[CII] emission and star formation in the spiral arms of M 31

¹ Observatoire de Bordeaux, L3AB (UMR 5804), CNRS/Université Bordeaux 1, BP 89, 2 rue de l'Observatoire, 33270 Floirac, France e-mail: nemesio.rodriguez@obs.u-bordeaux1.fr
² Université Denis Diderot (Paris VII) & Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
³ LERMA, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France

Received: 8 December 2005
Accepted: 13 March 2006

Abstract

Context.The [Cii] 158 μm line is the most important coolant of the interstellar medium in galaxies but substantial variations are seen from object to object. The main source of the emission at a galactic scale is still poorly understood and candidates range from photodissociation regions (PDRs) to the cold neutral or diffuse warm ionized medium. Previous studies of the [Cii] emission in galaxies have a resolution of several kpc or more so the observed emission is an average of different ISM components.

Aims.The aim of this work is to study, for the first time, the [Cii] emission at the scale of a spiral arm. We want to investigate the origin of this line and its use as a tracer of star formation.

Methods. We present [Cii] and [Oi]  observations of a segment of a spiral arm of M 31 using the Infrared Space Observatory. The [Cii]  emission is compared with tracers of neutral gas (CO, Hi) and star formation (Hα, Spitzer 24 μm).

Results. The similarity of the [Cii] emission with the Hα and 24 μm images is striking when smoothed to the same resolution, whereas the correlation with the neutral gas is much weaker. The [Cii] cooling rate per H atom increases dramatically from  erg s^-1 atom^-1 in the border of the map to  erg s^-1 atom^-1 in the regions of star formation. The [Cii]/FIR_42-122 ratio is almost constant at 2%, a factor 3 higher than typically quoted. However, we do not believe that M 31 is unusual. Rather, the whole-galaxy fluxes used for the comparisons include the central regions where the [Cii]/FIR ratio is known to be lower and the resolved observations neither isolate a spiral arm nor include data as far out in the galactic disk as the observations presented here. A fit to published PDR models yields a plausible average solution of and for the PDR emission in the regions of star formation in the arm of M 31.