Volume 635, March 2020
|Number of page(s)||6|
|Published online||01 April 2020|
NGC 3894: a young radio galaxy seen by Fermi-LAT
INAF – Istituto di Radioastronomia, Bologna, Italy
2 Dip. di Fisica e Astronomia, Università di Bologna, Bologna, Italy
3 College of Science, George Mason University, Resident at Naval Research Laboratory, Washington DC, USA
4 University of New Mexico, Albuquerque, NM, USA
5 INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Bologna, Italy
6 Naval Research Laboratory, Space Science Division, Washington DC, USA
Accepted: 28 February 2020
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 >9σ 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 βapp, NW = 0.132 ± 0.004 and βapp, 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.
Key words: Galaxy: evolution / galaxies: nuclei / galaxies: general / galaxies: jets / radio continuum: galaxies / gamma-rays: galaxies
© ESO 2020
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.