This week on HST

HST Programs: October 9 - October 15, 2006

Some selected highlights

GO 10793: A Survey for Supernovae in Massive High-Redshift Clusters

Images of supernova 2005cs in M51; this supernova happens to be a (slightly unusual) Type II

Supernovae are the most spectacular form of stellar obituary. In recent years, these celestial explosions have acquired even more significance through their use as distance indicators in mapping out the `dark energy' acceleration term of cosmic expansion. However, while there are well-established models for the two main types of supernovae - runaway fusion on the surface of a white dwarf in a binary system (type Ia), and implosion of the core of a massive star (type II) - substantive questions remain regarding the expected supernova rates, and the potential variation in those rates as a function of look-back time. This proposal aims to address that question through observations of galaxy clusters with redshifts in the range 0.5 < z < 0.9. Candidate supernovae are identified by ACS imaging in the F814W or F775W, with follow-up Keck spectroscopy used to determine the redshift of the parent galaxy.

GO 10816: The Formation History of Andromeda's Extended Metal-Poor Halo

HST ACS image of the outer bulge/inner halo resgions of M31

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 on HST, it has become possible to resolve main sequence late-F and G dwarfs in M31. This permits observations that probe stars with luminosities below the turnoff of the Galactic halo population, and substantial effort has been devoted to this program in recent cycles. The initial results suggested that M31's halo might be very different from our own - specifically, the data indicated the presence of a significant number of stars with both intermediate age (6-8 Gyrs) and intermediate metallicity ([Fe/H] ~ -0.5), rather than the >10 Gyrs and -5 < [Fe/H] < -1.5 values derived for the Galactic halo. However, all previous HST observations were at radial distances of less than 30 kpc from the M31 centre, and it has become clear that those data were contaminated by the M31 Bulge (or "spheroid" component). Recent observations indicate that a more traditional halo component dominates at larger radii. The present program aims to confirm that and derive reliable age, metallicity and density estimates, using the F606W and F814W filters on ACS to image several fields lying at radial distances from 22 to 35 kpc.

GO 10863: Magnifying the High-z Universe with the Bullet Cluster 1E0657-56

Combined optical and x-ray image of the "Bullet" galaxy cluster: the red outlines the distribution of hot, baryonic gas (from X-rays); the blue maps the underlying (dark matter) mass distribution.

The "Bullet Cluster", 1E0657-56, lying at a redshift z=0.296, actually comprises two galaxy clusters that appear to be undergoing a collision, perhaps leading eventually to a merger. Individual galaxies in each cluster are little affected by this process, passing through without interactions. Hot gas in the intracluster medium, however, collides dissipationally, and consequently its density distribution is offset from the underlying galaxies. The ongoing collision also likely accounts for the fact that 1E06757-56 has one of the hottest (T ~ 70 x 106 K) and most luminous gaseous components. Very recently, Clowe and collaborators ( see this link ) have taken advantage of this unusual event to provide the strongest evidence to date for the existence of dark matter on large scales. As the accompanying figure shows, they have used weak lensing to demonstrate that the underlying mass distribution remains centred on the galaxy distribution. This indicates, first, that the hot gas does not dominate the mass distribution in these clusters (as required by modified gravity theories), and, second that the dark matter component is non-dissipational. The present proposal aims to take advantage of another property of the Bullet cluster - its overall mass distribution. Models that this cluster should serve as an highly potent lensing system, allowing detection of galaxies at redshifts up to z=7. Moreover, the magnification introduced by the lensing is sufficient that those images should be within the range of state-of-the-art ground-based spectrographs, allowing us to probe the details of star formation at these early epochs. This proposal aims to use ACS to search for the distorted arcs and arclets that characterise lensed images.

GO 10886: The Sloan Lens ACS Survey: Towards 100 New Strong Lenses

ACS images of galaxy-galaxy Einstein ring lenses from the Sloan survey

Gravitational lensing is a consequence the theory of general relativity. Its importance as an astrophysical tool first became apparent with the realisation (in 1979) that the quasar pair Q0957+561 actually comprised two lensed images of the same background quasar. In the succeeding years, lensing has been used primarily to probe the mass distribution of galaxy clusters, using theoretical models to analyse the arcs and arclets that are produced by strong lensing of background galaxies, and the large-scale mass distribution, through analysis of weak lensing effects on galaxy morphologies. Gravitational lensing can also be used to investigate the mass distribution of individual galaxies. Until recently, the most common background sources were quasars. Galaxy-galaxy lenses, however, offer a distinct advantage, since the background source is extended, and therefore imposes a stronger constraints on the mass distribution of the lensing galaxy than a point-source QSO. The Sloan sky survey provides a powerful tool for identifying candidate galaxy-galaxy lenses; this program is using HST-ACS imaging to verify the nature of those candidates, and provide the angular resolution necessary to model the mass distribution.