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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

Kepler supernova

Type Ia supernovae are known as “standard candles” due to their consistency, allowing us to measure distances based on their brightness. But what if these explosions aren’t quite as consistent as we thought? Due scientific diligence requires careful checks, so a recent study investigates whether the metallicity of a supernova’s environment affects the peak luminosity of the explosion.

Metallicity Dependence?

Type Ia supernovae are incredibly powerful tools for determining distances in our universe. Because these supernovae are formed by white dwarfs that explode when they reach a uniform accreted mass, the supernova peak luminosity is thought to be very consistent. This consistency allows these supernovae to be used as standard candles to measure distances to their host galaxies.

But what if that peak luminosity is affected by a factor that we haven’t taken into account? Theorists have proposed that the luminosities of Type Ia supernovae might depend on the metallicity of their environments — with high-metallicity environments suppressing supernova luminosities. If this is true, then we could be systematically mis-measuring cosmological distances using these supernovae.

Testing Abundances

luminosity-metallicity dependence

Supernova brightnesses vs. the metallicity of their environments. Low-metallicity supernovae (blue shading) and high-metallicity supernovae (red shading) have an average magnitude difference of ~0.14. [Adapted from Moreno-Raya et al. 2016]

A team led by Manuel Moreno-Raya, of the Center for Energy, Environment and Technology (CIEMAT) in Spain, has observed 28 Type Ia supernovae in an effort to test for such a metallicity dependence. These supernovae each have independent distance measurements (e.g., from Cepheids or the Tully-Fisher relation).

Moreno-Raya and collaborators used spectra from the 4.2-m William Herschel Telescope to estimate oxygen abundances in the region where each of these supernovae exploded. They then used these measurements to determine if metallicity of the local region affects the luminosity of the supernova.

Determining Distances

The authors find that there are indeed differences in peak supernova luminosity based on metallicity of the local environment. Their observations support a trend in which more metal-rich galaxies host less luminous supernovae, whereas lower-metallicity galaxies host supernovae with greater luminosities — consistent with theoretical predictions.

This observational confirmation suggests that the metallicity of the progenitor may well play a role in peak supernova luminosity and, as a result, the distances at which we estimate they exploded. This systematic effect can, however, be easily corrected for in the distance-estimate procedure.

As the number of known supernovae is expected to drastically increase with the start of future large surveys such as the Large Synoptic Survey Telescope (LSST) or the Dark Energy Survey (DES), supernova distance measurements will soon be dominated by systematic errors rather than statistical ones. Correctly accounting for effects such as this apparent metallicity-dependence of supernovae continues to be important for accurately determining distances using Type Ia supernovae as indicators.

Citation

Manuel E. Moreno-Raya et al 2016 ApJ 818 L19. doi:10.3847/2041-8205/818/1/L19

eclipsing binary

In KIC 9246715, two red-giant stars — twins in nearly every way — circle each other in a 171-day orbit. This binary pair may be a key to learning about masses and radii of stars with asteroseismology, the study of oscillations in the interiors of stars.

Two Ways to Measure

In order to understand a star’s evolution, it is critical that we know its mass and radius. Unfortunately, these quantities are often difficult to pin down!

One of the few cases in which we can directly measure stars’ masses and radii is in eclipsing binaries, wherein two stars eclipse each other as they orbit. If we have a well-sampled light curve for the binary, as well as radial velocities for both stars, then we can determine the stars’ complete orbital information, including their masses and radii.

But there may be another way to obtain stellar mass and radius: asteroseismology. In asteroseismology, oscillations inside stars are used to characterize the stellar interiors. Conveniently, if a star with a convective envelope exhibits solar-like oscillations, these oscillations can be directly compared to those of the Sun. Mass and radius scaling relations — which use the Sun as a benchmark and scale based on the star’s temperature — can then be used to derive the mass and radius of the star.

Test Subjects from Kepler

Solar-like oscillations from KIC 9246715 are shown in red across different resonant frequencies. The oscillations of a single red-giant star with similar properties are shown upside down in grey for reference. [Rawls et al. 2016]

Solar-like oscillations from KIC 9246715 are shown in red across different resonant frequencies. The oscillations of a single red-giant star with similar properties are shown upside down in grey for reference. [Rawls et al. 2016]

Of course, scaling relations are only useful if we can test them! A team of scientists including Meredith Rawls (New Mexico State University) has identified 18 red-giant eclipsing binaries in the Kepler field of view that also exhibit solar-like oscillations — perfect for testing the scaling relations.

In a recent study led by Rawls, the team analyzed the first of these binaries, KIC 9246715. Using the Kepler light curves in addition to radial velocity measurements from high-resolution ground-based spectroscopy at the Fred Lawrence Whipple Observatory and Apache Point Observatory, Rawls and collaborators established that the two stars have masses of 2.17 and 2.15 solar masses, and radii of 8.4 and 8.3 solar radii.

Not Quite Twins?

Intriguingly, when the authors measured the stellar oscillations from the binary, they were only able to pick out one signal. Using the scaling relations, their measurements reveal that the star producing the oscillations has a mass of 2.17 solar masses and radius of 8.3 radii — consistent with both red giants in the system, within error bars. This provides excellent confirmation of the scaling relations for obtaining mass and radius, but it also raises a new question: why is only one star of this twin system producing oscillations?

Rawls and collaborators have an idea: one star might be more magnetically active than the other, causing the suppression of oscillations in the more active star. The authors’ observations and detailed modeling support this idea, but similar analyses of the rest of the red-giant eclipsing binaries identified in the Kepler field will help to determine if KIC 9246715 is unusual, or if this behavior is common among such systems.

Citation

Meredith L. Rawls et al 2016 ApJ 818 108. doi:10.3847/0004-637X/818/2/108

Several months ago, the discovery of WD 1145+017 was announced. This white dwarf appears to be orbited by planetary bodies that are actively disintegrating due to the strong gravitational pull of their host. A follow-up study now reveals that this system has dramatically evolved since its discovery.

Signs of Disruption

Potential planetary bodies orbiting a white dwarf would be exposed to a particular risk: if their orbits were perturbed and they passed inside the white dwarf’s tidal radius, they would be torn apart. Their material could then form a debris disk around the white dwarf and eventually be accreted.

Interestingly, we have two pieces of evidence that this actually happens:

  1. We’ve observed warm, dusty debris disks around ~4% of white dwarfs, and
  2. The atmospheres of ~25-50% of white dwarfs are polluted by heavy elements that have likely accreted recently.

But in spite of this indirect evidence of planet disintegration, we’d never observed planetary bodies actively being disrupted around white dwarfs — until recently.

Unusual Transits

In April 2015, observations by Kepler’s K2 mission revealed a strange transit signal around WD 1145+017, a white dwarf 570 light-years from Earth that has both a dusty debris disk and a polluted atmosphere. This signal was interpreted as the transit of at least one, and possibly several, disintegrating planetesimals.

In a recent follow-up, a team of scientists led by Boris Gänsicke (University of Warwick) obtained high-speed photometry of WD 1145+017 using the ULTRASPEC camera on the 2.4m Thai National Telescope. These observations were taken in November and December of 2015 — roughly seven months after the initial photometric observations of the system. They reveal that dramatic changes have occurred in this short time.

Rapid Evolution

light curve

A sample light curve from TNT/ULTRASPEC, obtained in December 2015 over 3.9 hours. Many varied transits are evident (click for a better view!). Transits labeled in color appear across multiple nights. [Gänsicke et al. 2016]

Initial observations of WD 1145+017 showed a significant transit dip (>10%) only every ~3.6 hours, on average. In contrast, in the current observations, every light curve is riddled with numerous transit events that have durations of 3–12 minutes and depths of 10–60%. Many of the transit features overlap, so there are now only short segments of the light curve that don’t appear to be attenuated by debris.

Gänsicke and collaborators use the new data to analyze the transiting bodies. Though some transits are consistent from night to night, most evolve in shape and depth, appearing and disappearing over the course of the observing campaign. This rapid variability, along with the large size of the transiting bodies (several times the size of the white dwarf), support the conclusion that the transiting objects are not solid bodies. Instead, they are likely clouds of gas and dust flowing from smaller bodies that are being disrupted.

Because astronomical timescales are often extremely long, the observations of WD 1145+047 are especially exciting — this is a rare chance to watch a system evolve in real time! Given how rapidly it appears to be changing, continued observations are sure to soon reveal more about the planetary bodies orbiting this white dwarf.

Citation

B. T. Gänsicke et al 2016 ApJ 818 L7. doi:10.3847/2041-8205/818/1/L7

Big news: the Laser Interferometer Gravitational-Wave Observatory (LIGO) has detected its first gravitational-wave signal! Not only is the detection of this signal a major technical accomplishment and an exciting confirmation of general relativity, but it also has huge implications for black-hole astrophysics.

What did LIGO see?

LIGO is designed to detect the ripples in space-time created by two massive objects orbiting each other. These waves can reach observable amplitudes when a binary system consisting of two especially massive objects — i.e., black holes or neutron stars — reach the end of their inspiral and merge.

LIGO has been unsuccessfully searching for gravitational waves since its initial operations in 2002, but a recent upgrade in its design has significantly increased its sensitivity and observational range. The first official observing run of Advanced LIGO began 18 September 2015, but the instruments were up and running in “engineering mode” several weeks before that. And it was in this time frame — before official observing even began! — that LIGO spotted its first gravitational wave signal: GW150914.

LIGO

One of LIGO’s two detection sites, located near Hanford in eastern Washington. [LIGO]

The signal, detected on 14 September, 2015, provides astronomers with a remarkable amount of information about the merger that caused it. From the detection, the LIGO team has extracted the masses of the two black holes that merged, 36+5-4 and 29+4-4 solar masses, as well as the mass of the final black hole formed by the merger, ~62 solar masses. The team also determined that the merger happened roughly a billion light-years away (at a redshift of z~0.1), and the direction of the signal was localized to an area of ~600 square degrees (roughly 1% of the sky).

Why is this detection a big deal?

This is the first direct detection of gravitational waves, providing spectacular further confirmation of Einstein’s general theory of relativity. But the implications of GW150914 go far beyond this confirmation. This detection is a huge deal for astrophysics because it’s the first direct evidence we’ve had that:

  1. “Heavy” stellar-mass black holes exist.
    We’ve reliably measured black holes of masses up to 10–20 solar masses in X-ray binaries (binary systems in which a single neutron star or black hole accretes matter from a donor star). But this is the first proof we’ve found that stellar-mass black holes of >25 solar masses can form in nature.
  2. Binaries consisting of two black holes can form in nature.
    As we’ll discuss shortly, there are two theorized mechanisms for the formation of these black-hole binaries. Until now, however, there was no guarantee that either of those mechanisms worked!
  3. These black-hole binaries can inspiral and merge within the age of the universe.
    The formation of a black-hole binary is no guarantee that it will merge on a reasonable timescale: if the binary forms with enough separation, it could take longer than the age of the universe to merge. This detection proves that black-hole binaries can form with small enough separation to merge on observable timescales.

What can we learn from GW150914?

LIGO/Virgo sensitivity

Expected increase in sensitivity for LIGO/Virgo detectors is shown as a function of total system mass (x-axis) and surveyed volume (y-axis). The red star indicates the mass of GW150914. [Abbott et al. 2016]

For starters, we can throw out the lower estimates we had on merger rates. This event provides a new inferred binary-black-hole merger rate for the low-redshift universe of 2–400 Gpc-3 yr-1.

Another interesting conclusion about this binary system is that it probably formed in a low-metallicity environment (~ <1/2 solar metallicity). We infer this based on our current understanding of massive-star winds (which drive mass loss) and their dependence on metallicity: had the environment been high-metallicity, it is unlikely that such large black holes would have been able to form.

What can we learn from future gravitational-wave detections?

One of the key questions we’d like to answer is: how do binary black holes form? Two primary mechanisms have been proposed:

  1. A binary star system contains two stars that are each massive enough to individually collapse into a black hole. If the binary isn’t disrupted during the two collapse events, this forms an isolated black-hole binary.
  2. Single black holes form in dense cluster environments and then — because they are the most massive objects — sink to the center of the cluster. There they form pairs through dynamical interactions.

Now that we’re able to observe black-hole binaries through gravitational-wave detections, one way we could distinguish between the two formation mechanisms is from spin measurements. If we discover a clear preference for the misalignment of the two black holes’ spins, this would favor formation in clusters, where there’s no reason for the original spins to be aligned.

The current, single detection is not enough to provide constraints, but if we can compile a large enough sample of events, we can start to present a statistical case favoring one channel over the other.

What does GW150914 mean for the future of gravitational-wave detection?

The fact that Advanced LIGO detected an event even before the start of its first official observing run is certainly promising! The LIGO team estimates that the volume the detectors can probe will still increase by at least a factor of ~10 as the observing runs become more sensitive and of longer duration.

Virgo

Aerial view of the Virgo interferometer near Pisa, Italy. [Virgo Collaboration]

In addition, LIGO is not alone in the gravitational-wave game. LIGO’s counterpart in Europe, Virgo, is also undergoing design upgrades to increase its sensitivity. Within this year, Virgo should be able to take data simultaneously with LIGO, allowing for better localization of sources. And the launch of (e)LISA, ESA’s planned space-based interferometer, will grant us access to a new frequency range, opening a further window to the gravitational-wave sky.

The detection of GW150914 marks the dawn of a new field: observational gravitational-wave astronomy. This detection alone confirms much that was purely theory before now — and given that instrument upgrades are still underway, the future of gravitational-wave detection looks incredibly promising.

Bonus

This awesome video (produced by SXS lensing) shows an actual simulation of the black-hole merger GW150914. Time is slowed by a factor of 100, compared to the actual merger. The two black holes — of 29 and 36 solar masses — warp the space-time around them, causing the distorted view.

Citation

B.P. Abbott et al. 2016 ApJL 818 L22. doi:10.3847/2041-8205/818/2/L22

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