This week on HST

HST Programs: August 14 - August 20, 2006

Some selected highlights

GO 10632: Searching for galaxies at z>6.5 in the Hubble Ultra Deep Field

The Hubble Ultra-Deep Field (the UDF) is the deepest image so far obtained of the Cosmos. The original program comprised 412 orbits directed at a single ACS field within the Chandra Deep Field-South (CDF-S) GOODS area. Those 412 orbits were divided among four filters - F435W (56 orbits), F606W (56 orbits), F775W (150 orbits), and F850LP (150 orbits). A further 144 orbits were devoted to a 3x3 grid of F110W (J) and F160W (H) NICMOS images covering the same field.(GO program 9803). NICMOS was also employed in parallel with the ACS observations to obtain deep J- and H-band images of adjacent fields. The present proposal devotes 204 orbits to new observations of the prime and parallel fields, but with HST inverted; thus, the program will obtain ACS imaging (in F606W, F775W and F750LP) of the parallel fields, and deeper NICM (F110W, F160W) data for part of the original ACS UDF. The primary goal of the program is identify candidate very high redshift (6.5 < z < 8.5) sources, using photometric redshifts derived by combining the optical and near-infrared photometric catalogues.

GO 10634: White dwarf cooling physics: calibrating the clock

HST ACS images of white dwarfs in the globular cluster, M4

White dwarfs are the evolutionary end point for most stars with masses less than ~7 MSun. These compact degenerate objects lack any internal heat source, and therefore gradually cool from their initial temperatures of ~100,000-200,000K. As they cool, the luminosity decreases from Mbol ~ 2-3 (for the immediate post-PN object) to Mbol ~ 17 (for 10-12 Gyr-old Galactic halo white dwarfs). The rate of cooling can be predicted using sophisticated models of white dwarf interiors. These models show that the rates are mass dependent, but the overwhelming majority of field white dwarfs have masses in the range 0.6-0.7 MSun, reflecting the steep slope to the IMF above ~1 MSun (high mass stars are rare, so high mass remnants, like Sirius B, are also rare). Consequently, we can use the luminosity function of white dwarfs as a cosmic chronometer: specifically, the location of the low-luminosity cut-off can provide an estimate of the age of a given stellar population, and this technique has already been used to probe the age of the disk. Clearly, this is a useful tool, and it is important to verify the timescales predicted by the cooling tracks. The most effective means of doing so is to search for white dwarfs in clusters, where the main-sequence turn-off provides an independent age estimate. Deep HST observations have already been obtained for two globulars (NGC 6397 and M4). The current program is extending coverage to younger ages, using the ACS/WFC to survey five open clusters with ages between 0.6 to 2.2 Gyrs.

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 preence 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 10870: The Ring Plane Crossings of Uranus in 2007

Images of Uranus spanning 2000 to 2004, showing the rotation of the ring plane

Like the other Solar System gas giants, Uranus not only has an extensive number of satellite moons, but also possesses a ring system. Unlike the other giant planets, Uranus has a polar obliquity of 98o degrees, so its equator is close to perpendicular to the ecliptic plane. Consequently, from our vantage point on Earth, we view the north and south poles alternately during Uranus' 84-year circling of the Sun. Midway between the polar apparitions, of course, we view Uranus' equatorial plane - and see the ring system edge-on. The next ring plane crossing will occur in May and August 2007. At this juncture, the denser and more prominent rings will almost disappear from view, providing an opportunity to search for small satellite "shepherd" moons. These moons are expected to be present, acting as gravitational delineators, defining the radial size of the individual rings. Besides searching for the shepherds, the current HST program will use the High Resolution Camera on ACS to measure the thickness of the rings, and study the colours of the recently discovered fainter rings.

GO 10882: Emission Line Snapshots of 3CR Radio Galaxies

Extended structure in the radio galaxy, 3C 75

The Third Cambridge Catalogue of Radio Sources was compiled originally in 1959 from observations at 159 MHz made by the Cambridge Radio Astronomy Group using the Cambridge interferometer (Edge et al, Mem RAS, 68, 37). The addition of observations at 178 MHz led to the revised catalogue, the 3CR (Bennett, 1962, Mem RAS, 68, 137). This catalogue was the main basis for the Fanaroff-Riley classification scheme (FR-Is are double-lobed sources with small separation between the peaks; FR-IIs have a separation that exceeds half the largest size of the source), and includes many of the brightest known radio galaxies. As such, the 3CR sources have remained extremely important observational targets for understanding the nature and structure these highly energetic sources. Over the past several cycles, many 3CR galaxies have been observed at a variety of wavelengths by HST. Those observations have revealed new optical jets, dust lanes, face-on disks with optical jets, besides point-like nuclei whose properties support FR-I/BL Lac unified schemes. The present program is using the ramp filters on the ACS/WFC to obtain images covering the H-alpha and O[III]5007 emission lines, probing the star formation characteristics of these radio galaxies.

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Past weeks:

• Cycle 14 observations (from March 13 2006 to Jul 1 2006)
• Cycle 15 observations (from July 1 2006)

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page by Neill Reid, updated 25/7/2006