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arXiv · 1612.01996

Chandra Reveals Heavy Obscuration and Circumnuclear Star Formation in Seyfert 2 Galaxy NGC 4968

Stephanie M. LaMassa·Tahir Yaqoob·N. A. Levenson·Peter Boorman·Timothy M. Heckman·Poshak Gandhi·Jane R. Rigby·C. Megan Urry·Andrew F. Ptak
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Abstract

We present the Chandra imaging and spectral analysis of NGC 4968, a nearby (z = 0.00986) Seyfert 2 galaxy. We discover extended ($\sim$1 kpc) X-ray emission in the soft band (0.5 - 2 keV) that is neither coincident with the narrow line region nor the extended radio emission. Based on spectral modeling, it is linked to on-going star formation ($\sim$2.6-4 M$_{\sun}$ yr$^{-1}$). The soft emission at circumnuclear scales (inner $\sim$400 pc) originates from hot gas, with kT $\sim$ 0.7 keV, while the most extended thermal emission is cooler (kT $\sim$ 0.3 keV). We refine previous measurements of the extreme Fe K$\alpha$ equivalent width in this source (EW = 2.5$^{+2.6}_{-1.0}$ keV), which suggests the central engine is completely embedded within Compton-thick levels of obscuration. Using physically motivated models fit to the Chandra spectrum, we derive a Compton-thick column density ($N_{\rm H} > 1.25\times10^{24}$ cm$^{-2}$) and an intrinsic hard (2-10 keV) X-ray luminosity of $\sim$3-8$\times 10^{42}$ erg s$^{-1}$ (depending on the presumed geometry of the obscurer), which is over two orders of magnitude larger than that observed. The large Fe K$\alpha$ EW suggests a spherical covering geometry, which could be confirmed with X-ray measurements above 10 keV. NGC 4968 is similar to other active galaxies that exhibit extreme Fe K$\alpha$ EWs (i.e., $>$2 keV) in that they also contain ongoing star formation. This work supports the idea that gas associated with nuclear star formation may increase the covering factor of the enshrouding gas and play a role in obscuring AGN.

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Stephanie M. LaMassa, Tahir Yaqoob, N. A. Levenson, Peter Boorman, Timothy M. Heckman, Poshak Gandhi, Jane R. Rigby, C. Megan Urry, Andrew F. Ptak. 2016-12-06. Chandra Reveals Heavy Obscuration and Circumnuclear Star Formation in Seyfert 2 Galaxy NGC 4968. https://doi.org/10.3847/1538-4357%2F835%2F1%2F91

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