Different Star Formation Laws for Disks Versus Starbursts at Low and High Redshifts

Abstract

We present evidence that bona fide disks and starburst systems occupy distinct regions in the gas mass versus star formation (SF) rate plane, both for the integrated quantities and for the respective surface densities. This result is based on CO observations of galaxy populations at low and high redshifts, and on the current consensus for the CO luminosity to gas mass conversion factors. The data suggest the existence of two different star formation regimes: a long-lasting mode for disks and a more rapid mode for starbursts, the latter probably occurring during major mergers or in dense nuclear SF regions. Both modes are observable over a large range of SF rates. The detection of CO emission from distant near-IR selected galaxies reveals such bimodal behavior for the first time, as they allow us to probe gas in disk galaxies with much higher SF rates than are seen locally. The different regimes can potentially be interpreted as the effect of a top-heavy IMF in starbursts. However, we favor a different physical origin related to the fraction of molecular gas in dense clouds. The IR luminosity to gas mass ratio (i.e., the SF efficiency) appears to be inversely proportional to the dynamical (rotation) timescale. Only when accounting for the dynamical timescale, a universal SF law is obtained, suggesting a direct link between global galaxy properties and the local SF rate.

Dusty star-forming galaxies at high redshift

“…Moreover, Magnelli et al (2012) find an inverse relationship between the conversion factor and and dust temperature, which is consistent with the empirical inverse powerlaw correlation between X CO and gas surface density uncovered by Tacconi et al (2008) and Ostriker & Shetty (2011). Turning toward more 'normal' disk galaxies at high-z, Daddi et al (2010b) utilize dynamical arguments to infer α CO = 3.6±0.8 M pc −2 (K km s −1 ) −1 (i.e. only slightly less than the mean Milky Way value), while Magdis et al (2011) find a value α CO = 4.1 +3.3 −2.7 M pc −2 (K km s −1 ) −1 when considering dust-to-gas ratio based arguments.…”

“…A super-linear index in the KS relation suggests that very high Σ SFR galaxies have shorter gas depletion time scales than lower Σ SFR galaxies. On the other hand, other studies by Daddi et al (2010b) and Genzel et al (2010) suggest that the KS relation may be linear when including high-z galaxies, though these studies additionally include more quiescent galaxies on the SFR-M * main sequence.…”

“…There is still a debate in the literature as to whether the CO-H 2 conversion factor is bimodal or continuous (e.g. Daddi et al, 2010b;Genzel et al, 2010;Narayanan et al, 2012b). This effectively boils down to assuming that the physical conditions in all starburst galaxies are exactly the same (and all normal disk galaxies are exactly the same), versus assuming that there may be dispersion and variation in different galaxies.…”

“…For example, Daddi et al (2010b and Bothwell et al (2010) presented relatively large samples of CO-detected galaxies at high-z. These included both very luminous systems such as SMGs, as well as more moderate star-forming galaxies that lie on the SFR-M * main sequence.…”

ALMA observations of cold molecular gas filaments trailing rising radio bubbles in PKS 0745−191

“…Kennicutt 1998;Daddi et al 2010;Kennicutt & Evans 2012). HST FUV observations show a clear UV excess at the centre of the BCG extending to the W of the nucleus that is clearly coincident with the inner part of the SW filament before it bends at ∼ 1 arcsec radius (Fig.…”