Constant disk stability key to galactic star forming efficiency

Star formation in galaxies is a local process whereby stars form out of molecular Hydrogen (H₂). Detailed multiwavelength studies of the gas and stellar components of nearby galaxies have found the H₂ star formation efficiency (SFE_H2) – the ratio of star formation rate to the H₂ mass – to be approximately constant. Similarly, the atomic Hydrogen (HI) star formation efficiency (SFE_HI) has also been observed to be uniform across 5 orders of magnitude in galaxy stellar masses. Since the fraction of HI and H₂ is known to vary within a galaxy, as well as between galaxies, it is unclear what is driving the observed uniformity in SFE_HI.

A new model, by CAASTRO Affiliate Dr Ivy Wong (ICRAR-UWA) and colleagues, is the first to link the uniform SFE_HI observed in low-redshift galaxies to star-forming disks with constant marginal stability. In their simple model, disk stability is derived from a two-fluid approach: one fluid representing the gas component and the other the stellar component. The researchers tested two versions of their model with differing prescriptions for determining the molecular gas fraction, based on either the hydrostatic pressure or the stellar surface density of the modelled disk. For high-mass galaxies such as the Milky Way, they found that both prescriptions were able to reproduce the observed SFE_HI. However, the hydrostatic prescription was the only prescription that was able to reproduce the observed SFE_HI for both low- and high-mass galaxies. The primary driver of the disk structure in the model is the amplitude of the rotational velocity, Vmax, while the specific angular momentum of the galaxy may play a role in explaining the weak correlation between SFE_HI and the effective surface brightness of the disk. The team was also able to reproduce the observed star formation properties of a proto-Milky Way at higher redshifts when the Universe was at its peak star formation period.

The success of their model at reproducing the observed star formation properties in galaxies at low and high redshifts suggests that star formation in galaxies is largely governed by the formation and stabilisation of their disks.

Publication details:

Ivy Wong et al. in The Monthly Notices of the Royal Astronomical Society (2016): “Characterising uniform star formation efficiencies with marginally-stable galactic disks