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À propos de : Relating dust, gas, and the rate of star formation in M 31        

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  • Relating dust, gas, and the rate of star formation in M 31
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  • Aims. We investigate the relationships between dust and gas, and study the star formation law in M 31. Methods. We have derived distributions of dust temperature and dust opacity across M 31 at 45 $\arcsec$ resolution using the Spitzer data. With the opacity map and a standard dust model we de-reddened the H α emission yielding the first H α map of M 31 corrected for extinction. We compared the emissions from dust, H α, HI, and H 2 by means of radial distributions, pixel-to-pixel correlations, and wavelet cross-correlations. We calculated the star formation rate and star formation efficiency from the de-reddened H α emission. Results. The dust temperature steeply decreases from 30 K near the center to 15 K at large radii. The mean dust optical depth at the H α wavelength along the line of sight is about 0.7. The radial decrease in the dust-to-gas ratio is similar to that of the oxygen abundance. Extinction is nearly linearly correlated with the total gas surface density within limited radial intervals. On scales <2 kpc, cold dust emission is best correlated with that of neutral gas, and warm dust emission with that of ionized gas. The H α emission is slightly better correlated with emission at 70  μm than at 24  μm. The star formation rate in M 31 is low. In the area 6 kpc <R< 17 kpc, the total SFR is $\simeq$0.3 ${M}_{odot}$ yr -1. A linear relationship exists between surface densities of SFR and H 2. The Kennicutt-Schmidt law between SFR and total gas has a power-law index of 1.30 ± 0.05 in the radial range of R = 7-11 kpc increasing by about 0.3 for R = 11-13 kpc. Conclusions. The better 70  μm-H α than 24  μm-H α correlation plus an excess in the 24  μm/70  μm intensity ratio indicates that other sources than dust grains, e.g. those of stellar origin, contribute to the 24  μm emission. The lack of H 2 in the central region could be related to the lack of HI and the low opacity/high temperature of the dust. Since neither SFR nor SFE is well correlated with the surface density of H 2 or total gas, other factors than gas density must play an important role in the formation of massive stars in M 31. The molecular depletion time scale of 1.1 Gyr indicates that M 31 is about three times less efficient in forming young massive stars than M 33.
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  • aa13593-09
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  • © ESO, 2010
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  • ESO
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