The 12 
m ISO-ESO-Sculptor
and 24 m Spitzer faint counts reveal a population of ULIRGs as dusty
massive ellipticals*
Evolution by types and cosmic star formation
1
Institut d'Astrophysique de Paris, UMR7095 CNRS, Université Pierre et Marie Curie – Paris 6, 98bis boulevard Arago, 75014 Paris, France e-mail: brigitte.rocca@iap.fr
2
Université Paris Sud, Bât 121, 91405 Orsay Cedex, France
3
Spitzer Science Center, California Institute of Technology, Mail Code 220-6, 1200 East California Boulevard, Pasadena, CA 91125, USA
Received:
16
March
2006
Accepted:
20
June
2007
Context.Multi-wavelength galaxy number counts provide clues to the
nature of galaxy evolution. The interpretation per galaxy type of the
mid-IR faint counts obtained with ISO and Spitzer, consistent with
the analysis of deep UV-optical-near IR galaxy counts, provide new
constraints on the dust and stellar emission. Discovering the nature
of new populations, such as high redshift ultra-luminous (≥10
)
infrared galaxies (ULIRGs), is also crucial for understanding galaxy
evolution at high redshifts.
Aims.We first present the faint galaxy counts at 12 μm from the catalogue of the ISO-ESO-Sculptor Survey (ISO-ESS) published in a companion article (Seymour et al. 2007a, A&A, 475, 791). They go down to 0.31 mJy after corrections for incompleteness. We verify the consistency with the existing ISO number counts at 15 μm. Then we analyse the 12 μm (ISO-ESS) and the 24 μm (Spitzer) faint counts, to constrain the nature of ULIRGs, the cosmic star formation history and time scales for mass buildup.
Methods.We show that the “normal” scenarios in our evolutionary code PÉGASE, which had previously fitted the deep UV-optical-near IR counts, are unsuccessful at 12 μm and 24 μm. We thus propose a new ULIRG scenario adjusted to the observed cumulative and differential 12 μm and 24 μm counts and based on observed 12 μm and 25 μm IRAS luminosity functions and evolutionary optical/mid-IR colours from PÉGASE.
Results.We succeed in simultaneously modelling the typical excess observed at 12 μm, 15 μm (ISO), and 24 μm (Spitzer) in the cumulative and differential counts by only changing 9% of normal galaxies (1/3 of the ellipticals) into ultra-bright dusty galaxies evolving as ellipticals, and interpreted as distant ULIRGs. These objects present similarities with the population of radio-galaxy hosts at high redshift. No number density evolution is included in our models even if minor starbursts due to galaxy interactions remain compatible with our results.
Conclusions.Higher spectral and spatial resolution in the mid-IR, together with submillimeter observations using the future Herschel observatory, will be useful to confirm these results.
Key words: infrared: galaxies / galaxies: evolution / galaxies: photometry / cosmology: observations
Based on observations with ISO, an ESA project with instruments funded by ESA member states (specially the PI countries France, Germany, The Netherlands, and the United Kingdom) with the participation of ISAS and NASA. The ESO-Sculptor Survey is based on observations collected at the European Southern Observatory (ESO), La Silla, Chile.
© ESO, 2007