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Research highlights from the journals of the American Astronomical Society
Updated: 2 weeks 6 days ago

How A Black Hole Lights Up Its Surroundings

Mon, 2017-10-23 12:00

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

Trapping Dust to Form Planets

Fri, 2017-10-20 18:45

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 well match the ALMA images.

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

Featured Image: Mixing Chemicals in Stars

Wed, 2017-10-18 12:00

How do stars mix chemicals in their interiors, leading to the abundances we measure at their surfaces? Two scientists from the Planetary Science Institute in Arizona, Tamara Rogers (Newcastle University, UK) and Jim McElwaine (Durham University, UK), have investigated the role that internal gravity waves have in chemical mixing in stellar interiors. Internal gravity waves — not to be confused with the currently topical gravitational waves — are waves that oscillate within a fluid that has a density gradient. Rogers and McElwaine used simulations to explore how these waves can cause particles in a star’s interior to move around, gradually mixing the different chemical elements. Snapshots from four different times in their simulation can be seen below, with the white dots marking tracer particles and the colors indicating vorticity. You can see how the particles move in response to wave motion after the first panel. For more information, check out the paper below!

Citation

T. M. Rogers and J. N. McElwaine 2017 ApJL 848 L1. doi:10.3847/2041-8213/aa8d13

Is S0-2 a Binary Star?

Tue, 2017-10-17 12:00

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Investigating the Binarity of S0-2: Implications for its Origins and Robustness as a Probe of the Laws of Gravity around a Supermassive Black Hole
Authors: D. S. Chu, T. Do, A. Hees, A. Ghez, et al.
First Author’s Institution: University of California, Los Angeles
Status: Submitted to ApJ, open access

The most exciting discoveries in astronomy all have something in common: they let us marvel at the fact that nature obeys laws of physics. The star S0-2 is one of these exciting discoveries. S0-2 (also known as S2) is a fast-moving star that has been observed to follow a full elliptical, 16-year orbit around the Milky Way’s central supermassive black hole, precisely according to Kepler’s laws of planetary motion. Serving as a test-particle probe of the gravitational potential, S0-2 provides some of the best constraints on the black hole’s mass and distance yet. S0-2 is the brightest of the S-stars, a group of young main-sequence stars concentrated within the inner 1” (0.13 ly) of the nuclear star cluster.

The next time S0-2 reaches its closest approach to the black hole, in 2018, there will exist a unique opportunity to detect a deviation from Keplerian motion — namely the relativistic redshift of S0-2’s radial (line-of-sight) velocity — in a direct measurement. In anticipation of this event, the authors of today’s paper investigate possible consequences of S0-2 being not a single star, but a spectroscopic binary, which would complicate this measurement.

Figure 1: Top: Radial velocity measurements of S0-2 over time. Bottom: Residual velocities after subtraction of the best-fit model for the orbital motion. [Chu et al. 2017]

To search for any periodicity in S0-2’s radial velocity curve that would indicate the presence of a companion star, the authors combine their most recent velocity measurements with previous ones obtained as part of monitoring programs carried out at both the WMKO in Hawaii and the VLT in Chile. The resulting data set consists of 87 measurements in total, which are spread over 17 years of observations and have a typical uncertainty of a few 10 km/s (Figure 1, top panel). When S0-2 passes the black hole, the relativistic redshift of its radial velocity is predicted to amount to roughly 200 km/s at closest approach, while the radial velocity is expected to change from +4000 to -2000 km/s. S0-2’s actual speed at this time will be close to 8000 km/s, about 2.7% of the speed of light.

Figure 2: Lomb-Scargle periodogram of S0-2’s residual radial velocity curve (see Fig. 1). No peak reaches the 95% confidence level, which implies that no statistically significant signature of a periodic variation is found in the observations. [Chu et al. 2017]

After having accounted for the long-term radial velocity variation due to the orbital motion of S0-2 (Figure 1, bottom panel), the authors create a Lomb-Scargle periodogram to search for short-term periodic signatures in the velocity residuals. A companion of S0-2 would need to have an orbital period shorter than 120 days at maximum, or the binary system would be too wide to remain stable against tidal forces so close to the black hole. The minimum orbital period could be no less than a few days, or the two stars would come into contact. Yet even in between these limits, the measured periodogram shows no statistically significant peak at any particular period (Figure 2).

However, this non-detection of a periodic signal places an upper limit on the radial velocity variations that could be caused by a possible companion of S0-2, which can be converted into a mass limit. For instance, at a period of 100 days, velocity changes larger than about 12 km/s would have been confidently detected. This implies a companion mass smaller than about 1.7 solar masses, assuming a reasonable total mass of the binary in the range of 14.1 to 20 solar masses.

To estimate the effect of such a companion on the prospective measurement of the relativistic redshift, the authors simulate observations of S0-2 extending into 2018, using a relativistic orbit model and assuming that S0-2 is in fact a binary. These data sets are then fit in the same way as the real data would normally be, using a model in which S0-2 is assumed to be a single star and the strength of the expected relativistic effect is described by a free parameter. The authors conclude that even if S0-2 is a binary, the relativistic redshift could still be detected with high statistical significance in 2018, although the measurement could come out slightly biased, depending on the specific configuration of the binary system (Figure 3).

Figure 3: The bias in estimating the parameter describing the strength of the relativistic redshift, if S0-2 is a binary but assumed to be a single star, for different realizations of plausible binary configurations. Shown are deviations from the expected value of this parameter in general relativity, which is 1, where for a Keplerian orbit it is 0. [Chu et al. 2017]

A continued monitoring beyond 2018 will provide further opportunities to detect relativistic effects on the on-sky motion of S0-2 as well, and it remains to be studied how a possible binarity would influence those particular measurements. The authors also note that if, in time, the search for spectroscopic binaries could be extended to the fainter S-stars too, a comprehensive study of their binary fraction should be able to distinguish between different proposed scenarios for their formation. So stay tuned!

About the author, Philipp Plewa:

I am currently a graduate student at the Max-Planck-Institute for Extraterrestrial Physics in Germany. My main interest is in developing new tools for high-precision infrared astrometry, with the aim of learning more about the supermassive black hole at the center of the Milky Way and the stars in its vicinity.

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