Marios Chatzikos

Deep Chandra, HST-COS, and Megacam Observations of the Phoenix Cluster: Extreme Star Formation and AGN Feedback on Hundred Kiloparsec Scales

United States. National Aeronautics and Space Administration (Contract HST-GO-13456.002A (Hubble))

Stout: Cloudy's Atomic and Molecular Database

We describe a new atomic and molecular database we developed for use in the spectral synthesis code Cloudy. The design of Stout is driven by the data needs of Cloudy, which simulates molecular, atomic, and ionized gas with kinetic temperatures and densities spanning the low-to high-density limits. The radiation field between photon energies 10−8 Ry and 100 MeV is considered, along with all atoms and ions of the lightest 30 elements, and ~102 molecules. For ease of maintenance, the data are stored in a format as close as possible to the original data sources. Few data sources include the full range of data we need. We describe how we fill in the gaps in the data or extrapolate rates beyond their tabulated range. We tabulate data sources both for the atomic spectroscopic parameters and for collision data for the next release of Cloudy. This is not intended as a review of the current status of atomic data, but rather a description of the features of the database which we will build upon.

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.

H-, He-like recombination spectra – II. l-changing collisions for He Rydberg states

Cosmological models can be constrained by determining primordial abundances. Accurate predictions of the He I spectrum are needed to determine the primordial helium abundance to a precision of <1 per cent in order to constrain big bang nucleosynthesis models. Theoretical line emissivities at least this accurate are needed if this precision is to be achieved. In the first paper of this series, which focused on H I, we showed that differences in l-changing collisional rate coefficients predicted by three different theories can translate into 10 per cent changes in predictions for H I spectra. Here, we consider the more complicated case of He atoms, where low-l subshells are not energy degenerate. A criterion for deciding when the energy separation between l subshells is small enough to apply energy-degenerate collisional theories is given. Moreover, for certain conditions, the Bethe approximation originally proposed by Pengelly & Seaton is not sufficiently accurate. We introduce a simple modification of this theory which leads to rate coefficients which agree well with those obtained from pure quantal calculations using the approach of Vrinceanu et al. We show that the l-changing rate coefficients from the different theoretical approaches lead to differences of ∼10 per cent in He I emissivities in simulations of H II regions using spectral code CLOUDY.

Implications of coronal line emission in NGC 4696

We announce a new facility in the spectral code CLOUDY that enables tracking the evolution of a cooling parcel of gas with time. For gas cooling from temperatures relevant to galaxy clusters, earlier calculations estimated the [Fe XIV] λ 5303 / [Fe X] λ 6375 luminosity ratio, a critical diagnostic of a cooling plasma, to slightly less th an unity. By contrast, our calculations predict a ratio ∼3. We revisit recent optical coronal line observations alon g the X-ray cool arc around NGC 4696 by Canning et al. (2011), which detected [Fe X] λ 6375, but not [Fe XIV] λ 5303. We show that these observations are not consistent with predictions of cooling flow models. Differential extinction could in principle accoun t for the observations, but it requires extinction levels (AV > 3.625) incompatible with previous observations. The non-detection of [Fe XIV] implies a temperature ceiling of 2.1 million K. Assuming cylindrical geometry and transonic turbulent pressure support, we estimate the gas mass at ∼1 million M⊙. The coronal gas is cooling isochorically. We propose that the coronal gas has not condensed out of the intracluster medium, but instead is the conductive or mixing interface between the X-ray plume and the optical filaments. We present a number of emissi on lines that may be pursued to test this hypothesis and constrain the amount of intermediate temperature gas in the system.

ON THE OBSERVABILITY OF OPTICALLY THIN CORONAL HYPERFINE STRUCTURE LINES

We present CLOUDY calculations for the intensity of coronal hyperfine lines in various environments. We model indirect collisional and radiative transitions, and quantify the collisionally excited line emissivity in the density-temperature phase space. As an observational aid, we also express the emissivity in units of that in the 0.4-0.7 keV band. For most hyperfine lines, knowledge of the X-ray surface brightness and the plasma temperature is sufficient for rough estimates. We find that the radiation fields of both Perseus A and Virgo A can enhance the populations of highly ionized species within 1 kpc. They can also enhance line emissivity within the cluster core. This could have implications for the interpretation of spectra around bright active galactic nuclei. We find the intensity of the 57Fe XXIV λ3.068 mm line to be about two orders of magnitude fainter than previously thought, at ~20 μK. Comparably bright lines may be found in the infrared. Finally, we find the intensity of hyperfine lines in the Extended Orion Nebula to be low, due to the shallow sightline. Observations of coronal hyperfine lines will likely be feasible with the next generation of radio and submillimeter telescopes.

Accurate determination of the free–free Gaunt factor – I. Non-relativistic Gaunt factors

Modern spectral synthesis codes need the thermally averaged free-free Gaunt factor dened over a very wide range of parameter space in order to produce an accurate prediction for the spectrum emitted by an ionized plasma. Until now no set of data exists that would meet this need in a fully satisfactory way. We have therefore undertaken to produce a table of very accurate non-relativistic Gaunt factors over a much wider range of parameters than has ever been produced before. We rst produced a table of non-averaged Gaunt factors, covering the parameter space 10 log i = 20 to +10 and 10 logw = 30 to +25. We then continued to produce a table of thermally averaged Gaunt factors covering the parameter space 10 log 2 = 6 to +10 and 10 logu = 16 to +13. Finally we produced a table of the frequency integrated Gaunt factor covering the parameter space 10 log 2 = 6 to +10. All the data presented in this paper are

Effects of External Radiation Fields on Line Emission--Application to Star-Forming Regions

A variety of astronomical environments contain clouds irradiated by a combination of isotropic and beamed radiation fields. For example, molecular clouds may be irradiated by the isotropic cosmic microwave background, as well as by a nearby active galactic nucleus. These radiation fields excite atoms and molecules and produce emission in different ways. We revisit the escape probability theorem and derive a novel expression that accounts for the presence of external radiation fields. We show that when the field is isotropic the escape probability is reduced relative to that in the absence of external radiation. This is in agreement with previous results obtained under ad hoc assumptions or with the two-level system, but can be applied to complex many-level models of atoms or molecules. This treatment is in the development version of the spectral synthesis code CLOUDY. We examine the spectrum of a Spitzer cloud embedded in the local interstellar radiation field and show that about 60% of its emission lines are sensitive to background subtraction. We argue that this geometric approach could provide an additional tool toward understanding the complex radiation fields of starburst galaxies.

Thermodynamically-consistent semi-classical ℓ-changing rates

We compare the results of the semi-classical (SC) and quantum- mechanical (QM) formalisms for angular-momentum changing transitions in Rydberg atom collisions given by Vrinceanu et al , with those of the SC formalism using a modified Monte Carlo realization. We find that this revised SC formalism agrees well with the QM results. This provides further evidence that the rates derived from the QM treatment are appropriate to be used when modelling recombination through Rydberg cascades, an important process in understanding the state of material in the early universe. The rates for ∆ l = ± 1 derived from the QM formalism diverge when integrated to sufficiently large impact parameter, b . Further to the empirical limits to the b integration suggested by Pengelly & Seaton, MNRAS 127 , 165 (1964), we suggest that the fundamental issue causing this divergence in the theory is that it does not fully cater for the finite time taken for such distant collisions to complete.

Ultraviolet emission lines of Si ii in cool star and solar spectra

Recent atomic physics calculations for Si II are employed within the Cloudy modelling code to analyse Hubble Space Telescope (HST) STIS ultraviolet spectra of three cool stars, Beta-Geminorum, Alpha-Centauri A and B, as well as previously published HST/GHRS observations of Alpha-Tau, plus solar quiet Sun data from the High Resolution Telescope and Spectrograph. Discrepancies found previously between theory and observation for line intensity ratios involving the 3s