NEARBY INTERACTING GALAXY PAIRS: DYNAMICS AND IONIZED GAS PROPERTIES

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Date
2016-09-07
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Publisher
Johns Hopkins University
Abstract
Galaxy merging is an important process in galaxy formation and evolution throughout the cosmic time. Mergers are expected to play a significant role in galaxy mass assembly and transformation of galaxy morphology. In this work, we study inter- acting pairs of galaxies in a relatively early stage of the merging process, after they first pass by each other but before they coalesce into a single galaxy. At this stage, interacting disk galaxies display peculiar morphologies, often induced by the strong gravitational tidal field experienced during the first passage. These peculiar features can be utilized to constrain the encounter parameters of the merging galaxies, such as time since first passage, and pericentric separation (Toomre & Toomre, 1972; Hi- bbard & Mihos, 1995; Barnes & Hibbard, 2009). Moreover, in the early stages of galaxy mergers the individual galaxies are still clearly separated making it possible to investigate the chain of physical processes that are caused by the interaction. This includes starbursts or active galactic nuclei (AGNs) triggered by the infall of gas into the cores (Mihos & Hernquist, 1996; Hopkins & Quataert, 2011), and the shocks that are produced by feedback from starbursts and AGNs (Cox et al., 2006b; Narayanan et al., 2008; Rich et al., 2015), or by collision of gaseous clouds in the interstellar medium (ISM) of the two disks as they pass through each other (Struck, 1997). We develop a novel automated method for modeling the dynamics of equal mass galaxy mergers that puts meaningful constraints on the system’s encounter parameters. In order to understand the systematics of the measured encounter parameters, we test our method against an independent set of galaxy merger simulations with known initial conditions. For a controllable subset of these tests, our automated method recovers parameters such as merger stage and initial disk orientations within 3σ of the correct value. We explore the effects of using different kinematic tracers on the inferred encounter parameters by applying our method to HI and Hα velocity maps of a well-studied galaxy merger system in the nearby Universe, NGC 4676 a.k.a the Mice galaxies. We show for the first time that constraints on the encounter parameters derived from HI and Hα kinematics are consistent suggesting that Hα velocity maps can also be used for dynamical modeling. In total we observe a sample of 22 galaxy mergers in this work. Nineteen of them have morphological indicators similar to the Mice galaxies (separate cores and strong tidal features), and the other three are recently coalesced systems where cores are united, but outskirts are still disturbed. We use SparsePak integral field unit (IFU) (Bershady et al., 2004) on the WIYN telescope at Kitt Peak National Observatory (KPNO) to observe the Hα emission over the entire visible regions of these galaxies, including faint tidal tails. Relatively high spectral resolution of our data allows us to investigate multiple kinematic components in the emission lines. We separate Hα emission of photo-ionized HII regions from shocked gas, measure the velocity maps of HII regions, and discuss the fraction and spatial distribution of shocks and their power source. We apply our dynamical modeling method on equal mass systems in our sample and obtain the first ever constraints on the encounter parameters of one of them. We find a trend between shocked gas fraction and the projected separation between the galaxies in pairs, similar to Rich et al. (2015). In our sample of interacting pairs, for the first time, we also find a trend between shocked gas fraction and the light ratio (mass ratio). These trends suggest that in most of the observed systems the gravitational tidal impulse at the time of the first passage is the dominant origin of shocks. Also for the first time, we investigate the correlation between shocked gas fraction and encounter parameters from dynamical modeling. We find that time until coalescence and pericentric separation are both strongly anti-correlated with the amount of shocks. However, larger statistical sample is required for understanding the physical details and timing of shock production during galaxy mergers. Clean separation of shocks from star forming regions also improves the accuracy of star formation rate measurements in merging galaxies. Large ongoing and upcoming IFU galaxy surveys such as MaNGA (Bundy et al., 2015) will provide spatially resolved spectra of large numbers of galaxies including many galaxy mergers. Tools and techniques developed for this thesis project are required steps for better utilization of these valuable datasets.
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Keywords
Galaxy Formation and Evolution, Galaxy Interaction
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