SAO: Weekly Science Updates

Find out the latest thinking about our universe.
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SAO: Gravitational Lensing of a Young Super-Luminous Galaxy

Postby bystander » Wed Jul 16, 2014 6:36 pm

Gravitational Lensing of a Super-Luminous Galaxy in the Young Cosmos
Smithsonian Astrophysical Observatory
Weekly Science Update | 2014 Mar 28

About fifteen years ago astronomers, using improved submillimeter wavelength telescopes, discovered a new class of very distant galaxies: submillimeter galaxies (SMGs). These objects are among the most luminous, rapidly star-forming galaxies known, and can shine brighter than a trillion Suns (about one hundred times more luminous than the Milky Way), but they are undetected in the visible. Their ultraviolet and optical light is absorbed by dust in the galaxies which is warmed and then emits in the submillimeter. SMGs are typically so distant that their light has been traveling for over ten billion years, more than 70% of the lifetime of the universe. Their power source is thought to be star formation, with some having rates as high as one thousand stars per year (in the Milky Way, the rate is more like a few stars per year), although the cause of such dramatic bursts is not understood.

Atomic and molecular lines are particularly important diagnostics of star formation, black hole activity, and interstellar gas properties. Furthermore, the shapes of the emission lines provide direct insights into the dynamics of the system. The observed far-infrared and submillimeter spectra of SMGs is dominated by such emission lines because the gas in their molecular clouds, as well as the dust, is exposed to ultraviolet flux from nearby young stars that stimulates the gas to glow. ...

[C II] and 12CO(1–0) Emission Maps in HLSJ091828.6+514223:
A Strongly Lensed Interacting System at z = 5.24
- T. D. Rawle et al
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SAO: Cosmology and the Spatial Distribution of Galaxies

Postby bystander » Wed Jul 16, 2014 7:23 pm

Cosmology and the Spatial Distribution of Galaxies
Smithsonian Astrophysical Observatory
Weekly Science Update | 2014 Apr 04

Perhaps the most astonishing and revolutionary discovery in cosmology was that galaxies are moving away from us. Hubble's 1929 paper provides the underpinning of the big bang picture of creation in which the universe is expanding, and has been for 13.8 billion years. Astronomers since then have been steadily working to refine this general picture, and in 1998, two teams (one led by CfA scientists) further astonished the world with their results showing that the universe would expand forever -- and not only that: it is accelerating outward. They used supernovae to probe the distant cosmos. These discoveries have led to more sophisticated questions, with a primary task today being to understand in detail the expansion history of the universe, that is, how the rate of expansion of the universe evolved from the time of the big bang to the way it is today. The answers to this question directly address the properties of the acceleration mechanism, the nature of dark matter, the evolution of galaxies in early times, and more.

Precision measurements of the cosmic distance scale are crucial for probing this behavior, and one particularly powerful method uses what are called baryon acoustic oscillations (BAO). Baryons refer to ordinary matter, and acoustic oscillations are sound waves. Sound waves caused by density fluctuations were bouncing through the cosmos during its first 400,000 years. Then, once ionized atoms became neutral, radiation no longer interacted strongly with matter and the cosmic microwave background was released. The intensity maps of the background radiation contain a record of these sound waves – the BAO. Astronomers calculate that at the time the cosmic background was produced, sound waves (traveling at the speed of sound) could have spread across a distance of about 500 million light-years, leaving in their wake a coherent record in the matter distribution that eventually condensed into galaxies and clusters of galaxies. Because the scale of this acoustic distortion is so large, many times the size of galaxy clusters, the BAO signature was only modestly altered subsequently as the universe evolved; simulations and theory suggest deviations are below 1%. The robustness of the scale of this distinctive clustering signature allows it to be used as a standard ruler to measure the cosmic distance scale, and indeed the imprint of the BAOs has been detected in a variety of observations of the structure of the nearby universe. ...

The Clustering of Galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey:
Measuring DA and H at z = 0.57 from the Baryon Acoustic Peak
in the Data Release 9 Spectroscopic Galaxy Sample
- Lauren Anderson et al

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SAO: Dramatic Gas Outflow from a Bright Galaxy Cluster

Postby bystander » Wed Jul 16, 2014 8:02 pm

Dramatic Gas Outflow from a Bright Galaxy Cluster
Smithsonian Astrophysical Observatory
Weekly Science Update | 2014 Apr 11

Galaxy clusters are groupings of several to thousands of galaxies. Most galaxies are members of a cluster. In our own case, the Milky Way is a member of the "Local Group," a band of about fifty galaxies whose other large member is the Andromeda galaxy about 2.3 million light-years away. The closest large cluster of galaxies to us is the Virgo Cluster, with about 2000 members; its center is about 50 million light-years away. The space between all these galaxies is not empty, but is filled with hot gas whose temperature is of order ten million kelvin, or even higher.

The clustering of galaxies influences how any particular member galaxy will evolve. Current models of galaxy clustering suffer from several glaring omissions. One relates to the fate of its hot, X-ray emitting gas. In the cores of clusters the gas is heated by jets from supermassive black holes, supernovae (the explosive deaths of stars), and other processes. The mystery is why this hot gas does not cool and then sink towards the center of the cluster. The common surmise has been that outflowing jets from supermassive black holes, or other kinds of feedback, inhibit the formation of such "cooling flows," but establishing the details of this mechanism has been elusive. ...

A 1010 Solar Mass Flow of Molecular Gas in the A1835 Brightest Cluster Galaxy - B. R. McNamara et al
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SAO: A New Map of the Galaxy in the Light of Atomic Hydrogen

Postby bystander » Wed Jul 16, 2014 8:42 pm

A New Map of the Galaxy in the Light of Atomic Hydrogen
Smithsonian Astrophysical Observatory
Weekly Science Update | 2014 Apr 18

Atomic hydrogen is the lightest and by far most abundant element in the universe. When it is exposed to ultraviolet light, its single electron can be stripped from the atom, a situation that arises near stars that are hot. When a free electron then reunites with a proton to form a neutral atom it emits light including the visible H-alpha emission line. A region of H-alpha emission - a nebula - thus signals the presence of ionized gas near a hot star.

Typically stars are hot either because they are young and massive or because their nuclear burning has progressed to a hotter phase; they include O-stars, Be stars, supergiants, luminous blue variable stars, Wolf-Rayet stars, as well as very young stars of all masses and compact interacting binary stars. Surveys that take images in H-alpha emission are thus excellent detectors of hot stars and associated phenomena, not least because nebulae are comparatively larger in size and easier to spot than are the point-like stars that heat them. ...

The VST Photometric Hα Survey of the Southern Galactic Plane and Bulge (VPHAS+) - J. E. Drew et al
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SAO: Orbit Flips in Exoplanet Systems

Postby bystander » Wed Jul 16, 2014 8:58 pm

Orbit Flips in Exoplanet Systems
Smithsonian Astrophysical Observatory
Weekly Science Update | 2014 Apr 25

The orbits of the planets in our solar system are almost circular (Kepler made the case for their actually being ellipses). This nearly circular, concentric property helps keep the solar system stable, since highly elliptical orbits could occasionally bring planets close enough together for their gravitational interactions to disrupt their paths. Orbital shapes are quantified by their eccentricity, a measure of the closest distance of a planet from the Sun compared to its largest distance (thus helping determine the annual variations in stellar illumination); the Earth’s eccentricity is small, 0.0167, and in December the Earth is only about 3% closer to the Sun than in June.

The northern hemisphere is cooler in December (not June) because the Earth's axis of rotation is tilted with respect to its orbital motion, and in December the north pole is pointed slightly away from the Sun. The size of this tilt (called the obliquity) is 23.4 degrees, and it was likely produced in a cataclysmic impact between the Earth and another large body about 4.5 billion years ago. The impact is also thought to have formed the moon, whose presence plays the important role of stabilizing the value of the tilt which otherwise might wobble. Mars, for example, has no large moon, and its obliquity – currently 25 degrees – wobbles by up to tens of degrees on time scales of only hundreds of thousands of years, driving profound climate changes on the planet as detected in the structure of its polar ice caps. Eccentricity and obliquity are thus key planetary parameters, and they are not necessarily constant but can evolve in time. ...

Eccentricity Growth and Orbit Flip in Near-coplanar Hierarchical Three-body Systems - Gongjie Li et al
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SAO: Mining Asteroids

Postby bystander » Thu Jul 17, 2014 8:19 pm

Mining Asteroids
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 May 02

Last month NASA held a forum to explore the possibility of a human mission to an asteroid, motivated in part by its longer-term goal: the human exploration of Mars. An asteroid mission, besides providing new science on the origins of the asteroids and the solar system, would also result in more practical results, from advanced rocket propulsion systems to enhanced detection techniques, and including the potential mining of precious metals. The first step in such a mission is likely to be the robotic recovery of a nearby asteroid, a near-Earth object (NEO). The current Asteroid Recovery Mission concept proposes to capture robotically a smallish NEO (between five and ten meters in diameter) and tow it into an orbit around the Earth where a crew of astronauts would rendezvous with it and retrieve samples. ...

If additional remote observations could be performed first, to further characterize the population, and if these observations used sensitive space-based telescopes with current capabilities, the scientists calculate that the search could be narrowed and the probe fleet reduced to fewer than half-dozen missions, at considerable cost savings. The paper concludes that the recovery of economically worthwhile ore-bearing asteroids is indeed possible, but that improved prior characterization would greatly enhance the effectiveness of the program.

How Many Assay Probes to Find One Ore-Bearing Asteroid? - Martin Elvis, Thomas Esty

NASA: Asteroid Initiative Opportunities Forum
NASA: SSERVI: Asteroid Grand Challenge
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SAO: Clusters of Massive New Stars

Postby bystander » Thu Jul 17, 2014 8:51 pm

Clusters of Massive New Stars
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 May 09

Over ninety percent of the stars in our galaxy were born in stellar nurseries, clusters of stars nestled deep within clouds of dust and molecular gas. These young natal environments are key targets for astronomers who study star formation because they retain imprints of the initial conditions that produced the stars and the dynamical environments under which they evolved. Clusters with massive stars (stars more than a few solar masses in size) are of particular interest since both the formation of massive stars and their impact on other cluster members are poorly understood, for several fundamental reasons. Massive stars begin hydrogen burning while they are still growing and hence quickly develop strong winds and ultraviolet radiation that inhibit their further growth while at the same time disrupting the nursery with shock waves and ionizing light. Moreover, because massive stars evolve quickly they do not linger in any particular stage of development long enough for easy study, and they remain obscured by the not-yet-disbursed natal dust. None of these issues applies to the formation of lower mass stars. ...

astronomers report finding 3021 young stars in their set of five clusters, a large enough sample to draw significant conclusions. The youngest group of these stars, a subset of 539, are found in regions where the cloud material is densest, supporting the general picture of cluster formation. The scientists also find that massive young stars form preferentially in filamentary (rather than spherical) structures that subsequently fragment, probably due to turbulence effects.

A Multiwavelength Study of Embedded Clusters in W5-east, NGC 7538, S235, S252 and S254-S258 - L. Chavarria et al
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SAO: Ice-Cold Water

Postby bystander » Thu Jul 17, 2014 9:07 pm

Ice-Cold Water
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 May 16

Warm water vapor in the interstellar medium (about at room temperature) behaves roughly as expected from chemical models. This was the conclusion reached from observations by the Infrared Space Observatory about fifteen years ago, subsequently confirmed by the Submillimeter Wave Astronomy Satellite. However, in the colder conditions that exist in molecular clouds, and in particular in the denser clouds where stars might form, most of the water is frozen as ice onto the surface of dust grains where its properties are much harder to understand. Moreover, since the molecule itself is produced mainly on the surfaces of such grains where atoms of hydrogen and oxygen meet and bond, the several processes underway on the grain surface are complex.

In order to test chemical models that include grain-surface chemistry, CfA astronomer Eric Keto and two colleagues observed water in the cold, dense cloud L1544 with the Herschel Space Observatory. Small clouds like this one are only about a light-year in size and contain about a hundred solar masses of cold (less than 15 kelvin) material. They have the advantage of being relatively simple: nearly spherical in shape and with no internal sources of heat (i.e., no stars). They are warmed from the outside by cosmic rays and by the ultraviolet background of starlight; from the inside they are cooled via radiation from molecules and dust. Their simplicity makes them uniquely powerful laboratories for testing complex hypotheses like the chemistry of cold water. ...

Chemistry and Radiative Transfer of Water in Cold, Dense Clouds - Eric Keto, Jonathan Rawlings, Paola Caselli
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SAO: Dark Stars

Postby bystander » Thu Jul 17, 2014 9:31 pm

Dark Stars
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 May 23

The vast majority of matter is the universe, about eighty-five percent of it, is so-called "dark matter." It consists not of ordinary atoms, but of some still unknown kind of particle. Understanding this ubiquitous yet mysterious substance is a prime goal of modern astrophysics. Dark matter is detected via its gravitational influence on stars and other normal matter, and some astronomers speculate that it might have another other property besides gravity in common with ordinary matter: It might come in two forms, matter and anti-matter, that annihilate on contact emitting high energy radiation.

Several hundred million years after the big bang, the first stars began to form as gravity gradually condensed the primordial material and heated it to temperatures able to trigger nuclear burning. Scientists have speculated that something roughly similar might also have occurred to dark matter: Gravity condensed it into cores that ultimately ignite, not with nuclear burning – dark matter is not atomic and has no (normal) nuclei – but rather via annihilation radiation. These so-called "dark stars" might have shone for some time as more and more dark matter accreted onto them, powering ongoing annihilation. They may even have heated up their environment in a way that inhibited the growth of the first generation of normal stars. ...

The Mutual Interaction Between Population III Stars and Self-Annihilating Dark Matter - Athena Stacy et al
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SAO: The Interface Region Imaging Spectrograph

Postby bystander » Thu Jul 17, 2014 10:08 pm

The Interface Region Imaging Spectrograph
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 May 30

The region between the Sun's surface and its hot, million-degree corona is a complex interface zone. Only a few thousand kilometers deep, within it the density of the gas drops by a factor of about one million, while the temperature increases from about five thousand to one million kelvin. Almost all of the mechanical energy that drives solar activity and solar atmospheric heating is converted into heat and radiation within this interface, with only a small amount leaking through to power coronal heating and drive the solar wind. Multiple physical processes act within this region to shape the intricate system of magnetic fields, energetic particles, and radiation that in turn power the corona and the solar wind.

Despite the importance of the interface region it remains poorly understood, in part just because it is so complex and in part because modeling the process requires measurements over a wide spectral range (from the visible to the extreme ultraviolet) with high enough spatial resolution to identify small features. As a result, it presents a challenging target for observers and modelers alike. The Interface Region Imaging Spectrograph (IRIS) mission was launched on 27 June 2013 to address these issues. IRIS is a NASA solar mission to which the CfA and its staff contributed its twenty centimeter telescope as well as a science team. IRIS advances previous rocket studies by using novel, high-throughput, and high-resolution instrumentation, efficient numerical simulation codes, and powerful, massively parallel supercomputers to aid interpretation of the data.

The CfA team consists of P.N. Cheimets, E.E. DeLuca, L. Golub, R. Gates, E. Hertz, S. McKillop, S. Park, T. Perry, W.A. Podgorski, K. Reeves, S. Saar, P. Testa, H. Tian, and M. Weber. They and their colleagues have just published the first paper from IRIS, detailing the instrumental performance and the initial sixty-day observing program. The paper reports that IRIS is working well, and achieving excellent spatial and spectral resolution, temporally resolved observations as short 1.5 seconds, and velocity information as small as one kilometer a second.

The Interface Region Imaging Spectrograph (IRIS) - B. De Pontieu et al

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SAO: The Properties of a Six-Meter Near Earth Object

Postby bystander » Fri Jul 18, 2014 1:56 pm

The Properties of a Six-Meter Near Earth Object
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 June 06

Near Earth Objects (NEOs) are asteroids (or comets) whose orbits sometimes bring them close to the earth's orbit. Thus they could potentially collide with the Earth, giving them considerably more parochial interest than most objects in astronomy. The 1908 Tunguska event, for example, that flattened over 2000 square kilometers in Russia was by some basic estimates caused by an asteroid about 60 meters in diameter. The asteroid that struck Siberia last spring (the Chelyabinsk meteor) was only about 40 meters in diameter. While it is relatively easy to detect an NEO in visible light by watching its movement across the sky from night to night, determining its size is more difficult. This is because the optical brightness of an NEO is the result of two factors, its size and its reflectivity. CfA astronomers have for several years been using the IRAC infrared camera on Spitzer to measure the infrared light emitted from NEOs, and modeling the flux to determine the reflectivity and thus the sizes of NEOs.

NASA has decided to support a human mission to an asteroid, with the first step likely being the robotic recovery of an NEO. The current Asteroid Recovery Mission (ARM) concept proposes to robotically grab a smallish NEO (between five and ten meters in diameter) and tow it into an orbit around the Earth where a crew of astronauts would rendezvous with it and retrieve samples. Finding a suitable one is tough, however: the sizes of some smallish NEOs have only recently been determined (by infrared techniques), and a suitable one needs to be in an orbit close enough now for remote characterization but then returning to the immediate neighborhood in a few years for the recovery mission itself. ...

Physical Properties of Near-Earth Asteroid 2011 MD - M. Mommert et al

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Mining Asteroids
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 May 02

NASA: Asteroid Initiative Opportunities Forum
NASA: SSERVI: Asteroid Grand Challenge
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SAO: Shocks in the Cygnus Loop Supernovae Remnant

Postby bystander » Fri Jul 18, 2014 2:27 pm

Shocks in the Cygnus Loop Supernovae Remnant
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 June 13

Supernova remnants (SNRs) play a vital role in the lifecycle of dust in the interstellar medium. As shockwaves from supernovae sweep up interstellar material, they heat the gas and dust, and destroy a significant fraction of the grains, releasing refractory elements back into the gas phase. The shock-heated dust emits strongly at infrared wavelengths where many atomic and molecular species emit diagnostically useful lines. Thus, infrared observations of SNR shocks are crucial for studying shock properties.

Most SNRs in the galaxy are obscured by dust and cannot be easily detected at ultraviolet or even optical wavelengths. In these cases, infrared emission offers the primary way to study the radiative shocks.The Cygnus Loop, a middle-aged remnant, is an ideal object for the study. It is bright and relatively nearby (about 1800 light-years away). The Cygnus Loop, so-called because of its dramatic optical appearance, exhibits a classical "shell" morphology and contains a rich set of shock excited emission lines across a broad wavelength range.

The proximity of the Cygnus Loop allows careful modeling of the shock diagnostic infrared lines in considerable detail. CfA astronomers John Raymond and Terrance Gaetz, along with their colleagues, used the infrared spectrometer on the Spitzer Space Telescope to examine the shocks here, and report finding a lack of dust emission, at least in the range they examined,presumably because the shocks destroyed the small grains that contribute at these wavelengths. By combining the IR spectrum some optical and UV spectra, they find among other details that the shock speed is about 150 km/sec and the age of the Loop is about 1000 years.

Spitzer IRS Observations of the XA Region in the Cygnus Loop Supernova Remnant - Ravi Sankrit et al
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SAO: Are Ultra-Luminous Galaxies Colliding?

Postby bystander » Fri Jul 18, 2014 2:54 pm

Are Ultra-Luminous Galaxies Colliding?
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 June 20
hs-1994-39-e.jpg

Ultra-luminous infrared galaxies (ULIRGs) are galaxies whose luminosity exceeds that of a trillion suns. By way of comparison, our Milky Way galaxy has a typical modest luminosity of only about ten billion suns. ULIRGs were discovered by an all-sky infrared survey satellite in the 1980's, and since then the origin(s) of their powerful emission has been widely debated. Extreme infrared activity is known to be associated with interacting galaxies, and optical imaging shows that many ULIRGs are indeed in collision. The two primary known sources of global energy production in galaxies are star formation and accretion activity around a massive black hole in the nucleus (a so-called active galactic nucleus). Both of them produce radiation that heats up the dust. Mergers might result in intense infrared emission.because they help to drive this kind of active star formation.

The issue takes on an even sharper focus because earlier in our universe - about ten billion years after the big bang when most of the stars in the current universe were formed - some evidence suggests that ultra-luminous galaxies are not merging - implying that the dominant star formation mechanisms did not involve mergers. These distant, bright galaxies are rich in molecular gas among other features, for example, and perhaps their active star formation has little to do with the merger scenario? Either way it remains unclear exactly what physical mechanisms power the luminosity. Determining whether these galaxies are in fact colliding or not is difficult because merging galaxies are identified by morphological distortions in their shapes, and in distant galaxies such distortions are difficult to see. ...

Comparison of the Morphological Properties Between Local and Z~1
Infrared Luminous Galaxies. Are Local and High−Z (U)LIRGS Different?
- Chao-Ling Hung et al
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SAO: Spinning Black Holes in Galactic Nuclei

Postby bystander » Fri Jul 18, 2014 3:10 pm

Spinning Black Holes in Galactic Nuclei
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 June 27

The nuclei of most galaxies contain a massive black hole. In our Milky Way, for example, the nuclear black hole contains about four million solar masses of material, and in other galaxies the black holes are estimated to have masses of hundreds of millions of suns, or even more. In dramatic cases, like quasars, these black holes are suspected of driving the observed bipolar jets of particles outward at nearly the speed of light. How they do this is not known, but scientists think that the spin of the black hole somehow plays a pivotal role.

A black hole is so simple (at least in traditional theories) that it can be completely described by just three parameters: its mass, its spin, and its electric charge. Even though it may have formed out of a complex mix of matter and energy, all the other specific details are lost when it collapses to a singular point. Astronomers are working to measure the spins of black hole in active galaxies in order to probe the connections between spin and jet properties.

One method for measuring black hole spin is X-ray spectra, by looking for distortions in the atomic emission line shapes from the very hot gas in the accreting disk of material around the black hole. Effects due to relativity in these extreme environments can broaden and skew intrinsically narrow emission lines into characteristic profiles that depend on the black hole spin value. ...

NuSTAR AND XMM-NEWTON Observations of NGC 1365: Extreme
Absorption Variability and a Constant Inner Accretion Disk
- D. J. Walton et al
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SAO: A Spider-Like Outflow in a Young Star Forming Region

Postby bystander » Fri Jul 25, 2014 2:13 pm

A Spider-Like Outflow in a Young Star Forming Region
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 July 04

Astronomers in the past decade have been able to examine some of the key physical processes underway in early stages of stellar gestation, thanks in part to submillimeter and infrared telescopes that can peer through the heavy obscuration of dust in the birth clouds. One of the key current mysteries is how new stars rid themselves of the spin that has accumulated as material in the cloud contracted to form them. The answer is probably lies in the bipolar jets of material observed shooting from the stars. These jets can channel the spinning material into outflows that escape and allow the contraction to continue. Indeed these outflows, often dramatically narrow and long, are commonly seen. How and when these flows develop, and how effective are they at enabling a young star to continue its growth, remain key areas of research.

The leading theories on the origin of protostellar outflows suggest that they are generated through the interaction of ionized material in the star’s disk and the magnetic fields present in the star and/or the disc. Protostellar winds then entrain the host cloud’s molecular gas, thereby producing molecular outflows. There is substantial observational evidence showing that molecular outflows from low-mass young stars tend to have relatively well collimated lobes early in their development but that they tend to develop wider opening angle flows at later stages. There is, however, no consensus on the detailed physics that produces this evolutionary trend in molecular outflows: a wandering jet axis (precession) could for example produce wider cavities as the protostar evolves. Alternatively, protostellar winds could have both collimated and wide-angle components whose relative strengths vary with age. ...

A spider-like outflow in Barnard 5 - IRS 1: The transition from a collimated jet to a wide-angle outflow? - Luis A. Zapata et al
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SAO: Computer Simulations of Colliding Galaxies

Postby bystander » Fri Jul 25, 2014 2:28 pm

Computer Simulations of Colliding Galaxies
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 July 11

The universe contains some galaxies more than a thousand times brighter than our own Milky Way galaxy. Many of them are practically invisible at optical wavelengths because the star formation that powers their emission is buried deep inside obscuring clouds of dust, but at infrared wavelengths the starlight-heated dust glows brightly. Galaxy collisions can enhance such star formation activity and produce these so-called infrared-luminous galaxies, but exactly what happens when galaxies merge and how they stimulate star formation is a subject that has been debated for at least fifty years since it was first hypothesized. Studying merging galaxies does more than sheds light on how galaxies evolve and form stars. Since they are so bright, they act as lanterns to cosmological epochs and their activity helps scientists study the early universe.

The general approach in studying mergers is to examine local examples to categorize their behaviors, and model these cases with computational codes that simulate the mergers. Twentieth century simulations included only the effects of gravity, but even so they provided valuable insights into the effects of mergers on galaxy morphologies, the formation of sweeping distortions seen as tidal tails and shells, and the kinematics of merger remnants. In the last few decades more sophisticated simulations have also included gas dynamical processes -- an important addition because the gas in galaxies constitutes a significant fraction of the mass, and unlike the stars, which are comparatively small and never collide, the gas can be heated, shocked or otherwise excited to dissipate energy. Modern merger simulations have also led to an improved understanding of the mechanisms that produce supermassive black holes at the galactic nucleus and that modify the galaxies' disks.

There are approximations used in the current simulations, largely because of a desire to reduce the time needed for a full computation. One such aspect of the most popular technique is to subdivide the mass in a galaxy into fixed-sized unit-cells to track the behavior of the gas, for example of dimensions about 150 light-years on a side. The problem is that any activity that takes place on a finer spatial scale might be inaccurately estimated. CfA astronomers Paul Torrey and Lars Hernquist and three colleagues have completed a study of mergers using a new simulation technique that among other things allows the dimensions and mass associated with a cell to change with time. In principle this should more accurately calculate the effects of shocks and other processes in the gas. Writing in the latest issue of Monthly Notices of the Royal Astronomical Society, the astronomers report that although the new code appears to work well and provide more precise and accurate results, the major conclusions from earlier studies about star formation and black hole growth are qualitatively verified. There are some quantitative differences of interest, but the overall result offers reassuring support for the earlier models, while being a step in the direction of even more precise calculations.

Galaxy Mergers on a Moving Mesh: A Comparison with Smoothed Particle Hydrodynamics - Christopher C. Hayward et al
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SAO: The Most Distant Known Star in the Milky Way

Postby bystander » Fri Jul 25, 2014 2:29 pm

The Most Distant Known Star in the Milky Way
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 July 18

Our Milky Way galaxy is a barred spiral galaxy, a flattened disk of stars, gas and dust about 100,000 light-years in diameter containing several hundred billion stars. About 0.5% of them are red giants, stars that have consumed most of their hydrogen fuel and whose outer layers have swollen and cooled. Although they are rare, red giants are large and luminous, as much as 10,000 times brighter than their smaller and more numerous stellar cousins. Our sun will evolve into a red giant star in another eight billion years or so, when its radius will have expanded to beyond the current orbit of Mercury.

The galactic disk is surrounded by a large spherical, diffuse halo roughly about 500,000 light-years in diameter (although its ill-defined edges could extend to much larger distances) and also contains red giant stars. Because red giant stars are big and luminous, they can be spotted at greater distances than many other stars, and so offer an important tool in the study of the remote regions of the galactic halo. CfA astronomers Nelson Caldwell and Warren Brown and five colleagues have just announced the discovery of the most distant known star in the Milky Way, the red giant ULAS J001535.72+015549.6 located about 900,000 light-years away. For comparison, the Large Magellanic Cloud, a small neighbor galaxy to the Milky Way, is five times closer. Before the discovery of this star (and a second slightly less distant red giant in the study) the seven most distant stars known were about 500,000 light-years away. ...

The Most Distant Stars in the Milky Way - John J. Bochanski et al
Hunting The Most Distant Stars in the Milky Way: Methods and Initial Results - John J. Bochanski et al

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SAO: Binary Stars in the Globular Cluster Messier 4

Postby bystander » Fri Jul 25, 2014 2:59 pm

Binary Stars in the Globular Cluster Messier 4
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 July 25

A globular cluster is a roughly spherical ensemble of stars, as many as several million of them, gravitationally bound together in groups whose diameters can be as small as only tens of light-years. To sense the dramatic implications of this dense packing, consider that the nearest star to the Sun, Proxima Centauri, is about four light-years away. Messier 4 (M4) is the closest globular cluster to Earth at a distance of about six thousand light-years, and a puzzle to astronomers. Normal gravitational effects should, over time, redistribute the stars in a globular cluster until they are more numerous towards the center, but while M4 shows a central concentration of stars it does not show evidence for a steep central cusp even though astronomers think enough time has passed.

To understand what is going on in this globular cluster, and to help understand how these clusters evolve in general, CfA astronomer Maureen van den Berg and her collaborators have undertaken a large and unprecedented set of deep images of M4 with the Hubble Space Telescope to look for binary stars, that is stars with companions. The dynamical interactions between the densely crowded stars in a globular cluster should disrupt many such binaries, but for reasons that are not understood about fifteen percent of the stars in M4 are binaries, at least based on monitoring brightness variations (a more typical number is two percent). Whether or not this unusual abundance is connected to the lack of a central cusp in stellar density is also not understood. ...

The M 4 Core Project with HST – III. Search for Variable Stars in the Primary Field - V. Nascimbeni et al
The M 4 Core Project with HST – II. Multiple Stellar Populations at the Bottom of the Main Sequence - A. P. Milone et al
The M 4 Core Project with HST – I. Overview and First Epoch - L. R. Bedin et al
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Re: SAO: Weekly Science Updates

Postby MargaritaMc » Sun Jul 27, 2014 4:01 pm

Great achievement, bystander, bringing these up to the current date! Many thanks.
M
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SAO: The Mysterious Center of our Galaxy

Postby bystander » Sat Aug 16, 2014 6:41 pm

The Mysterious Center of our Galaxy
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Aug 01

The center of our Milky Way galaxy is located about twenty-five thousand light years from Earth, in the direction of the constellation of Sagittarius. It is invisible to us in optical light because of extensive amounts of absorbing, intervening dust, but radiation at other wavelengths, including the infrared and radio, can penetrate the veiling material. At the heart of the galactic center is a supermassive black hole, SagA*, containing about four million solar-masses of material. The region is actively studied by astronomers because the origin, evolution, and perhaps future of the Milky Way (including the solar system, which orbits the center every few hundred million years) are determined in part by the properties of the Galactic Center. Moreover it is the closest galactic nucleus to us by far, enabling scientists interested in the bizarre nature of black holes to watch in detail, at least in principle, the physical activities underway in its immediate environment.

One problem – if you can call it that – is that SagA* is relatively passive and dim, with only slight flickering seen and thought to be the result of small blobs of material randomly accreting onto a disk around it. This passivity distinguishes SagA* from many other supermassive black holes that actively accrete and heat large amounts of material and eject powerful bipolar jets of fast-moving charged particles. Astronomers a few years ago were extremely excited, therefore, to spot a large cloud of gas (about three Earth-masses) moving quickly towards SagA*. Estimates projected that the cloud (known as G-2) might be “eaten” sometime around this summer, with the consequent accretion lighting up the region and permitting detailed modeling of black hole feeding mechanisms. Numerous groups began campaigns to monitor the activity of SagA*. ...

Spitzer/IRAC Observations of the Variability of Sgr A* and the Object G2 at 4.5 microns - J. L. Hora et al
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SAO: A Possible Signal from Dark Matter?

Postby bystander » Sat Aug 16, 2014 7:04 pm

A Possible Signal from Dark Matter?
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Aug 08

Galaxies are often found in groups or clusters, the largest known aggregations of matter and dark matter. The Milky Way, for example, is a member of the "Local Group" of about three dozen galaxies, including the Andromeda Galaxy located about 2 million light-years away. Very large clusters can contain thousands of galaxies, all bound together by gravity. The closest large cluster of galaxies to us, the Virgo Cluster with about 2000 members, is about 50 million light-years away.

The space between galaxies is not empty. It is filled with hot intergalactic gas whose temperature is of order ten million kelvin, or even higher. The gas is enriched with heavy elements that escape from the galaxies and accumulate in the intracluster medium over billions of years of galactic and stellar evolution. These intracluster gas elements can be detected from their emission lines in X-ray, and include oxygen, neon, magnesium, silicon, sulfur, argon, calcium, iron, nickel, and even chromium and manganese.

The relative abundances of these elements contain valuable information on the rate of supernovae in the different types of galaxies in the clusters since supernovae make and/or disburse them into the gas. Therefore it came as something of a surprise when CfA astronomers and their colleagues discovered a faint line corresponding to no known element. Esra Bulbul, Adam Foster, Randall Smith, Scott Randall and their team were studying the averaged X-ray spectrum of a set of seventy-three clusters (including Virgo) looking for emission lines too faint to be seen in any single one when they uncovered a line with no known match in a particular spectral interval not expected to have any features. ...

Detection of an Unidentified Emission Line in the Stacked X-Ray Spectrum of Galaxy Clusters - Esra Bulbul et al

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SAO: A Terrestrial Exoplanet's Diameter Measured to 1 %

Postby bystander » Sat Aug 16, 2014 7:13 pm

A Terrestrial Exoplanet's Diameter Measured to 1 Percent
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Aug 15

There are now about 1750 confirmed exoplanets, and several thousand candidates awaiting followup measurements. Most of them have been discovered by the Kepler satellite which looks for planetary transits: the slight dimming of starlight when an exoplanet crosses the face of the star as seen from Earth. The orbital period of the exoplanet is determined from multiple transits and its mass from the star's mass and stellar wobble. The duration and details of the transit dip provide a measure of the planet’s size, and together with the mass yields an average density -- and hence a clue to the planet’s composition (with low density around 1 gram/cm^3 implying a watery composition and high density like the Earth's of 5.5 grams/cm^3 implying a rocky planet). Measuring the planet’s density precisely is a difficult task, however, especially for smaller, Earth-sized planets, and so far just fifty-eight exoplanets have their masses known with a high degree of confidence.

CfA astronomers David Charbonneau, Francois Fressin, Li Zeng, and their colleagues teamed up with experts on the Spitzer Space Telescope to improve by a factor of two the diameter measurement of the Earth-sized exoplanet known as Kepler-93b. They used the Spitzer spacecraft’s pointing stability to position the star steadily on just one small region of one camera pixel, and the camera's precise infrared calibration to detect exceptionally small intensity variations. From seven transits, and complementary other data, they determined that the planet has a period of 4.72673978 +- 0.00000097 days and a radius of 1.481 +- 0.019 Earth-radii (an uncertainly of slightly over 1% corresponding to a mere 120 kilometers). Its mass is 3.8+- 1.5 Earth-masses, and so its density is 6.3 +- 2.6 grams-cm^-3: rocky and very similar to Earth’s average density. ...

Kepler-93b: A Terrestrial World Measured to within 120 km,
and a Test Case for a New Spitzer Observing Mode
- Sarah Ballard et al

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SAO: Hunting for Gravity Waves

Postby bystander » Mon Sep 08, 2014 3:26 am

Hunting for Gravity Waves
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Aug 22

Einstein's general theory of relativity predicts that accelerating masses should radiate gravity waves in a roughly similar way that accelerating electrical charges radiate electromagnetic (light) waves. One notable difference is that the gravitational force is intrinsically about trillion trillion trillion times weaker than the electromagnetic force, and so gravity waves are phenomenally weak. In fact, none has ever been measured in the laboratory, although their presence has been reliably inferred from the decaying orbital energy of binary stars as they radiate these waves into space. Astronomers expect that new generations of gravity wave detectors being built for space will be able not only to test relativity in new regimes, but also measure many important astrophysical phenomenon that are otherwise mysterious, from the motion, growth, and evolution of black holes to binary star evolution, and even details of the early universe.

The primary space gravity-wave mission planned is called eLISA ("evolved Laser Interferometer Space Antenna"). It is not scheduled for launch for another decade or more, but a proof-of-concept mission is scheduled for launch in 2015. One of the many issues facing the new gravity-wave instruments is their accurate testing and calibration. White-dwarf binary stars are expected to play this role. When a star like our Sun gets to be old, in another seven billion years or so, it will no longer be able to sustain burning its nuclear fuel. With only about half of its mass remaining it will shrink to a fraction of its radius and become a white dwarf star. White dwarfs are common, either isolated or in a multi-star system of some kind, the most famous one being the companion to the brightest star in the sky, Sirius. ...

A New Gravitational Wave Verification Source - Mukremin Kilic et al
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Re: SAO: Weekly Science Updates

Postby geckzilla » Mon Sep 08, 2014 3:34 am

Hmm, I wonder if the scale of that illustration is an accurate representation of the two stars' proportions and closeness to one another. Illustrations often emphasize dimensions in some way or another as well as the positions of things in order to convey an idea rather than something more realistic looking. If that's at all accurate then it would be very surprising to me that they can be that close.
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SAO: Sonograms of Young Stars

Postby bystander » Mon Sep 08, 2014 3:48 am

Sonograms of Young Stars
Smithsonian Astrophysical Observatory
Weekly Science Updates | 2014 Aug 29


The evolution of a star depends crucially on its initial birth mass and composition, and developments in its early lifetime. These initial properties determine, for example, the production of the chemical elements forged later on in the star's nuclear furnace, and its early angular momentum affects the subsequent distribution of its internal energy. Astronomers are therefore working to understand the physical processes underway in the very earliest stages of a star’s life after a dense clump of interstellar matter has contracted, warmed, and begun the stellar gestation processes.

In the early stages of a star's life, its central temperature and density are not yet high enough to initiate significant nuclear burning; its energy comes from the release of gravitational energy, and it circulates via gas motions. As the internal temperatures rise and the hot gas becomes more transparent, this internal energy begins to redistribute itself through radiation as well through gas motions. Then, as these and related processes compete for primacy, the star starts to vibrate slightly, an effect which can be observed as periodic variations in the star's surface through the luminosity and temperature. In some young stars it is possible to use "asteroseismology" – the measurement of these acoustic patterns - to explore the star’s structure and evolution. ...

Echography of young stars reveals their evolution - K. Zwintz et al

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