Fermi large area telescope detection of extended gamma-ray emission from the radio galaxy Fornax A

M. Ackermann, Deutsches Elektronen-Synchrotron (DESY)
M. Ajello, Clemson University
L. Baldini, Università di Pisa
J. Ballet, Astrophysique, Instrumentation et Modélisation de Paris-Saclay
G. Barbiellini, Istituto Nazionale di Fisica Nucleare, Sezione di Trieste
D. Bastieri, Istituto Nazionale Di Fisica Nucleare, Sezione di Padova
R. Bellazzini, Istituto Nazionale di Fisica Nucleare, Sezione di Pisa
E. Bissaldi, Istituto Nazionale di Fisica Nucleare, Sezione di Bari
R. D. Blandford, Kavli Institute for Particle Astrophysics and Cosmology
E. D. Bloom, Kavli Institute for Particle Astrophysics and Cosmology
R. Bonino, Istituto Nazionale di Fisica Nucleare, Sezione di Torino
T. J. Brandt, NASA Goddard Space Flight Center
J. Bregeon, Laboratoire Univers et Particules de Montpellier
P. Bruel, École polytechnique
R. Buehler, Deutsches Elektronen-Synchrotron (DESY)
S. Buson, NASA Goddard Space Flight Center
G. A. Caliandro, Kavli Institute for Particle Astrophysics and Cosmology
R. A. Cameron, Kavli Institute for Particle Astrophysics and Cosmology
M. Caragiulo, Istituto Nazionale di Fisica Nucleare, Sezione di Bari
P. A. Caraveo, INAF Istituto di Astrofisica Spaziale e Fisica Cosmica, Milan

Abstract

We report the Fermi Large Area Telescope detection of extended γ-ray emission from the lobes of the radio galaxy Fornax A using 6.1 years of Pass 8 data. After Centaurus A, this is now the second example of an extended γ-ray source attributed to a radio galaxy. Both an extended flat disk morphology and a morphology following the extended radio lobes were preferred over a point-source description, and the core contribution was constrained to be % of the total γ-ray flux. A preferred alignment of the γ-ray elongation with the radio lobes was demonstrated by rotating the radio lobes template. We found no significant evidence for variability on ∼0.5 year timescales. Taken together, these results strongly suggest a lobe origin for the γ-rays. With the extended nature of the γ-ray emission established, we model the source broadband emission considering currently available total lobe radio and millimeter flux measurements, as well as X-ray detections attributed to inverse Compton (IC) emission off the cosmic microwave background (CMB). Unlike the Centaurus A case, we find that a leptonic model involving IC scattering of CMB and extragalactic background light (EBL) photons underpredicts the γ-ray fluxes by factors of about ∼2-3, depending on the EBL model adopted. An additional γ-ray spectral component is thus required, and could be due to hadronic emission arising from proton-proton collisions of cosmic rays with thermal plasma within the radio lobes.