1.3. Other Ways of Addressing the Morphological K-correction

A number of authors have explored the effects of band-pass shifting using multicolor optical images to extrapolate to the rest-frame UV on a pixel-by-pixel basis. This has been done both using ground-based data (e.g., Hibbard & Vacca, 1997; Brinchmann et al. 1998), FOCA mid-UV images (Burgarella et al. 2001), UIT far-UV images (Kuchinski et al. 2000), and HST images of galaxies at moderate redshifts (Abraham et al. 1999; Bouwens, Broadhurst, & Silk 1998). The peculiarities in shape and size distributions measured in deep HST surveys are found to considerably exceed the effects of band-pass shifting. While this is a robust result, these methods may not capture the full range of rest-frame UV galaxy SEDs, and are less suitable for making detailed comparisons with the local universe.

For instance, there is considerable scatter in the mid-UV for galaxies of a given optical color. Donas, Milliard & Laget (1995) find a 3 magnitude range in (UV - B) colors at a given (B - R) color in a faint space-based galaxy sample, where a 2000Å filter was used to measure the UV flux. This UV/optical decoupling is confirmed by the spectroscopy of Treyer et al. (1998) and implies that the true evolutionary history of galaxies can be very different from that inferred from optical data alone, and that mid-UV observations are necessary for a better understanding.

Recent observations of the HDF-North with HST/NICMOS show that some galaxies out to z ~ 2.5 appear `regular' and fit into the Hubble sequence in the near-IR, but look more patchy and irregular at shorter wavelengths (Dickinson et al. 2001). Galaxies that look disturbed in the optical, on the other hand, in general also look very similar in the rest-frame UV. These distorted high-z galaxies also tend to be high SB objects in the UV, forming stars at a high rate. Nearby galaxies whose structure is dominated by star-formation also have similar morphologies in the rest-frame optical and mid-UV (e.g., Conselice et al. 2000), which suggests that some nearby star-bursts galaxies may be analogs to these `peculiar' high-z objects.