Tracing and isolating major mergers triggered in galaxy evolution : spatially resolved properties along the merger sequence

Abstract

Interactions and mergers have proved to be fundamental mechanisms for galaxy evolution. Galaxy-galaxy encounters could induce inflows of material as well as the enhancement of star formation. They could also lead to outflows by (stellar or active galactic nuclear) feedback. Despite the efforts to characterize and to understand the evolution of the galactic components in the course of the merging event, few studies have focused on the change of the spatially resolved properties of interacting galaxies. Spatially resolved observations are necessary for a comprehensive understanding of interacting and merging galaxies since the properties of their components could change drastically across the whole galaxy. To unveil how these components unfold as the interaction evolves, it is required to map these galactic properties in a sample of merging galaxies covering a wide range of the interaction parameter space (covering for example interaction stages stellar masses). To quantify the impact of mergers and to disentangle those processes induced by secular evolution from those related to mergers, it is also required to have a homogeneous sample of non-interacting galaxies (i.e., observed, reduced and analyzed using the same instrument, pipeline, and methodologies as for the interacting sample). The CALIFA (Calar Alto Legacy Integral Field Area) survey is the first integral field survey in the nearby universe that satisfies these requirements. This survey provides data cubes for $\sim$ 600 galaxies as a representative sample of the galactic population in the nearby Universe. It allows us to select a sample of interacting galaxies at different interaction stages and a well-match control sample, both sets of datacubes observed and reduced homogeneously. \\

In this thesis work we develop a common methodology to characterize and to analyze the spatially resolved information extracted from the CALIFA datacubes. In particular, we provide a method to measure the kinematic properties of velocity fields with no assumptions about the internal motions of galaxies. We also provide an estimate of the star formation and oxygen abundance of these two samples. Our studies are aimed at probing and to quantifying the impact of mergers on galaxy evolution. This is one of the first works aimed at statistically quantifying the impact of interactions and mergers in spatially resolved properties of galaxies. As a first step, we select a sample of 103 interacting galaxies at different stages of the interaction - from close pairs to merger remnants - and a sample of 80 non-interacting galaxies as control sample. We present our main three results as follows: \\

Using the methodology developed we determine the global major axis kinematic orientation (kinematic PA), the comparison between the receding and approaching kinematic sides (internal kinematic misalignment) and the radial deviations of the kinematic PA from a straight line ($\delta$PA$_{\mathrm{kin}}$) for the stellar and ionized gas velocity fields of the 80 non-interacting galaxies. When we compare the global kinematic with the morphological major axis orientations, we find that morpho-kinematic position angle differences are smaller than 22 degrees in 90\% of the sample for both stellar and ionised gas components. Moreover, internal kinematic misalignments are generally smaller than 16 degrees. The global kinematic orientation for the stellar and the ionized gas components present a tight relation, which are consistent with circular-flow pattern motions (90\% of the sample has differences smaller than 16 degrees). This relation also holds, generally in barred galaxies across the bar and galaxy disc scales. Our findings suggest that even in the presence of strong bars, both the stellar and the gaseous components tend to globally follow the gravitational potential of the disk.\\

Results for non-interacting galaxies can be used to assess the degree of external distortions in interacting galaxies. Comparing these results with the sample of 103 interacting galaxies, we find that around half of the interacting objects show morpho-kinematic PA misalignments that cannot be found in the control sample. In particular, we observe those misalignments mainly in galaxies with evident signatures of interaction. On the other hand, the level of alignment between the approaching and receding sides for both samples is similar, with most of the galaxies displaying internal misalignments comparable with those observed in the control sample. Radial deviations of the kinematic PA orientation from a straight line in the stellar component measured by $\delta$PA$_{\mathrm{kin}}$ are large for both samples. However, for a large fraction of interacting galaxies the ionised gas $\delta$PA$_{\mathrm{kin}}$ is larger than the typical values derived from isolated galaxies (48\%), indicating that this parameter is a good indicator to trace the impact of interaction and mergers in the internal motions of galaxies. By comparing the stellar and ionized gas kinematic PA, we find that 42\% (28/66) of the interacting galaxies have misalignments larger than 16$^{\circ}$, compared to 10\% from the control sample. Our results show the impact of interactions on the motion of stellar and ionized gas as well as the wide variety of their spatially resolved kinematic distributions. This study also provides a local universe benchmark for kinematic studies in merging galaxies at high redshift. \\

Finally, from the CALIFA datacubes we extract the H$\alpha$ equivalent width and the emission line flux maps, for several ions, of these two samples. H$\alpha$ equivalent width is a proxy for the specific star formation rate (sSFR, which provides an indication of the current star-formation activity compared to previous activity) while the 2D emission line flux distributions provide information about the gas-phase metallicity particularly the oxygen abundance. We confirm the moderate enhancement ($\times$ 2-3 times) of sSFR for interacting galaxies in central regions as reported by previous studies; however, this parameter is comparable to the control sample when observed over extended regions. We find that control and interacting star forming galaxies share similar oxygen abundances in their central regions, when normalized to their stellar masses. Oxygen abundances of these interacting galaxies seem to decrease with respect to the control objects at large aperture sizes in units of effective radius. Although the enhancement in central star formation and possible lower metallicities for interacting galaxies have been attributed to tidally induced inflows, our results suggest that other processes such as stellar feedback can contribute to the metal enrichment in interacting galaxies.