The Chemical Evolution of QSOs and the Implications for Cosmology and Galaxy Formation

We examine the chemical evolution of QSO broad-line gas by applying spectral synthesis and chemical enrichment models to the N V/C IV and N V/He II emission-line ratios. The models indicate that BLR metallicities are typically ~1 to perhaps ≳10 times solar. The enrichment must occur in ≲1 Gyr for sources where the redshift is ≳3 (if q0 = ½). The higher metallicity QSOs require star formation favoring massive stars (compared to the Galactic disk). These results imply that extensive evolution usually occurs before the QSOs become observable. Our models of the evolution are equivalent to models proposed for elliptical galaxies and for the bulges of disk galaxies. We conclude that the QSO phenomenon is preceded by vigorous star formation, exactly like that expected in massive, young galactic nuclei. The observed N V/C IV and N V/He II ratios can be several times larger in sources with high redshift and high luminosity. Systematically different physical conditions could contribute to these trends, but they could also result entirely from higher metallicities in the higher redshift/luminosity objects. We suggest that the high metallicities are related to higher QSO (and/or host galaxy) masses at large redshifts. This implies a mass- metallicity relation in QSOs analogous to the well-known relationship in nearby ellipticals. The trend with luminosity also suggests that metallicity differences can influence the observed global Baldwin effect. The evolution models predict a ~1 Gyr delay in the Fe enrichment due to Type Ia supernovae. The timescale for this delay is fixed by the (albeit uncertain) lifetimes of SN Ia precursors and is not sensitive to the IMF or star formation rates. The expected ˜1 Gyr delay could therefore be used as a clock to constrain QSO ages if accurate Fe abundances are measured. Age constraints could in turn constrain the cosmology (i.e., q0) when applied to high-redshift sources. One-zone photoionization models suggest that the delayed rise in Fe should be observable in, for example, the ratio of UV Fe II/Mg II emission lines. Most of the evolution models also predict Fe overabundances after ˜1-2 Gyr, with Fe/O and Fe/Mg up to several times solar. This overabundance might explain the strong Fe II emission observed in many QSOs and active galactic nuclei

The Chemical Enrichment of Gas in Broad Absorption Line QSOs: Rapid Star Formation in the Early History of Galaxies

Utilizing spectra from ground-based and Hubble Space Telescope observations, Korista et al. identified at least 12 broad absorption lines in the broad absorption line (BAL) QSO 0226-1024 (zem = 2.256), many for the first time. In the present paper we use seven of their measured column densities to investigate the BAL gas ionization equilibrium and metallicity. We assume that the metal abundances follow the rapid star formation (RSF) models of Hamann & Ferland or are simply scaled from solar ratios to (1) limit the free parameters in constraining the overall metallicity and (2) test the applicability of such a scenarios to the enrichment of BAL gas. Photoionization calculations which consider separately the highly ionized gas (C IV, N V, and O VI) and the doubly ionized metal gas (C III, N III, and O III), as well as those which consider the full set of ionic column densities, all indicate significant enhancements in the metal abundances over solar, up to ~10 Z⊙. The requirement for greater than solar metallicities is shown to be independent of the large range of ionizing continuum shapes and of the adopted enrichment scenario, and even larger metallicities could be derived if the metals are allowed to vary independently in a non-RSF enrichment scenario. Because the BALs in 0226 - 1024 that are measurable from the ground have column densities typical in BAL QSOs, the main result, Z \u3e Z⊙, should apply generally to the BAL QSO phenomenon. This result supports an enrichment scenario expected in the cores of massive galaxies in which early rapid star formation produces metal enriched gas. Metallicities enhanced over solar will increase the opacity of the outflowing BAL gas without significantly increasing the mass load of whatever is driving the outflow. This should have a significant impact on radiative acceleration models of BAL outflows which have, up to now, assumed solar metallicity

Broad NE VIII λ744 Emission from the Quasar PG 1148+549

We discuss the probable detection of broad Ne VIII λ774 emission from the zɛ = 0.978 quasar PG 1148+549, and we use spectral synthesis calculations to study the physical conditions in the line-forming gas. The theoretical predictions and the measured line wavelength both support Ne VIII λ774 as the most likely identification. Our calculations show that Ne VIII λ774 forms in hotter and more highly ionized gas than previously recognized in the broad emission line region. If the gas is photoionized by a standard active galactic nucleus continuum, the observed Ne VIII equivalent width, the Ne VIII/O VI flux ratio, and the assumption of cloud stability imply ionization parameters 5 ≲ U ≲ 30 and temperatures 5 × 104 K ≲ Tɛ ≲ 2 × 105 K. The large Ne VIII equivalent width also suggests that the emitting clouds cover ≲⅓ of the continuum source and have a total hydrogen column density NH 1022 cm-2. If the gas is instead collisionally ionized, Ne VIII could reside in stable clouds with equilibrium temperatures near 8 × 105 K. In either case, the Ne VIII-emitting clouds will appear as X-ray warm absorbers if they lie along our line of sight to the X-ray continuum source. Line thermalization can greatly lower some of the line fluxes and alter the line ratios. For example, C IV λ1549 can be thermalized easily, with doublet emission ratios λ1548/λ1550 near unity. However, our calculations indicate that Ne VIII λ774 is not thermalized and that its doublet emission ratio should be λ770/λ780 ≍ 2. Future observations of the widely separated Ne VIII doublet would test this prediction and constrain the space and column densities in the Ne VIII-emitting region. Finally, temperatures in the Ne VIII gas may be in the range needed for optically thin models of the UV continuum, i.e., the big blue bump. However, the diffuse thermal continuum flux from the Ne VIII- emitting region falls well short of the observed continua unless the measured lines are severely suppressed by thermalization at densities ≳5 × 1012 cm-3. This result supports Kriss\u27s claim that the optically thin blue-bump models have the serious problem of over-predicting the UV metal line fluxes

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

Chemical Abundances in Broad Emission Line Regions: The Nitrogen-loud\u27\u27 Quasi-Stellar Object Q0353-383

The intensity of the strong N V λ1240 line relative to C IV λ1549 or to He II λ1640 has been proposed as an indicator of the metallicity of quasi-stellar object (QSO) broad emission line regions, allowing abundance measurements in a large number of QSOs out to the highest redshifts. Previously, it had been shown that the (normally) much weaker lines N III] λ1750 and N IV] λ1486 could be used in the same way. The redshift 1.96 QSO 0353-383 has long been known to have N III] and N IV] lines that are far stronger relative to Lyα or C IV than in any other QSO. Because in this particular case these intercombination lines can be easily measured, this unusual object provides an ideal opportunity for testing whether the N V line is a valid abundance indicator. Using new observations of Q0353-383 made both with the Hubble Space Telescope in the ultraviolet and from the ground in the visible passband, we have carefully remeasured the emission lines and reanalyzed their strengths using the latest models of the QSO broad emission line region. We find that intensity ratios involving the strengths of N V, N IV], and N III] relative to lines of He, C, and O all indicate that nitrogen is overabundant relative to oxygen in Q0353-383 by a factor of ~15 compared to solar ratios. This agreement among the diagnostics supports the use of these lines for measuring broad emission line region chemical abundances. If nitrogen behaves like a secondary element, such that N/O~O/H, then the extreme nitrogen enhancement in Q0353-383 implies a metallicity of ~15 times the solar value. Even if Q0353-383 represents an extreme outlier in the N/O~O/H relation, the overall metallicity should still be at least 5 times solar. Unusually high metallicities in Q0353-383 might imply that we caught this object just as the gas-phase metallicity in the central part of its host galaxy has peaked, at a time when the interstellar gas supply is nearly exhausted and hence the fuel source for the central QSO is ready to shut off

Numerical Simulations of Fe II Emission Spectra

This paper describes the techniques that we have used to incorporate a large-scale model of the Fe+ ion and resulting Fe IIemission into CLOUDY, a spectral synthesis code designed to simulate conditions within a plasma and model the resulting spectrum. We describe the numerical methods we use to determine the level populations, mutual line overlap fluorescence, collisional effects, and the heating-cooling effects of the atom on its environment. As currently implemented, the atom includes the lowest 371 levels (up to 11.6 eV) and predicts intensities of 68,635 lines. We describe our data sources, which include the most recent transition probabilities and collision strengths. Although we use detailed fits to temperature-dependent collision strengths where possible, in many cases the uncertain g approximation is the only source for collision data. The atom is designed to be readily expanded to include more levels and to incorporate more accurate sets of collision and radiative data as computers grow faster and the atomic databases expand. We present several test cases showing that the atom goes to LTE in the limits of high particle and radiation densities. We give an overview of general features of the Fe II spectra as their dependencies on the basic parameters of our models (density, flux, microturbulent velocity, the Fe abundance, and Lyα pumping). Finally, we discuss several applications to active galactic nuclei to illustrate the diagnostic power of the Fe II spectrum and make some predictions for UV observations

High Metal Enrichments in Luminous Quasars

We discuss observations of the broad emission line in luminous intermediate redshift quasars, with emphasis on Q0207 -398. This object has relatively sharp lines, allowing us to deconvolve the profiles into distinct components. In this paper we examine the high ionization region, which we show must be matter-bounded, partially transparent to ionizing radiation. We measure a large N V λ1240/He II λ1640 ratio in Q0207-398, but detailed calculations predict this ratio to be ~1 for standard cloud properties. We show that no choice of incident continuum, ionizing photon flux, density, or even the appeal to nonradiative energy sources, can reproduce the observed N V, C IV, O VI, He II spectrum with solar abundances. The intensity of the N V line is sensitive to the abundance of the gas because the abundance of nitrogen, a secondary element goes up roughly as the square of the metallicity. The lowest metallicity capable of reproducing the N V spectrum in Q0207 - 398 is Z ~ 5 Z☉, consistent with that inferred to exist in the cores of massive galaxies after epochs of rapid star formation. Our final model of the high-ionization emission region is one with many properties similar to those inferred for broad absorption-line QSO absorption-line clouds

Very High Density Clumps and Outflowing Winds in QSO Broad-Line Regions

Intercomparison of the spectra of seven high-luminosity quasi-stellar objects (QSOs) shows that there is a gradation of properties in their broad emission-line regions (BELRs) that can be understood as differing mixtures of different gas components. Six of these seven objects have unusually narrow BELR profiles, which greatly facilitates the disentangling of blends and measuring of weak lines. In the QSO 0207 - 398, the BELR is made up of at least three kinematically distinct components; its spectrum is in fact a composite of the spectra of the more homogeneous BELRs in the QSOs at either end of our sequence of properties. This paper focuses on the properties of the line-emitting region in Q0207 - 398 dubbed component A, which has sharp (FWHM = 1000 km s-1), symmetric line profiles centered at zero velocity. We find that these lines are emitted in very dense (nH ~ 1012.5 cm-3) gas at a characteristic radius r ~ 1017.7 cm from the continuum source and which emits a low-ionization spectrum including strong A1 III λ1857. The second component, B, in Q0207 - 398 is the subject of a companion paper. It is characterized by high-ionization lines such as N V λ1240, O VI λ1034, and C IV λ1549 with profiles that peak at zero velocity but have a blue tail extending out to - 11,000 km s-1. It receives about the same incident flux as component A and therefore may lie at the same distance from the continuum source, but it is significantly less dense (nH ~ 1011 cm-3). The remaining line emission from Q0207 - 398 is attributed to a component C which has reasonably broad (FWHM = 2000 km s-1), symmetric line profiles centered at zero velocity. Most of the Lyα and C III] λ1909 emission comes from this region, but it also contributes to C IV, N V, and many other lines. The spectrum of component C is in fact quite similar to that of normal QSOs. We interpret component A as the dense source for radiatively accelerated, outward flowing gas which we see as component B. Component A may consist of the ablated atmospheres of stars which have strayed too close to the QSO nucleus. In addition, component A\u27s velocity width is essentially the same as that of the stellar population in the nucleus of M87. From the radius and velocity, we infer the presence of a central mass of only 107-108 M☉, if we are measuring virial motions and if the ionizing continuum is isotropic. An alternate possibility is that the narrow single-peaked component A profile comes from a rotating torus/accretion disk, probably seen face-on, in which case it is impossible to measure the central mass. We discuss the similarity between Mg II-type broad absorption line (BAL) QSOs and Q0207 - 398, which does not have BALs. In fact, the two Mg II-type BAL objects in our sample have spectra almost exactly like that of Q0207 - 398, except that component B is seen in absorption rather than in emission