Extragalactic magnetism with SOFIA (SALSA Legacy Program) -- IV: Program
  overview and first results on the polarization fraction
  We present the first data release of the Survey on extragALactic magnetiSm
with SOFIA (SALSA Legacy Program) with a set of 14 nearby ($<20$ Mpc) galaxies
with resolved imaging polarimetric observations using HAWC+ from $53$ to $214$
$\mu$m at a resolution of $5-18$" ($90$ pc $-$ $1$ kpc). We introduce the
definitions and background on extragalactic magnetism, and present the
scientific motivation and sample selection of the program. Here, we focus on
the general trends in the emissive polarization fraction. Far-infrared
polarimetric observations trace the thermal polarized emission of magnetically
aligned dust grains across the galaxy disks with polarization fractions of
$P=0-15$% in the cold, $T_{d} = [19,48]$ K, and dense, $\log_{10}(N_{HI+H_{2}})
= [19.96,22.91]$, interstellar medium. The spiral galaxies show a median
$\langle P_{154\mu m} \rangle = 3.3\pm0.9 $% across the disks. We report the
first polarized spectrum of starburst galaxies showing a minimum within
$89-154$ $\mu$m. The falling $53-154$ $\mu$m polarized spectrum may be due to a
decrease in the dust grain alignment efficiency produced by variations in dust
temperatures along the line-of-sight in the galactic outflow. We find that the
starburst galaxies and the star-forming regions within normal galaxies have the
lowest polarization fractions. We find that 50% (7 out of 14) of the galaxies
require a broken power-law in the $P-N_{HI+H_{2}}$ and $P-T_{d}$ relations with
three different trends. Group 1 has a relative increase of anisotropic random
B-fields produced by compression or shear of B-fields in the galactic outflows,
starburst rings, and inner-bar of galaxies; and Groups 2 and 3 have a relative
increase of isotropic random B-fields driven by star-forming regions in the
spiral arms, and/or an increase of dust grain alignment efficiency caused by
shock-driven regions or evolutionary stages of a galaxy.
