NGC 3894: a young radio galaxy seen by <i>Fermi</i>-LAT
G. Principe, Giulia Migliori, T. J. Johnson, F. D’Ammando, M. Giroletti, M. Orienti, C. Stanghellini, G. B. Taylor, E. Torresi, C. C. Cheung
Abstract
Context. According to radiative models, radio galaxies may produce γ -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 γ -ray counterpart in the Fourth Fermi -LAT source catalog. Here we present a study of the source in the γ -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 γ -rays with a significance >9 σ over the full period, and no significant variability has been observed in the γ -ray flux on a yearly time-scale. The spectrum is modeled with a flat power law (Γ = 2.0 ± 0.1) and a flux on the order of 2.2 × 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 β app, NW = 0.132 ± 0.004 and β 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° < θ < 21°) does not exclude a possible jet-like origin of the γ -ray emission.