The obscuring starburst of NGC 6221 and implications for the hard X-ray background

We present NGC 6221 as a case study of "X-ray-loud composite galaxies," whi ch appear similar to starbursts at optical wavelengths and resemble traditi onal 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 i s similar to Seyfert 1 galaxies, exhibiting a power-law continuum spectrum, a broad Fe K alpha line, and continuum variability on timescales of days a nd years. High-resolution images reveal that the detected active nucleus is relatively weak, not only at optical, but also at near-infrared wavelength s. An obscuring starburst, in which the interstellar gas and dust associate d with the starburst conceal the active nucleus, accounts for these peculia r features. We demonstrate quantitatively that obscuration by column densit y N-H = 10(22) cm(-2) combined with relatively weak intrinsic nuclear activ ity can produce an optical spectrum that is characteristic of the surroundi ng starburst alone. While optical surveys would not identify the active nuc lei that make these galaxies significant X-ray sources, such galaxies may, in fact, be important contributors to the X-ray background.