This week on HST

HST Programs: November 16 - November 22, 2015

Selected highlights

GO 13774: Feeding and feeback: The impact of AGN on the circumgalactic medium.

An artist's impression gaseous outflows driven by an active nucleus

Galaxy formation, and the overall history of star formation within a galaxy, clearly demands the presence of gas. The detailed evolution therefore is tied very closely to how gas is accreted, recycled, circulated through the halo and disk, and, perhaps, ejected back into the intergalactic medium. Active Galactic Nuclei (AGN) are believed to play a substantive role in driving feedback, particularly in the early universe, injecting energy into the Galactic ISM and ejecting gas into the circumgalactic medium (CGM). The presence or absence of an active nucleus may well lead to significant differences in the density distribution and velocity structure of the galactic CGM; the present proposal aims to address that question. Tracing the overall 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. This program is using the Cosmic Origins Spectrograph to probe gas in the circumgalactic medium to radii of 175 kpc around 10 moderately high luminosity AGN-dominated galaxies. The targets are drawn from the Sload Digital Sky Survey, and the observatons will probe the extent, composition and velocity structure of the gaseous halos.

GO 14096: RELICS: Reionization Lensing Cluster Survey

Hubble image and mass map for the cluster ACT-CL J0102-4915, one of the clusters included in the RELICS program

The overwhelming majority of galaxies in the universe are found in clusters. As such, those 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, and the Frontier Fields program, targeting six clusters for deep multi-colour imaging. RELICS is focused on using massive galaxy clusters as gravitational telescopes, searching for strongly lensed background galaxies drawn from the high redshift universe. Imaging 46 fields in 41 galaxy clusters, this program aims to identify galaxies with redshifts in the range 9 < z < 12. By targeting strongly-lensing clusters, standard models for galaxy evolution suggest that the program can deliver ~100 galaxies in that redshift range, together with more than 150 galaxies at z~8. A significant number of these galaxies should be brighter than H~25.5, and therefore accessible to more detailed follow-up observations. Conversly, the actual number of galaxies detected will set constraints on the galaxy number-redshift distribution, and the overall formation and assembly history.

GO 14143: Probing the nature and evolution of the oldest known planetary system through Lyman-alpha observations

An artist's impression of the Kepler 444 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. Transiting systems provide particularly powerful insight into exoplanet characteristics since they provide a direct measurement of size (at least relative to the star) as well as an opportunity to probe the atmospheric composition and temperature structure via the transmission and emissive spectra. Over the following decade the number of known systems grew slowly through extensive ground-based campaigns, but the launch of Kepler in March 2009 marked a turning point. In its four year survey, Kepler has identified more than 1030 confirmed transiting exoplanets, with a further ~4700 candidates and the potential for more discoveries through increasingly detailed analysis of the archival data. The present program focuses on one of Kepler's disoveries - Kepler 444, a K0 dwarf lying at a distance of ~36 parsecs from the Sun. Kepler 444 stands out not only in its having 5 known exoplanet companions, all smaller than Earth in size, at separations between 0.04 and 0.09 AU (periods of 3.6 to 9.7 days) and likely to have rocky compositions, but also in its intrinsic metallicity, which is substantially less than thatof the Sun. Analysing Kepler's light curve outside of planetary transits, asteroseismology indicates a likely age (based on the inferred core mass) of ~11.2 billion years. This is particularly interesting given thne apparent paucity of jovian-mass exoplanet companions in globular cluster systems. The Kepler 444 planets are hypothesised to have formed at significantly larger radii, migrating inward through the protoplanetary disk. During the migration, they are expected to have accreted substantial water ice tha should now be evaporating. The present program aims to search for a signature of that evapration, Lyman-alpha absorption due to the expected large gaseous cloud of hydrogen. The Space Telescope Institute Spectrograph will be used to search for that signatuer during transit.

GO 14227: The CANDELS Lyman-alpha Emission At Reionization (CLEAR) Experiment

Part of the GOODS/Chandra Deep Field South field, as imaged by HST

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. The present program builds on this foundation by adding 16 pointings within the CANDELS fields with the WFC3 G102 grism. The goal is to probe reionisation by measuring the strength of Lyman-alpha absorption in galaxies at redshifts between z=6.5 and z=8.2. The expectation is that the ovall absorption strength should decrease with decreasing redshift as the intergalactic medium is ionised, and the proportion of neutral gas decreases.