This week on HST

HST Programs: June 25 - July 1, 2007

Official start of Cycle 16 on July 1

Some selected highlights

GO 10818: Very Young Globular Clusters in M31 ?

Hubble WFPC2 image of the brightest M31 globular cluster, Mayall II or G1

Galactic globular clusters are members of the halo stellar population in the Milky Way, with an approximately spherical spatial distribution. They typically have between 106 and 107 stellar members, and analysis of their colour-magnitude diagrams shows that they are old systems that formed during the first extensive period of Galactic star formation. Similar clusters have been found in many other galaxies, notably the nearest large spiral galaxy, M31. The first such clusters were identified by Hubble, in 1932, and several hundred are currently catalogued, exceeding the ~150 clusters in the Galactic system. Ground-based observations are only capable of resolving the brightest stars in the outskirts of these systems, but HST has allowed astronomers to construct colour-magnitude diagrams that extend beyond 27th magnitude, or ~1.5 magnitudes below the red clump or horizontal branch. As in the Galaxy, the overwhelming majority of these systems are clearly old. However, recent observations appeared to identify a number of star clusters with morphologies matching globular clusters, but extremely blue colours, consistent with much younger stellar populations. The aim of the present HST program is to use WFPC2 to image these candidate young globulars and verify their nature.

GO 10872: Lyman Continuum Emission in Galaxies at z=1.2

Lyman alpha image of the radio galaxy, 4C41.17

In Big Bang cosmology, the early history of the unverise is characterised by three distinct phases: the initial expansion, during which time Big Bang nucleosynthesis occurs, and the universe cools from its initial exceedingly high temperatures; recombination, which occurs at a redshift z~1,100 (or an age of ~400,000 years), when the Universe was cool enough to allow neutral hydrogen to become dominant, leading to high opacity and the cosmic microwave background; and reionisation, when energy sources reionised hydrogen, reducing the opacity of the intergalactic medium and restoring transparency. Reionisation is generally believed to have occurred at a redshift between z~6 and z~20, with the ionising sources either (or both) the first generation of stars (Population III starbursts) and/or proto-quasars. The IGM remains ionised thereafter. A key issue in developing an understanding of this process is assessing how readily starburst-generated Lyman-alpha emission escapes from galaxies, and how starbursts contribute to reionisation at intermediate redshifts. This proposal aims to quantify this matter by targeting galaxies at redshifts 1 < z < 2 for observations at ultraviolet wavelengths with the Advanced Camera for Surveys Solar Blind Channel (ACS/SBC).

GO 10926: GRB afterglows and host galaxies at very high redshifts

Artist's impression of a gamma-ray burst

Gamma ray bursts are events that tap extraordinary energies (1045 to 1047 joules) in remarkably short periods of time. Several thousands bursts have been detected over the last 30+ years, and analyses indicate that they can be divided into two classes with durations longer or shorter than 2 seconds. The short bursts appear to release more high energy radiation, so the two subsets are known as long/soft and short/hard bursts.The short/hard bursts appear to arise from coalescing binary systems (probably pairs of neutron stars or black holes), but the long/soft bursts appear to originate in the collapse of very massive stars. The latter sources are therefore almost certainly associated with star formation, so they act as signposts to active star-forming regions in the high redshift universe. This program aims to use GRBs to identify young galaxies at redshifts z > 6. Gamma ray bursts are, by their nature, unpredictable; consequently, follow-up observations must be 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 follow-up observations to track the evolution over the following ~300 days. During the next week, NICMOS follow-up observations are scheduled of a GRB that was detected on 27 September 2006.

GO 11211: An Astrometric Calibration of Population II Distance Indicators

Measuring trigonometric parallax

Trigonometric parallax measurement remains the fundamental method of determining distances to astronomical objects. The best ground-based parallax measurements can achieve accuracies of ~1 milliarcsecond, comparable with the typical accuracies achieved by the ESA Hipparcos astrometric satellite. This level of accuracy allows us to obtain reliable distances and luminosities for main sequence stars, subgiants, red giants and even a number of metal poor subdwarfs. However, with an effective distance limit of 100-150 parsecs, the sampling volume includes less than a handful of rarer, shorter-lived celestial objects. In particular, there are no RR Lyraes or Cepheids, two of the principal calibrators in the extragalactic distance scale. There is only one instrument currently available that can achieve astrometry of higher accuracy - the Fine Guidance Sensors (FGS) on HST. The present team used the FGS to measure a parallax of 3.82+/10.2 milliarseconds for RR Lyrae, the nearest star of its type. this corresponds toa distance of 262 parsecs. The present program aims to improve the calibration by extending observations to four more relatively nearby RR Lyraes (XZ Cyg, UV Oct, RZ Cep and SU Dra) and two Pop II Cepheids (Kappa Pav and VY Pyx).