Cosmic rays and the emission line regions of active galactic nuclei

The effects that the synchrotron emitting relativistic electrons could have on the emission line regions which characterize active nuclei are discussed. Detailed models of both the inner, dense, broad line region and the outer, lower density, narrow line region are presented, together with the first models of the optically emitting gas often found within extended radio lobes. If the relativistic gas which produces the synchrotron radio emission is mixed with the emission line region gas then significant changes in the emission line spectrum will result. The effects of the synchrotron emitting electrons on filaments in the Crab Nebula are discussed in an appendix, along with a comparison between the experimental calculations, which employ the mean escape probability formalism, and recent Hubbard and Puetter models

The line continuum luminosity ratio in AGN: Or on the Baldwin Effect

The luminosity dependence of the equivalent width of CIV in active galaxies, the "Baldwin" effect, is shown to be a consequence of a luminosity dependent ionization parameter. This law also agrees with the lack of a "Baldwin" effect in Ly alpha or other hydrogen lines. A fit to the available data gives a weak indication that the mean covering factor decreases with increasing luminosity, consistent with the inference from X-ray observations. The effects of continuum shape and density on various line ratios of interest are discussed

Broad Line Region Clouds and the Absorbing Material in NGC 4151

The constraints imposed by X-ray, ultraviolet, and optical observations on the absorbing material in the Seyfert 1.5 galaxy NGC 4151 are discussed. These observations are compared to the predictions of photoionization models of both broad line region clouds, which Holt et al. identify as the source of the extinction, and to models of the narrow line region gas. The weakness of both C III] 1909 and the broad component of Hα relative to C IV 1549, together with the absence of an absorption Balmer jump, indicate that the ionization parameter for the broad line region is a factor ~ 30 times larger in NGC 4151 than is found in typical QSO clouds. The energy budget of the system, as deduced from model calculation and emission-line fluxes, demands essentially full coverage of the continuum source by both broad and narrow line region clouds. The continuum incident on the outer narrow line region clouds is dominated by hard (hν \u3e 5 keV) X-rays because of the effective low frequency absorption arising in the inner broad line region gas. The calculations suggest that narrow line region clouds have a composition similar to extragalactic giant H II regions, and the hydrogen line spectrum shows that dust is mixed with narrow line region gas. We argue that broad line region clouds are likely to be in a radiatively driven wind and show that these clouds are opaque to radio emission. Some other consequences of these calculations are also discussed

Cosmic Rays and the Emission-Line Regions of Active Galactic Nuclei

We discuss the effects that the synchrotron emitting relativistic electrons, which are known to exist throughout many distance scales in active nuclei, could have on the emission-line regions that characterize these objects. Detailed models of both the inner, dense, broad-line region and the outer, lower density, narrow-line region are presented, together with the first models of the optically emitting gas often found within extended radio lobes. We show that, in all cases, if the relativistic gas that produces the synchrotron radio emission is mixed with the gas in the emission-line region, then significant changes in the emission-line spectrum will result. For small cosmic-ray densities the main effect is to strengthen lines that are formed in neutral regions, where photoelectric heating rates are lowest. As the cosmic-ray density increases, so do both the temperature and the ionization, until the cooling peak near 105 K is reached and a thermal runaway occurs. The implications of our results for correlations between radio and optical properties of nuclei are discussed. We find that the addition of a flux of cosmic rays to a standard model of the broad-line region can effectively quench Fe II emission, in agreement with Grandi and Osterbrock\u27s discovery that radio-loud objects tend to be weak Fe II emitters. The models do not reproduce their correlation between radio properties and Balmer decrements. Models of low-density gas in the narrow-line region show that relativistic particles can raise the temperature in the [O III] zone by an amount sufficient to explain some observed λ5007/λ4363 ratios. Although the addition of a flux of cosmic rays is probably not the only way to explain these anomalies in the emission spectra of broad- and narrow-line regions, it does provide a mechanism that is both simple and natural for powering the optically emitting extended regions that lie in the extended radio lobes far from the central engine. The effects of the synchrotron emitting electrons on filaments in the Crab Nebula are discussed in an appendix, along with a comparison between our calculations, which employ the mean escape probability formalism, and recent Hubbard and Puetter models

Expanded Iron UTA spectra -- probing the thermal stability limits in AGN clouds

The Fe unresolved transition array (UTAs) produce prominent features in the 15-17?A wavelength range in the spectra of Active Galactic Nuclei (AGN). Here we present new calculations of the energies and oscillator strengths of inner- shell lines from Fe XIV, Fe XV, and Fe XVI. These are crucial ions since they are dominant at inflection points in the gas thermal stability curve, and UTA excitation followed by autoionization is an important ionization mechanism for these species. We incorporate these, and data reported in previous papers, into the plasma simulation code Cloudy. This updated physics is subsequently employed to reconsider the thermally stable phases in absorbing media in Active Galactic Nuclei. We show how the absorption profile of the Fe XIV UTA depends on density, due to the changing populations of levels within the ground configuration.Comment: ApJ in pres

The Origin of Fe II Emission in AGN

We used a very large set of models of broad emission line (BEL) clouds in AGN to investigate the formation of the observed Fe II emission lines. We show that photoionized BEL clouds cannot produce both the observed shape and observed equivalent width of the 2200-2800A Fe II UV bump unless there is considerable velocity structure corresponding to a microturbulent velocity parameter v_turb > 100 km/s for the LOC models used here. This could be either microturbulence in gas that is confined by some phenomenon such as MHD waves, or a velocity shear such as in the various models of winds flowing off the surfaces of accretion disks. The alternative way that we can find to simultaneously match both the observed shape and equivalent width of the Fe II UV bump is for the Fe II emission to be the result of collisional excitation in a warm, dense gas. Such gas would emit very few lines other than Fe II. However, since the collisionally excited gas would constitute yet another component in an already complicated picture of the BELR, we prefer the model involving turbulence. In either model, the strength of Fe II emission relative to the emission lines of other ions such as Mg II depends as much on other parameters (either v_turb or the surface area of the collisionally excited gas) as it does on the iron abundance. Therefore, the measurement of the iron abundance from the FeII emission in quasars becomes a more difficult problem.Comment: 23 pages. Accepted by Ap

H, He-like recombination spectra I : l-changing collisions for hydrogen

Hydrogen and helium emission lines in nebulae form by radiative recombination. This is a simple process which, in principle, can be described to very high precision. Ratios of He I and H I emission lines can be used to measure the He+/H+ abundance ratio to the same precision as the recombination rate coefficients. This paper investigates the controversy over the correct theory to describe dipole l-changing collisions (nl → nl0 = l ±1) between energy-degenerate states within an n-shell. The work of Pengelly & Seaton (1964) has, for half-a-century, been considered the definitive study which “solved” the problem. Recent work by Vrinceanu et al. (2012) recommended the use of rate coefficients from a semi-classical approximation which are nearly an order of magnitude smaller than those of Pengelly & Seaton (1964), with the result that significantly higher densities are needed for the nl populations to come into local thermodynamic equilibrium. Here, we compare predicted H I emissivities from the two works and find widespread differences, of up to ≈ 10%. This far exceeds the 1% precision required to obtain the primordial He/H abundance ratio from observations so as to constrain Big Bang cosmologies. We recommend using the rate coefficients of Pengelly & Seaton (1964) for l-changing collisions, to describe the H recombination spectrum, based-on their quantum mechanical representation of the long-range dipole interaction

The Origin of Fe II Emission in Active Galactic Nuclei

We used a very large set of models of broad emission line region (BELR) clouds in active galactic nuclei to investigate the formation of the observed Fe II emission lines. We show that photoionized BELR clouds cannot produce both the observed shape and observed equivalent width of the 2200-2800 Å Fe II UV bump unless there is considerable velocity structure corresponding to a microturbulent velocity parameter vturb≥100 km s-1 for the locally optimally emitting cloud models used here. This could be either microturbulence in gas that is confined by some phenomenon such as MHD waves or a velocity shear such as in the various models of winds flowing off the surfaces of accretion disks. The alternative way that we can find to simultaneously match both the observed shape and equivalent width of the Fe II UV bump is for the Fe II emission to be the result of collisional excitation in a warm, dense gas. Such gas would emit very few lines other than Fe II. However, since the collisionally excited gas would constitute yet another component in an already complicated picture of the BELR, we prefer the model involving turbulence. In either model, the strength of Fe II emission relative to the emission lines of other ions such as Mg II depends as much on other parameters (either vturb or the surface area of the collisionally excited gas) as it does on the iron abundance. Therefore, the measurement of the iron abundance from the Fe II emission in quasars becomes a more difficult problem