Galaxy Structure in the Ultraviolet: The Dependence of Morphological Parameters on Rest-Frame Wavelength

TL;DR

This study examines how galaxy morphological parameters change with rest-frame wavelength, especially in the UV, revealing that galaxies appear more late-type at shorter wavelengths and providing a large dataset for future comparisons.

Contribution

It quantifies the wavelength dependence of galaxy structure using CAS parameters across a large sample of nearby galaxies in the UV, aiding high-redshift galaxy studies.

Findings

Galaxies appear more late-type at shorter wavelengths.

E/S0 galaxies show UV features resembling spiral galaxies.

Provides a large dataset of CAS parameters in UV for future research.

Abstract

Evolutionary studies that compare galaxy structure as a function of redshift are complicated by the fact that any particular galaxy's appearance depends in part on the rest-frame wavelength of the observation. This leads to the necessity for a "morphological k-correction" between different pass-bands, especially when comparing the rest-frame optical or infrared (IR) to the ultraviolet (UV). This is of particular concern for high redshift studies that are conducted in the rest-frame UV. We investigate the effects of this "band-pass shifting" out of the UV by quantifying nearby galaxy structure via "CAS parameters" (concentration, asymmetry, and clumpiness). For this study we combine pan-chromatic data from the UV through the near-IR with GALEX (Galaxy Evolution Explorer) data of 2073 nearby galaxies in the "NUV" (~230 nm) and 1127 in the "FUV" (~150 nm), providing the largest study of this kind in the mid- to far-UV. We find a relationship between the CAS parameters and observed rest-frame wavelength that make galaxies appear more late-type at shorter wavelengths, particularly in the UV. The effect is strongest for E/S0 galaxies in the far-UV, which have concentrations and asymmetries that more resemble those of spiral and peculiar/merging galaxies in the optical. This may be explained by extended disks containing recent star formation. Here we also release the CAS values of the galaxies imaged in GALEX NUV and FUV for use in comparisons with deep HST imaging and JWST in the future.