Towards universal hybrid star formation rate estimators
  To compute the SFR of galaxies from the rest-frame UV it is essential to take
into account the obscuration by dust. To do so, one of the most popular methods
consists in combining the UV with the emission from the dust itself in the IR.
Yet, different studies have derived different estimators, showing that no such
hybrid estimator is truly universal. In this paper we aim at understanding and
quantifying what physical processes drive the variations between different
hybrid estimators. Doing so, we aim at deriving new universal UV+IR hybrid
estimators to correct the UV for dust attenuation, taking into account the
intrinsic physical properties of galaxies. We use the CIGALE code to model the
spatially-resolved FUV to FIR SED of eight nearby star-forming galaxies drawn
from the KINGFISH sample. This allows us to determine their local physical
properties, and in particular their UV attenuation, average SFR, average
specific SFR (sSFR), and their stellar mass. We then examine how hybrid
estimators depend on said properties. We find that hybrid UV+IR estimators
strongly depend on the stellar mass surface density (in particular at 70 and
100 micron) and on the sSFR (in particular at 24 micron and the TIR).
Consequently, the IR scaling coefficients for UV obscuration can vary by almost
an order of magnitude. This result contrasts with other groups who found
relatively constant coefficients with small deviations. We exploit these
variations to construct a new class of hybrid estimators based on observed UV
to near-IR colours and near-IR luminosity densities per unit area. We find that
they can reliably be extended to entire galaxies. The new estimators provide
better estimates of attenuation-corrected UV emission than classical hybrid
estimators. Naturally taking into account the variable impact of dust heated by
old stellar populations, they constitute a step towards universal estimators.
