The ultraviolet visibility and quantitative morphology of galactic disks at low and high redshift

We have used ultraviolet (200 nm) images of the local spiral galaxies M33: M51, M81, M100, M101 to compute morphological parameters of galactic disks a this wavelength: half-light radius ri,l, surface brightness distributions, asymmetries (A) and concentrations (C-A). The visibility and the evolution of the morphological parameters are studied as a function of the redshift. The main results are: local spiral galaxies would he hardly observed and classified if projected at high redshifts (z greater than or equal to 1) unless a strong luminosity evolution is assumed. Consequently, the non-detection of large galactic disks cannot be used without caution as a constraint on the evolution of galatic disks. Spiral galaxies observed in ultraviolet appear more irregular since the contribution from the young stellar population becomes predominent. When these galaxies are put in a (log A vs. log CIA) diagram, they move to the irregular sector defined at visible wavelengths. Moreover, the log A parameter is degenerate and cannot be used for an efficient classification of morphological ultraviolet types. The analysis of high redshift galaxies cannot be carried out in a reliable way so far and a multi-wavelength approach is required if one does not want to misinterpret the data.