Starship Asterisk*

Jason Hinkle wrote:
One widely-held theory of how galaxies grow and evolve throughout cosmic time is a process known as hierarchical growth. In this scenario, smaller galaxies merge with each other to form larger galaxies, which explains why the most massive galaxies we see live in the local universe. There are two major types of galaxies in the Universe: spirals and ellipticals, often called disk and spheroidal galaxies. For more details on galaxy evolution check out this astrobite.

Because galaxy mergers tidally distort pairs of galaxies, stars initially on ordered orbits within the galaxies tend to be disturbed and placed on randomized orbits. This means that the most massive galaxies, which have formed through multiple major mergers, tend to be spheroidal in nature. Despite this, roughly 10 percent of galaxies more massive than ~4 times the mass of the Milky Way have significant disks. Because the number of major mergers a galaxy experiences is highly dependent on its stellar mass, it is likely that all massive galaxies have gone through a similar number of large mergers throughout cosmic time. If this is true, then why do we still see massive galaxies with disks in the local universe?

In order to answer this question, the authors of this paper simulate the universe using the Horizon-AGN simulations. These simulations include both stellar and AGN feedback, which impart energy, momentum, and enriched material into the galaxy. With their simulation, the authors can trace the evolution of stars, black holes, and galaxy morphology in great detail over time. The merger histories of galaxies are traced from a redshift of z = 3 to z = 0.06 (corresponding to almost 11 billion years) in time steps of ~130 million years. ...

• Monthly Notices of the RAS (online 10 Apr 2020) DOI: 10.1093/mnras/staa970
• arXiv.org > astro-ph > arXiv:2004.00023 > 31 Mar 2020

Wynn Jacobson-Galan wrote:
The Universe reveals an amalgam of ways in which stars can die. We observe stars imploding, erupting and merging, yet the tidal disruption event (TDE) is one of the most tumultuous spectacles of stellar destruction we have discovered so far. This transient phenomenon begins with a star orbiting near a supermassive black hole (SMBH) in the galaxy center. Oblivious of its impending doom, the star’s trajectory pushes it too close to the SMBH’s sphere of gravitational influence and tidal forces begin to shred the stellar structure. The woeful star is now a fly in a supermassive spider’s web: the star will be ripped apart, spaghettified stellar gas coming to form an accretion disk. This then results in a violent eruption of radiation as bits of star fall into the central black hole (Figure 1).

While we discover hundreds of TDEs every year, the nature of how the star is disrupted and comes to form an accretion disk around a SMBH is still very much an open question. Theoretical predictions spanning the past two decades suggest that this infall of gas from the disrupted star can, however, be uniquely recognized in spectroscopic observations. For example, as shown in Figure 1 (A), a smoking-gun signature of the accretion of stellar material is a double-peaked H-alpha emission line that arises from excited Hydrogen being consumed by the SMBH. And now this exact process was observed!

In an exciting leap for the study of TDEs, the authors of today’s paper present the first confident detection of a newly formed accretion disk around a SMBH. The discovered explosion is a TDE called Astronomical Transient (AT) 2018hyz, which was observed spectroscopically by the team for over 300 days after the explosion was detected. In Figure 2 we see that by Day 51 the SMBH’s stellar consumption has revealed itself in the form of “horned” Hydrogen emission line profiles. ...

Alexander Iamartino wrote:
Pretend for a moment that you are a Tyrannosaurus Rex, living some 66 million years ago. You are the alpha predator, unrivaled in strength and intelligence, and you rule over a vast rainforest near the ocean. While feasting on a vanquished sauropod, you look to the sky to see a massive flaming sphere barreling towards the ocean. You watch as it crashes into the Earth. Had you survived the immediate impact effects, such as the 100-meter-tall mega tsunami, the ensuing famine and global winter would have surely claimed you (and 75% of all life on Earth). With this scenario in mind, consider the question: what is preventing an apocalyptic event like this from happening again?

We now know destructive events, like the one that killed the dinosaurs, are caused by asteroids and other celestial objects crashing into Earth. NASA’s Jet Propulsion Laboratory (JPL) and the Center for Near Earth Object Studies (CNEOS) officially define the various categories of objects that come in close proximity to Earth. A Near-Earth Object (NEO) is any small celestial body whose closest approach to the Sun is 1.3 AU. This definition is further divided into subcategories, such as Near-Earth comets (NEC), Near-Earth asteroids (NEA), and Potentially Hazardous Asteroids (PHA). ...

Wei Vivyan Yan wrote:
Starting from their birth, supermassive black holes (SMBHs) co-evolve with their host galaxies through active galactic nuclei (AGNs) feedback. In the central region of the obscured AGN, a donut-shaped thick wall of dust and gas surrounds the accretion disk. This wall, referred to as the “dusty torus”, separates the broad emission line (BEL) region and the narrow emission line (NEL) region. As a result, the dusty torus can absorb most, if not all, of the BELs.

The scale of the torus region (pc-scale to ~100 pc) we are talking about here is quite small compared to that of the entire host galaxy. Direct observations are therefore challenging. However, it is very important to understand what occurs in the central region because AGN outflows can strongly affect galactic evolution from the center.

Today’s paper provides us with more information about AGN outflows. ...

• arXiv.org > astro-ph > arXiv:2004.02436 > 06 Apr 2020

Keir Birchall wrote:
Supermassive black holes (SMBHs) are a quiet bunch. Most of their time is spent sitting silently at the centre of their host galaxy as it rotates around them. However they are known to come out of their shell occasionally. During the host galaxy’s life, gas and dust can find its way into the centre and be devoured by the SMBH. As with any greedy diner, indigestion can strike and, for an SMBH, it’s explosive. Radiation from across the electromagnetic spectrum belches from the SMBH as it transitions, temporarily, into an active galactic nucleus (AGN). Today’s paper seeks to understand whether galaxy mergers could drive gas into the centre and trigger this transition from SMBH to AGN.

To investigate this phenomenon, today’s authors use spectroscopic data from the SDSS DR7 & GAMA surveys. Combining these surveys allows them to construct a comprehensive galaxy sample: the SDSS provides a huge number of galaxies within the nearby universe and GAMA contains data for the more distant universe, spanning a much greater range of redshifts. To search for mergers, the authors need to classify the shapes of all these galaxies, but this sample is too large to do this all by eye. Instead they make use of a novel convolutional neural network (a type of machine learning algorithm) to perform this task. The network was trained on images of merging galaxies identified in Galaxy Zoo, which were then used to classify mergers in the SDSS and GAMA samples. Two sets of data were collected from the algorithm with a 1) looser and 2) stricter definition of a merger considered. Next, to classify AGN, the authors used the optical BPT diagram, however this technique is known to miss obscured AGN, so a Mid-IR (MIR) colour check was also applied to the data. ...

• arXiv.org > astro-ph > arXiv:2004.00680 > 01 Apr 2020

John Weaver wrote:
Seminal work beginning in the early 2000’s introduced a surprising and deeply puzzling phenomenon: although central supermassive black holes are about the size of the solar system and typically make up less than 1% of the galaxy total stellar mass, their masses are extensively and intimately correlated with observed properties of their host galaxy. Bulge luminosity and mass, as well as the velocity dispersion (randomness of the stellar orbits) appear to be greater in galaxies with more massive central supermassive black holes. Understanding this relationship appears to be a key stepping stone in forming a comprehensive picture of galaxy evolution as a whole.

Observations and simulations alike have endeavored to discover the causal mechanism linking galaxy properties and black hole mass. Gas outflows ejected by AGN accretion activity have been a leading contender. They drive gas out of the nuclear region of the galaxy which may have otherwise been available to form stars or to feed the accretion of material about the black hole. The wider implication is the suppression of star-formation (quenching) of the galaxy at large by starvation of cold molecular gas.

This functional picture of the link between AGN activity and galaxy growth begs for a focused study into the behavior and composition of these outflows. ...

• Monthly Notices of the RAS (online 16 Apr 2020) DOI: 10.1093/mnras/staa1018
• arXiv.org > astro-ph > arXiv:2004.04749 > 09 Apr 2020

Jenny Calahan wrote:
Out beyond the orbit of Neptune lie small solar system bodies called trans-Neptunian Objects (TNOs). They are rocky, icy, dirt balls that for the majority of their orbit lie far beyond Neptune, but whose perihelion exists within the orbit of Neptune, or less than about 30 AU.

This paper uses data from the Dark Energy Survey (DES), which while on the quest for searching for dark energy far beyond our solar system, has found some extreme trans-neptunian objects (eTNOs, basically very distant TNOs). Based on the observed TNOs from DES, we can see that their orbits appear to be aligned. As you can see in Figure 1, they appear to lie on one side of the sky, having similar ecliptic longitudes. That’s weird, because things in space tend to be symmetric, or isotropic. So, shortly after astronomers saw these weirdly aligned orbits, an interesting hypothesis came about. Maybe there is a super-Earth located way beyond the orbit of Neptune that is pushing these trans-neptunian objects to be in these aligned orbits. That hypothesised planet was deemed Planet 9 and is still being hunted for after about 4 years of searching.

• arXiv.org > astro-ph > arXiv:2003.08901 > 19 Mar 2020

Wynn Jacobson-Galan wrote:
Astrophysicists love clever titles, and in transient astronomy, we can get some fun ones! Transient sky surveys discover thousands of new explosions every year, with each one receiving a name based on when it was discovered. For example, in 2018, a peculiar transient called Astronomical Transient (AT) 2018cow was discovered and aptly deemed “the Cow” after the last letters of its International Astronomical Union (IAU) name. Coincidently, the Cow happened to be so unique that it received worldwide acclaim as one of the most exciting discoveries of 2018!

Following its discovery, the Cow became the prototypical transient in a new class of explosions called Fast Blue Optical Transients (FBOTs), whose name is reflective of their unique observational signatures. After detection, FBOTs rise to peak brightness in only a few days (i.e. fast!) and have extremely hot temperatures, which makes them appear bluer in color than typical supernova explosions. However, astrophysicists are still puzzled at how objects like the Cow are formed: a black hole shreds a white dwarf? Or maybe a massive star implodes to form an accreting black hole or magnetar? Either way, FBOTs present a fresh mystery that can only be solved by finding and studying similar events!

The authors of today’s paper present the discovery of a fuzzy new FBOT, ZTF18abvkwla, which was nicknamed the “Koala” after the last four letters of its official transient name. Furry animals keep making their way into astrophysics! Unlike earth-based koalas, this transient creature is anything but docile: observations spanning the electromagnetic spectrum revealed that the Koala was a luminous event whose turbulent explosion resulted in high temperatures and rapid ejection of stellar material. The Koala was first observed by the Zwicky Transient Factory and is located in a distant dwarf galaxy with a high star formation rate of 7 solar masses per year. The large number of new stars in the Koala’s host galaxy may indicate that this FBOT came from the explosion of a young massive star rather than from an older star system containing a white dwarf. ...

• arXiv.org > astro-ph > arXiv:2003.01222 > 02 Mar 2020 (v1), 13 Apr 2020 (v4)

Jason Hinkle wrote:
Stars spend most of their lives on the main sequence, where they happily fuse hydrogen into helium in their cores. After hydrogen fusion in a star’s core stops, the star leaves the main sequence and goes through several later stages of evolution. While massive stars with M > 8 solar masses (M_⊙︎) die and become either neutron stars or black holes, the majority of stars in the universe lose their outer layers and become planetary nebulae. At the centers of these objects are the cores of the now-dead stars, known as white dwarfs.

White dwarfs (WDs) are interesting for several reasons. They allow us to learn about the late stages of stellar evolution, glean details on stellar populations, and even gain insight into properties of exotic matter. There is a particular class of WDs that is especially intriguing, known as extremely-low mass (ELM) WDs, whose masses are less than 0.3 M_⊙︎. Based on our current understanding of stellar evolution, a single star cannot create such a wimpy WD. Therefore, these ELM WDs must lose a significant amount of their mass as they age. One way this occurs is when a binary companion steals mass from them. These objects are rare (with only ~100 known) and tend to live binary systems.

Even though their parent star is dead doesn’t mean that WDs don’t have any life left in them. Since they are old stellar cores, they are extremely hot, radiating significant amounts of high-energy ultraviolet light. Over time a single WD will just cool off and fade, but if a WD has a partner, they can emit gravitational waves as they orbit each other or even explode when they get too massive (more on these later). This paper presents the confirmation of the closest yet known ELM WD and what systems like it may tell us in the future. ...

• arXiv.org > astro-ph > arXiv:2004.07556 > 16 Apr 2020

Spencer Wallace wrote:
In terms of size, our sun is far from common, with the majority of stars in the Milky Way being only a fraction of the mass of the Sun. The smallest of these, called ultra-cool dwarfs (UCDs) range between about 10-100 times the mass of Jupiter (only 1-10% the mass of the sun). Despite their tiny proportions, UCDs are thought to make up somewhere between 15 and 30 percent of the total number of stars in our galaxy.

For the smallest UCDs (also called brown dwarfs), the distinction between planet and star can get rather fuzzy. These objects are too small and cold to burn hydrogen like a regular star. Instead, they are thought to be powered by deuterium or lithium fusion, or possibly even the energy released by slow gravitational contraction. Because these objects are powered differently than their larger cousins, the resulting protoplanetary disks that form around them are likely different as well. Planet formation around an UCD might therefore produce something rather exotic.

The authors of today’s paper seek to better understand what kinds of planets might form around these tiny stars. To do so, they use a planet population synthesis model to predict what the ‘average’ UCD planet should look like. At it’s heart, a planet population synthesis model predicts properties such as the size, composition, and orbits of the planets based on a range of input parameters related to the host star and the protoplanetary disk. This is a powerful way to answer questions such as: Can UCD planets form at all? Are they rocky or gaseous? How sensitive are these answers to our uncertainties about the planet formation process? ...

• arXiv.org > astro-ph > arXiv:2004.07239 > 15 Apr 2020

Jay Vijay Kalinani wrote:
Recent detection of gravitational waves (GW) from the merger of two neutron stars by the LIGO–Virgo interferometers along with their electromagnetic counterparts across the entire spectrum, has opened the floodgates for multi-messenger astrophysics. This event, famously known as GW170817, was also coincident with a short gamma-ray burst (SGRB) GW170817A, giving us smoking-gun evidence connecting binary neutron star (BNS) mergers to SGRBs. However, the nature of remnants left behind by BNS mergers that could act as possible central engines to power such highly relativistic SGRB jets still remains an open question. ...

In this paper, the authors propose a third alternative called the ’time-reversal’ scenario, where a supramassive NS is formed after the BNS merger, which then collapses to a BH-disk system to produce an SGRB jet. ...

• Astrophysical Journal Letters 798(2):L36 (2015 Jan 10) DOI: 10.1088/2041-8205/798/2/L36
• arXiv.org > astro-ph > arXiv:1411.2015 > 07 Nov 2014 (v1), 27 Jan 2015 (v4)

Michael Foley wrote:
Supernovae are some of the most brilliant events in astronomy. Capable of rivaling the luminosity of their entire host galaxy (the galaxy in which the supernova explodes), these explosions release huge amounts of energy back into the universe. The energy released by countless supernova explosions can have a huge impact on the evolution of the host galaxy. Modern simulations of galaxies, therefore, spend a lot of computational time trying to effectively model these events. However, there is still significant uncertainty surrounding many of the details of supernovae. Today’s paper looks at how these uncertainties can propagate to yield major uncertainties in the results of galaxy simulations. ...

• arXiv.org > astro-ph > arXiv:2004.03608 > 07 Apr 2020

Sumeet Kulkarni wrote:
The latest pair of merging black holes announced by LIGO-Virgo, the first made public from their latest observing run, is unlike any seen before

It’s hard to believe that over five years have passed since the first detection of gravitational waves (GWs) by LIGO. The once unimaginable feat has now become routine with advancements in detector technology and the joining of Virgo, the European gravitational-wave observatory. Together over the past year, they have issued public alerts for a total of 56 detection candidates in their third observing run (O3), which was suspended one month prematurely due to COVID-19.

There is reason to remain excited, however, as the collaboration has started rolling out results from the observing run following detailed analyses. ...

• arXiv.org > astro-ph > arXiv:2004.08342 > 17 Apr 2020

Lukas Zalesky wrote:
As we gaze deeply into the sky and push the capabilities of our observatories to their practical limits, we realize Edwin Hubble’s words: “The history of astronomy is a history of receding horizons,” (The Realm of the Nebulae). In all directions we meet a universe full of diverse galaxies that change dramatically the further we look back, and in particular, within the first few billion years after the Big Bang (Figure 1). In attempting to make sense of it all, astronomers act like cosmic archeologists and try to piece together a coherent framework which explains and connects the past.

Galaxies in the early universe are quite different from the ones we see today: they are almost ubiquitously forming stars at dramatic rates. But how exactly do they differ from each other? In today’s bite, we take a journey billions of years into the past to study star-forming galaxies during a time when the universe was a fraction of its current age. ...

• arXiv.org > astro-ph > arXiv:2004.11175 > 23 Apr 2020

Sunayana Bhargava wrote:
One of the key properties of our cosmos is that it is isotropic. This means the universe is expected to look the same whichever direction you look, and therefore has no preferred direction or centre. Another property states that the universe is homogeneous – everything looks roughly the same at every point in space. The fulfilment of these two properties over large distances forms the basis of the cosmological principle.

This fundamental pillar of our cosmological model is reinforced by observations of leftover light from the Big Bang, known as the cosmic microwave background. which is impressively uniform. However, measurements of its anisotropy at very small scales have been used to challenge the cosmological principle – see this Astrobite for more.

An important consequence of isotropy is that the expansion of the universe must be the same in every direction. Today’s paper investigates just how constant this expansion is using the largest objects in our cosmos. If the expansion is found to be different depending on where one looks, it could point towards a more anisotropic universe than previously believed, which could have implications for how our universe evolved. ...

• Astronomy & Astrophysics 636:A15 (Apr 2020) DOI: 10.1051/0004-6361/201936602
• arXiv.org > astro-ph > arXiv:2004.03305 > 07 Apr 2020

Ashley Piccone wrote:
This entire story begins with an O-type star: a star so massive and bright that it radiates enough energy to ionize a bubble of Hydrogen around it. Scientists have long hypothesized that ionized Hydrogen (HII) regions like this bubble can trigger supersonic compressions on the outer surface of their shell, creating filamentary structures of denser material that then condense to form stars. These filaments are especially good at forming massive stars, possibly because they create the necessary conditions to increase high mass star formation efficiency. The authors of this paper wanted to explore the effects of HII regions on creating filamentary structure and triggering star formation, so they dove into the tale of RCW 120, an HII region with dense structures along its outer edge. ...

• Astronomy & Astrophysics (Accepted: 06 April 2020) DOI: 10.1051/0004-6361/202037815
• arXiv.org > astro-ph > arXiv:2004.05604 > 12 Apr 2020

Will Saunders wrote:
Imagine a cross between Earth and Uranus—a planet with water and high insolation (sunlight) but tilted to Uranus’ crazy 98º. No planet like that exists in our solar system today, but one might have billions of years ago. Mars’ tilt varies wildly over millions of years and it is theorized that Mars once contained global oceans. The authors of today’s paper propose the fundamental question: would an earth-like, high obliquity planet be habitable? ...

• Icarus 340:113592 (Apr 2020) DOI: 10.1016/j.icarus.2019.113592

Michael Foley wrote:
In studying the universe, astronomers must increasingly make use of all the natural sciences. State-of-the-art simulations of other galaxies now include complicated chemical networks that track the formation and evolution of various elements throughout time and space. Such estimates of elemental abundances play a crucial role in getting accurate measurements of gas cooling, star formation, and other physics. This has proven difficult in large galaxy simulations, though, since finite computational resources limit their resolution. In other words, these simulations cannot currently probe the smaller scales at which chemical formation and evolution occur. Consequently, one must turn to what are known as “subgrid models.”

In today’s paper, the authors investigate the effects of including a subgrid model for turbulent metal diffusion in dwarf galaxies from the FIRE simulations (Fig 1). Let’s break this down. A subgrid model is any computation done below the resolution limit of the simulation. Subgrid, therefore, refers to something done on the scale of an individual resolution element (grid cell, particle, etc) or smaller. In astronomy jargon, metals refer to anything on the periodic table other than hydrogen and helium. Turbulent metal diffusion then means the movement of metals away from their point of origin via the turbulent motion of the surrounding gas. This is akin to giving the galaxy a good shake at each step in the simulation so that the metals spread out. ...

• Monthly Notices of the RAS 474(2):2194 (Feb 2018) DOI: 10.1093/mnras/stx2858
• arXiv.org > astro-ph > arXiv:1710.06533 > 18 Oct 2017 (v1), 26 Jun 2018 (v2)

Kate Storey-Fisher wrote:
When the Laser Interferometer Gravitational Wave Observatory (LIGO) turned on three years ago, it opened the window onto black hole collisions throughout the universe. These binary black holes (BBHs) emit gravitational waves (GWs) as they inspiral, and LIGO detects these spacetime ripples. Though observations of BBHs have begun streaming in, it turns out that the GW-windows are rather sparse and dirty (not unlike my New York City apartment).

The current LIGO run is part of the second generation of GW experiments; in the coming decades we will see the third generation (3G) come online, with the planned Einstein Telescope and Cosmic Explorer detectors. These are expected to detect tens of thousands of BBHs, an explosion of data compared to the relative pitch-blackness of the dozen or so LIGO BBH detections so far. However, this is still sparse compared to the millions of galaxies we have observed.

Further, we can only vaguely make out the location of BBH mergers. Through the eyes (or really, ears) of the detectors, we estimate smeared-out areas where the BBH might live. Current LIGO “localization regions” are between 30 and 2200 square degrees on the sky (for comparison, the largest constellation Hydra covers 1300 square degrees). 3G detectors will localize BBHs to within 1 square degree, but this is still a far cry from the pinpointed locations of galaxies.

Today’s authors take on the task of extracting information about BBHs through these less-than-ideal windows. They present a method to learn the relationship between BBHs, galaxies, and the underlying cosmic web—the large-scale structure of dark matter throughout the universe. This, in turn, provides insight into the origins of the black holes themselves. ...

• arXiv.org > astro-ph > arXiv:2005.01111 > 03 May 2020

Bryanne McDonough wrote:
Humanity’s dependence on electricity has been cast into further relief by the global COVID-19 pandemic. It is impossible to imagine modern life without it, lacking everything from lights to the internet. What would happen if that all disappeared over night? If all of our electrical grids failed essentially simultaneously? This is more than the setting of a dystopian novel: it is a real possibility. A large geomagnetic storm triggered by the sun could blow out the transformers that are an essential component of our electrical grids. And it’s happened before. ...

Alex Pizzuto wrote:
The engineering that goes into modern day astrophysical instruments is astounding. From mirrors that can detect a change in distance 10,000 times smaller than the size of a proton to telescopes that can detect individual light particles from millions of light years away, astrophysicists and astronomers constantly prove themselves to be vanguards of tech. Today’s authors discuss a new and exciting piece of technology to do astrophysics: old rocks. ...

• arXiv.org > hep-ph > arXiv:2004.08394 > 17 Apr 2020

Luna Zagorac wrote:
For the past few years, most astronomers have become closely acquainted with our new favorite bedtime story, The Tale of Two Hubbles. In this tale, our heroes measure two completely different values for the Hubble constant: one in the early universe from the Planck satellite dataset, one in the late universe from Type Ia supernovae. This discrepancy is important, as the Hubble constant is what astronomers use to measure how fast the universe is expanding, and is closely tied to our assumptions about how it all works (see these two bites for more information).

As additional measurements confirm this tension, it could mean our assumptions about the early universe are totally off, and that there is new physics there to be discovered. Alternatively, it might mean that we haven’t accounted for all the important effects that impact these measurements (for instance, voids have been investigated). In this paper, the authors consider how primordial magnetic fields might affect the Planck measurement. ...

• arXiv.org > astro-ph > arXiv:2004.09487 > 20 Apr 2020 (v1), 28 Apr 2020 (v2)

Ali Crisp wrote:
Besides the main celestial bodies in our solar system – the planets, moons, and dwarf planets (RIP Pluto) we’re familiar with – there are also around 150 million asteroids floating around. Among these are Trojan asteroids, special groups of asteroids that fall within the orbital paths of planets. As one may guess, the asteroids are named after the Trojans from Homer’s Iliad and Odyssey, with the largest asteroids getting specific names of heroes from the tales, such as Hektor and Achilles.

Though the term has become more general, the archetypal Trojan asteroids fall within the orbital path of Jupiter at its L4 and L5 Lagrange points. Lagrange points are, essentially, points where the gravitational forces from two bodies create stable positions for other objects like asteroids to sit. There are five Lagrange points for any two gravitationally interacting objects – creatively labeled L1 through L5. L1, L2, and L3 are only considered ‘meta-stable’, meaning that a slight push could cause the object to leave the point, while L4 and L5 are truly stable. A visualization of these points for the Earth-Sun system is shown in Figure 1. The group leading Jupiter’s orbit at L4 is sometimes referred to as the Greek camp, while the trailing group at L5 is the Trojan camp.

The origin of Jupiter’s Trojan asteroids, or the Jovian Trojans, is somewhat ambiguous, but they are thought to have been present since the early days of the solar system. They were likely captured, because their actual orbital distributions and inclinations don’t match those theorized by an in situ formation model. According to the paper, there are currently 5,553 known Jovian Trojans, but some proposed projects with the upcoming Vera C. Rubin Observatory and Wide Field Infrared Survey Telescope (WFIRST) aim to increase these numbers. The Lucy mission is also set to study the Jovian Trojans in more depth in the late 2020’s. ...

• arXiv.org > astro-ph > arXiv:2005.03635 > 07 May 2020

James Negus wrote:
Impermanence. The indelible signature of the Universe.

Transient events, such as energetic stellar explosions and chaotic mergers expose the variable essence of the cosmos. Observed from afar, these stunning events splatter polychromatic ink haphazardly across the cosmic canvas. Despite their violent nature, though, these events enrich the voids of space with the precious ingredients necessary to birth new stars. ...

While SNe Ia have been extensively studied, the properties of the companion star, the accretion dynamics of the progenitor system, and the features of the ultimate explosion remain inconclusive. To address this, the authors of today’s paper analyze the morphology of Type Ia supernova remnants (SNR Ia) and their impact on the CSM to better understand these events.

They focus their attention on the renowned Kepler’s Supernova (SN 1604) remnant and its impact on the CSM. ...

• Galaxies 8(2):38 (Jun 2020) DOI: 10.3390/galaxies8020038
• arXiv.org > astro-ph > arXiv:2004.14493 > 29 Apr 2020

Jamie Sullivan wrote:
Much like Voltron, different observational probes of cosmology are powerful on their own, but are at their strongest when combined together. This power comes from the ability to obtain better constraints by breaking parameter degeneracies, a jargon-y phrase we will unpack the meaning of in today’s bite.

The ΛCDM model – the standard model of cosmology – tells us all about the large-scale distribution of matter in the universe with just 5 parameters. However, we cannot directly measure the matter distribution directly since it is dominated by dark matter which produces no observable signal. Instead we have to turn to tracers of the dark matter, or observational probes. One such tracer is gravitational lensing of light from some background source by matter between us and the source. Another is the positions of galaxies themselves and their clustering. Whereas the lensing signal is a direct tracer of the matter distribution, galaxies are what are referred to as biased tracers, since they form only in very high-density regions – i.e. dark matter halos. So what we actually measure when we are trying to constrain cosmological parameter values is not the matter distribution at all, but rather the lensing and galaxy clustering signals.

Given a measured catalog of galaxies and a measured sky map of the lensing signal the quantities we actually measure are correlation functions (see also this bite) of these spatial distributions. At its core, a correlation function tells you how much more likely over random chance you are to find two objects separated by a fixed distance. In this case our objects are lensed photons (aka “cosmic shear”) and galaxies, but there is one more signal we can construct from our other two – this is the so-called “Galaxy-Galaxy Lensing” (GGL) signal. GGL is essentially a measure of cross-correlation between the matter and galaxy distributions, and is measurable when you have two populations of galaxies – a background population that acts as the source of gravitational lensing, and a foreground population that traces the matter responsible for the lensing signal.

So what do we do with all of these signals? In order to pump the maximum amount of information possible out of observational data, modern surveys such as DES employs a combined-probes analysis. A single observational probe can often only measure a combination of parameters of interest. Suppose probe 1 can only measure xy², but probe 2 can measure xy. By dividing the measurement of probe 2 by probe 1, we can obtain y, which then allows us to find x. If we only had one probe, we wouldn’t be able to separate the parameters, and therefore could not know either. This is exactly the situation we solve (i.e. “breaking degeneracies”) by using measurements of the same parameter from multiple probes!

A complication to the degeneracy-breaking picture arises for combining galaxy clustering and lensing, however. On large scales, it really is as simple as the division described above, as the galaxy correlation function is simply a constant factor (termed the “bias” b) multiplied by the matter correlation function measured by lensing. But on small scales this relationship no longer holds – the bias now becomes “scale-dependent” (a piece of jargon that will be important for later!), and some complicated function is necessary to connect the galaxy correlation function to the matter correlation function. How do cosmologists deal with this complication? ...

• arXiv.org > astro-ph > arXiv:2004.07811 > 16 Apr 2020