Stars form in giant molecular clouds, and feedback from massive stars within these clouds results in a star formation efficiency (SFE) of about 2%: only in the dense molecular cores does the SFE rise to about ~ 30%, as confirmed with the prestellar core mass function. In other words, there is a constant fraction of molecular gas turned into stars per free-fall timescale, as in fig. 17. A similar global SFE is observed in star-forming disk galaxies, in which the star formation rate SFR can be described by:
 |
(48) |
where ρgas is the gas density and tdyn is the dynamical time of the rotating disk.
 |
Figure 17. Observed star formation efficiency per dynamical time as a function of mean gas density. Each data point represents a different method of measuring the gas, which is sensitive to different densities. GMC indicates giant molecular clouds, IRDCs indicates infrared dark clouds, ONC is the Orion Nebula cluster, HCN represents extragalactic measurements. Figure from [94]. |
The reason for these relations resides in the gravitational instability of cold disks, while feedback physics, such as supernovae-driven turbulence, provides the observed efficiency of ~ 2% which reproduces the normalization, SFE, of the global star formation law in eq. 48. The star formation rate per unit area, ΣSFR, obeys the Kennicutt-Schmidt (KS) law, which can be expressed as:
 |
(49) |
where Σgas is the surface density of gas. Such a global
law applies also to starburst galaxies, as in fig. 18.
Figure 18. Star formation surface density
versus gas surface density per dynamical time. The slope of the solid
line represents the star formation efficiency SFE. Figure from
[95].
It is a remarkable coincidence that the SFE observed in giant molecular
clouds is similar to that seen globally in nearby (as well as in
distant) disk galaxies. Massive OB stars provide a common link, but
grain photoheating, winds and photo-ionization dominate in the former
case, and SNe in the latter case.
An investigation of the KS law reveals that the key ingredient that
regulates star formation is molecular gas, H2, with an
evident "knee" in the ΣSFR -
ΣHI+H2 distribution at the transition point
from a HI to an H2-dominated interstellar medium, as in
fig. 19.
Because of the saturation of ΣHI at ~
9M
Figure 19.
ΣSFR vs
ΣHI+H2. Diagonal dotted lines show lines of
constant star formation efficiency SFE. Figure from
[96].
Disk instabilities result in cloud formation and subsequent star
formation, and one needs to supply cold gas in order to maintain such a
cold disk. There is evidence for spiral galaxies to have reservoirs of
HI in their outer regions, for example in NGC 6946
[97]
and UGC 2082
[98],
pointing to recent gas accretion. In particular, the deep neutral
hydrogen survey HALOGAS with WSRT, presented in
[98],
has the goal of revealing the global characteristics of cold gas
accretion onto spiral galaxies in the local Universe. Recent examples of
extraplanar and HI gas reservoirs are
[99,
100,
101].
pc-2, this quantity as well as the total
ΣHI+H2 cannot be used to predict either
ΣSFR or the SFE in spiral galaxies
[96].
Indeed, in the outer parts of galaxies, where the molecular gas
H2 is reduced due to the UV radiation field and lower surface
density, the star formation rate per unit gas mass also declines.
