Dust-corrected surface photometry of M 31 from Spitzer far-infrared observations

¹ Tartu Observatory, 61602 Tõravere, Estonia e-mail: elmo@aai.ee
² Institute of Physics, Tartu University, Tähe 4, 51010 Tartu, Estonia e-mail: [atamm;peeter.tenjes]@ut.ee

Received: 26 March 2009
Accepted: 17 November 2009

Abstract

Aims. We create a model for recovering the intrinsic, absorption-corrected surface brightness distribution of a galaxy and apply the model to the nearby galaxy M 31.

Methods. We constructed a galactic model as a superposition of axially symmetric stellar components and a dust disc to analyse the intrinsic absorption effects. Dust column density is assumed to be proportional to the far-infrared flux of the galaxy. Along each line of sight, the observed far-infrared spectral energy distribution was approximated with modified black body functions corresponding to dust components with different temperatures, thereby allowing us to determine the temperatures and relative column densities of the dust components. We applied the model to the nearby galaxy M 31 using the Spitzer Space Telescope far-infrared observations for mapping dust distribution and temperature. A warm and a cold dust component were distinguished.

Results. The temperature of the warm dust in M 31 varies between 56 and 60 K and is highest in the spiral arms, while the temperature of the cold component is mostly 15-19 K and rises up to about 25 K at the centre of the galaxy. The intensity-weighted mean temperature of the dust decreases from  K in the centre to  K at  kpc and outwards. The scalelength of the dust disc is . We also calculated the intrinsic U, B, V, R, I, and L surface brightness distributions and the spatial luminosity distribution. The intrinsic dust extinction in the V-colour rises from at the centre to at  kpc and decreases smoothly thereafter. The calculated total extinction-corrected luminosity of M 31 is  , corresponding to an absolute luminosity  ±0.04 mag. Of the total B-luminosity, 20% (0.24 mag) is obscured from us by the dust inside M 31. The intrinsic shape of the bulge is slightly prolate in our best-fit model.