an orbital period of just 11 hours. ...

Absence of a Thick Atmosphere on the Terrestrial Exoplanet LHS 3844b ~ Laura Kreidberg et al
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A New Technique for Finding Newly Formed Exoplanets

Post by bystander » Fri Sep 20, 2019 9:02 pm

A New Technique for Finding Newly Formed Exoplanets
astrobites | Daily Paper Summaries | 2019 Sep 17
Jessica Roberts wrote:
Next year will mark the 25th anniversary of the first detection of a planet orbiting a main sequence star: 51 Peg b. Since this discovery, we have been finding these extrasolar planets (or exoplanets) at an exponential rate, passing the 4,000 milestone earlier this year. Of these thousands of planets, we have found some bigger than Jupiter and rocky planets more massive than Earth. As you might expect, most of the planets we have discovered are unlike any we have in our own Solar System. But all except a handful of these have one important characteristic in common: they are no longer undergoing formation. As most of these systems are older than a billion years, it is a challenge to piece together how these planets initially formed and evolved. We know they must have formed in a disk (known as a protoplanetary disk) that surrounded the forming star. We’ve observed many of these disks, and we have observed very massive planets forming within them. Still, more observations are needed if we want to learn how a planet goes from dust and gas particles to a rocky object potentially surrounded by a volatile-rich (not just hydrogen and helium) atmosphere.

This might seem like an easy task; we are clearly skilled at finding exoplanets. However, the disk makes it nearly impossible to find anything but the most massive planets forming. Methods such as the transit method or radial velocities rely on watching for changes in the star’s light due to the presence of a planet, but the disk can mask these signals. Our current direct imaging capabilities are already limited to only the brightest and hence most massive objects.

If we want to find Jupiter-mass and smaller planets, we need a new technique. This is where today’s authors come in. Since we have this massive and large disk, why not look for possible forming planets by observing the effect these planets would have on the motion of the gas particles, known as kinematical effects? As a planet orbits, it will push the gas around creating a lack of particles in one area and a build-up of particles in another. This local deviation in the gas structure in turn creates a difference in pressure, or pressure gradient. Without any pressure gradient, gas particles should orbit the star with a Keplerian, or orbital, velocity where the velocity decreases as you move further away from the star. However with this gradient, particles deviate from Keplerian orbits by speeding up or slowing down in order to fill the void created by the planet. A cartoon of this effect is shown in Figure 1 which plots the velocity differences as a function of distance from the star. Notice that gas particles at distances closer to the star than the planet slow down while those further away speed up in order to try to match the planet’s orbital speed. The authors hypothesize that if they can observe this effect in a disk, they will not only be able to confirm the existence of a planet but also determine its mass. ...

A Kinematical Detection of Two Embedded Jupiter Mass Planets in HD 163296 - Richard Teague et al Kinematic Evidence for an Embedded Protoplanet in a Circumstellar Disc - C. Pinte et al Kinematic Detection of a Planet Carving a Gap in a Protoplanetary Disk ~ C. Pinte et al
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An Ad-Mira-ble Distance Measurement

Post by bystander » Fri Sep 20, 2019 10:27 pm

An Ad-Mira-ble Distance Measurement
astrobites | Daily Paper Summaries | 2019 Sep 18
Tarini Konchady wrote:
If you want to figure out the fate of our Universe, the value of the Hubble Constant (H0) would be handy to have. H0 tells us how fast the Universe is expanding right now… I mean now… actually now — you get the picture. The Hubble Constant isn’t constant over time. Taken with other quantities its current value can tell us a lot about the Universe, such as its age and ultimate fate.

As great as H0 is though, it’s a bit tricky to measure. And to complicate things, the measured value of H0 changes with the measurement method. Currently, Planck measurements of the cosmic microwave background (CMB) return H0 = 67.4 ± 0.5 km/s/megaparsec (km/s/Mpc). This value is significantly smaller than the measurement obtained by using the distances and redshifts/velocities of distant galaxies, which is H0 = 74.03 ± 1.42 km/s/Mpc. The difference between these two measurements has been increasing since the 1990s (see Figure 1), leading astronomers on both sides to scrutinize their methods for unaccounted errors.

The method that uses galaxy distances and velocities relies heavily on how those distances are measured. Currently, distance measurements for the purposes of measuring H0 are closely tied to variable stars called Cepheids. To double-check the Cepheid-based distances, we need other objects to use as distance indicators.

One of these objects could be Mira variable stars (Miras). In this paper, the authors search for Miras in NGC 1559 using Hubble Space Telescope (HST) observations. NGC 1559 has hosted another better understood distance indicator — a Type Ia supernovae — making it a good place to test how Miras can perform as extragalactic distance indicators. ...

Hubble Space Telescope Observations of Mira Variables in the Type Ia Supernova Host
NGC 1559: An Alternative Candle to Measure the Hubble Constant
~ Caroline D. Huang et al
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(s)Pinning down the origins of black hole mergers

Post by bystander » Fri Sep 20, 2019 10:39 pm

(s)Pinning down the origins of black hole mergers
astrobites | Daily Paper Summaries | 2019 Sep 19
Philippa Cole wrote:
Everything you could possibly want to know about a black hole can be boiled down to three quantities – its mass, its spin, and its charge. We normally assume the charge of black holes to be negligible, and various methods have gotten pretty good at measuring the mass of black holes… but what about the spin? ...

The author’s of today’s paper try to tackle this problem by tracking the evolution of spinning massive stars in binary pairs as they collapse to form black holes, and then compare the effective spin prediction with observations from LIGO/Virgo. They do this by modifying a binary star evolution code to account for low metallicities, which allows the existence of massive enough stars that don’t lose too much of their mass over time, as well as considering two different models for the transfer of angular momentum from the stars to the black holes. ...

Black hole spins in coalescing binary black holes ~ Konstantin Postnov, Alexander Kuranov
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