DUSTY STARBURSTS AS A STANDARD PHASE IN GALAXY EVOLUTION David Elbaz CEA Saclay/DAPNIA/SAp F-91191 Gif-sur-Yvette Cedex, France [email protected]

Abstract

We discuss the implications of the ISO, SCUBA and Spitzer extragalactic surveys, combined with the COBE cosmic infrared background, on the star formation history of galaxies. A class of galaxies is emerging, similar to local luminous infrared galaxies, that appears to play a dominant role in the shaping of present-day galaxies. Instead of a class, these observations suggest that these "dusty starbursts" should be considered as a common phase experienced by most if not all galaxies, once or even several times in their lifetime.

Keywords:

Galaxies: starbursts - star formation rates - dust: extinction

1.

Introduction

It is widely accepted that in the local universe stars form in giant molecular clouds (GMCs) where their optical and mostly UV light is strongly absorbed by the dust which surrounds them. Whether extinction was already taking place in the more distant universe where galaxies are less metal rich was less obvious ten years ago. Galaxies forming stars at a rate larger than about 20 M an−1 were known to radiate the bulk of their luminosity above 5 µm thanks to IRAS, the so-called luminous (LIRGs, 12 > log(LIR /L ) ≥ 11) and ultra-luminous (ULIRGs, log(LIR /L ) ≥ 12) infrared (IR) galaxies. In the following, we will call these galaxies “dusty starbursts”. The bolometric luminosity of galaxies experiencing such large star formation rates is dominated by the radiation of their young and massive stars. In the local universe, such objects are very rare and indeed “dusty starbursts” radiate only 2 % of the bolometric luminosity of galaxies at z∼0. In the past, galaxies were more gaseous and formed the bulk of their present-day stars, hence we may expect to find more of these violent star formation events. Already IRAS observations indicated a rapid decline of the comoving number density of ULIRGs since z∼0.3 (Kim & Sanders 1998, see also Oliver et al 1996), but this was over a

2 small redshift range and with small number statistics. However, the idea that “dusty starbursts” should have been common in the past was not accepted until a combination of observations arised during the last ten years. Distant galaxies were expected to be only marginally affected by dust extinction by reference to local galaxies and because they were less metal rich. Star formation rates were commonly measured from optical emission lines uncorrected for extinction, such as OII or Hα, or the UV continuum. The first version of the cosmic star formation history of the universe (Madau et al 1995) was published without accounting for any extinction effect. However several independant sources converged towards another scenario, where most star formation that took place in the universe was obscured by dust, such as: 1 extragalactic source counts at 15 µm (Elbaz et al 1999, Metcalfe et al 2003, Gruppioni et al 2003, Rodighiero et al 2004, Fadda et al 2004) exhibit a slope which cannot be reconciled with model expectations unless strong evolution is advocated, either in luminosity and/or density of the mid infrared luminosity function, hence of the amount of star formation hidden by dust (e.g. Chary & Elbaz 2001). 2 the nearly simultaneous discovery of the cosmic infrared background (CIRB, Puget et al 1996, Fixsen et al 1998, Hauser & Dwek 2001 and references therein), at least as strong as the UV-optical-near IR one, whereas local galaxies only radiate about 30 % of their bolometric luminosity in the IR above λ ∼ 5 µm. 3 the 850 µm number counts from the SCUBA sub-millimeter bolometer array at the JCMT (Hughes et al 1998, Barger et al 1998, Smail et al 2002, Chapman et al 2003, and references therein) which also indicate a strong excess of faint objects in this wavelength range, implying that even at large redshifts dust emission must have been very large in at least the most active galaxies. 4 the most distant galaxies, individually detected thanks to the photometric redshift technique using their Balmer or Lyman break signature showed the signature of a strong dust extinction. The so-called “β-slope” technique (Meurer et al 1999) used to derive the intrinsic luminosity of these galaxies and correct their UV luminosity by factors of a few (typically between 3 and 7, Steidel et al 1999, Adelberger & Steidel 2000) was later on shown to even underestimate the SFR of LIRGs/ULIRGs (Goldader et al 2002). 5 the slope of the sub-mJy deep radio surveys (Haarsma et al 2000).

Dusty starbursts as a standard phase in galaxy evolution

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6 More recently, extragalactic source counts at 24 µm with MIPS onboard Spitzer confirmed the strong evolution found at 15 µm (Chary et al 2004, Papovich et al 2004).

It has now become clear that the cosmic history of star formation based on rest-frame UV or emission line indicators of star formation such as [0II] or [Hα] strongly underestimate the true activity of galaxies in the past if not corrected by strong factors due to dust extinction (Flores et al 2004, Liang et al 2004, Cardiel et al 2003). Although distant galaxies were less metal rich and much younger, they must have found the time to produce dust rapidly in order to efficiently absorb the UV light of their young stars.

Figure 1. Density of star formation per unit comoving volume as a function of redshift (or lookback time, upper axis), i.e. cosmic star formation history. Cosmology: H0 =70 km s−1 Mpc−1 , Ωm =0.3, ΩΛ =0.7. The origin of the data points is given in the upper-right box. Data are from 1500Å(Massarotti et al 2001, Madau et al 1998, Pascarelle et al 1998), 1700Å(Steidel et al 1999), 2000Å(Treyer et al 1998), 2800Å(Connolly et al 1997, Lilly et al 1996, Cowie et al 1999), 3000Å(Sawicki et al 1997), OII (Hammer et al 1997), Hα (Gallego et al 1995, Tresse & Maddox 1998, Glazebrook et al 1999, Yan et al 1999), 15 µm (Flores et al 1999), 850 µm (Hughes et al 1998), 21 cm (Haarsma et al 2000). left (a): empty symbols are only modestly corrected for dust extinction (except for Hα uncorrected) following the recipee of Ascasibar et al. (2002): A(1500-2000 Å)=1.2 mag and A(2880 Å, 3000 Å, OII)= 0.625 mag, i.e. factors of 3 and 1.8 respectively. The filled symbols are corrected by extinction or do not require any correction (as for the 15, 850 µm and 1.4 GHz) data). We used the corrections quoted by the authors except for Hα for which no correction was available. We applyied a correction of a factor 2.3 to this indicator, i.e. half the one observed for ISOCAM galaxies (see Hammer et al 2004, Liang et al 2004). right (b): only the filled points are represented and compared to the range of possible star formation histories derived from source counts in the mid IR, far IR, sub-mm and the CIRB by Chary & Elbaz (2001).

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Figure 2. Redshift evolution of the comoving density of stars or Ω∗ (divided by the critical density of the universe). Data from Dickinson et al (2003). The plain line is the best fit model of Chary & Elbaz (2001), including both luminosity and density evolution. The dashed lines represent the range of possible models within 1-σ of the observational constraints.

2.

Towards a coherent picture of the cosmic star formation history

We have compiled published versions of the comoving density of star formation for different star formation indicators references that we could find at the time of the conference in the Fig. 1 (see figure caption for the references). Fig. 1a shows the very large dispersion of all these measurements, leading the reader to the impression that they provide no valuable constraint on what really happened. However, once we take out data points providing only lower limits because they are not corrected for dust extinction, we get a much sharper scenario (Fig. 1b). The grey area represents the range of possible scenarios from Chary & Elbaz (2001) fitting the combination of mid IR, far IR, sub-mm counts, the cosmic infrared background (CIRB) and the redshift distribution of the 15 µm ISOCAM sources. A revised version including Spitzer MIPS 24 µm counts will soon be submitted. It does not require a major revision of this scenario. These data suggest that a strong evolution took place below z ∼ 2 that gave rise to a large fraction of present-day stars. The main actors of this scenario are the “dusty starbursts”, which are required to explain the source counts as well as the CIRB. An interesting test for this scenario consists in integrating the cosmic star formation history and to compare it to the observed evolution of the cosmic density of stars in the universe. This integral (for a Gould IMF,

Dusty starbursts as a standard phase in galaxy evolution

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Gould et al 1996) is compared to the compilation of measurements from Dickinson et al (2003) in the Fig. 2. We have assumed a fraction of 20 % of the counts and infrared background to be due to active galactic nuclei (AGNs, see Fadda et al 2002). Although the best fitting model from Chary & Elbaz (2001) slightly overpredicts the comoving stellar mass density above z ∼ 1, data are consistent with the broad region permitted by the range of valid models (region within the two dot-dashed lines). Note that the LIRGs by themselves provide about 63 % of present-day stars in this framework and fit the observed stellar mass density by themselves. The excess of stellar mass derived from the integration of the cosmic star formation history (Fig. 1b) could be produced by several causes: - a larger fraction of AGNs above z ∼ 1.5 - a top-heavy initial mass function in dusty starbursts - a change of the IR spectral energy distribution of galaxies that would imply that the SFR that we derived is overestimated for distant galaxies. However, the global agreement of the two cosmic histories - of star formation and of stellar mass density- suggest that we are getting close to a coherent picture in which dusty starbursts play a major in shaping present-day galaxies. In this scenario, SCUBA galaxies of a few mJy with redshifts measured around z ∼ 2.5 (Chapman et al 2003) are the tip of the iceberg, i.e. ULIRGs, while most of the evolution is due to LIRGs. Many questions still remain unsolved, among others: - what is the triggering mechanism for the distant dusty starbursts ? Major mergers are not numerous enough top explain such numbers of dusty starbursts in the past. Other dynamical processes such as "passing by" objects, may represent an important alternative to major mergers that should be carefully considered in hierarchical simulations. A recent study by Moy et al (2005, in prep.) indeed suggests that clustering plays a major role in triggering these phases. - what is the present-day counterpart of distant LIRGs ? A discussion of the relative contribution of dusty starbursts as a function of morphological type is provided by Hammer et al (2004), who suggest that the disks of spiral galaxies may have been destroyed and recen tly rebuilt. - Have we underestimated the contribution of Compton thick AGNs to infrared counts ? A population of such objects might not have been detected by XMM-Newton and Chandra but could still provide an important contribution to the peak of the cosmic X-ray background at 30 keV (see Worsley et al 2004).

Acknowledgments We wish to thank the Centre National d’Etudes Spatiales (CNES) for their financial support.

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