The Physical Evolution of Mass and Dust in Distant Galaxies

Abstract

Main clues on galaxy evolution are derived independently from stellar masses in the K-band and from dust thermal emissions at 12µm, both for high-z populations of galaxies.

As a first result (Rocca-Volmerange et al, 2004), the most massive galaxies known as radio powerful ellipticals, trace in the K-band Hubble diagram the 10¹²M_⊙ critic mass of the fragmentation predicted by the dissipative self gravitational models (Rees and Ostriker, 1977). Because these massive galaxies were already formed at z =4, they required an extremely short mass accumulation duration, implying strong constraints on the two time scales τ _freefall and τ _cooling . Finally the measured size of the cloud, ionized by these massive radio sources, is also compatible with the critic radius predicted by the theory.

On another wavelength domain dominated by dust, we are publishing the results of a deep survey observed at 12 µm with the ISOCAM camera (Rocca-Volmerange et al. and Seymour et al., preprints). The covered field is the ESO-Sculptor redshift survey (de Lapparent et al 1997) area. The limit flux, reaching 0.2mJy, allows to follow the evolution of the brightest galaxies and starbursts at high redshifts. The 12 µm source counts are interpreted by averaging inhomogeneities (the survey area is ≃ 800 arcmin²). We take into account cosmological and evolution corrections computed by a new version of the code PEGASE.3 (Fioc et al, in preparation), extended to the far-IR. As a second result, the evolution scenarios which allowed to significantly interpret the multispectral faint galaxy counts in the optical, also give predictions in the far-infrared compatible with the 12 µm observations, making robust our scenarios of galaxy evolution. They confirm that the main evolving component in this wavelength range is largely dominated by actively star forming spiral galaxies. More investigations at high-z are required to understand if only the most massive galaxies form by dissipative gravitational collapse. In the present status, the current evolution scenarios of spiral galaxies remain compatible with both the gravitational collapse and/or the hierarchical merging. The perspectives of multi spectral surveys are built to solve this debated question.