Re-analysis of the radio luminosity function of Galactic HII regions

TL;DR

This study re-analyzes the radio luminosity function of Galactic HII regions using updated data and methods, revealing different slopes in the luminosity distribution and a possible luminosity break, with implications for understanding Galactic ionizing sources.

Contribution

It provides a revised luminosity function of Galactic HII regions using a generalized Schmidt's estimator and recent absorption data, improving previous estimates and revealing potential luminosity breaks.

Findings

Luminosity function slopes: -2.23 in the fourth quadrant, -1.85 in the first quadrant.

Evidence of a luminosity break at 10^23.45 erg s^(-1) Hz^(-1).

HII regions contribute about 30% of the Galaxy's ionizing luminosity.

Abstract

We have re-analyzed continuum and recombination lines radio data available in the literature in order to derive the luminosity function (LF) of Galactic HII regions. The study is performed by considering the first and fourth Galactic quadrants independently. We estimate the completeness level of the sample in the fourth quadrant at 5 Jy, and the one in the first quadrant at 2 Jy. We show that the two samples (fourth or first quadrant) include, as well as giant and super-giant HII regions, a significant number of sub-giant sources. The LF is obtained, in each Galactic quadrant, with a generalized Schmidt's estimator using an effective volume derived from the observed spatial distribution of the considered HII regions. The re-analysis also takes advantage of recently published ancillary absorption data allowing to solve the distance ambiguity for several objects. A single power-law fit to the LFs retrieves a slope equal to -2.23+/-0.07 (fourth quadrant) and to -1.85+/-0.11 (first quadrant). We also find marginal evidence of a luminosity break at L_knee = 10^23.45 erg s^(-1) Hz^(-1) for the LF in the fourth quadrant. We convert radio luminosities into equivalent H_alpha and Lyman continuum luminosities to facilitate comparisons with extra-galactic studies. We obtain an average total HII regions Lyman continuum luminosity of 0.89 +/- 0.23 * 10^(53) sec^(-1), corresponding to 30% of the total ionizing luminosity of the Galaxy.