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Neutron-star X-ray binary

In X-ray binaries that consist of a neutron star and a companion star, gas funnels from the companion into an accretion disk surrounding the neutron star, spiraling around until it is eventually accreted. How do the powerful magnetic fields threading through the neutron star affect this accretion disk? Recent observations provide evidence that they may push the accretion disk away from the neutron star’s surface.

Truncated Disks

Theoretical models have indicated that neutron star accretion disks may not extend all the way in to the surface of a neutron star, but may instead be truncated at a distance. This prediction has been difficult to test observationally, however, due to the challenge of measuring the location of the inner disk edge in neutron-star X-ray binaries.

In a new study, however, a team of scientists led by Ashley King (Einstein Fellow at Stanford University) has managed to measure the location of the inner edge of the disk in Aquila X-1, a neutron-star X-ray binary located 17,000 light-years away.

Fe K line feature

Iron line feature detected by Swift (red) and NuSTAR (black). The symmetry of the line is one of the indicators that the disk is located far from the neutron star; if the inner regions of the disk were close to the neutron star, severe relativistic effects would skew the line to be asymmetric. [King et al. 2016]

Measurements from Reflections

King and collaborators used observations made by NuSTAR and Swift/XRT — both X-ray space observatories — of Aquila X-1 during the peak of an X-ray outburst. By observing the reflection of Aquila X-1’s emission off of the inner regions of the accretion disk, the authors were able to estimate the location of the inner edge of the disk.

The authors find that this inner edge sits at ~15 gravitational radii. Since the neutron star’s surface is at ~5 gravitational radii, this means that the accretion disk is truncated far from the star’s surface. In spite of this truncation, material still manages to cross the gap and accrete onto the neutron star — as evidenced by X-ray flaring (almost certainly caused by accretion) that occurred during the authors’ observations.

Magnetic Effects

What could cause the truncation of the disk? The authors believe the most likely factor is pressure from the neutron star’s sizable magnetic field, pushing the inner edge of the disk out. They calculate that a field strength of roughly 5*108 Gauss (for comparison, a typical refrigerator magnet has a field strength of ~100 G!) would be necessary to hold the inner edge this far out. This is consistent with previous estimates for the field of the neutron star in Aquila X-1.

The authors point out that magnetic field lines could also explain how the neutron star is still accreting material despite the gap between it and its disk: gas could be channeled along field lines from the inner edge of the disk — which is roughly co-rotating with the neutron star — onto the neutron star poles.

The observations of Aquila X-1’s truncated disk are an important step toward confirming models of how neutron stars’ magnetic fields interact with their accretion disks in X-ray binaries.

Citation

Ashley L. King et al 2016 ApJ 819 L29. doi:10.3847/2041-8205/819/2/L29

Galactic center

New radio images of the center of the Milky Way are providing an unprecedented view of the structure and processes occurring in the Galactic center.

Sgr A structure

JVLA images of Sgr A at 5.5 GHz. The large-scale, bright ring structure is Sgr A East, a supernova remnant. The mini-spiral structure along the lower-right edge of the ring is Sgr A West, and Sgr A* is located near the center of the mini-spiral structure. Click for a closer look! [Zhao et al. 2016]

Improved Radio View

A recent study led by Jun-Hui Zhao (Harvard-Smithsonian Center for Astrophysics) presents new images of the Galactic center using the Jansky Very Large Array (JVLA) at 5.5 GHz. The images center on the radio-bright zone at the core of our galaxy, with the field of view covering the central 13’ of the Milky Way — equivalent to a physical size of ~100 light-years.

Due to recent hardware and software improvements in the VLA, these images are much deeper than any previously obtained of the Galactic center, reaching an unprecedented 100,000:1 dynamic range. Not only do these observations provide a detailed view of previously known structures within the Sagittarius A radio complex in the Milky Way’s heart, but they also reveal new features that can help us understand the processes that formed this bright complex.

Features in Sagittarius A

Sgr A consists of three main components nested within each other: the supernova remnant Sgr A East, the mini-spiral structure Sgr A West (located off-center within the Sgr A East structure), and the compact radio source Sgr A* (located near the center of the mini-spiral). Sgr A* is the supermassive black hole that resides at the very center of the Milky Way.

The newest JVLA images reveal numerous filamentary sources that trace out two radio lobes, oriented nearly perpendicular to the Galactic plane and ~50 light-years in size. These are smaller radio counterparts to the enormous (on the scale of 30,000 light-years!) gamma-ray Fermi bubbles that have been observed to extend from the Galactic center. The bipolar radio structures appear to be due to winds emanating from Sgr A* itself, from a central cluster of massive stars, or from a combination of the two.

Top: Superposition of the JVLA image of Sgr A (blue) and a molecular line image taken with the SMA (red) that shows Sgr A*’s circumnuclear disk. Bottom left: Molecular emission is shown in contours, and the Sigma Front is traced by blue lines. Bottom right: The authors’ geometrical model for the supernova explosion and resulting emission. [Adapted from Zhao et al. 2016]

Top: superposition of the JVLA image of Sgr A (blue) and a molecular line image (red) showing Sgr A*’s circumnuclear disk. Bottom left: molecular emission is shown in contours, and the Sigma Front is traced by blue lines. Bottom right: a geometrical model for the supernova explosion and resulting emission. [Zhao et al. 2016]

Supernova Structures

The outermost shape of Sgr A East — which looks like an elliptical ring — is thought to be an expanding spherical shell from a past supernova explosion, appearing as an ellipse because of our angle of view. In the newest JVLA images, Zhao and collaborators identify a new structure inside of the ring that they term the “Sigma Front”.

The authors argue that this emission front — which is shaped like the capital Greek letter sigma — may be the reflection of the supernova blast wave bouncing off of the dense, clumpy circumnuclear molecular disk around Sgr A* (which encircles the mini-spiral, but isn’t visible in radio wavelengths). Under this assumption, they use the Sigma Front to constrain the geometry of the supernova explosion.

These new JVLA images contain a wealth of information in their detail, and analysis is only just beginning. Further examination of these images will continue to help us learn about the activity at the heart of our galaxy.

Citation

Jun-Hui Zhao et al 2016 ApJ 817 171. doi:10.3847/0004-637X/817/2/171

Circumbinary planets

What happens to Tattooine-like planets that are instead in unstable orbits around their binary star system? A new study examines whether such planets will crash into a host star, get ejected from the system, or become captured into orbit around one of their hosts.

Orbit Around a Duo

At this point we have unambiguously detected multiple circumbinary planets, raising questions about these planets’ formation and evolution. Current models suggest that it is unlikely that circumbinary planets would be able to form in the perturbed environment close their host stars. Instead, it’s thought that the planets formed at a distance and then migrated inwards.

One danger such planets face when migrating is encountering ranges of radii where their orbits become unstable. Two scientists at the University of Chicago, Adam Sutherland and Daniel Fabrycky, have studied what happens when circumbinary planets migrate into such a region and develop unstable orbits.

Producing Rogue Planets

Collisions and ejections

Time for planets to either be ejected or collide with one of the two stars, as a function of the planets’ starting distance (in AU) from the binary barycenter. Colors represent different planetary eccentricities. [Sutherland & Fabrycky 2016]

Sutherland and Fabrycky used N-body simulations to determine the fates of planets orbiting around a star system consisting of two stars — a primary like our Sun and a secondary roughly a tenth of its size — that are separated by 1 AU.

The authors find that the most common fate for a circumbinary planet with an unstable orbit is ejection from the system; over 80% of unstable planets were ejected. This has interesting implications: if the formation of circumbinary planets is common, this mechanism could be filling the Milky Way with a population of free-floating, “rogue” planets that no longer are associated with their host star.

The next most common outcome for unstable planets is collision with one of their host stars (most often the secondary), resulting in accretion of the planet onto the star. Only rarely do unstable planets make it through the 10,000-yr integration without being removed from the system via ejection or collision.

Tidal Effects

As a final experiment, the authors also added the effects of tidal stripping, which occurs when the stars of the binary tear away some of the planets’ mass during close encounters. They found that this alters the orbit of the planets that have close encounters with one of the stars, making it slightly more likely that they can be captured around a star.

How can we test these models? When a star tidally strips a planet or accretes a planet in a collision, this process leaves its mark on the star in the form of stellar pollution. By comparing the amount of planetary material in the two stars of a binary, it may be possible to confirm the rates predicted here — thereby answering the question of what happens to unstable Tattooines.

Citation

Adam P. Sutherland and Daniel C. Fabrycky 2016 ApJ 818 6. doi:10.3847/0004-637X/818/1/6

VCC 1287

A series of recent deep-imaging surveys has revealed dozens of lurking ultra-diffuse galaxies (UDGs) in nearby galaxy clusters. A new study provides key information to help us understand the origins of these faint giants.

What are UDGs?

There are three main possibilities for how UDGs — galaxies with the sizes of giants, but luminosities no brighter than those of dwarfs — formed:

  1. They are “tidal dwarfs”, created in galactic collisions when streams of matter were pulled away from the parent galaxies and halos to form dwarfs.
  2. They are descended from “normal” galaxies and were then altered by tidal interactions with the galaxy cluster.
  3. They are ancient remnant systems — large galaxies whose gas was swept away, putting an early halt to star formation. The gas removal did not, however, affect their large dark matter halos, which permitted them to survive in the cluster environment.

The key to differentiating between these options is to obtain mass measurements for the UDGs — how large are their dark matter halos? In a recent study led by Michael Beasley (Institute of Astrophysics of the Canary Islands, University of La Laguna), a team of astronomers has determined a clever approach for measuring these galaxies’ masses: examine their globular clusters.

Masses from Globular Clusters

halo mass vs. stellar mass

VCC 1287’s mass measurements put it outside of the usual halo-mass vs. stellar-mass relationships for nearby galaxies: it has a significantly higher halo mass than is normal, given its stellar mass. [Adapted from Beasley et al. 2016]

Beasley and collaborators selected one UDG, VCC 1287, from the Virgo galaxy cluster, and they obtained spectra of the globular clusters around it using the OSIRIS spectrograph on the Great Canary Telescope. They then determined VCC 1287’s total halo mass in two ways: first by using the dynamics of the globular clusters, and then by relying on a relation between total globular cluster mass and halo mass.

The two masses they found are in good agreement with each other; both are around 80 billion solar masses. This is an unprecedented factor of 3,000 larger than the stellar mass for the galaxy (obtained from the galaxy’s luminosity) — which means that VCC 1287 has an unusually large dark matter halo given its stellar population.

Clues to Origins

This result makes it unlikely that VCC 1287 is a tidal-dwarf system, since these usually have dark-matter fractions of less than 10%. The authors also don’t believe it is a tidally stripped system, since no obvious tidal features were revealed in their imaging. Instead, they think the most probable scenario is that VCC 1287 is a massive dwarf galaxy that had its star formation quenched by gas starvation as it fell into the Virgo cluster long ago.

To learn whether VCC 1287 is typical of UDGs, the authors encourage finding additional UDG masses using the same techniques outlined in this study. Additional observations of the globular-cluster populations for UDGs will significantly help understand these unusual galaxies.

Citation

Michael A. Beasley et al 2016 ApJ 819 L20. doi:10.3847/2041-8205/819/2/L20

Trans-Neptunian Object

How can we hope to measure the hundreds of thousands of objects in our distant solar system? A team of astronomers is harnessing citizen science to begin to tackle this problem!

RECON light curve

A light curve from an occultation collected by a RECON site in Quincy, California. As the object’s shadow passes, the background star’s light dims. [RECON/Charley Arrowsmith (Feather River College)]

Occultation Information

Estimates currently place the number of Kuiper belt objects larger than 100 km across at over 100,000. Knowing the sizes and characteristics of these objects is important for understanding the composition of the outer solar system and constraining models of the solar system’s formation and evolution.

Unfortunately, measuring small, dim bodies at large distances is incredibly difficult! One of the best ways to obtain the sizes of these objects is to watch as they occult a distant star. Timing the object as it passes across the face of the star can give us a good measure of its size and shape, when observed from multiple stations in the path of the shadow.

An Extended Network

Occultations by nearby objects (like main-belt asteroids) can be predicted fairly accurately, but those by trans-Neptunian objects are much more poorly constrained. Only ~900 trans-Neptunian objects have approximately known paths, and occultation-shadow predictions for these objects are often only accurate to ~1000km on the Earth’s surface. So how can we ensure that there’s a telescope in the right location, ready to observe when an occultation occurs?

RECON site map

Map of the 56 RECON sites distributed over 2000 km in the western United States. [Buie et al. 2016]

The simplest answer is to set up a huge network of observing stations, and wait for the shadows to come to the network. With this approach, even if the predicted path isn’t precisely known, some of the stations will still observe the occultation.

Due to the number of stations needed, this project lends itself perfectly to citizen science. In a recently published paper by Marc Buie (Southwest Research Institute) and John Keller (California Polytechnic State University), the team describes the Research and Education Collaborative Occultation Network (RECON).

RECON of Distant Objects

RECON consists of 56 communities in the western United States that have each been armed with a telescope, camera, and timing device. The observing groups include teachers and their students, amateur astronomers, and other community members, and telescopes are primarily located at schools.

Because the shadows from occultations generally travel from east to west, the communities are based in a roughly north-south network spanning 2000 km. They’re spaced no more than 50 km apart, providing enough coverage to obtain sizes for 100-km objects crossing the baseline.

RECON is a great example of how citizen science can be used to advance astronomy. The project reached full operating status in April 2015, and it has already conducted two official observing campaigns of trans-Neptunian objects, as well as roughly 30 additional campaigns, including training runs and local projects. The team is now publishing some of its first results in an upcoming paper, so keep an eye out for future publications to find out what they’ve learned!

Bonus

Check out this awesome video of an asteroid occulting a star, as observed by a RECON system. The grey field shows the actual video image collected by one of the RECON cameras, in which one of the two visible stars (the one on the right) is occulted. The asteroid itself is too dim for us to see. The inset at the top left shows the light curve collected during the occultation, and the upper right-hand corner shows an animation of the asteroid as it occults the star. [RECON]

Patroclus_small

Citation

Marc W. Buie and John M. Keller 2016 AJ 151 73. doi:10.3847/0004-6256/151/3/73

Globular cluster Fornax 5

Many satellites — dwarf galaxies and globular clusters — are thought to be orbiting our galaxy, but detecting them can be a tricky business. In particular, satellites can be disrupted by the galactic potential and spread out into streams, making them so diffuse that we’re unable to spot them in photometric observations.

In a recent study, a team of scientists led by John Vickers (Chinese Academy of Sciences) has cleverly worked around this difficulty by searching for groups of stars that have clustered velocities and metallicities differing from the background field.

Searching Through Stars

LAMOST stars

Radial velocity and metallicity of LAMOST stars near the physical location of Lamost 1. Circles are stars within 1.5° of the target location, small dots are stars within 5°. [Vickers et al. 2016]

The team trawled the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) catalog, which contains spectroscopic information for 2.5 million stars. Vickers and collaborators first hunted for stars that shared an approximate physical location and had similar velocities (because the stars of a satellite will maintain similar velocities even after the satellite is disrupted). Next, they discarded any of these clumps that didn’t also share a similar metallicity.

Vickers and collaborators then compared the resulting set of 21 candidate streams to catalogs of known globular clusters, open clusters, and nearby galaxies. Three of the candidate clumps, clustered in a 3° area on the sky, do not correspond to any known objects. The authors postulate that these are all part of a disrupted satellite, which they dub Lamost 1.

Characterizing a Former Cluster

Fitting the spectroscopic data for the member stars, the authors are able to estimate a number of characteristics of Lamost 1, with the best fit implying an age of 11 Gyr, a total mass of about 20,000 solar masses, and a distance from us of about 8,500 light-years.

Based on the stellar motions, the authors believe that the clump is on an eccentric orbit and is currently at its furthest distance from the Galactic center. They suggest that the elliptic orbit and advanced age of the clump indicate it is most likely to be a disrupted globular cluster, rather than a dwarf galaxy.

Interestingly, when the authors went back to search for a stellar overdensity corresponding to Lamost 1 in photometric data, they were unable to detect it. This reaffirms that their approach of searching for velocity and metallicity clumping is an important tool for discovering otherwise-invisible diffuse streams.

Bonus

Check out this cool graphic Vickers made using Stellarium and Aladin to demonstrate where in the sky the stars of Lamost 1 are located. Lamost 1’s stars are the red dots in the constellation Draco.

fig3

Citation

John J. Vickers et al 2016 ApJ 816 L2. doi:10.3847/2041-8205/816/1/L2

K2

Remember back in May 2013 when the second of Kepler’s reaction wheels failed, rendering it unable to control its precision pointing? As a result of a clever backup plan by intrepid scientists, Kepler is still going strong! This January, a paper was published describing some of the results from the first year of the extended Kepler mission, known as K2.

K2: A Second Chance

K2 histograms

Histograms of the K2 planet candidate sample (solid yellow) compared with planet candidates from the first four months of Kepler observations (blue diagonal lines). The histograms compare planet radius, orbital period, and brightness. [Vanderburg et al. 2016]

After an incredibly successful five years discovering transiting exoplanets, the failure of two of Kepler’s reaction wheels (which allow it to maintain its orientation) looked like it would shut down the mission. Luckily, the scientific community came up with the ingenious plan of stabilizing the telescope using the radiation pressure exerted by the Sun. Though this solution limits Kepler to observing within the ecliptic plane, it has provided a new life lease for the project.

Despite the significantly worsened pointing precision in the K2 mission, new analysis techniques have been developed that decouple the motion of the spacecraft from its observations, resulting in an observational precision for K2 that’s within 35% of the original precision achieved by Kepler.

Using these techniques, a team of scientists led by Andrew Vanderburg (Harvard–Smithsonian Center for Astrophysics) analyzed the publicly released data from the first year of the K2 mission. In a new study, they describe the results from the 59,174 targets that Kepler has observed in that time.

Planetary Candidates

Vanderburg and collaborators report that K2 has detected 234 planetary candidates around 208 stars in its first year. These candidates span a range of sizes from gas-giant to smaller than the Earth, and have orbital periods that range from hours to more than a month. The list includes:

  1. 26 candidates with sizes between 1 and 4 Earth radii, orbiting bright stars. These are well suited for precise radial velocity follow-up.
  2. 10 candidates with radii between 1.6 and 4 Earth radii that are likely to have gaseous envelopes. These are well suited for atmospheric characterization.
  3. 8 sub-Earth sized candidates, the smallest of which are about 0.75 times the size of Earth.

Vanderburg and collaborators make all of their data products (light curves, spectra, vetting diagnostics, etc.) publicly available. Their observations and data provide an excellent starting point for follow-up on the many potential planets discovered by K2 within the first year of its proposed three-year mission. And given this already long list of candidates, it’s clear that while Kepler’s power may have been reined in slightly, this telescope still has many more discoveries to show us.

Citation

Andrew Vanderburg et al 2016 ApJS 222 14. doi:10.3847/0067-0049/222/1/14

tidal disruption event

Tidal disruption events (TDEs) occur when a star passes a little too close to a supermassive black hole at the center of a galaxy. Tidal forces from the black hole cause the passing star to be torn apart, resulting in a brief flare of radiation as the star’s material accretes onto the black hole. A recent study asks the following question: do TDEs occur most frequently in an unusual type of galaxy?

A Trend in Disruptions

So far, we have data from eight candidate TDEs that peaked in optical and ultraviolet wavelengths. The spectra from these observations have shown an intriguing trend: many of these TDEs’ host galaxies exhibit weak line emission (indicating little or no current star-formation activity), and yet they show strong Balmer absorption lines (indicating star formation activity occurred within the last Gyr). These quiescent, Balmer-strong galaxies likely underwent a period of intense star formation that recently ended.

To determine if TDEs are overrepresented in such galaxies, a team of scientists led by Decker French (Steward Observatory, University of Arizona) has quantified the fraction of galaxies in the Sloan Digital Sky Survey (SDSS) that exhibit similar properties to those of TDE hosts.

Quantifying Overrepresentation

SDSS vs TDE-host galaxies

Spectral characteristics of SDSS galaxies (gray) and TDE candidate host galaxies (colored points): line emission vs. Balmer absorption. The lower right-hand box identifies the quiescent, Balmer-strong galaxies — which contain most TDE events, yet are uncommon among the galaxy sample as a whole. Click for a better look! [French et al. 2016]

French and collaborators compare the optical spectra of the TDE host galaxies to those of nearly 600,000 SDSS galaxies, using two different cutoffs for the Balmer absorption — the indicator of past star formation. Their strictest cut, filtering for very high Balmer absorption, selected only 0.2% of the SDSS galaxies, yet 38% of the TDEs are hosted in such galaxies. Using a more relaxed cutoff selects 2.3% of galaxies on average, yet includes the hosts of 75% of the TDEs.

This means that quiescent galaxies with strong past star formation are overrepresented in the TDE host galaxy sample by a factor of ~190 times. Quiescent galaxies with at least moderately strong past star formation are overrepresented among TDE hosts by a factor of ~33.

Why the Preference?

So why might these galaxies so frequently host TDEs? The authors propose an idea: many of these galaxies may have experienced recent galaxy–galaxy mergers. Such a merger could trigger a burst of star formation, perturb stellar orbits, and then eventually settle into a quiescent state with stars that are more likely to be centrally concentrated and with orbits that might lead them to pass close to the central black hole(s).

Future observations of more TDEs will certainly help to further evaluate this trend. But the current data certainly implies that TDEs are discriminating in their choice of host, providing interesting clues about the mechanisms driving their rates.

Citation

K. Decker French et al 2016 ApJ 818 L21. doi:10.3847/2041-8205/818/1/L21

Habitable zones

One of the main goals of exoplanet surveys like the Kepler mission is to find potentially habitable planets orbiting other stars. Finding planets in a star’s habitable zone, however, is easier when we know in advance where to look! A recent study has provided us with a starting point.

Defining the Zone

A habitable zone is defined as the range of distances from a star where liquid water could exist on an orbiting planet, given a dense enough planetary atmosphere. The habitable zone can be calculated from the star’s parameters, and the inner and outer edges of a habitable zone are set considering hypothetical planetary atmospheres of different composition.

Knowing the parameters of the habitable zones around nearby stars is important for current and future exoplanet surveys, as this information allows them to identify stars with habitable zones that can be probed, given the survey’s sensitivity. To provide this target selection tool, a team of scientists led by Colin Chandler (San Francisco State University) has created a catalog of the habitable zones of roughly 37,000 nearby, main-sequence stars.

Habitable-zone widths

Distribution of habitable-zone widths found in CELESTA, for conservative and optimistic measurements. [Chandler et al. 2016]

Selecting for Sun-Like Stars

The Catalog of Earth-Like Exoplanet Survey Targets, or CELESTA, was built starting with the Revised Hipparcos Catalog, a high-precision catalog of photometry and parallax measurements (which provides the star’s distance) for 117,955 bright, nearby stars. Chandler and collaborators combined these measurements with stellar models to determine parameters such as effective temperature, radius, and mass of the stars.

The authors exclude giant stars and cool dwarfs, choosing to focus on main-sequence stars within the temperature range 2600–7200K, more similar to the Sun. They test their derived stellar parameters by comparing to observational data from the Exoplanet Data Explorer (EDE), where available, and confirm that their photometrically derived stellar parameters agree well with the parameters in EDE, typically measured spectroscopically.

Providing Survey Targets

Survey sensitivity

Plot showing the number of stellar habitable zones that can be probed by a survey, based on how long the stars are observed in the surveys. Surveys listed as reference points are TESS at 27 days, K2 at 75 days, PLATO at 180 days, HARPS at 6 years, and AAPS at 15 years. [Chandler et al. 2016]

The final CELESTA catalog details the habitable zones of 37,354 bright, main-sequence stars. The stars’ habitable-zone widths are generally under 5 AU, with the majority falling between 1 and 1.5 AU. The authors also provide an estimate of how many of these habitable zones current surveys (like Kepler) and upcoming surveys (like the Transiting Exoplanet Survey Satellite, or TESS) will be able to probe, based on the duration of the surveys’ typical campaigns.

Though a planet’s potential for habitability relies on additional factors besides the location of its orbit, cataloging the locations of stellar habitable zones for nearby, observable stars is an important start. CELESTA is an excellent reference for this, and it will provide a living resource that the authors plan to continue to update with additional stars, as well as with improved-accuracy stellar measurements, expected from upcoming astrometric missions.

Citation

Colin Orion Chandler et al 2016 AJ 151 59. doi:10.3847/0004-6256/151/3/59

Reionization

During the period of reionization that followed the “dark ages” of our universe, hydrogen was transformed from a neutral state, which is opaque to radiation, to an ionized one, which is transparent to radiation. But what generated the initial ionizing radiation? The recent discovery of multiple distant galaxies offers evidence for how this process occurred.

Two Distant Galaxies

We believe reionization occurred somewhere between a redshift of z = 6 and 7, because Lyα-emitting galaxies drop out at roughly this redshift. Beyond this distance, we’re generally unable to see the light from these galaxies, because the universe is no longer transparent to their emission. This is not always the case, however: if a bubble of ionized gas exists around a distant galaxy, the radiation can escape, allowing us to see the galaxy.

This is true of two recently-discovered Lyα-emitting galaxies, confirmed to be at a redshift of z~7 and located near one another in a region known as the Bremer Deep Field. The fact that we’re able to see the radiation from these galaxies means that they are in an ionized HII region — presumably one of the earlier regions to have become reionized in the universe.

But on their own, neither of these galaxies is capable of generating an ionized bubble large enough for their light to escape. So what ionized the region around them, and what does this mean for our understanding of how reionization occurred in the universe?

A Little Help From Friends

BDF objects

Location in different filters of the objects in the Hubble Bremer Deep Field catalog. The z~7 selection region is outlined by the grey box. BDF-521 and BDF-3299 were the two originally discovered galaxies; the remaining red markers indicate the additional six galaxies discovered in the same region. [Castellano et al. 2016]

A team of scientists led by Marco Castellano (Rome Observatory, INAF) investigated the possibility that there are other, faint galaxies near these two that have helped to ionize the region. Performing a survey using deep field Hubble observations, Castellano and collaborators found an additional 6 galaxies in the same region as the first two, also at a redshift of z~7!

The authors believe these galaxies provide a simple explanation of the ionized bubble: each of these faint, normal galaxies produced a small ionized bubble. The overlap of these many small bubbles provided the larger ionized region from which the light of the two originally discovered galaxies was able to escape.

How normal is this clustering of galaxies found by Castellano and collaborators? The team demonstrates via cosmological modeling that the number density of galaxies in this region is a factor of 3–4 greater than would be expected at this distance in a random pointing of the same size.

These results greatly support the theoretical prediction that the first ionization fronts in the universe were formed in regions with significant galaxy overdensities. The discovery of this deep-field collection of galaxies strongly suggests that reionization was driven by faint, normal star-forming galaxies in a clumpy process.

Citation

M. Castellano et al 2016 ApJ 818 L3. doi:10.3847/2041-8205/818/1/L3

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