Broad-line Region Clouds Research Articles

Simultaneous Hα and dust reverberation mapping of 3C 120: Testing the bowl-shaped torus geometry

We monitored the Seyfert-1 galaxy 3C 120 between September 2014 and March 2015 at the Universitätssternwarte Bochum near Cerro Armazones in BVRIJK and a narrowband filter covering the redshifted Hα line. In addition we obtained a single contemporary spectrum with the spectrograph FAST at Mt. Hopkins. Compared to earlier epochs 3C 120 is about a factor of three brighter, allowing us to study the shape of the broad line region (BLR) and the dust torus in a high luminosity phase. The analysis of the light curves yields that the dust echo is rather sharp and symmetric in contrast to the more complex broad Hα BLR echo. We investigated how far this supports an optically thick bowl-shaped BLR and dust torus geometry. The comparison with several parameterizations of these models supports the following geometry: The BLR clouds lie inside the bowl closely above the bowl rim up to a halfcovering angle 0° < θ < 40° (measured against the equatorial plane). Then the BLR is spread over many isodelay surfaces, yielding a smeared and structured echo as observed. Furthermore, if the BLR clouds shield the bottom of the bowl rim against radiation from the nucleus, the hot dust emission comes essentially from the top edge of the bowl (40° < θ < 45°). Then, for small inclinations as for 3C120, the top dust edge forms a ring that largely coincides with a narrow range of isodelay surfaces, yielding the observed sharp dust echo. The scale height of the BLR increases with radial distance from the black hole (BH). This leads to luminosity dependent foreshortening effects of the lag. We discuss the implications and possible corrections of the foreshortening for the BH mass determination and consequences for the lag (size)–luminosity relationships and the difference from interferometric torus sizes.

Atom-Rydberg atom chemi-ionization/recombination processes in the hydrogen clouds in Broad Line Region of AGNs

Atom-Rydberg atom chemi-ionization/recombination processes in the hydrogen clouds in Broad Line Region of AGNs

Superluminous Transients at AGN Centers from Interaction between Black Hole Disk Winds and Broad-line Region Clouds

Abstract We propose that superluminous transients that appear at central regions of active galactic nuclei (AGNs) such as CSS100217:102913+404220 (CSS100217) and PS16dtm, which reach near- or super-Eddington luminosities of the central black holes, are powered by the interaction between accretion-disk winds and clouds in broad-line regions (BLRs) surrounding them. If the disk luminosity temporarily increases by, e.g., limit–cycle oscillations, leading to a powerful radiatively driven wind, strong shock waves propagate in the BLR. Because the dense clouds in the AGN BLRs typically have similar densities to those found in SNe IIn, strong radiative shocks emerge and efficiently convert the ejecta kinetic energy to radiation. As a result, transients similar to SNe IIn can be observed at AGN central regions. Since a typical black hole disk-wind velocity is ≃0.1c, where c is the speed of light, the ejecta kinetic energy is expected to be ≃1052 erg when ≃1 M ⊙ is ejected. This kinetic energy is transformed to radiation energy in a timescale for the wind to sweep up a similar mass to itself in the BLR, which is a few hundred days. Therefore, both luminosities (∼1044 erg s−1) and timescales (∼100 days) of the superluminous transients from AGN central regions match those expected in our interaction model. If CSS100217 and PS16dtm are related to the AGN activities triggered by limit–cycle oscillations, they become bright again in coming years or decades.

A new method to measure the virial factors in the reverberation mapping of active galactic nuclei

Based on the gravitational redshift, one prediction of Einstein's general relativity theory, of broad optical emission lines in active galactic nuclei (AGNs), a new method is proposed to estimate the virial factors $f$ in measuring black hole masses $M_{\rm{RM}}$ by the reverberation mapping of AGNs. The factors $f$ can be measured on the basis of two physical quantities, i.e. the gravitational redshifts $z_{\rm{g}}$ and full widths at half maxima $v_{\rm{FWHM}}$ of broad lines. In the past it has been difficult to determine the factors $f$ for individual AGNs. We apply this new method to several reverberation mapped Seyfert 1 galaxies. There is a correlation between $f$ and broad-line region (BLR) radius $r_{\rm{BLR}}$, $f=5.4 r_{\rm{BLR}}^{0.3}$, for the gravitationally redshifted broad lines He II, He I, H$\beta$ and H$\alpha$ in narrow-line Seyfert 1 galaxy (NLS1) Mrk 110. This correlation results from the radiation pressure influence of the accretion disc on the BLR clouds. The radiation pressure influence seems to be more important than usually thought in AGNs. Mrk 110 has $f \approx$ 8--16, distinctly larger than the mean $\langle f\rangle \approx 1$, usually used to estimate $M_{\rm{RM}}$ in the case of $v_{\rm{FWHM}}$. NGC 4593 and NLS1 Mrk 486 has $f\approx 3$ and $f\approx 9$, respectively. Higher $f$ values of several tens are derived for three other NLS1s. There is a correlation between $f$ and accretion rate $\mathscr{\dot M}_{f=1}$, $f=6.8\mathscr{\dot M}^{0.4}_{f=1}$ for five objects, where $\mathscr{\dot M}_{f=1}=\dot M_{\bullet}/L_{\rm{Edd}}c^{-2}$ as $f=1$ is assumed to estimate $M_{\rm{RM}}$ used in the Eddington luminosity $L_{\rm{Edd}}$, $\dot M_{\bullet}$ is the mass accretion rate, and $c$ is the speed of light. These larger $f$ values will produce higher $M_{\rm{RM}}$ values and lower Eddington ratios.

CHEMICAL EVOLUTION OF THE UNIVERSE AT 0.7 < z < 1.6 DERIVED FROM ABUNDANCE DIAGNOSTICS OF THE BROAD-LINE REGION OF QUASARS

ABSTRACT We present an analysis of Mg ii λ2798 and Fe ii UV emission lines for archival Sloan Digital Sky Survey (SDSS) quasars to explore the diagnostics of the magnesium-to-iron abundance ratio in a broad-line region cloud. Our sample consists of 17,432 quasars selected from the SDSS Data Release 7 with a redshift range of 0.72 < z < 1.63. A strong anticorrelation between the Mg ii equivalent width (EW) and the Eddington ratio is found, while only a weak positive correlation is found between the Fe ii EW and the Eddington ratio. To investigate the origin of these differing behaviors of Mg ii and Fe ii emission lines, we perform photoionization calculations using the Cloudy code, where constraints from recent reverberation mapping studies are considered. We find from calculations that (1) Mg ii and Fe ii emission lines are created at different regions in a photoionized cloud, and (2) their EW correlations with the Eddington ratio can be explained by just changing the cloud gas density. These results indicate that the Mg ii/Fe ii flux ratio, which has been used as a first-order proxy for the Mg/Fe abundance ratio in chemical evolution studies with quasar emission lines, depends largely on the cloud gas density. By correcting this density dependence, we propose new diagnostics of the Mg/Fe abundance ratio for a broad-line region cloud. In comparing the derived Mg/Fe abundance ratios with chemical evolution models, we suggest that α-enrichment by mass loss from metal-poor intermediate-mass stars occurred at z ∼ 2 or earlier.

Gamma-ray opacity of the anisotropic stratified broad-line regions in blazars

The GeV-range spectra of blazars are shaped not only by non-thermal emission processes internal to the relativistic jet but also by external pair-production absorption on the thermal emission of the accretion disc and the broad-line region (BLR). For the first time, we compute here the pair-production opacities in the GeV range produced by a realistic BLR accounting for the radial stratification and radiation anisotropy. Using photoionization modelling with the CLOUDY code, we calculate a series of BLR models of different sizes, geometries, cloud densities, column densities and metallicities. The strongest emission features in the model BLR are Ly$\alpha$ and HeII Ly$\alpha$. Contribution of recombination continua is smaller, especially for hydrogen, because Ly continuum is efficiently trapped inside the large optical depth BLR clouds and converted to Lyman emission lines and higher-order recombination continua. The largest effects on the gamma-ray opacity are produced by the BLR geometry and localization of the gamma-ray source. We show that when the gamma-ray source moves further from the central source, all the absorption details move to higher energies and the overall level of absorption drops because of decreasing incidence angles between the gamma-rays and BLR photons. The observed positions of the spectral breaks can be used to measure the geometry and the location of the gamma-ray emitting region relative to the BLR. Strong dependence on geometry means that the soft photons dominating the pair-production opacity may be actually produced by a different population of BLR clouds than the bulk of the observed broad line emission.

Kinematics and structure of clumps in broad-line regions in active galactic nuclei

We use the Jeans equations for an ensemble of collisionless particles to describe the distribution of broad-line region (BLR) cloud in three classes: (A) non disc (B) disc-wind (c) pure disc structure. We propose that clumpy structures in the brightest quasars belong to class A, fainter quasars and brighter Seyferts belong to class B, and dimmer Seyfert galaxies and all low-luminosity AGNs (LLAGNs) belong to class C. We derive the virial factor, $f$, for disc-like structures and find a negative correlation between the inclination angle, $\theta_{0}$, and $f$. We find similar behaviour for $f$ as a function of the FWHM and $\sigma_{z}$, the $z$ component of velocity dispersion. For different values of $\theta_{0}$ we find that $ 1.0 \lesssim f \lesssim 9.0 $ in type1 AGNs and $ 0.5 \lesssim f \lesssim 1.0 $ in type2 AGNs. Moreover we have $ 0.5 \lesssim f \lesssim 6.5 $ for different values of FWHM and $ 1.4 \lesssim f \lesssim 1.8 $ for different values of $ \sigma_{z} $. We also find that $ f $ is relatively insensitive to the variations of bolometric luminosity and column density of each cloud and the range of variation of $ f $ is in order of 0.01. Considering wide range of $ f $ we see the use of average virial factor $ \langle f \rangle $ is not very safe. Therefore we propose AGN community to divide a sample into a few subsamples based on the value of $ \theta_{0}$ and FWHM of members and calculate $ \langle f \rangle $ for each group separately to reduce uncertainty in black hole mass estimation.

SPACE TELESCOPE AND OPTICAL REVERBERATION MAPPING PROJECT. IV. ANOMALOUS BEHAVIOR OF THE BROAD ULTRAVIOLET EMISSION LINES IN NGC 5548

ABSTRACT During an intensive Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) UV monitoring campaign of the Seyfert 1 galaxy NGC 5548 performed from 2014 February to July, the normally highly correlated far UV continuum and broad emission line variations decorrelated for ∼60–70 days, starting ∼75 days after the first HST/COS observation. Following this anomalous state, the flux and variability of the broad emission lines returned to a more normal state. This transient behavior, characterized by significant deficits in flux and equivalent width of the strong broad UV emission lines, is the first of its kind to be unambiguously identified in an active galactic nucleus reverberation mapping campaign. The largest corresponding emission line flux deficits occurred for the high ionization, collisionally excited lines C iv and Si iv(+O iv]), and also He ii(+O iii]), while the anomaly in Lyα was substantially smaller. This pattern of behavior indicates a depletion in the flux of photons with relative to those near 13.6 eV. We suggest two plausible mechanisms for the observed behavior: (i) temporary obscuration of the ionizing continuum incident upon broad line region (BLR) clouds by a moving veil of material lying between the inner accretion disk and inner (BLR), perhaps resulting from an episodic ejection of material from the disk, or (ii) a temporary change in the intrinsic ionizing continuum spectral energy distribution resulting in a deficit of ionizing photons with energies >54 eV, possibly due to a transient restructuring of the Comptonizing atmosphere above the disk. Current evidence appears to favor the latter explanation.

High-frequency excess in the radio continuum spectrum of the type-1 Seyfert galaxy NGC 985

Abstract The Seyfert galaxy NGC 985 is known to show a high-frequency excess in its radio continuum spectrum at a milli-Jansky level on the basis of previous observations at 1.4–15 GHz; a steep spectrum at low frequencies (a spectral index, α = −1.10 ± 0.03) changes at ∼10 GHz into an inverted spectrum at higher frequencies (α = +0.86 ± 0.09). We conduct new observations at 15–43 GHz using the Very Large Array and at 100 GHz using the Nobeyama Millimeter Array. As a result, the high-frequency excess has been confirmed as continuing at even higher radio frequencies, up to 43 GHz. The non-detection at 100 GHz was not so strong a constraint, and therefore the spectral behavior above 43 GHz remains unclear. The astrometric position of the high-frequency excess component coincides with the optical position of the Seyfert nucleus and the low-frequency radio position to an accuracy of 0${^{\prime\prime}_{.}}$1, corresponding to ∼80 pc; the radio source size is constrained to be <0${^{\prime\prime}_{.}}$02, corresponding to <16 pc. We discuss the physical origin of the observed high-frequency excess component. Dust emission at the Rayleigh–Jeans regime, free–free emission from X-ray radiating high-temperature plasma, free–free emission from the ensemble of broad-line region clouds, or thermal synchrotron from hot accretion flow cannot be responsible for the observed radio flux. Compact jets under synchrotron self-absorption may be unlikely in terms of observed time scales. Alternatively, we cannot rule out the hypotheses of synchrotron jets free–free absorbed by a circumnuclear photo-ionized region, and self-absorbed nonthermal synchrotron from disk corona, as the origin of the high-frequency excess component.

ALMA observations of the submillimetre hydrogen recombination line from the type 2 active nucleus of NGC 1068

Hydrogen recombination lines at the submillimetre band (submm-RLs) can serve as probes of ionized gas without dust extinction. One therefore expects to probe the broad line region (BLR) of an obscured (type 2) active galactic nucleus (AGN) with those lines. However, admitting the large uncertainty in the continuum level, here we report on the non-detection of both broad and narrow H26$\alpha$ emission line (rest frequency = 353.62 GHz) towards the prototypical type 2 AGN of NGC 1068 with the Atacama Large Millimeter/submillimeter Array (ALMA). We also investigate the nature of BLR clouds that can emit submm-RLs with model calculations. As a result, we suggest that clouds with an electron density ($N_e$) of $\sim$ 10$^9$ cm$^{-3}$ can mainly contribute to broad submm-RLs in terms of the line flux. On the other hand, line flux from other density clouds would be insignificant considering their too large or too small line optical depths. However, even for the case of $N_e \sim 10^9$ cm$^{-3}$ clouds, we also suggest that the expected line flux is extremely low, which is impractical to detect even with ALMA.

TheChandra/HETG view of NGC 1365 in a Compton-thick state

We present the analysis of a Chandra High-Energy Transmission Grating (HETG) observation of the local Seyfert galaxy NGC 1365. The source, well known for its dramatic X-ray spectral variability, was caught in a reflection-dominated, Compton-thick state. The high spatial resolution afforded by Chandra allowed us to isolate the soft X-ray emission from the active nucleus, neglecting most of the contribution from the kpc-scale starburst ring. The HETG spectra thus revealed a wealth of He- and H-like lines from photoionized gas, whereas in larger aperture observations these are almost exclusively produced through collisional ionization in the circumnuclear environment. Once the residual thermal component is accounted for, the emission-line properties of the photoionized region close to the hard X-ray continuum source indicate that NGC 1365 has some similarities to the local population of obscured active galaxies. In spite of the limited overall data quality, several soft X-ray lines seem to have fairly broad profiles (~800-1300 km/s full-width at half maximum), and a range of outflow velocities (up to ~1600 km/s, but possibly reaching a few thousands km/s) appears to be involved. At higher energies, the K$\alpha$ fluorescence line from neutral iron is resolved with > 99 per cent confidence, and its width of ~3000 km/s points to an origin from the same broad-line region clouds responsible for eclipsing the X-ray source and likely shielding the narrow-line region.

Location of gamma-ray flaring region in quasar 4C +21.35

Abstract 4C +21.35 is a flat-spectrum-radio-quasar-type blazar, in which the rapid variability of very high energy (VHE, Eγ ≳ 100 GeV) emission as short as ∼ 10 min was observed by MAGIC Cherenkov telescopes, and the VHE spectrum extends up to at least 400 GeV. In this paper, by using a flat broad-line region (BLR) structure, we study the location and properties of the γ-ray-emitting region of 4C +21.35 under the constraints of multiwavelength data. We fit three quasi-simultaneous spectral energy distributions using a homogeneous one-zone leptonic model, in which a flat BLR with aperture angle α = 25° and a spherically symmetric hot dusty torus with a temperature of Tsub = 1200 K are assumed. The results show that the jet structure of 4C +21.35 is nearly conical with a half-opening angle of θj ≃ 0 $_{.}^{\circ}$29–0 $_{.}^{\circ}$6. Furthermore, the emitting region is located within the BLR clouds and approaches the outer radius of the BLR during the flaring states, while it is well beyond the dusty torus in quiescent state. The quiescent high-energy emission is dominated by the synchrotron self-Compton process, the high-energy emission during the flaring periods is dominated by Compton scattering of BLR and dusty torus photons with a value of the Compton-dominance parameter of about 30. Moreover, the fit to optical/ultraviolet data provides further support that the central black hole mass of 4C +21.35 is 6 × 108 M⊙.

On the orbital motion of cold clouds in broad-line regions

On the orbital motion of cold clouds in broad-line regions

THE MULTI-LAYER VARIABLE ABSORBERS IN NGC 1365 REVEALED BYXMM-NEWTONANDNuSTAR

Between July 2012 and February 2013, NuSTAR and XMM-Newton performed four long-look joint obser- vations of the type 1.8 Seyfert, NGC 1365. We have analyzed the variable absorption seen in these observations in order to characterize the geometry of the absorbing material. Two of the observations caught NGC 1365 in an unusually low absorption state, revealing complexity in the multi-layer absorber which had previously been hidden. We find the need for three distinct zones of neutral absorption in addition to the two zones of ionized absorption and the Compton-thick torus previously seen in this source. The most prominent absorber is likely associated with broad line region clouds with column densities of around $\sim\,$10$^{23}$ cm$^{-2}$ and a highly clumpy nature as evidenced by an occultation event in February 2013. We also find evidence of a patchy absorber with a variable column around $\sim\,10^{22}$ cm$^{-2}$ and a line of sight covering fraction of 0.3$-$0.9 which responds directly to the intrinsic source flux, possibly due to a wind geometry. A full-covering, constant absorber with a low column density of $\sim\,1 \times$ 10$^{22}$ cm$^{-2}$ is also present, though the location of this low density haze is unknown.

Revisiting correlations between broad-line and jet emission variations for AGNs: 3C 120 and 3C 273

We restudy the issue of cross-correlations between broad-line and jet emission variations, and aim to locate the position of radio (and gamma-ray) emitting region in jet of active galactic nuclei (AGNs). Considering the radial profiles of the radius and number density of clouds in a spherical broad-line region (BLR), we derive new formulae connecting jet emitting position $R_{\rm{jet}}$ to time lag $\tau_{\rm{ob}}$ between broad-line and jet emission variations, and BLR radius. Also, formulae are derived for a disk-like BLR and a spherical shell BLR. The model-independent FR/RSS method is used to estimate $\tau_{\rm{ob}}$. For 3C 120, positive lags of about 0.3 yr are found between the 15 GHz emission and the H$\beta$, H$\gamma$ and He II $\lambda 4686$ lines, including broad-line data in a newly published paper, indicating the line variations lead the 15 GHz ones. Each of the broad-line light curves corresponds to a radio outburst. $R_{\rm{jet}}=$1.1--1.5 parsec (pc) are obtained for 3C 120. For 3C 273, a common feature of negative time lags is found in the cross-correlation functions between light curves of radio emission and the Balmer lines, and as well Ly$\alpha \/\ \lambda 1216$ and C~{\sc iv} $\lambda 1549$ lines. $R_{\rm{jet}}=$ 1.0--2.6 pc are obtained for 3C 273. The estimated $R_{\rm{jet}}$ are comparable for 3C 120 and 3C 273, and the gamma-ray emitting positions will be within $\sim$ 1--3 pc from the central engines. Comparisons show that the cloud number density and radius radial distributions and the BLR structures only have negligible effects on $R_{\rm{jet}}$.

Studying the complex absorption profiles of Si IV in 21 HiBALQSO spectra

We investigate the physical conditions and kinematics of broad absorption line region clouds of Si IV in 21 HiBAL Quasars. We use the Danezis et al. method [1], [2], [3] in order to fit and analyze the broad absorption troughs of Si IV resonance lines in the UV region of the electromagnetic spectrum. We find that the BAL flow is not smooth but instead plasma clouds are formed in it. BAL troughs present multicomponent structure which indicates the existence of more than one absorbing cloud in the line of sight, where every absorbing cloud produces a Si IV doublet. We show that the blending of these doublets produces the apparent broad absorption troughs we observe. One of our main achievements is that we managed to decompose and deblend each complex absorption trough to the individual doublets that it consists of. Apart from that, we succeeded in deblending the resonance lines of every doublet. By achieving accurate fits to the BAL troughs we calculated some physical and kinematical parameters that describe the plasma clouds in the line of sight. These parameters are: the radial outflow velocities of the clouds, the random velocities of ions inside each plasma cloud, the apparent optical depth in the center of every absorption component, the FWHM and the equivalent width. As a final step we correlate these physical parameters in order to draw useful conclusions.

SELF-SHADOWING EFFECTS OF SLIM ACCRETION DISKS IN ACTIVE GALACTIC NUCLEI: THE DIVERSE APPEARANCE OF THE BROAD-LINE REGION

Supermassive black holes in active galactic nuclei (AGNs) undergo a wide range of accretion rates, which lead to diversity of appearance. We consider the effects of anisotropic radiation from accretion disks on the broad-line region (BLR), from the Shakura-Sunyaev regime to slim disks with super-Eddington accretion rates. The geometrically thick funnel of the inner region of slim disks produces strong self-shadowing effects that lead to very strong anisotropy of the radiation field. We demonstrate that the degree of anisotropy of the radiation fields grows with increasing accretion rate. As a result of this anisotropy, BLR clouds receive different spectral energy distributions depending on their location relative to the disk, resulting in diverse observational appearance of the BLR. We show that the self-shadowing of the inner parts of the disk naturally produces two dynamically distinct regions of the BLR, depending on accretion rate. These two regions manifest themselves as kinematically distinct components of the broad H$\beta$ line profile with different line widths and fluxes, which jointly account for the Lorentzian profile generally observed in narrow-line Seyfert 1 galaxies. In the time domain, these two components are expected reverberate with different time lags with respect to the varying ionizing continuum, depending on the accretion rate and the viewing angle of the observer. The diverse appearance of the BLR due to the anisotropic ionizing energy source can be tested by reverberation mapping of H$\beta$ and other broad emission lines (e.g., \feii), providing a new tool to diagnose the structure and dynamics of the BLR. Other observational consequences of our model are also explored.

On the dynamics of clouds in the broad-line region of AGNs with an ADAF atmosphere

We investigate orbital motion of spherical, pressure-confined clouds in the broad-line region (BLR) of active galactic nuclei (AGNs). The combined influence of gravity of the central object and the non-isotropic radiation of the central source are taking into account. While most of the previous studies assume that the pressure of the intercloud gaseous component is proportional to a power-law function of the radial coordinate, we generalize it to a case where the external pressure depends on both the radial distance and the latitudinal angle. Our prescribed pressure profile determines the radius and the column density of BLR clouds as a function of their location. We also discuss about stability of the orbits and a condition for the existence of bound orbits is obtained. We found that BLR clouds tend to populate the equatorial regions more than other parts simply because of the stability considerations. Although this finding is obtained for a particular pressure profile, we think, this result is valid as long as the pressure distribution of the intercloud medium decreases from the equator to the pole.

ANHST/COS OBSERVATION OF BROAD Lyα EMISSION AND ASSOCIATED ABSORPTION LINES OF THE BL LACERTAE OBJECT H 2356-309

Weak spectral features in BL Lacertae objects (BL Lac) often provide a unique opportunity to probe the inner region of this rare type of active galactic nucleus. We present a Hubble Space Telescope/Cosmic Origins Spectrograph observation of the BL Lac H 2356-309. A weak Ly$\alpha$ emission line was detected. This is the fourth detection of a weak Ly$\alpha$ emission feature in the ultraviolet (UV) band in the so-called "high energy peaked BL Lacs", after Stocke et al. Assuming the line-emitting gas is located in the broad line region (BLR) and the ionizing source is the off-axis jet emission, we constrain the Lorentz factor ($\Gamma$) of the relativistic jet to be $\geq 8.1$ with a maximum viewing angle of 3.6$^\circ$. The derived $\Gamma$ is somewhat larger than previous measurements of $\Gamma \approx 3 - 5$, implying a covering factor of $\sim$ 3% of the line-emitting gas. Alternatively, the BLR clouds could be optically thin, in which case we constrain the BLR warm gas to be $\sim 10^{-5}\rm\ M_{\odot}$. We also detected two HI and one OVI absorption lines that are within $|\Delta v| < 150\rm\ km\ s^{-1}$ of the BL Lac object. The OVI and one of the HI absorbers likely coexist due to their nearly identical velocities. We discuss several ionization models and find a photoionization model where the ionizing photon source is the BL Lac object can fit the observed ion column densities with reasonable physical parameters. This absorber can either be located in the interstellar medium of the host galaxy, or in the BLR.

CONSTRAINING UV CONTINUUM SLOPES OF ACTIVE GALACTIC NUCLEI WITH CLOUDY MODELS OF BROAD-LINE REGION EXTREME-ULTRAVIOLET EMISSION LINES

Understanding the composition and structure of the broad-line region (BLR) of active galactic nuclei (AGN) is important for answering many outstanding questions in supermassive black hole evolution, galaxy evolution, and ionization of the intergalactic medium. We used single-epoch UV spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope to measure EUV emission-line fluxes from four individual AGN with $0.49 \le z \le 0.64$, two AGN with $0.32 \le z \le 0.40$, and a composite of 159 AGN. With the Cloudy photoionization code, we calculated emission-line fluxes from BLR clouds with a range of density, hydrogen ionizing flux and incident continuum spectral indices. The photoionization grids were fit to the observations using single-component and locally optimally emitting cloud (LOC) models. The LOC models provide good fits to the measured fluxes, while the single-component models do not. The UV spectral indices preferred by our LOC models are consistent with those measured from COS spectra. EUV emission lines such as N IV \lambda 765, O II \lambda 833, and O III \lambda 834 originate primarily from gas with electron temperatures between 37000 K and 55000 K. This gas is found in BLR clouds with high hydrogen densities (n_H \ge 10^12 cm^-3) and hydrogen ionizing photon fluxes (\Phi_H \ge 10^22 cm^-2 s^-1).