The miniJPAS survey: Identification and characterization of the emission
  line galaxies down to $z < 0.35$ in the AEGIS field
  The Javalambre-Physics of the Accelerating Universe Astrophysical Survey
(J-PAS) is expected to map thousands of square degrees of the northern sky with
56 narrowband filters in the upcoming years. This will make J-PAS a very
competitive and unbiased emission line survey compared to spectroscopic or
narrowband surveys with fewer filters. The miniJPAS survey covered 1 deg$^2$,
and it used the same photometric system as J-PAS, but the observations were
carried out with the pathfinder J-PAS camera. In this work, we identify and
characterize the sample of emission line galaxies (ELGs) from miniJPAS with a
redshift lower than $0.35$. Using a method based on artificial neural networks,
we detect the ELG population and measure the equivalent width and flux of the
$H\alpha$, $H\beta$, [OIII], and [NII] emission lines. We explore the
ionization mechanism using the diagrams [OIII]/H$\beta$ versus [NII]/H$\alpha$
(BPT) and EW(H$\alpha$) versus [NII]/H$\alpha$ (WHAN). We identify 1787 ELGs
($83$%) from the parent sample (2154 galaxies) in the AEGIS field. For the
galaxies with reliable EW values that can be placed in the WHAN diagram (2000
galaxies in total), we obtained that $72.8 \pm 0.4$%, $17.7 \pm 0.4$% , and
$9.4 \pm 0.2$% are star-forming (SF), active galactic nucleus (Seyfert), and
quiescent galaxies, respectively. Based on the flux of $H\alpha$ we find that
the star formation main sequence is described as $\log$ SFR $[M_\mathrm{\odot}
\mathrm{yr}^{-1}] = 0.90^{+ 0.02}_{-0.02} \log M_{\star} [M_\mathrm{\odot}]
-8.85^{+ 0.19}_{-0.20}$ and has an intrinsic scatter of $0.20^{+
0.01}_{-0.01}$. The cosmic evolution of the SFR density ($\rho_{\text{SFR}}$)
is derived at three redshift bins: $0 < z \leq 0.15$, $0.15 < z \leq 0.25$, and
$0.25 < z \leq 0.35$, which agrees with previous results that were based on
measurements of the $H\alpha$ emission line.
