This week on HST

HST Programs: May 18 - May 24, 2015

Selected highlights

GO 13650: The MUSCLES Treasury Survey: Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems

SOHO image of an extremely strong solar flare

M dwarfs - at least, the subset of M dwarfs known as flare stars - are renowned for possessing extremely active chromospheres and coronae. Their discovery as highly variable objects happened largely by chance. Willem Luyten had noticed in 1924 that certain M dwarfs showed spectroscopic variability, with the occasional appearance of emission lines, while in the early 1940s van Maanen commented that two late-type dwarfs, Gl 412B (WX UMa) and Gl 285 (YZ CMi), had brightened by over a magnitude on a handful of parallax plates. The crucial observations came in 1948, when E.F Carpenter noticed that the fainter component of a wide binary system had brightened by more than 3 magnitudes in a matter of minutes. In the succeeding 50 years, these stars have been subjected to extensive observations, particularly at optical and X-ray wavelengths, and the underlying physical processes are relatively well understood. However, most attention has focused on the more active flare stars, and we still have a relatively uncertain grasp on the flare frequency among less active stars. This issue has acquired increased importance with the realisation that somewhere between 10 and 50% of M dwarfs host planetary systems. As the most populous stars in the Galaxy, this also makes M dwarfs the premier planet hosts. The habitable zones in those systems lie much closer to the parent star, and planets are correspondingly vulnerable to detrimental effects from enhanced UV radiation, particularly short-wavelength UV-C. This proposal uses the Cosmic Origins Spectrograph and the Space Telescope Imaging Spectrograph to obtain UV and optical spectra (1150-5700 Angstroms) of nearby M dwarfs, providing a broad sampling of the range of activity levels among these low mass dwarfs.

GO 13656: Unveiling the Dark Baryons: The First Imaging of Circumgalactic OVI in Emission

A computer simulation of galactic gas accretion and outflow

The processes of galaxy assembly and evolution reflect how gas is accreted, recycled, circulated through the halo and disk, 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 observations, primarily spectroscopy, where gas within nearby systems can be detected as absorption lines superimposed on the spectra of more distant objects, usually quasars. Hubble's Cosmic Origins Spectrograph has been used in this mannerto probe gas in the circumgalactic medium for a large sample of relatively local disk galaxies. The present program takes a different tack, using the Solar Blind Channel on the Advanced Camera for Surveys to search for far-UV emission due to highly ionised oxygen (O VI at 1032, 1038 Angstrom) in the circumgalactic medium of galaxies at moderate redshift. The O VI emission has low contrast and is extremely difficult to detect; the present program will test a possible method involving combining pairs of on-line and continuum images of an extreme starburst galaxy at redshift z=0.23; if successful, the continuum-subtracted image will provide crucial spatial information on the distribution of hot gas in these systems.

GO 13691: CHP-II: The Carnegie Hubble Program to Measure Ho to 3% Using Population II

RR Lyrae's light curve at visible wavelengths

The classical cosmic distance scale rests on a series of distance indicators that step outwards from the Milky Way, establishing reliable measurements to ever more distant galaxies. Cepheids have long been the prime calibrators in this process, but other pulsating variables, notably Mira AGB long-period variables and RR Lyrae variables, also make significant contributions, while stellar population characteristics, such as the location of the tip of the red giant branch (TRGB), also play a role. RR Lyrae variables are evolved, near-solar-mass stars that are passing through the instability strip where it crosses the horizontal branch. With periods of 0.5 to 1.5 days, they have long served as distance indicators for old stellar populations (Baade's Population II). They have been known in the Galactic field and in Galactic globular clusters for over 150 years, and they are also present in the older stellar populations of the dwarf spheroidal Galactic satellites. Cluster (or dsph) RR Lyraes are particularly interesting, since their metallicities and ages can be deduced from analysis of the colour-magnitude diagrams for those systems. They are significantly less luminous than Cepheids, nonetheless, near-infrared photometric monitoring has demonstrated that these stars delineate a period-luminosity relation at those wavelengths that has the potential to establish distances to better than 1.5% accuracy. Distances to stellar populations can also be derived from measuring the location of the TRGB, marking the point in intermediate- and low-mass star evolution where core temperatures are raised to the point thatn helium ignites in the triple-alpha reaction, and the star evolves rapidly onto the horizontal branch. The present programs combines deep optical/far-red/near-IR imaging of moderately distant galaxies that have hosted Type Ia supernovae and of nearby galaxies with RR Lyraes with WFC3-IR observations of 4 individual Galactic variables with trigonometric parallaxes to cosmntruct a distance ladder that is independent of the Cepheid calibration. The oevrall goal is to define H0 to 3% accuracy.

GO 13792: A Complete Census of the Bright z~9-10 Galaxies in the CANDELS Data Set

Four z>9 galaxy candidates from CANDELS GOODS-N (from Oesch et al, 2014)

Hubble has made significant contributions in many science areas, but galaxy formation, assembly and evolution is a topic that has been transformed by the series of deep fields obtained over the past 20 years. CANDELS, one of three Multi-Cycle Treasury Program executed in cycles 18 through 20, is one of the more recent additions to this genre.Building on past investment of both space- and ground-based observational resources, it covers five five fields including both the Great Observatory Origins Deep Survey (GOODS), centred on the northern Hubble Deep Field (HDF) in Ursa Major and the Chandra Deep Field-South in Fornax. In addition to deep HST data at optical and near-infrared wavelengths, the fields have been covered at X-ray wavelengths by Chandra (obviously) and XMM-Newton; at mid-infrared wavelengths with Spitzer; and ground-based imaging and spectroscopy using numerous telescopes, including the Kecks, Surbaru and the ESO VLT. This represents an accumulation of almost 1,000 orbits of HST time, and comparable scale allocations on Chandra, Spitzer and ground-based facilities. CANDELS added new optical and near-infrared observations with WFC3 and ACS (see this link for more details). Those data have been processed and analysed by both the CANDELS team and by other groups within the community. In particular, the present program builds on a joint analysis of HST and Spitzer data aimed at identifying galaxies at the highest redshifts, z ~ 9-10, representing structure within half a billion years of the Big Bang. Eight candidates have been identified, and those objects are targeted for follow-up J-band (F105W) imaging with WFC3-IR that will confirm the redshift.