A long-wavelength view on galaxy evolution from deep surveys by the Infrared Space Observatory

Abstract

Abstract.We discuss the constraints set on galaxy evolution by a variety of data from deep extragalactic surveys performed in the mid-IR and far-IR with the Infrared Space Observatory and with millimetric telescopes at longer wavelengths. These observations indicate extremely high rates of evolution for IR galaxies, exceeding those measured for galaxies at other wavelengths and comparable or larger than the rates observed for quasars. We also match the modelled integrated emission by IR galaxies at any redshifts with the observed spectral intensity of the extragalactic IR background (CIRB), as a further constraint. The multi-wavelength statistics on IR galaxies can be reconciled with each other by assuming for the bulk of the population spectral energy distributions (SED) as typical for starbursts, which we take as an indication that stellar (rather than AGN, see also Fadda et al. 2001) activity powers IR emission by faint galaxies. According to our model and following the analysis of Elbaz et al. (2001), the deep ISO surveys at 15 µm may have already resolved more than 50% of the bolometric CIRB intensity: the faint ISO 15 µm source samples, relatively easy to identify in deep optical images , can then allow to investigate the origin of the CIRB background. From our fits to the observed optical-IR SEDs, these objects appear to mostly involve massive galaxies hosting luminous starbursts (SF R ∼ 100 M /yr). The evolutionary scheme we infer from these data considers a bimodal star formation (SF), including a phase of long-lived quiescent SF, and enhanced SF taking place during transient events recurrently triggered by interactions and merging. We interpret the strong observed evolution as an increase with z of the rate of interactions between galaxies (density evolution) and an increase of their IR luminosity due to the more abundant fuel available in the past (luminosity evolution): both factors enhance the probability to detect a galaxy during the "active" phase at higher z. Very schematically, we associate the origin of the bulk of the optical/NIR background to the quiescent evolution, while the CIRB is interpreted as mostly due the dusty starburst phase. The latter possibly leads to the formation of galaxy spheroids, when the dynamical events triggering the starburst re-distribute already present stellar populations. The large energy contents in the CIRB and optical backgrounds are not easily explained, considering the moderate efficiency of energy generation by stars: a top-heavy stellar IMF associated with the starburst phase (and compared with a more standard IMF during the quiescent SF) would alleviate the problem. The evolution of the IR emissivity of galaxies from the present time to z ∼ 1 is so strong that the combined set of constraints by the observed z-distributions and the CIRB spectrum impose it to turn-over at z > 1: scenarios in which a dominant fraction of stellar formation occurs at very high-z are not supported by our analysis.

ISO deep far-infrared survey in the “Lockman Hole”

“…In scenario E, the SFR density is 3× the local value at z ∼ 0.5 and peaks at z ∼ 2.5 with a density of 10× the local value. The models by Rowan-Robinson (2001) and Franceschini et al (2001) are overlaid in the bottom panels by using the long-dashed and dash-dot-dot lines, respectively. In the models by Rowan-Robinson and Franceschini et al, the SFR densities rapidly increase with redshift, from peaks at z ∼ 1.…”

Evolution of the dusty infrared luminosity function fromz = 0toz = 2.3using observations fromSpitzer

“…The important contribution of infrared luminous galaxies (Luminous InfraRed Galaxies, LIRGs: 10 11 L < L IR < 10 12 L ; Ultra-Luminous InfraRed Galaxies, ULIRGs 10 12 L < L IR ) in the evolution of the star-formation rate (SFR) history of the Universe is now well established up to z ∼ 1 (Chary & Elbaz 2001;Franceschini et al 2001;Xu et al 2001;Elbaz et al 2002;Metcalfe et al 2003;Lagache et al 2004;Le Floc'h et al 2005;Magnelli et al 2009). Their contribution to the SFR density of the Universe increases with redshift up to z ∼ 1 where the bulk of the SFR density occurs in LIRGs.…”

Mid-infrared luminosity as an indicator of the total infrared luminosity of galaxies

“…Such studies play a crucial role in constructing and verifying IR galaxy evolution models (e.g., Granato et al 2000;Franceschini et al 2001;Takeuchi et al 2001a,b;Takagi et al 2003).…”