Sueli M. Viegas

Molecular hydrogen and (Fe II) in Active Galactic Nuclei

Near-infrared spectroscopy is used to study the kinematics and excitation mechanisms of H2 and (Fe uf769uf769) lines in a sample of mostly Seyfert 1 galaxies. The spectral coverage allows simultaneous observation of the JHK bands, thus eliminating the aperture and seeing effects that have usually plagued previous works. The H2 lines are unresolved in all objects in which they were detected while the (Fe uf769uf769) lines have widths implying gas velocities of up to 650 km s −1 . This suggests that, very likely, the H2 and (Fe uf769uf769) emission does not originate from the same parcel of gas. Molecular H2 lines were detected in 90% of the sample, including PG objects, indicating detectable amounts of molecular material even in objects with low levels of circumnuclear starburst activity. Analysis of the observations favors thermal excitation mechanisms for the H2 lines. Indeed, in NGC 3227, Mrk 766, NGC 4051 and NGC 4151, the molecular emission is found to be purely thermal but with heating processes that vary between the objects. Thermal excitation is also confirmed by the rather similar vibrational and rotational temperatures in the objects for which data were available. (Fe uf769uf769) lines are detected in all of the sample AGN. The (Fe uf769uf769) 1.254 µm/Paβ ratio is compatible with excitation of the (Fe uf769uf769) lines by the active nucleus in most Seyfert 1 galaxies, but in Mrk 766 the ratio implies a stellar origin. A correlation between H2/Brγ and (Fe uf769uf769)/Paβ is found for our sample objects supplemented by data from the literature. The correlation of these line ratios is a useful diagnostic tool in the NIR to separate emitting line objects by their level of nuclear activity. X-ray excitation models are able to explain the observed H2 and part of the (Fe uf769uf769) emission but fail to explain the observations in Seyfert 2 galaxies. Most likely, a combination of X-ray heating, shocks driven by the radio jet and circumnuclear star formation contributes, in different proportions, to the H2 and (Fe uf769uf769) lines observed. In most of our sample objects, the (Fe uf769uf769) 1.257 µm/1.644 µm ratio is found to be 30% lower than the intrinsic value based on current atomic data. This implies either that the extinction towards the (Fe uf769uf769)-emitting clouds is very similar in most objects or there are possible inaccuracies in the A-values in the Fe uf769uf769 transitions.

Outflows of Very Ionized Gas in the Centers of Seyfert Galaxies: Kinematics and Physical Conditions*

Mid-resolution spectra are used to deduce the size and kinematics of the coronal region in a sample of Seyfert galaxies by means of observations of the [Fe XI], [Fe X], [Fe VII], [Si VI], and [Si VII] lines. These coronal lines (CLs) extend from the unresolved nucleus up to a few tens to a few hundreds of parsecs. The region of the highest ionized ions studied, [Fe XI] and [Fe X], is the least spatially extended and concentrates at the center; intermediate-ionization lines extend from the nucleus up to a few tens to a few hundred parsecs; lower [O III]-like ions are known to extend to the kpc range. All together indicate a stratification in the ionized gas, usually interpreted in terms of nuclear photoionization as the driving ionization mechanism. However, CL profiles show various peculiarities: they are broader by a factor of 2 than lower ionization lines, the broadening being in terms of asymmetric blue wings, and their centroid position at the nucleus is blueshifted by a few hundred km s-1. Moreover, in NGC 1386 and NGC 1068, a double-peaked [Fe VII] line is detected in the nuclear and extended coronal region, this being the first report of this type of profile in CLs in active galactic nuclei. If interpreted as outflow signatures, the total broadening of the lines at zero-intensity levels implies gas velocities up to 2000 km s-1. Although the stratification of ions across the coronal region means that photoionization is the main power mechanism, the high velocities deduced from the profiles, the relatively large spatial extension of the emission, and the results from photoionization models indicate that an additional mechanism is at work. We suggest that shocks generated by the outflow could provide the additional required power for line formation.

Ionization Corrections for Low-Metallicity H II Regions and the Primordial Helium Abundance

Helium and hydrogen recombination lines observed in low-metallicity, extragalactic H II regions provide the data used to infer the primordial helium mass fraction, YP. The ionization corrections for unseen neutral helium (or hydrogen) are usually assumed to be absent; i.e., the ionization correction factor (ICF) is taken to be unity (ICF ≡ 1). In this paper we revisit the question of the ICF for H II regions ionized by clusters of young, hot, metal-poor stars. Our key result is that for the H II regions used in the determination of YP, there is a reverse ionization correction: ICF < 1. We explore the effect on the ICF of more realistic inhomogeneous H II region models and find that for those regions ionized by young stars, with hard radiation spectra, the ICF is reduced further below unity. In Monte Carlos using H II region data from the literature (Izotov & Thuan) we estimate a reduction in the published value of YP of order 0.003, which is roughly twice as large as the quoted statistical error in the YP determination.

Infrared Fe II emission in narrow-line Seyfert 1 galaxies

We obtained 0.8-2.4 μm spectra at a resolution of 320 km s-1 of four narrow-line Seyfert 1 galaxies in order to study the near-infrared properties of these objects. We focus on the analysis of the Fe II emission in that region and the kinematics of the low-ionization broad lines. We show that the 1 μm Fe II lines (λ9997, λ10501, λ10863, and λ11126) are the strongest Fe II lines in the observed interval. For the first time, primary cascade lines of Fe II arising from the decay of upper levels pumped by Lyα fluorescence are resolved and identified in active galactic nuclei. Excitation mechanisms leading to the emission of the 1 μm Fe II features are discussed. A combination of Lyα fluorescence and collisional excitation is found to be the main contributor. The flux ratio between near-IR Fe II lines varies from object to object, in contrast to what is observed in the optical region. A good correlation between the 1 μm and optical Fe II emission is found. This suggests that the upper z4F0 and z4D0 levels from which the bulk of the optical Fe II lines descend are mainly populated by the transitions leading to the 1 μm lines. The width and profile shape of Fe II λ11127, Ca II λ8642, and O I λ8446 are very similar but significantly narrower than Paβ, giving strong observational support to the hypothesis that the regions where Fe II, Ca II, and O I are produced are cospatial, interrelated kinematically, and most probably located in the outermost portion of the broad-line region.

The Evolution of Helium and Hydrogen Ionization Corrections as H II Regions Age

Helium and hydrogen recombination lines observed in low-metallicity, extragalactic, H II regions provide the data used to infer the primordial helium mass fraction, YP. When abundances are derived from observations, the correction for unseen neutral helium or hydrogen is usually assumed to be absent; i.e., the ionization correction factor (icf) is taken to be unity (icf ≡ 1). In a previous paper, we revisited the question of the icf for H II regions ionized by clusters of young, hot, metal-poor stars, confirming earlier work which had demonstrated a reverse ionization correction: icf < 1. In that work, the icf was calculated using more nearly realistic models of inhomogeneous H II regions, revealing that for those H II regions ionized by young stars with hard radiation spectra the icf is reduced even farther below unity compared with homogeneous models. Based on these results, our previous paper suggested that the published values of YP needed to be reduced by an amount of order 0.003. As star clusters age, their stellar spectra evolve and so, too, will their icfs. Here the evolution of the icf is studied, along with that of two alternate measures of the hardness of the radiation spectrum. The differences between the icf for radiation-bounded and matter-bounded models are also explored, along with the effect on the icf of the He/H ratio (since He and H compete for some of the same ionizing photons). Particular attention is paid to the amount of doubly ionized helium predicted, leading us to suggest that observations of, or bounds to, He++ may help to discriminate among models of H II regions ionized by starbursts of different ages and spectra. We apply our analysis to the Izotov & Thuan data set, utilizing the radiation softness parameter, the [O III]/[O I] ratio, and the presence or absence of He++, to find 0.95 icf 0.99. This suggests that the Izotov & Thuan estimate of the primordial helium abundance should be reduced by ΔY ≈ 0.006 ± 0.002, from 0.244 ± 0.002 to 0.238 ± 0.003.

Near-infrared coronal lines in narrow-line Seyfert 1 galaxies

We report spectroscopic observations in the wavelength region 0.8-2.4 μm aimed at detecting near-infrared coronal lines in a sample of five narrow-line and one broad-line Seyfert 1 galaxies. Our measurements show that [Si VI] 1.963 μm, [S IX] 1.252 μm, and [S VIII] 0.991 μm are present in most of the objects and are useful tracers of nuclear activity. Line ratios between coronal and low-ionization forbidden lines are larger in narrow-line Seyfert 1 galaxies. A positive correlation between FHWM and ionization potential of the forbidden lines is observed. Some coronal lines have widths similar to those of lines emitted in the broad-line region (BLR), indicating that part of their flux originates in gas close to the outer portions of the BLR. Most coronal lines are blueshifted relative to the systemic velocity of the galaxy, and this shift increases with the increase in line width. Asymmetries toward the blue are observed in the profiles of high-ionization Fe lines, suggesting that the emitting gas is related to winds or outflows, most probably originating in material that is being evaporated from the torus. This scenario is supported by models that combine the effects of shock ionization and photoionization by a central continuum source in the gas clouds. The agreement between the coronal line emission predicted by the models and the observations is satisfactory; the models reproduced the whole range of coronal line intensities observed. We also report the detection of [Fe XIII] 1.074, 1.079 μm in three of our objects and the first detection of [P II] 1.188 μm and [Ni II] 1.191 μm in a Seyfert 1 galaxy, Ark 564. Using the ratio [P II]/[Fe II], we deduced that most Fe present in the outer narrow-line region of Ark 564 is locked up in grains and that the influence of shocks is negligible.

The continuum and narrow line region of the NLS1 galaxy Mrk 766

We present the first spectroscopic observations in the interval 0.8-4.0 microns, complemented with HST/UV and optical spectroscopy, of the narrow line Seyfert 1 galaxy Mrk766. The NIR spectrum is characterized by permitted lines of HI, HeI, HeII and FeII, and forbidden lines of [SII], [SIII] and [FeII] among others. High ionized species such as [SiIX], [SiX], [SIX] and [MgVII] are also observed. The continuum has a complex shape, with contribution of the central engine, circumnuclear stellar population and dust. This last component is evidenced by the presence of an excess of emission peaking at 2.25 microns, fitted by blackbody function with T_bb=1200K. That temperature is close to the evaporation temperature of graphite grains. As such, it provides strong evidence of hot dust, probably very close to the nucleus. Consistent modeling of the line and broad band continuum spectrum by composite models, which account for the photoionizing flux of the central engine and shocks, shows that the shock velocities are between 100 and 500 km/s, the preshock densities between 100 and 1000 cm^-3 and the radiation fluxes from the active centre between 10^9 and 5x10^12 photons cm^-2 s^-1 eV^-1 at 1 Ryd with spectral indices

Temperature Fluctuations and Abundances in H II Galaxies

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Temperature of the Central Stars of Planetary Nebulae and the Effect of the Nebular Optical Depth

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A NEW GENERATION OF PHOTOIONIZATION CODES: THREE-DIMENSIONAL MODELS. THE BIPOLAR PLANETARY NEBULA IC 4406

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