J-PLUS: measuring ${\rm H}\alpha$ emission line fluxes in the nearby
  universe
  In the present paper we aim to validate a methodology designed to extract the
Halpha emission line flux from J-PLUS photometric data. J-PLUS is a multi
narrow-band filter survey carried out with the 2 deg2 field of view T80Cam
camera, mounted on the JAST/T80 telescope in the OAJ, Teruel, Spain. The
information of the twelve J-PLUS bands, including the J0660 narrow-band filter
located at rest-frame Halpha, is used over 42 deg2 to extract de-reddened and
[NII] decontaminated Halpha emission line fluxes of 46 star-forming regions
with previous SDSS and/or CALIFA spectroscopic information. The agreement of
the inferred J-PLUS photometric Halpha fluxes and those obtained with
spectroscopic data is remarkable, with a median comparison ratio R = 1.05 +-
0.25. This demonstrates that it is possible to retrieve reliable Halpha
emission line fluxes from J-PLUS photometric data. With an expected area of
thousands of square degrees upon completion, the J-PLUS dataset will allow the
study of several star formation science cases in the nearby universe, as the
spatially resolved star formation rate of nearby galaxies at z < 0.015, and how
it is influenced by the environment, morphology or nuclear activity. As an
illustrative example, the close pair of interacting galaxies NGC3994 and
NGC3995 is analyzed, finding an enhancement of the star formation rate not only
in the center, but also in outer parts of the disk of NGC3994.
