A large number of luminous infrared galaxies in the massive cluster Cl 0024+1654

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  • 11 October 2003
Abstract
Cl 0024+1654 was observed with the Infrared Space Observatory using ISOCAM. Forty seven sources were detected at 14.3 microns and nineteen of them are cluster galaxies. The remaining sources consist of four stars, one quasar, four foreground galaxies, one background galaxy and 18 sources with unknown redshift. The median infrared luminosity of the nineteen cluster galaxies is 1.3x10^+11 solar luminosities, with thirteen falling in the category of Luminous Infrared Galaxies. The [OII] star formation rates obtained for 6 cluster galaxies are one to two orders of magnitudes lower than the infrared values, implying that most of the star formation is missed in the optical because it is enshrouded by dust in the starburst galaxy. The colour-magnitude diagramme is given for the galaxies within the ISOCAM field-of-view. Only 30% of the galaxies that satisfy the Butcher-Oemler requirement were detected at 14.3 microns. The counterparts of about two-thirds of the 14.3 microns sources are blue, luminous, star-forming systems and the type of galaxy that is usually associated with the Butcher-Oemler effect. HST images of these galaxies reveal a disturbed morphology with a tendency for an absence of nearby companions. Surprisingly the counterparts of the remaining one third of the 14.3 microns sources lie on the main sequence of the colour-magnitude diagramme which implies that they are early type galaxies. However in HST images they all have nearby companions and appear to be involved in interactions and mergers. Dust obscuration may be the major cause of the 14.3 microns sources appearing on the cluster main sequence. Finally, the mid-infrared results on Cl 0024+1654 are compared with those from other clusters observed with ISOCAM.

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