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Issue A&A
Volume 476, Number 3, December IV 2007
Page(s) 1161 - 1178
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20078179



A&A 476, 1161-1178 (2007)
DOI: 10.1051/0004-6361:20078179

Star formation in M 33: Spitzer photometry of discrete sources

S. Verley1, L. K. Hunt2, E. Corbelli1, and C. Giovanardi1

1  Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
    e-mail: [simon;edvige;giova]@arcetri.astro.it
2  INAF - Istituto di Radioastronomia-Sezione Firenze, Largo E. Fermi 5, 50125 Firenze, Italy
    e-mail: hunt@arcetri.astro.it

(Received 28 June 2007 / Accepted 12 September 2007)

Abstract
Aims.Combining the relative vicinity of the Local Group spiral galaxy M 33 with the Spitzer images, we investigate the properties of infrared (IR) emission sites and assess the reliability of the IR emission as a star formation tracer.
Methods.The mid- and far-IR emission of M 33 was obtained from IRAC and MIPS images from the Spitzer archive. We compared the photometric results for several samples of three known types of discrete sources (HII regions, supernovae remnants and planetary nebulae) with theoretical diagnostic diagrams, and derived the spectral energy distribution (from 3.6 to 24 $\mu$m) of each type of object. Moreover, we generated a catalogue of 24 $\mu$m sources and inferred their nature from the observed and theoretical colours of the known type sources. We estimated the star formation rate in M 33 both globally and locally, from the IR emission and from the H$\alpha$ emission line.
Results.The colours of the typical IR emissions of HII regions, supernovae remnants and planetary nebulae are continuous among the different samples, with overlapping regions in the diagnostic diagrams. The comparison between the model results and the colours of HII regions indicates a dusty envelope at relatively high temperatures ~600 K, and moderate extinction AV $\la$ 10. The 24 $\mu$m sources IR colours follow the regions observationally defined by the three classes of known objects but the majority of them represent HII regions. The derived total IR luminosity function is in fact very similar to the HII luminosity function observed in the Milky Way and in other late type spirals. Even though our completeness limit is 5 $\times$ 1037 erg s-1, in low density regions we are able to detect sources five times fainter than this, corresponding to the faintest possible HII region. The 8 and 24 $\mu$m luminosities within the central 5 kpc of M 33 are comparable and of order 4 $\times$ 1028 erg s-1 Hz-1 ($\nu L_\nu$(8) = 1.5 $\times$ 1042 and $\nu L_\nu$(24) = 4.4 $\times$ 1041 erg s-1). We estimate the total IR emission in the same region of M 33 to be 109 $L_\odot$. The discrete sources account for about one third of the 24 $\mu$m emission while the rest is diffuse. From the IR emission, we derive a star formation rate for the inner disk equal to 0.2 $M_\odot$ yr-1, consistent with the star formation rate obtained from the H$\alpha$ emission.


Key words: galaxies: individual: M 33 -- galaxies: ISM -- galaxies: Local Group -- galaxies: spiral



© ESO 2007


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