Sings observations of spiral galaxies - Evidence for secular evolution

Abstract

One process in the secular evolution of galaxies is the formation of pseudo-bulges. The formation of these pseudo-bulges requires the inflow of gas to smaller radii. 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 micron 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. We find that in five of the 11 galaxies there is a central excess in the 8 micron and CO emission above the inner extrapolation of an exponential disk. In particular, all four barred galaxies in the sample have strong central excesses in both 8 micron and CO emission. This correlation suggests that the excess seen in the CO profiles is, in general, not simply due to a radial increase in the CO emissivity.

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