SAO: Weekly Science Updates 2018

Find out the latest thinking about our universe.
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What Powers the Most Luminous Galaxies?

Post by bystander » Fri Jul 06, 2018 3:10 pm

What Powers the Most Luminous Galaxies?
SAO Weekly Science Update | 2018 Jul 06
Galaxy-galaxy interactions have long been known to influence galaxy evolution. They are commonplace events, and a large majority of galaxies show signs of interactions, including tidal tails or other morphological distortions. The most dramatic collisions trigger the galaxies to light up, especially in the infrared, and they are some of the most luminous objects in the sky. Their brightness allows them to be studied at cosmological distances, helping astronomers reconstruct activity in the early universe.

Two processes in particular are responsible for the enhanced radiation: bursts of star formation or the fueling of the supermassive black hole at a galaxy’s core (an active galactic nuclei - AGN). Although in principle these two processes are quite different and should be readily distinguishable (AGN, for example, produce much hotter ultraviolet and X-ray radiation), in practice the discriminating features can be faint and/or obscured by dust in the galaxies. Astronomers therefore often use the shape of the galaxy’s entire emission profile from the ultraviolet to the far infrared (its spectral energy distribution - SED), to diagnose what is going on. The dust that absorbs much of the radiation also re-radiates it at the longer infrared wavelengths and computer codes can model and unravel the numerous physical effects. ...

The astronomers find that the AGN contribution in their sample of galaxies reaches as high as ninety percent of the total luminosity; in other cases it falls below twenty percent and is possibly negligible. The team makes efforts to relate the magnitude of the AGN contribution to the merger stage of the system (from beginning to coalescing stages), but their modest sample size limited the generality of the conclusions. They are expanding their analysis to several hundred other mergers in order to strengthen the conclusions.

The AGN Luminosity Fraction in Merging Galaxies - Jeremy Dietrich et al
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Gravitational Microlens Detection from Spitzer

Post by bystander » Fri Jul 13, 2018 5:10 pm

Gravitational Microlens Detection from Spitzer
SAO Weekly Science Update | 2018 Jul 13
su201828.jpg
Hubble Space Telescope images of a microlens system. The image on the left was
taken 3.7 years after an observed microlensing event; the one on the right was taken
8.9 years later after the moving foreground (lensing) source had changed position.
The lens and source components (A and B) are clearly resolved in the later image.
(Image Credit: NASA/Hubble)

The path of light from a star as it passes by a massive body, like an exoplanet, will be bent and an observer looking towards the star will see its image distorted. Like an object seen through the stem of a wineglass, the stellar image could even be deformed into two bright peaks. That mass could influence light in this way was first confirmed in 1919, but some of the more subtle effects have only been detected in the past twenty-five years. In one such process, microlensing, a flash of light is produced when the path of a moving cosmic body (perhaps otherwise unknown) passes fortuitously in front of a star and briefly increases the intensity of its light.

The Spitzer Space Telescope circles the Sun in an Earth-trailing orbit, and it is currently 1.66 astronomical units away from Earth (one AU is the average distance of the Earth form the Sun). Scientists had predicted that if it ever became possible to observe a microlensing flash from two well-separated vantage points, a parallax measurement (the apparent angular difference between the positions of the star as seen from the two separated sites) would determine the distance of the dark object. In fact, since 2014 Spitzer has been used successfully to measure the parallax for hundreds of microlensing events. In all these cases, Spitzer was used after ground-based observations had first identified a microlensing event underway. ...

OGLE-2017-BLG-1130: The First Binary Gravitational Microlens Detected From Spitzer Only - Tianshu Wang et al
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How Disc Galaxies Work

Post by bystander » Sat Jul 21, 2018 3:16 pm

How Disc Galaxies Work
SAO Weekly Science Update | 2018 Jul 20
Disc galaxies like our own Milky Way, characterized by a flattened disc of stars and gas (often with a central bulge of material as well) have a wide range of masses, spatial extents, and stellar content. Nonetheless all disc galaxies, both locally and in the distant Universe, share some strikingly similar properties. Most notable is that the star formation rate correlates tightly with the galaxy’s gas content, the gas motions (the "velocity dispersion"), and the dynamical lifetime (roughly, the time it takes for the galaxy to rotate once). Moreover, this curiously universal rate is remarkably small: only about one per cent of the gas in disc galaxies turns into stars over that timescale, with much of the activity concentrated in the galaxies’ central regions. Most simple models of star formation predict that gravity should be much more effective in forming stars as it compresses the gas in molecular clouds. Observations indicate that both the correlations and the inefficiency extend down to the scale of individual molecular clouds.

CfA astronomers Blakesley Burkhart and John Forbes and two colleagues have developed a new unified model for galaxy discs that explains these phenomena, and some others besides. The scientists show that the correlation of star formation rate with gas motion is not caused by these motions but rather is the result of the transport of material within the galaxy, which affects both. The model maintains a state of gas equilibrium and marginal gravitational stability by including in a galaxy the radial transport of gas towards its nucleus and also the turbulent feedback from star formation. These two considerations are relatively straightforward in principle but produce a dramatic improvement in the agreement between observations and theory, for example by explaining how the eventual quenching of the star formation happens. The new work also provides a natural explanation for the cosmic epochs at which galaxies build up bulges and discs.

A Unified Model for Galactic Discs: Star Formation, Turbulence Driving, and Mass Transport - Mark R. Krumholz et al
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Galaxies in the Early Universe

Post by bystander » Wed Aug 01, 2018 4:45 pm

Galaxies in the Early Universe
SAO Weekly Science Update | 2018 Jul 27
Image
The Spitzer infrared image of a distant luminous
infrared galaxy, overlaid with millimeter intensity
contours of the source from the Submillimeter
Array. Credit: NASA/Hill et al (MNRAS 2018)

Astronomers in the past decade have detected thousands of galaxies dating from epochs only a few billion years after the big bang. Many of them, discovered in deep optical and near-infrared surveys, are low-mass galaxies, and are faint and difficult to study. More luminous, massive star-forming galaxies should are present but these large objects are difficult to assemble so early in cosmic time and there are not as many of them. These star forming galaxies contain dust that absorbs stellar radiation and emits strongly at far infrared and submillimeter wavelengths; the Herschel Space Observatory surveys spotted about ten thousand candidate sources whose infrared colors suggested there were distant galaxies.

CfA astronomer Glen Petitpas was a member of a large team of astronomers who used the SCUBA-2 (Submillimeter Common User Bolometer Array) instrument to confirm 188 of the reddest of these sources as indeed being distant, dusty star formation galaxies, typically so far away that their light has been traveling towards us for over eleven billion years, and so bright that they must be making stars at a rate many thousands of times faster than does the Milky Way.

In a related paper, Petitpas was joined by CfA astronomers Mark Gurwell, Giovanni Fazio and David Wilner and their colleagues to use the Submillimeter Array to image some distant SCUBA-2 galaxies (although a set that is a bit closer than the above sample: their light has only been traveling for a bit over ten billion years). They targeted seventy galaxies with the SMA and detected sixty-two of them, including three pairs of galaxies that can be classified as colliding systems (the others may also include some mergers that are not spatially separable). The scientists show that the descendants of such huge galaxies in our local universe are probably massive elliptical galaxies, and that about ten percent of their stars probably formed in short bursts of activity in this earlier phase of their evolution.

High-Resolution SMA Imaging of Bright Submillimetre Sources from the SCUBA-2 Cosmology Legacy Survey - Ryley Hill et al Red, Redder, Reddest: SCUBA-2 Imaging of Colour-Selected Herschel Sources - S. Duivenvoorden et al
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Spitzer Infrared Observations of a Gravitational Wave Source

Post by bystander » Fri Aug 03, 2018 5:00 pm

Spitzer Infrared Observations of a Gravitational Wave Source
SAO Weekly Science Update | 2018 Aug 03
GW170817 is the name given to a gravitational wave signal seen by the LIGO and Virgo detectors on 17 August 2017. Lasting for about 100 seconds, the signal was produced by the merger of two neutron stars. The observation was then confirmed - the first time this has happened for gravitational waves - by observations with light waves: the preceding five detections of merging black holes did not have (and were not expected to have) any detectable electromagnetic signals. The light from the neutron star merger is produced by the radioactive decay of atomic nuclei created in the event. (Neutron star mergers do more than just produce optical light, by the way: they are also responsible for making most of the gold in the universe.) Numerous ground-based optical observations of the merger concluded that the decaying atomic nuclei fall into at least two groups, a rapidly evolving and fast moving one composed of elements less massive than Lanthanide Series elements, and one that is more slowly evolving and dominated by heavier elements.

Ten days after the merger, the continuum emission peaked at infrared wavelengths with a temperature of approximately 1300 kelvin, and continued to cool and dim. The Infrared Array Camera (IRAC) on the Spitzer Space Telescope observed the region around GW170817 for 3.9 hours in three epochs 43, 74 and 264 days after the event (SAO is the home of IRAC PI Fazio and his team). The shape and evolution of the emission reflect the physical processes at work, for example, the fraction of heavy elements in the ejecta or the possible role of carbon dust. Tracking the flux over time enables the astronomers to refine their models and understanding of what happens when neutron stars merge. ...

Spitzer Space Telescope Infrared Observations of the Binary Neutron Star Merger GW170817 - V. A. Villar et al
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Unraveling the Stellar Content of Young Clusters

Post by bystander » Thu Aug 16, 2018 2:55 pm

Unraveling the Stellar Content of Young Clusters
SAO Weekly Science Update | 2018 Aug 10
About twenty-five percent of young stars in our galaxy form in clustered environments, and stars in a cluster are often close enough to each other to affect the way they accrete gas and grow. Astronomers trying to understand the details of star formation, for example the relative abundance of massive stars to low mass ones, must take such complicated clustering effects into account. Measuring the actual demographics of a cluster is not easy either. Young stars are embedded within obscuring clouds of natal material. Infrared radiation can escape, however, and astronomers probe these regions at infrared wavelengths using the shape of the spectral energy distribution (the SED - the relative amounts of flux emitted at different wavelengths) to diagnose the nature of the young star: its mass, age, accretion activity, developing disk, and similar properties. One major complication is that the various telescopes and instruments used to measure an SED have large and different-sized beams that encompass multiple objects in a cluster. As a result, each point in an SED is a confused blend of emission from all the constituent stars, with the longest wavelength datapoints (from the largest beams) covering a spatial region perhaps ten times larger than the shortest wavelength points.

CfA astronomers Rafael Martinez-Galarz and Howard Smith and their two colleagues have developed a new statistical analysis technique to address the problem of confused SEDs in clustered environments. Using the highest spatial resolution images for each region, the team identifies the distinguishable stars (at least this many are in the cluster) and their emission at those wavelengths. They combine a Bayesian statistical approach with a large grid of modeled young stellar SEDs to determine the most probable continuation of each individual SED into the blended, longer-wavelength bands and thus leads to the determination of the most likely value of each star's mass, age, and environmental parameters. The resultant summed SED is not unique but is the most likely solution. ...

Unraveling the Spectral Energy Distributions of Clustered YSOs ~ Juan R. Martínez-Galarza et al
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The TESS Input Catalog

Post by bystander » Fri Sep 07, 2018 4:56 pm

The TESS Input Catalog
SAO Weekly Science Update | 2018 Aug 31
The Transiting Exoplanet Survey Satellite (TESS), launched on April 18, has as its core mission goal to discover small transiting exoplanets orbiting nearby bright stars, and to do so it will conduct a nearly all-sky photometric survey over the next two years. For 27.4 days at a time TESS will look at one region of the sky while its 64-million-pixel camera reads out once every 30 minutes in an effort to spot the slight dips in starlight that signal the transit of a planet across the face of a distant star. (Several hundred thousand of the pixels will read out in a two minute cadence to probe more closely high value targets.) At the end of 27.4 days TESS will point to another region of the sky and repeat.

TESS, however, needs a source catalog of likely stars in order to know which stars in the field to observe. The highly successful exoplanet mission Kepler similarly had a stellar source catalog. The TESS Input Catalog (TIC) is used not only to select optimal targets, it also is designed to provide the properties of each of the stars, properties necessary for determining stellar radii (and hence planetary radii) and other key facts about the possible exoplanet system. Not least, every TESS pixel sees a relatively large area of the sky (twenty arcseconds on a side) in which multiple objects could fall, and the catalog therefore needs to target only luminous stars that are not known to vary. Finally, the catalog will be used for objects with which to test the system performance and look for any false positives. ...

The TESS Input Catalog and Candidate Target List ~ TESS Target Selection Working Group
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Polaris, the North Star

Post by bystander » Fri Sep 07, 2018 5:07 pm

Polaris, the North Star
SAO Weekly Science Update | 2018 Sep 07
The North Star, Polaris, is a Cepheid variable: one whose mass, age and physical conditions generate periodic oscillations with a period proportional to the star's intrinsic luminosity. This extraordinarily useful property of Cepheid variables, discovered and calibrated at Harvard by Henrietta Leavitt starting in 1908, allows them to be used as cosmic distance calibrators. By comparing the intrinsic brightness as determined from the period (which is easily measured) with the measured brightness, the period-luminosity relationship, a precise distance can in principle be obtained. Cepheids in nearby galaxies that are receding from us provide the basis for the famous distance-velocity relationship of galaxies that underpins the expanding model of the universe (the "big bang" model). Cepheids are so important that they have also become benchmarks for testing our understanding of stellar evolution.

Polaris is not only famous as the beacon for early navigators, it is also the closest Cepheid to earth (445.5 light-years away), and a subject of intense study. It is a member of a triple system, and one source of confusion about its development has been the extent to which its companion stars could have affected its evolution. The star we can see by eye, Polaris Aa, has a close companion, Polaris Ab that orbits it in 29.59 years; a third star, Polaris B, orbits these two but is one hundred times farther away. Two more stars nearby, Polaris C and D, might also be faint companions. ...

The Orbit of the Close Companion of Polaris: Hubble Space Telescope Imaging, 2007 to 2014 ~ Nancy Remage Evans et al
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Chandra Detection of Diskless Young Stars

Post by bystander » Tue Sep 18, 2018 3:26 pm

Chandra Detection of Diskless Young Stars
SAO Weekly Science Update | 2018 Sep 14
Stars frequently form in crowded environments. By combining the resources of multi-wavelength missions like Chandra in the X-ray and Spitzer in the infrared, astronomers are able to resolve ambiguities and assemble a much more complete census of cluster content and the individual properties of the population. A case in point is the development of disks (possibly protoplanetary) around new stars. Disks form along with the new star and then evolve over a few million years before dissipating, perhaps leaving planets behind, and in clustered environments their development can be influenced by interactions with neighbors.

Stellar disks are warmed by their stars and were first spotted via the infrared emission from the warm dust. More evolved young stars without disks lack this characteristic infrared signature and thus can be identified as the the more evolved ones in a cluster. Young stars were also discovered to emit elevated levels of X-rays as compared to main-sequence stars because of their still-developing internal circulation. (In fact, young stars can have luminosities thousands of times brighter in X-rays than their older stellar counterparts.) In a crowded cluster environment, however, where other factors besides age are thought to be able to inhibit or disrupt the development of a disk, the X-ray emission offers an independent tool to identify those young stars without disks.

The Serpens South cluster of stars, estimated to be located about 900 light-years away in the direction of the constellation Serpens, is very young and its stars are heavily masked by the thick natal dust in their environment – indeed, it is thought to be among the youngest regions near us, making it an important test-bed for the study of disk evolution in clustered environments. ...

Chandra Detection of an Evolved Population of Young Stars in Serpens South ~ Elaine M. Winston et al
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A New Classification Scheme for Exoplanet Sizes

Post by bystander » Sat Sep 22, 2018 3:08 pm

A New Classification Scheme for Exoplanet Sizes
SAO Weekly Science Update | 2018 Sep 21
There are about 4433 exoplanets in the latest catalogs. Their radii have generally been measured by knowing the radius of their host star and then closely fitting the lightcurves as the planet transits across the face of the star. The radius of the host star is thus a key parameter and latest data release of the Gaia mission has enabled astronomers to improve the accuracy of stellar properties in its catalog very significantly – to a precision in radius of about 8% - for nearly one hundred and eight thousand stars in the Kepler exoplanet fields.

CfA astronomer Dimitar Sasselov was part of a team with three colleagues to use the new stellar results to refine the radial measurements of 4268 exoplanets. The large dataset and refined values enable the scientists to confirm some previous hints about the distribution of exoplanet sizes, namely, that the size distribution is not exactly uniform but rather some exoplanet sizes are less common than might be expected. In particular, there is a paucity of planets with radii slightly larger than about two Earth-radii, and other slight decreases again at sizes of about four and about ten Earth-radii.

The astronomers use their new database to define a new classification scheme for exoplanets. The smallest category consists of planets smaller than four Earth-radii, and within this group are two subgroups: those smaller than two Earth-radii and those between about two and four Earth-radii. These small planets are generally gas poor. The second category has between four and ten Earth-radii, and the team proposes they be called "transitional planets" since they form a bridge between the small class and the large gas giants. There is a relative paucity of objects in this class for reasons that are not well understood.

The third new grouping contains the gas giant planets, those with sizes larger than about ten Earth-radii and which are dominated by hydrogen and helium; these include Jupiter analogs, and even brown dwarf stars. The authors conclude by observing that the group of two-to-four Earth-radii planets are the ones most likely to have water- rich cores ("water worlds"). They propose that their results will help refine the list of objects selected for observational follow-ups including potentially habitable worlds.

Survival Function Analysis of Planet Size Distribution with GAIA Data Release 2 Updates ~ Li Zeng et al
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