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X-shaped radio source candidates

X-shaped radio galaxies (XRGs) have been used as a measure of the frequency with which supermassive black holes (SMBHs) in the centers of galaxies coalesce after galaxy mergers. But a new study suggests that this approach may be over-predicting the merger rate — as well as the gravitational wave background resulting from it.

Flipping Axes

The dominant contributor to the gravitational wave background in low frequencies is thought to be the coalescence of two SMBHs during galaxy mergers. Predictions of the strength of the gravitational wave background — and, accordingly, our ability to detect it — require us to know how often such mergers occur.

X examples

A closer examination of three candidate XRGs. Top panel: J0702+5002, a possible case of backflow deflection. Middle panel: J0845+4031, a possible case of jet-axis precession. Bottom panel: J1043+3131, a possible case of a genuine jet-axis reorientation due to SMBHs coalescing. [Roberts et al. 2015]

One approach to estimating merger frequency is to determine the abundance of extended radio sources with an X-shaped appearance. XRGs are predicted to contain SMBHs whose axes have been reoriented as a result of absorbing a second SMBH during a merger. Since these sources emit jets, we see both the old and the new axes of the emission, which form an X.

But there’s a problem with using XRGs to estimate the merger frequencies, according to a new study led by David Roberts of Brandeis University: many of the sources identified might not be caused by mergers. Roberts and his coauthors set out to examine and characterize the extended structure of known XRGs to determine how many are actually signposts of SMBH coalescence.

Alternate Causes

The authors used archival data from the Very Large Array, which was available for 52 sources identified as candidate XRGs in the NRAO FIRST survey. By examining the high-resolution images of the archival data, the authors determined that in most of the sources, the X shape was probably not caused by the sudden flip of jet axes during SMBH coalescence. Instead, it was caused either by slow precession or drift of the axes, or by deflection of backflows from the jets as they impact the thermal halo of the host galaxy.

In the end, only 11 of the 52 sources examined were determined to be genuine cases of sudden axis reorientations. This means only 1.3% of extended radio sources appear to be indicators of a recent galaxy merger — significantly lower than the 7% abundance predicted by previous studies. The merger rate, rather than being the ~1 Gyr-1 galaxy-1 previously predicted, is therefore closer to 0.13 Gyr-1 galaxy-1.

What can we learn from this about gravitational waves? If it’s indeed the case that the number of galaxy mergers has been overestimated by studies using XRGs as signposts, then the associated gravitational wave background may be substantially smaller than these studies have predicted.

Citation

David H. Roberts et al 2015 ApJ 810 L6. doi:10.1088/2041-8205/810/1/L6

Comet 67P

Comet 67P/Churyumov–Gerasimenko — made famous by the explorations of the Rosetta mission — has been displaying puzzling activity as it hurtles toward the Sun. However, recent modeling of the comet by a group of scientists from the Côte d’Azur University may now explain what’s causing 67P’s activity.

Shadowed Activity

67P temperature model

A model of comet 67P, with the colors indicating the rate of change of the temperature on the comet’s surface. The most rapid temperature changes are seen at the comet’s neck, in the same locations as the early activity seen in the Rosetta images. [Alí-Lagoa et al. 2015]

Between June and September of 2014, Rosetta observed comet 67P displaying “early activity” in the form of jets of dust emitted from near the “neck” of the comet (its narrowest point). Such activity is usually driven by the sublimation of volatiles from the comet’s surface as a result of sun exposure. But the neck of the comet is frequently shadowed as the comet rotates, and it receives significantly less sunlight than the rest of the comet. So why would the early activity originate from the comet’s neck?

The authors of a recent study, led by Victor Alí-Lagoa, hypothesize that it’s precisely because the neck is receiving alternating sunlight/shadows that it’s displaying activity. They suggest that thermal cracking of the surface of the comet is happening faster in this region, due to the rapid changes in temperature that result from the shadows cast by the surrounding terrain. The cracking exposes subsurface ices in the neck faster than in other regions, and the ensuing sublimation of that ice is what creates the activity we’re seeing.

Temperature Models

To test their hypothesis, the authors study the surface temperatures on comet 67P by means of a thermophysical model — a model used to calculate the temperatures on an airless body, both on and below the surface. The model takes into account factors like thermal inertia (how quickly the body’s temperature responds to changes in the incident energy), shadowing, and self-heating between parts of the surface in contact.

67P surface temperatures

Plot of the modeled temperature of two typical surfaces on the comet: one from the neck region (solid line) and one from the head region (dashed line). Unlike the head, the neck displays drastic drops in temperature as a result of shadowing. [Alí-Lagoa et al. 2015]

Using this model, the authors find that the temperatures behaved as they predicted: the shadows falling on the comet’s neck causes this region to experience very rapid temperature changes relative to the rest of the body. The authors also found a definite correlation between the regions of most rapid temperature variations and the regions of the comet that show signs of activity in Rosetta images. This provides strong evidence that thermal cracking is indeed taking place in the shadowed regions of the neck, gradually eroding away the surface.

Should this model prove correct, it’s a step toward understanding the evolution of comets like 67P. In addition, the results from this study imply that thermal cracking might happen faster than previously estimated in shadowed regions of other atmosphereless bodies, both near Earth and in the asteroid belt.

Citation

V. Alí-Lagoa et al 2015 ApJ 810 L22. doi:10.1088/2041-8205/810/2/L22

Cosmic web

Scientists have recently identified a connection between metal-poor regions in a set of dwarf galaxies and bursts of star-formation activity within them. These observations provide long-awaited evidence supporting predictions of how stars formed in the early universe and in dwarf galaxies today.

Metal-Poor Clues

The primary driver of star formation over cosmic history is thought to be the accretion onto galaxies of cold gas streaming from the cosmic web. The best way to confirm this model would be to observe a cloud of cosmic gas flowing into an otherwise-quiescent galaxy and launching a wave of star formation. But because cold gas doesn’t emit much radiation, it’s difficult to detect directly.

Now, a team of scientists have found a clever way around this problem: they searched galaxies for a correlation between areas of active star formation and metal-poor regions. Why? Because metal-poor regions could be a smoking gun indicating a recently accreted cloud of cold gas from the cosmic web.

Impacting Clouds

Metallicity and Star Formation Rates

Distribution of metallicity along the major axis of one of the target galaxies. The red bar in the top image shows the position of the spectrograph slit along the galaxy, with the arrow showing the direction of growing distance in the plot below. The plot shows the metallicity variation (red symbols) and star-formation rate (blue line) along the galaxy’s major axis. The metallicity drop coincides with the brightest knot of the galaxy. [Sánchez Almeida et al. 2015]

The authors of this study, led by Jorge Sánchez Almeida (Instituto de Astrofísica de Canarias and University of La Laguna, Spain), used the Great Canary Telescope to obtain high-quality spectra of ten dwarf galaxies with especially low average metallicities. They aligned the spectrograph slit along the major axes of the galaxies in order to measure abundances as a function of position within each galaxy.

The team found that, in nine out of the ten cases, the galaxies displayed sharp drops (by factors of 3–10) in metallicity along a portion of their lengths. The metallicity drops corresponded to bright knots representing starburst regions, in which surface star formation rates are larger than that of the rest of the galaxy by factors of 10–100.

The authors conclude that in these galaxies, a cold cosmic gas cloud with low metallicity impacted the galaxy’s outer region. This impact caused the cloud to compress, triggering the star formation we now observe. At the same time, the gas from the cloud diluted the region’s metallicity, resulting in the low abundances now measured. The authors determine that the cloud impacted within the last 100 Myr — otherwise enough time would have passed for the gas to mix azimuthally as it rotated around the galaxy.

 

Citation

J. Sánchez Almeida et al 2015 ApJ 810 L15. doi:10.1088/2041-8205/810/2/L15

Pre-transitional disk

The protoplanetary disk around DoAr 44 is fairly ordinary in most ways. But a recent study has found that this disk contains water vapor in its inner regions — the first such discovery for a disk of its type.

Drying Out Disks

DoAr 44 is a “transitional disk”: a type of protoplanetary disk that has been at least partially cleared of small dust grains in the inner regions of the disk. This process is thought to happen as a result of dynamical interactions with a protoplanet embedded in the disk; the planet clears out a gap as it orbits.

protoplanetary disks

A schematic of the differences between a full protoplanetary disk, a pre-transitional disk, and a transitional disk. [Catherine Espaillat]

Classical protoplanetary disks surrounding young, low-mass stars often contain water vapor, but transitional disks are typically “dry” — no water vapor is detected from the disk inner regions. This is probably because water vapor is easily dissociated by far-UV radiation from the young, hot star. Once the dust is cleared out from the inner regions of the disk, the water vapor is no longer shielded from the UV radiation, so the disk dries out.

Enter the exception: DoAr 44. The disk in this system doesn’t have a fully cleared inner region, which labels it “pre-transitional”. It’s composed of an inner ring out to 2 AU, a cleared gap between 2 and 36 AU, and then the outer disk. What makes DoAr 44 unusual, however, is that it’s the only disk with a large inner gap known to harbor detectable quantities of water vapor. The authors of this study ask a key question: where is this water vapor located?

Unusual System

Led by Colette Salyk (NOAO and Vassar College), the authors examined the system using the Texas Echelon Cross Echelle Spectrograph, a visiting instrument on the Gemini North telescope. They discovered that the water vapor emission originates from about 0.3 AU — the inner disk region, where terrestrial-type planets may well be forming.

Both dust-shielding and water self-shielding seem to have protected this water vapor from the harsh radiation of the central star, and the authors model this shielding to place constraints on the composition of the disk’s inner regions. They conclude that DoAr 44 has maintained similar physical and chemical conditions to classical protoplanetary disks in its terrestrial-planet forming regions, in spite of having formed a large gap.

Why has DoAr 44 succeeded at maintaining its water vapor, unlike other transition disks? The authors propose that gas might be migrating across the gap in the disk, replenishing the inner disk from the outer. Future observations are planned to help better understand the overall architecture of the gap, as well as the implications of these detections for any possible planets embedded in the disk.

Citation

Colette Salyk et al 2015 ApJ 810 L24. doi:10.1088/2041-8205/810/2/L24

Kuiper Belt

A feature of the Kuiper Belt known as the “kernel” has yet to be adequately explained by solar system formation models. In a recent study, a theorist at the Southwest Research Institute proposes a new explanation for how Neptune arrived at its current orbit — and how this planet’s migration in the early years of the solar system might have created the kernel.

Orbital Jump

The kernel is a concentration of orbits within the Kuiper Belt that all have semimajor axes of roughly a ≈ 44 AU, low eccentricities, and low inclinations. How this collection of objects formed — and why they exist where they do — is difficult to explain with current models, however. Kernel objects aren’t in resonance with any of the larger bodies, so why are they concentrated at that specific distance? In this study, David Nesvorný proposes that the kernel resulted from Neptune’s outward migration through the solar system.

In the currently favored model of our solar system’s formation, the outermost gas giant planets formed closer to the Sun and then migrated out to their current locations. Nesvorný ran a series of simulations of this migration to test the theory that a discontinuity in Neptune’s movement outward — i.e., a sudden jump in the planet’s orbital distance — could explain the presence of the Kuiper Belt’s kernel.

Gas Giant Orbits

Results from a previous study, in which the authors evolved the four gas giant planets plus a fifth giant planet (blue) initially on an orbit between Saturn and Uranus. At 18.3 Myr, a close encounter with the fifth planet causes Neptune’s orbit (pink) to jump outwards by ~0.4 AU, and the fifth planet is then ejected from the solar system by Jupiter. [Nesvorný 2015]

Resonant Population

Nesvorný was successful in finding a model that reproduced the kernel, as well as other observed features of the solar system today. In his model, Neptune began its journey closer to the Sun, at a distance of roughly 24 AU, and it migrated fairly rapidly outward to about 28 AU. As it traveled, it swept up bodies in the outer disk in a 2:1 resonance. These bodies migrated along with it, from an original location of around 40 AU out to 44 AU.

At this point, something occurred to cause Neptune’s orbit to suddenly jump outward by roughly half an AU. The 2:1 population, unable to keep up, were released from the resonance with Neptune here, and they remained orbiting at 44 AU to this day. These bodies are what we now observe as the Kuiper Belt kernel. Neptune, meanwhile, continued to migrate slowly, eventually reaching its current orbit of roughly 30 AU.

Fifth Planet

But what happened to make Neptune’s distance suddenly jump? Nesvorný speculates that Neptune may have had a close encounter with another giant planet — one not currently in our solar system. In a previous study, he inserted a fifth gas giant planet into his simulations with an initial orbit between Saturn and Uranus. Neptune’s orbit jumped by about half an AU when it interacted with the planet at 28 AU, and the fifth planet was subsequently ejected from the solar system by Jupiter.

Citation

David Nesvorný 2015 AJ 150 68. doi:10.1088/0004-6256/150/3/68

Tidal disruption event

In a tidal disruption event (TDE), an unfortunate star passes too close to a dormant supermassive black hole (BH) and gets torn apart by tidal forces, feeding the BH for a short time. Oddly, we’re not finding nearly as many TDEs — typically detected due to their distinctive observational signatures — as theory says we should. A recent study suggests that we might be missing many of these events, due to the way the streams of shredded stars fall onto the BHs.

Signatures of Shredding

When a BH tears a star apart, the star’s material is stretched out into what’s known as a tidal stream. That stream continues on a trajectory around the BH, with roughly half the material eventually falling back on the BH, whipping around it in a series of orbits. Where those orbits intersect each other, the material smashes together and circularizes, forming a disk that then accretes onto the BH.

What does a TDE look like? We don’t observe anything until after the tidal streams collide and the material begins to accrete onto the BH. At that point we observe a sudden peak in luminosity, which then gradually decreases (scaling roughly as time-5/3) as the tail end of what’s left of the star accretes and the BH’s food source eventually runs out.

So why have we only been observing about a tenth as many TDEs as theory predicts we should see? By studying the structure of tidal streams in TDEs, James Guillochon (Harvard-Smithsonian Center for Astrophysics) and Enrico Ramirez-Ruiz (UC Santa Cruz) have found a potential reason — and the culprit is general relativity.

Dark Years

The authors run a series of simulations of TDEs around black holes of varying masses and spins to see what form the resulting tidal streams take over time. They find that precession of the tidal stream due to the BH’s gravitational effects changes how the stream interacts with itself, and therefore what we observe. Some cases behave like what we expect for what’s currently considered a “typical” TDE — but some don’t.

tidal streams

Example from simulations of a tidal stream’s path around the black hole. Relativistic precession causes this tidal stream to wind around the black hole 13 times before it finally collides with itself. [Guillochon&Ramirez-Ruiz 2015]

For cases where the relativistic effects are small (such as BHs with masses less than a few 106 solar masses), the tidal stream collides with itself after only a few windings around the BH, quickly forming a disk. The disk forms far from the BH, however, so it takes a long time to accrete. As a result, the observed flare can take 100 times longer to peak than what’s typically expected for a TDE, so we might be failing to identify these sources as TDEs.

Furthermore, for cases where the BH is both massive and has a spin of a ≳ 0.2, the tidal stream doesn’t collide with itself right away. Instead, it can take many windings around the BH before the first intersection. In these cases, it may potentially be years after a star gets ripped apart before the material accretes and we’re able to observe the event!

 

Citation

James Guillochon and Enrico Ramirez-Ruiz 2015 ApJ 809 166. doi:10.1088/0004-637X/809/2/166

At the end of last year, the Sun’s large-scale magnetic field suddenly strengthened, reaching its highest value in over two decades. Here, Neil Sheeley and Yi-Ming Wang (both of the Naval Research Laboratory) propose an explanation for why this happened and what it predicts for the next solar cycle.

Magnetic Strengthening

Until midway through 2014, solar cycle 24 — the current solar cycle — was remarkably quiet. Even at its peak, it averaged only 79 sunspots per year, compared to maximums of up to 190 in recent cycles. Thus it was rather surprising when, toward the end of 2014, the Sun’s large-scale magnetic field underwent a sudden rejuvenation, with its mean field leaping up to its highest values since 1991 and causing unprecedentedly large numbers of coronal loops to collapse inward.

Yet in spite of the increase we observed in the Sun’s open flux (the magnetic flux leaving the Sun’s atmosphere, measured from Earth), there was not a significant increase in solar activity, as indicated by sunspot number and the rate of coronal mass ejections. This means that the number of sources of magnetic flux didn’t increase — so Sheeley and Wang conclude that flux must instead have been emerging from those sources in a more efficient way! But how?

Aligned Activity

WSO open flux and the radial component of the interplanetary magnetic field (measures of the magnetic flux leaving the Sun’s photosphere and heliosphere, respectively), compared to sunspot number (in units of 100 sunspots). A sudden increase in flux is visible after the peak of each of the last four sunspot cycles. [Sheeley & Wang 2015]

WSO open flux and the radial component of the interplanetary magnetic field (measures of the magnetic flux leaving the Sun’s photosphere and heliosphere, respectively), compared to sunspot number (in units of 100 sunspots). A sudden increase in flux is visible after the peak of each of the last four sunspot cycles. Click for a larger view! [Sheeley & Wang 2015]

The authors show that the active regions on the solar surface in late 2014 lined up in such a way that the emerging flux was enhanced, forming a strong equatorial dipole field that accounts for the sudden rejuvenation observed.

Interestingly, this rejuvenation of the Sun’s open flux wasn’t just a one-time thing; similar bursts have occurred shortly after the peak of every sunspot cycle that we have flux measurements for. The authors find that three factors (how the active regions are distributed longitudinally, their sizes, and the contribution of the axisymmetric component of the magnetic field) determine the strength of this rejuvenation. All three of these factors happened to contribute optimally in 2014.

As a final note, Sheeley and Wang suggest that the current strength of the axisymmetric component of the magnetic field can be used to provide an early indication of how active the next solar cycle might be. Using this method, they predict that solar cycle 25 will be similar to the current cycle in amplitude.

 

Citation

N. R. Sheeley Jr. and Y.-M. Wang 2015 ApJ 809 113. doi:10.1088/0004-637X/809/2/113

Maffei 1 and 2

Habitable zones are a hot topic in exoplanet studies: where, around a given star, could a planet exist that supports life? But if you scale this up, you get a much less common question: which type of galaxy is most likely to host complex life in the universe? A team of researchers from the UK believes it has the answer.

Criteria for Habitability

Led by Pratika Dayal of the University of Durham, the authors of this study set out to estimate the habitability of a large population of galaxies. The first step in this process is to determine what elements contribute to a galaxy’s habitability. The authors note three primary factors:

  1. Total number of stars
    More stars means more planets!
  2. Metallicity of the stars
    Planets are more likely to form in stellar vicinities with higher metallicities, since planet formation requires elements heavier than iron.
  3. Likelihood of Type II supernovae nearby
    Planets that are located out of range of supernovae have a higher probability of being habitable, since a major dose of cosmic radiation is likely to cause mass extinctions or delay evolution of complex life. Galaxies’ supernova rates can be estimated from their star formation rates (the two are connected via the initial mass function).

Hospitable Cosmic Giants

Number of habitable Earth-like planets

Lower panel: the number of Earth-like habitable planets (given by the color bar, which shows the log ratio relative to the Milky Way) increases in galaxies with larger stellar mass and lower star formation rates. Upper panel: the larger stellar-mass galaxies tend to be elliptical (blue line) rather than spiral (red line). Click for larger view. [Dayal et al. 2015]

Interestingly, these three conditions have previously been shown to be linked via something termed the “fundamental metallicity relation,” which relates the total stellar masses, metallicities, and star formation rates of galaxies. By using this relation, the authors were able to create predictions for the number of habitable planets in more than 100,000 galaxies in the local universe (cataloged by the Sloan Digital Sky Survey).

Based on these predictions, the authors find that the galaxies likely to host the largest number of habitable planets are those that have a mass greater than twice that of the Milky Way and star formation rates less than a tenth of that of the Milky Way.

These galaxies tend to be giant elliptical galaxies, rather than compact spirals like our own galaxy. The authors calculate that the most hospitable galaxies can host up to 10,000 times as many Earth-like planets and 1,000,000 times as many gas-giants (which might have habitable moons) as the Milky Way!

 

Citation

Pratika Dayal et al. 2015 ApJ 810 L2 doi:10.1088/2041-8205/810/1/L2

fallback disk

Did you know that the very first exoplanets ever confirmed were found around a pulsar? The precise timing measurements of pulsar PSR 1257+12 were what made the discovery of its planetary companions possible. Yet surprisingly, though we’ve discovered thousands of exoplanets since then, only one other planet has ever been confirmed around a pulsar. Now, a team of CSIRO Astronomy and Space Science researchers are trying to figure out why.

Formation Challenges

The lack of detected pulsar planets may simply reflect the fact that getting a pulsar-planet system is challenging! There are three main pathways:

  1. The planet formed before the host star became a pulsar — which means it somehow survived its star going supernova (yikes!).
  2. The planet formed elsewhere and was captured by the pulsar.
  3. The planet formed out of the debris of the supernova explosion.

The first two options, if even possible, are likely to be rare occurrences — but the third option shows some promise. In this scenario, after the supernova explosion, a small fraction of the material falls back toward the stellar remnant and is recaptured, forming what is known as a supernova fallback disk. According to this model, planets could potentially form out of this disk.

Disk Implications

Led by Matthew Kerr, the CSIRO astronomers set out to systematically look for these potential planets that might have formed in situ around pulsars. They searched a sample of 151 young, energetic pulsars, scouring seven years of pulse time-of-arrival data for periodic variation that could signal the presence of planetary companions. Their methods to mitigate pulsar timing noise and model realistic orbits allowed them to have good sensitivity to low-mass planets.

The results? They found no conclusive evidence that any of these pulsars have planets.

This outcome carries with it some significant implications. The pulsar sample spans 2 Myr in age, in which planets should have had enough time to form in debris disks. The fact that none were detected suggests that long-lived supernova fallback disks may actually be much rarer than thought, or they exist only in conditions that aren’t compatible with planet formation.

So if that’s the case, what about the planets found around PSR 1257+12? This pulsar may actually be somewhat unique, in that it was born with an unusually weak magnetic field. This birth defect might have allowed it to form a fallback disk — and, subsequently, planets — where the sample of energetic pulsars studied here could not.

 

Citation

M. Kerr et al. 2015 ApJ 809 L11 doi:10.1088/2041-8205/809/1/L11

Milky Way center

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

Clues from Abundances

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

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

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

Different Populations

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

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

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

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

Citation

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

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