The Mid-Infrared Properties of Starburst Galaxies from Spitzer-IRS Spectroscopy

December 2006 • 2006ApJ...653.1129B

Authors • Brandl, B. R. • Bernard-Salas, J. • Spoon, H. W. W. • Devost, D. • Sloan, G. C. • Guilles, S. • Wu, Y. • Houck, J. R. • Weedman, D. W. • Armus, L. • Appleton, P. N. • Soifer, B. T. • Charmandaris, V. • Hao, L. • Higdon, J. A. • Marshall, S. J. • Herter, T. L.

Abstract • We present 5-38 μm mid-infrared spectra at a spectral resolution of R~65-130 of a large sample of 22 starburst nuclei taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. The spectra show a vast range of starburst SEDs. The silicate absorption ranges from essentially no absorption to heavily obscured systems with an optical depth of τ_9.8μm~5. The spectral slopes can be used to discriminate between starburst and AGN powered sources. The monochromatic continuum fluxes at 15 and 30 μm enable a remarkably accurate estimate of the total infrared luminosity of the starburst. We find that the PAH equivalent width is independent of the total starburst luminosity L_IR as both continuum and PAH feature scale proportionally. However, the luminosity of the 6.2 μm feature scales with L_IR and can be used to approximate the total infrared luminosity of the starburst. Although our starburst sample covers about a factor of 10 difference in the [Ne III]/[Ne II] ratio, we found no systematic correlation between the radiation field hardness and the PAH equivalent width or the 7.7 μm/11.3 μm PAH ratio. These results are based on spatially integrated diagnostics over an entire starburst region, and local variations may be ``averaged out.'' It is presumably due to this effect that unresolved starburst nuclei with significantly different global properties appear spectrally as rather similar members of one class of objects.

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IPAC Authors
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Phil Appleton

Senior Scientist

Lee Armus

Senior Scientist