The Radial Distribution of the Interstellar Medium in Disk Galaxies: Evidence for Secular Evolution

Regan, Michael W.; Thornley, Michele D.; Vogel, Stuart N.; Sheth, Kartik; Draine, Bruce T.; Hollenbach, David J.; Meyer, Martin; Dale, Daniel A.; Engelbracht, Charles W.; Kennicutt, Robert C.; Armus, Lee; Buckalew, Brent; Calzetti, Daniela; Gordon, Karl D.; Helou, George; Leitherer, Claus; Malhotra, Sangeeta; Murphy, Eric; Rieke, George H.; Rieke, Marcia J.; & Smith, J. D.
2006, The Astrophysical Journal, 652, 1112


One possible way for spiral galaxies to internally evolve would be for gas to flow to the center and form stars in a central disk (pseudo-bulge). If the inflow rate is faster than the rate of star formation, a central concentration of gas will form. In this paper we present radial profiles of stellar and 8 μm emission from polycyclic aromatic hydrocarbons (PAHs) for 11 spiral galaxies to investigate whether the interstellar medium in these galaxies contains a central concentration above that expected from the exponential disk. In general, we find that the two-dimensional CO and PAH emission morphologies are similar, and that they exhibit similar radial profiles. We find that in 6 of the 11 galaxies there is a central excess in the 8 μm and CO emission above the inward extrapolation of an exponential disk. In particular, all four barred galaxies in the sample have strong central excesses in both 8 μm and CO emission. These correlations suggest that the excess seen in the CO profiles is, in general, not simply due to a radial increase in the CO emissivity. In the inner disk, the ratio of the stellar to the 8 μm radial surface brightness is similar for 9 of the 11 galaxies, suggesting a physical connection between the average stellar surface brightness and the average gas surface brightness at a given radius. We also find that the ratio of the CO to 8 μm PAH surface brightness is consistent over the sample, implying that the 8 μm PAH surface brightness can be used as an approximate tracer of the interstellar medium.

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