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 ЮМm 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 ЮМm 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 ЮМm 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 ЮМm 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 ЮМm PAH surface brightness is consistent over the sample, implying that the 8 ЮМm PAH surface brightness can be used as an approximate tracer of the interstellar medium.

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