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Astron. Astrophys. 354, L1-L5 (2000)

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1. Introduction

The recent detections of CO emission at cosmological distances provide hints about the physical structure of newly formed objects (Combes, Maoli & Omont 1999 and reference therein). A measure of the total gas and dust mass is indeed a very useful indicator of the object evolutionary status, because it provides an estimation of the fraction of the galaxy which has yet to be turned into stars at the epoch of observation. At high redshifts such measurements, therefore, provide hints about the occurrence of active star-formation processes and help in investigating models of galaxy formation (Silk & Spaans 1997).

Only a handful of distant objects was detected so far in CO, and most of them appear to be magnified by gravitational lenses (Table 1). For these objects the mass of molecular gas inferred from the CO intensities - corrected for gravitational amplification and using the Galactic CO to H2 conversion factor - turns out to be [FORMULA]% of the total dynamical mass, with typical values of M(H2) [FORMULA] [FORMULA]. These masses could be somewhat smaller if the CO to H2 conversion factor were higher than in the Galaxy. Large quantities of CO are expected when large FIR luminosities and dust content are detected. Most of the objects detected until now in CO were indeed selected because they were dust-rich systems.


Table 1. List of high-redshift objects detected to date in CO.

HR10 belongs to the class of objects with very red colours ([FORMULA]). Their faintness at optical/NIR wavelengths makes the redshift determination and the investigation of their nature difficult even with 4m class telescopes. HR10 is so far the only one with a measured spectroscopic redshift (Graham & Dey 1996). One of the main issues regarding ERGs is whether they are young and starbursting galaxies hidden in the optical by a large amount of dust or whether they are old passively evolving galaxies at [FORMULA]. Recent results indicate that both classes contribute to the population of ERGs (Cimatti et al 1999).
HR10 was first detected in the submm/mm continuum with the IRAM 30m equipped with the MPIfR bolometer and with the JCMT equipped with the SCUBA double arrays (Cimatti et al 1998). Subsequent observations confirmed the submm detection of this galaxy (Dey et al. 1999). The inferred properties of this object show its extraordinary nature: its dust mass is [FORMULA] [FORMULA] and its total FIR luminosity in the range 10 -2000 µm rest-frame is [FORMULA] L[FORMULA] (H0 = 50 Mpc/km/s). This places HR10 in the class of the ultra-luminous infrared galaxies and suggests the presence of a star-forming object with a SFR of [FORMULA] [FORMULA] yr-1 or even higher.

In this paper we present CO(2-1) and CO(5-4) observations of HR10 made with the IRAM Plateau de Bure interferometer. The observations are described in Sect. 2, the resulting detection of both lines are presented in Sect. 3, while implications of these measurements are reported in Sect. 4. Throughout the paper, we adopt H0 = 50 Mpc/km/s and q0 = 0.5.

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000