We present an analysis of the host properties of 85224 emission-line galaxies selected from the Sloan Digital Sky Survey. We show that Seyferts and LINERs form clearly separated branches on the standard optical diagnostic diagrams. We derive a new empirical classification scheme which cleanly separates star-forming galaxies, composite AGN-H ii galaxies, Seyferts and LINERs and we study the host galaxy properties of these different classes of objects. LINERs are older, more massive, less dusty and more concentrated, and they and have higher velocity dispersions and lower [OIII] luminosities than Seyfert galaxies. Seyferts and LINERs are most strongly distinguished by their [OIII] luminosities. We then consider the quantity L[OIII]/σ 4 , which is an indicator of the black hole accretion rate relative to the Eddington rate. Remarkably, we find that at fixed L[OIII]/σ 4 , all differences between Seyfert and LINER host properties disappear. LINERs and Seyferts form a continuous sequence, with LINERs dominant at low L/LEDD and Seyferts dominant at high L/LEDD . These results suggest that the majority of LINERs are AGN and that the Seyfert/LINER dichotomy is analogous to the high/low-state transition for X-ray binary systems. We apply theoretical photo-ionization models and show that pure LINERs require a harder ionizing radiation field with lower ionization parameter than Seyfert galaxies, consistent with the low and high X-ray binary states.

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The host galaxies and classification of active galactic nuclei

We have obtained moderate resolution (R = few thousand) spectra of the Na I λλ5890, 5896 (Na D) absorption line in a sample of 32 far-IR-bright starburst galaxies. In 18 cases, the Na D line in the nucleus is produced primarily by interstellar gas, while cool stars contribute significantly in the others. In 12 of the 18 "interstellar-dominated" cases the Na D line is blueshifted by over 100 km s-1 relative to the galaxy systemic velocity (the "outflow sources"), while no case shows a net redshift of more than 100 km s-1. The absorption-line profiles in these outflow sources span the range from near the galaxy systemic velocity to a maximum blueshift of ~400-600 km s-1. The outflow sources are galaxies systematically viewed more nearly face-on than the others. We therefore argue that the absorbing material consists of ambient interstellar material that has been entrained and accelerated along the minor axis of the galaxy by a hot starburst-driven superwind. The Na D lines are optically thick, but indirect arguments imply total hydrogen column densities of NH ~ few × 1021 cm-2. This implies that the superwind is expelling matter at a rate comparable to the star formation rate. This outflowing material is evidently very dusty: we find a strong correlation between the depth of the Na D profile and the line-of-sight reddening. Typical implied values are E(B-V) = 0.3-1 over regions several-to-10 kpc in size. We briefly consider some of the potential implications of these observations. The estimated terminal velocities of superwinds inferred from the present data and extant X-ray data are typically 400-800 km-1, are independent of the galaxy rotation speed, and are comparable to (substantially exceed) the escape velocities for L* (dwarf) galaxies. The resulting selective loss of metals from shallower potential wells can establish the mass-metallicity relation in spheroids, produce the observed metallicity in the intracluster medium, and enrich a general IGM to of order 10-1 solar metallicity. If the outflowing dust grains can survive their journey into the IGM, their effect on observations of cosmologically distant objects would be significant.

Absorption-Line Probes of Gas and Dust in Galactic Superwinds

We discuss moderate resolution spectra of the NaD absorption-line in a sample of 32 far-IR-bright starburst galaxies. In 18 cases, the line is produced primarily by interstellar gas, and in 12 of these it is blueshifted by over 100 km/s relative to the galaxy systemic velocity. The absorption-line profiles in these outflow sources span the range from near the galaxy systemic velocity to a maximum blueshift of 400 to 600 km/s. The outflows occur in galaxies systematically viewed more nearly face-on than the others. We therefore argue that the absorbing material consists of ambient interstellar gas accelerated along the minor axis of the galaxy by a hot starburst-driven superwind. The NaD lines are optically-thick, but indirect arguments imply total Hydrogen column densities of N_H = few X 10^{21} cm^{-2}. This implies that the superwind is expelling matter at a rate comparable to the star-formation rate. This outflowing material is very dusty: we find a strong correlation between the depth of the NaD profile and the line-of-sight reddening (E(B-V) = 0.3 to 1 over regions several-to-ten kpc in size). The estimated terminal velocities of superwinds inferred from these data and extant X-ray data are typically 400 to 800 km/s, are independent of the galaxy rotation speed, and are comparable to (substantially exceed) the escape velocities for $L_*$ (dwarf) galaxies. The resulting loss of metals can establish the mass-metallicity relation in spheroids, produce the observed metallicity in the ICM, and enrich a general IGM to 10$^{-1}$ solar metallicity. If the outflowing dust grains survive their journey into the IGM, their effect on observations of cosmologically-distant objects is significant.

We present new photometry of 16 local Seyferts including 6 Compton-thick sources in N-band filters around 12-microns, obtained with the VISIR instrument on the 8-m Very Large Telescope. The near-diffraction-limited imaging provides the least-contaminated core fluxes for these sources to date. Augmenting these with our previous observations and with published intrinsic X-ray fluxes, we form a total sample of 42 sources for which we find a strong mid-infrared:X-ray (12.3 micron:2-10 keV) luminosity correlation. Performing a physically-motivated sub-selection of sources in which the Seyfert torus is likely to be best-resolved results in the correlation L_{MIR} ~ L_X^{1.11+/-0.07}, with a reduction of the scatter in luminosities as compared to the full sample. Consideration of systematics suggests a range of 1.02-1.21 for the correlation slope. The mean 2 keV:12.3 micron spectral index (alpha_IX) is found to be -1.10+/-0.01, largely independent of luminosity. Observed 12-micron bolometric corrections range over ~10-30 if a known luminosity trend of intrinsic X-ray bolometric corrections is assumed. Comparison with ISO data spanning a larger luminosity range suggests that our correlation can be extended into the quasar regime. The fact that unobscured, obscured and Compton-thick sources all closely follow the same luminosity correlation has important implications for the structures of Seyfert cores. The typical resolution-limit of our imaging corresponds to ~70 pc at a median z=0.01, and the tightness of the correlation constrains any residual star-formation within these physical scales, for which we infer a mean upper-limit of <~40% of the remaining unresolved flux. We suggest that uncontaminated mid-IR continuum imaging of AGN is an accurate proxy for their intrinsic power.

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Resolving the mid-infrared cores of local Seyferts

We present new photometry of 16 local Seyferts including 6 Compton-thick sources in N -band filters around 12- μ m, obtained with the VISIR instrument on the 8-m Very Large Telescope. The near-diffraction-limited imaging provides the least-contaminated core fluxes for these sources to date. Augmenting these with our previous observations and with published intrinsic X-ray fluxes, we form a total sample of 42 sources for which we find a strong mid-infrared:X-ray (12.3  μ m:2–10 keV) luminosity correlation. Performing a physically-motivated subselection of sources in which the Seyfert torus is likely to be best-resolved results in the correlation L MIR  $\propto$ L X $^{1.11\pm 0.07}$, with a reduction of the scatter in luminosities as compared to the full sample. Consideration of systematics suggests a range of 1.02–1.21 for the correlation slope. The mean 2-keV:12.3- μ m spectral index ($\alpha_{\rm IX}$) is found to be -1.10$\;\pm\;$0.01, largely independent of luminosity. Indirectly-computed 12- μ m bolometric corrections range over ≈ 10–30 if a known luminosity trend of X-ray bolometric corrections is assumed. Comparison with ISO  data spanning a wider luminosity range suggests that our correlation can be extended into the quasar regime. That unobscured, obscured, and Compton-thick sources all closely follow the same luminosity correlation has important implications for the structures of Seyfert cores. The typical resolution-limit of our imaging corresponds to ~70 pc at a median $z=0.01$, and we use the tightness of the correlation to place constraints on the dominance of any residual emission sources within these physical scales. An upper-limit for any contaminating star formation of ≈ 40% of the unresolved flux is inferred, on average. We suggest that uncontaminated mid-IR continuum imaging of AGN is an accurate proxy for their intrinsic power.

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We present and discuss Hubble Space Telescope (HST) (WFPC2 and FOC) images and ultraviolet (GHRS) spectra plus ground-based optical spectra of three Seyfert 2 nuclei (NGC 7130, NGC 5135 and IC 3639). These galaxies, together with Mrk 477 (Heckman et al.), were selected on the basis of ultraviolet brightness from a bigger sample that comprises the 20 brightest Seyfert 2 nuclei, with the goal of studying the starburst-active galactic nucleus (AGN) connection and the origin of the so-called featureless continuum in Seyfert 2 nuclei. The data provide direct evidence of the existence of nuclear starbursts that dominate the ultraviolet light and that are responsible for the featureless continuum in these type 2 Seyfert nuclei. The GHRS spectra show absorption features formed in the photospheres (S V λ1501, C III λλ1426, 1428, Si III λ1417, and Si III + P III λ1341-1344) and in the stellar winds (C IV λ1550, Si IV λ1400, and N V λ1240) of massive stars. Signatures of massive stars are also clearly detected in their optical and near-UV spectra where the high-order Balmer series and He I lines are observed in absorption. These lines are formed in the photospheres of O and B stars, and thus they also provide strong independent evidence of the presence of massive stars in the nuclei of these Seyfert 2 nuclei. Interstellar absorption lines similar to those formed in the interstellar medium of starbursts are also observed. They are blueshifted by a few hundred km s-1 with respect to the systemic velocity, indicating that the interstellar gas is outflowing. These outflows are most likely driven by the nuclear starburst. These starbursts are dusty, compact, and powerful. They have sizes ranging from less than 100 pc to a few hundred parsecs (much smaller than that seen in the prototype Seyfert 2 galaxy NGC 1068). Their UV colors imply that they are heavily reddened (by 2 to 3 mag in the UV), and the implied bolometric luminosities are of order 1010 L☉. The bolometric luminosities of these starbursts are similar to the estimated bolometric luminosities of their obscured Seyfert 1 nuclei. The data on this small sample suggest that more powerful AGNs may be related to more powerful central starbursts. Comparing the HST spectra to IUE spectra obtained through apertures with projected sizes of 3-11 kpc (and to IRAS far-IR data) we estimate that the nuclear starbursts account for 6%-25% of the total intrinsic UV luminosity of the entire galaxy.

http://iopscience.iop.org/article/10.1086/306154/pdf

Ultraviolet-Optical Observations of the Seyfert 2 Galaxies NGC 7130, NGC 5135, and IC 3639: Implications for the Starburst-Active Galactic Nucleus Connection

Julian Krolik

Papers

Ultraviolet-Optical Observations of the Seyfert 2 Galaxies NGC 7130, NGC 5135, and IC 3639: Implications for the Starburst-Active Galactic Nucleus Connection