Gary J. Ferland

Cloudy 90: Numerical simulation of plasmas and their spectra

ABSTRACT CLOUDY is a large‐scale spectral synthesis code designed to simulate fully physical conditions within an astronomical plasma and then predict the emitted spectrum. Here we describe version 90 (C90) of the code, paying particular attention to changes in the atomic database and numerical methods that have affected predictions since the last publicly available version, C84. The computational methods and uncertainties are outlined together with the direction future development will take. The code is freely available and is widely used in the analysis and interpretation of emission‐line spectra. Web access to the Fortran source for CLOUDY, its documentation Hazy, and an independent electronic form of the atomic database is also described.

Atomic data for astrophysics. II. New analytic fits for photoionization cross sections of atoms and ions

We present a complete set of analytic fits to the nonrelativistic photoionization cross sections for the ground states of atoms and ions of elements from H through Si, and S, Ar, Ca, and Fe. Near the ionization thresholds, the fits are based on the Opacity Project theoretical cross sections interpolated and smoothed over resonances. At higher energies, the fits reproduce calculated Hartree-Dirac-Slater photoionization cross sections. {copyright} {ital 1996 The American Astronomical Society.}

ELEMENTAL ABUNDANCES IN QUASISTELLAR OBJECTS: Star Formation and Galactic Nuclear Evolution at High Redshifts

▪ Abstract Quasar (QSO) elemental abundances provide unique probes of high-redshift star formation and galaxy evolution. There is growing evidence from both the emission and intrinsic absorption li...

Atomic Data for Astrophysics. I. Radiative Recombination Rates for H-like, He-like, Li-like, and Na-like Ions over a Broad Range of Temperature

We present new calculations and analytic fits to the rates of radiative recombination toward H-like, He-like, Li-like, and Na-like ions of all elements from H through Zn ({ital Z}=30). The fits are valid over a wide range of temperature, from 3 K to 10{sup 9} K. {copyright} {ital 1996 The American Astronomical Society.}

Physical conditions in the Orion nebula and an assessment of its helium abundance

New long-slit spectrophotometric observations were obtained to redetermine the helium abundance of the Orion Nebula. The ionic ratio He(+)/H(+) is found to remain nearly constant at 0.088 {plus minus} 0.006. In the bright innermost region, the errors are largely systematic due to uncertainties in the reddening curve for Orion grains and deviations from case B emissivity. In the outer regions, the errors are dominated by statistical errors. Photoionization models are computed to determine the correction for the unobserved presence of neutral helium in regions where hydrogen is ionized. 114 refs.

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 q 0 =1/2). 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

A photon dominated region code comparison study

Aims. We present a comparison between independent computer codes, modeling the physics and chemistry of interstellar photon dominated regions (PDRs). Our goal was to understand the mutual differences in the PDR codes and their effects on the physical and chemical structure of the model clouds, and to converge the output of different codes to a common solution. Methods. A number of benchmark models have been created, covering low and high gas densities n = 10 3 , 10 5.5 cm −3 and far ultraviolet intensities χ = 10, 10 5 in units of the Draine field (FUV: 6 < h ν< 13.6 eV). The benchmark models were computed in two ways: one set assuming constant temperatures, thus testing the consistency of the chemical network and photo-processes, and a second set determining the temperature self consistently by solving the thermal balance, thus testing the modeling of the heating and cooling mechanisms accounting for the detailed energy balance throughout the clouds. Results. We investigated the impact of PDR geometry and agreed on the comparison of results from spherical and plane-parallel PDR models. We identified a number of key processes governing the chemical network which have been treated differently in the various codes such as the effect of PAHs on the electron density or the temperature dependence of the dissociation of CO by cosmic ray induced secondary photons, and defined a proper common treatment. We established a comprehensive set of reference models for ongoing and future PDR model bench-marking and were able to increase the agreement in model predictions for all benchmark models significantly. Nevertheless, the remaining spread in the computed observables such as the atomic fine-structure line intensities serves as a warning that there is still a considerable uncertainty when interpreting astronomical data with our models.Aims. We present a comparison between independent computer codes, modeling the physics and chemistry of photon dominated regions (PDRs). Our goal was to understand the mutual differences in the PDR codes and their effects on the physical and chemical structure of the model clouds, and to converge the output of different codes to a common solution. Methods. A number of benchmark models have been calculated, covering low and high gas densities n = 103, 105.5 cm−3 and far ultraviolet intensities χ = 10, 105 (FUV: 6 < h ν < 13.6 eV). The benchmark models were computed in two ways: one set assuming constant temperatures, thus testing the consistency of the chemical network and photo-reactions, and a second set determining the temperature self consistently by solving the thermal balance, thus testing the modeling of the heating and cooling mechanisms accounting for the detailed energy balance throughout the clouds. Results. We investigated the impact of PDR geometry and agreed on the comparison of results from spherical and plane-parallel PDR models. We identified a number of key processes governing the chemical network which have been treated differently in the various codes such as the effect of PAHs on the electron density or the temperature dependence of the dissociation of CO by cosmic ray induced secondary photons, and defined a proper common treatment. We established a comprehensive set of reference models for ongoing and future PDR modeling and were able to increase the agreement in model predictions for all benchmark models significantly. Nevertheless, the remaining spread in the computed observables such as the atomic fine-structure line intensities serves as a warning that the astronomical data should not be overinterpreted.

The VLT‐UVES survey for molecular hydrogen in high‐redshift damped Lyman α systems: physical conditions in the neutral gas

We study the physical conditions in damped Lyman α systems (DLAs), using a sample of 33 systems towards 26 quasi-stellar objects (QSOs) acquired for a recently completed survey of H2 by Ledoux, Petitjean & Srianand. We use the column densities of H2 in different rotational levels, together with those of C I ,C I ∗ ,C I ∗∗ ,C II ∗ and singly ionized atomic species to discuss the kinetic temperature, the density of hydrogen and the electronic density in the gas together with the ambient ultraviolet (UV) radiation field. Detailed comparisons are made between the observed properties in DLAs, the interstellar medium (ISM) of the Galaxy and the Large and Small Magellanic Clouds (LMCs and SMCs). The mean kinetic temperature of the gas corresponding to DLA subcomponents in which an H2 absorption line is detected, derived from the ortho-to-para ratio (OPR) (153 ± 78 K), is higher than that measured in the ISM (77 ± 17 K) and in Magellanic Clouds (82 ± 21 K). Typical pressure in these components (corresponding to T = 100‐300 K and n H = 10‐ 200 cm −3 ), measured using C I fine-structure excitation, are higher than what is measured along ISM sightlines. This is consistent with the corresponding higher values for N (H2, J = 2)/N (H2, J = 0) seen in DLAs. From the column densities of the high-J rotational levels, we derive that the typical radiation field in the H2-bearing components is of the order of or slightly higher than the mean UV field in the Galactic ISM. Determination of electron density in the gas with H2 and C I shows that the ionization rate is similar to that of a cold neutral medium (CNM) in a moderate radiation field. This, together with the fact that we see H2 in 13‐20 per cent of the DLAs, can be used to conclude that DLAs at z > 1.9 could contribute as much as 50 per cent star formation rate (SFR) density seen in Lyman break galaxies (LBGs). C II ∗ absorption line is detected in all the components where H2 absorption line is seen. The excitation of C II in these systems is consistent with the physical parameters derived from the excitation of H2 and C I .W edetect C II ∗ in about 50 per cent of the DLAs and, therefore, in a considerable fraction of DLAs that do not show H2 .I npart of the later systems, physical conditions could be similar to that in the CNM gas of the Galaxy. However, the absence of C I absorption line and the presence of Al III absorption lines with a profile similar to the profiles of singly ionized species suggest an appreciable contribution from a warm neutral medium (WNM) and/or partially ionized gas. The absence of H2, for the level of metallicity and dust depletion seen in these systems, is consistent with low densities (i.e. n H 1c m −3 ) for a radiation field similar to the mean Galactic UV field. Ke yw ords: galaxies: haloes ‐ galaxies: ISM ‐ quasars: absorption lines ‐ cosmology:

An Atlas of Computed Equivalent Widths of Quasar Broad Emission Lines

We present graphically the results of several thousand photoionization calculations of broad emission-line clouds in quasars, spanning 7 orders of magnitude in hydrogen ionizing flux and particle density. The equivalent widths of 42 quasar emission lines are presented as contours in the particle density-ionizing flux plane for a typical incident continuum shape, solar chemical abundances, and cloud column density of N(H) = 1023 cm-2. Results are similarly given for a small subset of emission lines for two other column densities (1022 and 1024 cm-2), five other incident continuum shapes, and a gas metallicity of 5 Z☉. These graphs should prove useful in the analysis of quasar emission-line data and in the detailed modeling of quasar broad emission-line regions. The digital results of these emission-line grids and many more are available over the Internet.

Rate Coefficients for Charge Transfer between Hydrogen and the First 30 Elements

We present analytic fits to charge exchange rate coefficients over the full range of temperatures which occurs in photoionized or shock-heated plasmas. We consider reactions between neutral hydrogen and all elements with parent ion charge {ital q}=1{minus}4 up to {ital Z}=30. Many rates were obtained from various sources in the literature. For reactions for which no data were available, we calculated rates using the Landau-Zener formalism. For these new reactions, we tabulate both total and state-specific rate coefficients. All are fitted with a consistent, accurate formula. These fits may be incorporated easily into spectral synthesis codes, and we make available an electronic form of our results. We draw attention to the most important reactions without high-quality rate coefficients to encourage further work. {copyright} {ital 1996 The American Astronomical Society.}