3.5. Morphological K-corrections
A primary problem in understanding the galaxy structure-redshift relationship is constraining the effects of the morphological k-correction, whereby galaxy structure changes as a function of wavelength. The main problem is that most deep high resolution imaging is done in the optical, probing up to = 1 µm, allowing for a sampling of the rest-frame optical, < 4000 Å, only up to z ~ 1.5. At redshifts higher than this we begin to sample rest-frame ultraviolet light from galaxies. For galaxies at z ~ 3, for example, the z-band filter (F850L) (the reddest GOODS and ACS Hubble Ultra Deep Field filter) samples ~ 2500 Å, the near ultraviolet, where only young stars with ages < 100 Myrs are sampled.
This is potentially a problem because both qualitatively, and quantitatively, galaxies have very different structures in their rest-frame optical and UV light (e.g., Bohlin et al. 1991; Kuchinski et al. 2001; Windhorst et al. 2002; Papovich et al. 2003). The largest differences are found for galaxies that are composed of old and young stellar components which are not spatially mixed, such as early type spirals or Hubble classifications Sa and Sb (e.g., Windhorst et al. 2002). The one type of galaxy that looks nearly identical at UV and optical wavelengths are starbursting galaxies, or galaxies whose structures are dominated by star formation (e.g., Conselice et al. 2000c; Windhorst et al. 2002).
Similarly, when examining galaxies at different redshifts, there are different types of morphological k-corrections, depending on the redshift and galaxy type. At z < 1.5 morphological k-corrections are not an issue as we are able to sample rest-frame optical light from galaxies. At higher redshifts the NICMOS camera on HST allows us to determine the rest-frame optical structures and morphologies, although only limited field coverage exists (e.g., Dickinson et al. 2000). The result of this imaging is that galaxies that look irregular and distorted in the rest-frame UV also appear distorted in the rest-frame optical (Teplitz et al. 1998; Thompson et al. 1999), with some possible and important exceptions (e.g., Giavalisco 2002; Labbe et al. 2003; Conselice et al. 2004a, b). While these are rare systems, they do exist, and examples of morphologically selected ellipticals are found out to z > 2. These normal galaxies will likely become more common as we probe deeper in the infrared with high resolution wide field imaging.