arXiv subjects

R. L. Porter

Publications and source records attributed to R. L. Porter.

26 records · Page 2Linked to original sources

arXiv2008

Collisional heating as the origin of filament emission in galaxy clusters

[Abridged] Photoionization, whether by starlight or other sources, has difficulty in accounting for the observed spectra of the optical filaments that often surround central galaxies in large clusters. Our first paper examined whether heating by energetic particles or dissipative MHD wave can account for the observations. Here we include atomic and low-ionization regions. The model of the hydrogen atom, along with all elements of the H-like iso-electronic sequence, is now fully nl-resolved. We show how the predicted HI spectrum differs from the pure recombination case. The second update is to the rates for H^0 - H2 inelastic collisions. We now use the values computed by Wrathmall et al. The rates are often much larger. We calculate the chemistry, ionization, temperature, gas pressure, and emission-line spectrum for a wide range of gas densities and collisional heating rates. We assume that the filaments are magnetically confined and free to move along field lines so that the gas pressure is equal to that of the surrounding hot gas. A mix of clouds, some being dense and cold and others hot and tenuous, can exist. The observed spectrum will be the integrated emission from clouds with different densities and temperatures but the same pressure. We assume that the gas filling factor is given by a power law in density. The power-law index is set by matching the observed intensities of IR H2 lines relative to optical HI lines. We conclude that the filaments are heated by ionizing particles, either conducted in from surrounding regions or produced in situ by processes related to MHD waves.

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arXiv2008

The origin of molecular hydrogen emission in cooling-flow filaments

The optical filaments found in many cooling flows in galaxy clusters consist of low density ($\sim 10^3 \pcc$) cool ($\sim 10^3$ K) gas surrounded by significant amounts of cosmic-ray and magnetic-field energy. Their spectra show anomalously strong low-ionization and molecular emission lines when compared with galactic molecular clouds exposed to ionizing radiation such as the Orion complex. Previous studies have shown that the spectra cannot be produced by O-star photoionization. Here we calculate the physical conditions in dusty gas that is well shielded from external sources of ionizing photons and is energized either by cosmic rays or dissipative MHD waves. Strong molecular hydrogen lines, with relative intensities similar to those observed, are produced. Selection effects introduced by the microphysics produce a correlation between the \htwo line upper level energy and the population temperature. These selection effects allow a purely collisional gas to produce \htwo emission that masquerades as starlight-pumped \htwo but with intensities that are far stronger. This physics may find application to any environment where a broad range of gas densities or heating rates occur.

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arXiv2007

A He I Case-B Recombination Code

Recent calculations of collisionless, Case-B, He I emissivities were performed by Bauman et al. (2005). The source code used in the calculation has been freely available online since that paper was published. A number of changes have been made to simplify the use of the code by third parties. Here I provide details on how to obtain, compile, and execute the program and interpret the results.

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arXiv2007

Revisiting He-like X-ray Emission Line Plasma Diagnostics

A complete model of helium-like line and continuum emission has been incorporated into the plasma simulation code Cloudy. All elements between He and Zn are treated, any number of levels can be considered, and radiative and collisional processes are included. This includes photoionization from all levels, line transfer including continuum pumping and destruction by background opacities, scattering, and collisional processes. The model is calculated self-consistently along with the ionization and thermal structure of the surrounding nebula. The result is a complete line and continuum spectrum of the plasma. Here we focus on the ions of the He I sequence and reconsider the standard helium-like X-ray diagnostics. We first consider semi-analytical predictions and compare these with previous work in the low-density, optically-thin limit. We then perform numerical calculations of helium-like X-ray emission (such as is observed in some regions of Seyferts) and predict line ratios as a function of ionizing flux, hydrogen density, and column density. In particular, we demonstrate that, in photoionized plasmas, the $R$-ratio, a density indicator in a collisional plasma, depends on the ionization fraction and is strongly affected by optical depth for large column densities. We also introduce the notion that the $R$-ratio is a measure of the incident continuum at UV wavelengths. The $G$-ratio, which is temperature-sensitive in a collisional plasma, is also discussed, and shown to be strongly affected by continuum pumping and optical depth as well. These distinguish a photoionized plasma from the more commonly studied collisional case.

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arXiv2006

He~I Emission in the Orion Nebula and Implications for Primordial Helium Abundance

We apply a recently developed theoretical model of helium emission to observations of both the Orion Nebula and a sample of extragalactic H II regions. In the Orion analysis, we eliminate some weak and blended lines and compare theory and observation for our reduced line list. With our best theoretical model we find an average difference between theoretical and observed intensities $ = 6.5%$. We argue that both the red and blue ends of the spectrum may have been inadequately corrected for reddening. For the 22 highest quality lines, with $3499 Å\leλ\le6678 Å$, our best model predicts observations to an average of 3.8%. We also perform an analysis of the reported observational errors and conclude they have been underestimated. In the extragalactic analysis, we demonstrate the likelihood of a large systematic error in the reported data and discuss possible causes. This systematic error is at least as large as the errors associated with nearly all attempts to calculate the primordial helium abundance from such observations. Our Orion analysis suggests that the problem does not lie in the theoretical models. We demonstrate a correlation between equivalent width and apparent helium abundance of lines from extragalactic sources that is most likely due to underlying stellar absorption. Finally, we present fits to collisionless case-B He I emissivities as well as the relative contributions due to collisional excitations out of the metastable $2s {}^{3}S$ term.

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arXiv2006

A Cloudy/Xspec Interface

We discuss new functionality of the spectral simulation code CLOUDY which allows the user to calculate grids with one or more initial parameters varied and formats the predicted spectra in the standard FITS format. These files can then be imported into the x-ray spectral analysis software XSPEC and used as theoretical models for observations. We present and verify a test case. Finally, we consider a few observations and discuss our results.

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arXiv2005

J-resolved He I emission predictions in the low-density limit

Determinations of the primordial helium abundance are used in precision cosmological tests. These require highly accurate He I recombination rate coefficients. Here we reconsider the formation of He I recombination lines in the low-density limit. This is the simplest case and it forms the basis for the more complex situation where collisions are important. The formation of a recombination line is a two-step process, beginning with the capture of a continuum electron into a bound state and followed by radiative cascade to ground. The rate coefficient for capture from the continuum is obtained from photoionization cross sections and detailed balancing, while radiative transition probabilities determine the cascades. We have made every effort to use today's best atomic data. Radiative decay rates are from Drake's variational calculations, which include QED, fine structure, and singlet-triplet mixing. Certain high-$L$ fine-structure levels do not have a singlet-triplet distinction and the singlets and triplets are free to mix in dipole-allowed radiative decays. We use quantum defect or hydrogenic approximations to include levels higher than those treated in the variational calculations. [Abstract clipped]

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arXiv2005

Theoretical He I Emissivities in the Case B Approximation

We calculate the He I case B recombination cascade spectrum using improved radiative and collisional data. We present new emissivities over a range of electron temperatures and densities. The differences between our results and the current standard are large enough to have a significant effect not only on the interpretation of observed spectra of a wide variety of objects but also on determinations of the primordial helium abundance.

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