This week on HST

HST Programs: April 25, 2011 - May 1, 2011

Selected highlights

GO 11598: How Galaxies Acquire their Gas: A Map of Multiphase Accretion and Feedback in Gaseous Galaxy Halos

A computer simulation of galactic gas accretion and outflow

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. The present program is using the Cosmic Origins Spectrograph to observe z>1 QSOs that lie at small angular separations from SDSS galaxies at redshifts between z=0.15 and 0.35. The sightlines run through the halos of the galaxies, and the QSOs therefore provide a pencilbeam backlight that probes hot gas in the foreground systems.

GO 11728: The Impact of Starbursts on the Gaseous Halos of Galaxies

A computer simulation of galactic gas accretion and outflow

The detailed history of galaxy formation and assembly depends on the availability of gas for star formation, and on how processes engendered by that star formation affects the distribution and intrinsic properties of that gas. In particular, feedback in the form of winds and ionising radiation from either young stellar associations or massive black holes may be capable of interfering with, and even interrupting, the overall assembly process. The present program aims to investigate this issue by probing the nature of halo gas in the vicinity of galaxies that have undergone recent (<100 Myr old) starbursts. The halo gas is expected to 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. The present program is using the Cosmic Origins Spectrograph to observe moderate-redshift QSOs that lie at small angular separations from the starburst galaxies. The sightlines run through the halos of the galaxies, and the QSOs therefore provide a pencilbeam backlight that probes hot gas in the foreground systems.

GO 12163: Structure and Stellar Content of the Nearest Nuclear Clusters in Late-Type Spiral Galaxies

the nearby spiral, NGC 300, one of the galaxies targetted in this program

Extensive observations over the past decade, primarily with HST, have shown that massive, young star clusters are present in the nuclear regions of almost all late-type spiral galaxies. It remains unclear, however, whether those clusters harbour (or generate) the black holes that are commonly found in the central regions of both ellipticals and spiral galaxies with significant bulge components, such as our Milky Way. The present program aims to address this issue through systematic observations of 10 relatively nearby bulgeless spirals. The aim is to combine multi-wavelength, high resolution imaging, using WFC3 on HST, with ground-based spectroscopy of the central regions of these systems. The resultant detailed colour-magnitude diagrams and spectral line strengths will be used in conjunction with stellar population models to constrain the star formation history and stellar content of the central regions, while the kinematics will probe the dynamics of the central regions, and set limits on the likely presence of central black holes in these systems.

GO 12165: Characterizing the UV environment of GJ1214b

Artist's impression of the GJ 1214 system

The first exoplanet, 51 Peg b, was discovered through radial velocity measurements in 1995. 51 Pegb was followed by a trickle, and then a flood of other discoveries, as astronomers realised that there were other solar systems radically different from our own, where "hot jupiters" led to short-period, high-amplitude velocity variations. Then, in 1999, came the inevitable discovery that one of those hot jupiters. HD 209458b, was in an orbit aligned with our line of sight to the star, resulting in transits. Since that date, the number of known transiting exoplanet systems has grown to more than 100 from ground-based observations, most detected through wide-field photometric surveys, while the high-sensitivity data provided by Kepler has added a further 1000+ systems. One of the most interesting is the nearby red dwarf star, GJ 1214, which has a transiting ~6.5 Earth-mass companion (a "super-Earth") in an orbit with period 1.58 days, semi-major axis 0.014 AU. At such small separations, the planetary atmosphere is vulnerable to evaporation from high energy radiation from the parent star. In the case of GJ 1214, the primary is a relatively cool mid-type M dwarf, spectral type M4.5, mass ~0.2 solar masses and temperature ~3000K. However, such stars can be subject to significant flare activity. The present HST program aims to quantify GJ 1214's characteristics in that regard throuhg UV spectrosocpy with the Space Telescope Imaging Spectrograph (STIS).