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 35years 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 =2.00.1) and a flux on the order of 2.2x10(-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 (app, NW)=0.132 +/- 0.004 and beta (app, SE)=0.065 +/- 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 +/- 5 years. The estimated range of viewing angle (10 degrees< < 21) does not exclude a possible jet-like origin of the gamma -ray emission.