Rest-frame UV versus optical morphologies of galaxies using Sersic profile fitting: the importance of morphological K-correction

TL;DR

This study compares the UV and optical morphologies of intermediate redshift galaxies, revealing that morphological K-correction significantly affects merger identification and shape analysis, with UV images biasing towards elongated and clumpy structures.

Contribution

It demonstrates the impact of morphological K-correction on galaxy classification, highlighting the importance of multi-wavelength analysis for accurate morphological studies.

Findings

Sersic index n is smaller in UV than optical, affecting merger counts.

UV images show higher ellipticity and elongated structures compared to optical.

Rest-frame UV biases may influence high-redshift galaxy morphology interpretations.

Abstract

We show a comparison of the rest-frame UV morphologies of a sample of 162 intermediate redshift (median redshift 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 WFPC2 (F300W) as part of the parallel observations of the Hubble Ultra Deep Field campaign overlapping with the HST/ACS GOODS dataset. We perform single component Sersic fits in both WFPC2/F300W (rest-frame UV) and ACS/F850LP (rest-frame optical) bands and deduce that the Sersic 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 starforming 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 LBGs might just be a bias related to the use of rest-frame UV datasets in high-z studies.