Features RSS

Milky Way center

A recent study suggests that the stars in the central parsec of our galaxy are not a single, roughly solar-metallicity population, as previously thought. Instead, these stars have a large variation in metallicities — which has interesting implications for the formation history of the Milky Way’s nuclear star cluster.

Clues from Abundances

Why do we care about the metallicity of stars and stellar populations? Metallicity measurements can help us to separate multiple populations of stars and figure out when and where they were formed.

Measurements of the chemical abundances of stars in the Milky Way have demonstrated that there’s a metallicity gradient in the galaxy: on average, it’s below solar metallicity at the outer edges of the disk and increases to above solar metallicity within the central 5 kpc of the galaxy.

So far, measurements of stars in the very center of the galaxy are consistent with this galactic trend: they’re all slightly above solar metallicity, with little variation between them. But these measurements exist for only about a dozen stars within the central 10 pc of the galaxy! Due to the high stellar density in this region, a larger sample is needed to get a complete picture of the abundances — and that’s what this study set out to find.

Different Populations

Led by Tuan Do (Dunlap Fellow at the University of Toronto and member of the Galactic Center Group at UCLA), the authors of this study determined the metallicities of 83 late-type giant stars within the central parsec of the galaxy. The metallicities were found by fitting the stars’ K-band spectra from observations by the NIFS instrument on the Gemini North telescope.

In contrast to the previous studies, the authors found that the 83 stars exhibited a wide range of metallicities, from a tenth of solar metallicity all the way to super-solar metallicities.

The abundances of the low-metallicity stars they found are consistent with globular cluster metallicities, suggesting that these stars (about 6% of the sample) may have arrived in the nuclear star cluster as a result of the infall of globular clusters. The super-solar metallicity stars were likely formed closer to the galactic center or from the disk.

The authors point out that current models of the star formation history and initial mass function of the nuclear stellar cluster — which typically assume a uniform population of stars with roughly solar metallicity — may need to be revisited in light of these results.

Citation

Tuan Do et al. 2015 ApJ 809 143 doi:10.1088/0004-637X/809/2/143

Mrk 231

Could a pair of supermassive black holes (SMBHs) be lurking at the center of the galaxy Mrk 231? A recent study finds that this may be the case — and the unique spectrum of this galaxy could be the key to discovering more hidden binary SMBH systems.

Where Are the Binary Supermassive Black Holes?

It’s believed that most, if not all, galaxies have an SMBH at their centers. As two galaxies merge, the two SMBHs should evolve into a closely-bound binary system before they eventually merge. Given the abundance of galaxy mergers, we would expect to see the kinematic and visual signatures of these binary SMBHs among observed active galactic nuclei — yet such evidence for sub-parsec binary SMBH systems remains scarce and ambiguous. This has led researchers to wonder: is there another way that we might detect these elusive systems?

A collaboration led by Chang-Shuo Yan (National Astronomical Observatories, Chinese Academy of Sciences) thinks that there is. The group suggests that these systems might have distinct signatures in their optical-to-UV spectra, and they identify a system that might be just such a candidate: Mrk 231.

A Binary Candidate

Mrk 231 model

Proposed model of Mrk 231. Two supermassive black holes, each with their own mini-disk, orbit each other in the center of a circumbinary disk. The secondary black hole has cleared gap in the circumbinary disk as a result of its orbit around the primary black hole. [Yan et al. 2015]

Mrk 231 is a galaxy with a disturbed morphology and tidal tails — strong clues that it might be in the final stages of a galactic merger. In addition to these signs, Mrk 231 also has an unusual spectrum for a quasar: its continuum emission displays an unexpected drop in the near-UV band.

Yan and her collaborators propose that the odd behavior of Mrk 231’s spectrum can be explained if the center of the galaxy houses a pair of SMBHs — each with its own mini accretion disk — surrounded by a circumbinary accretion disk. As the secondary SMBH orbits the primary SMBH (with a period of 1.2 years and a mass ratio of 38:1, according to the team’s models), it clears a gap in the circumbinary disk. The collaborators showed that this gap in the disk will cause a decrease in the continuum emission of the system consistent with the observed drop in Mrk 231’s UV spectrum.

If the collaboration’s models of Mrk 231 are confirmed, this would demonstrate the feasibility of finding other active binary SMBH systems by looking for similar deficits in the optical-to-UV spectra.

Citation

Chang-Shuo Yan et al. 2015 ApJ 809 117. doi:10.1088/0004-637X/809/2/117

Saturn Hexagon

For over three decades, we’ve been gathering observations of the mysterious hexagonal cloud pattern encircling Saturn’s north pole. Now, researchers believe they have a model that can better explain its formation.

Fascinating Geometry

Saturn’s northern Hexagon is a cloud band circling Saturn’s north pole at 78° N, first observed by the Voyager flybys in 1980–81. This remarkable pattern has now persisted for more than a Saturn year (29.5 Earth years).

Saturn's Hexagon

Eight frames demonstrating the motion within Saturn’s Hexagon. Click to watch the animation! The view is from a reference frame rotating with Saturn. [NASA/JPL-Caltech/SSI/Hampton University]

Observations by Voyager and, more recently, Cassini have helped to identify many key characteristics of this bizarre structure. Two interesting things we’ve learned are:

  1. The Hexagon is associated with an eastward zonal jet moving at more than 200 mph.
    The cause of the Hexagon is believed to be a jet stream, similar to the ones that we experience on Earth. The path of the jet itself appears to follow the hexagon’s outline.
  2. The Hexagon rotates at roughly the same rate as Saturn’s overall rotation.
    While we observe individual storms and cloud patterns moving at different speeds within the Hexagon, the vertices of the Hexagon move at almost exactly the same rotational speed as that of Saturn itself.

Attempts to model the formation of the Hexagon with a jet stream have yet to fully reproduce all of the observed features and behavior. But now, a team led by Raúl Morales-Juberías of the New Mexico Institute of Mining and Technology believes they have created a model that better matches what we see.

Simulating a Meandering Jet

The team ran a series of simulations of an eastward, Gaussian-profile jet around Saturn’s pole. They introduced small perturbations to the jet and demonstrated that, as a result of the perturbations, the jet can meander into a hexagonal shape. With the initial conditions of the team’s model, the meandering jet is able to settle into a stable hexagonal shape that rotates with very nearly the same period as Saturn’s rotational period.

The formation of this hexagon depends on factors such as the initial amplitude and curvature of the jet. The model’s treatment of the wind profile within Saturn’s atmosphere is another key component that allowed them to match the observed characteristics of the Hexagon, such as its shape, vorticity behavior, temperature gradient, and seasonal stability.

Bonus

The gif below shows part of an animation the authors produced of the jet evolution in their model. You can see a hexagon begin to develop at around 230 days into the simulation, and by about 400 days it becomes stable and non-rotating (we’re looking at it from a reference frame rotating with Saturn). The full animation can be viewed here. [Morales-Juberías et al., 2015]

Hexagon model

Citation

R. Morales-Juberías et al. 2015 ApJ 806 L18 doi:10.1088/2041-8205/806/1/L18

Fermi-LAT sky map

Since the release of the second Fermi-LAT catalog in 2012, astronomers have been hunting for 3FGL J1906.6+0720, a gamma-ray source whose association couldn’t be identified. Now, personal-computer time volunteered through the Einstein@Home project has resulted in the discovery of a pulsar that has been hiding from observers for years.

A Blind Search

Identifying sources detected by Fermi-LAT can be tricky: the instrument’s sky resolution is limited, so the position of the source can be hard to pinpoint. The gamma-ray source 3FGL J1906.6+0720 appeared in both the second and third Fermi-LAT source catalogs, but even after years of searching, no associated radio or X-ray source had been found. A team of researchers, led by Colin Clark of the Max Planck Institute for Gravitational Physics, suspected that the source might be a gamma-ray pulsar. To confirm this, however, they needed to detect pulsed emission — something inherently difficult given the low photon count and the uncertain position of the source.

The team conducted a blind search for pulsations coming from the general direction of the gamma-ray source. Two things were needed for this search: clever data analysis and a lot of computing power. The data analysis algorithm was designed to be adaptive: it searched a 4-dimensional parameter space that included a safety margin, allowing the algorithm to wander if the source was at the edge of the parameter space. The computing power was contributed by tens of thousands of personal computers volunteered by participants in the Einstein@Home project, making much shorter work out of a search that would have required dozens of years on a single laptop.

Sky position of pulsar

The sky region around the newly discovered pulsar. The dotted ellipse shows the 3FGL catalog 95% confidence region for the source. The data analysis algorithm was designed to search an area 50% larger (given by the dashed ellipse), but it was allowed to “walk away” within the gray shaded region if the source seemed to be at the edge. This adaptive flexibility is what allowed the pulsar’s actual location — shown by the solid ellipse, inset — to be found. [Clark et al., 2015]

Teasing the Signal From Misinformation

This new search method turned out to be exactly what was needed: the team was able to identify pulsed emission coming from a point located significantly outside of the source’s original confidence region. This source, now named PSR J1906+0722, has been determined to be a young, energetic, isolated pulsar, and the team was able to identify its spin parameters and pulse history from the last six years.

So why was this mysterious pulsar so hard to find? Its misidentified position, most likely due to a secondary source that had been lumped with the pulsar into a single source, was the biggest factor. Additionally, the pulsar had an enormous glitch one year into Fermi observations of it, making it even more difficult to pin down its pulse profile.

The team’s success in spite of these complications indicates the value of their methods for identifying other young, gamma-ray, radio-quiet pulsars that are likely hidden in the Fermi-LAT catalog.

 

Citation

C. J. Clark et al. 2015 ApJ 809 L2 doi:10.1088/2041-8205/809/1/L2

hot Jupiter

Two researchers at the University of Chicago have recently developed a new theory to explain an apparent dichotomy in the orbits of planets around cool vs. hot stars. Their model proposes that the spins of cool stars are affected when they ingest hot Jupiters (HJs) early in their stellar lifetimes.

A Puzzling Dichotomy

In exoplanet studies, there is a puzzling difference observed between planet orbits around cool and hot (those with Teff ≥ 6250 K) stars: the orbital planes of planets around cool stars are primarily aligned with the host star’s spin, whereas the orbital planes of planets around hot stars seem to be randomly distributed.

Previous attempts to explain this dichotomy have focused on tidal interactions between the host star and the planets observed in the system. Now Titos Matsakos and Arieh Königl have taken these models a step further — by including in their calculations not only the effects of observed planets, but also those of HJs that may have been swallowed by the star long before we observed the systems.

Modeling Meals

Plots of the distribution of the obliquity λ for hot Jupiters around cool hosts (upper plot) and hot hosts (lower plot). The dashed line shows the initial distribution, the bins show the model prediction for the final distribution after the systems evolve, and the black dots show the current observational data. [Matsakos & Königl, 2015]

Plots of the distribution of the obliquity λ for hot Jupiters around cool hosts (upper plot) and hot hosts (lower plot). The dashed line shows the initial distribution, the bins show the model prediction for the final distribution after the systems evolve, and the black dots show the current observational data. [Matsakos & Königl, 2015]

The authors’ model assumes that as HJs are formed and migrate inward through the protoplanetary disk, they stall out near the star (where they have periods of ~2 days) and get stranded as the gas disk evaporates around them. Tidal interactions can cause these planets to become ingested by the host star within 1 Gyr.

Using Monte Carlo simulations, the authors model these star-planet tidal interactions and evolve a total of 10^6 systems: half with hot (Teff = 6400 K), main-sequence hosts, and half with cool (Teff = 5500 K), solar-type hosts. The initial obliquities — the angle between the stellar spin and the planets’ orbital angular momentum vectors — are randomly distributed between 0° and 180°.

The authors find that early stellar ingestion of planets might be very common: to match observations, roughly half of all stellar hosts must ingest an HJ early in their lifetimes!

This scenario results in a good match with observational data: about 50% of cool hosts’ spins become roughly aligned with the orbital plane of their planets after they absorb the orbital angular momentum of the HJ they ingest. Hot stars, on the other hand, generally retain their random distributions of obliquity, because their angular momentum is typically higher than the orbital angular momentum of the ingested planet.

 

Citation

Titos Matsakos and Arieh Königl 2015 ApJ 809 L20 doi:10.1088/2041-8205/809/2/L20

Local Group

When a dwarf galaxy falls into the halo of a large galaxy like the Milky Way, how is star formation in the dwarf affected? A collaboration led by Andrew Wetzel (California Institute of Technology and Carnegie Observatories) recently set out to answer this question using observations of nearby galaxies and simulations of the infall process.

Observed Quenching

Isolated dwarf galaxies tend to be gas-rich and very actively star-forming. In contrast, most dwarf galaxies within 300 kpc of us (the Milky Way’s virial radius) contain little or no cold gas, and they’re quiescent: there’s not much star formation happening.

And this isn’t just true of the Milky Way; we observe the same difference in the satellite galaxies surrounding Andromeda galaxy. Once a dwarf galaxy has moved into the gravitational realm of a larger galaxy, the satellite’s gas vanishes rapidly and its star formation is shut off — but how, and on what timescale?

Known dwarf galaxies

The known dwarf galaxies in the Local Group (out to 1.6 Mpc) are plotted by their distance from their host vs. their stellar mass. Blue stars indicate actively star-forming dwarfs and red circles indicate quiescent ones. Credit: Wetzel et al. 2015.

Timescales for Quiescence

To answer these questions, the authors explored the process of galaxy infall using Exploring the Local Volume in Simulations (ELVIS), a suite of cosmological N-body simulations intended to explore the Local Group. They combined the infall times from the simulations with observational knowledge of the fraction of nearby galaxies that are currently quiescent, in order to determine what timescales are required for different processes to deplete the gas in the dwarf galaxies and quench star formation.

Based on their results, two types of quenching culprits are at work: gas consumption (where a galaxy simply uses up its immediate gas supply and doesn’t have access to more) and gas stripping (where external forces like ram pressure remove gas from the galaxy).

These processes operate at different rates for different sizes of galaxies. The authors argue that for galaxies with stellar mass larger than 109 solar masses, the primary means of quenching is gas consumption. The timescale for this mechanism to quench the largest galaxies is roughly 5 Gyr. For galaxies with stellar mass smaller than 109 solar masses, gas stripping takes over, and star-formation is quenched within 1 Gyr for the smallest galaxies.

Neither quenching mechanisms operates efficiently for galaxies with stellar mass right around 109 solar masses, though, so these galaxies can sustain star formation for much longer. This could explain why the Magellanic clouds (which both have stellar mass of roughly 109 solar masses) are still star-forming despite being within the Milky Way’s halo!

 

Citation

Andrew R. Wetzel et al. 2015 ApJ 808 L27 doi:10.1088/2041-8205/808/1/L27

RGG 118

A team of astronomers have reported the detection of the smallest black hole (BH) ever observed in a galactic nucleus. The BH is hosted in the center of dwarf galaxy RGG 118, and it weighs in at 50,000 solar masses, according to observations made by Vivienne Baldassare of University of Michigan and her collaborators.

Small Discoveries

Why is the discovery of a small nuclear BH important? Some open questions that this could help answer are:

  • Do the very smallest dwarf galaxies have BHs at their centers too?
    Though we believe that there’s a giant BH at the center of every galaxy, we aren’t sure how far down the size scale this holds true.
  • What is the formation mechanism for BHs at the center of galaxies?
  • What’s the behavior of the M-sigma relation at the low-mass end?
    The M-sigma relation is an observed correlation between the mass of a galaxy’s central BH and the velocity dispersion of the stars in the galaxy. This relation is incredibly useful for determining properties of distant BHs and their galaxies empirically, but little data is available to constrain the low-mass end of the relation.
M-sigma relation

M-sigma relation, plotting systems with dynamically-measured black hole masses. RGG 118 is plotted as the pink star. The solid and dashed lines represent various determinations of scaling relations. Credit: Baldassare et al. 2015.

Identifying a Black Hole

RGG 118 was identified as a candidate host for an accreting, nuclear BH from the catalog of dwarf galaxies observed in the Sloan Digital Sky Survey. Baldassare and her team followed up with high-resolution spectroscopy from the Clay telescope in Chile and Chandra x-ray observations.

Using these observations, the team determined that RGG 118 plays host to a massive BH at its center based on three clues: 1) narrow emission line ratios, which is a signature of accretion onto a massive BH, 2) the presence of broad emission lines, indicating that gas is rotating around a central BH, and 3) the existence of an X-ray point source at the nucleus of the galaxy.

The spread in the broad emission lines was what allowed Baldassare and collaborators to estimate the mass of the BH, placing it firmly on the extrapolation of the M-sigma relation. In addition to helping us further understand this relation, this unique BH also constrains nuclear BH formation: we know that pathways must produce seeds at least this large! The group hopes that continued analysis of Sloan candidates might allow for the discovery of more such BHs at the centers of dwarf galaxies.

 

Citation

Vivienne F. Baldassare et al. 2015 ApJ 809 L14 doi:10.1088/2041-8205/809/1/L14

IR Galactic Center

Could stars be forming in the inhospitable environment near Sagittarius A* in the heart of the Milky Way? A possible signature of low-mass star formation has recently been found just two light-years from the black hole at the center of our galaxy — a region that was previously thought to be too hostile for such activity.

Searching for Signatures

Previous observations of the central few light-years of the Milky Way had focused on a population of about 200 massive, young and very bright stars in tight orbits around Sgr A*. These stars are only a few million years old and prompted scientists to wonder: have they somehow managed to form in situ, in spite of their close proximity to the black hole, or did they form further out and then migrate in?
Motivated by this mystery, Farhad Yusef-Zadeh of Northwestern University and collaborators looked for evidence of even younger stars close to Sagittarius A*, which would demonstrate that star formation in the area is an ongoing process. Using the Very Large Array (VLA), the collaboration discovered several small sources in one arm of activity near Sgr A*.

Proplyds

This 34-GHz image provides a close-up view of two protoplanetary disk candidates (labeled P26 and P8) located near Sgr A*. These objects are outlined on the right side by a bow shock caused by impacting stellar wind that streams from the young, hot stars closer to the Galactic center. The disks are thought to contain recently-formed, low-mass stars. (Credit: Yusef-Zadeh et al., 2015)

Heated Disks

The team identified these sources as candidate photoevaporative protoplanetary disks, or “proplyds” — areas of dense, ionized gas and dust surrounding young, newly formed stars. The proplyd candidates are between 10,000 and 100,000 years old, and they lie along the edge of a large molecular cloud. It is likely that this cloud produced the disks by providing a reservoir of gas to feed the star-formation activity.
The region surrounding these proplyds is blasted with harsh ultraviolet radiation streaming from hot stars orbiting close to Sgr A*. The gas of the proplyds is heated and stripped away by this radiation, forming bow shocks around the disks. Both the proplyds themselves and the bow shocks surrounding them are visible in Yusef-Zadeh’s observations.

Potential for Planet Formation

Unlike the young massive stars that have previously been identified in the galactic center, the proplyd candidates in this study are associated with low-mass stars. This has led to speculation that it may in fact be easier for low-mass stars to form in the hostile surroundings of the black hole than it is for them to form elsewhere in the Milky Way. In addition, the rate at which material is lost from such proplyds is expected to be low, so there is a chance for the disk to eventually form planets. With that comes the tantalizing possibility that as telescope resolution and data-analysis techniques improve, we may even be able to watch planet formation occur near Sgr A*.

Citation

F. Yusef-Zadeh et al. 2015 ApJ 801 L26 doi:10.1088/2041-8205/801/2/L26

Bonus:

Check out the authors’ video abstract below, which shows the locations of the proplyd candidates relative to Sgr A* and provides more information about how the observations were made and analyzed.

CR7

Elusive Population

Population III stars — the theoretical generation of extremely metal-poor stars that should have been formed in the early universe before metals existed — have been conspicuously absent in observations. But a team led by David Sobral (Institute of Astrophysics and Space Sciences, University of Lisbon, and Leiden Observatory) may have changed this paradigm with their recent detection of an extremely bright galaxy in the early universe.

The team’s broad survey of distant galaxies using ESO’s Very Large Telescope provides a glimpse of the universe as it was only 800 million years after the Big Bang. The survey uncovered several unusually bright galaxies — including the brightest galaxy ever seen at this distance, an important discovery by itself.

But further scrutiny of this galaxy, named CR7, produced an even more exciting find: a bright pocket of the galaxy contained no sign of any metals. Follow-up with other telescopes confirmed this initial detection.

Formation Waves

Sobral and his team postulate that we are observing this galaxy at just the right time to have caught a cluster of Population III stars — the bright, metal-free region of the galaxy — at the end of a wave of early star formation. The observations of CR7 also suggest the presence of regular stars in clumps around the metal-free pocket. These older, surrounding clusters may have formed stars first, helping to ionize a local bubble in the galaxy and allowing us to now observe the light from CR7.

It was previously thought that Population III stars might only be found in small, dim galaxies, making them impossible for us to detect. But CR7 provides an interesting alternative: this galaxy is bright, and the candidate Population III stars are surrounded by clusters of normal stars. This suggests that these first-generation stars might in fact be easier to detect than was originally thought.

Additional follow-up observations with other telescopes will help to confirm the identity of these stars. In particular, the James Webb Space Telescope is expected to further advance the pursuit of the earliest galaxies and stars in the universe.

Citation

David Sobral et al. 2015 ApJ 808 139 doi:10.1088/0004-637X/808/2/139

 

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

1 … 93 94 95 96