04 October 2001 CfA Colloquium

Title: HST/WFPC2 mid-UV imaging of nearby galaxies, and what they would look like in deep NGST images at z=1--15.

Speaker: Rogier Windhorst


We present a systematic imaging survey of 37 nearby galaxies with the HST WFPC2 in the mid-UV filter F300W (centered at 2930 Ang), as well as in the I-band (filter F814W or 8230 Ang). Eleven galaxies located in the HST Continuous Viewing Zone were also imaged in the F255W filter (centered at 2550 Ang). Our goal is to provide a local comparison sample to interpret the morphology of faint and distant galaxies from the HST deep fields, which are often observed in their rest-frame mid-UV.

Our sample is carefully selected for size and surface brightness. Together with 17 galaxies imaged in F300W available in the HST Archive, our survey covers a wide range of Hubble types and inclinations. Complementary data at other wavelengths are available for our galaxy sample: most objects have ground-based UBVRI images, some have JHK images, and 15 have far-UV images from the ASTRO/UIT mission. Mid-UV images (2000--3200 Ang) have been the missing ingredient thus far.

1) Early-type galaxies show a significant change in surface brightness from the mid-UV to the red, reflecting a general lack of young stellar populations. Three ellipticals become nearly point sources in the mid-UV (e.g., dominated by LINER's, Seyferts), leading us to wonder to what extent the apparently strong cosmological evolution of weak AGN in early-type galaxies is due to a ``morphological K-correction'', whereby the surrounding early-type host is no longer visible at higher redshifts.

2) Mid-type spiral galaxies and star-forming galaxies can appear as different types in the mid-UV. There is a considerable range in the scale and surface brightness of individual star-forming regions. A comparison of F300W to F814W images yields good sensitivity to dust features.

3) The late-type, irregular, peculiar and merging galaxies imaged are a heterogeneous mixture. Of these galaxies that we observed, the majority show a F300W morphology that is similar to that in F814W, but with important differences due to recognizable dust-features absorbing the F300W light, and hot stars, star-clusters, or star-formation ``ridges'' that are better visible in the mid-UV. A few late-type galaxies yield significantly different classifications in F300W and F814W.

Finally, we present simulations using these mid-UV images to show what these galaxies would look like to NGST in the redshift range z=1--15, if they existed as such in that redshift range. Other future plans with these data are outlined.

Lunch with the students will be on Friday, October 5 at 12:00 in A-101.