astrobites: Daily Paper Summaries 2019

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Dwarf Galaxies without Dark Matter

Post by bystander » Fri Sep 20, 2019 10:52 pm

Dwarf Galaxies without Dark Matter
astrobites | Daily Paper Summaries | 2019 Sep 20
Jessica May Hislop wrote:
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
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Aliens Among Us

Post by bystander » Tue Oct 01, 2019 3:45 pm

Aliens Among Us
astrobites | Daily Paper Summaries | 2019 Sep 23
Jenny Calahan wrote:
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
viewtopic.php?t=39796#p295572
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Cosmic Archaeology from an Ancient Pulsating Star

Post by bystander » Tue Oct 01, 2019 3:59 pm

Cosmic Archaeology from an Ancient Pulsating Star
astrobites | Daily Paper Summaries | 2019 Sep 24
Oliver Hall wrote:
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
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Is the Milky Way Gaining or Losing Mass?

Post by bystander » Tue Oct 01, 2019 4:06 pm

Is the Milky Way Gaining or Losing Mass?
astrobites | Daily Paper Summaries | 2019 Sep 25
Michael Foley wrote:
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
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Inconstant, Fine Structures

Post by bystander » Tue Oct 01, 2019 4:13 pm

Inconstant, Fine Structures
astrobites | Daily Paper Summaries | 2019 Sep 26
Caitlin Doughty wrote:
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
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Re: (s)Pinning down the origins of black hole mergers

Post by BDanielMayfield » Tue Oct 01, 2019 7:00 pm

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.
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
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The Pulsar Search Collaboratory

Post by bystander » Wed Oct 09, 2019 5:27 pm

The Pulsar Search Collaboratory: Making Pulsar
Science Accessible to High School Students

astrobites | Daily Paper Summaries | 2019 Oct 01
Haley Wahl wrote:
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
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Twinkle, Twinkle, Little Resonant Planetary System

Post by bystander » Wed Oct 09, 2019 5:54 pm

Twinkle, Twinkle, Little Resonant Planetary System
astrobites | Daily Paper Summaries | 2019 Oct 03
Kate Storey-Fisher wrote:
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
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Building Planets Around a Black Hole

Post by bystander » Wed Oct 09, 2019 6:03 pm

Building Planets Around a Black Hole
astrobites | Daily Paper Summaries | 2019 Oct 07
Ellis Avallone wrote:
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
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Making a Splash

Post by bystander » Wed Oct 09, 2019 6:13 pm

Making a Splash
astrobites | Daily Paper Summaries | 2019 Oct 08
Tomer Yavetz wrote:
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 viewtopic.php?t=38852
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Second Radio Outburst from Magnetar XTE J1810-197

Post by bystander » Sun Oct 13, 2019 3:30 pm

A Second Mysterious Radio Outburst from Magnetar XTE J1810-197
astrobites | Daily Paper Summaries | 2019 Oct 10
Haley Wahl wrote:
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
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An Orphaned Exomoon and the Mystery of Tabby’s Star?

Post by bystander » Sun Oct 13, 2019 3:47 pm

Can an Orphaned Exomoon Help Solve the Mystery of Tabby’s Star?
astrobites | Daily Paper Summaries | 2019 Oct 11
Jamie Wilson wrote:
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
viewtopic.php?t=35401
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