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RE: Interacting Galaxies
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Title: A First Look at Galaxy Flyby Interactions: Characterizing the Frequency of Flybys in a Cosmological Context
Authors: Manodeep Sinha, Kelly Holley-Bockelmann

Hierarchical structure formation theory is based on the notion that mergers drive galaxy evolution, so a considerable framework of semi-analytic models and N-body simulations has been constructed to calculate how mergers transform a growing galaxy. However, galaxy mergers are only one type of major dynamical interaction between halos -- another class of encounter, a close flyby, has been largely ignored. We analyse a 50 Mpc/h, 1024^3 collisionless cosmological simulation and find that the number of close flyby interactions is comparable to, or even surpasses, the number of mergers for halo masses \ga 10^{11}\,{h^{-1} solar masses at z \la 2. Halo flybys occur so frequently to high mass halos that they are continually perturbed, unable to reach a dynamical equilibrium. We also find tentative evidence that at high redshift, z \ga 14, flybys are as frequent as mergers. Our results suggest that close halo flybys can play an important role in the evolution of the earliest dark matter halos and their galaxies, and can still influence galaxy evolution at the present epoch.

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Overlapping Galaxy Pairs
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Title: Galaxy Zoo: A Catalogue of Overlapping Galaxy Pairs for Dust Studies
Authors: William C. Keel, Anna Manning, Benne W. Holwerda, Massimo Mezzoprete, Chris J. Lintott, Kevin Schawinski, Pamela Gay, Karen L. Masters

Analysis of galaxies with overlapping images offers a direct way to probe the distribution of dust extinction and its effects on the background light. We present a catalogue of 1990 such galaxy pairs selected from the Sloan Digital Sky Survey (SDSS) by volunteers of the Galaxy Zoo project. We highlight subsamples which are particularly useful for retrieving such properties of the dust distribution as UV extinction, the extent perpendicular to the disk plane, and extinction in the inner parts of disks. The sample spans wide ranges of morphology and surface brightness, opening up the possibility of using this technique to address systematic changes in dust extinction or distribution with galaxy type. This sample will form the basis for forthcoming work on the ranges of dust distributions in local disk galaxies, both for their astrophysical implications and as the low-redshift part of a study of the evolution of dust properties. Separate lists and figures show deep overlaps, where the inner regions of the foreground galaxy are backlit, and the relatively small number of previously-known overlapping pairs outside the SDSS DR7 sky coverage.

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Title: Tidal Tails of Interacting Galaxies
Authors: Timothy Harrison

A computer simulation of two galaxies, passing in parabolic orbits, was made to show their interaction and the tidal patterns formed. The galaxies were modelled as a point masses surrounded by 5 densely packed, concentric rings of test masses in circular motion, which represented the outer disk of the galaxy. Several features were noticeable in the simulations, including: a bridge forming between the two galaxies during the interaction, mass being lost to the perturbing galaxy and universe, and an arching counterarm emanating away from the bridge. The results suggest the process was primarily kinematic. Quantitative analysis was made on the counterarm length dependence on galaxy separation and mass ratio. In both cases linear relationships were found with arm length increasing for smaller separations and for larger perturbing masses. The effects were greatly reduced in a retrograde interaction. The images were compared to astrophysical observations and qualitative features matched.

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Title: Gemini Spectroscopic Survey of Young Star Clusters in Merging/Interacting Galaxies. IV. Stephan's Quintet
Authors: G. Trancho, I. Konstantopoulos, N. Bastian, K. Fedotov, S. Gallagher, B. Mullan, J. Charlton

We present a spectroscopic survey of 21 young massive clusters and complexes and one tidal dwarf galaxy candidate (TDG) in Stephan's Quintet, an interacting compact group of galaxies. All of the selected targets lie outside the main galaxies of the system and are associated with tidal debris. We find clusters with ages between a few and 125 Myr and confirm the ages estimated through HST photometry by Fedotov et al. (2011), as well as their modelled interaction history of the Quintet. Many of the clusters are found to be relatively long-lived, given their spectrosopically derived ages, while their high masses suggest that they will likely evolve to eventually become intergalactic clusters. One cluster, T118, is particularly interesting, given its age (~ 125 Myr), high mass (~ 2 x 10^6 solar masses) and position in the extreme outer end of the young tidal tail. This cluster appears to be quite extended (Reff ~ 12 - 15 pc) compared to clusters observed in galaxy disks (Reff ~ 3 - 4 pc), which confirms an effect we previously found in the tidal tails of NGC 3256, where clusters are similarly extended. We find that star and cluster formation can proceed at a continuous pace for at least ~ 150 Myr within the tidal debris of interacting galaxies. The spectrum of the TDG candidate is dominated by a young population (~ 7 Myr), and assuming a single age for the entire region, has a mass of at least 10^6 solar masses.

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Project for Astronomy. Images from NGC 3395 (and 3396), NGC 3294, and NGC 2859. A special bonus - features stellar cannibalism

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Galaxy mergers, which were more common in the early cosmos than they are today, are thought to be one of the main driving forces for cosmic evolution, turning on quasars - the bright halos around supermassive black holes lurking in young galaxies - sparking frenetic bouts of star births and explosive stellar deaths, or even stealthy mergers that result in new galaxies.
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Interacting galaxies are found throughout the Universe, sometimes dramatic collisions that trigger bursts of star formation, on other occasions as stealthy mergers that form new galaxies
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Astronomy textbooks typically present galaxies as staid, solitary, and majestic island worlds of glittering stars. But galaxies have a wild side. They have flirtatious close encounters that sometimes end in grand mergers and overflowing "maternity wards" of new star birth as the colliding galaxies morph into wondrous new shapes. Today, in celebration of the Hubble Space Telescope's 18th launch anniversary, 59 views of colliding galaxies constitute the largest collection of Hubble images ever released to the public. This new Hubble atlas dramatically illustrates how galaxy collisions produce a remarkable variety of intricate structures in never-before-seen detail.
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Title: The Spitzer Spirals, Bridges, and Tails Interacting Galaxy Survey: Interaction-Induced Star Formation in the Mid-Infrared
Authors: Beverly J. Smith (ETSU), C. Struck (ISU), M. Han**** (ETSU), P. N. Appleton (SSC), V. Charmandaris (UCrete), W. T. Reach (SSC)

We present Spitzer mid-infrared imaging of a sample of 35 tidally-distorted pre-merger interacting galaxy pairs selected from the Arp Atlas. We compare their global mid-infrared properties with those of normal galaxies from the SINGS Spitzer Legacy survey, and separate the disk emission from that of the tidal features. The (8.0 micron) - (24 micron), (3.6 micron) - (24 micron), and (5.8 micron) - (8.0 micron) colours of these optically-selected interacting galaxies are redder on average than those of spirals, implying enhancements to the mass-normalized star formation rates (SFRs) of a factor of ~2. Furthermore, the 24 micron emission in the Arp galaxies is more centrally concentrated than that in the spirals, suggesting that gas is being concentrated into the inner regions and fuelling central star formation. No significant differences can be discerned in the shorter wavelength Spitzer colours of the Arp galaxies compared to the spirals, thus these quantities are less sensitive to star formation enhancements. No strong trend of Spitzer colour with pair separation is visible in our sample; this may be because our sample was selected to be tidally disturbed. The tidal features contribute < 10% of the total Spitzer fluxes on average. The SFRs implied for the Arp galaxies by the Spitzer 24 micron luminosities are relatively modest, ~1 M(sun)/yr on average.

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