2009MNRAS.399..264B

In this paper, we investigate the relevance of using the ¹²CO-line emissions as indicators of star formation rates (SFRs). For the first time, we present this study for a relatively large number of ¹²CO transitions (12) as well as over a large interval in redshift (from z ∼ 0 to 6). For the nearby sources (D ≤ 10 Mpc), we have used our previously published homogeneous sample of ¹²CO data, mixing observational and modelled line intensities. For higher z sources (z ≥ 1), we have collected ¹²CO observations from various papers and completed the data set of line intensities with model predictions which we also present in this paper. Finally, for increasing the statistics, we have included recent ¹²CO(1-0) and ¹²CO(3-2) observations of intermediate-z sources. Linear regressions have been calculated for identifying the tightest SFR-¹²CO-line luminosity relationships. We show that the total ¹²CO, the ¹²CO(5-4), the ¹²CO(6-5) and the ¹²CO(7-6) luminosities are the best indicators of SFRs (as measured by the far-infrared luminosity). Comparisons with theoretical approaches from Krumholz & Thompson and Narayanan et al. are also performed in this paper. Although in general agreement, the predictions made by these authors and the observational results we present here show small and interesting discrepancies. In particular, the slope of the linear regressions for J_upper≥ 4 ¹²CO lines is not similar between theoretical studies and observations. On the one hand, a larger high-J ¹²CO data set of observations might help to better agree with models, increasing the statistics. On the other hand, theoretical studies extended to high-redshift sources might also reduce such discrepancies.