This week on HST

HST Programs: October 1 - October 7, 2007

Some selected highlights

GO 11101: The Relevance of Mergers for Fueling AGNs - Answers from QSO Host Galaxies

Composite optical/radio image of CenA, the elltipical merger that harbours the nearest AGN

Quasars are highly energetic sources that can achieve luminosities substantially exceeding 1012 LSun. These objects are generally believed to be powered by accretion onto a central supermassive black hole, with M > 107 MSun. Many QSOs reside within galaxies that are morphologically similar to elliptical galaxies, which are predominantly gas poor at th present epoch. This raises the issue of how one fuels the central, active black hole. One possibility is through mergers, with the QSO host assimilating smaller, gas-rich neighbours. Many nearby ellipticals are known to exhibit characteristics signatures of mergers - tidal tails, dust lanes and shells. Cen A is the classic example, which also happens to support a weak AGN and mild star formation within the dust lane. The present proposal builds on previous HST observations of five (much more distant) QSO host galaxies, where the ACS imaging revealed evidence for tidal features. This program will use WFPC2 to image a further 13 low-redshift QSO host galaxies. Observations of PG2349-014 (z=0.174) are scheduled for this week.

GO 11201: Systemic and Internal motions of the Magellanic Clouds: Third Epoch Images

The Large Magellanic Cloud (upper left) with the Small Magellanic Cloud (right) and the (foreground) Galactic globular cluster47 Tucanae

The Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) are the most massive satellites of the Milky Way galaxy. The orbital motions of these systems can be used to probe the mass distribution of Milky Way, and backtracking the orbits can shed light on how the three systems have interacted, In particular, the well known Magellanic Stream, stretching between the two Clouds, is thought to be a product either of interactions between the Clouds, or of ram-stripping of gas from the LMC on its last passage through the Plane of the Milky Way. The present program builds on observations obtained at two epochs with the now-defunct (but perhaps soon to be revived) ACS High Resolution Camera (ACS/HRC). The previous programs targeted known QSOs lying behind the Clouds; the QSOs serve as fixed reference points for absoltue astrometry of the numerous foreground LMC/SMC stars. First epoch observations were made in late 2002 (GO 9462), with the follow-up imaging in late 2004 (GO 10130). The tangential motions of the Clouds amount to only a few milliarcseconds, but the high spatial resolution and high stability of HST imaging makes such measurements possible, even with only a 2-year baseline. Surprisingly, the initial results suggest that the 3-D motions of both clouds are much higher than expected, suggesting either that the LMC/SMC/MW is either dynamically very young, or unbound. The present program will use WFPC2 to obtain third-epoch data in the same fields, providjng a crucial test of the initial results

GO 11203: A Search for Circumstellar Disks and Planetary-Mass Companions around Brown Dwarfs in Taurus

Brown dwarfs are often labelled as "failed stars" - objects that set on their way to become happy, long-lived stellar sources, but, for whatever reason, were prevented from acquiring sufficient mass to achieve central temperatures high enough to ignite hydrogen fusion. But why does this failure occur? Over the last few years, there have been suggestions that brown dwarfs are, in a sense, formed dynamically - ripp'd untimely from the cosy womb of stellar formation by gravitational interactions engendered by encounters with other young stellar systems. The main alternative model envisages brown dwarfs as not-so-happy (if we choose to anthropomorphise) accidents of lower income parental clouds, which simply don't have the necessary resources to set their offspring on the path to starhood. One means of differentiating between these two scenarios is to look for disks around young brown dwarfs: if brown dwarfs are just undersized stars, then they should have disks, but those disks should not have survived the traumatic process of cosmic ejection. The present program is using WFPC2 to survey 32 brown dwarf candidates in the Taurus star-forming region, and will use the high angular resolution of the Planetary camera to search for evidence for residual disks.

GO 11292: 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 the Planetary camera on WFPC2 to measure the thickness of the rings, and study the colours of the recently discovered fainter rings.