2001/04/18 by N. A. Levenson, R. Cid Fernandes, Jr., R. Cid Fernandes +6 · 27 citations
Physics and Astronomy · #Active galactic nucleus #Astronomy #Astrophysical Phenomena and Observations #Astrophysics #Astrophysics and Cosmic Phenomena #Galaxies: Formation, Evolution, Phenomena #Galaxy #Infrared #Interstellar medium #Luminous infrared galaxy #Physics #Wavelength #X-ray #astro-ph
paper · pdf · open access · doi:10.1086/321656
published in The Astrophysical Journal 557(1), 54-66 (IOP Publishing) · 13 pages including 10 figures and 3 tables; to appear in the ApJ, Aug. 10, 2001
arxiv created 2001/04/18 · openalex publication_date 2001/08/10 · arxiv updated 2012/08/27 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
We present NGC 6221 as a case study of X-rayÈloud composite galaxies,ÏÏ 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 Ka 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 cm~2 combined with relatively weak intrinsic nuclear activity can produce an optical spec-NH 1022trum that is characteristic of the surrounding starburst alone. While optical surveys would not identify the active nuclei that make these galaxies signiÐcant X-ray sources, such galaxies may, in fact, be impor-tant contributors to the X-ray background.