This week on HST


HST Programs: May 19 - May 25, 2014

Program Number Principal Investigator Program Title
13297 Giampaolo Piotto, Universita degli Studi di Padova The HST Legacy Survey of Galactic Globular Clusters: Shedding UV Light on Their Populations and Formation
13305 Carolin Villforth, University of St. Andrews Do mergers matter? Testing AGN triggering mechanisms from Seyferts to Quasars
13313 Mederic Boquien, University of Cambridge Determining attenuation laws down to the Lyman break in z~0.3 galaxies
13315 Marc W. Buie, Southwest Research Institute Pluto Satellite Orbits in Support of New Horizons
13327 Eileen T Meyer, Space Telescope Science Institute Proper Motions at 500 Mpc: Measuring Superluminal motions in Optical Jets with HST
13330 Bradley M Peterson, The Ohio State University Mapping the AGN Broad Line Region by Reverberation
13332 Seth Redfield, Wesleyan University A SNAP Survey of the Local Interstellar Medium: New NUV Observations of Stars with Archived FUV Observations
13344 Adam Riess, The Johns Hopkins University A 1% Measurement of the Distance Scale with Perpendicular Spatial Scanning
13352 Matthew A. Malkan, University of California - Los Angeles WFC3 Infrared Spectroscopic Parallel Survey WISP: A Survey of Star Formation Across Cosmic Time
13396 Sarah V. Badman, Lancaster University Dual views of Saturn's UV aurora: revealing magnetospheric dynamics
13410 Cristina Pallanca, Universita di Bologna COSMIC-LAB: a BSS orbiting a NS? The companion to the supermassive NS in NGC6440.
13412 Tim Schrabback, Universitat Bonn, Argelander Institute for Astronomy An ACS Snapshot Survey of the Most Massive Distant Galaxy Clusters in the South Pole Telescope Sunyaev-Zel'dovich Survey
13444 Bart P. Wakker, University of Wisconsin - Madison Constraining the size of intergalactic clouds with QSO pairs
13451 Frederick Hamann, University of Florida A Study of PG Quasar-Driven Outflows with COS
13459 Tommaso L. Treu, University of California - Santa Barbara The Grism Lens-Amplified Survey from Space {GLASS}
13462 Brian E. Wood, Naval Research Laboratory Tracking the Winds of Red Giants from the Star to the ISM
13463 Kailash C. Sahu, Space Telescope Science Institute Detecting and Measuring the Masses of Isolated Black Holes and Neutron Stars through Astrometric Microlensing
13482 Britt Lundgren, University of Wisconsin - Madison The Evolving Gas Content of Galaxy Halos: A Complete Census of MgII Absorption Line Host Galaxies at 0.7 < z < 2.5
13491 Todd Tripp, University of Massachusetts - Amherst Directly Probing >10^6 K Gas in Lyman Limit Absorbers at z > 2
13495 Jennifer Lotz, Space Telescope Science Institute HST Frontier Fields - Observations of Abell 2744
13517 Matthew A. Malkan, University of California - Los Angeles WFC3 Infrared Spectroscopic Parallel Survey WISP: A Survey of Star Formation Across Cosmic Time
13632 John T. Clarke, Boston University Seasonal Dependence of the Escape of Martian Water

Selected highlights

GO 13344: A 1% Measurement of the Distance Scale with Perpendicular Spatial Scanning


HST WFPC2 image of NGC 4639, one of the Cepheid-rich spiral galaxies used to calibrate SNe Ia
The cosmic distance scale and dark energy are two key issues in modern astrophysics, and HST has played a vital role in probing both. On the one hand, HST has been involved in cosmic distance measurements since its inception, largely through the H0 Key Project, which used WFPC2 to identify and photometer Cepheids in 31 spiral galaxies at distances from 60 to 400 Mpc. On the other, HST is the prime instrument for investigating cosmic acceleration by searching for and following Type Ia supernovae at moderate and high redshift. These two cosmological parameters are directly related, and recent years have seen renewed interest in improving the accuracy of H0 with the realization that such measurements, when coupled with the improved constraints from the Cosmic Microwave Background, provide important constraints on cosmic acceleration and the nature of Dark Energy. Previous HST programs have focused on identifying and measuring light curves for cepheids in external galaxies (eg GO 10802 , GO 11570 ) or quantifying the effects of variations in intrinsic stellar parameters, such as metallicity (eg GO 10918 , GO 11297 ). The present program focuses on the Galactic Cepheids that form the foundation for the whole distance ladder, employing a revived version of an old technique to determine accurate astrometry, and hence trigonometric parallaxes and reliable distances. The technique is drift-scanning - tracking HST during the observation so that stars form trails on the detector. This mode of observations was available in the early years of HST's operations, and has been revived primarily as a means of obtaing high signal-to-noise grism spectroscolpic data of stars hosting transiting exoplanets. However, the same technique can be used in imaging mode, and the extended trails allow multiple measurements of position differences for stars in the field. The net result is a significant improvement in the relative precision of the final astrometry. The present program, a continuation of Cycle 20 program GO 12879, targets 11 Galactic cepheids and aims for astrometric accuracies of 20 micro-arcseconds.

GO 13444: Constraining the size of intergalactic clouds with QSO pairs


Radio imaging of the Magellanic Stream and intergalactic clouds in the vicinity of the Milky Way (NRAO)
Galaxy formation, and the overall history of star formation within a galaxy, clearly demands the presence of gas. The detailed evolution therefore depends on how gas is accreted, recycled, circulated through the halo and, perhaps, ejected back into the intergalactic medium. Tracing that evolutionary history is difficult, since gas passes through many different phases, some of which are easier to detect than others. During accretion and, probably, subsequent recycling, the gas is expected to be reside predominantly at high temperatures. The most effective means of detecting such gas is through ultraviolet spectroscopy, where gas within nearby systems can be detected as absorption lines superimposed on the spectra of more distant objects, usually quasars. Such observations show the presence of extensive clouds of material in the intergalactic medium in the low redshift universe. The aim of the present program is to set constraints on the spatial size of these objects by using the Cosmic Origins Spectrograph to obtain spectra of QSOs lying at relatively small angular separation on the sky. These QSO pairs lie only 2 to 5 arcminutes apart, corresponding to linear separations of ~20-50 kpc. Matching the absorption spectra in these objects, and searching for lines that match, can therefore constrain the size of foreground IGM clouds.

GO 13495: HST Frontier Fields - Observations of Abell 2744


Pandora's Cluster, Abell 2744: the Chandra X-ray image, tracking hot gas, is plotted in red; the inferred dark matter distribution in blue
The overwhelming majority of galaxies in the universe are found in clusters. As such, these systems offer an important means of tracing the development of large-scale structure through the history of the universe. Moreover, as intense concentrations of mass, galaxy clusters provide highly efficient gravitational lenses, capable of concentrating and magnifying light from background high redshift galaxies to allow detailed spectropic investigations of star formation in the early universe. Hubble imaging has already revealed lensed arcs and detailed sub-structure within a handful of rich clusters. At the same time, the lensing characteristics provide information on the mass distribution within the lensing cluster. The present program builds on the highly successful CLASH program,which used 17-colour ACS/WFC3 images to map 25 galaxy clusters, tracing the mas profile and the dark matter distribution. in addition, the observations identified several lensed galaxies at redshifts that enter the JWST domaine, with the most distant object lying at a redshift z~11, within a few hundred million years of the Big Bang. The Frontier Fields program is a large-scale Director's Discretionary program that capitalises on the latter characteristic by targeting 4-6 strong-lensing galaxy clusters for very deep optical and near-infrared imaging. WFC3 and ACS will be used to observe the clusters, with simultaneous imaging obtained in parallel of a nearby "blank" field. Since the observations need to made at a specific orientation, they are being taken in two sets, ~6 months apart, alternating between detectors. Abell 2744, Pandora's Cluster, is the first target: images of the cluster with WFC3-IR were obtained in late 2013, together with ACS images of the blank field; HST is now starting the second epoch, with ACS covering the cluster and WFC3-IR centred on the parallel field.

GO 13632: Seasonal Dependence of the Escape of Martian Water


HST images of Mars oppositions
Mars lies at an average distance of 228 million kilometres, or 1.52 AU, from the Sun, and has an orbital period of 687 days. As one of earth's nearest neighbours, it has long attracted extensive attention and has been the target of more than 40 missions since the initial fly-by attempts by the USSR in the early 1960s ( see full list ). Mars currently hosts four orbiters and four landers, and the most recent mission, MAVEN, was launched in November 2013 and will arrive at Mars in September of this year. MAVEN's prime mission is to probe the structure and composition of Mars' upper atmosphere. With a mass only just over 10% that of the Earth, Mars has long since lost the atmosphere that permitted liquid water to exist on the surface, but there is a residual very low density component, evident in the sky coloration seen by the landers. HST cannot observe Mars on Maven's arrivial. Howeverf, in preparation for the visit, the current program aims to use the ACS Solar Blind Camera and the Space Telescope Imaging Spectrograph (STIS) to obtain UV imaging and spectra to measure the distribution of ocygen and hydrogen in the outer atmosphere. These observations will sample the Mars atmosphere during a different seasonal phase, extending the in-situ coverage provided by Maven.

Past weeks:
page by Neill Reid, updated 18/5/2014
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