Rest-Frame UV Versus Optical Morphologies of Galaxies Using Sérsic Profile Fitting: The Importance of Morphological K-Correction

Abstract

We show a comparison of the rest-frame UV morphologies of a sample of 162 intermediate-redshift (z _median = 1.02) galaxies with their rest-frame optical morphologies. We select our sample from the deepest near-UV image obtained with the Hubble Space Telescope (HST) using the Wide Field Planetary Camera 2 (WFPC2; F300W) as part of the parallel observations of the Hubble Ultra Deep Field campaign overlapping with the HST/ACS Great Observatories Origins Deep Survey data set. We perform single-component Sérsic fits in both WFPC2/F300W (rest-frame UV) and ACS/F850LP (rest-frame optical) bands and deduce that the Sérsic index n is estimated to be smaller in the rest-frame UV compared to the rest-frame optical, leading to an overestimation of the number of merger candidates by ~40%-100% compared to the rest-frame optical depending upon the cutoff in n employed for identifying merger candidates. This effect seems to be dominated by galaxies with low values of n(F300W) <= 0.5 that have a value of n(F850LP) ~ 1.0. We argue that these objects are probably clumpy star-forming galaxies or minor mergers, both of which are essentially contaminants, if one is interested in identifying major mergers. In addition, we also find evidence that the axis ratio b/a is lower, i.e., ellipticity (1 - b/a) is higher in rest-frame UV compared to the rest-frame optical. Moreover, we find that in the rest-frame UV, the number of high ellipticity (e >= 0.8) objects are higher by a factor of ~2.8 compared to the rest-frame optical. This indicates that the reported dominance of elongated morphologies among high-z Lyman Break Galaxies might just be a bias related to the use of rest-frame UV data sets in high-z studies.