6 The role of dusty star-forming EROs

The observed star formation rate (SFR) of each star-forming ERO has been estimated both from the [OII]$\lambda$3727 line emission and the 2800 Å  continuum luminosities using the relations of Kennicutt (1998). The average SFRs at $z_{\rm mean}=1.096$ are SFR([OII]) = 3.6 $M_{\odot}$ yr^-1 and SFR( $L_{2800})= 1.8 M_{\odot}$ yr^-1. Adopting the comoving volume included between the observed $z_{\rm min}$ and $z_{\rm max}$, the corresponding SFR densities are SFRD([OII])=0.0011 and SFRD(L₂₈₀₀) = 0.0005 $M_{\odot}$ yr^-1 Mpc^-3. Such SFRDs clearly represent lower limits because no corrections for dust extinction and for incompleteness have been applied. However, such SFRs may also be partly overestimated if dust-obscured AGN activity is present in some EROs as suggested by the possible [NeV]$\lambda$3426 emission (Fig. 1).

If we conservatively adopt an average $E(B-V)\sim0.5$ (see Sect. 4) and apply the corresponding extinction corrections, the ERO SFRD becomes $\sim$0.015 $M_{\odot}$ yr^-1 Mpc^-3, formally corresponding to a contribution of about 20% to the global SFRD of the universe at $z\sim 1$ (without counting EROs) corrected for dust extinction ($\sim 0.08$ $M_{\odot}$ yr^-1 Mpc^-3, as discussed by Somerville et al. 2001). Even if the uncertainties are large because of the assumptions on the SFR estimators and on the adopted extinction curve, our result strongly suggests that EROs may be important in the cosmic star formation budget at $z\sim 1$.

Our results also suggest that the ERO selection provides the possibility to uncover the population of high-z dusty star-forming galaxies in a way complementary to the surveys for submillimeter/millimeter-selected galaxies. In fact, if the dereddened SFRs of star-forming EROs are in the range of 50-150 $M_{\odot}$ yr^-1 (possible for $E(B-V)\sim0.5$-0.7), this would suggest that their far-infrared luminosities are generally below 10¹² $L_{\odot}$ (adopting the relationship SFR[$M_{\odot}$ yr^-1] $=4.5\times10^{-44}L_{\rm FIR}$[erg s^-1]; Kennicutt 1998). Such a scenario would explain the origin of the low detection rates of EROs in submillimeter/millimeter continuum follow-up observations, typically sensitive to detect ultra-luminous infrared galaxies (ULIGs, L>10¹² $L_{\odot}$) at z $\gtrsim$ 1 (e.g. Mohan et al. 2001).

Figure 2: The average rest-frame spectra (smoothed with a 3 pixel boxcar) of old passively evolving (top; $z_{\rm mean}=1.000$) and dusty star-forming EROs (bottom; $z_{\rm mean}=1.096$) with $Ks\leq 20$.

Figure 3: The average spectra of EROs (thin lines) and template galaxies (thick lines) (see text for details). The spikes at $\lambda \approx$ 3100 Å  are due to the noise in the SB6 spectrum.

Acknowledgements

We thank the referee, P. McCarthy, for the constructive comments, and the VLT support astronomers for their kind assistance. AC warmly thanks ESO (Garching) for the hospitality during his visits, A. Franceschini for useful discussion and B. Poggianti for providing the VLIG spectrum.