This week on HST

HST Programs: August 20 - August 26, 2012

Selected highlights

GO 12451: Through a Lens, Darkly - New Constraints on the Fundamental Components of the Cosmos

The cluster MACS J1206.2-0.47, imaged by HST as part of the CLASH program

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 aims to capitalise fully on HST's imaging capabilities, utilising the refurbished Advanced Camera for Surveys and the newly-installed Wide-Field Camera 3 to obtain 17-colour imaging of 25 rich clusters. The data will be use to map the mass profiles of the clusters and probe the characteristics of the high-redshift lensed galaxies. Since ACS and WFC3 can be operated in parallel, the program will also use parallel imaging in offset fields to search for high-redshift supernovae. The present observations target the cluster MACS0416-2403 at z=0.42.

GO 12521: The frequency and Demographics of Dual Active Galactic Nuclei

The dual AGN in the core of NGC 7358/Markarian 739 (from the galaxy zoo)

Dual AGNs are galaxies that host a pair of actively-accreting supermassive black holes within their central regions. These systems are generally expected to arise through major mergers between large galaxies, with each SMBH representing the core of one of the parent galaxies. Relatively few such systems are known, which is surprising given the expected frequency of these systems within a standard lambda-CDM cosmological model. This scarcity might arise from a number of sources, including basic problems with the formation model, variatioins in the accretion history of the individual SMBHs, and observational difficulties in resolving separate sources. The present prorgam aims to use the high angular resolution provided by HST WFC3 UVIS and IR observations to target AGNs that have double-peaked emission lines (specifically, [OIII] emission). These data offer the poetntial to expand searches to significantly smaller radial separations, likely corresponding to later stages in the evolution of a merger. the distribution of separatiosn can set constraints on the ovall likely frequency of these objects.

Brown dwarfs are likely to have complex atmospheric structures that resemble Jupiter

Brown dwarfs are failed stars - objects that form like stars, by gravitational collapse within giant molecular clouds, but which have insufficient mass to raise the central temperature above 107 K, and which therefore are unable to ignite hydrogran fusion and maintain a long-lived central energy source. As such, these objects reach a maximum surface temperature of perhaps 3,000K some tens of millions of years after their formation, and subsequently cool and fade into oblivion. As they cool, they move through spectral types M, L and T, with the oldest brown dwarfs now likely to have temperatures close to 300K and emergent spectra characterised by water and ammonia bands, the putative signatures of the spectral class Y. As these dwarfs cool from L to T (~1500 to ~1200K), the atmospheres undergo significant changes, with heavier elements condensing to form dust. That dust can form clouds, perhaps giving the dwarf's surface a banded appearance, similar to Jupiter. or leading to the generation of localised cloud features. Since the rotation periods for these obejcts are usually a matter of a few hours, the appearance and disappearance of asymmetric features might lead to periodic photometric or spectroscopic variability, and at least photometric variability has been detected for a handful of ultracool dwarfs. The clouds themselves may appear and disappear over relatively short timescales, leading to longer-term photometric variations at particular wavelengths. A Cycle 18 program (GO 12314) focuses on a handful of brown dwarfs near the L/T transition, using the WFC3 grism to obtain multi-orbit, high-accuracy monitoring of their spectral behaviour. That program succeeded in recovering significant variability, with periods from 5-10 hours. The present program builds on those results by extending coverage to a much larger sample of late-L/early-T dwarfs. As a SNAP prorgam, each will be observed for only ~40 minutes. insufficient to determine periodicities, but, given the high precision of HST, sufficient to identify new candidates and gain some insight into the frequenciy of the phenomenon.

GO 12283/12568: WISP - A Survey of Star Formation Across Cosmic Time

A region of massive star formation

Star formation is the key astrophysical process in determining the overall evolution of galactic systems, the generation of heavy elements, and the overall enrichment of interstellar and intergalactic material. Tracing the overall evolution through a wide redshift range is crucial to understanding how gas and stars evolved to form the galaxies that we see around us now. The present program builds on the ability of HST to carry out parallel observations, using more than one instrument. While the Cosmic Origins Spectrograph is focused on obtaining ultraviolet spectra of unparalleled signal-to-noise, this program uses the near-infrared grisms mounted on the Wide-Field Camera 3 infrared channel to obtain low resolution spectra between 1 and 1.6 microns of randomly-selected nearby fields. The goal is to search for emission lines characteristic of star-forming regions. In particular, these observations are capable of detecting Lyman-alpha emission generated by star formation at redshifts z > 5.6. A total of up to 40 "deep" (4-5 orbit) and 20 "shallow" (2-3 orbit) fields will be targeted in the course of this observing campaign.