We present a detailed, photoionization modeling analysis of XMM-Newton/Reflection Grating Spectrometer observations of the Seyfert 2 galaxy NGC 1068. The spectrum, previously analyzed by Kinkhabwala et al. (2002), reveals a myriad of soft-Xray emissi
on lines, including those from H- and He-like carbon, nitrogen, oxygen, and neon, and M- and L-shell iron. As noted in the earlier analysis, based on the narrowness of the radiative recombination continua, the electron temperatures in the emission-line gas are consistent with photoionization, rather than collisional ionization. The strengths of the carbon and nitrogen emission lines, relative to those of oxygen, suggest unusual elemental abundances, which we attribute to star-formation history of the host galaxy. Overall, the emission-lines are blue-shifted with respect to systemic, with radial velocities ~ 160 km/s, similar to that of [O III] 5007, and thus consistent with the kinematics and orientation of the optical emission-line gas and, hence, likely part of an AGN-driven outflow. We were able to achieve an acceptable fit to most of the strong emission-lines with a two-component photoionization model, generated with Cloudy. The two components have ionization parameters and column densities of logU = -0.05 and 1.22, and logN(H) = 20.85 and 21.2, and covering factors of 0.35 and 0.84, respectively. The total mass of the X-ray gas is roughly of an order of magnitude greater than the mass of ionized gas determined from optical and near-IR spectroscopy, which indicates that it may be the dominant component of the narrow line region. Furthermore, we suggest that the medium which produces the scattered/polarized optical emission in NGC~1068 possesses similar physical characteristics to those of the more highly-ionized of the X-ray model components.
We present a study of reddening and absorption towards the Narrow Line Regions (NLR) in active galactic nuclei (AGN) selected from the Revised Shapley-Ames, 12mu, and Swift/Burst Alert Telescope samples. For the sources in host galaxies with inclinat
ions of b/a > 0.5, we find that mean ratio of [O III] 5007A, from ground-based observations, and [O IV] 28.59mu, from Spitzer/Infrared Spectrograph observations, is a factor of 2 lower in Seyfert 2s than Seyfert 1s. The combination of low [O III]/[O IV] and [O III] 4363/5007 ratios in Seyfert 2s suggests more extinction of emission from the NLR than in Seyfert 1s. Similar column densities of dusty gas, NH ~ several X 10^21 cm^-2, can account for the suppression of both [O III] 5007A and [O III] 4363A, as compared to those observed in Seyfert 1s. Also, we find that the X-ray line OVII 22.1A is weaker in Seyfert 2s, consistent with absorption by the same gas that reddens the optical emission. Using a Hubble Space Telescope/Space Telescope Imaging Spectrograph slitless spectrum of the Seyfert 1 galaxy NGC 4151, we estimate that only ~ 30% of the [O III] 5007A comes from within 30 pc of the central source, which is insufficient to account for the low [O III]/[OIV] ratios in Seyfert 2s. If Seyfert 2 galaxies have similar intrinsic [OIII] spatial profiles, the external dusty gas must extend further out along the NLR, perhaps in the form of nuclear dust spirals that have been associated with fueling flows towards the AGN.
There are several key open questions as to the nature and origin of AGN including: 1) what initiates the active phase, 2) the duration of the active phase, and 3) the effect of the AGN on the host galaxy. Critical new insights to these can be achieve
d by probing the central regions of AGN with sub-mas angular resolution at UV/optical wavelengths. In particular, such observations would enable us to constrain the energetics of the AGN feedback mechanism, which is critical for understanding the role of AGN in galaxy formation and evolution. These observations can only be obtained by long-baseline interferometers or sparse aperture telescopes in space, since the aperture diameters required are in excess of 500 m - a regime in which monolithic or segmented designs are not and will not be feasible and because these observations require the detection of faint emission near the bright unresolved continuum source, which is impossible from the ground, even with adaptive optics. Two mission concepts which could provide these invaluable observations are NASAs Stellar Imager (SI; Carpenter et al. 2008 & http://hires.gsfc.nasa.gov/si/) interferometer and ESAs Luciola (Labeyrie 2008) sparse aperture hypertelescope.
We have examined the physical conditions within a bright emission-line knot in the inner narrow-line region (NLR) of the Seyfert 2 galaxy Mrk 573 using optical spectra and photoionization models. The spectra were obtained with the Hubble Space Telesc
ope/Space Telescope Imaging Spectrograph with the G430L and G750M gratings. Comparing the spatial emission-line profiles, we found [Fe X] 6734 barely resolved, [O III] 5007 centrally peaked, but broader than [Fe X], and [O II] 3727 the most extended. The spectra reveal that [Fe X] is broader in velocity width and blue-shifted compared with lines from less ionized species. Our estimate of the bolometric luminosity indicates that the active galactic nucleus (AGN) is radiating at or above its Eddington Luminosity, which is consistent with its identification as a hidden Narrow-Line Seyfert 1. We were able to successfully match the observed emission line ratios with a three-component photoionization model. Two components, one to account for the [O III] emission and another in which the [Fe X] arises, are directly ionized by the AGN, while [O II] forms in a third component, which is ionized by a heavily absorbed continuum. Based on our assumed ionizing continuum and the model parameters, we determined that the two directly-ionized components are ~ 55 pc from the AGN. We have found similar radial distances for the central knots in the Seyfert 2 galaxies Mrk 3 and NGC 1068, but much smaller radial distances for the inner NLR in the Seyfert 1 galaxies NGC 4151 and NGC 5548. Although in general agreement with the unified model, these results suggest that the obscuring material in Seyfert galaxies extends out to at least tens of parsecs from the AGN.
We present a study of the distribution of [O III] $lambda$5007 and [O II] $lambda$3727 emission in the Narrow Line Region (NLR) of the Seyfert 1 galaxy NGC 4151. While the NLR of NGC 4151 exhibits an overall structure consistent with the unified mode
l of Seyfert galaxies, narrow-band [O III] and [O II] images obtained with the Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope reveal significant emission from outside the the emission-line bi-cone. The [O III]/[O II] ratios are lower in these regions, consistent with a weaker ionizing flux. We performed a photoionization modeling analysis of the emission-line gas within a series of annuli, centered on the the central continuum source, with inner radii from 13 to 90 pc. The gas is ionized by radiation that has been attenuated by a relatively highly-ionized absorber (HABS), which completely covers the central source, and a lower-ionization absorber (LABS), which has a covering factor ranging from 0 to 1. We found that the [O III]/[O II] ratios are well fit by assuming that, within each segment of an annulus, some fraction of the NLR gas is completely within the shadow of LABS, while the rest is irradiated by the continuum filtered only by HABS. This suggests that the structure of the NLR is due to filtering of the ionizing radiation by ionized gas, consistent with disk-wind models. One possible scenario is that the low-ionization absorbers are dense knots of gas swept up by a wind.
We present a photoionization model study of the effects of micro-turbulence and dissipative heating on emission lines for number and column densities, elemental abundances, and ionizations typical for the narrow emission line regions (NLRs) of Seyfer
t galaxies. Earlier studies of NLR spectra generally found good agreement between the observations and the model predictions for most strong emission lines, such as [O III] $lambda$5007, [O II] $lambda$3727, [N II] $lambda$6583, [Ne III] $lambda$3869, and the H and He recombination lines. Nevertheless, the strengths of lines from species with ionization potentials greater than that of He$^{+}$(54.4 eV), e.g. N$^{+4}$ and Ne$^{+4}$, were often under-predicted. Among the explanations suggested for these discrepancies were (selectively) enhanced elemental abundances and contributions from shock heated gas. Interestingly, the NLR lines have widths of several 100 km s$^{-1}$, well in excess of the thermal broadening. If this is due to micro-turbulence, and the turbulence dissipates within the emission-line gas, the gas can be heated in excess of that due to photoionization. We show that the combined effects of turbulence and dissipative heating can strongly enhance N V $lambda$1240 (relative to He II $lambda$1640), while the heating alone can boost the strength of [Ne V] $lambda$3426. We suggest that this effect is present in the NLR, particularly within $sim$ 100 pc of the central engine. Finally, since micro-turbulence would make clouds robust against instabilities generated during acceleration, it is not likely to be a coincidence that the radially outflowing emission-line gas is turbulent.
