Statistical properties of circumnuclear HII regions in nearby galaxies

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Abstract
We analyze the statistical properties of the circumnuclear HII regions of a sample of 52 nearby galaxies ($v < 1000km/s) from archival HST/NICMOS H-band and Pa alpha (1.87micron) observations at unprecedented spatial resolutions of between 1 and 30pc. We find HII regions in the central regions of most galaxies, and more than a hundred in 8 galaxies. In contrast to disk HII regions, the physical properties (luminosity and size) of individual circumnuclear HII regions do not vary strongly with the morphological type of the host galaxy, nor does the number of circumnuclear HII regions per unit area. The H alpha luminosity within the central kpc, as derived from HII region emission, is significantly enhanced in early-type (S0/a-Sb) galaxies. We find evidence that bars increase the circumnuclear star formation, presumably by funneling gas from the disk towards the nucleus. Barred galaxies exhibit enhanced luminosities of the brightest HII region, the central kpc H alpha luminosities (an effect mostly due to the early type galaxies in our sample), and the star formation rates per unit stellar mass (which could also be understood as the integral equivalent widths of Pa alpha) over the central kpc with respect to non-barred galaxies. We fit the luminosity functions and diameter distributions of the circumnuclear HII regions in 8 galaxies where we can catalog enough HII regions to do so in a meaningful way. We use power laws and find that the fitted slopes of the HII region LF are exactly in the previously found ranges, and even confirm a trend with steeper slopes in galaxies of earlier morphological type. This implies that the physical processes giving rise to enhanced star formation in the circumnuclear regions of galaxies must be similar to those in disks.

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