J-PLUS: Uncovering a large population of extreme [OIII] emitters in the
  local Universe
  Over the past decades, several studies have discovered a population of
galaxies undergoing very strong star formation events, called extreme emission
line galaxies (EELGs). In this work, we exploit the capabilities of the
Javalambre Photometric Local Universe Survey (J-PLUS), a wide field multifilter
survey, with 2000 square degrees observed. We use it to identify EELGs at low
redshift by their [OIII]5007 emission line. We intend to provide with a more
complete, deep, and less biased sample of local EELGs. We select objects with
an excess of flux in the J-PLUS mediumband $J0515$ filter, which covers the
[OIII] line at z$<$0.06. We remove contaminants (stars and higher redshift
systems) using J-PLUS and WISE infrared data, with SDSS spectra as a benchmark.
We perform spectral energy distribution fitting to estimate the properties of
the galaxies: line fluxes, equivalent widths (EWs), masses, etc. We identify
466 EELGs at ${\rm z} < 0.06$ with [OIII] EW over 300 \text{\AA} and $r$-band
mag. below 20, of which 411 were previously unknown. Most show compact
morphologies, low stellar masses ($\log (M_{\star}/M_{\odot}) \sim
{8.13}^{+0.61}_{-0.58}$), low dust extinction
($E(B-V)\sim{0.1}^{+0.2}_{-0.1}$), and very young bursts of star formation
(${3.0}^{+2.7}_{-2.0}$ Myr). Our method is up to $\sim$ 20 times more efficient
detecting EELGs per Mpc$^3$ than broadband surveys, and as complete as
magnitude-limited spectroscopic surveys (and reaching fainter objects). The
sample is not directly biased against strong H$\alpha$ emitters, in contrast
with broadband surveys. We demonstrate the capability of J-PLUS to identify,
following a clear selection process, a large sample of previously unknown EELGs
showing unique properties. A fraction of them are likely similar to the first
galaxies in the Universe, but at a much lower redshift, which makes them ideal
targets for follow-up studies.
