Features Archives - Page 95 of 122

Features RSS

22 November 2017

Analog of the Milky Way and the Magellanic Clouds

A hunt for merging dwarf galaxies has yielded an intriguing result: 180 million light-years away, a galaxy very similar to the Milky Way — with two dwarf-galaxy satellites just like our own Magellanic clouds.

Unusual Satellites

The Large and Small Magellanic clouds, as observed from Earth. [ESO/S. Brunier]

The Large and Small Magellanic clouds (LMC and SMC), the only bright and star-forming satellite galaxies around the Milky Way, have proven unusual in the universe: satellite pairs of LMC–SMC mass are neither common in observation nor typically produced in numerical simulations of galaxy formation and evolution.

Since the probability of having such an interacting pair of satellites in a massive halo is so low, this raises questions about how our system came about. Did the Magellanic clouds form independently around the Milky Way and then interact? Were they more recently captured as an already-merging pair of dwarf galaxies? Or is there some other explanation?

If we could find other systems that look like the LMC–SMC–Milky-Way system, we might be able to learn more about pairs of dwarf galaxies and how they interact near the halos of large galaxies like the Milky Way. Conveniently, two researchers from Yonsei University in South Korea, Sanjaya Paudel and Chandreyee Sengupta, have now identified exactly such a system.

The UGC 4703 pair of dwarf galaxies show a stellar bridge connecting them — a sign of their past interaction, when tidal forces stripped material from them as they passed each other. [Adapted from Paudel & Sengupta 2017]

An Interacting Pair

Hunting for merging dwarf galaxies in various environments, Paudel and Sengupta found UGC 4703, an interacting pair of dwarf galaxies that are located near the isolated spiral galaxy NGC 2718. This pair of satellites around the massive spiral bear a striking resemblance to the LMC–SMC system around the Milky-Way.

The authors performed a multi-wavelength study of the system using archival images from the Sloan Digital Sky Survey, The Galaxy Evolutionary Explorer spacecraft, and the Spitzer Space Telescope. They also gather new observations of the H I gas distribution in the system using the Giant Metrewave Radio Telescope in India.

Paudel and Sengupta find that NGC 2718 and the Milky Way have similar stellar masses, and the stellar mass ratio of the UGC 4703 interacting pair is around 5:1, similar to the mass ratio of the LMC to the SMC. The separation of the UGC 4703 pair is also roughly the same as that of the LMC and SMC: ~70,000 light-years.

Similarities and Differences

The H I gas distribution in UGC 4703 reveals both similarities and differences between this system and the LMC–SMC system. [Paudel & Sengupta 2017]

The stellar bridge connecting the components of the UGC 4703 system are a sign of their past interaction, but a comparison of the optical and H I morphology between the UGC 4703 pair and the LMC–SMC pair suggests that the UGC 4703 galaxies are either interacting more slowly than the Magellanic clouds or that the interaction is at a more advanced stage than we see with the LMC–SMC.

Understanding these similarities and differences between the LMC–SMC–Milky-Way system and this analog are an important first step to studying dwarf galaxy pairs as they interact near the massive halos of their large spiral hosts. In the future, further observations of UGC 4703 and detailed modeling of the system may help continue to puzzle out how our own Magellanic clouds came about.

Citation

Sanjaya Paudel and C. Sengupta 2017 ApJL 849 L28. doi:10.3847/2041-8213/aa95bf

20 November 2017

Kepler Planets Tend to Have Siblings of the Same Size

After 8.5 years of observations with the Kepler space observatory, we’ve discovered a large number of close-in, tightly-spaced, multiple-planet systems orbiting distant stars. In the process, we’ve learned a lot about the properties about these systems — and discovered some unexpected behavior. A new study explores one of the properties that have surprised us: planets of the same size tend to live together.

Orbital architectures for 25 of the authors’ multiplanet systems. The dots are sized according to the planets’ relative radii and colored according to mass. Planets of similar sizes and masses tend to live together in the same system. [Millholland et al. 2017]

Ordering of Systems

From Kepler’s observations of extrasolar multiplanet systems, we have seen that the sizes of planets in a given system aren’t completely random. Systems that contain a large planet, for example, are more likely to contain additional large planets rather than additional planets of random size. So though there is a large spread in the radii we’ve observed for transiting exoplanets, the spread within any given multiplanet system tends to be much smaller.

This odd behavior has led us to ask whether this clustering occurs not just for radius, but also for mass. Since the multiplanet systems discovered by Kepler most often contain super-Earths and mini-Neptunes, which have an extremely large spread in densities, the fact that two such planets have similar radii does not guarantee that they have similar masses.

If planets don’t cluster in mass within a system, this would raise the question of why planets coordinate only their radii within a given system. If they do cluster in mass, it implies that planets within the same system tend to have similar densities, potentially allowing us to predict the sizes and masses of planets we might find in a given system.

Insight into Masses

Led by NSF graduate research fellow Sarah Millholland, a team of scientists at Yale University used recently determined masses for planets in 37 Kepler multiplanet systems to explore this question of whether exoplanets in a multiplanet system are more likely to have similar masses rather than random ones.

Millholland and collaborators find that the masses do show the same clustering trend as radii in multiplanet systems — i.e., sibling planets in the same system tend to have both masses and radii that are more similar than if the system were randomly assembled from the total population of planets we’ve observed. Furthermore, the masses and radii tend to be ordered within a system when the planets are ranked by their periods.

median planetary radius vs host star metallicity

The host star’s metallicity is correlated with the median planetary radius for a system. [Adapted from Millholland et al. 2017]

The authors note two important implications of these results:

The scatter in the relation between mass and radius of observed exoplanets is primarily due to system-to-system variability, rather than the variability within each system.
Knowing the properties of a star and its primordial protoplanetary disk might allow us to predict the outcome of the planet formation process for the system.

Following up on the second point, the authors test whether certain properties of the host star correlate with properties of the planets. They find that the stellar mass and metallicity have a significant effect on the planet properties and the structure of the system.

Continuing to explore multiplanet systems like these appears to be an excellent path forward for understanding the hidden order in the broad variety of exoplanets we’ve observed.

Citation

Sarah Millholland et al 2017 ApJL 849 L33. doi:10.3847/2041-8213/aa9714

17 November 2017

LIGO Finds Lightest Black-Hole Binary

Wednesday evening the Laser Interferometer Gravitational-wave Observatory (LIGO) collaboration quietly mentioned that they’d found gravitational waves from yet another black-hole binary back in June. This casual announcement reveals what is so far the lightest pair of black holes we’ve watched merge — opening the door for comparisons to the black holes we’ve detected by electromagnetic means.

A Routine Detection

The chirp signal of GW170608 detected by LIGO Hanford and LIGO Livingston. [LIGO collaboration 2017]

After the fanfare of the previous four black-hole-binary merger announcements over the past year and a half — as well as the announcement of the one neutron-star binary merger in August — GW170608 marks our entry into the era in which gravitational-wave detections are officially “routine”.

GW170608, a gravitational-wave signal from the merger of two black holes roughly a billion light-years away, was detected in June of this year. This detection occurred after we’d already found gravitational waves from several black-hole binaries with the two LIGO detectors in the U.S., but before the Virgo interferometer came online in Europe and increased the joint ability of the detectors to localize sources.

Mass estimates for the two components of GW170608 using different models. [LIGO collaboration 2017]

Overall, GW170608 is fairly unremarkable: it was detected by both LIGO Hanford and LIGO Livingston some 7 ms apart, and the signal looks not unlike those of the previous LIGO detections. But because we’re still in the early days of gravitational-wave astronomy, every discovery is still remarkable in some way! GW170608 stands out as being the lightest pair of black holes we’ve yet to see merge, with component masses before the merger estimated at ~12 and ~7 times the mass of the Sun.

Why Size Matters

With the exception of GW151226, the gravitational-wave signal discovered on Boxing Day last year, all of the black holes that have been discovered by LIGO/Virgo have been quite large: the masses of the components have all been estimated at 20 solar masses or more. This has made it difficult to compare these black holes to those detected by electromagnetic means — which are mostly under 10 solar masses in size.

GW170608 is the lowest-mass of the LIGO/Virgo black-hole mergers shown in blue. The primary mass is comparable to the masses of black holes we have measured by electromagnetic means (purple detections). [LIGO-Virgo/Frank Elavsky/Northwestern]

One type of electromagnetically detected black hole are those in low-mass X-ray binaries (LMXBs). LMXBs consist of a black hole and a non-compact companion: a low-mass donor star that overflows its Roche lobe, feeding material onto the black hole. It is thought that these black holes form without significant spin, and are later spun up as a result of the mass accretion. Before LIGO, however, we didn’t have any non-accreting black holes of this size to observe for comparison.

Now, detections like GW170608 and the Boxing Day event (which was also on the low end of the mass scale) are allowing us to start exploring spin distributions of non-accreting black holes to determine if we’re right in our understanding of black-hole spins. We don’t yet have a large enough comparison sample to make a definitive statement, but GW170608 is indicative of a wealth of more discoveries we can hope to find in LIGO’s next observing run, after a series of further design upgrades scheduled to conclude in 2018. The future of gravitational wave astronomy continues to look promising!

Citation

“GW170608: Observation of a 19 Solar-mass Binary Black Hole Coalescence,” B. P. Abbott et al 2017 ApJL 851 L35. doi:10.3847/2041-8213/aa9f0c

15 November 2017

A Look at the Milky Way’s Outskirts

Studying the large-scale structure of the Milky Way is difficult given that we’re stuck in its interior — which means we can’t step back for a broad overview of our home. Instead, a recent study uses distant variable stars to map out a picture of what’s happening in the outskirts of our galaxy.

Mapping with Tracers

Phase-folded light curve for two of the RR Lyrae stars in the authors’ sample, each with hundreds of observations over 7 years. [Cohen et al. 2017]

Since observing the Milky Way from the outside isn’t an option, we have to take creative approaches to mapping its outer regions and measuring its total mass and dark matter content. One tool used by astronomers is tracers: easily identifiable stars that can be treated as massless markers moving only as a result of the galactic potential. Mapping the locations and motions of tracers allows us to measure the larger properties of the galaxy.

RR Lyrae stars are low-mass, variable stars that make especially good tracers. They pulsate predictably on timescales of less than a day, creating distinctive light curves that can easily be distinguished and tracked in wide-field optical imaging surveys over long periods of time. Their brightness makes them detectable out to large distances, and their blue color helps to separate them from contaminating stars in the foreground.

Best of all, RR Lyrae stars are very nearly standard candles: their distances can be determined precisely with only knowledge of their measured light curves.

Locations on the sky of the several hundred outer-halo RR Lyrae stars in the authors’ original sample. The red curve shows the location of the Sagittarius stream, an ordered structure the authors avoided so as to only have unassociated stars in their sample. [Cohen et al. 2017]

Distant Variables

In a new study led by Judith Cohen (California Institute of Technology), the signals of hundreds of distant RR Lyrae stars were identified in observations of transient objects made with the Palomar Transient Factory (PTF) survey. Cohen and collaborators then followed up with the Keck II telescope in Hawaii to obtain spectra for a narrower sample of 122 RR Lyrae stars.

The stars in the sample lie at whopping distances of ~150,000–350,000 light-years from us. For comparison, we’re about 25,000 light-years from the center of the galaxy, and the stellar disk of the galaxy is only thought to be perhaps 100,000 light-years across — so these variable stars lie firmly in the Milky Way’s outer halo. The spectra of the stars reveal their radial velocity, providing us with precise measurements of how objects in the outer halo move.

More Space in the Suburbs?

Histogram with distance for the ~450 RR Lyrae stars in the authors’ broader sample. When the authors include their estimates for the completeness of their sample, the best fit scales with distance as r^-4, shown by the red line. [Cohen et al. 2017]

After reporting the velocity dispersions that they measure — which can be used to make more precise estimates of the Milky Way’s total mass — Cohen and collaborators discuss the stellar density implied by their sample. They find that the density of stars in the outer halo of the Milky Way scales with their distance as r^-4. This is similar to the drop-off in density we’ve measured in the inner halo, and it contradicts some studies that have predicted a much sharper drop in stellar density in the Milky Way’s outermost regions.

The work presented in this study goes a long way toward building our view of the galaxy’s outer halo. Future catalogs like the Pan-STARRS RR Lyrae catalog and upcoming surveys like LSST should also significantly increase the tracer sample size and measurement accuracy, further allowing us to map out the outskirts of the Milky Way.

Citation

Judith G. Cohen et al 2017 ApJ 849 150. doi:10.3847/1538-4357/aa9120

8 November 2017

Carrying Energy to the Corona with Waves

The solar corona has a problem: it’s weirdly hot! A new study explores how magnetic waves might solve the mystery of the unusually hot corona by transporting energy to the outer atmosphere of the Sun.

The Problem with the Corona

The temperatures of different layers of the Sun. Click for a closer look. [ISAS/JAXA]

The corona, the outer layer of the Sun’s atmosphere, has typical temperatures of 1–3 million K — significantly hotter than the cool 5,800 K of the photosphere, the surface of the Sun far below it. Since temperatures ordinarily drop the further you get from the heat source (in this case, the Sun’s atom-fusing center), this so-called “coronal heating problem” poses a definite puzzle.

As is the case for many astronomical mysteries, the answer may have something to do with magnetic fields. Alfvén waves, magnetohydrodynamic waves that travel through magnetized plasma, could potentially carry energy from the convective zone beneath the Sun’s photosphere up into the solar atmosphere. There, the Alfvén waves could turn into shock waves that dissipate their energy as heat, causing the increased temperature of the corona.

The Daniel K. Inouye Solar Telescope, located on the summit of Haleakala in Hawaii, is scheduled to be completed in 2018. [Ekrem Canli]

Predicting Observations

Alfvén waves as a means of delivering heat to the corona makes for a nice picture, but there’s a lot of work to be done before we can be certain that this is the correct model. Observational evidence of Alfvén waves has thus far been limited to specific conditions — and the observations have not yet been enough to convince us that Alfvén waves can deliver enough energy to explain the corona’s temperature.

Lucas Tarr, a scientist at the Naval Research Laboratory, argues that upcoming solar telescopes may make it easier to detect these waves — but first we need to know what to look for! In a recent study, Tarr uses a simplified analytic model to show which frequencies of waves are likely to carry power when magnetic field lines in the corona are pertubed.

A Promising Future

The power carried by Alfvén waves as a function of frequency, as a result of an initial perturbation, plotted for several different initial conditions (such as the size of the perturbation or the length of the loop on which it is introduced). [Tarr 2017]

Tarr modeled the effects of a minor perturbation — like a local magnetic reconnection event in the corona — on a coronal arcade, a common structure of magnetic field loops found in the corona. Tarr determined that such a disturbance would peak in power at a low frequency (maybe tens of millihertz, or oscillations on scales of minutes), but a substantial portion of the power is carried by waves of higher frequencies (0.5–4 Hz, or oscillations on scales of seconds).

Tarr’s findings confirm that with the cadence and sensitivity of current instrumentation, we would not expect to be able to detect these Alfvén waves. The results do indicate, however, that high-cadence observations with future telescope technology — like the instrumentation at the upcoming Daniel K. Inouye Solar Telescope, which should be completed in 2018 — may have the ability to reveal the presence of these waves and confirm the model of Alfvén waves as the means by which the Sun achieves its mysteriously hot corona.

Citation

Lucas A. Tarr 2017 ApJ 847 1. doi:10.3847/1538-4357/aa880a

6 November 2017

Did Triton Destroy Neptune’s First Moons?

Neptune’s moon system is not what we would expect for a gas giant in our solar system. Scientists have now explored the possibility that Neptune started its life with an ordinary system of moons that was later destroyed by the capture of its current giant moon, Triton.

An Odd System

Our current understanding of giant-planet formation predicts a period of gas accretion to build up the large size of these planets. According to models, the circumplanetary gas disks that surround the planets during this time then become the birthplaces of the giant planets’ satellite systems, producing systems of co-planar and prograde (i.e., orbiting in the same direction as the planet’s rotation) satellites similar to the many-moon systems of Jupiter or Saturn.

Triton’s orbit is tilted relative to the inner Neptunian satellite orbits. [NASA, ESA, and A. Feild (STScI)]

Neptune, however, is quirky. This gas giant has surprisingly few satellites — only 14 compared to, say, the nearly 70 moons of Jupiter — and most of them are extremely small. One of Neptune’s moons is an exception to this, however: Triton, which contains 99.7% of the mass of Neptune’s entire satellite system!

Triton’s orbit has a number of unusual properties. The orbit is retrograde — Triton orbits in the opposite direction as Neptune’s rotation — which is unique behavior among large moons in our solar system. Triton’s orbit is also highly inclined, and yet the moon’s path is nearly circular and lies very close to Neptune.

The distribution of impact velocities in the authors’ simulations for primordial satellite interactions with Triton, in three cases of different satellite mass ratios. In the low-mass case — a third of the mass ratio of the Uranian satellite system — 88% of simulations ended with Triton surviving on its high-inclination orbit. The survival rate was only 12% in the high-mass case. [Adapted from Rufu et al. 2017]

How did this monster of a satellite get its strange properties, and why is Neptune’s system so odd compared to what we would expect for a gas giant’s satellites? Two scientists, Raluca Rufu (Weizmann Institute of Science, Israel) and Robin Canup (Southwest Research Institute), propose an explanation in which Triton long ago wreaked havoc on a former system of satellites around Neptune.

Destruction After Capture

Rufu and Canup explore the scenario in which Neptune once had an ordinary, prograde system of moons around it that resembled those of the other gas giants. Triton, the authors suggest, may have been a former Kuiper belt object that was then captured by Neptune. The ensuing interactions between retrograde Triton and Neptune’s original, prograde satellite system may have then resulted in the destruction of this original system, leaving behind only Triton and Neptune’s other current satellites.

Nereid, a small irregular moon of Neptune, orbits at an average distance of more than 15 times that of Triton. Models of Triton’s orbital evolution must also account for the preservation of satellites like this one. [NASA]

Using N-body simulations that model a newly captured Triton and a likely primordial prograde system of moons, Rufu and Canup show that if the moons have a mass ratio similar to that of Uranus’s system or smaller, Triton’s interactions with it have a substantial likelihood of reproducing the current Neptunian satellite system. They even demonstrate that the interactions decrease Triton’s initial semimajor axis quickly enough to prevent smaller, outer satellites like Nereid from being kicked out of the system.

If the authors’ picture is correct, then it neatly explains why Neptune’s satellite system looks so unusual compared to Jupiter’s or Saturn’s — which means that our models of how primordial systems of moons form around gas giants still hold strong.

Citation

Raluca Rufu and Robin M. Canup 2017 AJ 154 208. doi:10.3847/1538-3881/aa9184

3 November 2017

Tracing the Fuel for Forming Stars

Huge reservoirs of cold hydrogen gas — the raw fuel for star formation — lurk in galaxies throughout the universe. A new study examines whether these reservoirs have always been similar, or whether those in distant galaxies are very different from those in local galaxies today.

Left: Optical SLOAN images of the five HIGHz galaxies in this study. Right: ALMA images of the molecular gas in these galaxies. Both images are 30” wide. [Adapted from Cortese et al. 2017]

Molecular or Atomic?

The formation of stars is a crucial process that determines how galaxies are built and evolve over time. We’ve observed that star formation takes place in cold clouds of molecular gas, and that star-formation rates increase in galaxies with a larger surface density of molecular hydrogen — so we know that molecular hydrogen feeds the star-forming process.

But not all cold gas in the interstellar medium of galaxies exists in molecular form. In the local universe, only around 30% of cold gas is found in molecular form (H2) and able to directly feed star formation; the rest is atomic hydrogen (H I). But is this true of galaxies earlier in the universe as well?

Studying Distant Galaxies

Cosmological simulations have predicted that earlier in our universe’s history, the ratio of molecular to atomic hydrogen could be larger — i.e., more cold hydrogen may be in a form ready to fuel star formation — but this prediction is difficult to test observationally. Currently, radio telescopes are not able to measure the atomic hydrogen in very distant galaxies, such as those at the peak of star formation in the universe, 10 billion years ago.

Recently, however, we have measured atomic hydrogen in closer galaxies: those at a redshift of about z ~ 0.2–0.4, a few billion years ago. One recent study of seven galaxies at this distance, using a sample from a survey known as COOL BUDHIES, showed that the hydrogen reservoirs of these galaxies are dominated by molecular hydrogen, unlike in the local universe. If this is true of most galaxies at this distance, it would suggest that gas reservoirs have drastically changed in the short time between then and now.

But a team of scientists from the International Centre for Radio Astronomy Research in Australia, led by Luca Cortese, has now challenged this conclusion.

Top: molecular vs. atomic hydrogen gas in galaxies between z = 0 and z = 1.5. Bottom: the evolution of the molecular-to-atomic mass ratio with redshift. [Adapted from Cortese et al. 2017]

Adding to the Sample

Cortese and collaborators combined observations from the Atacama Large Millimeter/submillimeter Array (ALMA) and Arecibo to estimate the ratio of molecular to atomic hydrogen in five HIGHz-survey massive star-forming galaxies at a redshift of z ~ 0.2. They then combine these results with those of the COOL BUDHIES survey; they argue that, since the two surveys use different selection criteria, the combination of the two samples provides a fairer view of the overall population of star-forming galaxies at z ~ 0.2.

Intriguingly, the HIGHz galaxies do not show the molecular-gas dominance that the COOL BUDHIES galaxies do. Cortese and collaborators demonstrate that the addition of the HIGHz galaxies to the sample reveals that the gas reservoirs of star-forming disks 3 billion years ago are, in fact, still the same as what we see today, suggesting that star formation in galaxies at z ~ 0.2 is likely fueled in much the same way as it is today.

As telescope capabilities increase, we may be able to explore whether this continues to hold true for more distant galaxies. In the meantime, increasing our sample size within the range that we can observe will help us to further explore how galaxies have formed stars over time.

Citation

Luca Cortese et al 2017 ApJL 848 L7. doi:10.3847/2041-8213/aa8cc3

25 October 2017

Little Eyes on Large Solar Motions

Images taken during the solar eclipse in 2012. The central color composite of the eclipsed solar surface was captured by SDO, the white-light view of the solar corona around it was taken by the authors, and the background, wide-field black-and-white view is from LASCO. The white arrows mark the “atypical” structure. [Alzate et al. 2017]

It seems like science is increasingly being done with advanced detectors on enormous ground- and space-based telescopes. One might wonder: is there anything left to learn from observations made with digital cameras mounted on ~10-cm telescopes?

The answer is yes — plenty! Illustrating this point, a new study using such equipment recently reports on the structure and dynamics of the Sun’s corona during two solar eclipses.

A Full View of the Corona

The solar corona is the upper part of the Sun’s atmosphere, extending millions of kilometers into space. This plasma is dynamic, with changing structures that arise in response to activity on the Sun’s surface — such as enormous ejections of energy known as coronal mass ejections (CMEs). Studying the corona is therefore important for understanding what drives its structure and how energy is released from the Sun.

Though there exist a number of space-based telescopes that observe the Sun’s corona, they often have limited fields of view. The Solar Dynamics Observatory AIA, for instance, has spectacular resolution but only images out to 1/3 of a solar radius above the Sun’s limb. The space-based coronagraph LASCO C2, on the other hand, provides a broad view of the outer regions of the corona, but it only images down to 2.2 solar radii above the Sun’s limb. Piecing together observations from these telescopes therefore leaves a gap that prevents a full picture of the large-scale corona and how it connects to activity at the solar surface.

Same as the previous figure, but for the eclipse in 2013. [Alzate et al. 2017]

To provide this broad, continuous picture, a team of scientists used digital cameras mounted on ~10-cm telescopes to capture white-light images from the solar surface out to several solar radii using a natural coronagraph: a solar eclipse. The team made two sets of observations: one during an eclipse in 2012 in Australia, and one during an eclipse in 2013 in Gabon, Africa. In a recent publication led by Nathalia Alzate (Honolulu Community College), the team now reports what they learned from these observations.

Building Atypical Structures

The authors’ image processing revealed two “atypical” large-scale structures with sharp edges, somewhat similar in appearance to what is seen near the boundaries of rapidly expanding polar coronal holes. But these structures, visible in the southeast quadrant of the images taken during both eclipses, were not located near the poles.

By analyzing their images along with space-based images taken at the same time, Alzate and collaborators were able to determine that the shape the structures took was instead a direct consequence of a series of sudden brightenings due to low-level flaring events on the solar surface. These events were followed by small jets, and then very faint, puff-like CMEs that might otherwise have gone unnoticed.

Impact of the passage of a series of puff-like CMEs (shown in the LASCO time sequence in the bottom panels) on coronal structures. [Alzate et al. 2017]

The fact that such innocuous transient events in the Sun’s lower atmosphere can be enough to influence the corona’s large-scale structure for timescales of 12–48 hours is a significant discovery. There are roughly 3 CMEs per day during solar maximum, suggesting that atypical structures like the ones discovered in these images are likely very common. These results therefore have a significant impact on our understanding of the solar corona — which goes to show that there’s still a lot we can learn with small telescopes!

Citation

Nathalia Alzate et al 2017 ApJ 848 84. doi:10.3847/1538-4357/aa8cd2

23 October 2017

How A Black Hole Lights Up Its Surroundings

How do the supermassive black holes that live at the centers of galaxies influence their environments? New observations of a distant active galaxy offer clues about this interaction.

Signs of Coevolution

Plot demonstrating the m-sigma relation, the empirical correlation between the stellar velocity dispersion of a galactic bulge and the mass of the supermassive black hole at its center. [Msigma]

We know that the centers of active galaxies host supermassive black holes with masses of millions to billions of suns. One mystery surrounding these beasts is that they are observed to evolve simultaneously with their host galaxies — for instance, an empirical relationship is seen between the growth of a black hole and the growth of its host galaxy’s bulge. This suggests that there must be a feedback mechanism through which the evolution of a black hole is linked to that of its host galaxy.

One proposed source of this coupling is the powerful jets emitted from the poles of these supermassive black holes. These jets are thought to be produced as some of the material accreting onto the black hole is flung out, confined by surrounding gas and magnetic fields. Because the jets of hot gas and radiation extend outward through the host galaxy, they provide a means for the black hole to influence the gas and dust of its surroundings.

In our current model of a radio-loud active galactic nuclei, a region of hot, ionized gas — the narrow-line region — lies beyond the sphere of influence of the supermassive black hole. [C.M. Urry and P. Padovani]

Clues in the Narrow-Line Region

The region of gas thought to sit just outside of the black hole’s sphere of influence (at a distance of perhaps a thousand to a few thousand light-years) is known as the narrow line region — so named because we observe narrow emission lines from this gas. Given its hot, ionized state, this gas must somehow be being pummeled with energy. In the canonical picture, radiation from the black hole heats the gas directly in a process called photoionization. But could jets also be involved?

In a recent study led by Ákos Bogdán, a team of scientists at the Harvard-Smithsonian Center for Astrophysics used X-ray observations of a galaxy’s nucleus to explore the possibility that its narrow-line region is heated and ionized not only by radiation, but also by the shocks produced as radio jets collide with their surrounding environment.

Heating from Jets

Chandra X-ray data for Mrk 3, with radio contours overplotted. Both wavelengths show S-shaped morphology of the jets, with the X-ray emission enveloping the radio emission. A strong shock is present in the west and a weaker shock toward the east. [Bogdán et al. 2017]

Bogdán and collaborators analyzed deep Chandra X-ray observations of the center of Mrk 3, an early-type galaxy located roughly 200 million light-years away. Chandra’s imaging and high-resolution spectroscopy of the galaxy’s narrow-line region allowed the team to build a detailed picture of the hot gas, demonstrating that it shows similar S-shaped morphology to the gas emitting at radio wavelengths, but it’s more broadly distributed.

The authors demonstrate the presence of shocks in the X-ray gas both toward the west and toward the east of the nucleus. These shocks, combined with the broadening of the X-ray emission and other signs, strongly support the idea that collisions of the jets with the surrounding environment heat the narrow-line-region gas, contributing to its ionization. The authors argue that, given how common small-scale radio jets are in galaxies such as Mrk 3, it’s likely that collisional ionization plays an important role in how the black holes in these galaxies impart energy to their surrounding environments.

Citation

Ákos Bogdán et al 2017 ApJ 848 61. doi:10.3847/1538-4357/aa8c76

20 October 2017

Trapping Dust to Form Planets

Growing a planet from a dust grain is hard work! A new study explores how vortices in protoplanetary disks can assist this process.

When Dust Growth Fails

Top: ALMA image of the protoplanetary disk of V1247 Orionis, with different emission components labeled. Bottom: Synthetic image constructed from the best-fit model. [Kraus et al. 2017]

Gradual accretion onto a seed particle seems like a reasonable way to grow a planet from a grain of dust; after all, planetary embryos orbit within dusty protoplanetary disks, which provides them with plenty of fuel to accrete so they can grow. There’s a challenge to this picture, though: the radial drift problem.

The radial drift problem acknowledges that, as growing dust grains orbit within the disk, the drag force on them continues to grow as well. For large enough dust grains — perhaps around 1 millimeter — the drag force will cause the grains’ orbits to decay, and the particles drift into the star before they are able to grow into planetesimals and planets.

A Close-Up Look with ALMA

So how do we overcome the radial drift problem in order to form planets? A commonly proposed mechanism is dust trapping, in which long-lived vortices in the disk trap the dust particles, preventing them from falling inwards. This allows the particles to persist for millions of years — long enough to grow beyond the radial drift barrier.

Observationally, these dust-trapping vortices should have signatures: we would expect to see, at millimeter wavelengths, specific bright, asymmetric structures where the trapping occurs in protoplanetary disks. Such disk structures have been difficult to spot with past instrumentation, but the Atacama Large Millimeter/submillimeter Array (ALMA) has made some new observations of the disk V1247 Orionis that might be just what we’re looking for.

Schematic of the authors’ model for the disk of V1247 Orionis. [Kraus et al. 2017]

Trapped in a Vortex?

ALMA’s observations of V1247 Orionis are reported by a team of scientists led by Stefan Kraus (University of Exeter) in a recent publication. Kraus and collaborators show that the protoplanetary disk of V1247 Orionis contains a ring-shaped, asymmetric inner disk component, as well as a sharply confined crescent structure. These structures are consistent with the morphologies expected from theoretical models of vortex formation in disks.

Kraus and collaborators propose the following picture: an early planet is orbiting at 100 AU within the disk, generating a one-armed spiral arm as material feeds the protoplanet. As the protoplanet orbits, it clears a gap between the ring and the crescent, and it simultaneously triggers two vortices, visible as the crescent and the bright asymmetry in the ring. These vortices are then able to trap millimeter-sized particles.

Gas column density of the authors’ radiation-hydrodynamic simulation of V1247 Orionis’s disk. [Kraus et al. 2017]

The authors run detailed hydrodynamics simulations of this scenario and compare them (as well as alternative theories) to the ALMA observations of V1247 Orionis. The simulations support their model, producing sample scattered-light images that match well the one-armed spiral observed in previous scattered-light images of the disk.

How can we confirm V1247 Orionis provides an example of dust-trapping vortices? One piece of supporting evidence would be the discovery of the protoplanet that Kraus and collaborators theorize triggered the potential vortices in this disk. Future deeper ALMA imaging may make this possible, helping to confirm our picture of how dust builds into planets.

Citation

Stefan Kraus et al 2017 ApJL 848 L11. doi:10.3847/2041-8213/aa8edc

Previous 1 … 93 94 95 96 97 … 122 Next