A comprehensive study of the color distributions of dwarf early-type galaxies

Abstract

The color of a galaxy contains important information about its evolution. The aim of this thesis is to understand the physics of dwarf early-type galaxies (dEs) more by studying their colors using high resolution photometric images to not only study the outer parts of these galaxies, but also to zoom into the very inner regions as small as a few parsec to investigate their center in nearby clusters as well. By calculating colors, we explore color-magnitude diagrams of dwarf early-type galaxies and investigate what information can be extracted from the position of a galaxy on the color-magnitude diagram. In addition, we use the color profile of each galaxy to find out how its color changes from the innermost part of a galaxy to its outer part. Besides color, we measure more photometric properties such as effective radius, magnitude, etc. For this study we used a sample of dwarf early-type galaxies in the Fornax cluster. To examine how the environment can affect the color of a dwarf early-type galaxy we compare our sample to similar ones in the Virgo and Coma clusters. The environmental differences between these clusters can guide us to find the effective processes responsible for the formation and evolution of these galaxies. Among the nearby clusters, Virgo is the most studied. After Virgo with a distance of 16.5 Mpc, Fornax, with a distance of 20 Mpc is the nearest and largest concentration of galaxies in the South (Blakeslee et al. 2009). The Fornax cluster is less massive, smaller and has a shallower potential well whereas Virgo is a very rich and active cluster with all kinds of galaxy-galaxy and galaxy-environment interactions such as ram-pressure stripping, tidal stirring, and galaxy harassment operating there. Comparing the cluster’s environment, Coma is a relaxed, dense and rich cluster at a distance of 100 Mpc (Carter et al. 2008). The density of a galaxy’s environment plays an important role in determining its star formation and quenching, and from there in determining its color. The major observations which are used in this study are the Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) surveys. The HST observation of these clusters are called the ACS Fornax Cluster Survey (ACSFCS), the ACS Virgo Cluster Survey (ACSVCS), and the HST/ACS Coma Cluster Survey. The ACSVCS and ACSFCS Surveys targeted an unbiased sample of 143 early-type galaxies in the Virgo and Fornax clusters (100 in Virgo and 43 in Fornax). The images are available in HST Mikulski archive. For the Coma cluster, only a limited part of the cluster was observed, mostly in the central regions as the ACS camera failed during the later stage of the survey. We limited our samples to a specific magnitude range which is mostly refers to as the bright dEs. In addition, to understand whether the blue star formation and dusty regions on the color maps of a dE can predict the presence of molecular gas, we selected a sample of carbon monoxide (CO) detected galaxies in the Fornax cluster and study their colors. CO is the most commonly observed tracer molecule of hydrogen (H​2) (Narayanan & Krumholz 2014). The galaxies with CO emission contain gas that can be used to form stars. To inspect their color maps and color profiles, the images of these galaxies in the Fornax Deep Survey (FDS) are used. The FDS is a new survey of the Fornax cluster which contains 573 galaxies in the optical bands using the ESO VLT Survey Telescope (VST). The imaging is done in the u, g, r and i-bands using the OmegaCAM instrument attached to the VST.