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Register a new userThe Compton-Thick Seyfert 2 Nucleus of NGC3281: Torus Constraints from the 9.7$mu$m Silicate Absorption
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Dinalva Sales Aires
Publication date
2011
fields
Physics
and research's language is
English
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We present mid infrared (Mid-IR) spectra of the Compton-thick Seyfert 2 galaxy NGC,3281, obtained with the Thermal-Region Camera Spectrograph (T-ReCS) at the Gemini South telescope. The spectra present a very deep silicate absorption at 9.7,$mu$m, and [S{sc,iv]},10.5,$mu$m and [Ne{sc,ii]},12.7,$mu$m ionic lines, but no evidence of PAH emission. We find that the nuclear optical extinction is in the range 24 $leq$ A$_{V}$ $leq$ 83,mag. A temperature T = 300,K was found for the black-body dust continuum component of the unresolved 65,pc nucleus and at 130,pc SE, while the region at 130,pc reveals a colder temperature (200,K). We describe the nuclear spectrum of NGC,3281 using a clumpy torus model that suggests that the nucleus of this galaxy hosts a dusty toroidal structure. According to this model, the ratio between the inner and outer radius of the torus in NGC,3281 is $R_0/R_d$ = 20, with {bf 14} clouds in the equatorial radius with optical depth of $tau_{V}$ = 40,mag. We would be looking in the direction of the torus equatorial radius ($i$ = {bf 60$^{circ}$}), which has outer radius of R$_{0},sim$ 11,pc. The column density is N$_{H}approx$,{bf 1.2},$times,10^{24},cm^{-2}$ and iron K$alpha$ equivalent width ($approx$ 0.5 - 1.2,keV) are used to check the torus geometry. Our findings indicate that the X-ray absorbing column density, which classifies NGC,3281 as a Compton-thick source, may also be responsible for the absorption at 9.7,$mu$m providing strong evidence that the silicate dust responsible for this absorption can be located in the AGN torus.
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The dusty torus plays a vital role in unifying active galactic nuclei (AGNs). However, the physical structure of the torus remains largely unclear. Here we present a systematical investigation of the torus mid-infrared (MIR) spectroscopic feature, i.e., the 9.7 um silicate line, of $175$ AGNs selected from the Swift/BAT Spectroscopic Survey (BASS). Our sample is constructed to ensure that each of the $175$ AGNs has Spizter/IRS MIR, optical, and X-ray spectroscopic coverage. Therefore, we can simultaneously measure the silicate strength, optical emission lines, and X-ray properties (e.g., the column density and the intrinsic X-ray luminosity). We show that, consistent with previous works, the silicate strength is weakly correlated with the hydrogen column density ($N_mathrm{H}^mathrm{X}$), albeit with large scatters. For X-ray unobscured AGNs, the silicate-strength-derived $V$-band extinction and the broad-H$alpha$-inferred one are both small; however, for X-ray obscured AGNs, the former is much larger than the latter. In addition, we find that the optical type 1 AGNs with strong X-ray absorption on average show significant silicate absorption, indicating that their X-ray absorption might not be caused by dust-free gas in the broad-line region. Our results suggest that the distribution and structure of the obscuring dusty torus are likely to be very complex. We test our results against the smooth and clumpy torus models and find evidence in favor of the clumpy torus model.
We explore the origin of mid-infrared (mid-IR) dust extinction in all 20 nearby (z < 0.05) bona-fide Compton-thick (N_H > 1.5 x 10^24 cm^-2) AGN with hard energy (E > 10 keV) X-ray spectral measurements. We accurately measure the silicate absorption features at lambda~9.7um in archival low-resolution (R~57-127) Spitzer Infrared Spectrograph (IRS) spectroscopy, and show that only a minority (~45%) of nearby Compton-thick AGN have strong Si-absorption features (S_9.7 = ln(f_{int}/f_{obs}) > 0.5) which would indicate significant dust attenuation. The majority (~60%) are star-formation dominated (AGN:SB<0.5) at mid-IR wavelengths and lack the spectral signatures of AGN activity at optical wavelengths, most likely because the AGN emission-lines are optically-extinguished. Those Compton-thick AGN hosted in low-inclination angle galaxies exhibit a narrow-range in Si-absorption (S_9.7 ~ 0-0.3), which is consistent with that predicted by clumpy-torus models. However, on the basis of the IR spectra and additional lines of evidence, we conclude that the dominant contribution to the observed mid-IR dust extinction is dust located in the host galaxy (i.e., due to disturbed morphologies; dust-lanes; galaxy inclination angles) and not necessarily a compact obscuring torus surrounding the central engine.
We present new NuSTAR and Chandra observations of NGC 3393, a galaxy reported to host the smallest separation dual AGN resolved in the X-rays. While past results suggested a 150 pc separation dual AGN, three times deeper Chandra imaging, combined with adaptive optics and radio imaging suggest a single, heavily obscured, radio-bright AGN. Using VLA and VLBA data, we find an AGN with a two-sided jet rather than a dual AGN and that the hard X-ray, UV, optical, NIR, and radio emission are all from a single point source with a radius <0.2. We find that the previously reported dual AGN is most likely a spurious detection resulting from the low number of X-ray counts (<160) at 6-7 keV and Gaussian smoothing of the data on scales much smaller than the PSF (0.25 vs. 0.80 FWHM). We show that statistical noise in a single Chandra PSF generates spurious dual peaks of the same separation (0.55$pm$0.07 vs. 0.6) and flux ratio (39$pm$9% vs. 32% of counts) as the purported dual AGN. With NuSTAR, we measure a Compton-thick source (NH=$2.2pm0.4times10^{24}$ cm$^{-2}$) with a large torus half-opening angle, {theta}=79 which we postulate results from feedback from strong radio jets. This AGN shows a 2-10 keV intrinsic to observed flux ratio of 150. Using simulations, we find that even the deepest Chandra observations would severely underestimate the intrinsic luminosity of NGC 3393 above z>0.2, but would detect an unobscured AGN of this luminosity out to high redshift (z=5).
Mid-infrared (MIR) spectra observed with Gemini/Michelle were used to study the nuclear region of the Compton-thick Seyfert 2 (Sy 2) galaxy Mrk 3 at a spatial resolution of $sim$200 pc. No polycyclic aromatic hydrocarbons (PAHs) emission bands were detected in the N-band spectrum of Mrk 3. However, intense [Ar III] 8.99 $mu$m, [S IV] 10.5 $mu$m and [Ne II] 12.8 $mu$m ionic emission-lines, as well as silicate absorption feature at 9.7$mu$m have been found in the nuclear extraction ($sim$200 pc). We also present subarcsecond-resolution Michelle N-band image of Mrk 3 which resolves its circumnuclear region. This diffuse MIR emission shows up as a wings towards East-West direction closely aligned with the S-shaped of the Narrow Line Region (NLR) observed at optical [O III]$lambda$5007AA image with Hubble/FOC. The nuclear continuum spectrum can be well represented by a theoretical torus spectral energy distribution (SED), suggesting that the nucleus of Mrk 3 may host a dusty toroidal structure predicted by the unified model of active galactic nucleus (AGN). In addition, the hydrogen column density (N$_H,=,4.8^{+3.3}_{-3.1}times,10^{23}$ cm$^{-2}$) estimated with a torus model for Mrk 3 is consistent with the value derived from X-ray spectroscopy. The torus model geometry of Mrk 3 is similar to that of NGC 3281, both Compton-thick galaxies, confirmed through fitting the 9.7$mu$m silicate band profile. This results might provide further evidence that the silicate-rich dust can be associated with the AGN torus and may also be responsible for the absorption observed at X-ray wavelengths in those galaxies.
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