NGC 3894: a young radio galaxy seen by Fermi-LAT
  Context. According to radiative models, radio galaxies may produce gamma-ray
emission from the first stages of their evolution. However, very few such
galaxies have been detected by the Fermi Large Area Telescope (LAT) so far.
  Aims. NGC 3894 is a nearby (z = 0.0108) object that belongs to the class of
compact symmetric objects (CSOs, i.e., the most compact and youngest radio
galaxies), which is associated with a gamma-ray counterpart in the Fourth
Fermi-LAT source catalog. Here we present a study of the source in the
gamma-ray and radio bands aimed at investigating its high-energy emission and
assess its young nature.
  Methods. We analyzed 10.8 years of Fermi-LAT data between 100 MeV and 300 GeV
and determined the spectral and variability characteristics of the source.
Multi-epoch very long baseline array (VLBA) observations between 5 and 15 GHz
over a period of 35 years were used to study the radio morphology of NGC 3894
and its evolution.
  Results. NGC 3894 is detected in gamma-rays with a significance >9 sigma over
the full period, and no significant variability has been observed in the
gamma-ray flux on a yearly time-scale. The spectrum is modeled with a flat
power law ($\Gamma$ = 2.0$\pm$0.1) and a flux on the order of 2.2 $\times$
10$^{-9}$ ph cm$^{-2}$ s$^{-1}$. For the first time, the VLBA data allow us to
constrain with high precision the apparent velocity of the jet and counter-jet
side to be $\beta_{\mathrm{app,NW}}$ = 0.132$\pm$0.004 and
$\beta_{\mathrm{app,SE}}$ = 0.065$\pm$0.003, respectively.
  Conclusions. Fermi-LAT and VLBA results favor the youth scenario for the
inner structure of this object, with an estimated dynamical age of 59$\pm$5
years. The estimated range of viewing angle (10{\deg} < $\theta$ < 21{\deg})
does not exclude a possible jet-like origin of the gamma-ray emission.
