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

Free access article

Issue		A&A Volume 453, Number 1, July I 2006


Page(s)		77 - 82
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20054662

A&A 453, 77-82 (2006)
DOI: 10.1051/0004-6361:20054662

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

N. J. Rodriguez-Fernandez^{1, 2}, J. Braine¹, N. Brouillet¹ and F. Combes³

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 $\mu$ m 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 (H $\alpha$ , Spitzer 24 $\mu$ m).
Results. The similarity of the [CII] emission with the H $\alpha$ and 24 $\mu$ m 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 ${\sim} 2.7 \times 10^{-26}$ erg s^-1 atom^-1 in the border of the map to ${\sim} 1.4 \times 10^{-25}$ 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 $G_0 \sim 100$ and $n \sim 3000$ for the PDR emission in the regions of star formation in the arm of M 31.

Key words: galaxies: spiral -- galaxies: ISM -- infrared: ISM -- stars: formation -- ISM: molecules -- galaxies: individual: Messier 31

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