Volume 428, Number 2, December III 2004
|Page(s)||409 - 423|
|Published online||26 November 2004|
Mid-IR emission of galaxies in the Virgo cluster and in the Coma supercluster
IV. The nature of the dust heating sources
Laboratoire d'Astrophysique de Marseille, BP 8, Traverse du Siphon, 13376 Marseille Cedex 12, France e-mail: Alessandro.Boselli@oamp.fr
2 Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France e-mail: James.Lequeux@obspm.fr
3 Universitá degli Studi di Milano-Bicocca, P.zza della Scienza 3, 20126 Milano, Italy e-mail: Giuseppe.Gavazzi@mib.infn.it
Accepted: 27 July 2004
We study the relationship between the mid-IR (5–18 μm) emission of late-type galaxies and various other star formation tracers in order to investigate the nature of the dust heating sources in this spectral domain. The analysis is carried out using a sample of 123 normal, late-type, nearby galaxies with available data at several frequencies. The mid-IR luminosity (normalized to the H-band luminosity) correlates better with the far-IR luminosity than with more direct tracers of the young stellar population such as the Hα and the UV luminosity. The comparison of resolved images reveals a remarkable similarity in the Hα and mid-IR morphologies, with prominent HII regions at both frequencies. The mid-IR images, however, show in addition a diffuse emission not associated with HII regions nor with the diffuse Hα emission. This evidence indicates that the stellar population responsible for the heating of dust emitting in the mid-IR is similar to that heating big grains emitting in the far-IR, including relatively evolved stars responsible for the non-ionizing radiation. The scatter in the mid-IR vs. Hα, UV and far-IR luminosity relation is mostly due to metallicity effects, with metal-poor objects having a lower mid-IR emission per unit star formation rate than metal-rich galaxies. Our analysis indicates that the mid-IR luminosity is not an optimal star formation tracer in normal, late-type galaxies.
Key words: galaxies: spiral / galaxies: ISM - stars: formation / infrared: ISM
© ESO, 2004
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