This week on HST

HST Programs: February 12 - February 18, 2007

Some selected highlights

GO 10871: Observations of the Galilean Satellites in Support of the New Horizons Flyby

HST WFPC2 images of the four Galilean satellites

Io, Europa, Ganymede and Callisto, the four "Medicean Planets", have played a key role in developing our understanding of the Solar System since their discovery by Galileo on January 7, 1610. Their varying configurations from night to night led to his conclusion that they were "sfour, erratic sidereal bodies performing their revolutions around Jupiter", providing clear evidence for change in the immutable heavens. These four moons have been the subject of extensive scrutiny over the succeeding four centuries, from ground-based telescopes, from space and from interplanetary probes, notably the Voyagers and Galileo. In February, the New Horizons spacecraft will fly by Jupiter, using the strong gravitational field to propel it on its way to Pluto. During the fly-by, New Horizons will carry out a number of experiments ( see this link ). HST is participating in a campaign of observations associated with the fly-by, targeting Io, Europa and Ganymede. With the demise of the ACS/HRC, WFPC2 will be used to obtain optical and near-UV images, probing, in particular, surface features and extended pluems associated with volcanic activity on Io. These observations will be coupled with ACS/SBC UV images that will map auroral activity on these satellites during the fly-by.

GO 10918: Reducing Systematic Errors on the Hubble Constant: Metallicity Calibration of the Cepheid PL Relation

M101, the spiral galaxy targetted in this program

Cepheids are the original distance indicator, and remain the primary calibrator for the extragalactic distance scale. Most investigations tie the zeropoint for the latter scale to the Large Magellanic Cloud, which has a large population of Cepheid variables that provide a well-populated period-luminosity-colour (PLC) relation. However, the Cepheid period-Luminosity relation has long been suspected of being dependent on metallicity. The LMC Cepheids have a lower average metallicity than both the Galactic Cepheids that define the local calibration and the Cepheids in more distant galaxies used to derive H0 - setting the stage for potential systematic errors in the distance scale. The present program aims to address this issue through HST ACS and WFPC2 imaging of Cepheids in several fields in the relatively nearby spiral galaxy, M101. The M101 disk has a metallicity gradient, and the targeted fields have been chosen to sample a range of metallicities. Comparison of the perid-luminosity relations from the separate fields will allow direct measurement of any systematic metallicity-based variations. The present set of observations will use WFPC2 to target Cepheids in the outermost field n M101.

GO 11080: Exploring the Scaling Laws of Star Formation

A 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.