1204.0785 (M. W. L. Smith et al.)

The Herschel Exploitation of Local Galaxy Andromeda (HELGA) II: Dust and Gas in Andromeda    [PDF]

M. W. L. Smith, S. A. Eales, H. L. Gomez, J. Roman Duval, J. Fritz, R. Braun, M. Baes, J. A. D. L Blommaert, G. J. Bendo, M. Boquien, A. Boselli, D. L. Clements, A. R. Cooray, L. Cortese, I. de Looze, G. P. Ford, W. K. Gear, K. D. Gordon, G. Gentile, J. Kirk, V. Lebouteiller, S. Madden, E. Mentuch, B. O'Halloran, M. J. Page, B. Schulz, L. Spinoglio, J. Verstappen, C. D. Wilson

We present a dust analysis of Andromeda (M31), using Herschel images sampling the entire far-infrared peak (100-500 micron) observed as part of the HELGA survey. We fit a modified-blackbody model to ~4000 quasi-independant pixels and find that a variable dust-emissivity index (beta) is required to adequately fit the data. We find no significant long-wavelength excess above this model which would suggest the presence of a cold dust component. The gas-to-dust ratio has an exponential dependence with radius, increasing from ~20 in the centre to ~70 in the star-forming ring at 10kpc. The gas-to-dust gradient is consistent with the metallicity gradient if a constant fraction of metals is taken up by the dust grains. In the main 10kpc star-forming ring an average beta of ~1.9 is determined, in good agreement with values determined for the Milky Way. However, in contrast to the Milky Way, we find significant radial variations in beta, which increases from 1.9 at 10kpc to a peak value of ~2.5 at a radius of 3.1kpc and then decreases to 1.7 in the centre of the galaxy. The dust temperature is fairly constant in the 10kpc ring with values between 17-20K, but increases strongly in the bulge to values around 30K. In the inner 3.1kpc we find the dust temperature is highly correlated with the 3.6 micron flux, suggesting the old stars in the bulge are the dominant source of dust heating. At radii greater than 3.1kpc there is a weak correlation between the star formation rate and dust temperature. We were unable to detect any `dark gas', possibly due to the gas mass being largely dominated by the atomic component or line-of-sight averaging affects. We obtained an estimate of the CO X-factor by minimising the dispersion in the gas-to-dust ratio obtaining a value of (1.9+/-0.4)x10^20 cm^-2 [K kms^-1]^-1 (or expressed as alpha(CO) = 4.1+/-0.9 Msun pc^-2 [K kms^-1]^-1).

View original: http://arxiv.org/abs/1204.0785