This week on HST

HST Programs: September 15 - September 21, 2014

Selected highlights

GO 13330: Mapping the AGN Broad Line Region by Reverberation

Simulations of the appearance and velocity structure within an AGN disk (see Keith Horne's webpage ).

Active galaxies (AGNs) are generally luminous systems, characterised by the presence of strong nuclear emission lines of numerous species including H, He I, He II, and Fe, Ca, O, C and S over a range of ionisations. These features originate from gas clouds in the nuclear regions, with the energy supplied through accretion onto a central massive black hole. The high-temperature, rapidly-rotating gas clouds nearest the central engine are responsible for producing broad emission lines (hence, the "Broad Line Region"). The structure of the BLR can be discerned using a technique known as reverberation mapping: variations in the accretion rate lead to fluctuations in luminosity; those variations lead, in turn, to variations in the photoionisation of the BLR, and corresponding changes in spectral line strengths and velocities; monitoring those changes, and correlating them with the photometric variability of the central source, measures the light travel time from nucleus to BLR gas, and hence maps the size of the BLR. The present prorgam will use the Cosmic Origins Spectrograph to undertake systematic monitoring of the nuclear regions of the Seyfert I galaxy, NGC 5548. The first observations were taken on February 2nd 2014 and the program reached its conclusion in late Jul, having taken observations at a cadence of ~one orbit per day for 179 days. The present observations target white dwarf reference stars.

GO 13364: LEGUS: Legacy ExtraGalactic UV Survey

UGC 4305 = Holmberg II - Arp 268, one of the star-forming galaxies targeted by LEGUS

Understanding the global architecture of star formation is a key step towards understanding the morphological evolution of galaxies and the characteristics of the underlying stellar populations. HST has devoted extensive resources to observations of nearby galaxies over its lifetime, including detailed surveys of a handful of systems, notably the PHAT survey of M31, with the enhanced imaging capabilities made available following SM4. Most programs, however, have focused on optical, far-red and, to a lesser extent, near-infrared wavelengths. While those observations provide high-quality colour-magnitude data that enable an exploration of relatively mature populations, they are less effective at probing active star-formation sites populated by young, high-mass stars. Those regions are most prominent at ultraviolet wavelengths, and the present program aims to capitalise on the past heritage of HST observations by adding near-UV imaging for 50 nearby galaxies. The 50 targets are drawn from a catalogue of 400 systems within ~11 Mpc of the Milky Way, and have been selected to provide a fair sampling of the wide variety of galactic systems within that volume. The program will image star-forming regions at near-UV and blue wavelengths using the WFC3 UVIS channel with the F275W, F336W and F438W filters, supplemented by F55W and F814W (V and I) where necessary. The present observations are targeting NGC 5194, the dwarf companion of M51; these observations compelte the program.

GO 13459: The Grism Lens-Amplified Survey from Space {GLASS}

HST imaging of the galaxy cluster MACS0717.5-3745

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. Hubble is currently undertaking deep imaging observations of up to 6 galaxy clusters as part of the Frontier Fields Director's Discretionary Time program (GO 13495). The present program supplements those visual and near-infrared data by adding imaging spectrophotometry at near-infrared wavelengths, using the low-resolution G102 and G141 grisms on the WFC3-IR camera. In all, the program targets 10 clusters drawn from both the Frontier Fields sample and from the larger-scale (but less deep) multicolour CLASH program. The goal is to identify and characterise galaxies at relatively high redshifts, close to the epoch of reionisation. The grism data should provide low-resolution spectra for 150-200 galaxies at redshifts z>6, with the potential to detect Lyman alpha emission at redshifts up to z~8.5. The present observations target the Frontier Fields cluster, MACS0717.5+3745.

GO 13794: Seasonal Dependence of the Escape of Water from the Martian Atmosphere

HST imaging of Mars - high clouds are visible in the upper atmosphere

Mars has been in the news recently for several reasons. The rover Curiousity continus to explore the surface; it is seeing the close passage of the Oort cloud comet, Comet 2013A Siding Spring, discovered by Robert McNaught last year; and this week sees the arrival of MAVEN, the Mars Atmosphere and Volatile EvolutioN planetary probe launched in 2013 and scheduled for orbit insertion on September 21. MAVEN's mission is to study the tenuous Martian atmosphere, using the onboard instruments to probe the upper atmosphere and determine its rate of escape. Hubble offers some limited opportunities to tackle the same questions, and the current progam aims to complement MAVEN's in situ measurements through ultraviolet imaging and spectroscopy with the Advanced Camera for Surveys' Solar Blind channel (ACS/SBC) and the Space Telescope Imagign Spectrograph. Those observations can detect emission from hydrogen and oxygen, and previous observations have revealed significant changes with the martian seasons. The present observations are part of a series that aim to characterise the atmospheric structure as Mars passes through its seasonal changes. In addition, these observations, taken while most of the Martian satellites have been powered down for the Comet Siding Spring encounter, offer the prospect of testing whether the comet's tail might have some detectable impact on the extended Martian atmosphere.