astrobites: Daily Paper Summaries 2019

[bystander]

Dwarf Galaxies without Dark Matter
astrobites | Daily Paper Summaries | 2019 Sep 20

The effect of dark matter on galaxies was first observed back in the 1920s by various astronomers. When studying the rotation of the outer parts of the galaxy, they could see it was going much faster than it should be given the mass of the galaxy (estimated from the light emitted by the stars). It was therefore inferred that there must be more mass in the galaxy that we can’t see. Moreover, it wasn’t just a little bit of mass that we couldn’t see, it was 5 times the mass of the visible stars within the galaxy.

Within the standard cosmological model, dark matter makes up approximately 27% of the Universe, whereas normal matter, known as baryons, only makes up 4.6%. It is thought that dark matter is spread all through our Universe. It is not smoothly distributed however. It is formed of filaments, very much like a very disorganised spiders web. These filaments join together at certain points, and it’s at these points where we have a lot of dark matter where the biggest galaxies form. A large group of dark matter particles causes a deep potential well, in which gas can fall in and form the first stars and build up a whole galaxy. The more dark matter there is, the deeper the potential well and the larger the galaxy that forms there.

In order to determine the dark matter content, we can define the ‘half optical light radius’ which is the radius at which half of the optical light of the galaxy is contained. For low mass galaxies known as ‘dwarf galaxies’ within the local group, whilst they have less dark matter than very massive galaxies, they are still dominated by dark matter even within this half optical light radius.

Today’s paper presents 19 galaxies that seem to be mostly baryons well beyond the half optical light radius, rather than being dominated by dark matter as expected. 14 of these galaxies are isolated galaxies, meaning they have no nearby larger galaxies which may be affecting them. ...

A population of dwarf galaxies deficient in dark matter ~ Qi Guo et al

• arXiv.org > astro-ph > arXiv:1908.00046 > 31 Jul 2019

[bystander]

Aliens Among Us
astrobites | Daily Paper Summaries | 2019 Sep 23

What if I told you that we have the opportunity to directly study other solar systems? You’d be like, “guuurrrlll, say whaaaat??” And then I’d say:

Similar to how we can find chunks of Mars or pieces of the astroid belt on Earth, we have rocks from other solar systems flying around interstellar space, and a few just so happen to come into our solar system. This was only recently proven with the discovery of Interstellar Object (ISO) ‘Oumuamua. ‘Oumuamua was ejected from a different solar system and zoomed right into ours. Slipping between the Sun and Earth, it was detected as it started its journey back outside the solar system. ‘Oumuamua was the first object of its kind to be discovered, and it brings up the question, how many bits of other solar systems may be floating around and near by us? The answer to that question can have wide implications in our understanding of solar system formation, planet formation, and even compositions of other solar systems.

Today’s paper utilizes the ‘Oumuamua detection in addition to a recent high-resolution protoplanetary disk survey, DSHARP, to predict the number of future ISO detections. To put that number into context, the authors predict how many ISOs the new LSST survey might be able to see. ...

Hidden Planets: Implications from 'Oumuamua and DSHARP ~ Malena Rice, Gregory Laughlin

• arXiv.org > astro-ph > arXiv:1909.06387 > 13 Sep 2019

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[bystander]

Cosmic Archaeology from an Ancient Pulsating Star
astrobites | Daily Paper Summaries | 2019 Sep 24

The gravitational constant, G, is one of the core fundamental constants of physics, appearing in Newton’s laws of gravitational motion, and therefore in the fundamental theory of gravity. While people historically questioned whether it truly is a constant, Einstein’s theory of general relativity states that G must be constant no matter where in space, or time, you find yourself. However modern work in string theory, which aims to reconcile the theory of gravity with the other fundamental forces of nature, says that the gravitational ‘constant’ can, in fact, vary over extremely long cosmic timescales.

If the gravitational constant was changing in time, we might be able to detect it in systems whose evolution has strongly relied on gravity, such as stars. If gravity was weaker in the past, that would have affected the evolution of a star, changing how it appears today. Measurements of the rate of change of G have been performed in this way using helioseismology, white dwarfs, and globular clusters, as well as studies of the cosmic microwave background. All these experiments draw the same conclusions; that G changes at a completely negligible rate (specifically, by no more than a fraction of a trillionth a year, where the universe is only 13 billion years old).

Today’s authors present a new test to more closely approximate the variation of G on truly cosmic timescales. The target of the authors’ study is KIC 7970740, a low-mass solar-like star on the main sequence which is, most importantly, roughly 11 billion years old! Thanks to high-quality measurements by the Kepler space telescope, this star also has a well measured set of clear stellar pulsations that allow for an asteroseismic analysis, and is one of the oldest stars for which this is possible. ...

Asteroseismic constraints on the cosmic-time variation of the gravitational constant
from an ancient main-sequence star ~ Earl Patrick Bellinger, Jørgen Christensen-Dalsgaard

• arXiv.org > astro-ph > arXiv:1909.06378 > 13 Sep 2019

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Is the Milky Way Gaining or Losing Mass?
astrobites | Daily Paper Summaries | 2019 Sep 25

Galaxies are so large that it can be hard to imagine them changing over time. However, we believe that galaxies are living and breathing entities, accreting and ejecting mass all throughout their lives. The Milky Way is no exception. Characterizing the rates of mass flow and the mass loading factor for galaxies, though, is crucial to understanding the details of this ‘galactic fountain’ model. In today’s paper, the authors provide new estimates of these rates for the Milky Way. They also present the first estimate of the mass loading factor (the ratio of material flowing out of the galaxy to the star formation rate) for the outflowing material from the entire Milky Way disk. Essentially, this measures how efficiently the Milky Way recycles the gas it takes from its surroundings. These are very cool results, so let’s break down exactly what they mean. ...

The Mass Inflow and Outflow Rates of the Milky Way ~ Andrew J. Fox et al

• arXiv.org > astro-ph > arXiv:1909.05561 > 12 Sep 2019

[bystander]

Inconstant, Fine Structures
astrobites | Daily Paper Summaries | 2019 Sep 26

Galaxies tend to interact with one another in a dramatic and destructive fashion. They exert enormous gravitational force over large scales and when they are drawn together, their shapes can be distorted to the point of being unrecognizable. Once galaxies are close enough that they can no longer stay separate, they end up merging together over millions or even billions of years.

Fine structural traces of mergers are clues to these encounters: tails of gas, streams of stars, and peculiar shell-like structures that appear out of place, all classified under an umbrella term of “stellar substructure” (see Figure 1 for an example). Is it possible to learn about a galaxy’s history of mergers by searching for these substructures in images?

The bad news is that our short lifespans prevent us from observing mergers in “real-time”: the timescales involved are simply too great. The good news is that the authors of today’s paper have circumnavigated this obstacle by using simulations to do the “observing” instead! By examining the different kinds of substructures in simulations, they have determined how such features are created and how long they survive. ...

Probing the merger history of red early-type galaxies
with their faint stellar substructures ~ B. Mancillas et al

• arXiv.org > astro-ph > arXiv:1909.07500 > 16 Sep 2019

[BDanielMayfield]

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.

Those are just the three intrinsic properties of black holes. But we can want to know (and even can discover) many more things about a BH. Things like location, orbital trajectory, mass accretion rate, effects on surroundings, etc. Also, I WANT to know what's behind the event horizon, even though that might never be known.

Bruce

[bystander]

The Pulsar Search Collaboratory: Making Pulsar
Science Accessible to High School Students
astrobites | Daily Paper Summaries | 2019 Oct 01

Some people think that in order to do “real” science, you need to be in a fancy lab with sophisticated equipment at a university, but that’s not true. Citizen science projects such as Galaxy Zoo and SETI@Home are working to make science more accessible to everyone, and new opportunities are popping up every day. Astronomers at West Virginia University have created a program that allows students as young as 13 to do real science with real telescope data right from their own home!

The Pulsar Search Collaboratory, or PSC, is a program that allows high school students and teachers to analyze pulsar data from the Green Bank Telescope (GBT). Pulsars are very compact corpses of dead stars; they’re basically like taking something with the mass of our Sun (which is 99.99% of the total mass of our solar system), crushing it into the size of Manhattan, and spinning it as fast as a blender. These stars give off radio waves like a lighthouse which cross our sightline as the pulsar rotates (see Figure 1). ...

The Pulsar Search Collaboratory: Current Status and Future Prospects ~ Harsha Blumer et al

• arXiv.org > physics > arXiv:1909.05104 > 11 Sep 2019

[bystander]

Twinkle, Twinkle, Little Resonant Planetary System
astrobites | Daily Paper Summaries | 2019 Oct 03

In the song Twinkle, Twinkle, Little Star, the interval between the first two notes and the second two notes is a perfect fifth. What in the world does this have to do with astronomy? It turns out that this is the same ratio as the orbital frequencies of planets in the K2-138 solar system.

The K2-138 system was first discovered by citizen scientists through the Exoplanet Explorers program in 2018. From the lightcurve of the star, these scientists noted dips in brightness indicating four transiting planets. Upon a more detailed inspection of the lightcurve, two more planets were detected.

Adding to the excitement of identifying a system with so many planets, the five inner planets were found to be in a resonant chain. This means that the orbital periods of the planets are successive ratios of each other, in this case following a 3:2 pattern (if the innermost planet has a 2-day period the next would have a 3-day period). The periods of the first and second K2-138 planets are 2.35 and 3.56 days, which is very close to 3:2; the second and third planets have a similar ratio, and so on. This makes K2-138 the longest known 3:2 resonance chain—and also makes it pitch-perfect in its rendition of Twinkle, Twinkle (see Figure 1).

Today’s paper is a follow-up analysis of the K2-138 system using spectral data from the HARPS spectrograph. This radial velocity (RV) data allows the authors to characterize the system in detail, including the masses and densities of the planets. ...

Exoplanet characterisation in the longest known resonant chain:
the K2-138 system seen by HARPS ~ T. A. Lopez et al

• Astronomy & Astrophysics (accepted 20 Sep 2019) DOI: 10.1051/0004-6361/201936267
• arXiv.org > astro-ph > arXiv:1909.13527 > 30 Sep 2019

[bystander]

Building Planets Around a Black Hole
astrobites | Daily Paper Summaries | 2019 Oct 07

Nearly 400 years ago, it was hypothesized that the planets in our solar system formed from the leftover material that formed the Sun. This hypothesis is now widely accepted as the standard model for solar system formation. We have even seen this process in action within other stellar systems thanks to radio telescopes like ALMA.

We continuously focus on planets which form around stars. But what if planets could form around other astronomical bodies? After all, stars aren’t the only objects in the universe which become surrounded by a tumultuous disk of gas and dust during their lives.

Active Galactic Nuclei (AGN) exist at the center of galaxies. The standard model for an AGN states that it is consists of a supermassive black hole and a hot accretion disk, both of which are surrounded by a donut-shaped (or torus-shaped) region of gas and dust. This configuration is shown in Figure 1. Today’s paper takes a look at how a planet could possibly form within the dusty torus around an AGN. ...

Planet Formation around Super Massive Black Holes in the Active
Galactic Nuclei ~ Keiichi Wada, Yusuke Tsukamoto, Eiichiro Kokubo

• arXiv.org > astro-ph > arXiv:1909.06748 > 15 Sep 2019 (v1), 24 Sep 2019 (v2)

[bystander]

Making a Splash
astrobites | Daily Paper Summaries | 2019 Oct 08

The Milky Way (MW) has had a relatively quiet and peaceful life. Every now and again a dwarf galaxy comes by and causes a small perturbation, but generally speaking, our galaxy has been left to do its own thing. This has allowed for the formation of a thin stellar disk in which the vast majority of the MW’s stars reside, including the Sun (see Figure 1).

Other galaxies have not been so lucky. Many of them show signs of having gone through a major merger in their recent past — in other words, the galaxy we see today is the result of a catastrophic collision between two galaxies of similar sizes. Major mergers tend to produce puffed up spheroidal or elliptical galaxies, and have far-reaching effects on the structure and the star formation rates within the galaxies. Such a catastrophic collision is actually expected to happen between the MW and our nearest neighbor, Andromeda, but only in approximately four billion years (here’s a great simulation of what that might look like!).

But has the MW always been so isolated? Recent observations suggest otherwise. It appears that the MW did in fact undergo a major merger, approximately ten billion years ago. Today’s paper focuses on the after-effects of this merger, and in particular on a group of stars — referred to as the Splash — whose orbits were dramatically changed as a result of the collision. ...

The Biggest Splash ~ Vasily Belokurov et al

• arXiv.org > astro-ph > arXiv:1909.04679 > 10 Sep 2019

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A Second Mysterious Radio Outburst from Magnetar XTE J1810-197
astrobites | Daily Paper Summaries | 2019 Oct 10

Pulsars. Fast Radio Bursts. Magnetars. The world of high-energy stellar astrophysics has no shortage of weird objects that do not always behave like we think they should. From the mysterious workings inside a neutron star to the unknown reason behind why some fast radio bursts repeat, these sources continue to surprise and mystify us. Now, the world of magnetars, stars with incredibly high magnetic fields, just got a little more interesting.

Magnetars (short for “magnetic stars”) are neutron stars with some of the strongest magnetic fields in the universe. Their magnetic field strengths are on the order of ~10¹⁵ Gauss; to put this in perspective, the magnetic field of the Earth that shields us from the Sun’s rays and produces auroras is about 0.5 Gauss. If a magnetar was at a distance from Earth equal to that of the moon, it could strip the information off of all of the credit cards on the planet. Magnetars also very young stars and emit variable X-ray radiation and transient radio emission and that was all, until XTE J1810-197 came along.

In 2006, magnetar XTE J1810-197 (which is also classified as an X-ray pulsar because in addition to having a very strong magnetic field, it intermittently emits X-rays) was found to be emitting radio pulses after a very strong outburst of energy in the radio frequency regime. At the beginning of this outburst, the pulsar had a nearly flat spectral index. The spectral index tells you how much the total power from the source is dependent on frequency, so if the spectral index was flat, it means that the power emitted was about the same at all frequencies. During that burst, radio emission came in spikes that lasted about 10 milliseconds. After the outburst, the source faded in power and essentially went off before it was re-observed 13 years later and a second radio burst was detected by the authors of today’s work. In Figure 1, you can see how the power emitted by the source declined over time but increased over frequency. Similar “spiky” short-duration radio pulses have been seen in high-energy phenomena such as giant pulses (essentially really bright radio pulses that occasionally come from some sources) and fast radio bursts (FRBs). With these similarities, the bursts from this magnetar could suggest a common origin for these phenomena. Let’s look a bit further into what the authors found from this mysterious source! ...

Distinct Properties of the Radio Burst Emission from the Magnetar XTE J1810–197 ~ Yogesh Maan et al

• Astrophysical Journal Letters 882(1):L9 (2019 Sep 01) DOI: 10.3847/2041-8213/ab3a47
• arXiv.org > astro-ph > arXiv:1908.04304 > 12 Aug 2019 (v1), 09 Sep 2019 (v2)

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Can an Orphaned Exomoon Help Solve the Mystery of Tabby’s Star?
astrobites | Daily Paper Summaries | 2019 Oct 11

Tabby’s Star – more correctly known as KIC 8462852 or Boyajian’s Star – first shot to fame back in 2015, causing quite a stir among astronomers and the public alike thanks to its strange fluctuations in brightness. The problem was that we had never seen a star behave quite like this before. Various explanations were suggested, but the one which most caught the public’s attention was the possibility that giant artificial structures, such as Dyson swarms, built by an advanced civilisation to harness energy from the star, could be the cause.

Nowadays (and perhaps disappointingly), most scientists are fairly sure there is a natural explanation for the weird activity. But if little green men aren’t responsible, then what exactly is causing this unusual behaviour? Several hypotheses have since been considered ranging from disintegrating comets and planetesimals to magnetic activity within the star itself, but so far none have managed to fully explain all aspects of the observations.

Now, a team of researchers from Columbia University think they might have a more consistent explanation: An exomoon, ripped away from its parent planet, slowly disintegrating in orbit around the star (Figure 1). ...

Orphaned Exomoons: Tidal Detachment and Evaporation Following an
Exoplanet-Star Collision ~ Miguel Martinez, Nicholas C. Stone, Brian D. Metzger

• Monthly Notices of the RAS 489(4):5119 (Nov 2019) DOI: 10.1093/mnras/stz2464
• arXiv.org > astro-ph > arXiv:1906.08788 > 20 Jun 2019

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[bystander]

Are pulsars having a (gravitational) wobble?
astrobites | Daily Paper Summaries | 2019 Oct 15

Gravitational waves (GW) are ripples in space-time caused by some of the most energetic processes in the universe. Typically, these waves occur when large, dense bodies like black holes or neutron stars begin accelerating towards each other. The decreasing orbit (inspiral) followed by the collision (merger) releases energy in the form of these waves, which propagate outwards from the site of the event at exactly the speed of light (a prediction of general relativity and later confirmed by the LIGO detector). ...

Although we can readily detect gravitational wave events, there are outstanding issues with accurately pinpointing their origin on the sky. ... Today’s authors are now pointing at another observable tracer for gravitational wave events of the past and future: pulsars. ...

Pulsars are a class of neutron stars, which emit beams of high-energy radiation, visible only when the beam is pointed in the direction of the observer. Neutron stars have the additional property of having short, regular rotational periods, thus they appear to pulse. These periods can be milliseconds to seconds, based on the size of the pulsar, and can be measured extremely precisely.

In today’s paper, the authors suggest that gravitational waves passing through pulsars can cause them to wobble, affecting the pulsar’s moment of inertia, which in turn affects its spin rate. In other words, the speed at which a pulsar is observed to be spinning can be changed by an amount that is detectable by telescopes, and can therefore be used as a sign for a GW event. Interestingly, pulsars might themselves be able to produce a certain kind of gravitational wave, which you can find out about in this Astrobite. The bite you are currently reading, however, focuses only on the use of pulsars as GW tracers. ...

Re-visiting Gravitational Wave Events via Pulsars ~ Minati Biswal, Shreyansh S. Dave, Ajit M. Srivastava

• arXiv.org > astro-ph > arXiv:1909.04476 > 10 Sep 2019

[bystander]

A New Look at SN 1987A
astrobites | Daily Paper Summaries | 2019 Oct 18

Three decades ago, astronomers witnessed the closest supernova since Kepler’s Supernova in 1604. Dubbed SN 1987A, this bright stellar explosion remained visible to the naked eye in the Southern Hemisphere for several months. Due to its relative proximity (at least in astronomical terms) in the nearby Large Magellanic Cloud, SN 1987A presented the first chance for modern astronomy to investigate the supernova process in exquisite detail. As a result, it has been continuously monitored by astronomers using telescopes across the electromagnetic spectrum. Today’s astrobite focuses on new radio observations of SN 1987A taken with the Atacama Large Millimeter/submillimeter Array (ALMA). ...

High Angular Resolution ALMA images of Dust and Molecules in the SN 1987A Ejecta ~ Phil Cigan et al

• arXiv.org > astro-ph > arXiv:1910.02960 > 07 Oct 2019

[bystander]

Plenty of Gas Left in Giant Dead Disk Galaxies
astrobites | Daily Paper Summaries | 2019 Oct 21

A concept fundamental in astronomy is that stars form from cold, dense molecular gas clouds. Applied to the formation and evolution of galaxies, star-formation is understood as being directly supported by an abundance of this cold molecular gas. We find that in star-forming galaxies — typically disk-like with blue spiral arms – there is a large reservoir of this cold molecular gas from which rapid star-formation can be sustained. Quiescent galaxies on the other hand – typically elliptical-like with red colors – do not actively form stars. Hence, they have been long thought to have only a scarce supply of cold molecular gas, likely as a result of either vigorous star-formation in the past, some sort of gas reservoir removal event, or a combination of the two.

There is overwhelming evidence that star-forming and quiescent galaxies inhabit very different environments in our universe. While star-forming galaxies are generally found alone or in small groups, quiescent galaxies are often found residing in large galaxy clusters with tens to hundreds of cluster members. This so-called morphology-density relation tells us something fundamental about the role of environment in shaping galaxy properties. In particular, it suggests that the cluster environment influences the ceasing (or quenching) star-formation.

The largest and most massive galaxies typically reside near the center of a cluster. We label them “central” galaxies to contrast them with smaller “satellite” galaxies with reside in the cluster outskirts. Most massive central galaxies are quiescent. ...

Today’s paper discusses new and surprising findings about the gas content of these massive central galaxies, and why they are kept from forming stars. ...

Nearly All Massive Quiescent Disk Galaxies Have a Surprisingly Large Atomic Gas Reservoir ~ Chengpeng Zhang et al

• arXiv.org > astro-ph > arXiv:1910.02093 > 04 Oct 2019

[bystander]

How to Glo up: Black Hole-Neutron Star Mergers
astrobites | Daily Paper Summaries | 2019 Oct 22

Recent gravitational wave (GW) detections have generated a lot of excitement around the merger of neutron stars (NSs) with black holes (BHs). However, while the first detection of merging neutron stars was a treasure trove of observational data both gravitational and electromagnetic, all BH-NS merger candidates so far remain shrouded in darkness. No ‘smoking gun’ electromagnetic (EM) counterpart to a NS-BH merger event has been observed yet. Joint GW-EM detections are important because different messengers carry different pieces of information, helping us paint a coherent picture of the source.

Perhaps this isn’t shocking: a BH-NS merger may not produce any EM counterpart at all, let alone one that’s bright enough to observe! The BH can simply swallow the NS whole, without the telltale kilonova glow.

So when can we expect to find a counterpart? And would these kilonovae look very different from the ones produced in binary NS mergers? Questions abound regarding the nature of the EM emission from BH-NS mergers. Today’s paper investigates the EM counterparts of these events for a range of binary properties, revealing interesting (and sometimes subtle!) patterns. ...

Electromagnetic Counterparts of Black Hole-Neutron Star Mergers:
Dependence on the Neutron Star Properties ~ C. Barbieri et al

• arXiv.org > astro-ph > arXiv:1908.08822 > 23 Aug 2019

[bystander]

Not Far in the Dark
astrobites | Daily Paper Summaries | 2019 Oct 23

At the end of March last year, the Dragonfly team announced the discovery that galaxy NGC 1052-DF2 had almost no dark matter (see this Astrobite for more on the discovery). This announcement set off a flurry of responses, since the existence of an object like NGC 1052-DF2 has enormous implications for models of galaxy formation and behavior (see this Astrobite for a great summary of some of the initial responses).

To determine how much dark matter a galaxy has, you have to compare its stellar mass (which comes from estimating how many stars it has) and its dynamical mass (which comes from measuring how the contents of a galaxy are moving). Measurements of stellar mass and dynamical mass are extremely dependent on distance, which was the basis of some criticisms of the NGC 1052-DF2 discovery paper.

Tip of the Red Giant Branch (TRGB) stars (see Figure 1) are stars that have just run out of hydrogen and started to burn helium. Being at this turning point in their lives gives TRGB stars a characteristic brightness and color, meaning that they can be used to measure distances (like here). In this paper, members of the Dragonfly team use TRGB stars to measure the distance to NGC 1052-DF4, another seemingly dark matter-deficient galaxy. ...

A Tip of the Red Giant Branch Distance to the Dark Matter Deficient Galaxy
NGC 1052-DF4 from Deep Hubble Space Telescope Data ~ Shany Danieli et al

• arXiv.org > astro-ph > arXiv:1910.07529 > 16 Oct 2019

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[bystander]

Not all Black Holes that Wander are Lost
astrobites | Daily Paper Summaries | 2019 Oct 24

At the centre of the most massive galaxies resides a supermassive black hole around which everything rotates. Typically, these black holes are identified by measuring the velocity and shape of the orbits of a galaxy’s innermost stars as they rotate around its centre. However, this approach is less effective when we consider much lighter galaxies like dwarf galaxies. These galaxies are much fainter than their high mass counterparts so their stellar populations can’t be sufficiently resolved by current missions. Instead, a lot of recent work in the field focuses on identifying the energetic process a massive black hole undergoes when it accretes gas and dust; the central region of the galaxy becomes an active galactic nucleus (AGN). By identifying the incidence of AGN, a lower limit on the distribution of black holes in dwarf galaxies can be obtained. Today’s authors adopt this approach to further study their black hole population and produce some surprising results. ...

A New Sample of (Wandering) Massive Black Holes in Dwarf Galaxies
from High Resolution Radio Observations ~ Amy Reines et al

• arXiv.org > astro-ph > arXiv:1909.04670 > 10 Sep 2019

[bystander]

A Discovery of a Triple AGN System
astrobites | Daily Paper Summaries | 2019 Oct 25

An AGN, or Active Galactic Nucleus, is the epitome of a fascinating and truly awesome astrophysical object. An AGN consists of a black hole that happens to be surrounded by gas and dust that is close enough to form an accretion disk and fall into the black hole. As a result, the black hole (now AGN) forms these enormous, extremely bright, and hottt jets. AGNs are quite useful for those interested in learning more about black holes (who isn’t), but black holes on their own don’t emit any sort of radiation. It’s only when they come in contact with or are in front of other things that we can directly detect, can we then indirectly learn about this incredible astrophysical phenomena. Astronomers are particularly interested in super massive black holes, which seem to be in the center of the vast majority of galaxies. Super massive black holes also seem to co-evolve with their host galaxy, at least according to the most popular theory of cosmology: ΛCDM. It’s quite interesting actually, to think that a single super massive black hole in the center of the galaxy, while being very massive for a single object, could potentially govern the evolution of its entire host galaxy which is much much larger and more massive. For example, the super massive black hole in the center of the Milky Way is about 3 million solar masses while being about a light minute in diameter but compare that to the Milky Way, and it is 150 BILLION solar masses and over 50,000 light years wide. How could that relatively small black hole have such an impact on an entire galaxy?

Since AGNs are super bright, astronomers can detect them from very far away. This allows us to explore how supermassive blackholes affect galaxy evolution across cosmic time. AGN only exist where there is an excess amount of dust and gas in a galaxy. Nowadays, when we look at nearby galaxies, they are lacking in dust and gas compared to galaxies at redshift of 2. Galaxies that are at redshift of 2 mean that we are looking back to a time where the galaxy was only about a quarter of its current age. Back then, galaxies contained a TON of gas and dust that was used for star formation, and was fuel for AGN. This is a key redshift that astronomers want to probe to understand how galaxies evolve to what they are today.

One of the main drivers of galaxy evolution are galaxy mergers. The math is simple: galaxy + galaxy = bigger galaxy. When two galaxies collide, the stars inside of them don’t run into each other, but the gas and dust in each galaxy do collide and interact, producing starburst events and, you guessed it, powering AGN! So if we wanted to hunt down some AGN, mergers are a fantastic place to start. Cosmological simulations even predict that ~16% of all mergers actually contain three galaxies, or a triple merger. This offers up the possibility of a triple merging system where each of the galaxies involved in the collision have an active galactic nucleus. If we believe our modern theory of the universe (ΛCDM), triple AGN systems must exist, but since there is going to be a lot of dust in the system, they could be obscured behind thick sheets of dust. That plus their rarity makes these systems difficult to detect. Today’s paper offers up a proposed detection of such an object. ...

A Triple AGN in a Mid-Infrared Selected Late Stage Galaxy Merger ~ Ryan W. Pfeifle et al

• arXiv.org > astro-ph > arXiv:1908.01732 > 05 Aug 2019 (v1), 07 Aug 2019 (v3)

[bystander]

Ancient Aurorae: Assyrian and Babylonian Astrologers
Recorded the Oldest-Known Solar Storms
astrobites | Daily Paper Summaries | 2019 Oct 28

Humans have always been star-gazers. Since time immemorial we have looked to the heavens and tried to make sense of what we saw there. Thousands of years ago the motions of heavenly bodies and the appearances of comets and meteors were believed to be omens that determined the fates of kings and predicted the downfall of empires. Official court astrologers were employed to read these portents in the sky and divine their (hopefully favourable) meanings.

Many of these ancient observations of celestial events still survive as written records in cuneiform tablets inscribed over two millennia ago. Cuneiform was one of the earliest systems of writing and was typically inscribed on rectangular clay tablets using a blunt reed to produce wedge-shaped marks. Cuneiform tablets have been discovered at archaeological sites all across the Near and Middle East, such as at the ancient Assyrian city of Ninevah (modern-day northern Iraq) – once the largest city in the world – and were used for almost everything, including recording celestial events, documenting laws and religious beliefs and even for entire literary works – such as the famous Epic of Gilgamesh. ...

A team led by Hisashi Hayakawa, a researcher at Osaka University and the lead author of today’s paper, carried out a survey of tablets dating from the 8th and 7th centuries B.C. looking for references to aurorae, that might match evidence inferred from tree ring samples. ...

The Earliest Candidates of Auroral Observations in Assyrian Astrological Reports:
Insights on Solar Activity around 660 BCE ~ Hisashi Hayakawa et al

• Astrophysical Journal Letters 884(1):L18 (2019 Oct 10) DOI: 10.3847/2041-8213/ab42e4
• arXiv.org > astro-ph > arXiv:1909.05498 > 12 Sep 2019

[bystander]

How to Un-squash a Galaxy
astrobites | Daily Paper Summaries | 2019 Oct 29

Around 90% of the Milky Way’s mass is in the form of invisible dark matter. This makes it extremely difficult to determine the total mass or the shape of our galaxy.

But that certainly doesn’t stop astrophysicists from trying! A variety of creative methods have been devised to overcome the inability to see dark matter: the positions and the velocities of satellite galaxies can be used to estimate the mass of the Milky Way; the width of stellar streams can help determine the galaxy’s shape; even the very nature of dark matter can be deduced by studying the velocities of metal-poor stars.

All of these methods rely on the answers to a few fundamental questions: what do existing theories of dark matter predict about a galaxy’s gravitational potential? Can those theories produce galaxies that look like the Milky Way? Do the resulting potentials agree with the kinematics of observable tracers like satellite galaxies, stellar streams, and metal-poor stars? Simulations are the perfect tool for answering these questions.

Today’s authors are specifically interested in what simulations can tell us about the shape of our galaxy. ...

Dark Matter Halo Shapes in the Auriga Simulations ~ Jesus Prada et al

• Monthly Notices of the RAS (online 12 Oct 2019) DOI: 10.1093/mnras/stz2873
• arXiv.org > astro-ph > arXiv:1910.04045 > 09 Oct 2019

[bystander]

Searching for Supernova Survivors
astrobites | Daily Paper Summaries | 2019 Oct 30

Surviving a supernova (SN) may sound crazy, since supernovae (SNe) are among the most energetic events in space. Type Ia SNe result from the explosion of a white dwarf, and just one of these events can temporarily outshine an entire galaxy. So how could anything survive such an explosion?

Well, there are two types of Type Ia SNe, both caused by white dwarfs hitting the Chandrasekhar mass limit – single degenerate (SD) and double degenerate (DD). DD Type Ia’s are caused by the merger of two white dwarfs, which upon merging, will pretty much annihilate one another and cause a SN. However, a SD Type Ia SN only involves one white dwarf. In this case, there is no merger – instead, the white dwarf has a non-degenerate (a.k.a., not a white dwarf) companion which it has drawn too much mass from, causing the white dwarf to explode. Since only one star (called the ‘progenitor’) is doing the exploding in this SD scenario, perhaps that companion will live long enough to tell its story…

The authors of today’s paper set out to look for potential companions dancing around SN remnants, the shell of material left over by a SN explosion. These companions stars, which could be main sequence (MS) stars, red giant stars, or helium stars, may have lost their outer layers in the deadly explosion but could live on as a dense core. These surviving cores should be identifiable – they probably move differently as a result of the explosion, and likely look different in color. ...

Search for Surviving Companions of Progenitors of Young LMC Type Ia Supernova Remnants ~ Chuan-Jui Li et al

• arXiv.org > astro-ph > arXiv:1910.01093 > 02 Oct 2019 (v1), 23 Oct 2019 (v2)

[bystander]

Hidden monster or spooky cosmological coincidence?
astrobites | Daily Paper Summaries | 2019 Oct 31

To the elation of gravitational wave hunters everywhere, the LIGO/Virgo collaborations went public in April of this year. This means that live alerts are sent out when their detectors find something significant. As a result, other teams with telescopes that are sensitive to different types of radiation can point their antennae in the direction of the gravitational radiation. If something luminous was involved in the gravitational wave production, then they’ll hope to see any radiation given off at other frequencies, like x-rays, or radio waves.

This is how the first confirmed neutron-star merger was detected. LIGO/Virgo picked up the gravitational wave signal, and then a number of facilities, including the NASA Fermi telescope and the Dark Energy Survey, confirmed the detection in all ranges of the electromagnetic spectrum. This led to tonnes of exciting science which you can read about here, here and here.

Today’s paper focuses on an unusual alert from LIGO/Virgo. It was a detection of black holes merging, so there was no electromagnetic counterpart to be found (although this might not always be true…) However, the thing that caught today’s author’s eyes was that there was not one, but two merger events within 20 minutes of each other, and seemingly from similar places in the sky. ...

The Two LIGO/Virgo Binary Black Hole Mergers on 2019 August 28 Were
Not Strongly Lensed ~ Leo P. Singer, Daniel A. Goldstein, Joshua S. Bloom

• arXiv.org > astro-ph > arXiv:1910.03601 > 08 Oct 2019

[bystander]

The Harvard Computers
astrobites | Daily Paper Summaries | 2019 Nov 04

Imagine you are a successful and intellectual astronomer. You spend your days mapping the universe and making some of the greatest breakthroughs in stellar classification. Despite this, you receive little to no respect from society and your countless scientific contributions go unrecognized. Does this seem relatable to you? You must be one of the Harvard Computers! ...

[bystander]

Elusive black holes: Have we found the ‘middle sibling’?
astrobites | Daily Paper Summaries | 2019 Nov 06

In the past few years, black holes have gone from being far-fetched concepts of science fiction to being observed in many different ways by many different surveys, such as the supermassive black hole (SMBH) at the centre of the Milky Way as well as the recent pioneering observations of the SMBH at the centre of M87. These are examples of SMBHs, expected to reside in the centre of every massive galaxy, and so these are the older, wiser and more experience siblings of the black hole family.

In contrast to SMBHs which have been observed to reach masses of 40 billion solar masses, stellar mass black holes in the range of 7 to 50 solar masses have been observed by LIGO through detections of gravitational waves from the merging of black hole pairs. Whilst not young, their relatively small masses make them the toddlers of the black hole world. Stellar mass black holes are the final stage of the evolution of individual massive stars. Black holes even smaller than this are also theoretically predicted and are even suggested to be numerous enough to be a dark matter candidate. But putting all of this together poses a surprising question.

We know that when massive stars die, they form stellar mass black holes (up to around 100 solar masses). To build up their mass, black holes merge and also accrete mass from their surrounding stars and gas. Therefore it’s expected that we should see less massive black holes in the early universe, compared to today.

But we have a mass gap. This mass gap between stellar mass and supermassive black holes are known as intermediate-mass black holes (IMBHs), having masses from around 100 to 100,000 solar masses, but the weird thing is we don’t really observe them. These ‘middle siblings*’ are the grumpy teenagers who are either hiding somewhere or they just don’t exist.

*analogy butchered and tortured from previous astrobites here and here!

There is a well observed black hole mass-stellar mass scaling relation showing that most galaxies have a SMBH of the order of 0.1% of their stellar mass. Therefore, when considering the local Universe, the place to look for IMBHs is not in the centres of massive galaxies, but instead they could be hiding in lower mass objects such dwarf galaxies or stellar/globular clusters, which brings us on to today’s paper where the authors present strong evidence for the presence of an IMBH residing in a star cluster located in the outer region of a large lenticular galaxy. ...

A Luminous X-ray Outburst from an Intermediate-Mass Black Hole in an Off-Centre Star Cluster ~ Dacheng Lin et al

• Nature Astronomy 2(8):656 (Aug 2018) DOI: 10.1038/s41550-018-0493-1
• arXiv.org > astro-ph > arXiv:1806.05692 > 14 Jun 2018

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