Edward J. Shaya

Far-infrared photometry of Titan and Iapetus

Titan was observed in four broad passbands between 35 and 150 microns. The brightness temperature in this interval is roughly constant at 76 plus or minus 3 K. Integrating Titans spectrum from 5 to 150 microns yields an effective temperature of 86 plus or minus 3 K. Both the bright and dark hemispheres of Iapetus were observed in one broadband filter with a flux-weighted mean wavelength approximately equal to 66 microns. The brightness temperatures for these two sides of Iapetus are 96 plus or minus 9 K and 114 plus or minus 10 K, respectively. The bright-side Bond albedo is calculated to be 0.61(+0.16 -0.22).

Optimizing the tip of the red giant branch distance estimator

Until very recently, our knowledge of the local peculiar velocity field has been severely hampered by the lack of reliable distance measurements. HST has dramatically changed this situation, allowing astronomers to obtain accurate distances to more than 150 nearby galaxies. This number could easily reach 400 if enough observing time would be dedicated to snapshot observation of the objects in the catalog of Karachentsev et al. (2004). Such a dense grid of objects correctly placed in their 3D position would provide key information on the amplitude of peculiar motions, the radial domain of bound groups, the clustering and morphological segrega- tion properties of galaxies, and the incidence of extreme dwarfs galaxies. The key instrument to measure distances with HST is the Tip of the Red Giant Branch technique. The full exploitation of this powerful distance estimator requires a deeper understanding of the possible sources of errors and biases, such as the absolute calibration of the I-band magnitude of the tip and its dependency on age and metallicity of the underlying population, the possible contamination by AGB stars, the breakdown of the methodology in sparsely populated colour-magnitude diagrams and when the tip is close to the photometric limit.

The Influence of Environment on Galaxy Formation

A schema of galaxy formation is developed in which the environmental influence of large-scale structure, plays a dominant role. This schema was motivated by the observation that the fraction of E and So galaxies is much higher in clusters than in low-density regions, and by an inference that those spirals that are found in clusters probably have fallen in relatively recently from the low-density regions. It is proposed that the tidal field of the Local Supercluster acts to determine the morphology of galaxies through two complementary mechanisms. In the first place, the supercluster can apply torques to protogalaxies. Galaxies which collapsed while expanding away from the central cluster decoupled from the external tidal field and conserved the angular momentum that they acquired before collapse. Galaxies which formed in the cluster while the cluster collapsed continued to feel the tidal field. In the latter case, the spin of outer collapsing layers can be halted and reversed, and tends to cancel the spin of inner layers. The result is a reduction of the total angular momentum content of the galaxy. In addition, the supercluster tidal field can regulate accretion of fresh material onto the galaxies since the field creates a Roche limit about galaxies and material beyond this limit is lost. Any material that has not collapsed onto a galaxy by the time. the galaxy falls into a cluster will be tidally stripped.