We present a new catalog of H II regions in M31. The full disk of the galaxy (∼24 kpc from the galaxy center) is covered in a 2.2 deg2 mosaic of 10 fields observed with the Mosaic Camera on the Mayall 4 m telescope as part of the Local Group Galaxies survey. We used HIIphot, a code for automated photometry of H II regions, to identify the regions and measure their fluxes and sizes. A 10σ detection level was used to exclude diffuse gas fluctuations and star residuals after continuum subtraction. That selection limit may result in missing some faint H II regions, but our catalog of 3691 H II regions is still complete to a luminosity of LHα= 1034 erg/s. This is five times fainter than the only previous CCD-based study which contained 967 objects in the NE half of M31. We determined the Hα luminosity function (LF) by fitting a power law to luminosities larger than LHα = 1036.7 and determined a slope of 2.52±0.07. The in-arm and inter-arm LFs peak at different luminosities but they have similar bright-end slopes. The inter-arm regions are less populated (40% of total detected regions) and constitute only 14% of the total luminosity of LHα= 5.6x1040 erg/s (after extinction correction and considering 65% contribution from diffused ionized gas). A star formation rate of 0.44 M☉/yr was estimated from the Hα total luminosity; this value is consistent with the determination from the Spitzer 8 µm image. We removed all known and potential planetary nebulae, yet we found a double-peaked LF. The inter-arm older population suggests a starburst between 15 and 20 million years ago. This result is in agreement with UV studies of the star formation history in M31 which found a star formation rate decrease in the recent past. We found a fair spatial correlation between the H II regions and stellar clusters in selected star-forming regions. Most of the matched regions lie within the arm regions.
galaxies: individual: M31 - H II regions
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<Available at CDS (J/AJ/142/139): table2.dat table3.dat>