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

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4. Implications

The analysis of the spectral energy distribution of HR10 shows the presence of thermal emission at rest-frame [FORMULA] with a range of dust temperatures between 30 and 45 K. The implied total dust mass is [FORMULA] [FORMULA] (for a dust emissivity index [FORMULA] of 2, Cimatti et al 1998). Therefore the resulting gas-to-dust mass ratio for HR10 ranges between 200 and 400, as local spirals (Andreani, Casoli & Gerin 1995), ULIRGs (Solomon et al. 1997) and also sub-mm selected luminous sources show (Frayer et al. 1999).
The total rest-frame far-IR luminosity in the range [FORMULA] is [FORMULA] [FORMULA] (Cimatti et al 1999) as estimated taking into account the ISO upper limits at 90 and 170 [FORMULA] (Ivison et al 1997). When these latter are not considered and the 450 [FORMULA] detection is included the luminosity turns out to be a factor of 3 larger (Dey et al. 1999). The ratio [FORMULA] lies therefore in the range [FORMULA] (K km s-1 pc2)-1, which agrees with the relation found for nearby luminous galaxies (Sanders & Mirabel 1996), whose emission is mainly powered by star-formation. Objects whose FIR emission is dominated by an AGN - as the hyperluminous Infrared Galaxies - show much larger [FORMULA] and do not even show up in CO (e.g., Evans et al. 1998). This indicates that the overall FIR emission by HR10 is dominated by star formation. Assuming that most of the FIR luminosity is due to recent OB star formation activity, the star formation rate turns out to be [FORMULA] [FORMULA]/yr.
Star formation efficiency is usually measured by the ratios [FORMULA] and [FORMULA] (see e.g. Young 1999). While the former shows indeed quite a high value (16-44) similar to that of merging local systems (Young 1999), the latter is of only 0.007 and very likely indicates a large extinction affecting the H[FORMULA] emission.

With the values above for molecular mass and SFR this active phase of gas depletion lifetime should have lasted at least:


The large value of gas conversion into stars (with respect to local galaxies) could be consistent with two possible scenarios: either a genuinely young galaxy in the process of active star-formation (and the detected amount of gas seems enough to feed it), or the presence of a large amount of gas could be the result of a merging process of two discs (in this latter case the resulting galaxy will have a mass of a present-day massive elliptical).

Most of the properties of HR10 suggest that the `locus', which best characterizes it, is that of local ULIRGs (Hughes, Dunlop, Rawlings 1997). HR10 follows also the expected tight correlation between the infrared flux and the radio continuum: in fact the logarithmic ratio of FIR (60 [FORMULA]) and radio (1.5GHz) continuum flux density (in HR10 rest-frame) [FORMULA] again falls within the value of nearby starbursts (Sanders & Mirabel 1996). Furthermore, the ratio between the line (2-1) and (5-4) and the FIR luminosities, [FORMULA] and [FORMULA], agree with a model of CO emission in high redshift galaxies, based on an extrapolation of the properties of local ULIRGs (Blain et al. 1999).

With the detected line width and the upper limit on the CO source size, given by the effective beam width [FORMULA], the upper limit to the total dynamical mass contained within the CO emitting region is:


where [FORMULA] is the observed deconvolved line width, i is the inclination and R is the linear diameter of the source ([FORMULA]). The resulting dynamical mass is a factor of 5 larger than the estimation of the molecular mass. The two values would coincide if the CO emission were concentrated within the inner 10 kpc.

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Online publication: January 31, 2000