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


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 5935-5950

Subject Categories

Astronomy, Astrophysics and Cosmology


Geosciences, Multidisciplinary



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