Letter to the Editor

A composite H ii region luminosity function in Hα of unprecedented statistical weight

¹ Centre for Astrophysics, University of Central Lancashire, Preston PR1 2HE, UK
² Centre for Astrophysics Research, University of Hertfordshire, Hatfield, Herts AL10 9AB, UK e-mail: j.knapen@star.herts.ac.uk
³ Instituto de Astrofísica de Canarias, 38200 La Laguna, Spain
⁴ Consejo Superior de Investigaciones Científicas, Spain

Received: 31 July 2006
Accepted: 15 September 2006

Abstract

Context.Statistical properties of H ii region populations in disk galaxies yield important clues to the physics of massive star formation.

Aims.We present a set of H ii region catalogues and luminosity functions for a sample of 56 spiral galaxies in order to derive the most general form of their luminosity function.

Methods.H ii region luminosity functions are derived for individual galaxies which, after photometric calibration, are summed to form a total luminosity function comprising 17 797 H ii regions from 53 galaxies.

Results.The total luminosity function, above its lower limit of completeness, is clearly best fitted by a double power law with a significantly steeper slope for the high luminosity portion of the function. This change of slope has been reported in the literature for individual galaxies, and occurs at a luminosity of (L in erg s^-1) which has been termed the Strömgren luminosity. A steep fall off in the luminosity function above is also noted, and is related to an upper limit to the luminosities of underlying massive stellar clusters. Detailed data are presented for the individual sample galaxies.

Conclusions.The luminosity functions of H ii regions in spiral galaxies show a two slope power law behaviour, with a significantly steeper slope for the high luminosity branch. This can be modelled by assuming that the high luminosity regions are density bounded, though the scenario is complicated by the inhomogeneity of the ionized interstellar medium. The break, irrespective of its origin, is of potential use as a distance indicator for disc galaxies.