Calibrating star-formation rate prescriptions at different scales (10 pc
  to 1 kpc) in M31
  We calibrate commonly used star formation rate (SFR) prescriptions using
observations in five kpc-sized fields in the nearby galaxy Andromeda (M31) at
10\,pc spatial resolution. Our observations at different scales enable us to
resolve the star-forming regions and to distinguish them from non star-forming
components. We use extinction corrected H$\alpha$ from optical integral field
spectroscopy as our reference tracer and have verified its reliability via
tests. It is used to calibrate monochromatic and hybrid (H$\alpha$+a$\times$IR
and FUV+b$\times$IR) SFR prescriptions, which use FUV (GALEX), 22\,$\mu$m
(WISE) and 24\,$\mu$m (MIPS). Additionally, we evaluate other multi-wavelength
infra-red tracers. Our results indicate that the SFR prescriptions do not
change (in M31) with spatial scales or with subtraction of the diffuse
component. For the calibration factors in the hybrid SFR prescriptions, we find
a$\approx$0.2 and b$\approx$22 in M31, which are a factor of 5 higher than in
the literature. As the fields in M31 exhibit high attenuation and low dust
temperatures, lie at large galacto-centric distances, and suffer from high
galactic inclination compared to measurements in other galaxies, we propose
that the fields probe a dust layer extended along the line of sight that is not
directly spatially associated with star-forming regions. This (vertically)
extended dust component increases the attenuation and alters the SFR
prescriptions in M31 compared to literature measurements. We recommend that SFR
prescriptions should be applied with caution at large galacto-centric distances
and in highly inclined galaxies, due to variations in the relative (vertical)
distribution of dust and gas.
