SAO: Weekly Science Updates 2017

[bystander]

The Remarkable Jet of the Quasar 4C+19.44
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Oct 13

[img3="A Chandra X-ray image of the quasar 4C19.44. The overlayed contours show the radio emission (the dimension 100kpc corresponds to 329,000 light-years; the extremely bright core produces a line of bright pixels as an artifact). Credit: NASA/Chandra, VLA, and Harris et al."]https://ned.ipac.caltech.edu/level5/Sep ... gure11.jpg[/img3][hr][/hr]

Quasars are galaxies with massive black holes at their cores. So much energy is being radiated from near the nucleus of a quasar that it is much brighter than the rest of the entire galaxy. Much of that radiation is at radio wavelengths, produced by electrons ejected from the core at speeds very close to that of light, often in narrow, bipolar jets that are hundreds of thousands of light-years long. The fast-moving charged particles can also scatter photons of light, kicking them up in energy into the X-ray range. Even after more than two decades of study, however, there is still no clear conclusion as to the physical mechanism actually responsible for the X-ray emission. In more powerful quasars, it does appear that this scattering process dominates. In lower power jets, however, the emission characteristics suggest that the X-ray emission is dominated by magnetic field effects, not scattering.

The lead author of a new paper on the remarkable jet in the quasar 4C+19.44 is CfA astronomer Dan Harris, who very sadly passed away in December, 2015, after a long and productive career. His CfA teammates on this project, Dan Schwartz with Nicholas Lee and Aneta Siemiginowska, worked to finish the research together with an international team of colleagues. The scientists undertook a detailed, high spatial resolution study of the straight, three hundred thousand light-year long jet in this quasar using multiwavelength data from the Chandra (X-ray), Spitzer (infrared), and Hubble (optical) space observatories as well as from the Very Large Array (radio).

The combination of multiwavelength observations with high spatial resolution enabled the team to measure the characteristics of the emission systematically in ten distinct knots along the jets. They find that both the magnetic field strength and the particle velocities are (remarkably) quite constant all along the length of this jet, at least when presuming the scattering process dominates. But the scientists are not able to exclude magnetic effects as producing some of the X-ray emission. They do conclude, however, that for the magnetic process to be active, any electrons contributing to it must belong to a separate population that is distinct from the electrons that dominate the scattering.

A Multi-Band Study of the Remarkable Jet in Quasar 4C+19.44 - Daniel Harris et al

• Astrophysical Journal 846(2):119 (2017 Sep 10) DOI: 10.3847/1538-4357/aa845c
  arXiv.org > astro-ph > arXiv:1708.01500 > 04 Aug 2017

[bystander]

The Atmospheres of Water Worlds
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Oct 20

[img3="Artist's illustration of a hypothetical ocean planet with two natural satellites. Astronomers have calculated the rates of evaporation of water from ocean planets under a variety of stellar wind scenarios, and conclude that ocean exoplanets around M stars are likely to loose their water in a relatively brief time.
Credit: Luciano Mendez, Wikimedia Commons"]https://upload.wikimedia.org/wikipedia/ ... mendez.JPG[/img3][hr][/hr]

There are currently about fifty known exoplanets with diameters that range from Mars-sized to several times the Earth's and that also reside within their stars' habitable zone – the orbital range within which their surface temperatures permit water to remain liquid. A "water world" is an extreme case, an exoplanet defined as being covered by a deep ocean, perhaps as deep as hundreds of kilometers, and among these fifty are several that might be candidates for this category. Astronomers note that at least two of the terrestrial planets in our solar system, Earth and Venus, may possibly also have been water worlds early in their evolution.

One of the critical factors in determining if a planet could really be habitable is the presence of an enduring atmosphere. The deep oceans on a water world offer a reservoir for water vapor for its atmosphere, and so scientists have been trying to calculate how stable an exoplanet’s ocean and atmosphere are, especially to effects like evaporation by winds from the star. Since most of the fifty known examples orbit close to their small, host M stars, they are heavily exposed to stellar winds and related stellar space weather events even though their temperatures may be moderate.

CfA astronomer Manasvi Lingam was a member of a team of astronomers who modeled the effects of the stellar wind on a water world under a variety of possible scenarios. They include effects of stellar magnetic fields, coronal mass ejections, and atmospheric ionization and ejection. Their computer simulations are in good agreement with the current Earth-Sun system, but in some of the more extreme possibilities, as for example might exist on the set of exoplanets around M-stars, the situation is very different and the escape rates may be as much as or more than one thousand times greater. The result means that even a water world, if it orbits an M-dwarf star, could lose its atmosphere after about one billion years, a relatively brief time for possible development of life. ...

The Dehydration of Water Worlds via Atmospheric Losses - Chuanfei Dong et al

• Astrophysical Journal Letters 847(1):L4 (2017 Sep 20) DOI: 10.3847/2041-8213/aa8a60
  arXiv.org > astro-ph > arXiv:1709.01219 > 05 Sep 2017

[bystander]

Measuring the Distance to the Far Side of the Galaxy
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Oct 27

[img3="An artist's view of the Milky Way showing the relative positions of the Earth and a water maser star formation region on the far side whose distance has now been precisely determined by radio interferometric parallax techniques. Credit: Bill Saxton, NRAO/ AUI/NSF; Robert Hurt, NASA"]https://www.cfa.harvard.edu/sites/www.c ... 201741.jpg[/img3][hr][/hr]

The size and shape of our home galaxy, the Milky Way, reflect not only its current structure but also its evolutionary history, providing details that form the basis for our understanding of all galaxies. The information is also important because it helps enable astronomers determine distances to objects in the Milky Way relative to other object's distances. Distance is often the main uncertainty when calculating a star’s inherent (not apparent) luminosity, its mass, or other physical attributes. Conversely, knowing the precise distances to objects in the Milky Way enables astronomers to construct a coherent picture of the galaxy's size and shape. Currently, for many objects in the galaxy, and especially for molecular clouds and others not bright in the optical, astronomers distances by measuring their velocities and fitting them to a rotating model of the galaxy thus roughly associating velocities with the corresponding "kinematic" distances.

The distances to nearby stars are precisely and accurately determined using the technique of parallax. When a celestial body is seen from different, widely separated viewing points, its position with respect to more distant background stars or galaxies varies: this angular difference is its parallax. Parallax is used to triangulate the distances to stars by measuring their apparent angular shifts six months apart, when the earth is on opposite sides of its orbit around the Sun. This traditional technique has in the past worked mostly for nearby stars because their angular parallaxes are comparatively large and the stars are bright enough to be clearly seen, and hence easy to measure. The technique is so straightforward that astronomers have been trying for nearly a century (since the Milky Way was recognized to be a galaxy) to piece together a more complete picture using increasingly precise angular measurements. ...

Mapping spiral structure on the far side of the Milky Way - Alberto Sanna et al

• Science 358(6360):227 (13 Oct 2017) DOI: 10.1126/science.aan5452
  arXiv.org > astro-ph > arXiv:1710.06489 > 17 Oct 2017

http://asterisk.apod.com/viewtopic.php?t=37659

[bystander]

A New Kind of Quantum Computer
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Nov 03

[img3="A photograph of Google's 1000+ qubit computer chip under development. CfA scientists and their colleagues have proposed a new way to use photons of light instead of silicon chips as qubits, opening the door to new technologies. Credit: Google"]https://www.cfa.harvard.edu/sites/www.c ... 201742.jpg[/img3][hr][/hr]

Quantum mechanics incorporates some very non-intuitive properties of matter. Quantum superposition, for example, allows an atom to be simultaneously in two different states with its spin axis pointed both up and down, or combinations in between. A computer that uses quantum mechanical manipulation of atoms or particles therefore has many more possible options than a conventional one that works with "zeros" and "ones" and has only two choices, called bits. A quantum computer's memory uses instead what are called quantum bits - qubits - and each qubit can be in a superposition of these two states. As a result, theoretical physicists estimate a quantum computer with only about one hundred of these qubits could in principle exceed the computing power of the powerful current classical computers. Building a quantum computer is therefore one of the main technological goals in modern physics and astrophysics.

CfA physicist Hannes Pichler, of the CfA's Institute for Theoretical Atomic, Molecular and Optical Physics (ITAMP), and three colleagues have proposed a new way to build a quantum computer using just a single atom. Light quanta (photons) can be used as information carriers and act as qubits, but to use them in a quantum computer they must interact with each other. Under normal conditions, however, light does not interact with itself and so the challenge is to create correlations between them. The key idea of their new paper is to allow light photons from an atom to interact with their own mirror image reflections Photons that the atom emits are reflected by the mirror and can interact again with the atom but with a very slight time delay. That delay, the scientists show, results in the combined waveform of the photons being so complex that in principle any quantum computation can be achieved by simply measuring the emitted photons.

The theoretical discovery is not only a conceptual breakthrough in quantum optics and information, it opens the door to new technology. In particular, the proposed single atom setup is appealing since it minimizes the resources needed and relies only on elements that have already been demonstrated in state-of the-art experiments. ...

Universal Photonic Quantum Computation via Time-Delayed Feedback - Hannes Pichler et al

• Proceedings of the NAS 114(43):11362 (24 Oct 2017) DOI: 10.1073/pnas.1711003114

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Jupiter's Surprising Southern Aurora
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Nov 10

[img3="New X-ray observations show the that auroras on Jupiter behave differently at each pole. Astronomers using data from the Chandra and XMM missions have obtained the first X-ray images of Jupiter's south pole aurora and discovered that, contrary to expectations, it is unlike its northern counterpart. Credit: X-ray: NASA/CXC/UCL/W.Dunn et al, Optical: South Pole: NASA/JPL-Caltech/SwRI/MSSS/Gerald Eichstädt /Seán Doran; North Pole: NASA/JPL-Caltech/SwRI/MSSS"]https://www.cfa.harvard.edu/sites/www.c ... 201743.jpg[/img3][hr][/hr]

Aurorae are seen in all of the planets in our solar system that have magnetic fields, including of course the Earth, and have even been detected around brown dwarf stars (known for having strong magnetic fields in their upper layers). Only Jupiter's north pole aurora, however, has been spatially resolved in X-rays, and the bright hot spot is seen to pulse as charged particles around Jupiter, funneled by the magnetic field lines, smash into atoms in the atmosphere and emit bursts of X-rays. Surprisingly, however, no similar aurora had been clearly identified from Jupiter's south pole, although they had been expected assuming Jupiter’s magnetic field lines connected north and south polar regions (like the Earth's are connected).

CfA astronomers William Dunn and Ralph Kraft and a team of colleagues now report clearly identifying Jupiter’s southern X-ray aurora. The scientists used data from the Chandra X-ray Observatory and XMM-Newton missions from 2007 and 2016, when the Jupiter’s south pole was suitably oriented toward Earth. Jupiter's aurorae are complex. The visible ones result primarily from the excitation of hydrogen atoms and make it appear pinkish-purple, but the moon Io injects a large amount of volcanic material into Jupiter's environment which leads to the production of X-ray emission. The several sources of activity make Jupiter's aurorae the brightest in the solar system.

In contrast, the X-ray emission from the polar regions is much more variable and confined, and less well understood. The new results find, for example, that the periodic brightening occurs at different rates in the north and south, in contrast to the current models which expect that the magnetic processes are coherent. In fact, the south appears to host a persistent X-ray hot spot. The implications are that the physical processes at work are different and more complex than the solar wind effects that produce the Earth’s aurorae.

The Independent Pulsations of Jupiter’s Northern and Southern X-ray Auroras - W. R. Dunn et al

• Nature Astronomy 1:758 (30 Oct 2017) DOI: 10.1038/s41550-017-0262-6

http://asterisk.apod.com/viewtopic.php?t=37712

[bystander]

Dusty Protoplanetary Disks
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Nov 17

[img3="An ALMA image of the planet-forming disk around the young, Sun-like star TW Hydrae. The inset image zooms in on the gap nearest to the star, which is at the same distance as the Earth is from the Sun. Astronomers have made a study of 284 disks in three nearby regions to study how they evolve.
Credit: S. Andrews (Harvard-Smithsonian CfA), ALMA (ESO/NAOJ/NRAO)"]https://cdn.eso.org/images/screen/eso1611b.jpg[/img3][hr][/hr]

Planetary systems form out of disks of gas and dust around young stars. How the formation proceeds, however, is complex and poorly understood. Many physical processes are involved including accretion onto the star, photoevaporation of material of the disk, interactions of the disk with planetary embryos, growth of the dust grains, settling of the dust to the midplane of the disk, and more. To unravel these various factors, observations of protoplanetary disks at multiple wavelengths are used; the submillimeter wavelength range in particular offers a way to peer through most of the disk to estimate dust masses directly.

Surveys of star forming regions with facilities such as Submillimeter Array (SMA) and ALMA have determined that disks have typical masses of 0.1%–0.5% of the host star. CfA astronomers Sean Andrews and David Wilner were members of a team that used these two facilities to study systematically the dust in 284 protoplanetary disks in three nearby star formation regions. They find clear evidence for grain growth from the spectral shape of the emission, at least for two of the regions. The result, which is in agreement with previous work, is indicative of the early stages of planet formation. They also find that the average temperature of the dust is between about 40-50K in all the examples. Their study suggest that the disks in the three regions are all similar, though with individual variations, a result that was mildly surprising since evolutionary effects were thought to become more apparent. The team now plans additional observations to compile better and more complete statistics. ...

Far-Infrared to Millimeter Data of Protoplanetary Disks: Dust Growth
in the Taurus, Ophiuchus, and Chamaeleon I Star-forming Regions - Álvaro Ribas et al

• Astrophysical Journal 849(1):63 (2017 Nov 01) DOI: 10.3847/1538-4357/aa8e99
  arXiv.org > astro-ph > arXiv:1710.08426 > 23 Oct 2017

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The Mysterious Star MWC349
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Nov 24

[img3="An infrared 3-color image of the region of star formation which includes the mysterious maser star MWC349 (the very bright star at the right). New spectroscopic results find that the star is probably not physically associated with a nearby star whose age (assuming it was a binary companion) had been used to constrain the age of MWC349 itself. As a result, MWC349 might be a very young massive star.
Credit: Strelnitski et al, 2013 ApJ"]https://www.cfa.harvard.edu/sites/www.c ... 201744.jpg[/img3][hr][/hr]

Molecular clouds in interstellar space can sometimes produce natural masers (the radio wavelength analogs of lasers) that shine with bright, narrow beams of radiation. Regions of active star formation generate some of the most spectacular such masers -- in one case radiating as much energy in a single spectral line as does our Sun in its entire visible spectrum. In these sources, the maser radiation comes from molecules like water or OH that are excited by collisions and the radiation environment around the young stars.

In 1989, maser emission from atoms of atomic hydrogen gas was discovered around the star MWC349, a source with a disk of ionized material that is seen edge-on and a bipolar outflow. The emission is extremely bright and varies in time, the result of sensitivity to changes in the detailed excitation processes. Subsequent observations over the decades have found numerous masing hydrogen lines around this star, allowing scientists to model the emitting region more carefully. MWC349 has two particularly mysterious features. First, it is an almost unique example of a hydrogen maser source, Despite decades of searching for similar sources, only a few other examples have been found, and they are all much fainter and less dramatic. The second is that its age is not known: it has been suggested to be a very young star still approaching the main-sequence, or an evolved star well into its main-sequence evolution. The consensus so far -- that it is old -- comes from the fact that there is another star only a few arcseconds away from it and possibly a binary companion. That star is older, about five million years, and so the MWC349 itself must be that old as well. But the evidence is confusing, and the rare and unusual nature of the object makes its age classification an astronomical priority.

CfA astronomers Howard Smith and Jessica Mink, together with four colleagues, used the TRES spectrograph of the 1.5-meter Tillinghast Reflector of the Fred L. Whipple Observatory and the Keck/HIRES spectrograph to measure carefully the velocities of the two stars from the wavelengths of their spectral lines. They find that the two differ by about thirty-five kilometers per second, making the binary companion scenario unlikely. The scientists conclude that probably the second star is just a coincidental alignment. The result reopens the possibility that MWC349 is a young, massive star in a region of active star formation.

MWC349 A and B Are Not Gravitationally Bound: New Evidence - P. M. Drew et al

• arXiv.org > astro-ph > arXiv:1711.06734 > 17 Nov 2017 (v1), 22 Nov 2017 (v2)

[bystander]

A Microlensing Event Seen from Three Positions in Space
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Dec 01

[img3="A graphic of the current orbital position of the Spitzer Space Telescope. Astronomers observed a microlensing event from three different locations in space - Spitzer, the Earth, and the Kepler K-2 satellite - and used them to measure for the first time, in principle without ambiguity, the mass and location of a microlensing body 77 Jupiter-masses in size. Credit: NASA/JPL-Caltech/Spitzer"]https://www.cfa.harvard.edu/sites/www.c ... 201745.jpg[/img3][hr][/hr]

The path of a light beam will be bent by the presence of mass, an effect explained by General Relativity, and a massive body can therefore act like a lens - a so called "gravitational lens" – to distort the image of an object seen behind it. Scientists first confirmed this prediction quantitatively during the now famous total eclipse of 29 May 1919 by observing starlight bent by the mass of the Sun. Microlensing is the name given to a related phenomenon: the short flash of light produced when a cosmic body, acting as a gravitational lens, changes the intensity of visible light from a more distant, background star as the body's motion fortuitously moves in front of it.

About thirty years ago, scientists predicted that if it ever became possible to observe a microlensing flash from two well-separated vantage points, a parallax measurement would pin down the distance of the dark object. The Spitzer Space Telescope is currently orbiting the Sun at the distance of the Earth but trailing the Earth at a location about one-quarter of the way around in its orbital path. A years ago, CfA astronomer Jennifer Yee led a team to make the first parallax microlensing measurement of a small stellar object using both Spitzer and ground-based telescopes. One complication was that measurements made with only two vantage points leaves a possible ambiguity in the result – but a three point measurement would eliminate that uncertainty.

In a new paper, Yee and a large team of her colleagues report the first microlensing event seen from three well-separated points: Spitzer, the Earth, and the Kepler “K2” mission, which has an orbit similar to that of Spitzer but which currently trails the Earth about one-sixth of the way around in its orbital path. The lensing object, known as MOA-2016-BLG-290, was determined from these measurements to be an extremely low mass star of about .07 solar-masses (seventy-seven Jupiter-masses), and situated about twenty-two thousand light-years away in our galaxy. The result, besides detecting an object intermediate in mass between a star and a planet, demonstrates the power of microlensing parallax measurements predicted decades ago. ...

An Isolated Microlens Observed from K2, Spitzer, and Earth - Wei Zhu et al

• Astrophysical Journal Letters 849(2):L31 (2017 Nov 10) DOI: 10.3847/2041-8213/aa93fa
  arXiv.org > astro-ph > arXiv:1709.09959 > 26 Sep 2017

[bystander]

The Initial Mass Function
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Dec 08

[img3="The elliptical galaxy NGC 1600, approximately 200 million light-years away – shown in the center of the Hubble image and highlighted in the box. Astronomers have concluded from the study of this and similar galaxies that the relative populations of stars of different masses in a cluster of stars (the IMF) is influenced by the distribution of velocities in the cluster. Credit: NASA, ESA, Digital Sky Survey 2"]https://cdn.spacetelescope.org/archives ... c1607a.jpg[/img3][hr][/hr]

The gas and dust in giant molecular clouds gradually come together under the influence of gravity to form stars. Precisely how this occurs, however, is incompletely understood. The mass of a star, for example, is by far the most important factor constraining its future evolution, but astronomers do not clearly understand what determines the exact mass of a newly forming star. One aspect of this problem is simply knowing how many stars of each size there are, that is, knowing the distribution of stellar masses in a large cluster of stars. The initial mass function (IMF) describes this distribution, and is currently based on an average from observations of stars in our Milky Way.

The observed IMF has relatively few massive stars (i.e., ones more massive than the sun). Sun-sized stars are comparatively abundant. Stars somewhat smaller than the sun are even more common, but then stars of decreasing mass (down to one-tenth of the sun's mass or even less) decrease in numbers. The precise statistics for low mass stars are somewhat uncertain because they are faint and hard to detect. The theoretical basis for the IMF is also being debated, as is whether the IMF of the Milky Way is representative of the IMF elsewhere in the universe. The relative abundance of elements (the “metallicity’) in the collapsing cloud, for example, has been suggested as one way to modify the IMF. The idea of a universal IMF, however, has been a cornerstone of stellar theory for decades, but recently there has been considerable effort to test and challenge this assumption, made possible in part by sensitive instruments capable of measuring stars that are smaller and/or fainter. Since stars of different masses have atmospheres showing different spectral features, spectroscopy of a distant cluster whose individual stars cannot be resolved can nevertheless reveal the proportions of stars of different masses within it from the proportions of these features.

CfA astronomer Charlie Conroy and four colleagues are conducting a study of the IMF with the Keck telescope and its spectrometer. They do find some variations in the IMF and, contrary to some expectations, they conclude that metallicity is not the sole driver of these variations. Instead, they conclude that the velocities of the material in the star clusters seems to be a key factor. The result, which now will be followed up with more measurements, is important because it suggests a different theoretical framework is needed to explain the origin of the IMF.

Initial Mass Function Variability (or Not) among Low-velocity Dispersion, Compact Stellar Systems - Alexa Villaume et al

• Astrophysical Journal Letters 850(1):L14 (2017 Nov 20) DOI: 10.3847/2041-8213/aa970f
  arXiv.org > astro-ph > arXiv:1710.11144 > 30 Oct 2017

http://asterisk.apod.com/viewtopic.php?t=36022

[bystander]

Starspots
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Dec 15

[img3="One of the largest sunspots in the last nine years as seen by NASA's Solar Dynamics Observatory. An image of Earth has been added for scale. Astronomers have used the Kepler satellite to characterize "starspots" on 2244 stars. Credit: NASA/GSFC/SDO"]https://www.nasa.gov/sites/default/file ... arth_0.jpg?[/img3][hr][/hr]

Sunspots are regions on the Sun's photosphere that appear darker than surrounding areas because they are cooler, usually by one or two thousand degrees Celsius. These spots are temporary phenomena caused by magnetic activity that results from the Sun's rotation and the complex circulation of hot gas below its surface, and they are accompanied by solar flares, mass ejections and other energetic phenomena. Other stars have similar regions, called starspots, and there have been some suggestions that in comparison the Sun is comparatively quiescent. Starspots are interesting to stellar astronomers because they are informed by the star’s rotation and circulation, details that are otherwise difficult to discern. Although starspots are too small to be imaged directly by current telescopes, they can be inferred from variations in a star's light.

CfA astronomer Raphaelle Haywood and two colleagues have analyzed archival data from the Kepler satellite to uncover some of the general properties of starspots. Kepler's primary goal was the detection of exoplanets from periodic variations in starlight due to transits, but its steady monitoring of starlight revealed many other temporal phenomena, including starspots. From a set of over thirty-four thousand main-sequence stars, the astronomers selected 2244 with rotation periods between 9.5 and 20.5 days. They were able to group the observed starspot behavior into three categories based on their persistence, and reached three significant conclusions. One is that our Sun is not unusually quiet. The others are that bigger starspots live longer, and that the starspots on cooler stars decay more slowly. The conclusions are consistent with current models of starspots, but this work is the first large-scale survey of the phenomenon.

A Kepler Study of Starspot Lifetimes with Respect to Light-Curve Amplitude and Spectral Type - Helen A. C. Giles et al

• Monthly Notices of the RAS 472(2):1618 (Dec 2017) DOI: 10.1093/mnras/stx1931
  arXiv.org > astro-ph > arXiv:1707.08583 > 26 Jul 2017

[bystander]

The Toothbrush Cluster
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Dec 22

[img3="A multiwavelength false-color image of the "Toothbrush" cluster of galaxies, 1RXS J0603.3+4214. The intensity in red shows the radio emission, blue is X -ray, and the background color composite is optical emission. Astronomers studying the cluster with new radio observations combined with other wavelengths have been able to confirm the galaxy merger scenario and estimate the magnetic field strength in the shocks. (Credit: RJ van Weeren et al)"]https://www.cfa.harvard.edu/sites/www.c ... 201749.jpg[/img3][hr][/hr]

Most galaxies lie in clusters containing from a few to thousands of objects. Our Milky Way, for example, belongs to a cluster of about fifty galaxies called the Local Group whose other large member is the Andromeda galaxy about 2.3 million light-years away. Clusters are the most massive gravitationally bound objects in the universe and form (according to current ideas) in a “bottoms-up” fashion with smaller structures developing first and larger groupings assembling later in cosmic history. Dark matter plays an important role in this growth process. Exactly how they grow, however, appears to depend on several competing physical processes including the behavior of the intracluster gas. There is more mass in this gas than there is in all the stars of a cluster’s galaxies, and the gas can have a temperature of ten million kelvin or even higher. As a result, the gas plays an important role in the cluster’s evolution. The hot intracluster gas contains rapidly moving charged particles that radiate strongly at radio wavelengths, sometimes revealing long filamentary structures.

The “Toothbrush” galaxy cluster, 1RXS J0603.3+4214, hosts three of these radio structures as well as a large halo. The most prominent radio feature extends over more than six million light years, with three distinct components that resemble the brush and handle of a toothbrush. The handle is particularly enigmatic because, besides being large and very straight, it is off center from the axis of the cluster. The halo is thought to result from turbulence produced by the merger of galaxies, although some other possibilities have been suggested.

CfA astronomers Reinout van Weeren, Bill Forman, Felipe Andrade-Santos, Ralph Kraft, and Christine Jones and their colleagues used the Very Large Array (VLA) facility to observe the relativistic particles in the cluster with precise, sensitive radio imaging, which they compared with Chandra X-ray and other datasets. In the radio, the Toothbrush has a very narrow ridge, created by a huge shock resulting from the merger, and at least thirty-two previously undetected compact sources. The halo’s radio and X-ray morphologies are very similar and lend support to the merger scenario. Astronomers are also able to estimate the strength of the magnetic field, and combined with other results, use it to conclude that the merger scenario is most suitable.

Deep VLA observations of the cluster 1RXS J0603.3+4214 in the frequency range 1-2 GHz - K. Rajpurohit et al

• arXiv.org > astro-ph > arXiv:1712.01327 > 04 Dec 2017

[bystander]

The Geometry of Nuclear Black Hole Accretion Disks
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Dec 29

[img3="A Hubble image of the Circinus galaxy with its active galactic nucleus (AGN). Astronomers have measured the sizes of the accreting regions around the supermassive black holes in four distant AGN using techniques of reverberation mapping. Credit: NASA, Andrew S. Wilson (U. Maryland); Patrick L. Shopbell (Caltech); Chris Simpson (Subaru); Thaisa Storchi-Bergmann and F. K. B. Barbosa (UFRGS); and Martin J. Ward (U. Leicester)"]https://www.cfa.harvard.edu/sites/www.c ... 201750.jpg[/img3][hr][/hr]

Supermassive black holes with millions or even billions of solar-masses of material are found at the nuclei of most galaxies, including our Milky Way. A torus of dust and gas orbits around the black hole (at least according to most theories) and radiates in ultraviolet light when material falling toward the black hole heats the disk to millions of degrees. The accretion process can also power the ejection of jets of rapidly moving charged particles. Such actively accreting supermassive black holes in galaxies are called active galactic nuclei (AGN).

Astronomers who model the physical processes in one of these huge dynamos start with the gas motions and geometry of the region. The gas motions can be measured straightforwardly from emission lines in the gas, typically optical lines of hydrogen that are excited by the uv radiation. As for geometry, simple calculations estimate that the radius of line-emitting gas should be a few thousand astronomical units (one AU is the average distance of the Earth from the Sun). Because most AGN are too far away to be able to measure dimensions this small, astronomers have come to rely on the technique of "reverberation mapping." Radiation from the accretion disk is highly variable. Since it takes time for the uv to travel from the accretion disk near the black hole out to the line-emitting gas, there is a delay between an event seen in the continuum and then in the hydrogen lines

CfA astronomer Anna Pancoast and a team of her colleagues analyzed reverberation mapping data of four AGN to study their geometries and, in particular, the volume of hot gas known for its rapid motions, the so-called broad line region because the spectral lines have widths corresponding to as much as three thousand kilometers per second. The scientists find the geometry of this gas, at least in these four AGN, is well-described as coming from thick disks seen nearly face-on, with median radii from about 1600 AU to 4000 AU, and each with a black hole whose mass is about seventy million solar-masses (with an estimated precision for each one of about 50%). The new work was successful in modeling the observations and nearly doubles the size of the AGN sample modeled with this technique. The sample is still small, however, and more observations are being planned. ...

The Structure of the Broad-line Region in Active Galactic Nuclei. II. Dynamical
Modeling of Data From the AGN10 Reverberation Mapping Campaign - C. J. Grier et al

• Astrophysical Journal 849(2):146 (2017 Nov 10) DOI: 10.3847/1538-4357/aa901b
  arXiv.org > astro-ph > arXiv:1705.02346 > 05 May 2017 (v1), 27 Sep 2017 (v2)