Astron. Astrophys. 324, L13-L16 (1997)

Letter to the Editor

Mid-IR colors and star formation in Virgo and Coma galaxies ^*

A. Boselli ¹, J. Lequeux ², A. Contursi ², G. Gavazzi ³, O. Boulade ⁴, F. Boulanger ⁵, D. Cesarsky ⁵, C. Dupraz ⁶, S. Madden ⁴, M. Sauvage ⁴, F. Viallefond ² and L. Vigroux <sup>4

1</sup> Laboratoire d'Astronomie Spatiale, Traverse du Siphon, BP 8, F-13376 Marseille CEDEX 12, France
² Observatoire de Paris, 61 Av. de l'Observatoire, F-75014 Paris, France
³ Università degli studi di Milano, Dipartimento di Fisica, Via Celoria 16, I-20121 Milano, Italy
⁴ Service d'Astrophysique, Centre d'Etudes de Saclay, F-91191 Gif sur Yvette, France
⁵ Institut d'Astrophysique Spatiale, Bat. 121, Université Paris-Sud, F-91405 Orsay, France
⁶ Radioastronomie, Ecole Normale Supérieure, 24 Rue Lhomond, F-75231 Paris CEDEX 05, France

Received 17 January 1997 / Accepted 23 May 1997

Abstract

We present observations of the global emission of a sample of spiral and irregular galaxies in the Virgo cluster and the Coma supercluster. These observations have been made with the camera ISOCAM on board the Infrared Space Observatory, through two broad filters centered respectively at 6.75 and 15.0 µm. The corresponding fluxes are corrected for the stellar contribution in order to obtain the pure interstellar contribution; the stellar contribution is estimated from the near-infrared fluxes using as template some S0 and elliptical galaxies observed by chance in the ISO fields. The fluxes at 6.75 and 15.0 µm are then normalized to the stellar mass of the galaxy, and compared to the normalized far-UV fluxes at 2000 Å  which are good indicators of the star-formation rate (SFR). We find that, while the mid-IR fluxes are proportional to the SFR when it is not too large, the galaxies with a high SFR have relatively less emission at 6.75 and 15 µm. It appears that the carriers of the Unidentified Infrared Bands which are responsible for almost all the emission at 6.75 µm and for a part of the emission at 15 µm have different properties in these galaxies or are partly destructed by the UV field. At 15 µm, there is a contribution of very small, three-dimensional grains in galaxies with a high SFR.

Key words: Galaxies: general Galaxies: ISM Galaxies: spiral Infrared: galaxies Infrared: ISM: continuum

* Based on observations with ISO, an ESA project with instruments founded by ESA member states (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with participation of ISAS and NASA

Send offprint requests to: A. Boselli boselli@astrsp-mrs.fr

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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