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Galaxies of similar stellar mass to our own don’t all have the same bulge and black hole masses. So what determines how much mass will end up in the bulge and the black hole at the center of a Milky-Way-like galaxy?

The Role of Mergers

One theory is that major and minor mergers build up the bulge and black-hole masses for some galaxies. It’s often argued that massive, centrally concentrated “classical” bulges are caused by merger activity, whereas less massive, more disk-like “pseudobulges” might be caused by other means, such as violent disk instabilities in early gas-rich disks, or misaligned infall of gas throughout cosmic time.

halo vs. bulge mass

Bulge mass (top) and BH mass (bottom) as a function of stellar halo mass. Red denotes galaxies with low-mass pseudobulges, black shows galaxies with higher-mass classical bulges. The grey shaded area in the bottom plot shows what would be expected if there were a 1:1 correlation between bulge mass and stellar halo mass. [Bell et al. 2017]

A team of scientists led by Eric Bell (University of Michigan) set out to test the role of major and minor mergers in bulge formation by examining the stellar halos of a sample of 18 Milky-Way-mass galaxies — six with classical bulges expected to have grown through mergers and 12 with pseudobulges expected to have grown through a variety of other mechanisms.

Halos as Historical Record

Stellar halos offer a useful way of tracking the merger history of a galaxy. It’s believed that as major mergers with larger satellites occur, a galaxy’s stellar halo will increase in both mass and metallicity as it retains the stars of the satellite.

Bell and collaborators first verify this picture in their sample by plotting the stellar halo metallicities against the stellar halo masses. This check reveals a strong correlation between the two properties that’s consistent with the outcomes from simulations — so the stellar halos indeed encode the merger history of the galaxies. This means that from their halos, we can infer the masses of the largest satellites accreted by these galaxies.

Laboratories for Quiet Accretion

The authors then search for any indication of correlation between the stellar halo mass and the galaxy’s bulge mass or black hole mass. They find that their galaxy sample has a wide range in stellar halo masses that don’t correlate significantly with the bulge-to-total ratio, bulge mass, or black hole mass of the galaxy. This is true not only for the pseudobulges, but also for the classical bulges.

M81

The galaxy M81 has a massive classical bulge but an anemic stellar halo containing only 2% of its total stellar mass. This galaxy may be a useful laboratory for studying quiet accretion events. [Subaru Telescope (NAOJ)/HST/R. Colombari/R. Gendler]

This outcome suggests that not even the classical bulges form primarily via minor and major merging activity. Instead, the bulges all form from a variety of mechanisms: a few are likely created by mergers, but the remainder are probably caused by quieter means like secular evolution, disk instabilities or misaligned gas accretion.

These findings challenge the classical models of massive bulge formation and suggest that more detailed simulations and observations are necessary to unravel how the bulges and black holes at the centers of Milky-Way-like galaxies are grown. In particular, the galaxies with massive classical bulges but without massive stellar halos (the galaxy M81 is suggested as an example) may be ideal laboratories for studying quiet growth mechanisms.

Citation

Eric F. Bell et al 2017 ApJL 837 L8. doi:10.3847/2041-8213/aa6158

Dust clouds around star

Twenty-three new objects have been added to the growing collection of stars observed to have unusual dips in their light curves. A recent study examines these stars and the potential causes of their strange behavior.

An Influx of Data

The primary Kepler mission provided light curves for over 100,000 stars, and its continuation K2 is observing another 20,000 stars every three months. As we enter an era where these enormous photometric data sets become commonplace — Gaia will obtain photometry for millions of stars, and LSST billions — it’s crucial that we understand the different categories of variability observed in these stars.

Unusual light curves

The authors find three different types of light curves among their 23 unusual stars. Scallop-shell curves (top) show many undulations; persistent flux-dip class curves (middle) have discrete triangularly shaped flux dips; transient, narrow dip class curves (bottom) have only one dip that is variable in depth. The authors speculate a common cause for the scallop-shell and persistent flux-dip stars, and a different cause for the transient flux-dip stars. [Stauffer et al. 2017]

After filtering out the stars with planets, those in binary systems, those with circumstellar disks, and those with starspots, a number of oddities remain: a menagerie of stars with periodic variability that can’t be accounted for in these categories. Some of these stars are now famous (for instance, Boyajian’s star); some are lesser known. But by continuing to build up this sample of stars with unusual light curves, we have a better chance of understanding the sources of variability.

Building the Menagerie

To this end, a team of scientists led by John Stauffer (Spitzer Science Center at Caltech) has recently hunted for more additions to this sample in the K2 data set. In particular, they searched through the light curves from stars in the ρ Oph and Upper Scorpius star-forming region — a data set that makes up the largest collection of high-quality light curves for low-mass, pre-main-sequence stars ever obtained.

In these light curves, Stauffer and collaborators found a set of 23 very low-mass, mid-to-late-type M dwarfs with unusual variability in their light curves. The variability is consistent with the stars’ rotation period where measured — which suggests that whatever causes the dips in the light curve, it’s orbiting at the same rate as the star spins.

Causes of Variability?

Comparison to Upper Sco

These plots show how the properties of these 23 stars compare to those of the rest of the stars in their cluster (click for a closer look!). For all but the rotation rate, they are typical. But the stars with scallop-shaped light curves have among the shortest periods in Upper Sco, with some near the theoretical break-up for stars of their age. [Stauffer et al. 2017]

The authors categorize the 23 stars into two main groups.

  1. The first group consists of 19 stars with short periods; more than half of them rotate within a factor of two of their predicted breakup period! Many of these show sudden changes in their light-curve morphology, often after a stellar flare. The authors propose that the variability in these light curves might be caused by warm coronal gas clouds that are organized into a structured toroidal shape around the star.
  2. The second group consists of the remaining four stars, which have slightly longer periods. The light curves show a single short-duration flux dip — with highly variable depth and shape — superposed on normal, spotted-star light curves. The authors’ best guess for these four stars is that there are clouds of dusty debris circling the star, possibly orbiting a close-in planet or resulting from a recent collisional event.

Stauffer and collaborators are currently developing more detailed models for these stars based on the possible variability scenarios. The next step, they state, is to determine if the gas in these structures have properties necessary to generate the light-curve features we see.

Citation

John Stauffer et al 2017 AJ 153 152. doi:10.3847/1538-3881/aa5eb9

AGN model

The structure immediately around a supermassive black hole at the heart of an active galaxy can tell us about how material flows in and out of these monsters — but this region is hard to observe! A new study provides us with clues of what might be going on in these active and energetic cores of galaxies.

In- and Outflows

In active galactic nuclei (AGN), matter flows both in and out. As material flows toward the black hole via its surrounding accretion disk, much of this gas and dust can then be expelled from the vicinity via highly collimated jets.

corona-jet relations

Top: The fraction of X-rays that is reflected decreases as jet power increases. Bottom: the distance between the corona and the reflecting part of the disk increases as jet power increases. [Adapted from King et al. 2017]

To better understand this symbiosis between accretion and outflows, we examine what’s known as the “corona” — the hot, X-ray-emitting gas that’s located in the closest regions around the black hole. But because the active centers of galaxies are generally obscured by surrounding gas and dust, it’s difficult for us to learn about the structure of these inner regions near the black hole.

Where are the X-rays of the corona produced: in the inner accretion flow, or at the base of the jet? How far away is this corona from the disk? And how does the corona’s behavior relate to that of the jet?

Reflected Observations

To address some of these questions, a group of scientists led by Ashley King (Einstein Fellow at Stanford University) has analyzed X-ray observations from NuSTAR and XMM-Newton of over 40 AGN. The team examined the reflections of the X-rays off of the accretion disk and used two measurements to learn about the structure around the black hole:

  1. the fraction of the corona’s X-rays that are reflected by the disk, and
  2. the time lag between the original and reflected X-rays, which reveals the distance from the corona to the reflecting part of the disk.
model images

A visualization of the authors’ model for an AGN. The accretion disk is red, corona is green, and jet is blue. The corona shines on the disk, causing the inner regions (colored brighter) to fluoresce, “reflecting” the radiation. As the accretion rate increases from the top to the bottom panel, the jet power increases and the dominant reflective part of the disk moves outward due to the ionization of the inner region (which puffs up into a torus). [Adapted from King et al. 2017]

King and collaborators find two interesting relationships between the corona and the jet: there is an inverse correlation between jet power and reflection fraction, and there is a correlation between jet power and the distance of the corona from the reflecting part of the disk the disk. These observations indicate that there is a relationship between changes in the corona and jet production in AGN.

Modeling the Corona

The authors use these observations to build a self-consistent model of an AGN’s corona. In their picture, the corona is located at the base of the jet and moves mildly relativistically away from the disk, propagating into the large-scale jets.

As the velocity of the corona increases, more of its radiation is relativistically beamed away from the accretion disk, which decreases the fraction of X-rays that are reflected — explaining the inverse correlation between jet power and reflection fraction.

At the same time, the increased mass accretion further ionizes the inner disk region, pushing the dominant reflection region to further out in the disk — which explains the correlation between jet power and the distance from corona to reflection region.

King and collaborators show that this model is fully consistent with the X-ray observations of the 40 AGN they examined. Future X-ray observations of the strongest radio jet sources will help us to further pin down what’s happening at the heart of active galaxies.

Citation

Ashley L. King et al 2017 ApJ 835 226. doi:10.3847/1538-4357/835/2/226

Breakthrough Starshot

The Breakthrough Starshot Initiative made headlines last year when the plan was first announced to send tiny spacecraft to our nearest stellar neighbors. But just how feasible is this initiative? A new study looks at just one aspect of this plan: whether we can propel the spacecraft successfully.

Propelling a Fleet

Alpha Centauri

The Alpha Centauri star system, which consists of Alpha (left) and Beta (right) Centauri as well as Proxima Centauri (circled). [Skatebiker]

The goal behind the Breakthrough Starshot Initiative is to build a fleet of tiny, gram-scale spacecraft to travel to the Alpha Centauri star system — a system in which a planet was recently discovered around Proxima Centauri, the star nearest to us.

To propel the spacecraft, the team plans to attach a reflective sail to each one. When a high-power laser beam is pointed at that sail from Earth, the impulse of the photons bouncing off the sail can accelerate the lightweight spacecraft to a decent fraction of the speed of light, allowing it to reach the Alpha Centauri system within decades.

Among the many potential engineering challenges for such a mission, one interesting one is examined in a recent study by Zachary Manchester and Avi Loeb of Harvard University: how do we keep the spacecraft’s light sail centered on the laser beam long enough to accelerate it?

4-Gaussian beam

Beam profile (left) and corresponding potential function (right) for a laser beam made up of four Gaussians. With this configuration, the potential well pushes the spacecraft back to the center if it drifts toward the edges of the well. [Manchester & Loeb 2017]

The Search for Stability

Manchester and Loeb argue that any slight perturbations to the light sail’s position relative to the laser beam — in the form of random disturbances, misalignments, or manufacturing imperfections — could cause it to slide off the beam, preventing it from continuing to accelerate. Ideally, the project would use a sail that could be passively stable: the sail wants to stay centered on the beam, rather than requiring active interference to keep it there.

The scenario that’s been proposed and studied in the past is that of a conical sail propelled by a Gaussian beam. But Manchester and Loeb perform analytic stability calculations to show that such a system will not, in fact, be stable — if the beam gets knocked off the center of the sail, it will not be able to recover its centered position.

Spheres on the Go

beam-riding

Sail position during beam-riding simulations for a spherical sail on the 4-Gaussian beam. Left: When the sail begins with a 5-cm offset from the center of the beam, it oscillates around the center but successfully remains bounded in the x-y plane (rather than drifting off the beam). Right: When noise is added to the beam, the sail oscillates more, but it still remains stable and bounded over several minutes of acceleration. [Manchester & Loeb 2017]

So if a conical sail won’t work, what will instead? Manchester and Loeb propose an intriguing alternative: a light sail in the shape of a spherical shell around the spacecraft, propelled by a beam that is constructed from the sum of four Gaussians. This more complex configuration has the benefit that if the spacecraft is knocked off the center of the beam, it will experience a restoring force that pushes it back to the center. The spherical shape of the sail means that it won’t destabilize if it’s tilted.

The authors perform a series of numerical simulations to test this configuration, demonstrating that it remains stable even when they introduce deliberate noise into the beam. The simulations show that the beam can stay successfully centered on the spherical sail for at least several minutes — sufficient for the spacecraft to be accelerated to a sizable fraction of the speed of light.

So does this approach make Starshot feasible? It may be a step in the right direction, but challenges still remain. We can undoubtedly look forward to seeing further clever innovations as planning for this project continues!

Citation

Zachary Manchester and Abraham Loeb 2017 ApJL 837 L20. doi:10.3847/2041-8213/aa619b

magnetic fields in compact objects

White dwarfs, the compact remnants left over at the end of low- and medium-mass stars’ lifetimes, are often found to have magnetic fields with strengths ranging from thousands to billions of times that of Earth. But how do these fields form?

Multiple Possibilities

Around 10–20% of white dwarfs have been observed to have measurable magnetic fields with a wide range of strengths. There are several theories as to how these fields might be generated:

  1. The fields are “fossil”.
    The original weak magnetic fields of the progenitor stars were amplified as the stars’ cores evolved into white dwarfs.
  2. The fields are caused by binary interactions.
    White dwarfs that formed in the merger of a binary pair might have had a magnetic field amplified as a result of a dynamo that was generated during the merger.
  3. The fields were produced by some other internal physical mechanism during the cooling of the white dwarf itself.

In a recent publication, a team of authors led by Jordi Isern (Institute of Space Sciences, CSIC, and Institute for Space Studies of Catalonia, Spain) explored this third possibility.

Dynamos from Crystallization

mantle width

The inner and outer boundaries of the convective mantle of carbon/oxygen white dwarfs of two different masses (top vs. bottom panel) as a function of luminosity. As the white dwarf cools (toward the right), the mantle grows thinner due to the crystallization and settling of material. [Isern et al. 2017]

As white dwarfs have no nuclear fusion at their centers, they simply radiate heat and gradually cool over time. The structure of the white dwarf undergoes an interesting change as it cools, however: though the object begins as a fluid composed primarily of an ionized mixture of carbon and oxygen (and a few minor species like nickel and iron), it gradually crystallizes as its temperature drops.

The crystallized phase of the white dwarf is oxygen-rich — which is denser than the liquid, so the crystallized material sinks to the center of the dwarf as it solidifies. As a result, the white dwarf forms a solid, oxygen-rich core with a liquid, carbon-rich mantle that’s Rayleigh-Taylor unstable: as crystallization continues, the solids continue to sink out of the mantle.

By analytically modeling this process, Isern and collaborators demonstrate that the Rayleigh-Taylor instabilities in the convective mantle can drive a dynamo large enough to generate the magnetic field strengths we’ve observed in white dwarfs.

dynamo scaling relation

Magnetic field density as a function of the dynamo energy density. The plots show Earth and Jupiter (black dots), T Tauri stars (cyan), M dwarf stars (magenta), and two types of white dwarfs (blue and red). Do these lie on the same scaling relation? [Isern et al. 2017]

A Universal Process?

This setup — the solid core with an unstable liquid mantle on top — is exactly the structure expected to occur in planets such as Earth and Jupiter. These planets’ magnetic fields are similarly thought to be generated by convective dynamos powered by the cooling and chemical separation of their interiors — and the process can also be scaled up to account for the magnetic fields of fully convective objects like T Tauri stars, as well.

If white-dwarf magnetic fields are generated by the same type of dynamo, this may be a universal process for creating magnetic fields in astrophysical objects — though other processes may well be at work too.

Citation

Jordi Isern et al 2017 ApJL 836 L28. doi:10.3847/2041-8213/aa5eae

If you examined images of planetary nebulae, you would find that many of them have an appearance that is too “messy” to be accounted for in the standard model of how planetary nebulae form. So what causes these structures?

unlikely category

Examples of planetary nebulae that have a low probability of having been shaped by a triple stellar system. They are mostly symmetric, with only slight departures (labeled) that can be explained by instabilities, interactions with the interstellar medium, etc. [Bear and Soker 2017]

A Range of Looks

At the end of a star’s lifetime, in the red-giant phase, strong stellar winds can expel the outer layers of the star. The hot, luminous core then radiates in ultraviolet, ionizing the gas of the ejected stellar layers and causing them to shine as a brightly colored “planetary nebula” for a few tens of thousands of years.

Planetary nebulae come in a wide variety of morphologies. Some are approximately spherical, but others can be elliptical, bipolar, quadrupolar, or even more complex.

It’s been suggested that non-spherical planetary nebulae might be shaped by the presence of a second star in a binary system with the source of the nebula — but even this scenario should still produce a structure with axial or mirror symmetry.

A pair of scientists from Technion — Israel Institute of Technology, Ealeal Bear and Noam Soker, argue that planetary nebulae with especially messy morphologies — those without clear axial or point symmetries — may have been shaped by an interacting triple stellar system instead.

maybe category

Examples of planetary nebulae that might have been shaped by a triple stellar system. They have some deviations from symmetry but also show signs of interacting with the interstellar medium. [Bear and Soker 2017]

Departures from Symmetry

To examine this possibility more closely, Bear and Soker look at a sample of thousands planetary nebulae and qualitatively classify each of them into one of four categories, based on the degree to which they show signs of having been shaped by a triple stellar progenitor. The primary signs the authors look for are:

  1. Symmetries
    If a planetary nebula has a strong axisymmetric or point-symmetric structure (i.e., it’s bipolar, elliptical, spherical, etc.), it was likely not shaped by a triple progenitor. If clear symmetries are missing, however, or if there is a departure from symmetry in specific regions, the morphology of the planetary nebula may have been shaped by the presence of stars in a close triple system.
  2. Interaction with the interstellar medium
    Some asymmetries, especially local ones, can be explained by interaction of the planetary nebula with the interstellar medium. The authors look for signs of such an interaction, which decreases the likelihood that a triple stellar system need be involved to produce the morphology we observe.
Examples of planetary nebulae that are extremely likely to have been shaped by a triple stellar system. They have strong departures from asymmetry and don’t show signs of interacting with the interstellar medium. [Bear and Soker 2017]

Examples of planetary nebulae that are extremely likely to have been shaped by a triple stellar system. They have strong departures from symmetry and don’t show signs of interacting with the interstellar medium. [Bear and Soker 2017]

Influential Trios

From the images in two planetary nebulae catalogs — the Planetary Nebula Image Catelog and the HASH catalog — Bear and Soker find that 275 and 372 planetary nebulae are categorizable, respectively. By assigning crude probabilities to their categories, the authors estimate that the total fraction of planetary nebulae shaped by three stars in a close system is around 13–21%.

The authors argue that in some cases, all three stars might survive. This means that we may be able to find direct evidence of these triple stellar systems lying in the hearts of especially messy planetary nebulae.

Citation

Ealeal Bear and Noam Soker 2017 ApJL 837 L10. doi:10.3847/2041-8213/aa611c

illustration of dust and gas swirling around a bright, newly forming star.

The dusty disks around young stars make the news regularly due to their appeal as the birthplace of early exoplanets. But how do disks like these first form and evolve around their newly born protostars? New observations from the Atacama Large Millimeter/submillimeter Array (ALMA) are helping us to better understand this process.

Formation from Collapse

Stars are born from the gravitational collapse of a dense cloud of molecular gas. Long before they start fusing hydrogen at their centers — when they are still just hot overdensities in the process of contracting — we call them protostars. These low-mass cores are hidden at the hearts of the clouds of molecular gas from which they are born.

ALMA

Aerial image of the Atacama Large Millimeter/submillimeter Array. [EFE/Ariel Marinkovic]

During this contraction phase, before a protostar transitions to a pre-main-sequence star (which it does by blowing away its outer gas envelope, halting the star’s growth), much of the collapsing material will spin into a centrifugally supported Keplerian disk that surrounds the young protostar. Later, these circumstellar disks will become the birthplace for young planets — something for which we’ve seen observational evidence in recent years.

But how do these Keplerian disks — which eventually have scales of hundreds of AU — first form and grow around protostars? We need observations of these disks in their early stages of formation to understand their birth and evolution — a challenging prospect, given the obscuring molecular gas that hides them at these stages. ALMA, however, is up to the task: it can peer through to the center of the gas clouds to see the emission from protostellar cores and their surroundings.

ALMA observations of the protostar Lupus 3 MMS. The molecular outflows from the protostar are shown in panel a. Panel b shows the continuum emission, which has a compact component that likely traces a disk surrounding the protostar. [Adapted from Yen et al. 2017]

ALMA observations of the protostar Lupus 3 MMS. The molecular outflows from the protostar are shown in panel a. Panel b shows the continuum emission, which has a compact component that likely traces a disk surrounding the protostar. [Adapted from Yen et al. 2017]

New Disks Revealed?

In a recent publication led by Hsi-Wei Yen (Academia Sinica Institute of Astronomy and Astrophysics, Taiwan), a team of scientists presents results from ALMA’s observations of three very early-stage protostars: Lupus 3 MMS, IRAS 15398–3559, and IRAS 15398–2429. ALMA’s spectacular resolution allowed Yen and collaborators to infer the presence of a 100-AU Keplerian disk around Lupus 3 MMS, and signatures of infall on scales of <30 AU around the other two sources.

The authors construct models of the sources and show that the observations are consistent with the presence of disks around all three sources: a 100-AU disk around a 0.3 solar-mass protostar in the Lupus system, a 20-AU disk around a 0.01 solar-mass protostar in IRAS 15398–3559, and 6-AU disk around a 0.03 solar-mass protostar in IRAS 15398–2429.

By comparing their observations to those of other early-stage protostars, the authors conclude that in the earliest protostar stage, known as the Class 0 stage, the protostar’s disk grows rapidly in radius. As the protostar ages and enters the Class I stage, the disk growth stagnates, changing only very slowly after this.

These observations mark an important step in our ability to study the gas motions on such small scales at early stages of stellar birth. Additional future studies will hopefully allow us to continue to build this picture!

Citation

Hsi-Wei Yen et al 2017 ApJ 834 178. doi:10.3847/1538-4357/834/2/178

SN 2014C

For the first time ever, we’ve been able to watch the complete metamorphosis of an unusual explosion from one type of supernova to another. What do our observations of SN 2014C mean for our understanding of how massive stars end their lives?

Categorizing Explosions

Supernovae — the explosions that mark the end of massive stellar lifetimes — are broadly categorized into two types: Type I supernovae, which do not show evidence of hydrogen in their spectra, and Type II supernovae, which do.

The majority of supernovae in both categories have the same cause: the fuel in the star’s core is exhausted, and the core subsequently collapses under its own gravity. But a supernova will appear as a Type I or Type II depending upon whether or not the star had already lost its outer hydrogen envelope long before the explosion.

SN 2014C

This plot summarizes key observational features of SN 2014C across the spectrum. The X-ray and radio evolution of SN 2014C is shown compared to other Type Ib/c supernovae (center panel). SN 2014C shows an uncommon rise in luminosity at late times, as well as the onset of Hα emission (bottom panel) when its shock plows into the hydrogen shell. [Margutti et al. 2017]

Supernova Surprise

SN 2014C has muddied this categorization, however. When this unusual explosion was first observed in December 2014, it looked like a Type Ib supernova: there was no sign of hydrogen in its spectrum. But over a timescale of roughly a year — during which it was continuously monitored across all wavelengths — SN 2014C transformed into a Type IIn supernova.

In a new study led by Raffaella Margutti (CIERA, Northwestern University), data is presented from close monitoring of SN 2014C over this entire transition period. The intriguing outcome? After about a year, SN 2014C’s expanding shock appears to have plowed into a substantial shell of hydrogen gas encasing the supernova.

Implications of an Unexpected Collision

How did this shell of hydrogen get there? Margutti and collaborators show that it must have been thrown off of the dying star in an eruptive episode decades to centuries before the supernova explosion.

This presents an interesting problem, because theoretical models of core-collapse supernovae typically assume that when a star loses its hydrogen envelope, this happens gradually over a long period of time, via a mechanism like strong stellar winds. If some stars are instead eruptively throwing off their outer layers long before they explode as supernova, we may need to make some serious changes in how we model stellar evolution and deaths in the future.

supernovae with late-time re-brightenings

A few Type Ib/c supernovae that display late-time radio re-brightenings with similarities to SN 2014C. [Margutti et al. 2017]

Is SN 2014C Alone?

Margutti and collaborators suggest that SN 2014C might be a type of bridge supernova. In this picture, the difference between ordinary Type Ib/c supernovae and Type IIn supernovae (which show signs of interaction with a dense medium right away) lies only in when the star’s hydrogen envelope is thrown off: shortly before explosion for Type IIn supernovae, or decades to centuries beforehand for Type Ib/c supernovae.

When the authors search a sample of 183 Type Ib/c supernovae, they find that roughly 10% of the supernovae in their sample show signs of late-time interactions similar to SN 2014C, lending support to this picture.

We can certainly hope to gather observations of other supernovae behaving like SN 2014C in the future! In the meantime, our full look at SN 2014C’s transformation has given us plenty of new information to consider.

Citation

Raffaella Margutti et al 2017 ApJ 835 140. doi:10.3847/1538-4357/835/2/140

Centaurus A

We believe that supermassive black holes evolve in tandem with their host galaxies — but how do the two communicate? Observations from the Atacama Large Millimeter/submillimeter Array (ALMA) have revealed new clues about how a monster black hole talks to its galaxy.

Central galaxy in Phoenix cluster

A Hubble image of the central galaxy in the Phoenix cluster. [Adapted from Russell et al. 2017]

Observing Feedback

Active galactic nuclei (AGN), the highly luminous centers of some galaxies, are thought to radiate due to active accretion onto the supermassive black hole at their center.

It’s long been suspected that the radiation and outflowing material — which often takes the form of enormous bipolar radio jets emitted into the surroundings — influence the AGN’s host galaxy, affecting star formation rates and the evolution of the galaxy. This “AGN feedback” has been alternately suggested to trigger star formation, quench it, and truncate the growth of massive galaxies.

The details of this feedback process, however, have yet to be thoroughly understood — in part because it’s difficult to obtain detailed observations of how AGN outflows interact with the galactic gas surrounding them. Now, a team of scientists led by Helen Russell (Institute of Astronomy in Cambridge, UK) has published the results of a new, high-resolution look at the gas in a massive galaxy in the center of the Phoenix cluster.

Many Uses for Fuel

The Phoenix cluster, a nearby (z = 0.596) group of star-forming galaxies, is the most luminous X-ray cluster known. The central galaxy in the cluster is especially active: it hosts a starburst of 500–800 solar masses per year, the largest starburst found in any galaxy below a redshift of z = 1.

The star formation in this galaxy is sustained by an enormous reservoir of cold molecular gas — roughly 20 billion solar masses’ worth. This reservoir also powers the galaxy’s central black hole, fueling powerful radio jets that extend into the hot atmosphere of the galaxy and blow a giant bubble into the hot gas at each pole.

molecular gas by ALMA

ALMA observations of the molecular gas in the central galaxy of the Phoenix cluster. The bubbles blown by the radio jets are indicated by the dashed white contours. Extended filaments of molecular gas can be seen to wrap around these cavities. [Adapted from Russell et al. 2017]

ALMA Spots Filaments

ALMA’s observations of this reservoir show that extended filaments of molecular gas wrap around the peripheries of the radio bubbles. These filaments span 10–20 kpc (~30–60 thousand light-years) and have a mass of several billion solar masses. The velocity gradients along them are smooth, suggesting that the gas is moving in an ordered flow around the bubble.

Russell and collaborators suggest that these observations indicate that the clouds of molecular gas were either lifted by the radio bubbles as they inflated, or they formed in place via instabilities caused by the inflating bubbles.

Either way, the data provide clear confirmation that the jets from the black hole affect the location and motion of the cold gas in the surrounding galaxy. This is a beautiful piece of direct evidence showing how supermassive black holes might be communicating with their galaxies.

Citation

H. R. Russell et al 2017 ApJ 836 130. doi:10.3847/1538-4357/836/1/130

hot Jupiter

A tiny telescope has discovered a scalding hot world orbiting its star 1,300 light-years from us. KELT-16b may only be around for a few more hundreds of thousands of years, however.

Don’t Underestimate Tiny Telescopes

KELT-North

The KELT-North telescope in Arizona. This tiny telescope was responsible for the discovery of KELT-16b. [Vanderbilt University]

In an era of ever larger observatories, you might think that there’s no longer a place for small-aperture ground-based telescopes. But small ground-based telescopes have been responsible for the discovery and characterization of around 250 exoplanets so far — and these are the targets that are especially useful for exoplanet science, as they are more easily followed up than the faint discoveries made by telescopes like Kepler.

The Kilogree Extremely Little Telescope (KELT) consists of two telescopes — one in Arizona and one in South Africa — that each have a 4.2-centimeter aperture. In total, KELT observes roughly 70% of the entire sky searching for planets transiting bright hosts. And it’s recently found quite an interesting one: KELT-16b. In a publication led by Thomas Oberst (Westminster College in Pennsylvania), a team of scientists presents their find.

KELT-16b light curves

Combined follow-up light curves obtained for KELT-16b from 19 transits. The best-fit period is just under a day. [Oberst et al. 2017]

A Hot World

KELT-16b is what’s known as a hot Jupiter. Using the KELT data and follow-up observations of 19 transits, Oberst and collaborators estimate KELT-16b’s radius at roughly 1.4 times that of Jupiter and its mass at 2.75 times Jupiter’s. Its equilibrium temperature is a scalding 2453 K — caused by the fact that it orbits so close to its host star that it completes each orbit in a mere 0.97 days!

This short period is extremely unusual: there are only five other known transiting exoplanets with periods shorter than a day. KELT-16b is orbiting very close to its host, making it subject to extreme irradiation and strong tidal forces.

Based on KELT-16b’s orbit, Oberst and collaborators estimate that the planet began a runaway inspiral by the age of 1 billion years. Now, at ~3.1 billion years old, KELT-16b is orbiting at a radius of just over 3 stellar radii above its host’s surface. The authors estimate that KELT-16b’s continuing inward spiral could end in the planet’s destruction by tidal forces in as little as another 550,000 years.

What We Can Learn from KELT-16b

transiting exoplanets

KELT-16b in context with other transiting-exoplanet discoveries on a diagram of planet radius vs. period. Only five other planets have been found with periods shorter than a day. [Oberst et al. 2017]

This highly irradiated world makes for an especially useful target due to its short period (which means we can observe many transits) and bright host (which means follow-up observations are more convenient and have a large signal-to-noise ratio).

In particular, with followup observations of KELT-16b from missions like Hubble, Spitzer, and eventually the James Webb Space Telescope, we can learn more about open questions in exoplanet atmospheric processes — like how heat is transferred vertically through the atmosphere, or what happens at the day-to-night terminator line on such a highly irradiated planet.

In addition, by studying KELT-16b, we can hope to gain overall insight into hot Jupiter formation and migration. The ease of observing this planet and the wealth of information it can provide will likely make it one of the top-studied exoplanets. KELT-16b has a lot to teach us before it’s torn apart!

Citation

Thomas E. Oberst et al 2017 AJ 153 97. doi:10.3847/1538-3881/153/3/97

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