The Obscuring Starburst of NGC 6221 and Implications for the Hard X-ray Background

Details The Obscuring Starburst of NGC 6221 and Implications for the Hard X-ray Background The Obscuring Starburst of NGC 6221 and Implications for the Hard X-ray Background

2001-04-18

arxiv.org/abs/astro-ph/0104316v1

Astrophysical Journal, 557, 54 (2001)

Astronomy and Astrophysics

Astrophysics

13 pages including 10 figures and 3 tables; to appear in the ApJ, Aug. 10, 2001

Scientific paper

10.1086/321656

We present NGC 6221 as a case study of ``X-ray loud composite galaxies,'' which appear similar to starbursts at optical wavelengths and resemble traditional active galactic nuclei in X-rays. The net optical spectrum of NGC 6221 is properly characterized as a starburst galaxy, but in X-rays, NGC 6221 is similar to Seyfert 1 galaxies, exhibiting a power-law continuum spectrum, a broad Fe K alpha line, and continuum variability on timescales of days and years. High-resolution images reveal that the detected active nucleus is relatively weak, not only at optical, but also at near-infrared wavelengths. An obscuring starburst, in which the interstellar gas and dust associated with the starburst conceal the active nucleus, accounts for these peculiar features. We demonstrate quantitatively that obscuration by column density N_H = 10^{22} cm^{-2} combined with relatively weak intrinsic nuclear activity can produce an optical spectrum characteristic of the surrounding starburst alone. While optical surveys would not identify the active nuclei that make these galaxies significant X-ray sources, such galaxies may in fact be important contributors to the X-ray background.

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