Astronomy and Astrophysics – Astronomy
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21810602a&link_type=abstract
American Astronomical Society, AAS Meeting #218, #106.02; Bulletin of the American Astronomical Society, Vol. 43, 2011
Astronomy and Astrophysics
Astronomy
Scientific paper
We present a new catalog of HII regions in M31. The galaxy is observed as part of the Local Group Galaxies survey. We used HIIphot, a code for automated photometry of HII regions, to identify HII regions and measure their fluxes and sizes. HIIphot fits boundaries with irregular morphologies and can avoid overlapping in crowded regions. A S/N=10 detection level was used to exclude diffuse gas fluctuations and star residuals after continuum subtraction. Our catalog of 4368 objects is complete to a luminosity of L(Hα)=10e34 erg/s. This is five times fainter than the only previous CCD-based study which contained 981 objects in the NE half of M31. We have detected very faint regions created by individual OB stars and resolved most of them in crowded regions. They are responsible for 80% of the total Hα emission. We determined the HII luminosity function (LF) by fitting a power law to luminosities larger than 10e36.7 and determined a slope of 2.52 ±0.07.
A second peak at L(Hα)=10e35 suggests a star burst between 15 and 20 million years ago which is consistent with UV star formation history studies. No significant difference was observed between in-arm and inter-arm LFs, but the inter-arm regions are less populated (37% of total detected regions) and constitute only 12% of the total luminosity of L(Hα)= 5.6e40 erg/s(after extinction correction). A star formation rate of 0.42 M_sun/yr was estimated from the Hα total luminosity which is consistent with the determination from the Spitzer 8μm data. We found a poor spatial correlation between the HII regions and young clusters. We conclude that these clusters are older than the lifetime of the HII regions. which is in agreement with the UV studies of star forming history in M31 which suggest a decrease in star formation rate in the recent past.
Azimlu Mohaddesseh
Barmby Pauline
Marciniak Ryan
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