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Planetary-mass binary

An object previously identified as a free-floating, large Jupiter analog turns out to be two objects — each with the mass of a few Jupiters. This system is the lowest-mass binary we’ve ever discovered.

Tracking Down Ages

TW Hydrae Association

2MASS J11193254–1137466 is thought to be a member of the TW Hydrae Association, a group of roughly two dozen young stars moving together in the solar neighborhood. [University of Western Ontario/Carnegie Institution of Washington DTM/David Rodriguez]

Brown dwarfs represent the bottom end of the stellar mass spectrum, with masses too low to fuse hydrogen (typically below ~75-80 Jupiter masses). Observing these objects provides us a unique opportunity to learn about stellar evolution and atmospheric models — but to properly understand these observations, we need to determine the dwarfs’ masses and ages.

This is surprisingly difficult, however. Brown dwarfs cool continuously as they age, which creates an observational degeneracy: dwarfs of different masses and ages can have the same luminosity, making it difficult to infer their physical properties from observations.

We can solve this problem with an independent measurement of the dwarfs’ masses. One approach is to find brown dwarfs that are members of nearby stellar associations called “moving groups”. The stars within the association share the same approximate age, so a brown dwarf’s age can be estimated based on the easier-to-identify ages of other stars in the group.

An Unusual Binary

Recently, a team of scientists led by William Best (Institute for Astronomy, University of Hawaii) were following up on such an object: the extremely red, low-gravity L7 dwarf 2MASS J11193254–1137466, possibly a member of the TW Hydrae Association. With the help of the powerful adaptive optics on the Keck II telescope in Hawaii, however, the team discovered that this Jupiter-like object was hiding something: it’s actually two objects of equal flux orbiting each other.

Keck binary

Keck images of 2MASS J11193254–1137466 reveal that this object is actually a binary system. A similar image of another dwarf, WISEA J1147-2040, is shown at bottom left for contrast: this one does not show signs of being a binary at this resolution. [Best et al. 2017]

To learn more about this unusual binary, Best and collaborators began by using observed properties like sky position, proper motion, and radial velocity to estimate the likelihood that 2MASS J11193254–1137466AB is, indeed, a member of the TW Hydrae Association of stars. They found roughly an 80% chance that it belongs to this group.

Under this assumption, the authors then used the distance to the group — around 160 light-years — to estimate that the binary’s separation is ~3.9 AU. The assumed membership in the TW Hydrae Association also provides binary’s age: roughly 10 million years. This allowed Best and collaborators to estimate the masses and effective temperatures of the components from luminosities and evolutionary models.

Planetary-Mass Objects

2MASS J1119–1137 among ultracool dwarfs

The positions of 2MASS J11193254–1137466A and B on a color-magnitude diagram for ultracool dwarfs. The binary components lie among the faintest and reddest planetary-mass L dwarfs. [Best et al. 2017]

The team found that each component is a mere ~3.7 Jupiter masses, placing them in the fuzzy region between planets and stars. While the International Astronomical Union considers objects below the minimum mass to fuse deuterium (around 13 Jupiter masses) to be planets, other definitions vary, depending on factors such as composition, temperature, and formation. The authors describe the binary as consisting of two planetary-mass objects.

Regardless of its definition, 2MASS J11193254–1137466AB qualifies as the lowest-mass binary discovered to date. The individual masses of the components also place them among the lowest-mass free-floating brown dwarfs known. This system will therefore be a crucial benchmark for tests of evolutionary and atmospheric models for low-mass stars in the future.

Citation

William M. J. Best et al 2017 ApJL 843 L4. doi:10.3847/2041-8213/aa76df

Mercury

If space rocks are unpleasant to encounter, space dust isn’t much better. Mercury’s cratered surface tells of billions of years of meteoroid impacts — but its thin atmosphere is what reveals its collisional history with smaller impactors. Now new research is providing a better understanding of what we’re seeing.

Micrometeoroids Ho!

The inner solar system is bombarded by micrometeoroids, tiny particles of dust (on the scale of a tenth of a millimeter) emitted by asteroids and comets as they make their closest approach to the Sun. This dust doesn’t penetrate Earth’s layers of atmosphere, but the innermost planet of our solar system, Mercury, doesn’t have this convenient cushioning.

Just as Mercury is affected by the impacts of large meteoroids, it’s also shaped by the many smaller-scale impacts it experiences. These tiny collisions are thought to vaporize atoms and molecules from the planet’s surface, which quickly dissociate. This process adds metals to Mercury’s exosphere, the planet’s extremely tenuous atmosphere.

Modeling Populations

Impactor source distributions

Distribution of the directions from which meteoroids originate before impacting Mercury’s surface, as averaged over its entire orbit. Local time of 12 hr corresponds to the Sun-facing side. A significant asymmetry is seen between the dawn (6 hrs) and dusk (18 hrs) rates. [Pokorný et al. 2017]

The metal distribution in the exosphere provides a way for us to measure the effect of micrometeoroid impacts on Mercury — but this only works if we have accurate models of the process. A team of scientists led by Petr Pokorný (The Catholic University of America and NASA Goddard SFC) has now worked to improve our picture of micrometeoroid impact vaporization on Mercury.

Pokorný and collaborators argue that two meteoroid populations — Jupiter-family comets (short-period) and Halley-type comets (long-period) — contribute the dust for the majority of micrometeoroid impacts on Mercury. The authors model the dynamics and evolution of these two populations, reproducing the distribution of directions from which micrometeoroids strike Mercury during its yearly orbit.

impact geometry

Schematic of Mercury in its orbit around the Sun. The dawn side leads the orbital motion, while the dusk side trails it.

Geometry of an Orbit

Mercury’s orbit is unique in our solar system: it circles the Sun twice for every three rotations on its own axis — so if you were on Mercury, you’d see a single day pass over the span of two years. As with all prograde planets, the edge leading the Mercury’s orbit marks the dawn terminator, while the edge trailing the planet’s orbital motion marks the dusk terminator.

Pokorný and collaborators find a significant asymmetry in the impact vaporization that occurs on Mercury’s dawn side versus its dusk side. This is due to impact geometry (since the dusk side is shielded from impacts in the direction of motion) and seasonal variation of the dust/meteoroid environment around the planet. The authors show that the source of impact vaporization shifts toward the nightside as Mercury approaches aphelion, and toward the dayside when the planet approaches the Sun.

Importance of Long-Period Comets

seasonal variations

Seasonal variations of the relative vaporization rate from the authors’ model (black line) compared to measurements of Mercury’s exospheric abundance of Ca. The contribution of long-period comets is shown by the blue line. [Pokorný et al. 2017]

The dawn/dusk asymmetry and the seasonal variations predicted by the model are all nicely consistent NASA’s MESSENGER spacecraft observations of the metal distribution in Mercury’s exosphere.

What makes Pokorný and collaborators’ model work so well? Their inclusion of the long-period, Halley-type comets is key: the high impact velocity of the micrometeoroids produced by this family play a significant role in shaping the impact vaporization rate of Mercury’s surface.

This work successfully demonstrates that we can use measurements of Mercury’s exosphere as a unique tool to constrain the dust population in the inner solar system.

Citation

Petr Pokorný et al 2017 ApJL 842 L17. doi:10.3847/2041-8213/aa775d

magnetar

The origin of the mysterious fast radio bursts has eluded us for more than a decade. With the help of a particularly cooperative burst, however, scientists may finally be homing in on the answer to this puzzle.

A Burst Repeats

FRB 121102 host

The host of FRB 121102 is placed in context in this Gemini image. [Gemini Observatory/AURA/NSF/NRC]

More than 20 fast radio bursts — rare and highly energetic millisecond-duration radio pulses — have been observed since the first was discovered in 2007. FRB 121102, however, is unique in its behavior: it’s the only one of these bursts to repeat. The many flashes observed from FRB 121102 allowed us for the first time to follow up on the burst and hunt for its location.

Earlier this year, this work led to the announcement that FRB 121102’s host galaxy has been identified: a dwarf galaxy located at a redshift of z = 0.193 (roughly 3 billion light-years away). Now a team of scientists led by Cees Bassa (ASTRON, the Netherlands Institute for Radio Astronomy) has performed additional follow-up to learn more about this host and what might be causing the mysterious flashes.

HST view of FRB 121102 host

Hubble observation of the host galaxy. The object at the bottom right is a reference star. The blue ellipse marks the extended diffuse emission of the galaxy, the red circle marks the centroid of the star-forming knot, and the white cross denotes the location of FRB 121102 ad the associated persistent radio source. [Adapted from Bassa et al. 2017]

Host Observations

Bassa and collaborators used the Hubble Space Telescope, the Spitzer Space Telecsope, and the Gemini North telecsope in Hawaii to obtain optical, near-infrared, and mid-infrared observations of FRB 121102’s host galaxy.

The authors determined that the galaxy is a dim, irregular, low-metallicity dwarf galaxy. It’s resolved, revealing a bright star-forming region roughly 4,000 light-years across in the galaxy’s outskirts. Intriguingly, the persistent radio source associated with FRB 121102 falls directly within that star-forming knot.

Bassa and collaborators also found that the properties of the host galaxy are consistent with those of a type of galaxy known as extreme emission line galaxies. This provides a tantalizing clue, as these galaxies are known to host both hydrogen-poor superluminous supernovae and long-duration gamma-ray bursts.

Linking to the Cause

What can this tell us about the cause of FRB 121102? The fact that this burst repeats already eliminates cataclysmic events as the origin. But the projected location of FRB 121102 within a star-forming region — especially in a host galaxy that’s similar to those typically hosting superluminous supernovae and long gamma-ray bursts — strongly suggests there’s a relation between these events.

gamma-ray burst

Artist’s impression of a gamma-ray burst in a star-forming region. [NASA/Swift/Mary Pat Hrybyk-Keith and John Jones]

The authors propose that this observed coincidence, supported by models of magnetized neutron star birth, indicate an evolutionary link between fast radio bursts and neutron stars. In this picture, neutron stars or magnetars are born as long gamma-ray bursts or hydrogen-poor supernovae, and then evolve into fast-radio-burst-emitting sources.

This picture may finally explain the cause of fast radio bursts — but Bassa and collaborators caution that it’s also possible that this model applies only to FRB 121102. Since FRB 121102 is unique in being the only burst discovered to repeat, its cause may also be unique. The authors suggest that targeted searches of star-forming regions in galaxies similar to FRB 121102’s host may reveal other repeating burst candidates, helping us to unravel the ongoing mystery of fast radio bursts.

Citation

C. G. Bassa et al 2017 ApJL 843 L8. doi:10.3847/2041-8213/aa7a0c

galactic center minispiral

continuum minispiral

An image of the continuum emission from the galactic center minispiral, previously taken by ALMA at 100 GHz. This image labels the structures of the minispiral: a bar and multiple arcing arms, and the black hole Sgr A* near the center. [Tsuboi et al. 2017]

The region around Sgr A*, the 4-million-solar-mass black hole at the heart of our galaxy, is a complex and dynamic place. New Atacama Large Millimeter/submillimeter Array (ALMA) observations of the Milky Way’s center now reveal more about this harsh, inhospitable environment.

A New View

One of the prominent structures at the heart of the Milky Way is a bundle of ionized gas streams located surrounding Sgr A* within the close distance of 6.5 light-years. These streams take the form of a bar and a series of arms that make it look much like a tiny spiral galaxy — earning it the name of “the galactic center minispiral.”

Where did this gas come from? What’s happening to it now? And what can it tell us about the environment about Sgr A*? A team of scientists led by Masato Tsuboi (Japan Aerospace Exploration Agency) has now obtained new ALMA images of the minispiral that are helping us to answer these questions.

electron temperatures and densities

Electron temperature in K (numbers in yellow) and density in cm-3 (numbers in red) from the new ALMA observations of the ionized gas streamers. [Tsuboi et al. 2017]

Clues from Gas

Tsuboi and collaborators imaged the gas within the galactic center minispiral and its surroundings as part of the first ALMA observation cycle. This powerful telescope’s images allowed the team to observe the streamers of ionized gas within the arms of the minispiral and determine their velocities. The authors were then able to use these measurements to identify which gas components are related and the speeds and directions of motion for the different components.

Besides tracking the dynamics of the ionized gas in the minispiral, the team also confirmed that the electron temperatures and densities in the streamers increase with proximity to Sgr A*. We would expect these increases to cause the arms to expand laterally closer to the black hole — but that’s not what’s observed. Instead, the arms remain closely confined.

This discrepancy tells us something about the environment around the minispiral: there must be surrounding, ambient ionized gas that’s pushing on the streamers, providing the external pressure to keep them confined.

An Explanation for Proplyds

proplyds in minispiral

The ALMA observations of ionized gas (top panel) line up nicely with the JVLA detections of candidate proplyds near Sgr A* (bottom panel, with the ionized gas emission from the top panel shown as contours). [Tsuboi et al. 2017]

Lastly, Tsuboi and collaborators compare their ALMA observations of the ionized gas in the minispiral with previous observations of the galactic center made with the Jansky Very Large Array. The JVLA observations revealed the presence of compact half-shell-like structures that may be “proplyds” — protostars being photoevaporated by the hot radiation coming from the central star cluster around Sgr A*.

These candidate proplyds have posed an astronomical puzzle: between Sgr A*’s strong tidal forces and the radiation being emitted from the central star cluster, conditions are extremely inhospitable to star formation. So how did these proplyds get there?

Tsuboi and collaborators’ observations may shed some light on this. Lining up the new ALMA images with the old JVLA ones, it’s clear that the proplyds are all concentrated along the ionized gas streamer of the northeastern arm in the minispiral. This suggests that the protostars may have formed further away from Sgr A*, and they were brought to their present-day location as the streamer fell inwards toward the black hole.

Citation

Masato Tsuboi et al 2017 ApJ 842 94. doi:10.3847/1538-4357/aa74e3

cloudy exoplanet

Direct imaging of exoplanets was once only possible for the brightest of planets orbiting the dimmest of stars — but improving technology is turning this into an increasingly powerful technique. In a new study, direct-imaging observations of the Jupiter-like exoplanet 51 Eridani b provide tantalizing clues about its atmosphere.

Direct Imaging of 51 Eri b

51 Eri b

Images of 51 Eri b from GPI (top) and Keck (bottom). [Rajan et al. 2017]

While transit detections remain the best way to discover large quantities of new exoplanets, direct imaging provides a unique advantage: you can measure the light from the exoplanet itself. With proper constraints on the host star, it therefore becomes possible to measure the spectrum of the planet’s atmosphere.

One target for this technique is 51 Eri b, a Jupiter-like exoplanet located roughly 100 light-years away. This object was the first exoplanet directly imaged by the Gemini Planet Imager Exoplanet Survey, a project that used the Gemini Planet Imager (GPI) instrument in Chile to search for exoplanets around 600 young nearby stars.

A team of scientists led by Abhijith Rajan (Arizona State University) has now made new near-infrared observations of 51 Eri b: spectroscopy in the K band using GPI, and photometry in the Ms band with a camera on the Keck I telescope in Hawaii. Rajan and collaborators combined this new data with past observations and modeling to better characterize the 51 Eri b’s properties.

51 Eri b color

Color–magnitude diagrams for brown dwarfs and imaged exoplanets (click for a closer look). The color of 51 Eri b (marked with red star) places it among late T dwarfs, but it is redder than most comparable-temperature brown dwarfs. The tracks for the L–T transition for two different planet masses are shown. [Rajan et al. 2017]

Cloudy Transition

One intriguing aspect of 51 Eri b is the challenge of determining its spectral type. Though its spectrum is consistent with that of a T dwarf, photometry shows that it’s unusually red for this spectral type. There may be a reason for this, however: clouds.

Rajan and collaborators find that the best fitting models for 51 Eri b’s spectra all have an atmosphere consisting of patchy clouds. This result holds true both for models with the salt and sulfide clouds expected to condense in the atmospheres of mid-to-late T dwarfs, and for models with the iron and silicate clouds common in atmospheres of redder L-dwarfs.

The authors hypothesize that 51 Eri b may be in the process of transitioning from a warmer L-type body to a cooler T-type body. As an L-type planet cools, holes and low-opacity patches appearing in an initially uniform cloud deck could cause the transition of the planet’s spectrum to T-type.

An Unusual Start?

prospects with JWST

The ten best-fitting cloudy (red) and cloudless (blue) atmospheres over the wavelength range of JWST, illustrating that JWST will likely be able to differentiate between atmospheric models for 51 Eri b. [Rajan et al. 2017]

In addition to examining 51 Eri b’s atmosphere, Rajan and collaborators use its luminosity to explore how it may have formed. They demonstrate that 51 Eri b is one of the only directly imaged planets that’s consistent with what’s known as the cold-start scenario, in which planets slowly grow via accretion onto a solid core.

While much remains to be learned about 51 Eri b, these new results provide an excellent step in the right direction. The authors also show that future observations — such as with the James Webb Telescope — will allow us to further differentiate between models describing this planet. 51 Eri b’s intriguing atmosphere makes it a prime target to revisit as our observational capabilities continue to improve.

Citation

Abhijith Rajan et al 2017 AJ 154 10. doi:10.3847/1538-3881/aa74db

AGN torus

Supermassive black holes are thought to grow in heavily obscured environments. A new study now suggests that many of the brightest supermassive black holes around us may be escaping our detection as they hide in these environments.

Unified AGN

The geometric dependence of AGN types in the unified AGN model. Type 1 AGN are viewed from an angle where the central engine is visible. In Type 2 AGN, the dusty torus obscures the central engine from view. [Urry & Padovani, 1995]

A Torus Puzzle

The centers of galaxies with bright, actively accreting supermassive black holes are known as active galactic nuclei, or AGN. According to a commonly accepted model for AGN, these rapidly growing black holes and their accretion disks are surrounded by a thick torus of dust. From certain angles, the torus can block our direct view of the central engines, changing how the AGN appears to us. AGN for which we can see the central engine are known as Type 1 AGN, whereas those with an obscured central region are classified as Type 2.

Oddly, the fraction of AGN classified as Type 2 decreases substantially with increasing luminosity; brighter AGN seem to be more likely to be unobscured. Why? One hypothesis is that the torus structure itself changes with changing AGN luminosity. In this model, the torus recedes as AGN become brighter, causing fewer of these AGN to be obscured from our view.

But a team of scientists led by Silvia Mateos (Institute of Physics of Cantabria, Spain) suggests that we may instead be missing the bigger picture. What if the problem is just that many of the brightest obscured AGN are too well hidden?

Geometry Matters

Missing sources

Type 2 AGN fraction vs. torus covering factor for AGN in the authors’ three luminosity bins. The black line shows the 1-to-1 relation describing the expected Type 2 AGN fraction; the black data points show the observed fraction. The red points show the best-fit model including the “missing” AGN, and the inset shows the covering-factor distribution for the missing sources. [Mateos et al. 2017]

Mateos and collaborators built a sample of nearly 200 X-ray-observed AGN from the Bright Ultra-hard XMM-Newton Survey (BUXS). They then determined the intrinsic fraction of these AGN that were obscured (i.e., classified as Type 2) at a given luminosity, for redshifts between 0.05 ≤ z ≤ 1.

The team next used clumpy torus models to estimate the distributions of AGN covering factors, the geometric factor that describes the fraction of the sky around the AGN central engine that’s obscured.

The pointing directions for AGN should be randomly distributed, and geometry then dictates that the covering factor distributions combined over the total AGN population should match the intrinsic fraction of AGN classified as Type 2 AGN. Instead, the sample from BUXS reveals a “missing” population of high-covering-factor tori that we have yet to detect in X-rays.

Missing Sources

When they include the missing AGN, Mateos and collaborators find that the total fraction of Type 2 AGN is around 58%. They also show that more of these AGN are missing at higher luminosities. By including the missing ones, the total fraction of obscured AGN therefore has a much weaker dependence on luminosity than we thought — which suggests that the receding torus model isn’t necessary to explain observations.

Mateos and collaborators’ results support the idea that the majority of very bright, rapidly accreting supermassive black holes at redshifts of z ≤ 1 live in nuclear environments that are extremely obscured. These black holes are so well embedded in their environments that they’ve escaped detection in X-ray surveys thus far.

Citation

S. Mateos et al 2017 ApJL 841 L18. doi:10.3847/2041-8213/aa7268

first stars

Primordial supermassive stars might be responsible for the earliest supermassive black holes in our universe. But just how big can a star grow before it inevitably collapses into a black hole?

The Puzzle of Distant Quasars

quasar

Artist’s illustration of a high-redshift quasar, a black hole feeding on the material around it. [ESO/M. Kornmesser]

Quasars — supermassive black holes that are actively feeding — have been observed with enormous sizes (billions of solar masses) at very large distances (redshifts of z > 6). These monsters pose a problem: how could they have accreted so much mass in so little time since the beginning of the universe?

One theory is that these black holes formed from the direct collapse of stars. The larger the original star before collapse, the better the chances that the resulting black hole will be able to grow quickly. But even theorized Pop III stars (which have hundreds of solar masses) would have to accrete at rates higher than believed possible to achieve the black-hole masses we observe so quickly. For this reason, the commonly invoked explanation now is supermassive stars.

Early Giants

Supermassive stars are theoretical stars that formed in the very different environment of the early universe.

star formation

A composite infrared and X-ray image showing a molecular cloud and newly formed stars around Cepheus B. [X-ray: NASA/CXC/PSU/K. Getman et al.; IR: NASA/JPL-Caltech/CfA/J. Wang et al.]

In ordinary star formation, halos of gas cool primarily due to emission by molecules. When these clouds cool, they fragment and then collapse into normal-sized stars.

In the supermassive star-formation scenario, hydrogen molecules in primordial halos are broken down — possibly by ultraviolet radiation from nearby star formation. This prevents the halos from cooling by molecular emission, instead allowing them to grow to an enormous 107–108 solar masses before they start cooling due to atomic emission. At this point they finally collapse to form a star.

Stars forming via this scenario quickly grow to be very massive, as the halo material falls onto the core at catastrophic rates of 0.01–10 solar masses per year. After a short period of this rapid accretion, the supermassive star then collapses into a black hole due to instability. But how massive could such a star grow before its collapse?

Simulating Growth

To answer this question, a team of scientists led by Tyrone Woods (Monash University, Australia) ran stellar-evolution simulations of the birth, growth, and eventual collapse of accreting, non-rotating supermassive stars. Their simulations included the effects of nuclear burning and captured the hydrodynamics of the instability that causes the stars to collapse into black holes, allowing the authors to follow the whole evolution.

final mass before collapse

The final mass at collapse of a star as a function of its accretion rate. Most stars collapse due to instability during hydrogen burning. [Woods et al. 2017]

Woods and collaborators found that for accretion rates above 0.1 solar masses per year, the supermassive stars generally collapsed into black holes at masses of 150,000–330,000 solar masses. Since the final mass at collapse grows only logarithmically with accretion rate, the upper end of this range represents an approximate upper limit on the mass of supermassive stars.

This also sets the maximum mass of the supermassive black holes formed by direct collapse of stars in the early universe. At hundreds of thousands of solar masses, these first quasars provide much more plausible seeds than Pop III stars for growing the billion-solar-mass monsters we observe at high redshifts. Supermassive stars may indeed be the key to the formation of the first and most luminous quasars in our universe.

Citation

T. E. Woods et al 2017 ApJL 842 L6. doi:10.3847/2041-8213/aa7412

Tycho SNR

The outlined regions mark the 57 knots in Tycho selected by the authors for velocity measurements. Magenta regions have redshifted line-of-sight velocities (moving away from us); cyan regions have blueshifted light-of-sight velocities (moving toward us). [Williams et al. 2017]

The Tycho supernova remnant was first observed in the year 1572. Nearly 450 years later, astronomers have now used X-ray observations of Tycho to build the first-ever 3D map of a Type Ia supernova remnant.

Signs of Explosions

Supernova remnants are spectacular structures formed by the ejecta of stellar explosions as they expand outwards into the surrounding interstellar medium.

One peculiarity of these remnants is that they often exhibit asymmetries in their appearance and motion. Is this because the ejecta are expanding into a nonuniform interstellar medium? Or was the explosion itself asymmetric? The best way we can explore this question is with detailed observations of the remnants.

Histograms of the velocity in distribution of the knots in the X (green), Y (blue) and Z (red) directions (+Z is away from the observer). They show no evidence for asymmetric expansion of the knots. [Williams et al. 2017]

Enter Tycho

To this end, a team of scientists led by Brian Williams (Space Telescope Science Institute and NASA Goddard SFC) has worked to map out the 3D velocities of the ejecta in the Tycho supernova remnant. Tycho is a Type Ia supernova — thought to be caused by the thermonuclear explosion of a white dwarf in a binary system that was destabilized by mass transfer from its companion.

After ~450 years of expansion, the remnant now has the morphological appearance of a roughly circular cloud of clumpy ejecta. The forward shock wave from the supernova, however, is known to have twice the velocity on one side of the shell as on the other.

To better understand this asymmetry, Williams and collaborators selected a total of 57 knots in Tycho’s ejecta, spread out around the remnant. They then used 12 years of Chandra X-ray observations to measure both the knots’ proper motion in the plane of the sky and their line-of-sight velocity. These two measurements were then combined to build a full 3D map of the motion of the ejecta.

3D hydrodynamical simulations of Tycho, stopped at the current epoch. These show that both initially smooth (top) and initially clumpy (bottom) ejecta models are consistent with the current observations of the morphology and dynamics of Tycho’s ejecta. [Adapted from Williams et al. 2017]

Symmetry and Clumps

Williams and collaborators found that the knots have total velocities that range from 2400 to 6600 km/s. Unlike the forward shock of the supernova, Tycho’s ejecta display no asymmetries in their motion — which suggests that the explosion itself was symmetric. The more likely explanation is a density gradient in the interstellar medium, which could slow the shock wave on one side of the remnant without yet affecting the motion of the clumps of ejecta.

As a final exploration, the authors attempt to address the origin of Tycho’s clumpiness. The fact that some of Tycho’s ejecta knots precede its outer edge has raised the question of whether the ejecta started out clumpy, or if they began smooth and only clumped during expansion. Williams and collaborators matched the morphological and dynamical data to simulations, demonstrating that neither scenario can be ruled out at this time.

This first 3D map of a Type Ia supernova represents an important step in our ability to understand these stellar explosions. The authors suggest that we’ll be able to expand on this map in the future with additional observations from Chandra, as well as with new data from future X-ray observatories that will be able to detect fainter emission.

Citation

Brian J. Williams et al 2017 ApJ 842 28. doi:10.3847/1538-4357/aa7384

protogalaxies

How do magnetic fields form and evolve in early galaxies? A new study has provided some clever observations to help us answer this question.

The Puzzle of Growing Fields

Dynamo theory is the primary model describing how magnetic fields develop in galaxies. In this picture, magnetic fields start out as weak seed fields that are small and unordered. These fields then become ordered and amplified by large-scale rotation and turbulence in galaxy disks and halos, eventually leading to the magnetic fields we observe in galaxies today.

protogalaxy schematic

Schematic showing how to indirectly measure protogalactic magnetic fields. The measured polarization of a background quasar is altered by the fields in a foreground protogalaxy. Click for a closer look! [Farnes et al. 2017/Adolf Schaller/STSCI/NRAO/AUI/NSF]

To test this model, we need observations of the magnetic fields in young protogalaxies. Unfortunately, we don’t have the sensitivity to be able to measure these fields directly — but a team of scientists led by Jamie Farnes (Radboud University in the Netherlands) have come up with a creative alternative.

The key is to find early protogalaxies that absorb the light of more distant background objects. If a protogalaxy lies between us and a distant quasar, then magnetic fields of the protogalaxy — if present — will affect the polarization measurements of the background quasar.

Observing Galactic Building Blocks

redshift distributions

Top: Redshift distribution for the background quasars in the authors’ sample. Bottom: Redshift distribution for the foreground protogalaxies the authors are exploring. [Farnes et al. 2017]

Farnes and collaborators examined two types of foreground protogalaxies: Damped Lyman-Alpha Absorbers (DLAs) and Lyman Limit Systems (LLSs). They obtained polarimetric data for a sample of 114 distant quasars with nothing in the foreground (the control sample), 19 quasars with DLAs in the foreground, and 27 quasars with LLSs in the foreground. They then used statistical analysis techniques to draw conclusions about the magnetic fields in the foreground protogalaxies.

Farnes and collaborators were unable to detect either coherent or random magnetic fields in DLAs. LLSs, however, showed some evidence of coherent magnetic fields and significant evidence of incoherent magnetic fields. The observations show that the magnetized gas in LLSs must be highly turbulent on a scale of ~5–20 parsecs — similar to turbulence scales in the Milky Way.

Support for Dynamos

What do these observations imply? Both support the dynamo theory of magnetic field growth in galaxies!

Polarization fraction

Polarization fraction distributions (top) and their logarithms (bottom) for sources with and without protogalaxies in the foreground (pink for DLAs, blue for LLSs, and grey for no intervenor). Statistical analysis reveals that the distribution for LLSs differs from the control sample, indicating the presence of magnetized gas. [Adapted from Farnes et al. 2017]

The DLAs appear to consist of mostly non-turbulent quiescent gas; no dynamo action is currently occurring in these protogalaxies. The LLSs, on the other hand, appear to be growing their random magnetic fields via a turbulent dynamo. The fields have not yet had enough time to become ordered like the fields of more evolved galaxies, however.

Farnes and collaborators’ data indicate that magnetic fields are indeed being gradually built up in early galaxies by dynamos. They also suggest that DLAs may represent an earlier galactic evolutionary stage than LLSs, as DLAs haven’t yet had the time to develop their magnetic fields to a detectable level.

A future increase in sample size will certainly help improve our understanding of the field formation process. In the meantime, the data in this study provide the first observational picture of magnetic field evolution in galaxies, lending excellent support to theoretical models.

Citation

J. S. Farnes et al 2017 ApJ 841 67. doi:10.3847/1538-4357/aa7060

tidal disruption event

When a passing star is torn apart by a supermassive black hole, it emits a flare of X-ray, ultraviolet, and optical light. What can we learn from the infrared echo of a violent disruption like this one?

Stellar Destruction

F01004–2237

Optical (black triangles) and infrared (blue circles and red squares) observations of F01004–2237. Day 0 marks the day the optical emission peaked. The infrared emission rises steadily through the end of the data. [Dou et al. 2017]

Tidal disruption events occur when a star passes within the tidal radius of a supermassive black hole. After tidal forces pull the star apart, much of the stellar matter falls onto the black hole, radiating briefly in X-ray, ultraviolet and optical as it accretes. This signature rise and gradual fall of emission has allowed us to detect dozens of tidal disruption events thus far.

One of the recently discovered candidate events is a little puzzling. Not only does the candidate in ultraluminous infrared galaxy F01004–2237 have an unusual host — most disruptions occur in galaxies that are no longer star-forming, in contrast to this one — but its optical light curve also shows an unusually long decay time.

Now mid-infrared observations of this event have been presented by a team of scientists led by Liming Dou (Guangzhou University and Department of Education, Guangdong Province, China), revealing why this disruption is behaving unusually.

dusty torus

Schematic of a convex dusty ring (red bows) that absorbs UV photons and re-emits in the infrared. It simultaneously scatters UV and optical photons into our line of sight. The dashed lines illustrate the delays at lags of 60 days, 1, 2, 3, 4, and 5 years. [Adapted from Dou et al. 2017]

A Dusty Solution?

The optical flare from F01004–2237’s nucleus peaked in 2010, so Dou and collaborators obtained archival mid-infrared data from the WISE and NEOWISE missions from 2010 to 2016. The data show that the galaxy is quiescent in mid-infrared in 2010 — but in data from three years later, the infrared emission has significantly increased, and it continues to brighten steadily through the end of the data.

What’s going on? The supermassive black hole in the nucleus of F01004–2237 is likely shrouded by dust! The optical and ultraviolet radiation from the disruption is absorbed by the dust surrounding the black hole. This light is then reemitted as infrared radiation — which we see as a delayed echo of the flare, since the light had to travel out to the surrounding dust before being reemitted and traveling to us.

Modeling Echoes

Dust ring model

A fit of the data (points) to light curves (dashed lines) generated by one of the authors’ dust ring models. [Adapted from Dou et al. 2017]

Dou and collaborators show that the observations of F01004–2237 can be explained if the black hole is surrounded by a thick torus of at least 7 solar masses’ worth of dust, with a radius of at least 3 light-years. Such a large dust mass so close to the supermassive black hole implies that these dust grains can’t have been newly formed — so they must have already been there from the dusty torus of the galactic nucleus.

The authors point out that this dusty ring solves one of the mysteries of this disruption candidate: because the dust also scatters some of the optical light, this explains why the optical light curve didn’t decay as quickly as we’d expect.

Conveniently, the authors’ model of this event can be easily tested: it predicts a sharp decrease in the mid-infrared flux in the near future. Continued monitoring of F01004–2237 in mid-infrared channels should therefore soon be able to confirm our picture of this event. If we’re correct, these observations provide us with an excellent opportunity to learn about the environments around supermassive black holes.

Citation

Liming Dou et al 2017 ApJL 841 L8. doi:10.3847/2041-8213/aa7130

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