This week on HST

HST Programs: July 16 - July 22, 2012

Selected highlights

GO 12105, 12107: A Panchromatic Hubble Andromeda Treasury

M31: the Andromeda spiral galaxy

M31, the Andromeda galaxy, is the nearest large spiral system to the Milky Way (d ~ 700 kpc), and, with the Milky Way, dominates the Local Group. The two galaxies are relatively similar, with M31 likely the larger system; thus, Andromeda provides the best opportunity for a comparative assessment of the structural properties of the Milky Way. Moreover, while M31 is (obviously) more distant, our external vantage point can provide crucial global information that complements the detailed data that we can acquire on individual members of the stellar populations of the Milky Way. With the advent on the ACS and, within the last 2 years, WFC3 on HST, it has become possible to resolve main sequence late-F and G dwarfs, permitting observations that extend to sub-solar masses in M31's halo and disk. Initially, most attention focused on the extended halo of M31 (eg the Cycle 15 program GO 10816 ), with deep imaging within a limited number of fields revealing the complex metallicity structure within that population. With the initiation of the present Multi-Cycle Treasury program, attention switches to the M31 disk. "PHAT" is conducting a multi-waveband survey of approximately one third of disk and bulge, focusing on the north-east quadrant. Observations extend over Cycles 19, 20 and 21, and will provide a thorough census of upper main-sequence stars, open clusters, associations and star forming regions, matching the stellar distribution against the dust and gas distribution.

SN 1999bw in NGC 3198, one of the transients targeted by this proposal

Searching for transient objects, particularly supernovae, novae, cataclysmic variables and flare stars, has factored prominently in astronomy since at least the cataloguing of "guest stars" by the Chinese over 2000 years ago. For most of the 19th and 20th centuries, this field was largely the pursuit of amateur astronomers, coupling visual scans of the heavens with pesonal encyclopaedic knowledge of star patterns. However, the development of large-format, highly-sensitive digital imaging devices oevr the past decade or more has opened the subject for investment by professional observatories. Several transient surveys are currently underway, notably the Berekeley Automated Supernova Survey, using the Leischner Observatory in Lafeyette, california; the Palomar Transient Factory, using the 48-inch Oschin Schmidt of Palomar Sky Survey fame, now equipped with multiple CCDs; and Pan-STARRS, a dedicated survey telescope operating on Haleakala. Further down the road lies LSST, the Large Synoptic Survey Telescope, the highest ground-based priority from the Astro2010 Decadal Survey. In the meantime, the increasingly extensive catalogues generated by the ongoing surveys have turned up a variety of unusual transients. The present program focuses on one such example, a set of luminous spernova-like transients, detected in external galaxies, where pre-outburst observations show no evidence for a progenitor. The hypothesis is that these originate in highly obscured AGB stars. This program couples WFC3-IR J and H observations with Chandra and Spitzer measurements to probe the underlying nature of these sources.

Combined optical (ESO, wide-field; HST, segment) X-ray (Chandra)images of the spiral galaxy, M83

M83 is a grand design barred spiral galaxy lying almost face-on at a distance of ~4.5 Mpc from the Milky Way. Lying in the southern constellation of Hydra, the galaxy was originally discovered by the French astronomer Nicolas Louis de LaCaille from observations at the Cape of Good Hope in 1752, before being catalogued by Messier in 1781. The galaxy lies towards the centre of one of the two sub-groups of the Centaurus A association, and has a total luminosity estimated as ~2 x 1010LSun, or comparable with that of the Milky Way. As a nearby galaxty, M83 has been well observed by both amateurs and professonal astronomers, and has been the site of six SNe over the last century (SN 1923A, 1945B, 1950B, 1957D, 1968L and 1983N). At least four of these supernovae are classed as Type II, indicating their origin as massive stars (SN45B has few observations and SN1983N was a peculiar type I) and testifying tot he extensive star formation currently under way within the galaxy. The present program aims to investigate the global star-formation characteristics through combining multi-band WFC3-UVIS observations with a deep (750 ksec) imaging observatons with the Chandra x-ray satellite. The multi-colour observations will enable fine-scale age-dating of the star forming regions, potentially enabling spatial mapping of the star-formation history. Similarly, the HST data will enable investigation of the immediate environment of the many X-ray binaries detected in the Chandra observations. Taken together, these data will provide a valuable archival reference for investigating future SNe detected within this near neighbour of the Milky Way.

A projection onto the supergalactic plane of the distribution of galaxies in the local volume (from The Atlas of the Universe )

The distribution of galaxies within the local universe provides a map of the local gravitational field, and potential insight into the evolutionary history of local structure. Reliable distances have been determined to most of the larger systems within that volume, but there are many lower-mass galaxies whose distances are still uncertain to factors of 3 to 5. Over the years, a wider range of distance estimators has been put in play to amp the distribution, with techniques ranging from detailed determinations of variable star light curves (Cepheids, RR Lyraes, Pop II Cepheids, long period variables and miras) to measuring the surface brihtness fluctuations of largely-unresolved stellar populatios. One of the most-used, and reasonably effective, methods is to determine the brightness of the tip of the first red giant branch in the colour-magnitude diagram. Stars evolving up the red giant branch undergo hydrogen shell burning; that terminates when the helium core ignites; the location of that ignition is only weakly dependent on stellar composition; and the location of the tip can be determined by constructing a luminosity function for red giant branch stars, since asymptotic giant branch (second gioant branch) stars are much less frequent, leading to a sharp drop in number density at higher luminosities. That measurement demands observations that resolve individual stars in these galaxies, and HST is the most effective means of obtaining such high angular resoluton data. The present program is a SNAP survey focused primarily on galaxies believed to lie at distances between 4 and 7 Mpc. The Advanced Camera for Surveys is being used to F660W and F814W images for those targets, permitting construction of (I, (V-I)) colour-magnitude diagrams for the older stellar population in those systems.