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Disk galaxies

What were galaxies like in the first 500 million years of the universe? According to simulations by Yu Feng (UC Berkeley) and collaborators, the earliest massive galaxies to form were mostly disk-shaped, rather than the compact clumps previously predicted.

Early-Galaxy Models

Current models for galaxy formation predict that small perturbations in the distribution of matter in the early universe collapsed to form very compact, irregular, clumpy first galaxies. Observations support this: the furthest out that we’ve spotted disk-shaped galaxies is at z=3, whereas the galaxies we’ve observed from earlier times — up to redshifts of z=8–10 — are very compact.

But could this be a selection effect, arising from the rarity of large galaxies in the early universe? Current surveys at high redshift have thus far only covered relatively small volumes of space, so it’s not necessarily surprising that we haven’t yet spotted any large disk galaxies. Similarly, numerical simulations of galaxy formation are limited in the size of the volume they can evolve, so resulting models of early galaxy formation also tend to favor compact clumpy galaxies over large disks.

An Enormous Simulation

Pushing at these limitations, Feng and his collaborators used the Blue Waters supercomputer to carry out an enormous cosmological hydrodynamic simulation called BlueTides. In this simulation, they track 700 billion particles as they evolve in a volume of 400 comoving Mpc/h — 40 times the volume of the largest previous simulation and 300 times the volume of the largest observational survey at these redshifts.

What they find is that by z=8, a whopping 70% of the most massive galaxies (over 7 billion solar masses each) were disk-shaped, though they are more compact, gas-rich, and turbulent than present-day disk galaxies like the Milky Way. The way the most massive galaxies formed in the simulation also wasn’t expected: rather than resulting from major mergers, they were built from smooth accretion onto the disks from nearby filaments.

These simulations suggest we still have a lot to learn about the structure of galaxies in the early universe and how they formed. Luckily, future telescope projects should help us out: Feng and collaborators estimate that the WFIRST satellite, for instance, should have the capability to detect 8000 disk galaxies of the type BlueTides predicts — compared to the weak 30% chance of finding a single one in the current largest-area Hubble survey!

 

BlueTides

This graphic shows the scope of the BlueTides simulation; click to get a better look! The insets show the environment of the most massive black hole and the most massive disk galaxy at different scales. Crosses mark the positions of supermassive black holes. Credit: bluetides-project.org

Citation

Yu Feng et al. 2015 ApJ 808 L17. doi:10.1088/2041-8205/808/1/L17

Pluto geologic activity

New Horizons scientists Kelsi Singer and Alan Stern predicted that Pluto may have subsurface activity, in this study published even before New Horizon’s recent observations of Pluto’s strangely uncratered surface areas.

Where Does the Nitrogen Come From?

Pluto’s surface and atmosphere contain a significant amount of nitrogen, but the gas leaks out of Pluto’s atmosphere at an tremendous rate — estimated at about 1.5 × 1012-13 grams per year (roughly 200-2000 tons/hr!). But if the nitrogen has been escaping at this rate since the solar system was formed, the entire atmospheric reservoir of would have been lost long before now. So what is resupplying Pluto’s nitrogen?

Singer and Stern explore several possible sources:

  1. Delivery by comet impact
    The authors calculate that over the 4-billion-year span since Pluto’s formation, it has been impacted by a total of 600 million comets of varying sizes, all likely containing nitrogen. But their estimates show that the amount of nitrogen this would supply falls several orders of magnitude shy of explaining the escape rate.
  2. Excavation by cratering
    Could comet impacts simply expose nitrogen buried in reservoirs just beneath Pluto’s surface? That method, too, falls short of resupplying atmospheric nitrogen escape by at least an order of magnitude, even using the most generous estimates.
  3. Internal activity
    Unless the believed atmospheric loss rate of Pluto is overestimated, the authors conclude that Pluto must experience some sort of internal activity such as cryovolcanism that brings nitrogen from below its surface up and into the atmosphere.

The Study in Context of Current Events

Singer and Stern wrote and submitted this paper before the New Horizons spacecraft’s recent flyby of Pluto. Data from this mission has recently provided surprise after surprise — from images of smooth, crater-free regions on Pluto’s surface to evidence of sheets of carbon monoxide, methane, and nitrogen ices flowing like glaciers.

These clues support Singer and Stern’s theories of internal activity, but raise new questions about the nature of that activity! As data from New Horizons keeps streaming in (in fact, atmospheric data from the Alice instrument is expected to pin down the atmospheric loss rate very soon), we can hope to continue to piece this picture together.

 

Citation:

Kelsi N. Singer and S. Alan Stern 2015 ApJ 808 L50 doi:10.1088/2041-8205/808/2/L50

BH-NS eccentric merger

When a neutron star (NS) has a glancing encounter with a black hole (BH), its spin has a significant effect on the outcome, according to new simulations run by William East of Stanford University and his collaborators.

Spotting an Eccentric Merger

In a traditional BH-NS merger, the two objects orbit each other quasi-circularly as they spiral in. But there’s another kind of merger that’s possible in high-density environments like galactic nuclei or globular clusters: a dynamical capture merger, in which a NS and BH pass each other just close enough that the gravity of the black hole “catches” the NS, leading the two objects to merge with very eccentric orbits.

During an eccentric merger, the NS can be torn apart — at which point some fraction of the tidally-disrupted material will escape the system, while some fraction instead accretes back onto the BH. Knowing these fractions is important for being able to model the expected electromagnetic signatures for the merger: the unbound material can power transients like kilonovae, whereas the accreting material may be the cause of short gamma-ray bursts. The amount of material available for events like these would change their observable strengths.

Testing the Effects of Spin

To see whether NS spin has an impact on the behavior of the merger, East and collaborators use a general-relativistic hydrodynamic code to simulate the glancing encounter of a BH and a NS with dimensionless spin between a=0 (non-spinning) and a=0.756 (rotation period of 1 ms). They also vary the separation of the first encounter.

The group finds that changing the NS’s spin can change a number of outcomes of the merger. To start with, it can affect whether the NS is captured by the BH, or if the encounter is glancing and then both objects carry on their merry way. And if the NS is trapped by the BH and torn apart, then the higher the NS’s spin, the more matter outside of the BH ends up unbound, instead of getting trapped into an accretion disk around the BH.

As a result of these simulations, the authors argue that the spin of NSs in dynamical capture mergers is crucially important for correctly modeling the observational signatures that might come out of them.

 

Citation:

William E. East et al. 2015 ApJ 807 L3 doi:10.1088/2041-8205/807/1/L3

Kuiper belt analog

A debris disk just discovered around a nearby star is the closest thing yet seen to a young version of the Kuiper belt. This disk could be a key to better understanding the interactions between debris disks and planets, as well as how our solar system evolved early on in its lifetime.

Hunting for an analog

The best way to understand how the Kuiper belt — home to Pluto and thousands of other remnants of early icy planet formation in our solar system — developed would be to witness a similar debris disk in an earlier stage of its life. But before now, none of the disks we’ve discovered have been similar to our own: the rings are typically too large, the central star too massive, or the stars exist in regions very unlike what we think our Sun’s birthplace was like.

A collaboration led by Thayne Currie (National Astronomical Observatory of Japan) has changed this using the Gemini Planet Imager (GPI), part of a new generation of extreme adaptive-optics systems. The team discovered a debris disk of roughly the same size as the Kuiper belt orbiting the star HD 115600, located in the nearest OB association. The star is only slightly more massive than our Sun, and it lives in a star-forming region similar to the early Sun’s environment. HD 115600 is different in one key way, however: it is only 15 million years old. This means that observing it gives us the perfect opportunity to observe how our solar system might have behaved when it was much younger.

A promising future

GPI’s spatially-resolved spectroscopy, combined with measurements of the reflectivity of the disk, have led the team to suspect that the disk might be composed partly of water ice, just as the Kuiper belt is. The disk also shows evidence of having been sculpted by the motions of giant planets orbiting the central star, in much the same way as the outer planets of our solar system may have shaped the Kuiper belt.

The observations of HD 115600 are some of the very first to emerge from GPI and the new generation of planet-hunting instruments. The detection of this disk provides a promising outlook on what we can expect to discover in the future with these systems.

Citation:

Thayne Currie et al. 2015 ApJ 807 L7 doi:10.1088/2041-8205/807/1/L7

M85

Two surprisingly small heavy-weights have been discovered around galaxies in the nearby Virgo cluster by a team led by undergrads Michael Sandoval and Richard Vo and their advisor Aaron Romanowsky of San Jose State University. Setting a new record, these two objects now hold the title of the densest galaxy and the densest free-floating stellar system ever observed.

Classification Difficulties

What is the difference between large star clusters and small galaxies? Once thought to be distinct categories, the decade-old discovery of a new class of object, ultracompact dwarfs (UCDs), blurred the line between them somewhat: UCDs sit awkwardly between the two categories in size, mass and luminosity. So what are UCDs? It’s hard to say — in part because their full range of possible parameters has yet to be carefully explored.

Sandoval and his team set out to address this problem by combing through archival data from the Sloan Digital Sky Survey, searching for objects that display properties between those of star clusters and galaxies. Their search yielded two especially interesting objects: one around the galaxy M59, and the other around M85 (see figure 2). Follow-up observations with Subaru Telescope and the Southern Astrophysical Research telescope provided additional imaging and spectroscopic information.

Density plot

Plot of stellar surface mass density vs. mass of known stellar systems. The data include the two new objects (M85-HCC1 and M59-UCD3) as well as globular clusters, UCDs, and compact elliptical galaxies. Credit: Sandoval et al. 2015

Record-Breakers

What makes these two discoveries so unusual? Both are remarkably dense compared to similar objects! The first, M59-UCD3, was categorized as an ultracompact dwarf galaxy — but it’s significantly more dense than any other galaxy discovered. The night sky in M59-UCD3 would appear to contain roughly a million stars, compared to the few thousand we see overhead here in the Solar neighborhood.

M85-HCC1 is another ten times denser than even that! It’s such an unusual stellar system that it defies classification in the usual categories, which is why Sandoval and collaborators created a new name for this type of object: hypercompact cluster.

In spite of the differences between these two stellar systems, the team argues that there is evidence that they were formed the same way. They believe that both objects are galactic centers that have been tidally stripped of all of the outlying stars and gas, leaving only the dense cores behind. They argue that this could be caused by mergers of M59 and M85 with intermediate mass galaxies. If true, searching for more of these unique objects could provide us with clues to how galaxies were assembled.

Citation:

Michael Sandoval et al. 2015 ApJ 808 L32 doi:10.1088/2041-8205/808/1/L32

Bonus:

Check out this cool visualization from the authors of how tidal stripping of a small galaxy might happen. This is one theory of how UCDs are formed.

HL Tau

The exciting results of the highest-resolution test campaign yet attempted by the Atacama Large Millimeter/submillimeter Array (ALMA) are detailed in a recent set of four papers.

Juno

Animation (click to watch) of the asteroid Juno as seen in mm wavelengths by ALMA’s Long Baseline Campaign. Image credit: ALMA (NRAO/ESO/NAOJ)

ALMA’s array of antennas can be configured so that the baseline of the simulated telescope is as small as 150 m or as large as 15 km across. In its smaller configurations, ALMA studies the large-scale structure of cold objects in the Universe — and this is how the array has been used since it began its first operations in 2011. But now ALMA has begun to test its long-baseline configuration, in which it is able to make its highest-resolution observations and study the small-scale structure of objects in detail.

The Targets

ALMA’s Long Baseline Campaign, run in late 2014, observed five science targets using 22–36 antennas arranged with a baseline of up to the full 15 km. The targets were selected to push the limits of ALMA’s capabilities: each target has a small angular size (less than two arcseconds) with fine-scale structure that is largely unresolved in previous observations. Two of the targets, the variable star Mira and the active galaxy 3C138, were primarily used for calibration and comparisons of ALMA data to those of other telescopes. The remaining three targets not only demonstrated ALMA’s capabilities, but also resulted in new science discoveries.

SDP.81

ALMA’s highest resolution observation yet, of the gravitationally lensed galaxy SDP.81. The maximum resolution of this image is 23 milliarcseconds. Image credit: ALMA (NRAO/ESO/NAOJ); B. Saxton NRAO/AUI/NSF

  • Juno is one of the largest asteroids in our solar system’s main asteroid belt. ALMA’s observations of Juno were made when the asteroid was approximately 295 million km from Earth, and the ten images ALMA took have been stitched together into a brief animation that show the asteroid tumbling through space as it orbits the Sun. The resolution of these images — enough to study the shape and even some surface features of the asteroid! — are unprecedented for this wavelength.
  • HL Tau is a young star surrounded by a protoplanetary disk. ALMA’s detailed observations of this region revealed remarkable structure within the disk: a series of light and dark concentric rings indicative of planets caught in the act of forming. Studying this system will help us understand how multi-planet solar systems like our own form and evolve.
  • The star-forming galaxy SDP.81 — located so far away that the light we see was emitted when the Universe was only 15% of its current age — is gravitationally-lensed into a cosmic arc, due to the convenient placement of a nearby foreground galaxy. The combination of the lucky alignment and ALMA’s high resolution grant us a spectacularly detailed view of this distant galaxy, allowing us to study its actual shape and the motion within it.

The observations from ALMA’s first test of its long baseline demonstrate that ALMA is capable of doing the transformational science it promised. As we gear up for the next cycle of observations, it’s clear that exciting times are ahead!

Citation:

ALMA Partnership et al. 2015 ApJ 808 L1, L2, L3 and L4. Focus on the ALMA Long Baseline Campaign

core collapse simulation

What happens at the very end of a massive star’s life, just before its core’s collapse? A group led by Sean Couch (California Institute of Technology and Michigan State University) claim to have carried out the first three-dimensional simulations of these final few minutes — revealing new clues about the factors that can lead a massive star to explode in a catastrophic supernova at the end of its life.

A Giant Collapses

In dying massive stars, in-falling matter bounces off the of collapsed core, creating a shock wave. If the shock wave loses too much energy as it expands into the star, it can stall out — but further energy input can revive it and result in a successful explosion of the star as a core-collapse supernova.

In simulations of this process, however, theorists have trouble getting the stars to consistently explode: the shocks often stall out and fail to revive. Couch and his group suggest that one reason might be that these simulations usually start at core collapse assuming spherical symmetry of the progenitor star.

Adding Turbulence

Couch and his collaborators suspect that the key is in the final minutes just before the star collapses. Models that assume a spherically-symmetric star can’t include the effects of convection as the final shell of silicon is burned around the core — and those effects might have a significant impact!

To test this hypothesis, the group ran fully 3D simulations of the final three minutes of the life of a 15 solar-mass star, ending with core collapse, bounce, and shock-revival. The outcome was striking: the 3D modeling introduced powerful turbulent convection (with speeds of several hundred km/s!) in the last few minutes of silicon-shell burning. As a result, the initial structure and motions in the star just before core collapse were very different from those in core-collapse simulations that use spherically-symmetric initial conditions. The turbulence was then further amplified during collapse and formation of the shock, generating pressure that aided the shock expansion — which should ultimately help the star explode!

The group cautions that their simulations are still very idealized, but these results clearly indicate that the 3D structure of massive stellar cores has an important impact on the core-collapse supernova mechanism.

Citation

Sean M. Couch et al. 2015 ApJ 808 L21 doi:10.1088/2041-8205/808/1/L21

Asteroid Itokawa

Recent observations of asteroid (335433) 2005 UW163 have added a new member to the mysterious category of “super-fast rotators” — asteroids that rotate faster than should be possible, given current theories of asteroid composition.

Asteroids come in sizes of a few meters to a few hundred kilometers, and can spin at rates from 0.1 to nearly 1000 revolutions per day. Current theories suggest that asteroids smaller than 150m are mostly monolithic (made up of a single rock), whereas asteroids larger than 150m are usually what’s known as a “rubble pile” — a collection of rock fragments from past collisions, bound together into a clump by gravity. “Rubble pile” asteroids have an important structural limitation: they can’t spin faster than once every 2.2 hours without flying apart as the centripetal force overcomes the force of gravity.

Asteroid 2005 UW163 violates this rule: its diameter is 690m, but it rotates once every 1.29 hours. This discovery was made by a team of scientists using telescopes at the Palomar Observatory in California to conduct a large survey of the rotation rates of nearby asteroids. The group, led by Chan-Kao Chang of Taiwan’s National Central University, discovered 11 super-fast rotator candidates — of which asteroid 2005 UW163 is the first to have its rotation rate confirmed by additional observations.

The category of super-fast rotators poses an interesting problem: how are they able to spin so quickly without flying apart? Either the density of these asteroids is unexpectedly high (roughly four times the density of typical “rubble pile” asteroids), or else there must be additional forces besides gravity at work to help hold the asteroid together, such as bonds between the rocks. Future observations of super-fast rotators will help us better understand the peculiar structure of these rocky neighbors.

Citation:

Chan-Kao Chang et al. 2014 ApJ 791 L35 doi:10.1088/2041-8205/791/2/L35

Protoplanetary disk

A team of scientists led by Catherine Walsh (Leiden Observatory) has found evidence of two planets orbiting a young, hot star located relatively nearby, at just 335 light years from Earth. The star, HD 100546, is surrounded by a disk of gas and dust — which is a prime environment for forming planets. Previous observations have hinted at two planets potentially hiding in this disk, but directly imaging planets in disks is very difficult. Walsh’s team took a different approach to finding these subtle planets: rather than looking for them directly, the group instead looked for gaps in the dust of the disk.

The team examined the signature of dust particles in the disk around HD 100546 using observations from the Atacama Large Millimeter/Submillimeter Array (ALMA), a telescope located in Chile. They found that rather than forming a solid disk, the dust particles have settled into two nested rings with a gap between them. That gap is telltale evidence of a planet: planets embedded in disks tend to clear out a path as they orbit, accreting the gas and dust onto themselves.

Walsh’s team followed up the discovery by using different models of planetary formation to try to reproduce ALMA’s observations. They found that the best model required there to be two planets in HD 100546’s system: one located very close to the star, and a second, newly-forming planet located within the disk gap.

This model is consistent with previous estimates of where the two suspected planets might be located, which is promising news for exoplanet enthusiasts. HD 100546 could be our first opportunity to study a planet caught in the act of forming — which is an important step toward understanding how planets are created out of disks around young stars.

Citation:

Catherine Walsh et al. 2014 ApJ 791 L6 doi:10.1088/2041-8205/791/1/L6

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