This week on HST

HST Programs: June 11 - June 17, 2007

Some selected highlights

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 10908: Gotcha - Gamma-ray bursts at high redshifts

The life history of a gamma-ray burst

Gamma ray bursts are described colloquially as the biggest bangs since the Big Bang. Originally detected by US spy satellites in the 1960s, these short-lived bursts of high energy radiation resisted characterisation for over 30 years. It is only within the last decade that the Galactic vs. Extragalactic debate on their origins has been setled in favour of the latter. Generically, gamma ray bursts are believed to originate in the death throes of an extremely massive star, as it collapses to form either a black hole or a highly magnetised neutron star. Most occur at moderate to high redshifts; indeed, GRBs have proven a surprisingly effective way of finding galaxies at very high redshifts. The aim of this program is to identify host galaxies at z > 6. Gamma ray bursts are, by their nature, unpredictable; consequently, the observations are made in Target of Opportunity mode. The proposal triggers with the detection of an appropriate burst by the Swift satellite, and couples multicolour HST NICMOS imaging with ground-based radio and optical follow-up observations to track its evolution over the following ~300 days. During the next week, NICMOS follow-up observations are scheduled of a GRB that was detected on 22 December 2006.

GO 11080: Exploring the Scaling Laws of Star Formation

Composite BVJ image of NGC 972, one of the galaxies targeted in this program

Understanding the star formation process is crucial to our developing viable theories of galaxy formation. The present program aims to investigate this process by obtaining NICMOS Paschen-alpha images of the central regions of an all-sky sample 84 nearby spiral and S0 galaxies. The near-infrared images will be combined with CO and HI interferometric radio maps, and, where such data are available, mid-infrared imaging by Spitzer, probing the relative distributions of gas, dust and star formation in these galaxies. In addition to studying how the star formation rate (SFR) varies as a function of gas density, the NICMOS data will permit high spatial-resolution investigations of where (and whether) Schmidt-type star-formation "laws" (power-law relations between the SFR and gas density) are valid in external galaxies.

GO 11310: The high-amplification microlensing event OGLE-2007-BLG-224: a substellar lens in the Galactic disk or a low-mass stellar lens in the halo?

A microlensed souce in the Galactic bulge

Gravitational lensing is a consequence of general relativity, and the effects were originally quantified by Einstein himself in the mid-1920s. In the 1930s, Fritz Zwicky suggested that galaxies could serve as lenses, but lower mass objects can also also lens background sources. Bohdan Paczynski pointed out in the mid-1980s that this offered a means of detecting dark, compact objects that might contribute to the dark-matter halo. Paczcynski's suggestion prompted the inception of several large-scale lensing surveys, notably MACHO, OGLE, EROS and DUO. These wide-field imaging surveys target high density starfields towards the Magellanic Clouds and the Galactic Bulge, and have succeeded in identifying numerous lensing events. Statistical analysis, however, strongly suggests that the distribution of event durations and the overall number of lenses are inconsistent with a dark matter component. In particular, most, perhaps all, Bulge events are probably generated by normal stellar and brown dwarf members of the Galactic disk passing in front of a more distant Bulge star. Under those circumstances, it can become possible to directly detect both the background source and the lens if we allow sufficient time for the relative motion to separate the two images. (This has already been achieved for one LMC source, MACHO 5, where HST observations, taken 10 years after the lensing event, reveal the lens as a foreground red dwarf.) The present program aims to detect the lens for Bulge event, OGLE-2007-BLG-224/MOA-2007-BLG-163. The is the strongest lensing event detected so far, with a maximum amplification of ~4,000-fold. Based on the timescale of the light curve, the lens is either a low-mass M dwarf/subdwarf or a brown dwarf. HST images are being taken with WFPC2 this coming week as a reference. The field will be re-observed in ~6 months, when the lens should be separated from the source. If the lensing object is an M dwarf, then it will clearly visible on the second epoch images.