Emission Line Intensity Ratios Research Articles

The ionizing radiation of Seyfert 2 galactic nuclei

We report the discovery of a nonrandom trend in the dispersion of emission-line intensity ratios for Seyfert 2 galaxies. The sense of this pattern suggests the influence of a single physical parameter, the hardness of the ionizing continuum, which controls the heating energy per ionizing photon. We compare the observed line ratios with new photoionization calculations and find that the observed distributions can be reproduced if the ionizing continuum is parametrized by a power law. Our results also suggest an inverse correlation between luminosity and continuum hardness for Seyfert 2 nuclei; if true, this trend extends a similar pattern known in quasars and Seyfert 1 galaxies to active galactic nuclei of lower luminosity. Samples of Seyfert 2 nuclei with improved selection uniformity are desirable for elaboration of these findings.

PHYSICAL PROPERTIES OF EXTENDED EMISSION-LINE REGIONS IN ACTIVE AND COOLING-FLOW GALAXIES

ABSTRACT This dissertation utilizes primarily optical observations of extended emission-line regions in active and cooling-flow galaxies to clarify the nature of this nebulosity and its underlying sources of energy. In particular, I employ case studies of Seyfert galaxies in order to elucidate nuclear properties as seen in light that is apparently scattered into our line of sight, and to provide direct measurements of emission-line intensity ratios resulting from a radially diluted, photoionizing continuum for comparison with theory. The apparent Seyfert 2 galaxy NGC 4388 displays a weak, spatially extended, broad-line emission that is most readily understood as scattered light from a hidden Seyfert 1 nucleus. IC 5135 exhibits both Seyfert 2 and starburst activity in its nucleus, and the influence of both can be traced in nuclear emission-line profiles as well as line-intensity ratios of spatially resolved nebulosity. I also report results bearing on star formation and the rate of mass accretion in the cluster cooloing-flow galaxy NGC 1275, as revealed from spectroscopy of the nebular filaments in this system. While preferential low-mass star formation has been invoked to reconcile high X-ray inferred accretion rates in cooling flows with a paucity of apparent cooled matter, data reported here reveal a site of high-mass star formation in material believed to arise from the flow. Detailed scrutiny of this region suggests, moreover, that the initial mass function of star formation in this locale is unlikely to be biased unusually toward low-mass star formation. These results provide additional evidence that the true rate of mass deposition may be less than implied by a simple interpretation of the X-ray data. The same data set is used to derive improved upper temperatures. The ability of these measurements to constrain the accretion rate is limited by exclusion of the galaxy center, which is host to a Seyfert nucleus. Finally, I present results of a narrow-band imaging survey of X-ray selected, early-type galaxies that are examined for evidence linking nebulosity in these sources to cooling-flow or weak Seyfert phenomena. These results suggest that optical nebulosity in these systems probably bears a multiparameter relationship to cooling flows, active nuclei, and accretion from external sources.

Temperature-sensitive line ratios of the lithium-like ion O vi

The intensity ratios of emission lines from the lithium-like ion Ovi are calculated as a function of electron temperature using recent computations of collision strengths by Zhang, Sampson, and Fontes (1990). The line ratios are strongly dependent on temperature and are appropriate as a diagnostic of hot thin plasmas such as in the solar transition region and corona.

The excited-state population distribution of titanium atoms in A d.c. Argon plasma jet at atmospheric pressure

The distribution of excited-state titanium atoms in an argon plasma at atmospheric pressure has been studied by spectroscopic measurements. The upper levels of the argon atoms were found to be in equilibrium with the continuum; electron collisions are dominant for these levels and the Boltzmann temperature compared favorably with the LTE temperature obtained from the extrapolated electron density. The distribution of titanium atoms was found to be far from equilibrium. The temperature determined from the intensity ratios of titanium emission lines differs by a factor of more than two from the electron temperature obtained from argon emission-line data. The titanium data were corrected by using a modified diffusion model. The resulting electron temperature is within 14% of that determined from a Boltzmann plot of the argon emission lines. This study suggests a procedural method to use emission-line data of metal atoms in a low-temperature plasma to obtain a better estimate of the electron temperature and to predict other population densities for situations in which the energy separation of the excited state from the ionization level is sufficiently large so that radiation processes are dominant over excited-state depopulations.

Models of the continuum of quasars and AGNs in the EUV and adjacent energy ranges

Abstract It is in the extreme ultraviolet band (1200 A − 1 keV) that the energy emitted by quasars and AGNs is most intense. No direct information however is yet available in this range. What constraints on the EUV continuum flux are set by the observations of the continuum in the optical, UV and X-ray ranges, and by the observations of optical/UV emission line intensity ratios? What constraints are set by plausible models of the UV/X spectrum? What are the signatures of an accretion disk in this range? We examine these questions and conclude that direct observations of the flux variations and the spectral shape of the EUV continuum are essential to make progress in this field.

Physical considerations of the nuclear region of NGC 3256

The nuclear region of the peculiar galaxy NGC 3256 has been studied spectroscopically in the visual domain of the spectrum. Its spectral characteristics indicate low excitation, high oxygen, and nitrogen abundances besides a comparative excess of N with respect to O and a rather low electron density. Emission line intensity ratios correspond to anHii region spectrum compatible with a star-burst nucleus.

Ratio of Balmer line intensities resulting from dissociative excitation of molecular hydrogen in an ionizing plasma

We have estimated the cross section for the production of excited hydrogen atoms from the emission cross sections for Lyman and Balmer lines when molecular hydrogen is dissociated into excited atoms. We have constructed a collisional-radiative model for the system of ground-state hydrogen molecules, excited hydrogen atoms, and protons, in a plasma and have calculated the population density distribution of the excited states. The resulting emission-line intensity ratios for the Balmer series lines are plotted against electron density. We expect these results to be useful for studies of plasmas containing molecular hydrogen, such as occur in the outer region of tokamak plasmas.

Electron density from Mg VIII emission lines

Mg VIII emission line intensity ratio I (430.47)/I (436.62) has been studied in detail and is found to be an excellent candidate for diagnosing the quiet Sun and the coronal holes.

New density diagnostic method based on emission line intensity ratio of neutral hydrogen in an ionizing phase plasma

For high temperature hydrogen plasmas in the ionizing phase, the population density of the excited levels has been calculated for a wide range of electron densities and temperatures. In the calculation we employed the most recent and reliable cross-section data for excitation between discrete levels and the ionization cross-section data from these levels. The line intensity ratios calculated under conditions of optical thinness are presented graphically for the Lyman and Balmer series lines. The intensity ratios are found to be rather insensitive to the electron temperature for temperatures higher than 10 eV. These values may be used for determining the electron density in the region of 1017 ⪅ ne ⪅ 1021 m−3, which is relevant to fusion research plasmas. Radiation loss from the plasma and energy loss from the plasma electrons are also presented for a wide range of electron densities and temperatures.

Optical and ultraviolet spectra of active galactic nuclei: Searching for accretion disks

The most important characteristics of the optical, ultraviolet (UV), and (to a lesser extent) near-infrared spectra of active galactic nuclei are described. These include the broad and narrow emission lines, a nonstellar continuum having approximately power-law shape from near-infrared to red wavelengths, and a large excess of radiation over the power law at visual and UV wavelengths. Aspects of the spectra that may indicate the existence and properties of accretion disks are examined in greatest detail. Specifically, the “big blue bump” is probably thermal radiation from an accretion disk. Its presence may contribute to the observed inverse correlation between UV continuum luminosity and equivalent width of UV emission lines (the “Baldwin effect”). The excess UV radiation should also affect some of the emission-line intensity ratios. Various inconsistencies in current photoionization models, such as that between the observed and inferred shapes of the ionizing continuum, can be eliminated if the UV photons are emitted anisotropically whereas the X-rays are isotropic. Variability of the intensity ratios may reveal changes in the accretion rate. If the disk produces emission lines, their profiles should be double-peaked, although emission originating from the standard broad-line clouds may mask this effect. At high velocities within the line profiles, however, the relative strengths of lines should nevertheless differ from those produced by the standard clouds. Finally, some evidence for disks and thick tori is provided by certain correlations between optical and radio properties of quasars, as well as by spectropolarimetric studies of type 2 Seyfert galaxies.

Spectrophotometry of six broad absorption line QSOs

Spectrophotometric observations of six broad absorption-line QSOs (BALQSOs) are presented. The continua and emission lines are compared with those in the spectra of QSOs without BALs. A statistically significant difference is found in the emission-line intensity ratio for (N V 1240-A)/(C IV 1549-A). The median value of (N V)/(C IV) for the BALQSOs is two to three times the median for QSOs without BALs. The absorption features of the BALQSOs are described, and the column densities and limits on the ionization structure of the BAL region are discussed. If the dominant ionization mechanism is photoionization, then it is likely that either the ionizing spectrum is steep or the abundances are considerably different from solar. Collisional ionization may be a significant factor, but it cannot totally dominate the ionization rate.

Energy levels of Er3+in Cs2NaErCl6

The analysis of the energy level data available for Cs2NaErCl6 is given, the mean error in fitting 31 levels being 16·4 cm-1. The eigenvectors obtained from the calculation are used to compute spontaneous emission probabilities and line intensity ratios for the 4 I 13/2 → 4 I 15/2 magnetic-dipole transitions of Er3+ in Cs2NaErCl6. The liquid-helium-temperature infrared luminescence spectrum for this transition of ErCl3- 6 in cubic elpasolites is reported for the first time and the line locations and relative intensities are in agreement with calculation.

STarburst Models for AGNs

A significant fraction of all spiral galaxies exhibit some type of “activity” in their nuclear regions as evidenced by the presence of emission lines in their optical spectrum (Keel, 1982; Cetty-Veron and Veron, 1985). It has become standard to classify emission-line galaxies into two main groups: “active” having Seyfert or Liner nuclei and “inactive”, having Starburst or HII-region like nuclei. Two main classification criteria are used, one based in the widths (Khachikian and Weedman 1974) the other in the intensity ratios of the nuclear emission lines (Baldwin, Phillips and Terlevich, 1981).

S V line ratios in the sun

In the present prediction of level populations and emission line intensity ratios for electron densities and temperatures appropriate to the sun, on the basis of new atomic data for S V, the electron impact collision rates for spin-forbidden transitions, and the intercombination transition spontaneous radiative rate, are noted to be substantially larger than previously ascertained. The S V intensity ratio is shown to be a useful electron density diagnostic for log N(e) greater than 11.5 ratios deduced from observations obtained with a slit spectrograph aboard Skylab generally agree with the theoretical values presented.

C II emission lines formed in optically thin plasmas

New Einstein A-coefficients and electron impact collision rates are presented for transitions between the three lowest ls of C II. The former are based on complex configuration interaction wave functions, while the latter use such wave functions in an eight-state R-matrix calculation of the collision process. These new atomic data are then used to calculate populations for the corresponding seven fine-structure levels on the assumption of an isotropic optically thin plasma. Theoretical emission-line intensity ratios deduced from these populations are in excellent agreement with observed low- and high-density limits.

Ca XVII line ratios in solar flares

New electron impact collision rates are presented for transitions between the 10 lowest states of Ca XVII. These data are then used to predict Ca XVII level populations and emission-line intensity ratios for electron densities and temperatures appropriate to solar flares. A comparison is made with observations obtained with the NRL spectro-heliograph aboard Skylab. For the intercombination to resonance line ratio the agreement is good. For other triplet to singlet ratios agreement is less satisfactory, possibly due to uncertainties in the observational data.

SI III line ratios in the sun

New atomic data for Si III have been used to predict level populations and emission-line intensity ratios for electron densities and temperatures appropriate to the solar transition region. The electron impact collision rates used here are substantially larger than those previously published owing to delineation of the complex resonance structures in the low-energy collision strengths. This together with small changes in the spontaneous radiative rates produces significant changes in the calculated intensity ratios. Generally good agreement is found with observations obtained using the Naval Research Laboratory slit spectrograph aboard Skylab, electron densities from three Si III ratios and from other methods normally agreeing to 0.2 dex or better for a wide variety of solar features. For a fourth ratio, incorporating lines with a wide wavelength separation, the agreement is less satisfactory, possibly owing to uncertainties in the observational data.

On the intensity ratio of emission lines of Nai D1 to D2 in prominences

Spectroscopic observations of the Nai D emission lines of prominences were made with the Domeless Solar Telescope in Hida Observatory. When active prominences are bright in the D2 emission line, the intensity ratio of D1 to D2 is found to deviate significantly from the theoretical ratio of the optically-thin case. On the other hand, the intensity ratio is close to the theoretical ratio for the most part of quiescent prominences. Furthermore, the full widths at half maximum intensity of the D2 emission line for active prominences become wider than those of the D1 line, as the intensity of the D2 line gets higher. These observed features clearly show that the emitting region of the Nai D lines is optically thick in some types of prominences.

Electric fields in coronal magnetic loops

We analyze spectra taken with the 40 cm coronograph at Sacramento Peak Observatory, for evidence of Stark effect on Balmer lines formed in coronal magnetic structures. Several spectra taken near the apex of a bright post-flare loop prominence show significant broadening from H10 to the limit of Balmer line visibility in these spectra, at about H20 The most likely interpretation of the increasing width is Stark broadening, although unresolved blends of Balmer emissions with metallic lines could also contribute to the trend. Less significant broadening is seen in 3 other post-flare loops, and the data from 5 other active coronal condensations observed in this study show no broadening tendency at all, over this range of Balmer number. The trend clearly observed in one post-flare loop requires an ion density of ni ≃ 2 × 1012 cm−3, if it is to be explained entirely as Stark effect caused by pressure broadening. But mean electron densities measured directly from the Thomson scattering at λ3875 in the same SPO spectra, yield ne ≲ 3−7 × 1010 cm−3 for the same condensations observed within that loop. Comparison of this evidence from electron scattering, with densities derived from emission measures and line-intensity ratios, argues against a volume filling factor small enough to reconcile the values of ni and ne derived in this study. This discrepancy leads us to suggest that the Stark effect observed in these loops, and possibly also in flares, could be caused by macroscopic electric fields, rather than by pressure broadening. The electric field required to explain the Stark broadening in the brightest post-flare loop observed here is approximately 170 V cm−1. We suggest an origin for such an electric field and discuss its implications for coronal plasma dynamics.

CLASSIFICATION PARAMETERS FOR THE EMISSION-LINE SPECTRA OF EXTRAGALACTIC OBJECTS.

ABSTRACT An investigation is made of the merits of various emission-line intensity ratios for classifying the spectra of extragalactic objects. It is shown empirically that several combinations of easily-measured lines can be used to separate objects into one of four categories according to the principal excitation mechanism: normal H II regions, planetary nebulae, objects photoionized by a power-law continuum, and objects excited by shock-wave heating. A two-dimensional quantitative classification scheme is suggested.