Galaxy evolution in extreme environments

Sammanfattning: Interactions play a crucial role in determining how galaxies evolve in terms of their morphologies and star formation histories, since the majority of the known galaxies in the nearby Universe are found to exist in groups. Compact groups of galaxies are of particular interest as they contain a small number of galaxies in dense configurations and host repeated interactions between them, enabling us to capture signatures of these interactions (e.g., tidal tails, shocks) for probing the underlying physics driving their evolution. While interactions themselves trigger intense star formation through gas inflows, compressive tides and turbulence, repeated interactions can spare no sufficient time for the galaxies to resume their normal star formation mode after their initial starburst and before the subsequent interactions. Such situations can dramatically affect the star formation regime of galaxies and give rise to three possible scenarios for subsequent interactions: a saturation effect in the star formation as the galaxies achieved high star formation rates already; a quenching effect where the initial interaction has stripped off the gas making it unavailable for forming further stars; a boost in starburst where the subsequent interactions are stronger than initial ones stimulating the burst itself. The aim of this project is to study the effect of repeated interactions on star formation in a compact galaxy group and Stephan’s Quintet being one of the widely studied specimens in this context because of the remarkable tidal, hydrodynamic features (such as shockwaves resulting from galaxy collisions with the intragroup debris) and starburst episodes that it exhibits. However, reproducing all these features with a single hypothesized idea is challenging and the complexity in Quintet modeling necessitated modeling a simpler compact group of 3 galaxy members as the main focus is to understand the physics of star formation in repeated galaxy encounters. Stephan’s Quintet and the general compact group are simulated using an adaptive mesh refinement hydrodynamic code RAMSES, the initial conditions for which are obtained using the MAGI code. To reproduce the features of the Quintet before exploring its physics, the model is simulated at a low resolution of about 1 kpc. Through the exploration of orbital parameter space, the proposed model resulted in some of the tidal features and the spatial locations of the galaxies to reasonably agree with observations, while the gas features such as the HI distribution outside the galaxies, separated stellar and gaseous tidal tails and the shock wave, could not be reproduced yet. Pertaining to the general group of 3 members, the simulations are performed at a resolution of about 12 pc with star formation and stellar feedback. The model considers an interaction of the main spiral galaxy with another spiral, shortly followed by another interaction of the main galaxy with an elliptical. A boost in starburst is observed by about 80-120 fold increase with respect to the star formation rate (SFR) prior to the interaction, in the central re- gions driven by gas inflows. Furthermore, the effect of interaction with the elliptical galaxy is not only causing an early onset of the starburst but also an enhancement in the SFR. Different orbital configurations that are less extreme than the initial are also considered. However, they do not yield quenching or saturation in the SFR either, but exhibit trends in the physics of star formation similar to that of the initial configuration. Furthermore, it is noticed that these starburst galaxies remain in an excited state (characterized by short depletion times) despite the SFR resuming pre-interaction values. I find that high velocity dispersion due to gas inflows and stellar feed- back is the likely cause for this long-lasting excitation. In a general setting, the models discussed in this document are imperfect imitations of real galaxies in the sense that they do not incorporate all the physical phenomena on multi-scales, for example, Active Galactic Nuclei (AGN), an important feedback form in massive galaxies is not included. Therefore, high resolution mod- els complemented with multi-wavelength and deep optical surveys revealing the signatures of repeated interactions in compact galaxy groups are needed to better constrain their interaction histories and thereby understand the physical processes governing their evolutions.