A Compton-thick AGN in the barred spiral galaxy NGC 4785

Gandhi, P., Yamada, S., Ricci, C., Asmus, D., Mushotzky, R.F., Ueda, Y., Terashima, Y. and La Parola, V. (2015) A Compton-thick AGN in the barred spiral galaxy NGC 4785. Monthly Notices of the Royal Astronomical Society, 449 (2), 1845-1855. (doi:10.1093/mnras/stv344).

Abstract

We present X-ray observations of the active galactic nucleus (AGN) in NGC 4785. The source is a local Seyfert 2 which has not been studied so far in much detail. It was recently detected with high significance in the 15–60?keV band in the 66-month Swift/BAT (Burst Array Telescope) all sky survey, but there have been no prior pointed X-ray observations of this object. With Suzaku, we clearly detect the source below 10?keV, and find it to have a flat continuum and prominent neutral iron fluorescence line with equivalent width ?1?keV. Fitting the broad-band spectra with physical reflection models shows the source to be a Compton-thick AGN with NH of at least 2 × 1024?cm?2 and absorption-corrected 2–10?keV X-ray power L2?10 ? few times 1042 erg s?1. Realistic uncertainties on L2?10 computed from the joint confidence interval on the intrinsic power-law continuum photon index and normalization are at least a factor of 10. The local bona fide Compton-thick AGN population is highly heterogeneous in terms of WISE mid-infrared source colours, and the nucleus of NGC 4785 appears especially sub-dominant in the mid-infrared when comparing to other Compton-thick AGN. Such sources would not be easily found using mid-infrared selection alone. The extent of host galaxy extinction to the nucleus is not clear, though NGC 4785 shows a complex core with a double bar and inner disc, adding to the list of known Compton-thick AGN in barred host galaxies.

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Contributors

Author: P. Gandhi

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