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Paper: Star Formation and the Interstellar Medium in Nearby Tidal Streams (SAINTS)
Volume: 423, Galaxy Wars: Star Formation and Stellar Populations in Interacting Galaxies
Page: 271
Authors: Higdon, S. J. U.; Higdon, J. L.; Smith, B. J.; Hancock, M.; Struck, C.
Abstract: We compare Spitzer Infrared Spectrograph observations of SQ-A and SQ-B in Stephan’s Quintet, Ambartzumian’s knot in Arp 105, Arp 242-N3, Arp 87-N1, a bridge star forming region, NGC 5291 N, and NGC 5291 S. The PAHs tend to be mainly neutral grains with a typical size of 50–100 carbon atoms. The interstellar radiation field is harder than in typical starburst galaxies, being similar to that found in dwarf galaxies. The neon line ratios are consistent with a recent episode of star formation ≤ 5 million years ago. We detect emission from ∼106 Msun of warm H2 in SQ-A and Arp 87N1 and ∼105 Msun in SQ-B. These results are similar to those derived for the tidal dwarf galaxies (TDGs) NGC 5291 N and NGC 5291 S and are consistent with emission from photodissociation regions. Using our 8 μm images of 14 interacting systems we identify 62 tidal star forming knots (TSFKs). The estimated stellar masses range from super star cluster (104–106 Msun) to TDG (∼109 Msun) sizes. The median stellar mass is 108 Msun. The stellar mass, with some scatter, scales with the 8 μm luminosity and tends to be an order of magnitude smaller than the KISS sample of star forming dwarfs. An exception to this are the TSFKs in Arp 242 which have stellar masses similar to the KISS dwarfs. The TSFKs have “notched” 3.6 – 8 μm spectral energy distributions (SEDs) characteristic of star forming regions. The TSFKs form two distinct clumps in a mid-infrared color diagram. There are 38 red-TSFKs with [4.5] – [8.0] > 3 and [3.6] – [4.5] < 0.4. This populations has significantly enhanced non-stellar emission, most likely due to PAHs and/or hot dust, relative to normal spirals and the KISS sample of dwarfs. The second group of 21 sources has 1.2 < [4.5] – [8.0] < 3 and [3.6] – [4.5] < 0.4. This population overlaps with the colors of star forming dwarf and spiral galaxies. The redder [4.5] – [8.0] population tends to have the sources with a rising 8-24 μm SED while the blue population tends to contain the sources with a descending SED. The rising SED is typical of spiral and starburst galaxies with a dominant 40–60 K dust component and the declining SED probably indicates a dominant hot dust component.
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