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A new type of galaxy has just been added to the galaxy zoo: a small, compact, and old elliptical galaxy that shows signs of a monster black hole actively accreting material in its center. What can this unusual discovery tell us about how compact elliptical galaxies form?

A New Galactic Beast

Compact elliptical galaxies are an extremely rare early-type dwarf galaxy. Consistent with their name, compact ellipticals are small, very compact collections of ancient stars; these galaxies exhibit a high surface brightness and aren’t actively forming stars.

SDSS J085431.18+173730.5

Optical view of the ancient compact elliptical galaxy SDSS J085431.18+173730.5 (center of image) in an SDSS color composite image. [Adapted from Paudel et al. 2016]

Most compact ellipticals are found in dense environments, particularly around massive galaxies. This has led astronomers to believe that compact ellipticals might form via the tidal stripping of a once-large galaxy in interactions with another, massive galaxy. In this model, once the original galaxy’s outer layers are stripped away, the compact inner bulge component would be left behind as a compact elliptical galaxy. Recent discoveries of a few isolated compact ellipticals, however, have strained this model.

Now a new galaxy has been found to confuse our classification schemes: the first-ever compact elliptical to also display signs of an active galactic nucleus. Led by Sanjaya Paudel (Korea Astronomy and Space Science Institute), a team of scientists discovered SDSS J085431.18+173730.5 serendipitously in Sloan Digital Sky Survey data. The team used SDSS images and spectroscopy in combination with data from the Canada-France-Hawaii Telescope to learn more about this unique galaxy.

Puzzling Characteristics

SDSS J085431.18+173730.5 presents an interesting conundrum. Ancient compact ellipticals are supposed to be devoid of gas, with no fuel left to trigger nuclear activity. Yet SDSS J085431.18+173730.5 clearly shows the emission lines that indicate active accretion onto a supermassive black hole of ~2 million solar masses, according to the authors’ estimates. Paudel and collaborators show that this mass is consistent with the low-mass extension of the known scaling relation between central black-hole mass and brightness of the host galaxy.

BH mass v. bulge brightness scaling relation

Central black hole mass vs. bulge K-band magnitude. SDSS J085431.18+173730.5 (red dot) falls right on the low-mass extension of the observed scaling relation. It has similar properties to M32, another compact elliptical galaxy. [Adapted from Paudel et al. 2016]

To add to the mystery, SDSS J085431.18+173730.5 has no nearby neighbors: like the few other isolated compact ellipticals recently discovered, there are no massive galaxies in the immediate vicinity that could have led to its tidal stripping. So how was this puzzling ancient galaxy formed?

The authors of this study support a previously proposed “flyby” scenario: isolated compact ellipticals may simply be tidally stripped systems that ran away from their hosts. Paudel and collaborators suggest that SDSS J085431.18+173730.5 might have long ago interacted with NGC 2672 — a galaxy group located a whopping 6.5 million light-years away — before being flung out to its current location.

Further studies of this unique galaxy’s emission profile, as well as efforts to learn about its underlying stellar population and central kinematics, will hopefully help us to better understand not only the origins of this galaxy, but how all compact ellipticals form and evolve.

Citation

Sanjaya Paudel et al 2016 ApJ 820 L19. doi:10.3847/2041-8205/820/1/L19

TYC-2505-672-1

A new record holder exists for the longest-period eclipsing binary star system: TYC-2505-672-1. This intriguing system contains a primary star that is eclipsed by its companion once every 69 years — with each eclipse lasting several years!

120 Years of Observations

In a recent study, a team of scientists led by Joseph Rodriguez (Vanderbilt University) characterizes the components of TYC-2505-672-1. This binary star system consists of an M-type red giant star that undergoes a ~3.45-year-long, near-total eclipse with a period of ~69.1 years. This period is more than double that of the previous longest-period eclipsing binary!

Rodriguez and collaborators combined photometric observations of TYC-2505-672-1 by the Kilodegree Extremely Little Telescope (KELT) with a variety of archival data, including observations by the American Association of Variable Star Observers (AAVSO) network and historical data from the Digital Access to a Sky Century @ Harvard (DASCH) program.

In the 120 years spanned by these observations, two eclipses are detected: one in 1942-1945 and one in 2011-2015. The authors use the observations to analyze the components of the system and attempt to better understand what causes its unusual light curve.

Characterizing an Unusual System

TYC-2505-672-1 light curve

Observations of TYC-2505-672-1 plotted from 1890 to 2015 reveal two eclipses. (The blue KELT observations during the eclipse show upper limits only.) [Rodriguez et al. 2016]

By modeling the system’s emission, Rodriguez and collaborators establish that TYC-2505-672-1 consists of a 3600-K primary star — that’s the M giant — orbited by a small, hot, dim companion that’s a toasty 8000 K. But if the companion is small, why does the eclipse last several years?

The authors argue that the best model of TYC-2505-672-1 is one in which the small companion star is surrounded by a large, opaque circumstellar disk. Rodriguez and collaborators suggest that the companion could be a former red giant whose atmosphere was stripped from it, leaving behind the small, hot core shrouded by a large, cool disk of stripped gas. The large size of the disk causes the eclipse of the primary to last for years, as viewed from Earth.

The authors estimate the properties such a disk would need to produce the observed light curve. They find that if the companion were surrounded by a disk several AU in diameter, it could orbit at a distance of ~20-30 AU from the primary and reproduce the emission we see.

The next eclipse of TYC-2505-672-1 will begin in April 2080. We needn’t wait until then to gather more information about this system, however! Radial velocity measurements will help establish the masses of the two components, and high-cadence UV observations could reveal more about the evolutionary state of the system. Studying this extreme binary provides an excellent opportunity to learn more about the environments in late-life star systems.

Citation

Joseph E. Rodriguez et al 2016 AJ 151 123. doi:10.3847/0004-6256/151/5/123

Stephan's Quintet

In this age of large astronomical surveys, one major scientific bottleneck is the analysis of enormous data sets. Traditionally, this task requires human input — but could computers eventually take over? A pair of scientists explore this question by testing whether computers can classify galaxies as well as humans.

classification disagreement

Examples of disagreement: galaxies that Galaxy-Zoo humans classified as spirals with >95% agreement, but the computer algorithm classified as ellipticals with >70% certainty. Most are cases where the computer got it wrong — but not all of them. [Adapted from Kuminski et al. 2016]

Limits of Citizen Science

Galaxy Zoo is an internet-based citizen science project that uses non-astronomer volunteers to classify galaxy images. This is an innovative way to provide more manpower, but it’s still only practical for limited catalog sizes. How do we handle the data from upcoming surveys like the Large Synoptic Survey Telescope (LSST), which will produce billions of galaxy images when it comes online?

In a recent study by Evan Kuminski and Lior Shamir, two computer scientists at Lawrence Technological University in Michigan, a machine learning algorithm known as Wndchrm was used to classify a dataset of Sloan Digital Sky Survey (SDSS) galaxies into ellipticals and spirals. The authors’ goal is to determine whether their algorithm can classify galaxies as accurately as the human volunteers for Galaxy Zoo.

Automatic Classification

After training their classifier on a small set of spiral and elliptical galaxies, Kuminski and Shamir set it loose on a catalog of ~3 million SDSS galaxies. The classifier first computes a set of 2,885 numerical descriptors (like textures, edges, and shapes) for each galaxy image, and then uses these descriptors to categorize the galaxy as spiral or elliptical.

Rate of agreement

Rate of agreement of the computer classification with human classification (for the Galaxy Zoo “superclean” subset) for different ranges of computed classification certainties. For certainties above 54% for spirals and 80% for ellipticals, the agreement is over 98%. [Kuminski et al. 2016]

In addition, the classifier calculates a certainty level for each classification, with the certainties adding to 100%: a galaxy categorized as spiral at 85% certainty is categorized as elliptical at 15% certainty. This provides a quantity/quality tradeoff, allowing for the creation of subcatalogs by cutting at specific certainty levels. Selecting for a high level of certainty decreases the sample size, but increases the sample’s classification accuracy.

Comparing the Outcome

To evaluate the accuracy of the algorithm’s findings, the authors examined SDSS galaxies that had also been classified by Galaxy Zoo. In particular, they used a 45,000-galaxy subset that consists only of “superclean” Galaxy Zoo galaxies — meaning the human volunteers who categorized them were in agreement at a level of 95% or higher.

Numbers of galaxies

Number of spiral and elliptical galaxies classified above different certainty levels. Cutting at the 54% certainty level for spirals and 80% for ellipticals leaves ~900,000 and ~600,000 spiral and elliptical galaxies, respectively. [Kuminski et al. 2016]

In this set, Kuminski and Shamir found that if they draw a cut-off at the 54% certainty level for spiral galaxies and the 80% certainty level for ellipticals, they find 98% agreement between the computer classification of the galaxies and the human classification via Galaxy Zoo. Applying these cuts to the entire sample resulted in the identification of ~900,000 spiral galaxies and ~600,000 ellipticals, representing the largest catalog of its kind.

The authors acknowledge that completeness is a problem; half the data had to be cut to achieve this level of accuracy. Sacrificing some data can still result in very large catalogs, however — and as surveys become more powerful and large databases become more prevalent, algorithms such as this one will likely become critical to the scientific process.

Citation

Evan Kuminski and Lior Shamir 2016 ApJS 223 20. doi:10.3847/0067-0049/223/2/20

Planet Nine

Recent studies have identified signs of an unseen, distant ninth planet in our solar system. How might we find the elusive Planet Nine? A team of scientists suggests the key might be cosmology experiments.

A Hypothetical Planet

Kuiper orbits

Orbits of six distant Kuiper-belt objects. Their clustered perihelia and orbital orientations suggest they may have been shepherded by a massive object, hypothesized to be Planet Nine. [Caltech/Robert Hurt]

Early this year, a study was published that demonstrated that the clustered orbits of distant Kuiper belt objects (and several other features of our solar system) can be explained by the gravitational tug of a yet-undiscovered planet. This hypothetical Planet Nine is predicted to be a giant planet similar to Neptune or Uranus, with a mass of more than ~10 Earth masses, currently orbiting ~700 AU away.

In a recent study, a team of scientists led by Nicolas Cowan (McGill University in Canada) has estimated the blackbody emission expected from Planet Nine. The team proposes how we might be able to search for this distant body using its heat signature.

Heat from an Icy World

Cowan and collaborators first estimate Planet Nine’s effective temperature, based on the solar flux received at ~700 AU and assuming its internal heating is similar to Uranus or Neptune. They find that Planet Nine’s effective temperature would likely be an icy ~30–50 K, corresponding to a blackbody peak at 50–100 micrometers.

Planet Nine search space

Search space for Planet Nine. Based on its millimeter flux and annual parallax motion, several current and future cosmology experiments may be able to detect it. Experiments’ resolution ranges are shown with blue boxes. [Cowan et al. 2016]

How can we detect an object with emission that peaks in this range? Intriguingly, cosmology experiments monitoring the cosmic microwave background (CMB) radiation are optimized for millimeter flux. At a wavelength of 1mm, Cowan and collaborators estimate that Planet Nine would have a very detectable flux level of ~30 mJy. The authors propose that CMB experiments with high enough resolution (~5m telescopes and larger) could have the ability to detect Planet Nine!

There’s one major catch: how can we differentiate between Planet Nine and the ~4000 foreground asteroids that are brighter than 30 mJy at millimeter wavelengths?

Cowan and collaborators argue that this can be done using a combination of asteroid databases and parallax measurements. The authors calculate that Planet Nine should move roughly a few arcseconds per day, mostly due to parallax. In comparison, asteroids will move ~10 arcminutes per day in a combination of proper motion and parallax — an order of magnitude faster than Planet Nine.

Resolution Constraints

To hunt down Planet Nine, we therefore need telescopes that can not only resolve a 30 mJy point source, but can also resolve an annual parallax motion of ~5 arcminutes per year.

The authors demonstrate that several current and planned CMB experiments have the resolution and ability to detect Planet Nine, provided that they map large swatches of the sky and return to the same regions every few months. These experiments include CCAT, the South Pole Telescope, the Atacama Cosmology Telescope, CMB-S4, and even possibly Planck.

With the astronomical community coming together to brainstorm ways to track down this elusive possible planet, the use of CMB experiments is an intriguing option. And even if Planet Nine is discovered by other means, measuring its heat signature will teach us more about the internal workings of giant planets.

Citation

Nicolas B. Cowan et al 2016 ApJ 822 L2. doi:10.3847/2041-8205/822/1/L2

Fornax cluster

Traditionally, dense cluster centers are cannibalistic environments, with larger galaxies stripping stars from smaller interlopers in minor mergers and dynamical harassment. A recent survey of the Fornax cluster, one example of such an environment, reveals how this cluster may have been built.

Clues in Halos

Fornax cluster context

Context for the southern constellation Fornax (“the furnace”). The Fornax cluster is marked with a red circle. [ESO, IAU and Sky & Telescope]

Deep surveys of dense cluster environments are necessary because the imprint of mass assembly is hidden in galactic halos, the faint outer regions of galaxies. Deep observations can reveal answers to questions about how the galaxies in these extreme environments formed and evolved — for instance, did the majority of the galaxies’ stars form in situ, or were they accreted from interactions with other galaxies?

The Fornax Deep Survey (FDS) is just such a campaign. FDS uses the European Southern Observatory’s VLT Survey Telescope to obtain deep photometry of the entire 26 square degrees of the Fornax cluster, a spectacular galaxy cluster located 65 million light-years away.

Central Observations

The FDS team plans to release the full results from the survey soon. For now, in an initial study led by Enrichetta Iodice (INAF’s Astronomical Observatory of Capodimonte, Italy), the team presents their first findings from the two square degrees around NGC 1399, a supergiant elliptical galaxy in the cluster center.

The two main results from this study are:

  1. The discovery of a faint stellar bridge between NGC 1399 and a nearby galaxy, NGC 1387.
  2. The characterization of NGC 1399’s light profile, which shows that the galaxy consists of two main components separated by a strong break. The bright central galaxy is likely composed of stars that formed in situ, whereas the exponential outer component is a stellar halo composed of stars likely captured from accretion events.

What do these points tell us about the history of the center of the Fornax cluster? These observations are indications that the Fornax cluster was built up by mergers and accretion events.

A Violent Past

The light profile the authors found is consistent with those of simulated galaxies whose halos were formed through the multiple accretion of progenitors. This suggests that the stellar halo of NGC 1399 has been through a major merging event.

This enlarged view of NGC 1399 and 1387 in the g band (top) and g–i band (bottom) gives a better view of the faint stellar stream connecting the two galaxies. [Iodice et al. 2016]

This enlarged view of NGC 1399 and 1387 in the g band (top) and g–i band (bottom) gives a better view of the faint stellar stream connecting the two galaxies. North is up and east is left. [Iodice et al. 2016]

The faint stellar bridge is likely a sign of an ongoing interaction between NGC 1399 and NGC 1387, in which NGC 1387’s outer envelope on its east side is being stripped away. But besides this indication, there is little evidence for recent merger activity, which would usually produce a significant number of luminous stellar streams and tidal tails.

The authors argue that this means that any major mergers in the Fornax cluster center probably happened in an early formation epoch. The cluster is now in a more dynamically evolved stage, in which most of the gravitational interactions between galaxies have already taken place.

Follow-up kinematics studies will be crucial to further interpreting these photometric observations from the center of the Fornax cluster. In the meantime, keep an eye out for future results from FDS!

Citation

E. Iodice et al 2016 ApJ 820 42. doi:10.3847/0004-637X/820/1/42

M-dwarf planets

M-dwarf stars are excellent targets for planet searches because the signal of an orbiting planet is relatively larger (and therefore easier to detect!) around small, dim M dwarfs, compared to Sun-like stars. But are there better or worse stars to target within this category when searching for habitable, Earth-like planets?

Confusing the Signal

Radial velocity campaigns search for planets by looking for signatures in a star’s spectra that indicate the star is “wobbling” due to the gravitational pull of an orbiting planet. Unfortunately, stellar activity can mimic the signal of an orbiting planet in a star’s spectrum — something that is particularly problematic for M dwarfs, which can remain magnetically active for billions of years. To successfully detect planets that orbit in their stars’ habitable zones, we have to account for this problem.

In a recent study led by Elisabeth Newton (Harvard-Smithsonian Center for Astrophysics), the authors use literature measurements to examine the rotation periods for main-sequence, M-type stars. They focus on three factors that are important for detecting and characterizing habitable planets around M dwarfs:

  1. Whether the habitable-zone orbital periods coincide with the stellar rotation
    False planet detections caused by stellar activity often appear as a “planet” with an orbital period that’s a multiple of the stellar rotation period. If a star’s rotation period coincides with the range of orbital periods corresponding to its habitable zone, it’s therefore possible to obtain false detections of habitable planets.
  2. How long stellar activity and rapid rotation last in the star
    All stars become less magnetically active and rotate more slowly as they age, but the rate of this decay depends on their mass: lower-mass stars stay magnetically active for longer and take longer to spin down.
  3. Whether detailed atmospheric characterization will be possible
    It’s ideal to be able to follow up on potentially habitable exoplanets, and search for biosignatures such as oxygen in the planetary atmosphere. This type of detection will only be feasible for low-mass dwarfs, however, due to the relative size of the star and the planet.

An Ideal Range

rotation period vs. mass

Stellar rotation period as a function of stellar mass. The blue shaded region shows the habitable zone as a function of stellar mass. For M dwarfs between ~0.25 and ~0.5 solar mass, the habitable-zone period overlaps with the stellar rotation period. [Newton et al. 2016]

Newton and collaborators find that stars in the mass range of 0.25 to 0.5 solar mass (stellar class M1V-M4V) are non-ideal targets, because their stellar rotation periods (or a multiple thereof) coincide with the orbital periods of their habitable zones. In addition, atmospheric characterization will only be feasible in the near future for stars with mass less than ~0.25 solar mass.

On the other hand, dwarfs with mass less than ~0.1 solar masses (stellar classes later than M6V) will retain their stellar activity and faster rotation rates throughout most of their lifetimes, making them non-ideal targets as well.

When searching for habitable exoplanets, the best targets are therefore the mid M dwarfs in the mass range of 0.1 to 0.25 solar mass (stellar class M4V-M6V). Building a sample focused on these stars will reduce the likelihood that planets found in the stars’ habitable zones are false detections. This will hopefully produce a catalog of potentially habitable exoplanets that we can eventually follow up with atmospheric observations.

Citation

Elisabeth R. Newton et al 2016 ApJ 821 L19. doi:10.3847/2041-8205/821/1/L19

Arp 220

Recent reanalysis of data from the Fermi Gamma-ray Space Telescope has resulted in the first detection of high-energy gamma rays emitted from a nearby galaxy. This discovery reveals more about how supernovae interact with their environments.

Colliding Supernova Remnant

After a stellar explosion, the supernova’s ejecta expand, eventually encountering the ambient interstellar medium. According to models, this generates a strong shock, and a fraction of the kinetic energy of the ejecta is transferred into cosmic rays — high-energy radiation composed primarily of protons and atomic nuclei. Much is still unknown about this process, however. One open question is: what fraction of the supernova’s explosion power goes into accelerating these cosmic rays?

In theory, one way to answer this is by looking for gamma rays. In a starburst galaxy, the collision of the supernova-accelerated cosmic rays with the dense interstellar medium is predicted to produce high-energy gamma rays. That radiation should then escape the galaxy and be visible to us.

Pass 8 to the Rescue

Observational tests of this model, however, have been stumped by Arp 220. This nearby ultraluminous infrared galaxy is the product of a galaxy merger ~700 million years ago that fueled a frenzy of starbirth. Due to its dusty interior and extreme levels of star formation, Arp 220 has long been predicted to emit the gamma rays produced by supernova-accelerated cosmic rays. But though we’ve looked, gamma-ray emission has never been detected from this galaxy … until now.

In a recent study, a team of scientists led by Fang-Kun Peng (Nanjing University) reprocessed 7.5 years of Fermi observations using the new Pass 8 analysis software. The resulting increase in resolution revealed the first detection of GeV emission from Arp 220!

Acceleration Efficiency

scaling relation

Gamma-ray luminosity vs. total infrared luminosity for LAT-detected star-forming galaxies and Seyferts. Arp 220’s luminosities are consistent with the scaling relation. [Peng et al. 2016]

Peng and collaborators argue that this emission is due solely to cosmic-ray interactions with interstellar gas. This picture is supported by the lack of variability in the emission, and the fact that Arp 220’s gamma-ray luminosity is consistent with the scaling relation between gamma-ray and infrared luminosity for star-forming galaxies. The authors also argue that, due to Arp 220’s high gas density, all cosmic rays will interact with the gas before escaping.

Under these two assumptions, Peng and collaborators use the gamma-ray luminosity and the known supernova rate in Arp 220 to estimate how efficiently cosmic rays are accelerated by supernova remnants in the galaxy. They determine that 4.2 ± 2.6% of the supernova remnant’s kinetic energy is used to accelerate cosmic rays above 1 GeV.

This is the first time such a rate has been measured directly from gamma-ray emission, but it’s consistent with estimates of 3-10% efficiency in the Milky Way. Future analysis of other ultraluminous infrared galaxies like Arp 220 with Fermi (and Pass 8!) will hopefully reveal more about these recent-merger, starburst environments.

Citation

Fang-Kun Peng et al 2016 ApJ 821 L20. doi:10.3847/2041-8205/821/2/L20

47 Tuc

Observations of globular clusters — gravitationally-bound, spherical clusters of stars that orbit galaxies as satellites — are critical to studies of galactic and stellar evolution. What type of galaxies host the largest total number of globular clusters in today’s universe? A recent study answers this question.

GCs per host gal luminosity

Total number of globular clusters vs. host galaxy luminosity for a catalog of ~400 galaxies of all types. [Harris 2016]

Globular Favoritism

Globular clusters can be found in the halos of all galaxies above a critical brightness of about 107 solar luminosities (in practice, all but the smallest of dwarfs). The number of globulars a galaxy hosts is related to its luminosity: the Milky Way is host to ~150 globulars, the slightly brighter Andromeda galaxy may have several hundred globulars, and the extremely bright giant elliptical galaxy M87 likely has over ten thousand.

But the number of galaxies is not evenly distributed in luminosity; tiny dwarf galaxies are extremely numerous in the universe, whereas giant ellipticals are far less common. So are most of the universe’s globulars found around dwarfs, simply because there are more dwarfs to host them? Or are the majority of globular clusters orbiting large galaxies? A scientist at McMaster University in Canada, William Harris, has done some calculations to find the answer.

Finding the Peak

Harris combines two components in his estimates:

  1. The Schechter function, a function that describes the relative number of galaxies per unit luminosity. This function drops off near a characteristic luminosity roughly that of our galaxy.
  2. Empirical data from ~400 galaxies that describe the average number of globulars per galaxy as a function of galaxy luminosity.
Where are the GCs?

Relative number of globular clusters in all galaxies at a given luminosity, for metal-poor globulars only (blue), metal-rich globulars only (red), and all globulars (black). The curves peak around the Schechter characteristic luminosity, and metal-poor globulars outnumber metal-rich ones 4 to 1. [Harris 2016]

He finds that globular clusters are most commonly found in galaxies within a surprisingly narrow range around the characteristic luminosity of the Schechter function. This means that, at the current time, the collection of galaxies similar in brightness to the Milky Way or Andromeda host the largest total number of globulars in the universe.

Metal-Poor Dominance

Harris extends these calculations by examining two subpopulations of globulars: blue (metal-poor) and red (metal-rich). Metal-poor globular clusters are found in all galaxies, but metal-rich ones reside preferentially in massive, bright galaxies. Strikingly, Harris finds that this preference results in metal-poor globulars making up almost 80% of all globular clusters in the universe, outnumbering the metal-rich ones by nearly 4 to 1.

This result implies that the earliest stages of hierarchical galaxy mergers — when most of the available gas was low-metallicity — provided the most favorable conditions for the formation of dense, massive star clusters. This early environment birthed the majority of the globular clusters we see today.

Citation

William E. Harris 2016 AJ 151 102. doi:10.3847/0004-6256/151/4/102

J1211

The recent discovery of a hyper-velocity binary star system in the halo of the Milky Way poses a mystery: how was this system accelerated to its high speed?

Accelerating Stars

Unlike the uniform motion in the Galactic disk, stars in the Milky Way’s halo exhibit a huge diversity of orbits that are usually tilted relative to the disk and have a variety of speeds. One type of halo star, so-called hyper-velocity stars, travel with speeds that can approach the escape velocity of the Galaxy.

How do these hyper-velocity stars come about? Assuming they form in the Galactic disk, there are multiple proposed scenarios through which they could be accelerated and injected into the halo, such as:

  1. Ejection after a close encounter with the supermassive black hole at the Galactic center
  2. Ejection due to a nearby supernova explosion
  3. Ejection as the result of a dynamical interaction in a dense stellar population.

Further observations of hyper-velocity stars are necessary to identify the mechanism responsible for their acceleration.

J1211’s Surprise

J1211 orbit

Models of J1211’s orbit show it did not originate from the Galactic center (black dot). The solar symbol shows the position of the Sun and the star shows the current position of J1211. The bottom two panels show two depictions (x-y plane and r-z plane) of estimated orbits of J1211 over the past 10 Gyr. [Németh et al. 2016]

To this end, a team of scientists led by Péter Németh (Friedrich Alexander University, Erlangen-Nürnberg) recently studied the candidate halo hyper-velocity star SDSS J121150.27+143716.2. The scientists obtained spectroscopy of J1211 using spectrographs at the Keck Telescope in Hawaii and ESO’s Very Large Telescope in Chile. To their surprise, they discovered the signature of a companion in the spectra: J1211 is actually a binary!

Németh and collaborators found that J1211, located roughly 18,000 light-years away, is moving at a rapid ~570 km/s relative to the galactic rest frame. The binary system consists of a hot (30,600 K) subdwarf and a cool (4,800 K) companion star in a wide orbit, likely separated by several AU.

An Unknown Past and Future

Why are these new observations of J1211 such a big deal? Because all the acceleration scenarios for a star originating in the Galactic disk fail in the case of J1211. The authors find by modeling J1211’s motion that the system can’t have originated in the Galactic center, so interactions with the supermassive black hole are out. And supernova explosions or dynamical interactions would tear the wide binary apart in the process of accelerating it. Németh and collaborators suggest instead that J1211 was either born in the halo population or accreted later from the debris of a destroyed satellite galaxy.

J1211’s speed is so extreme that its orbit could be either bound or unbound. Interestingly, when the authors model the binary’s orbit, they find that the assumed mass of the Milky Way’s dark-matter halo determines whether J1211’s orbit is bound. This means that future observations of J1211 may provide a new way to probe the Galactic potential and determine the mass of the dark matter halo, in addition to revealing unexpected origins of high-velocity halo stars.

Citation

Péter Németh et al 2016 ApJ 821 L13. doi:10.3847/2041-8205/821/1/L13

supernova

Supernovae — enormous explosions associated with the end of a star’s life — come in a variety of types with different origins. A new study has examined how the brightest supernovae in the Universe are produced, and what limits might be set on their brightness.

Ultra-Luminous Observations

Recent observations have revealed many ultra-luminous supernovae, which have energies that challenge our abilities to explain them using current supernova models. An especially extreme example is the 2015 discovery of the supernova ASASSN-15lh, which shone with a peak luminosity of ~2*1045 erg/s, nearly a trillion times brighter than the Sun. ASASSN-15lh radiated a whopping ~2*1052 erg in the first four months after its detection.

How could a supernova that bright be produced? To explore the answer to that question, Tuguldur Sukhbold and Stan Woosley at University of California, Santa Cruz, have examined the different sources that could produce supernovae and calculated upper limits on the potential luminosities of each of these supernova varieties.

Explosive Models

Sukhbold and Woosley explore multiple different models for core-collapse supernova explosions, including:

  1. Prompt explosion
    A star’s core collapses and immediately explodes.
  2. Pair instability
    Electron/positron pair production at a massive star’s center leads to core collapse. For high masses, radioactivity can contribute to delayed energy output.
  3. Colliding shells
    Previously expelled shells of material around a star collide after the initial explosion, providing additional energy release.
  4. Magnetar
    The collapsing star forms a magnetar — a rapidly rotating neutron star with an incredibly strong magnetic field — at its core, which then dumps energy into the supernova ejecta, further brightening the explosion.

They then apply these models to different types of stars.

Setting the Limit

ASASSN-15lh

The authors show that the light curve of ASASSN-15lh (plotted in orange) can be described by a model (black curve) in which a magnetar with an initial spin period of 0.7 ms and a magnetic field of 2*1013 Gauss deposits energy into ~12 solar masses of ejecta. Click for a closer look! [Adapted from Sukhbold&Woosley 2016]

The authors find that the maximum luminosity that can be produced by these different supernova models ranges between 5*1043 and 2*1046 erg/s, with total radiated energies of 3*1050 to 4*1052 erg. This places the upper limit on the brightness of a supernova at about 5 trillion times the luminosity of the Sun.

The calculations performed by Sukhbold and Woosley confirm that, of the options they explore, the least luminous events are produced by prompt explosions. The brightest events possible are powered by the rotational energy of a newly born magnetar at the heart of the explosion.

The energies of observed ultra-luminous supernovae are (just barely) contained within the bounds of the mechanisms explored here. This is even true of the extreme ASASSN-15lh — which, based on the authors’ calculations, was almost certainly powered by an embedded magnetar. If we were to observe a supernova more than twice as bright as ASASSN-15lh, however, it would be nearly impossible to explain with current models.

Citation

Tuguldur Sukhbold and S. E. Woosley 2016 ApJ 820 L38. doi:10.3847/2041-8205/820/2/L38

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