---
abstract: 'Dusty starbursts were more numerous around $z\sim$ 1 than today and appear to be responsible for the majority of cosmic star formation over the Hubble time. We suggest that they represent a common phase within galaxies in general which is triggered by the growth of cosmic structure.'
author:
- 'D. Elbaz, E. Moy'
---

Deep surveys - infrared - galaxy formation

Introduction
============

Dusty starbursts producing stars at a rate of about 50 M$_{\odot}$ yr$^{-1}$ were very efficiently detected by ISOCAM onboard the Infrared Space Observatory (ISO) at 15$\mu$m below $z\sim$ 1.3 where the broad bump due to aromatic features in the mid-IR (PAHs, polycyclic aromatic features) remains within the ISOCAM filter. These luminous infrared (LIR) galaxies with infrared luminosities greater than $L_{\rm IR}(8-1000\,\mu {\rm m})=\,10^{11}$ L$_{\odot}$ were producing more than thirty times their present-day comoving infrared luminosity density at $z\sim$ 1 than today, when it is only a few percent of the bolometric luminosity radiated by galaxies in the optical to near-infrared (Elbaz [*et al.*]{} 2002, Chary & Elbaz 2001). More distant LIR galaxies were detected in the sub-millimeter with SCUBA (Chapman et al. 2003 and references therein), with IR luminosities of several 10$^{12}$ L$_{\odot}$ due to sensitivity limits, but their large number density confirms the same trend as observed with ISO, i.e. that the bulk of the cosmic history of star formation was dominated by dusty star formation and can only be derived when carefully accounting for this dusty starbursting phase taking place in most if not all galaxies. This is also suggested by the fact that these galaxies contribute to about two thirds of the cosmic infrared background (CIRB, Elbaz et al. 2002), which contains about half or more of the total energy radiated by galaxies through the Hubble time, and dominate the cosmic history of star formation (Chary & Elbaz 2001). Are these galaxies dominated by star formation or nuclear activity ? What is triggering their strong activity ? Is it triggered by external interactions or did it happen naturally within isolated galaxies ?

![[**a)**]{} 15$\mu$m versus radio continuum (1.4 GHz) rest-frame luminosities. Small filled dots: sample of 109 local galaxies from ISOCAM and NVSS. Filled dots with error bars: 17 HDFN galaxies ($z\sim$0.7, radio from VLA or WSRT). Open dots with error bars: 7 CFRS-14 galaxies ($z\sim$0.7, Flores et al. 1999, radio from VLA). Open diamonds: 137 ELAIS galaxies ($z\sim$ 0-0.4). [**a)**]{} histogram of the redshift distribution of field (white) and ISOCAM (dark) galaxies which belong to redshift peaks as defined by Cohen et al. (2000).[]{data-label="FIG:radio"}](elbaz_mykonos_fig1.eps){width="12cm"}

Origin of the luminosity of luminous infrared galaxies at z=1
=============================================================

Among the fields surveyed in the mid-IR with ISOCAM, the Hubble Deep Field North (HDFN) and its Flanking Fields (total area of 27 $arcmin^2$), is the best one to study the contribution of active galactic nuclei (AGN) to the IR luminosity of these LIR galaxies. Thanks to the deepest soft to hard X-ray survey ever performed with Chandra in the HDFN, it is possible to pinpoint AGNs including those affected by dust extinction. Among a total number of 95 sources detected at 15$\mu$m in this field (Aussel et al. 1999, 2003 in prep.; with 47 above a completeness limit of 0.1 mJy), only 5 sources were classified as AGN dominated on the basis of their X-ray properties (Fadda et al. 2002). Hence, unless a large number of AGNs are so dust obscured that they were even missed with the 2 Megaseconds Chandra survey, the vast majority of ISOCAM LIR galaxies are powered by star formation. We are then left with the following question: how can one derive a total IR luminosity on the sole basis of a mid-IR measurement ? We showed in Elbaz et al. (2002) that in the local universe, there exists a very strong correlation between the mid-IR and total IR luminosity of galaxies over three decades in luminosity. In order to test whether these correlations remain valid up to $z\sim$ 1, it is possible to use another correlation existing between the radio and total IR luminosity in the local universe and check whether both the mid-IR and radio give a consistent prediction for the total IR luminosity. In the Fig. \[FIG:radio\]a, we have reproduced the plot from Elbaz et al. (2002) complemented with galaxies detected within the ELAIS survey (Rowan-Robinson et al. 2003). Except at low luminosities were the contribution of cirrus to the IR luminosity becomes non negligible, the 1.4 GHz and 15$\mu$m rest-frame luminosities are correlated up to $z\sim$ 1 and therefore predict very consistent total IR luminosities from which star formation rates as well as the contribution of these objects to the CIRB can be computed, leading to the results mentioned in the introduction.

![HST-ACS images of LIR galaxies with 11 $\leq$ log(L$_{\rm
IR}/L_{\odot}$) $\leq$ 12 (LIRGs) and $z\sim$ 0.7. The double-headed arrow indicates the physical size of 50 kpc. The IR luminosity increases from left to right and from top to bottom.[]{data-label="FIG:camgal"}](elbaz_mykonos_fig2.eps){width="12cm"}

The next question concerns the physical origin of this dusty starburst phase in galaxies (see Elbaz & Cesarsky 2003). In order to address it we have plotted in the Fig \[FIG:radio\]b the histogram of the redshift distribution of the ISOCAM galaxies with a spectroscopic redshift and above the completeness limit of 0.1 mJy in the HDFN. For comparison, only the redshifts where Cohen et al. (2000) found a redshift peak is plotted for field galaxies. One can clearly see that except for 3 ISOCAM galaxies, all are located within these redshift peaks. We have performed Monte-Carlo simulations to compute the probability that a random sample of optical galaxies of the same size than that of ISOCAM galaxies would fall in as many redshift peaks and we found that it is of the order of one percent, and even less if we restrict ourselves to the biggest redshift peaks (Moy et al., in prep). Is this result in agreement with expectations ? As showed by Franceschini et al. (2001), ISOCAM galaxies are relatively massive, hence more subject to belong to denser regions. Moreover LIR galaxies are mostly triggered by interactions in the local universe and one would naturally expect the same to take place in the more distant universe, hence in connection with cosmic structure formation. Thanks to the deepest existing survey with the ACS camera onboard the HST (GOODS survey, Giavalisco et al. 2003), we were able to study the morphology of ISOCAM galaxies in this field centered on the HDFN. A first result follows our expectations: most of these galaxies present a disturbed morphology and this is truer as a function of increasing IR luminosity (see Fig. \[FIG:camgal\]). A second result was less expected: several LIR galaxies appear to be relatively big disk galaxies, which could be affected by a neighboring dwarf galaxy or a passing-by galaxy but they are not major mergers in the main phase of interacting. This is a very important result that should be carefully considered by authors of galaxy formation simulations. A large fraction of present-day stars may originate from a phase of violent star formation triggered by passing-by galaxies even within disk galaxies which could remain as such in the present-day universe. Another important consequence of this type of physical origin for the dusty starburst phase is that there are not enough major mergers in simulations to explain the large excess of LIR galaxies in the distant universe and the fact that they dominate the cosmic star formation history. Indeed each individual galaxy could have experienced several encounters with other galaxies at the time of structure formation which would induce a series of bursts of star formation, each of them responsible for only a few percent of the final stellar mass of the galaxy.

[1]{}

Aussel, H., Cesarsky, C.J., Elbaz, D., Starck, J.L. 1999, A&A 342, 313

Chapman, S.C., Blain, A.W., Ivison, R.J., Smail, I. 2003, Nature 422, 695

Chary, R.R., Elbaz, D. 2001, ApJ 556, 562

Cohen, J.G., Hogg, D.W., Blandford, R., et al. 2000, ApJ 538, 29

Elbaz, D., Cesarsky, C.J., Fadda, D., et al. 1999, A&A 351, L37

Elbaz, D., Cesarsky, C.J., Chanial, P., et al. 2002, A&A, 384, 848

Elbaz, D., Cesarsky, C.J. 2003, Science 300, 270

Fadda, D., Flores, H., Hasinger, G., et al. 2002, A&A 383, 838

Flores, H., Hammer, F., Thuan, T.X., et al. 1999, ApJ 517, 148

Franceschini, A., Aussel, H., Cesarsky, C.J., Elbaz, D., Fadda, D. 2001, A&A 378, 1

Giavalisco, M., et al. 2003, ApJ (in press, astro-ph$/$0309105)

Rowan-Robinson, M., et al. 2003, MNRAS (submitted, astro-ph$/$0308283)
