The eﬀects of galaxy interactions on star formation
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This dissertation explores the effects of galaxy interactions on star formation through three separate projects. In the first two projects, we examine enhanced star formation by studying the star cluster populations of the interacting galaxies Arp 284 (NGC 7714/5) and Arp 261, using data from the Hubble Space Telescope along with ancillary data from the Spitzer Space Telescope and Galaxy Evolution Explorer to obtain broader wavelength coverage. Combined with Starburst99 evolutionary synthesis models, we estimate the ages and masses of the clusters. The mass and luminosity distributions are found to be in good agreement with other systems from the literature.
The clusters in Arp 284 are predominantly young, with ages less than 20 Myr, though observational limits make the significance of this result uncertain. Older clusters, though not numerous, have nearly the same spatial distribution within the imaged portion of NGC 7714 as young clusters. The cluster population in the bridge connecting the galaxies appears to be older, though the data in this part of the system are too limited to draw firm conclusions. The ages of the giant H II regions in NGC 7714 are generally older than those of their constituent clusters, possibly indicating that the young clusters we detect are surrounded by their dispersed predecessors. We call this the "jewels in the crown" effect.
The age distribution of the Arp 261 cluster population is more difficult to interpret because the metallicity of the galaxies is currently unknown, making the ages highly uncertain. Despite these uncertainties, it is clear that the majority of the clusters have ages ~20 Myr or less. We also find more evidence of the jewels in the crown effect in this system. The cluster age distributions in the features of this system have significant implications for its dynamical history. Radio data from the NVSS already indicates that the Taffy-like collision scenario suggested by the optical morphology may not be correct. Analysis of optical spectra, which have already been obtained, will allow us to determine the metallicity of the galaxies and improve our estimates of the cluster ages.
In the final project, we examine the suppression of star formation in the bridge between the Taffy galaxies using strong, resolved emission from warm H2. Relative to the continuum and faint PAH emission, the H2 emission in the system is the strongest in the bridge, where the purely rotational lines of H2 dominate the mid-infrared spectrum in a way very reminiscent of the group-wide shock in the strongly interacting group Stephan's Quintet. We use excitation diagrams to characterize the warm molecular gas, finding an average surface mass of ~5 Msol pc-2 and typical excitation temperatures of 150-175~K. H2 emission is also seen in the galaxy disks, although there the emission is more consistent with that seen in normal star forming galaxies. We investigate several possible heating mechanisms for the bridge gas, but favor the conversion of mechanical energy from the head-on collision via turbulence and shocks as the main heating source. Since the cooling time for the warm H2 is short, shocks must be permeating the bridge region in order to continue heating the molecular hydrogen.