Seyfert 2 galaxies in the GeV band: jets and starburst

TL;DR

This study reports the first detection of high energy gamma-ray emission from a Seyfert 2 galaxy, NGC 1068, revealing active nucleus contribution and opening new avenues for understanding gamma-ray sources.

Contribution

The paper provides the first evidence of gamma-ray emission from a Seyfert 2 galaxy, distinguishing between starburst and active nucleus contributions in high energy emission.

Findings

Detected gamma-ray excess in NGC 1068 and NGC 4945 with high significance.

NGC 4945's emission aligns with starburst activity, NGC 1068's suggests active nucleus dominance.

A leptonic model explains NGC 1068's spectral energy distribution.

Abstract

The Fermi/LAT collaboration recently reported the detection of starburt galaxies in the high energy gamma-ray domain, as well as radio-loud narrow-line Seyfert 1 objects, which were previously unusual extragalactic suspects for high energy emission. Motivated by the presence of sources close to the location of composite starburst/Seyfert 2 galaxies in the first year Fermi/LAT catalogue, we studied high energy gamma-ray emission from such objects, aiming at disentangling the emission of starburst and Seyfert activity. We analysed 1.6 years of Fermi/LAT data from NGC 1068 and NGC 4945, which count among the brightest Seyfert 2 galaxies. We found an excess of high energy gamma-rays of 8.3 sigmas and 9.2 sigmas for 1FGL J0242.7+0007 and 1FGL J1305.4-4928, which are found to be consistent with the position of the Seyfert 2 galaxies NGC 1068 and NGC 4945, respectively. While the high energy flux of NGC 4945 is consistent with starburst activity, that of NGC 1068 is an order of magnitude above expectations, suggesting dominant emission from the active nucleus. We show that a leptonic scenario can account for the multi-wavelength spectral energy distribution of NGC 1068. High energy gamma-ray emission is thus revealed for the first time in a Seyfert 2 galaxy. If this result is confirmed in other objects, new perspectives would be opened up into the GeV band, with the discovery of a new class of high energy gamma-ray emitters.