Herschel Exploitation of Local Galaxy Andromeda (HELGA) III: The Star
  Formation Law in M31
  We present a detailed study of how the Star Formation Rate (SFR) relates to
the interstellar medium (ISM) of M31 at ~140pc scales. The SFR is calculated
using the far-ultraviolet and 24um emission, corrected for the old stellar
population in M31. We find a global value for the SFR of 0.25+/-0.05Msun/yr and
compare this with the SFR found using the total far-infrared (FIR) luminosity.
There is general agreement in regions where young stars dominate the dust
heating. Atomic hydrogen (HI) and molecular gas (traced by carbon monoxide, CO)
or the dust mass is used to trace the total gas in the ISM. We show that the
global surface densities of SFR and gas mass place M31 amongst a set of low-SFR
galaxies in the plot of Kennicutt (1998b). The relationship between SFR and gas
surface density is tested in six radial annuli across M31, assuming a power law
relationship with index, N. The star formation law using total gas traced by HI
and CO gives a global index of N=2.03+/-0.04, with a significant variation with
radius; the highest values are observed in the 10kpc ring. We suggest that this
slope is due to HI turning molecular at ~10Msun/pc2. When looking at H2
regions, we measure a higher mean SFR suggesting a better spatial correlation
between H2 and SF. We find N~0.6 with consistent results throughout the disk -
this is at the low end of values found in previous work and argues against a
superlinear SF law on small scales.
