This week on HST

HST Programs: March 5 - March 11, 2012

Selected highlights

GO 12328: 3D-HST: A Spectroscopic Galaxy Evolution Treasury

A grism spectrum of a high-redshift source

Throughout its twenty-plus year career, HST has strongly influenced our understanding of numerous research areas within astronomy and astrophysics. Among those, galaxy evolution stands out as a discipline that has been essentially revolutionised by Hubble observations. The original Hubble Deep Field, the product of 10 days observation in December 1995 of a single pointing of Wide Field Planetary Camera 2, demonstrated conclusively that galaxy formation was a far from passive process. The images revealed numerous blue disturbed and irregular systems, characteristic of star formation in galaxy collisions and mergers. Building on this initial progam, the Hubble Deep Field South (HDFS) provided matching data for a second southern field, allowing a first assessment of likely effects due to field to field cosmic variance; the Hubble Ultra-Deep Field (UDF) probed to even fainter magitude with the Advanced Camera for Surveys (ACS); recent deep near-infrared imaging with WFC3 has pushed the redshift limit to beyond z~8; and the Multi-Cycle Treasury CANDELS program is providing a tiered set of observations that complement, in areal coverage and depth, the deep UDF observation. All of these programs rely on deep, high-sensitivity, high resolution imaging. However, WFC3 is also eqipped with grism/prism optical elements, and therefore offers the potential for low-resolution spectroscopy at optical and near-infrared wavelengths. The present program capitalises on that ability, using the near-infrared WFC3/G141 and far-red ACS/G800L grisms to survey subsets of the GOODS-South, AEGIS, UDS and COSMOS fields covered by the CANDELS survey. These data will provide spectroscopic means of probing star formation at redshifts in the range 1 < z < 3.5.

GO 12476: Measuring the Hubble Flow Constant

NGC 4911, from DSS scans of POSS II IIIaJ plate material

Cepheid variable stars have been the prime extragalactic distance indicator since Henrietta Leavitt's discovery of the period-luminosity relation described by Cepheids in the Small Magellanic Cloud. It was Hubble's identification of Cepheids in NGC 6822 that finally established that at least some nebulae were island universes. Cepheids and the extragalactic distance scale figure largely in HST's history, notably through the Hubble Constant Program, one of the initial Key Projects. Hubble has accumulated WFPC2 and NICMOS observations of Cepheids in 31 galaxies. All of those galaxies lie within 25 Mpc; thus, both the Key Project's derivation of H0 = 72 +/- 8 km/sec/Mpc and the competing value, H0 = 56 +/- 7 km/sec/Mpc, (an offset of 1.5 sigma), rely on secondary indicators to take measurements to the far-field Hubble flow. The aim of the present project is to extend covrage to Cepheids within the Coma cluster. The present program has its genesis in a Cycle 15 Treasury program designed to use the high sensitivity and high resolution of the Adnaced Camera for Surveys to search for Cepheids in two spiral galaxies in the cluster, NGC 4911 and NGC 4921. If Coma lies at a distance of 100 Mpc ( (m-M)=35.0), then long-period Cepheids (P~50 days) have mean apparent magnitudes of V~29 - challenging observations even for ACS. Unfortunately, the ACS failure in January 2007 left the original program barely 40% complete. However, the initial dataset was sufficient to identify 50 long-period Cepheids candidates in NGC 8921. The present program aims to build on those incomplete results by using the UVIS channel on Wide-field Camera 3 to obtain cadenced observations in the F350LP and F606W filters, and determine periods for verified variable stars..

GO 12528: Probing the nature of LBVs in M31 and M33 - Blasts from the Past

GR290, Romano's star, a luminous blue varibale in M33

Luminous Blue Variables (LBVs) are just what they sound like - bright blue stars that show low-level (few tenths of a magnitude), irregular changes in brightness over timescales of years, punctuated by substantial outbursts, when the star brightens by several magnitudes. The prototype is S Doradus, the brightest star in the Large Magellanic Cloud (MV < -10, so brighter than many dwarf galaxies), and there are only around 20 other such stars known, including P Cygni and eta Carinae. They are massive hypergiants, with radii over 100 times that of the Sun and system masses that may exceed 100 MSun. At least some of these systems are binary stars. The outbursts are very likely caused by substantial mass loss events, perhaps stimulated by the binary companion, and can lead to the star's brightness increasing over a hundredfold over a period of 5-10 years. The classic example is Eta Carinae, which was originally catalogued by Edmond Halley as 4th magnitude in 1677, but had brightened by 3-4 magnitudes by 1730, returning to its previous brightness by 1782. There have been a number of subsequent outbursts, and detailed, high-resoution imaging ahs revealed a complex series of nebular shells surrounding the stellar system, indicative of substantial mass-loss events. The present HST program aims to probe the LBV phenomenon through high spatial-resolution spectroscopy of the 6 known LBVs in the nearby spirals, M31 and M33. The goal is to use those spectral data to probe the outburst history of those stars, as traced by the surrounding circumstellar material, and hence shed light on the long term evolution of these systems.

GO 12551: Imaging Disk-Planet interactions in the beta Pictoris Disk

The extended disk around the M dwarf, AU Mic

Planet formation occurs in circumstellar disks around young stars. Most of the gaseous content of those disks dissipates in less than 10 million years, leaving dusty debris disks that are detectable through reflect light at near-infrared and, to a lesser extent, optical wavelengths. The disk structure is affected by massive bodies (i.e. planets and asteroids), which, through dynamical interactions and resonances, can produce rings and asymmetries. The relatively nearby A-type star Beta Pictoris has one of the most prominent, and best studied, circumstellar disks. Age estimates range from 8 to 20 million years, and the disk shows evidence for significant sub-structure that has been attributed to planetary companions. Beta Pic has been imaged extensively by HST in the past, including the data contributing to the accompanying figure. The present program aims to obtain follow-up observations with the STIS coronagraph, and will match those images against the now 15-year old original data, searching for changes that might signal the presence of planetary comapnions.