EDP Sciences
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Volume 403, Number 2, May IV 2003
Page(s) 501 - 522
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20030351

A&A 403, 501-522 (2003)
DOI: 10.1051/0004-6361:20030351

Infrared spectroscopy of faint 15  $\mu$m sources in the Hubble Deep Field South: First hints at the properties of the sources of the IR background

A. Franceschini1, S. Berta1, D. Rigopoulou2, H. Aussel3, 4, C. J. Cesarsky5, D. Elbaz6, R. Genzel2, E. Moy2, S. Oliver7, M. Rowan-Robinson8 and P. P. Van der Werf9

1  Dipartimento di Astronomia, Vicolo Osservatorio 5, 35122 Padova, Italy
2  Max Planck Institute fuer Extraterrestrische Physik, Garching bei Muenchen, Germany
3  Osservatorio Astronomico di Padova, Italy
4  Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA
5  European Southern Observatory, Karl-Schwarzschild-str. 2, 85740 Garching, Germany
6  Centre d'Études de Saclay, Service d'Astrophysique, Orme des Merisiers, 91191 Gif-sur-Yvette, France
7  Astronomy Center, CPES, University of Sussex, Falmer, Brighton BN1 9QJ, UK
8  ICSTM, Astrophysics Group, Blackett Laboratory, Prince Consort Rd., London, SW2 1BZ, UK
9  Leiden Observatory, PO Box 9513, 2300 RA, Leiden, The Netherlands

(Received 16 July 2002 / Accepted 7 March 2003 )

We present a spectroscopic analysis of a sample of 21 galaxies with z = 0.2-1.5 drawn from a 25 square arcmin ultra-deep ISOCAM survey at $\lambda_{\rm eff}=15~\mu$m centered in the WFPC-2 Hubble Deep Field South. Near-infrared spectra are reported for 18 ISO sources, carried out with ISAAC on the VLT, aimed at detecting the redshifted H $_\alpha+$[NII]. Additional optical data come from the ESO VLT/FORS2 and NTT/EMMI, primarily targeting [OII], [OIII] and H $_\beta$ for further physical insight. Although not numerous in terms of areal density in the sky, this population of very luminous IR sources has been recently found to be responsible for a substantial fraction of the extragalactic background light energy density. H $_\alpha$ line emission is detected in virtually all the observed objects down to a flux limit of $7 \times 10^{-17}$ erg cm -2 s -1 (corresponding to $L_{\rm H_\alpha} > 10^{41}$ erg s -1 at z = 0.6 for H0 = 65, $\Omega_\Lambda=0.7$ and $\Omega_m = 0.3$). Our analysis (including emission line, morphology, and SED properties) shows clear evidence for AGN activity in only two of these sources: one type-I (with broadened H $_\alpha$ at z=1.57) and one type-II quasars (with inverted [NII]/H $_\alpha$ ratio at z=1.39), while we suspect the presence of an AGN in two further sources (an Ultra-Luminous IR Galaxy, ULIRG, at z=1.27 and a luminous galaxy at z=0.69). The H $_\alpha$ luminosities indicate star formation rates ( SFR) in the remaining sources between 0.5 and 20  $M_{\odot}/\rm yr$, assuming a Salpeter IMF between 0.1 and 100  $M_{\odot}$ and without extinction corrections. We find good correlations between the mid-IR, the radio and H $_\alpha$ luminosities, confirming the mid-IR light as a good tracer of star formation (while the  SFR based on H $_\alpha$ flux show some large scatter and offset, which are still to be understood). We have estimated the baryonic masses in stars with a newly-developed tool fitting the overall optical-IR continuum, and found that the host galaxies of ISO sources are massive members of groups with typically high rates of SF ( $SFR\sim 10$ to 300  $M_{\odot}/\rm yr$). We have finally compared this ongoing SF activity with the already formed stellar masses to estimate the timescales  $t_{\rm SF}$ for the stellar build-up, which turn-out to be widely spread in these objects between 0.1 Gyrs to more than 10 Gyr. The faint ISOCAM galaxies appear to form a composite population, including moderately active but very massive spiral-like galaxies, and very luminous ongoing starbursts, in a continuous sequence. From the observed  $t_{\rm SF}$ and assuming typical starburst timescales, we infer that, with few exceptions, only a fraction of the galactic stars can be formed in any single starburst event, while several of such episodes during a protracted SF history are required for the whole galactic build-up.

Key words: galaxies: interactions -- galaxies: starburst

Offprint request: A. Franceschini, franceschini@pd.astro.it

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