Author:
Veilleux S.,Rupke D. S. N.,Kim D.-C.,Genzel R.,Sturm E.,Lutz D.,Contursi A.,Schweitzer M.,Tacconi L. J.,Netzer H.,Sternberg A.,Mihos J. C.,Baker A. J.,Mazzarella J. M.,Lord S.,Sanders D. B.,Stockton A.,Joseph R. D.,Barnes J. E.
Abstract
ABSTRACT
We report the results from a comprehensive study of 74 ultraluminous infrared
galaxies (ULIRGs) and 34 Palomar-Green (PG) quasars within z ∼
0.3 observed with the Spitzer Infrared Spectrograph (IRS). The
contribution of nuclear activity to the bolometric luminosity in these systems
is quantified using six independent methods that span a range in wavelength and
give consistent results within ∼±10%–15% on average. This agreement suggests
that deeply buried active galactic nuclei (AGNs) invisible to
Spitzer IRS but bright in the far-infrared are not common
in this sample. The average derived AGN contribution in ULIRGs is ∼35%–40%,
ranging from ∼15%–35% among “cool”
(f
25/f
60 ⩽ 0.2)
optically classified H ii-like and LINER ULIRGs to ∼50 and ∼75% among
warm Seyfert 2 and Seyfert 1 ULIRGs, respectively. This number exceeds ∼80% in
PG QSOs. ULIRGs fall in one of three distinct AGN classes: (1) objects with
small extinctions and large polycyclic aromatic hydrocarbon (PAH) equivalent
widths are highly starburst-dominated; (2) systems with large extinctions and
modest PAH equivalent widths have larger AGN contributions, but still tend to be
starburst-dominated; and (3) ULIRGs with both small extinctions and small PAH
equivalent widths host AGN that are at least as powerful as the starbursts. The
AGN contributions in class 2 ULIRGs are more uncertain than in the other
objects, and we cannot formally rule out the possibility that these objects
represent a physically distinct type of ULIRGs. A morphological trend is seen
along the sequence (1)–(2)–(3), in general agreement with the standard ULIRG−QSO
evolution scenario and suggestive of a broad peak in extinction during the
intermediate stages of merger evolution. However, the scatter in this sequence,
including the presence of a significant number of AGN-dominated systems prior to
coalescence and starburst-dominated but fully merged systems, implies that black
hole accretion, in addition to depending on the merger phase, also has a strong
chaotic/random component, as in local AGNs.
Publisher
American Astronomical Society
Subject
Space and Planetary Science,Astronomy and Astrophysics
Cited by
392 articles.
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