A hard X-ray view of luminous and ultra-luminous infrared galaxies in GOALS - I. AGN obscuration along the merger sequence
Journal article, 2021
The merger of two or more galaxies can enhance the inflow of material from galactic scales into the close environments of active galactic nuclei (AGNs), obscuring and feeding the supermassive black hole (SMBH). Both recent simulations and observations of AGN in mergers have confirmed that mergers are related to strong nuclear obscuration. However, it is still unclear how AGN obscuration evolves in the last phases of the merger process. We study a sample of 60 luminous and ultra-luminous IR galaxies (U/LIRGs) from the GOALS sample observed by NuSTAR. We find that the fraction of AGNs that are Compton thick (CT;N-H >= 10(24)cm(-2) ) peaks at at a late merger stage, prior to coalescence, when the nuclei have projected separations (d(sep)) of 0.4-6 kpc. A similar peak is also observed in the median N-H [[(1.6 +/- 0.5) x 10(24) cm(-2)].]. The vast majority (85(-9)(+7) per cent)) of the AGNs in the final merger stages (d(sep) less than or similar to 10 kpc) are heavily obscured (N-H = 10(23) cm(-2)), and the median N-H of the accreting SMBHs in our sample is systematically higher than that of local hard X-ray-selected AGN, regardless of the merger stage. This implies that these objects have very obscured nuclear environments, with the gas almost completely covering the AGN in late mergers. CT AGNs tend to have systematically higher absorption-corrected X-ray luminosities than less obscured sources. This could either be due to an evolutionary effect, with more obscured sources accreting more rapidly because they have more gas available in their surroundings, or to a selection bias. The latter scenario would imply that we are still missing a large fraction of heavily obscured, lower luminosity (L2-10 less than or similar to 10(43) erg s(-1)) AGNs in U/LIRGs.
galaxies: active
quasars: general
galaxies: Seyfert
infrared: galaxies
X-rays: general
Author
C. Ricci
Beijing University of Technology
Diego Portales University
G. C. Privon
National Radio Astronomy Observatory
R. W. Pfeifle
George Mason University
L. Armus
California Institute of Technology (Caltech)
K. Iwasawa
University of Barcelona
Catalan Institution for Research and Advanced Studies
N. Torres-Alba
Clemson University
S. Satyapal
George Mason University
F. E. Bauer
Space Science Institute
Pontificia Universidad Catolica de Chile
Millennium Institute of Astrophysics
E. Treister
Pontificia Universidad Catolica de Chile
L. C. Ho
Beijing University of Technology
Susanne Aalto
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
P. Arevalo
University of Valparaíso
L. Barcos-Munoz
National Radio Astronomy Observatory
V Charmandaris
Foundation for Research and Technology Hellas (FORTH)
University of Crete
T. Diaz-Santos
Foundation for Research and Technology Hellas (FORTH)
University of Crete
A. S. Evans
National Radio Astronomy Observatory
University of Virginia
T. Gao
Beijing Normal University
H. Inami
Hiroshima University
M. J. Koss
Eureka Scientific
G. Lansbury
European Southern Observatory Santiago
S. T. Linden
University of Massachusetts
A. Medling
ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D)
University of Toledo
D. B. Sanders
University of Hawaii
Y. Song
University of Virginia
D. Stern
California Institute of Technology (Caltech)
Y. Ueda
Kyoto University
S. Yamada
Kyoto University
Monthly Notices of the Royal Astronomical Society
0035-8711 (ISSN) 1365-2966 (eISSN)
Vol. 506 4 5935-5950Subject Categories
Astronomy, Astrophysics and Cosmology
Geology
Geosciences, Multidisciplinary
DOI
10.1093/mnras/stab2052