Study o the origin of Hi asymmetries of highly isolated galaxy CIG 96 by means of deep optical and Hi observations

Abstract

Throughout the years, many studies have addressed the numerous questions raised when trying to understand how the galaxies assemble and evolve. This topic constitutes one of the most studied fields of modern astronomy and, as we expand our knowledge of the different elements of a galaxy and their evolutionary roles, new problems arise. Some of them may be solved with deeper and higher resolution observations that let us detect the fine print of the processes shown in the different evolutionary stages. The Analysis of the interstellar Medium of Isolated GAlaxies project (AMIGA) seeks to provide a better understanding on the formation and evolution of galaxies focusing on those that lie in relatively uninhabited environments in comparison with denser ones, including the field galaxies. AMIGA has been performing a systematic multi−wavelength study of a selected and statistically significant sample of extremely isolated galaxies based on a refined version of the Catalog of Isolated Galaxies (CIG). In absence of companions, the CIG galaxies constitute an ideal laboratory to study secular evolution and. Since the foundation, the AMIGA project has been investigating the internal and environmental processes of these galaxies, as well as the implication in the theories of galaxy formation and evolution. This project constitutes the framework for the present dissertation. Asymmetries in the atomic hydrogen (HI) of galaxies are often caused by the interaction with close companions. The AMIGA project has demonstrated that isolated galaxies show the lowest levels of asymmetry in their HI integrated profiles compared to other samples, even field galaxies. However, some galaxies present high asymmetry ratios whose origins are poorly understood. Aiming to investigate the source of the asymmetries, we selected a sample that consists of 184 isolated galaxies from the CIG catalog. Out of them, the highly isolated galaxy CIG 96 (NGC864) shows a 16% asymmetry level in its HI integrated profile, representing a model case and main target of this study. In order to try to reveal the causes of the asymmetry, we performed deep optical observations of CIG 96 with the CAHA1.23m, CAHA2.2m and VST (with the OmegaCAM wide field camera) telescopes. We reach surface brightness limits of μCAHA2.2m =27.5 mag arcsec−2 (Cousins R) and μV ST =28.7 mag arcsec−2 (SDSS r) that show the pseudoring of the galaxy and its star forming regions in detail. Additionally, a wavelet filtering of the HI data cube from our deep observations with VLA/EVLA telescope allowed us to reach a column density of NHI =8.9×1018 cm−2 (5σ, 28”×28” beam), the lowest of any isolated galaxy to date. We confirm that the HI extends farther than 4×r25 in all directions and also detect two previously undetected gaseous structures (∼106M⊙) in the outskirts of the galaxy. The SDSS g−r colour index image from CAHA1.23m shows extremely blue colours in certain regions of the pseudoring (suggesting regions with star formation) where NHI >8.5×1020 cm−2, whereas the rest show red colours. Galactic cirrus contaminate some regions of the field, setting an unavoidable detection limit at 28.5 mag arcsec−2 (SDSS r). At the current surface brightness (SB) and HI column density (NHI) limit levels, we detect no stellar link within 1◦×1◦ neither gaseous link within 40’×40’ between CIG 96 and any companion. The isolation criteria rule out interactions with other similar sized galaxies for at least ∼2.7 Gyr. Using existing stellar evolution models, the age of the pseudoring is estimated to be 1 Gyr or older. Undetected companions previously accreted and cold gas accretion remain as the main hypothesis to explain its optical pseudoring and HI features. The deep optical and HI observations have let us unveil previously undetected features of CIG 96, showing they are a key tool for the study of the HI distribution asymmetry and their causes in isolated galaxies. Hence, all the deep optical data of CIG 96 and other targets presented in this dissertation guarantee the continuation of the project, they may potentially open new research paths within the AMIGA project and, in the end, help us to expand our knowledge on galaxy formation and evolution.