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The Larson-Tinsley Effect in the UV: Interacting vs. `Normal' Spiral Galaxies

The Larson-Tinsley Effect in the UV: Interacting vs. `Normal' Spiral Galaxies. Beverly Smith (East Tennessee State University) Curt Struck (Iowa State University). Enhanced Star Formation in Interacting Galaxies. -H-alpha Equivalent Widths:

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The Larson-Tinsley Effect in the UV: Interacting vs. `Normal' Spiral Galaxies

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  1. The Larson-Tinsley Effect in the UV: Interacting vs. `Normal' Spiral Galaxies Beverly Smith (East Tennessee State University) Curt Struck (Iowa State University)

  2. Enhanced Star Formation in Interacting Galaxies -H-alpha Equivalent Widths: Kennicutt et al. (1987); Barton et al. (2000) -H-alpha Luminosity per Surface Area: Bushouse (1987) -L(FIR)/L(B): Kennicutt et al. (1987); Bushouse et al. (1988) -Spitzer L(24 microns)/L(3.6 microns): Smith et al. (2007) and many other references These studies imply a factor of two enhancement in the mass-normalized star formation rate of interacting galaxies compared to non-interacting galaxies NOTE: This is an AVERAGE enhancement. There is a large range, with some interacting galaxies being quite quiescent, and others having stronger star formation.

  3. Larson and Tinsley (1978) Compared Optical UBV Colors of Arp Atlas (1966) Galaxies with those of galaxies in the Hubble Atlas. Found a larger scatter in the UBV colors of the interacting galaxies. Concluded this was due to bursts of star formation

  4. What about the UV? -GALEX since 2003: NUV (1750-2800 Angstroms) and FUV (1350-1705 Angstroms) -Traces somewhat older and lower mass stars than H-alpha: Measure of star formation over a longer timescale than H-alpha. -Good tracer of star formation in regions with low SFR, such as outer disks. -BUT....the UV is strongly affected by extinction from dust

  5. The Spirals, Bridges, and Tails (SB&T) Sample -42 pre-merger binary pairs from Arp Atlas with strong tidal features -Have Spitzer IR and GALEX UV Images (Smith et al. 2007, 2010) -Most have SDSS optical ugriz images -Compared the Spitzer [3.6 micron] – [24 micron] colors to those of a subset of the SINGS sample, that had been culled to remove the galaxies with nearby massive companions -The SB&T sample showed an enhancement in L(24)/L(3.6), implying an increase in mass-normalized star formation rate

  6. GALEX IMAGES OF SB&T GALAXIES Arp 202: Arp Atlas Arp 202: GALEX (Smith et al. 2010); Hancock et al. (2009)

  7. Arp 105 GALEX NUV Arp 105 (`the Guitar') SDSS Optical

  8. Tidal features are blue in NUV-g compared to their parent disks (Smith et al. 2010) Filled regions are lower limits

  9. A Comparison Sample of Spiral Galaxies -Only a subset of the culled SINGS sample used earlier has good UV/optical colors; sample too small -Built our own sample -Started with the `GALEX Ultraviolet Atlas of Nearby Galaxies' (Gil de Paz et al. 2007) -Used NASA Extragalactic Database (NED) to eliminate galaxies with massive companions -Eliminated E/S0 and Irr from both comparisons and SB&T -Selected galaxies between 6 and 143 Mpc to match SB&T -Cross-correlated with the SDSS -Final sample: 121 `normal' spiral galaxies -The two sets have similar large-scale environments -The g luminosity distributions for the two samples similar

  10. Solid line: mean values of g-r for the comparison sample, in 0.2 magnitude bins of NUV – g. Dotted line: same for the SB&T sample. Note: larger scatter for the SB&T galaxy From Smith and Struck (2010, AJ, submitted)

  11. g - r vs. u - g: Larger scatter in the SB&T sample vs. the comparison sample From Smith and Struck (2010, AJ, submitted)

  12. Mean NUV-g for the SB&T spiral disks is 0.26 magnitudes redder than for the comparison spirals, and the rms deviation from the mean is 90% bigger.

  13. NUV-g vs. g-r color-color plots, with Starburst99 (Leitherer et al. 1999) population synthesis models. -Instantaneous bursts plus an older population. -Solid lines: fixed burst strength, increasing age. Dotted lines: fixed age, increasing burst strength. Arrow: Av = 1 reddening

  14. CONCLUSIONS: There is BOTH more star formation AND more extinction in interacting galaxies, on average -Interactions can drive gas into the galaxy centers -Earlier studies showed that star formation is more centrally concentrated in interacting galaxies vs. normal spirals (Bushouse 1987; Smith et al. 2007) -L(FIR)/L(H-alpha) higher for interacting than normal galaxies (Bushouse 1987; Bushouse et al. 1988)

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