A Chandra Survey of the Nearest ULIRGs: Obscured AGN or Super-Starbursts?

Astrophysics, abstract
astro-ph/0304222

From: Andrew Ptak <ptak@pha.jhu.edu>

Date: Fri, 11 Apr 2003 18:08:15 GMT   (473kb)

Authors:
A. Ptak (1),
T. Heckman (1),
N. A. Levenson (2),
K. Weaver (1 and 3),
D. Strickland (1) ((1) Johns Hopkins University, (2) University of Kentucky, (3) NASA/GSFC)

Comments: 22 pages, 8 figures, formatted with emulateapj.sty, accepted for
publication in August 2003 ApJ

We present initial results from a Chandra survey of a complete sample of the
8 nearest (z <= 0.04) ultraluminous IR galaxies (ULIRGs), and also include the
IR-luminous galaxy NGC 6240 for comparison. In this paper we use the hard
X-rays (2-8 kev) to search for the possible presence of an obscured AGN. In
every case, a hard X-ray source is detected in the nuclear region. If we divide
the sample according to the optical/IR spectroscopic classification (starburst
vs. AGN), we find that the 5 “starburst” ULIRGs have hard X-ray luminosities
about an order-of-magnitude smaller than the 3 “AGN” ULIRGs. NGC 6240 has an
anomalously high hard X-ray luminosity compared to the “starburst” ULIRGs.
The Fe Kalpha line is convincingly detected in only two ULIRGs. The weakness of
the Fe-K emission in these ULIRGs generally suggests that the hard X-ray
spectrum is not dominated by reflection from high N_H neutral material. The
hard X-ray continuum flux ranges from a few X 10^3 to a few X 10^-5 of the
far-IR flux, similar to values in pure starbursts, and several
orders-of-magnitude smaller than in Compton-thin AGN. The upper limits on the
ratio of the Fe Kalpha to far-IR flux are below the values measured in
Compton-thick type 2 Seyfert galaxies. While very large column densities of
molecular gas are observed in the nuclei of these galaxies, we find no evidence
that the observed X-ray sources are obscured by Compton-thick material. Thus,
our new hard X-ray data do not provide direct evidence that powerful “buried
quasars” dominate the overall energetics of most ultraluminous infrared
galaxies.

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