1.2. UV Morphology of Nearby Galaxies as Benchmark for High Redshift Classifications
Observations of high redshift galaxies in the optical spectral region detect light that was emitted as UV luminosity in these galaxies' rest-frames. They therefore primarily trace high surface brightness ("SB") regions populated by high densities of young ( 1 Gyr) hot stellar populations. The dominance of young stellar complexes in producing UV light from star-forming galaxies was recognized from early satellite UV photometric observations (e.g., Code & Welch 1982; Israel et al. 1986), IUE far-UV spectra (e.g., Kinney et al. 1993, and references therein), as well as imaging of a few key objects from sounding rockets (e.g., Carruthers et al. 1978; Stecher et al. 1982). These results were confirmed when UV images became available from the FOCA balloon observations (Millard et al. 1992) and the Astro/UIT flights. The latter imaged ~ 100 nearby galaxies in the far-UV (1500Å) and ~ 40 in the mid-UV (2500Å) at a resolution of ~ 3 FWHM (Bohlin et al. 1991; Hill et al. 1992; Kuchinski et al. 2000; Marcum et al. 2001). Morphological K-corrections derived from these UIT images (and from earlier sounding rocket data) were first investigated by Bohlin et al. (1991) and Giavalisco et al. (1996). This, as well as the FOCA data, showed that galaxy morphology changes quite dramatically below 3600-4000Å, where the hot (young) stellar population - located mainly in spiral arms and H II-regions - dominates the spectral energy distribution (SED) and where bulges essentially disappear (see also Burgarella et al. 2001; Kuchinski et al. 2001). Galaxies therefore often appear to be of later Hubble type the further one looks into the rest-frame UV. Qualitatively, this is easy to understand: in the optical/near-IR, we see the accumulated luminous phases of long-lived (> 1 Gyr) stars, which emit most of their energy at longer wavelengths, whereas the mid-UV samples the star-formation rate (SFR) averaged over the past Gyr or less. The mid-UV includes the longest wavelengths where young stars can dominate the integrated galaxy light and traces primarily presently active star forming regions, or those regions where star-formation has only recently shut down.
Far-UV images of nearby early-type galaxies are not always appropriate for high redshift comparisons; they may be affected by the "UV upturn", a spectral feature that is likely caused by hot, low-mass, old stars (O'Connell et al. 1992; O'Connell 1999), and that only appears at relatively recent epochs (z 0.3; Burstein et al. 1988; Greggio & Renzini 1990; Brown et al. 2000). However, middle-aged stellar systems can be very dim in the UV compared to either older or younger objects, as in the case in, e.g., M32. In galaxies of all types the far-mid-UV emission can also be modulated by dust.