Speaker Name:
Benne Holwerda
Speaker Affiliation:
Astronomy Department, UCT
Talk Subject:
Dust Extinction Effects in Spiral Galaxies
Date:
10/15/2008 - 13:00
Venue:
Postgraduate Seminar Room, Otto Beit Building
The distribution of dust in spiral disks affects ever measurement of the spiral's properties (e.g., estimates of star-formation and history, and distance scales). Dust extinction as a function of galactic radius, arm or interarm region, and Hubble type are useful both to correct measurements of disks and to trace the cold, dusty ISM. The distribution of dust extinction in spirals could very well evolve over the lifetime of spiral disks.
The current project is to probe the distribution and variance of disk opacity out to a redshift of 1 using the same types of objects: spiral galaxies overlapping a more distant galaxy, preferably elliptical. In those cases, one can measure the extinction in the spiral in the overlap region. The dominant assumption is symmetry of both galaxies.
In order to accurately determine the opacity of spiral disks as a function of radius, redshift and Hubble type, a large sample of occulting pairs is needed. Until recently, only few were known and these had already been extensively studied. We obtained a first larger sample from the SDSS spectra, selecting elliptical galaxy spectra with lower-redshift emission lines. This netted 86 bona-fide pairs to a redshift of z = 0.2. The radial dependence of the disk opacity can be directly compared to that of local galaxies.
Recently, the team found a pair serendipitously imaged with the Hubble Space Telescope. The data-quality and favorable geometry of this pair has led us to use it as a test case for the analysis of occulting pairs, resulting in an extinction map and the effective extinction law through a spiral disk. Now that we know of many more pairs, I will briefly discuss future work using occulting pairs and what we hope to learn from these.