Abstract

Galaxy interactions and mergers play an important role in the hierarchical formation and evolution of galaxies. Studies in the nearby universe show a higher star formation rate (SFR) and active galactic nuclei (AGN) fraction in interacting and merging galaxies than in their isolated counterparts, indicating that such interactions are important contributors to star formation and black hole growth. A large fraction of massive galaxies is thought to be affected by galaxy mergers at high redshifts because the galaxy merger rate increases with redshift. We use deep observations and cosmological simulations to study the role of galaxy mergers and interactions in enhancing SFR and AGN activity in galaxies at $0.5 < z < 3.0$, covering the peak of cosmic star formation and AGN activity ($z\sim2$). For the observational study, we use deep CANDELS and COSMOS observations to compile the largest known sample of major spectroscopic galaxy pairs (2381 with $V < 5000$ km/s). We also select visually identified samples of 61 galaxy pairs of non-blended interactions, 100 galaxy pairs of blended interactions, and 66 galaxy mergers. We compare the AGN fraction and star formation rates (SFRs) of the galaxy pairs and mergers with their corresponding stellar mass-, redshift-, and environment-matched control sample of isolated galaxies to estimate the interaction-induced AGN fraction enhancement and the SFR enhancement, respectively. We identify X-ray and IR AGN in our samples. We do not see a significant AGN fraction enhancement (ratio of AGN fraction in the pair sample to control sample) at any separations in any luminosity or redshift bins for both X-ray and IR selected AGN samples. In the closest projected separation bin ($<25 $\thinspace kpc, $\Delta V <1000$\thinspace km s$^{-1}$), we find enhancement of factors of 0.94$^{+0.21}_{-0.16}$ and 1.00$^{+0.58}_{-0.31}$ for X-ray and IR-selected AGN, respectively. We estimate SFRs of galaxies using the SED fitting tool MAGPHYS or SED3FIT for galaxies in our sample. We estimate the highest SFR enhancement (ratio of average SFR of the pair sample to average SFR of control sample) of 1.21$^{+0.08}_{-0.09}$ ($\sim2.3\sigma$) in the closest projected separation bin ($<25$\thinspace kpc, $\Delta V <1000$\thinspace km s$^{-1}$) in our galaxy pair sample compared to the stellar mass-, redshift-, and environment-matched control sample of isolated galaxies. We find a $2.2\sigma$ higher SFR enhancement of $\sim1.5$ at low $z$ ($z<1$) as compared to high $z$ ($z>1$) in the smallest projected separation bin. We estimate an enhancement of 1.86$^{+0.29}_{-0.18}$ ($\sim3\sigma$) in our visually identified merger sample. While we see a slightly high enhancement for these advanced stage mergers, it is still a factor of $\sim2$ lower than seen in the local universe. Both our pair sample and visually identified merger sample suggest a reduction in interaction-induced SFR enhancement levels with increasing redshift. We conduct a similar study on the effect of galaxy interactions on the SFR and AGN activity of galaxy pairs using the IllustrisTNG hydrodynamical cosmological simulations (specifically the TNG100-1 simulation run). We estimate SFR enhancements, AGN accretion rate enhancements, and AGN fraction enhancements in massive major galaxy pairs compared to a stellar mass-, redshift-, and environment-matched control sample of isolated galaxies. For a fair comparison between the simulated enhancement results with the observed results, we take into account three different selection effects and study how each of them affect the results. These selection effects are: using projected space (2D compared to 3D) to select galaxy pairs and control samples, applying an observational cut based on the SFR of galaxies before selecting the pairs and control samples, and using an X-ray luminosity cut based on X-ray observations in the COSMOS field for taking depth of X-ray observations into account. We see an overall trend of increased SFR and AGN accretion rate enhancements with decreasing separations in all our redshift bins, showing that galaxy interactions enhance SFR and AGN activity of galaxies. At the closest separations ($<25$\thinspace kpc), the SFR enhancement in the simulated pair sample (1.49 $\pm$ 0.04) is $\sim3 \sigma$ higher than the observed SFR enhancement (1.21$^{+0.07}_{-0.09}$) in spectroscopic galaxy pairs. For the complete X-ray AGN sample from the IllustrisTNG simulation, We do not see a significant AGN fraction enhancement at any separation at any redshifts, consistent with our observation results. We only see significant AGN fraction enhancement for high luminosity AGN in simulations (L$_X>10^{43}$\thinspace erg\thinspace s$^{-1}$). These high L$_x$ AGN fraction enhancement levels at different redshifts show a trend of decreasing enhancement levels with increasing redshift. We conclude that the efficiency of galaxy interactions in triggering AGN activity is weaker at high redshifts compared to the local universe. Our results suggest there is a slight trend of decreased SFR enhancement and high luminosity AGN fraction enhancement levels with increased redshift. This effect is relatively stronger at low redshifts and significantly dilutes or disappears at high redshift.

Publication Date

7-22-2021

Document Type

Dissertation

Student Type

Graduate

Degree Name

Astrophysical Sciences and Technology (Ph.D.)

Department, Program, or Center

School of Physics and Astronomy (COS)

Advisor

Jeyhan Kartaltepe

Advisor/Committee Member

Andrew Robinson

Campus

RIT – Main Campus

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