• Le Floch, Emeric
• Puget, Jean-Loup
• Papovich, Casey
• Smith, Jd
• Lagache, Guilaine
• Dole, Herve

Abstract

Locally 0.5% of galaxies with Log(L(V))>10 Lo have Spectral Energy Distribution (SED) typical of Luminous Infrared Galaxies (LIRGs). This changes dramatically at higher redshift: in deep surveys, ISO at 15 microns detect about 15% of the MB<-20 galaxies (LIRGs at median redshift of 0.65) and SPITZER at 24 microns detect about 30% of field galaxies (starbursts and LIRGs at median redshift of about 1, Ultra-LIRGs up to redshift 3). In the infrared, LIRGs become the dominant population contributing to the comoving energy density beyond z~0.5-0.6 and represent 70% of the star-forming activity at z~1. It is thus clear this population contributes an important part of the whole galaxy build-up in the Universe. The determination of the SEDs of these galaxies at cosmological distance remains an open question despite a lot of work in the last few years. First studies on the 24 microns SPITZER selected galaxies, together with interpretations of the deep number counts by empirical models, suggest that PAH features remain prominent in LIRGs/ultra-LIRGs up to z~2.5. Establishing proper SEDs is one of the challenges of the coming decade. The unique capabilities of the IRS provide unprecedent access to this difficult investigation. We propose to observe with the IRS low resolution spectroscopy mode 40 starburst LIRGs at 0.1<z<0.6 that are identified in the 24 microns FLS and BOOTES cosmological surveys and have also spectroscopic redshifts. We impose an upper luminosity cutoff (Log(L)<12 Lo) to probe only the dominant population contributing to the very strong evolution of the comoving infrared energy density up to z~1. This cutoff prevents us to go to higher redshift. Measuring the mid-IR SEDs up to z=0.6 in normal starbursting galaxies will allow, for the first time to (1) test the strength of PAHs in distant LIRGs and (2) combined with 70 and 160 microns detections, conduct detailed studies of SED variations with the physical LIRGs properties such as redshift, metallicity and environnement....

Topics

• density
• impedance spectroscopy
• energy density
• strength
• forming