Log NH Research Articles

Narrow absorption line outflow in Seyfert 1 galaxy J1429+4518: outflow’s distance from the central source and its energetics

ABSTRACT In the Hubble Space Telescope/Cosmic Origins Spectrograph spectrum of the Seyfert 1 galaxy 2MASX J14292507+4518318, we have identified a narrow absorption line outflow system with a velocity of −151 km s−1. This outflow exhibits absorption troughs from the resonance states of ions like C iv, N v, S iv, and Si ii, as well as excited states from C ii* and Si ii*. Our investigation of the outflow involved measuring ionic column densities and conducting photoionization analysis. These allow the total column density of the outflow to be estimated as log NH = 19.84 cm−2, its ionization parameter to be log UH = −2.0, and its electron number density to be log ne = 2.75 cm−3. These measurements enabled us to determine the mass-loss rate and the kinetic luminosity of the outflow system to be $\dot{M}$ = 0.22 $\mathrm{ M}_{\odot } \, \mathrm{ yr}^{-1}$ and $\log \dot{E_{\mathrm{ K}}}$ = 39.3 erg s−1, respectively. We have also measured the location of the outflow system to be at ∼275 pc from the central source. This outflow does not contribute to the active galactic nucleus (AGN) feedback processes due to the low ratio of the outflow’s kinetic luminosity to the AGN’s Eddington luminosity ($\dot{E_{\mathrm{ K}}}/{L_{\mathrm{ Edd}}}\approx 0.00025 {{\, \rm per\, cent}}$). This outflow is remarkably similar to the two bipolar lobe outflows observed in the Milky Way by XMM–Newton and Chandra.

Comparing the host galaxy ages of X-ray selected AGN in COSMOS

We explore the properties of the host galaxies of X-ray selected AGN in the COSMOS field using the Chandra Legacy sample and the LEGA-C survey VLT optical spectra. Our main goal is to compare the relative ages of the host galaxies of the obscured and unobscured AGN by means of the calcium break Dn(4000) and the Hδ Balmer line. The host galaxy ages are examined in conjunction with other properties such as the galaxy stellar mass, and star-formation rate as well as the AGN Eddington ratio. Our sample consists of 50 unobscured or mildly obscured (NH < 1023 cm−2) and 23 heavily obscured AGN (NH > 1023 cm−2) in the redshift range z = 0.6 − 1. We take specific caution to create control samples in order to match the exact luminosity and redshift distributions for the obscured and unobscured AGN. The majority of unobscured AGN appear to live in young galaxies in contrast to the obscured AGN which appear to live in galaxies located between the young and old galaxy populations. This finding may be in contrast to those evolutionary AGN unification models which postulate that the AGN begin their life in a heavy obscuration phase. The host galaxies of the obscured AGN have significantly lower levels of specific star-formation. At the same time the obscured AGN have lower Eddington ratios indicating a link between the star-formation and the black hole accretion. We find that the distribution of the stellar masses of the host galaxies of obscured AGN is skewed towards higher stellar masses in agreement with previous findings. Our results on the relative age of obscured AGN are valid when we match our obscured and unobscured AGN samples according to the stellar mass of their host galaxies. All the above results become less conspicuous when a lower column density (log NH(cm−2) = 21.5 or 22) is used to separate the obscured and unobscured AGN populations.

Absence of nuclear polycyclic aromatic hydrocarbon emission from a compact starburst: The case of the type-2 quasar Mrk 477

Mrk 477 is the closest type-2 quasar, at a distance of 163 Mpc. This makes it an ideal laboratory for studying the interplay between nuclear activity and star formation with a great level of detail and signal-to-noise. In this Letter we present new mid-infrared (mid-IR) imaging and spectroscopic data with an angular resolution of 0.4″ (∼300 pc) obtained with the Gran Telescopio Canarias instrument CanariCam. The N-band (8–13 μm) spectrum of the central ∼400 pc of the galaxy reveals [S IV]λ10.51 μm emission, but no 8.6 or 11.3 μm polycyclic aromatic hydrocarbon (PAH) features, which are commonly used as tracers of recent star formation. This is in stark contrast with the presence of a nuclear starburst of ∼300 pc in size, an age of 6 Myr, and a mass of 1.1×108 M⊙, as constrained from ultraviolet Hubble Space Telescope observations. Considering this, we argue that even the more resilient, neutral molecules that mainly produce the 11.3 μm PAH band are most likely being destroyed in the vicinity of the active nucleus despite the relatively large X-ray column density, log NH = 23.5 cm−2, and modest X-ray luminosity, 1.5×1043 erg s−1. This highlights the importance of being cautious when using PAH features as star formation tracers in the central region of galaxies to evaluate the impact of feedback from active galactic nuclei.

Harvesting the Ly α forest with convolutional neural networks

ABSTRACT We develop a machine learning based algorithm using a convolutional neural network (CNN) to identify low H i column density Ly α absorption systems (log NH i/cm−2 < 17) in the Ly α forest, and predict their physical properties, such as their H i column density (log NH i/cm−2), redshift (zH i), and Doppler width (bH i). Our CNN models are trained using simulated spectra (S/N ≃ 10), and we test their performance on high quality spectra of quasars at redshift z ∼ 2.5−2.9 observed with the High Resolution Echelle Spectrometer on the Keck I telescope. We find that ${\sim}78{{\ \rm per\ cent}}$ of the systems identified by our algorithm are listed in the manual Voigt profile fitting catalogue. We demonstrate that the performance of our CNN is stable and consistent for all simulated and observed spectra with S/N ≳ 10. Our model can therefore be consistently used to analyse the enormous number of both low and high S/N data available with current and future facilities. Our CNN provides state-of-the-art predictions within the range 12.5 ≤ log NH i/cm−2 < 15.5 with a mean absolute error of Δ(log NH i/cm−2) = 0.13, Δ(zH i) = 2.7 × 10−5, and Δ(bH i) = 4.1 km s−1. The CNN prediction costs < 3 min per model per spectrum with a size of 120 000 pixels using a laptop computer. We demonstrate that CNNs can significantly increase the efficiency of analysing Ly α forest spectra, and thereby greatly increase the statistics of Ly α absorbers.

Photoionisation modelling of the X-ray emission line regions within the Seyfert 2 AGN NGC 1068

Aims.We investigate the photoionised X-ray emission line regions (ELRs) within the Seyfert 2 galaxy NGC 1068 to determine if there are any characteristic changes between observations taken 14 years apart.Methods.We comparedXMM-Newtonobservations collected in 2000 and 2014, simultaneously fitting the reflection grating spectrometer and EPIC-pn spectra of each epoch, for the first time, with the photoionisation model,PION, inSPEX.Results.We find that fourPIONcomponents are required to fit the majority of the emission lines in the spectra of NGC 1068, with log ξ = 1−4, log NH > 26 m−2, andvout = −100 to −600 km s−1for both epochs. Comparing the ionisation state of the components shows almost no difference between the two epochs, while there is an increase in the total equivalent column density. To estimate the locations of these plasma regions from the central black hole, we compare distance methods, excluding the variability arguments as there is no spectral change between observations. Although the methods are unable to constrain the distances for each plasma component, the locations are consistent with the narrow line region, with the possibility of the higher ionised component being part of the broad line region; we cannot conclude this for certain, but the photoionisation modelling does suggest this is possible. In addition, we find evidence for emission from collisionally ionised plasma, while previous analysis had suggested that collisional plasma emission was unlikely. However, althoughPIONis unable to account for the Fe XVIIemission lines at 15 and 17 Å, we do not rule out that photoexcitation is a valid processes to produce these lines as well.Conclusions.NGC 1068 has not changed, both in terms of the observed spectra or from our modelling, within the 14 year time period between observations. This suggests that the ELRs are fairly static relative to the 14 year time frame between observations, or there is no dramatic change in the spectral energy distribution, resulting from a lack of black hole variability.

Lyman-alpha radiation pressure: an analytical exploration

ABSTRACT We study radiation pressure due to Ly α line photons, obtaining and exploring analytical expressions for the force-multiplier, MF(NH, Z) = Fα/(Lα/c), as a function of gas column density, NH, and metallicity, Z, for both dust-free and dusty media, employing a WKB approach for the latter case. Solutions for frequency offset emission to emulate non-static media moving with a bulk velocity v have also been obtained. We find that, in static media, Ly α pressure dominates over both photoionization and dust-mediated UV radiation pressure in a very wide parameter range (16 < log NH < 23; −4 < log [Z/Z⊙] < 0). For example, it overwhelms the other two forces by $\lower.5ex\hbox{$\,\, \buildrel\gt \over \sim \,\,$}10$ (300) times in standard (low-Z) star-forming clouds. Thus, in agreement with previous studies, we conclude that Ly α pressure plays a dominant role in the initial acceleration of the gas around luminous sources, and must be implemented in galaxy formation, evolution and outflow models and simulations.

Significant H i and Metal Differences around the z = 0.83 Lens Galaxy toward the Doubly Lensed Quasar SBS 0909+532

Abstract We report a large difference in neutral hydrogen (H i) and metal column densities between the two sight lines probing opposite sides of the lensing galaxy at z lens = 0.83 toward the doubly lensed quasar SBS 0909+532. Using archival HST-STIS and Keck HIRES spectra of the lensed quasar images, we measure log N H I = 18.77 ± 0.12 cm−2 toward the brighter image (A) at an impact parameter of r A = 3.15 kpc and log N H I = 20.38 ± 0.20 cm−2 toward the fainter image (B) at an impact parameter of r B = 5.74 kpc. This difference by a factor of ∼41 is the highest difference between sight lines for a lens galaxy in which H i has been measured, suggesting patchiness and/or anisotropy on these scales. We estimate an average Fe abundance gradient between the sight lines to be ≥+0.35 dex kpc−1. The N Fe II/N Mg II ratios for the individual components detected in the Keck HIRES spectra have supersolar values for all components in sight line A and for 11 out of 18 components in sight line B, suggesting that Type Ia supernovae may have contributed to the chemical enrichment of the galaxy’s environment. Additionally, these observations provide complementary information to detections of cold gas in early-type galaxies and the tension between these and some models of cloud survival.

Physical conditions and redshift evolution of optically thin C iii absorbers: low-z sample

ABSTRACT We present a detailed analysis of 99 optically thin C iii absorption systems at redshift 0.2 ≤ z ≤ 0.9 associated with neutral hydrogen column densities in the range 15 ≤ log $N({\rm{H\,{\small I}}})$ (cm−2) ≤ 16.2. Using photoionization models, we infer the number density (nH), C-abundance ([C/H]), and line-of-sight thickness (L) of these systems in the ranges −3.4 ≤ log nH (cm−3) ≤ −1.6, −1.6 ≤ [C/H] ≤ 0.4, and 1.3 pc ≤ L ≤ 10 kpc, respectively, with most of the systems having sub-kpc scale thickness. We combine the low-z and previously reported high-z (2.1 ≤ z ≤ 3.3) optically thin C iii systems to study the redshift evolution and various correlations between the derived physical parameters. We see a significant redshift evolution in nH, [C/H], and L. We compare the redshift evolution of metallicity in C iii systems with those of various types of absorption systems. We find that the slope of [C/H] versus z for C iii absorbers is steeper compared to the redshift evolution of cosmic metallicity of the damped Ly α sample (DLA) but consistent with that of sub-DLAs. We find the existence of strong anticorrelation between L and [C/H] for the combined sample with a significance level of 8.39σ. We see evidence of two distinct [C/H] branch C iii populations (low-[C/H] branch, $[\mathrm{ C/H}]\, \le -1.2$, and high-[C/H] branch, $[\mathrm{ C/H}]\, \gt -1.2$) in the combined C iii sample when divided appropriately in the L versus $N({\rm{C\,{\small III}}})$ plane. Further studies of C iii absorbers in the redshift range 1.0 ≤ z ≤ 2.0 are important to map the redshift evolution of these absorbers and gain insights into the time evolution physical conditions of the circumgalactic medium.

The COS CGM Compendium. III. Metallicity and Physical Properties of the Cool Circumgalactic Medium at z ≲ 1

Abstract We characterize the metallicities and physical properties of cool, photoionized gas in a sample of 152 z ≲ 1 strong Lyα forest systems (SLFSs, absorbers with 15 < log N H i < 16.2). The sample is drawn from our Cosmic Origins Spectrograph (COS) circumgalactic medium compendium (CCC), an ultraviolet survey of H i-selected circumgalactic gas around z ≲ 1 galaxies that targets 261 absorbers with 15 < log N H i < 19. We show that the metallicity probability distribution function of the SLFSs at z ≲ 1 is unimodal, skewed to low metallicities with a mean and median of [X/H] = −1.47 and −1.18 dex. Very metal-poor gas with [X/H] < −1.4 represents about half of the population of absorbers with 15 < log N H i ≲ 18, while it is rare at higher N H i . Thus, there are important reservoirs of primitive (though not pristine) diffuse ionized gas around z ≲ 1 galaxies. The photoionized gas around z ≲ 1 galaxies is highly inhomogeneous based on the wide range of metallicities observed (−3 ≲ [X/H] ≲ +0.4) and the fact that there are large metallicity variations (factors of 2 to 25) for most of the closely spaced absorbers (Δv ≲ 300 km s−1) along the same sightlines. These absorbers show a complex evolution with redshift and H i column density, and we identify subtle cosmic evolution effects that affect the interpretation of metallicity distributions and comparison with other absorber samples. We discuss the physical conditions and cosmic baryon and metal budgets of the CCC absorbers. Finally, we compare the CCC results to recent cosmological zoom simulations and explore the origins of the 15 < log N H i < 19 absorbers within the Evolution and Assembly of GaLaxies and their Environments (EAGLE) high-resolution simulations.

INTERSTELLAR ABSORPTION TOWARDS THE NOVAE V339 DEL AND V5668 SGR

We present a search and study of interstellar (IS) absorption features in the observed spectra of two Novae V339 Del and V5668 Sgr. We obtained high resolution spectra (R ≈ 20, 000) in the wavelength range between 3800 and 8800 Å of both novae with the TIGRE telescope. Common IS features of Na I and Ca II were identified in both novae, and the Ca II H and K features of Nova V339 Del show substructures, while a blueshifted absorption feature was found in the spectra of Nova V5668 Sgr. Absorption of K I was identified only in the spectra of Nova V5668 Sgr. The DIBs 6196, 6203, 6379, 6614 and 7562 were found in both novae, and in the spectra of Nova V5668 Sgr we could also identify the DIBs 5780, 5797 and 6660. We present the equivalent widths of all features and determined the column densities of Ca II, K I and hydrogen in the direction of Nova V5668 Sgr (log NCa II = 12.50, log NK I = 11.55, log NH = 18.5).

A population of high-velocity absorption-line systems residing in the Local Group

Aims. We investigated the ionisation conditions and distances of Galactic high-velocity clouds (HVCs) in the Galactic halo and beyond in the direction of the Local Group (LG) barycentre and anti-barycentre, by studying spectral data of 29 extragalactic background sources obtained with the Cosmic Origins Spectropgraph (COS) installed on the Hubble Space Telescope (HST). Methods. We model column-densities of low, intermediate, and high ions such as Si II, C II, Si III, Si VI, and C IV, and use these data to construct a set of Cloudy ionisation models. Results. In total, we found 69 high-velocity absorption components along the 29 lines of sight. The components in the direction of the LG barycentre span the entire range of studied velocities, 100 ≲ |vLSR| ≲ 400 km s−1, while those in the anti-barycentre sample have velocities up to about 300 km s−1. For 49 components, we infer the gas densities. In the direction of the LG barycentre, the gas densities exhibit a wide range from log nH = −3.96 to −2.55, while in the anti-barycentre direction the densities are systematically higher, log nH > −3.25. The barycentre absorbers can be split into two groups based on their density: a high-density group with log nH > −3.54, which can be affected by the Milky Way radiation field, and a low-density group (log nH ≤ −3.54). The latter has very low thermal pressures of P/k < 7.3 K cm−3. Conclusions. Our study shows that part of the absorbers in the LG barycentre direction trace gas at very low gas densities and thermal pressures. These properties indicate that the absorbers are located beyond the virial radius of the Milky Way. Our study also confirms results from earlier, single-sightline studies, suggesting the presence of a metal-enriched intragroup medium filling the LG near its barycentre.

On the emergence of thousands of absorption lines in the quasar PG 1411+442: a clumpy high-column density outflow from the broad emission-line region?

Abstract Quasar outflows are fundamental components of quasar environments that might play an important role in feedback to galaxy evolution. We report on the emergence of a remarkable new outflow absorption-line system in the quasar PG1411+442 (redshift ∼0.089) detected in the UV and visible with the Hubble Space Telescope Cosmic Origins Spectrograph and the Gemini Multi-Object Spectrograph, respectively. This new ‘transient’ system contains thousands of lines, including Fe ii and Fe ii∗ from excited states up to 3.89 eV, H i∗ Balmer lines, Na i D λλ5890, 5896, and the first detection of He i∗ λ5876 in a quasar. The transient absorber is spatially inhomogeneous and compact, with sizes ≲0.003 pc, based on covering fractions on the quasar continuum source ranging from ∼0.45 in strong UV lines to ∼0.04 in Na i D. cloudy photoionization simulations show that large total column densities log NH(cm−2) ≳ 23.4 and an intense radiation field ≲0.4 pc from the quasar are needed to produce the observed lines in thick zones of both fully ionized and partially ionized gas. The densities are conservatively log nH(cm−3) ≳ 7 based on Fe ii∗, H i∗, and He i∗ but they might reach log nH(cm−3) ≳ 10 based on Na i D. The transient lines appear at roughly the same velocity shift, v ∼ −1900 km s−1, as a ‘mini-BAL’ outflow detected previously, but with narrower Doppler widths, b ∼ 100 km s−1, and larger column densities in more compact outflow structures. We propose that the transient lines identify a clumpy outflow from the broad emission-line region that, at its current speed and location, is still gravitationally bound to the central black hole.

HST/COS Observations of the Warm Ionized Gaseous Halo of NGC 891

Abstract The metallicity of galactic gaseous halos provides insights into accretion and feedback of galaxies. The nearby edge-on galaxy NGC 891 has a multicomponent gaseous halo and a background active galactic nucleus (AGN; LQAC 035+042 003) projected 5 kpc above the disk near the minor axis. Against the UV continuum of this AGN, we detect lines from 13 ions associated with NGC 891 in new Hubble Space Telescope/Cosmic Origin Spectrograph spectra. Most of the absorption is from the warm ionized gas with log T = 4.22 ± 0.04, log n H = −1.26 ± 0.51, and log N H = 20.81 ± 0.20. The metallicity of volatile elements (i.e., C, N, and S) is about half solar ([X/H] ≈ −0.3 ± 0.3), while Mg, Fe, and Ni show lower metallicities of [X/H] ≈ −0.9. The absorption system shows the depletion pattern seen for warm Galactic diffuse clouds, which is consistent with a mixture of ejected solar-metallicity disk gases and the hot X-ray-emitting halo (Z = 0.1–0.2 Z ⊙). The warm ionized gases are about five times more massive than the cold H i-emitting gases around the galactic center, which might lead to accretion with a mean rate of 102 M ⊙ yr−1 for a period of time. We also detect low-metallicity (≈0.1 Z ⊙) gases toward LQAC 035+042 003 at 110 km s−1 (a high-velocity cloud) and toward another sight line (3C 66A; 108 kpc projected from NGC 891) at 30 km s−1. This low-metallicity material could be the cold mode accretion from the intergalactic medium or the tidal disruption of satellites in the NGC 891 halo.

X-ray-bright optically faint active galactic nuclei in the Subaru Hyper Suprime-Cam wide survey

Abstract We construct a sample of X-ray-bright optically faint active galactic nuclei by combining Subaru Hyper Suprime-Cam, XMM-Newton, and infrared source catalogs. Fifty-three X-ray sources satisfying i-band magnitude fainter than 23.5 mag and X-ray counts with the EPIC-PN detector larger than 70 are selected from 9.1 deg2, and their spectral energy distributions (SEDs) and X-ray spectra are analyzed. Forty-four objects with an X-ray to i-band flux ratio FX/Fi > 10 are classified as extreme X-ray-to-optical flux sources. Spectral energy distributions of 48 among 53 are represented by templates of type 2 AGNs or star-forming galaxies and show the optical signature of stellar emission from host galaxies in the source rest frame. Infrared/optical SEDs indicate a significant contribution of emission from dust to the infrared fluxes, and that the central AGN is dust obscured. The photometric redshifts determined from the SEDs are in the range of 0.6–2.5. The X-ray spectra are fitted by an absorbed power-law model, and the intrinsic absorption column densities are modest (best-fit log NH = 20.5–23.5 cm−2 in most cases). The absorption-corrected X-ray luminosities are in the range of 6 × 1042–2 × 1045 erg s−1. Twenty objects are classified as type 2 quasars based on X-ray luminsosity and NH. The optical faintness is explained by a combination of redshifts (mostly z > 1.0), strong dust extinction, and in part a large ratio of dust/gas.

VLT/UVES observations of extremely strong intervening damped Lyman-αsystems

We present a detailed analysis of three extremely strong intervening DLAs (log N(HI)>=21.7) observed towards quasars with VLT/UVES. We measure overall metallicities of [Zn/H]~-1.2, -1.3 and -0.7 at respectively zabs=2.34 towards SDSS J2140-0321 (log N(HI) = 22.4+/-0.1), zabs=3.35 towards SDSS J1456+1609 (log N(HI) = 21.7+/-0.1) and zabs=2.25 towards SDSS J0154+1935 (log N(HI) = 21.75+/-0.15). We detect H2 towards J2140-0321 (log N(H2) = 20.13+/-0.07) and J1456+1609 (log N(H2) = 17.10+/-0.09) and argue for a tentative detection towards J0154+1935. Absorption from the excited fine-structure levels of OI, CI and SiII are detected in the system towards J2140-0321, that has the largest HI column density detected so far in an intervening DLA. This is the first detection of OI fine-structure lines in a QSO-DLA, that also provides us a rare possibility to study the chemical abundances of less abundant atoms like Co and Ge. Simple single phase photo-ionisation models fail to reproduce all the observed quantities. Instead, we suggest that the cloud has a stratified structure: H2 and CI likely originate from both a dense (log nH~2.5-3) cold (80K) and warm (250K) phase containing a fraction of the total HI while a warmer (T>1000 K) phase probably contributes significantly to the high excitation of OI fine-structure levels. The observed CI/H2 column density ratio is surprisingly low compared to model predictions and we do not detect CO molecules: this suggests a possible underabundance of C by 0.7 dex compared to other alpha elements. The absorber could be a photo-dissociation region close to a bright star (or a star cluster) where higher temperature occurs in the illuminated region. Direct detection of on-going star formation through e.g. NIR emission lines in the surrounding of the gas would enable a detailed physical modelling of the system.

VLT/X-shooter spectroscopy of the GRB 120327A afterglow

We present a study of the environment of the Swift long gamma-ray burst GRB 120327A at z ~2.8 through optical spectroscopy of its afterglow. We analyzed medium-resolution, multi-epoch spectroscopic observations (~7000 - 12000, corresponding to ~ 15 - 23 km/s, S/N = 15- 30 and wavelength range 3000-25000AA) of the optical afterglow of GRB 120327A, taken with X-shooter at the VLT 2.13 and 27.65 hr after the GRB trigger. The first epoch spectrum shows that the ISM in the GRB host galaxy at z = 2.8145 is extremely rich in absorption features, with three components contributing to the line profiles. The hydrogen column density associated with GRB 120327A has log NH / cm^(-2) = 22.01 +/- 0.09, and the metallicity of the host galaxy is in the range [X/H] = -1.3 to -1.1. In addition to the ground state lines, we detect absorption features associated with excited states of CII, OI, SiII, FeII, and NiII, which we used to derive information on the distance between the host absorbing gas and the site of the GRB explosion. The variability of the FeI\lambda2396 excited line between the two epochs proves that these features are excited by the GRB UV flux. Moreover, the distance of component I is found to be dI=200+100-60 pc, while component II is located closer to the GRB, at dII=100+40-30 pc. These values are among the lowest found in GRBs. Component III does not show excited transitions, so it should be located farther away from the GRB. The presence of H2 molecules is firmly established, with a molecular fraction in the range f=4 X 10^(-7) - 10^(-4). This particularly low value can be attributed to the small dust content. This represents the third positive detection of molecules in a GRB environment.

Broad-band study of hard X-ray-selected absorbed active galactic nuclei

In this paper we report on the broad-band X-ray properties of a complete sample of absorbed Seyfert galaxies hard X-ray selected with INTEGRAL. Our sample is composed of 33 sources, of which 15 are newly discovered active galactic nuclei (AGN) above 20 keV (IGR sources), while 18 are already known type 2 AGN (‘known’). For 17 sources (15 IGR + 2 ‘known’ sources) we have performed a broad-band analysis using both XMM–Newton, and INTEGRAL-IBIS data. To have a full view of the complete sample we have then complemented the analysis of the 16 remaining sources with already existing broad-band studies in the same range. The high-quality broad-band spectra are well reproduced with an absorbed primary emission with a high-energy cut-off and its scattered fraction below 2–3 keV, plus the Compton reflection features (Compton hump and Fe line emission). This study permitted a very good characterization of the primary continuum and, in turn, of all the spectral features. A high-energy cut-off is found in 30 per cent of the sample, with an average value below 150 keV, suggesting that this feature has to be present in the X-ray spectra of obscured AGN. The hard X-ray selection favours the detection of more obscured sources, with the log NH average value of 23.15 (standard deviation of 0.89). The diagnostic plot NH versus Foss(2–10 keV)/F(20–100 keV) allowed the isolation of the Compton-thick objects, and may represent a useful tool for future hard X-ray observations of newly discovered AGN. We are unable to associate the reflection components (both continuum and Fe line) with the absorbing gas as a torus (as envisaged in the Unified Model), a more complex scenario being necessary. In the Compton-thin sources, a fraction (but not all) of the Fe K line needs to be produced in a gas located closer to the black hole than the Compton-thick torus, and this is possibly associated with the optical broad-line region, responsible also for the absorption. We still need a Compton-thick medium (not intercepting the line of sight) likely associated to a torus, which contributes to the Fe line intensity and produces the observed reflection continuum above 10 keV. The so-called Iwasawa–Taniguchi effect cannot be confirmed with our data. Finally, the comparison with a sample of unobscured AGN shows that type 1 and type 2 (once corrected for absorption) Seyfert are characterized by the same nuclear/accretion properties (luminosity, bolometric luminosity, Eddington ratio), supporting the ‘unified’ view.

THE MID-INFRARED HIGH-IONIZATION LINES FROM ACTIVE GALACTIC NUCLEI AND STAR-FORMING GALAXIES

We used Spitzer/IRS spectroscopic data on 426 galaxies including quasars, Seyferts, LINER and HII galaxies to investigate the relationship among the mid-IR emission lines. There is a tight linear correlation between the [Ne V]14.3 um and 24.3 um (97.1 eV) and the [O IV]25.9 um (54.9 eV) high-ionization emission lines. The correlation also holds for these high-ionization emission lines and the [Ne III]15.56 um (41 eV) emission line, although only for active galaxies. We used these correlations to calculate the [Ne III] excess due to star formation in Seyfert galaxies. We also estimated the [O IV] luminosity due to star formation in active galaxies and determined that it dominates the [O IV] emission only if the contribution of the active nucleus to the total luminosity is below 5%. We find that the AGN dominates the [O IV] emission in most Seyfert galaxies, whereas star-formation adequately explains the observed [O IV] emission in optically classified HII galaxies. Finally we computed photoionization models to determine the physical conditions of the narrow line region where these high-ionization lines originate. The estimated ionization parameter range is -2.8 < log U < -2.5 and the total hydrogen column density range is 20 < log nH (cm-2) < 21.

THESUZAKUVIEW OF THESWIFT/BAT ACTIVE GALACTIC NUCLEI. II. TIME VARIABILITY AND SPECTRA OF FIVE “HIDDEN” ACTIVE GALACTIC NUCLEI

The fraction of Compton thick sources is one of the main uncertainties left in understanding the AGN population. The Swift Burst Alert Telescope (BAT) all-sky survey, for the first time gives us an unbiased sample of AGN for all but the most heavily absorbed sources (log NH > 25). Still, the BAT spectra (14 - 195 keV) are time-averaged over months of observations and therefore hard to compare with softer spectra from the Swift XRT or other missions. This makes it difficult to distinguish between Compton-thin and Compton-thick models. With Suzaku, we have obtained simultaneous hard (> 15 keV) and soft (0.3 - 10 keV) X-ray spectra for 5 Compton-thick candidate sources. We report on the spectra and a comparison with the BAT and earlier XMM observations. Based on both flux variability and spectral shape, we conclude that these hidden sources are not Compton-thick. We also report on a possible correlation between excess variance and Swift BAT luminosity from the 16 d binned light curves, which holds true for a sample of both absorbed (4 sources), unabsorbed (8 sources), and Compton thick (Circinus) AGN, but is weak in the 64 day binned BAT light curves.

CALIBRATION OF [O IV] 26 μm AS A MEASURE OF INTRINSIC ACTIVE GALACTIC NUCLEUS LUMINOSITY

We compare [O IV] 25.89 μm emission line luminosities with very hard (10-200 keV) X-rays from Swift, INTEGRAL, and BeppoSAX for a complete sample of 89 Seyferts from the Revised Shapley-Ames sample. Using Seyfert 1s, we calibrate [O IV] as a measure of active galactic nucleus (AGN) intrinsic luminosity, for particular use in high-obscuration environments. With this calibration, we measure the average decrement in 14-195 keV X-ray to [O IV] luminosity ratio for Seyfert 2s compared to type 1s. We find a decrement of 3.1 ± 0.8 for Seyfert 2s, and a decrement of 5.0 ± 2.7 for known Compton-thick Seyfert 2s. These decrements imply column densities of approximately log NH = 24.6 cm–2 and 24.7 cm–2, respectively. Thus, we infer that the average Seyfert 2 is more highly obscured and intrinsically more luminous than would be inferred even from the very hard X-rays. We demonstrate two applications of the hard X-ray to [O IV] ratio. For the extremely obscured NGC 1068, we measure a column density of log NH = 25.3-25.4 cm–2. Finally, by comparing [O IV] luminosities to total infrared luminosities for 12 bright ultraluminous infrared galaxies, we find that four have substantial AGN contributions.