The galactic distribution and luminosity function of ultracompact HII regions.

Astronomy and Astrophysics – Astrophysics

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Stars: Early-Type, Formation, Mass Function, Hii Regions, Galaxy: Stellar Contents, Structure

Scientific paper

The distribution of ultracompact HII regions in the IRAS Point Source Catalog in both galactic longitude and flux are studied in this paper, to derive the radial surface density profile in the galactic disk and the luminosity function under the assumptions of cylindrical symmetry and a position-independent luminosity function. An advantage of this approach is that the surface density profile is derived independently of kinematical information, allowing us to probe the inner regions of the galactic disk where kinematical distances become unreliable. An interesting result of our analysis is the discovery of a star forming ring about 2 kpc from the galactic center, possibly associated with the Inner Lindblad Resonance as observed in external galaxies with strong bars. On the other hand, comparison with the mass function in the solar neighbourhood allows us to estimate the average lifetime of ultracompact HII regions, which appears to be less than 10^5^ years, contrary to previous estimates. The luminosity function also suggests that this lifetime is not sensitive to the mass of the exciting star. The supernova rate in the galactic disk as derived from our results is found to be compatible with other current estimates.

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