lines good indicators of the star formation rate in galaxies ?

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.