Study of morphology and star-formation activity of merging dwarf galaxies in the field environment

Abstract

Strong evidence has been found in the past decades that dwarf galaxies have evolved over time. There are a few unanswered questions regarding the role of dwarf galaxy merger or interactions in galaxy evolution. This thesis focuses mainly on providing a new understanding of the evolution of dwarf galaxies by merger interactions. We studied 25 merging dwarf galaxies of different morphology: 15 compact (labeled Cd-𝑋, 𝑋 = 1, ..., 15) and 10 extended (labeled Ed-𝑋, 𝑋 = 1, ..., 10) morphologies and nearly half of them are located in group and field environments, respectively. In this work, we extensively utilized photometric and spectroscopic database from the Sloan Digital Sky Survey (SDSS) to measure galaxies’ structural and star-formation properties. These compact dwarf galaxies are: LEDA3111971, LEDA3093079, PGC1653806, LEDA4207520, LEDA3461301, PGC0029004, PGC030055, PGC030133, MRK0631, UM454, MRK0750, UGC06850, LEDA2793637, LEDA2214957 and LEDA1129746. Similary, the extended dwarf galaxies are: UGC00993, NGC2604, UGC06433, NGC4765, NGC5058, MRK1481, UGCA375, UGC09588, MRK0689 and PGC057169. The redshift (𝑧) of these candidate dwarf galaxies lies in the range 0.0024 to 0.0197. We performed ellipse fitting in the SDSS 𝑔-band images and measured galaxies’ size and mean surface brightness by using both parametric and non-parametric methods. For the parametric method, we used Sérsic function modeling of a one-dimensional light profile along the major axis obtained from the ellipse fit. On the other hand, we adopted the Petrosian method to calculate the overall galaxy size, magnitude and mean surface brightness. The SDSS optical spectrum is used to calculate star formation rate (SFR) and metallicity by analyzing emission lines. The SFR is found to vary from 0.0004 M⊙yr−1 to 0.4755 M⊙yr−1 for the 15 merging compact dwarf galaxies. For the 10 extended merging dwarf galaxies, the values of SFR are found in the range 0.0022 M⊙yr−1 to 0.1913 M⊙yr−1. It increases from 0.0008 M⊙yr−1 to 0.9048 M⊙yr−1 for Cd-galaxies and 0.0035 M⊙yr−1 to 0.2647 M⊙yr−1 for Ed-galaxies when taking into account extinction correction. As expected, the SFR is found to be more (almost 2 times in Cd galaxies and 1.5 times in Ed galaxies) after the extinction correction. The SFR is found to increase with redshift. This reveals the fact that the distant merging dwarf galaxies are gas-rich and metal-rich. Additionally, a weak correlation between redshift and the extinction coefficient is noticed in the merging dwarf galaxies. The abundance of metals is found to be quite different in these two types of dwarf galaxies. From the spectroscopic calculations, the metallicity of the compact merging galaxies is found to be in the range 8.11 dex to 9.19 dex while this value is 8.32 dex to 8.90 dex in the case of extended merging dwarf galaxies. This result vii indicates that the merging compact dwarf galaxies show a wider variety of metal-rich environments than that of the extended dwarf field. As a whole, the metallicity is found to increase slowly with the redshift. The effective radius, half-light radius and the Sérsic index of dwarf galaxies are found to lie in the range (1.56 to 81.09) arcsec, (0.47 to 4.39) arcsec and (0.4 to 2.9) respectively. It is noticed that the compact dwarf is located below the main sequence defined by the local star-forming dwarf galaxies on average. The mass-metallicity relation reveals that the compact galaxy shows different behaviors than the extended merging dwarf being significantly metal-rich compared to the comparison sample. On the other hand, extended dwarf galaxies well follow the mass metallicity relation provided by normal star-forming dwarf galaxies with a bit higher scattering. It is found that more evolved merging dwarfs are expected to have low star-formation activity and higher metal contents than less evolved extended merging dwarfs. Additionally, we verified that the merging dwarfs form compact elliptical structures.