J-PLUS: The Javalambre Photometric Local Universe Survey
  J-PLUS is an ongoing 12-band photometric optical survey, observing thousands
of square degrees of the Northern hemisphere from the dedicated JAST/T80
telescope at the Observatorio Astrof\'isico de Javalambre. T80Cam is a 2 sq.deg
field-of-view camera mounted on this 83cm-diameter telescope, and is equipped
with a unique system of filters spanning the entire optical range. This filter
system is a combination of broad, medium and narrow-band filters, optimally
designed to extract the rest-frame spectral features (the 3700-4000\AA\ Balmer
break region, H$\delta$, Ca H+K, the G-band, the Mgb and Ca triplets) that are
key to both characterize stellar types and to deliver a low-resolution
photo-spectrum for each pixel of the sky observed. With a typical depth of AB
$\sim 21.25$ mag per band, this filter set thus allows for an indiscriminate
and accurate characterization of the stellar population in our Galaxy, it
provides an unprecedented 2D photo-spectral information for all resolved
galaxies in the local universe, as well as accurate photo-z estimates
($\Delta\,z\sim 0.01-0.03$) for moderately bright (up to $r\sim 20$ mag)
extragalactic sources. While some narrow band filters are designed for the
study of particular emission features ([OII]/$\lambda$3727,
H$\alpha$/$\lambda$6563) up to $z < 0.015$, they also provide well-defined
windows for the analysis of other emission lines at higher redshifts. As a
result, J-PLUS has the potential to contribute to a wide range of fields in
Astrophysics, both in the nearby universe (Milky Way, 2D IFU-like studies,
stellar populations of nearby and moderate redshift galaxies, clusters of
galaxies) and at high redshifts (ELGs at $z\approx 0.77, 2.2$ and $4.4$, QSOs,
etc). With this paper, we release $\sim 36$ sq.deg of J-PLUS data, containing
about $1.5\times 10^5$ stars and $10^5$ galaxies at $r<21$ mag.
