Galaxy evolution from deep multi-wavelength infrared surveys: a prelude toHerschel

Abstract

Context. Studies of the generation and assembly of stellar populations in galaxies largely benefit from far-IR observations, considering that the IR flux is a close prior to the rate of star formation (the bulk of which happens in dust-obscured environments). At the same time, major episodes of nuclear AGN accretion are also dust-obscured and visible in the IR. Aims. At the end of the Spitzer cryogenic mission and the onset of the Herschel era, we review our current knowledge of galaxy evolution at IR wavelengths, and model it to achieve as far as a complete view of the evolution of cosmic sources. We also develop new tools for the analysis of background fluctuations to constrain source counts in regimes of high confusion, as it happens for the Herschel sub-mm surveys. Methods. We analysed a wide variety of new data on galaxy evolution and high-redshift source populations from Spitzer cosmological surveys, and confront them with complementary data from mm ground-based observations and constraints from the far-IR diffuse radiation, as well as preliminary results from Herschel surveys. Results. These data confirm earlier indications about a very rapid increase in galaxy volume emissivity with redshift up to z 1 [ρ(z) ∝ (1 + z) 4 ], the fastest evolution rate observed for galaxies at any wavelengths. The observed Spitzer counts require a combination of fast evolution for the dominant population and a bumpy spectrum with substantial PAH emission at z ∼ 1 to 2. Number counts at long wavelengths (70 through 1100 μm) confirm these results. All the present data require that the fast observed evolution from z = 0 to 1 flattens around redshift 1 and then keeps approximately constant up to z 2.5 at least. Our estimated redshift-dependent bolometric comoving energy density keeps lower at z > ∼ 1.5 than some previously published results based on either large extinction corrections, or large spectral extrapolations.Conclusions. The present-day IR/sub-mm data provide evidence of a distinct population of very luminous galaxies becoming dominant at z > 1. Their cosmological evolution, peaking around z 2, shows a faster decay with cosmic time than lower luminosity systems, whose maximal activity is set around z 1, then supporting an earlier phase of formation for the most luminous and massive galaxies. From a comparison of our results on the comoving IR emissivity with recent estimates of the redshift-dependent stellar mass functions of galaxies, we find that the two agree well with each other if we assume standard recipes for star formation (a universal Salpeter IMF) and standard fractions (∼20−30%) for the contribution of obscured AGN accretion. Systematic exploitation of the forthcoming Herschel survey data will be important for confirming all this.

The HerMES SPIRE submillimeter local luminosity function

“…Our measurements are slightly above most modeling predictions we considered at all wavelengths, and particularly at the luminosities close to the knee of the LLF which we probed with greater statistical significance. At these luminosities, however, Xu et al (2001) better reproduce the observed space densities than Negrello et al (2007) and Franceschini et al (2010). Our measurements of the IR LLF suggest a flatter lowluminosity slope than previously reported by either Sanders et al (2003) for an IRAS 60-μm-selected sample or Rodighiero et al (2010) for a 24-μm-selected one, but agree with predictions by Lagache et al (2004).…”

“…Each source is assigned the maximum Franceschini et al (2010) are: dashed blue line are spirals, dot-dashed cyan line are starbursts, dashed red line are high-luminosity starbursts, dot-dot-dot-dashed green line are Type-I AGNs, and the black solid line is the total prediction. Spirals are predicted to make up the bulk of local sources detected by SPIRE, as confirmed by SED fitting.…”

HerMES: SPIRE galaxy number counts at 250, 350, and 500 μm

“…Models all agree on the general trends, with a very strong evolution of the brightend (>10 11 L ) of the luminosity function and they yield approximately the same comoving number density of infrared luminous galaxies as a function of redshift. We compare the number counts with eight models, one pre-Spitzer (Xu et al 2003), two based on the ISO, SCUBA and Spitzer first results (Lagache et al 2004;Negrello et al 2007) and 5 being more constrained by deep Spitzer, SCUBA, AzTEC, and recent BLAST observations Valiante et al 2009;Franceschini et al 2009). The differences between the models are in several details, different assumptions leading sometimes to equally good fits to the current data.…”

“…The differences between the models are in several details, different assumptions leading sometimes to equally good fits to the current data. For example, Valiante et al (2009) conclude that it is necessary to introduce both an evolution in the AGN contribution and an evolution in the luminosity-temperature relation, while Franceschini et al (2009) reproduce the current data with only 4 galaxy populations and only one template for each population. We also compare with two semi-analytic models those of Lacey et al (2010) and Wilman et al (2010).…”

“…Exceptions are the models of Negrello et al (2007), Valiante et al (2009), Franceschini et al (2009 and Pearson & Khan (2009). Of these only Valiante et al (2009) can fit the rise from (20 < S < 100) mJy.…”