SESSIONS IX and X: SURVEYS OF PRE-MERGER PAIRS

Beverly Smith

East Tennessee State University

We have been conducting a GALEX UV, Spitzer IR, and ground-based optical imaging study of about 3 dozen pre-merger interacting galaxies selected from the Arp Atlas (the `Spirals, Bridges, and Tails' Survey). A parallel effort in numerical modeling of these encounters is also underway. The observed UV extent of the tidal features in these systems is often larger than in the optical or IR, and we have found a number of new candidate tidal dwarf galaxies in this sample. Other interesting morphologies such as accretion tails and `beads on a string' are also visible in this sample.

Douglas Swartz

NASA/MSFC

The results of a census of ultraluminous X-ray sources (ULXs) detected in Chandra observations of Arp interacting galaxy pairs is described. The census represents up to a three-fold increase in the number of the brightest known ULX candidates. This will determine to high statistical significance whether or not there exists an upper limit to the luminosity of ULXs or if the power law slope of their luminosity function at lower luminosities continues unabated to higher values. This will have profound implications for the physics underlying the ULX phenomena, in particular for the mass distribution of accreting black holes.

Betsy Barton

University of California, Irvine

Using cosmological dark matter simulations as a guide, my collaborators and I have developed a technique to make fair comparisons between the star-forming properties of galaxies in isolated pairs and the properties of their progenitors, isolated galaxies. We use the giant Sloan Digital Sky Survey to make precise measurement of the frequency and strength of triggered star formation in pre-merging galaxy interactions. The triggering of star formation happens for a large fraction --- if not all --- of galaxies on the blue sequence that experience close passes.

Sarah Higdon

Georgia Southern University

Zwicky proposed that dwarf galaxies might form out of self-gravitating clumps within the tidal tails of interacting galaxies, and indeed, concentrations of gas and stars with dwarf galaxy sizes, HI masses and optical luminosities are sometimes found in tidal features. Computer simulations show that these `tidal dwarf galaxies' (TDGs) can persist as independent objects for several Gyrs. Optically-faint but gas-rich TDGs represent new star forming systems with little pre-existing stellar populations, and as such are useful templates of star formation in the early universe. Even when tidal streams do not produce independent galaxies, they may still be important in enriching the intergalactic medium (IGM) with metals and dust. Dwarf galaxies are the most common galaxy type in the current epoch and TDGs may make up 1/3 of the dwarf population in compact groups. The Star Formation and the ISM in Nearby Tidal Streams (SAINTS) project has identified 16 tidal star forming regions in 12 interacting pairs for spectral observations using the Spitzer Space Telescope. The star forming environments are characterized using mid-infrared diagnostic tools and the results are compared to samples of sprial and dwarf galaxies.

Jessica Werk

University of Michigan / Columbia University

Isolated HII regions in the gaseous outskirts of galaxies provide a unique opportunity to probe the star-formation history, evolution, and origin of the gas. Following a systematic search of a sample of gas-rich galaxies, and a separate sample of interacting, disturbed galaxies (HI Rogues), we have catalogued and spectroscopically confirmed over 50 HII regions located far beyond (r > 2 X r25) the main optical components of galaxies. Here, we will discuss their frequency, stellar populations, and gaseous environments.