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 the continuum 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 AGN. We find the intensity of the $^{57}$Fe XXIV {\lambda}3.068 mm to be about two orders of magnitude fainter than previously thought, at about 20 {\mu}K. 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 sub-mm telescopes.Comment: 48 pages; 13 figures; 9 tables; additional images available at the online journa

The [CII] 158\u3cem\u3eμ\u3c/em\u3em Line Emission in High-Redshift Galaxies

Gas is a crucial component of galaxies, providing the fuel to form stars, and it is impossible to understand the evolution of galaxies without knowing their gas properties. The [CII] fine structure transition at 158 μm is the dominant cooling line of cool interstellar gas, and is the brightest of emission lines from star forming galaxies from FIR through metre wavelengths, almost unaffected by attenuation. With the advent of ALMA and NOEMA, capable of detecting [CII]-line emission in high-redshift galaxies, there has been a growing interest in using the [CII] line as a probe of the physical conditions of the gas in galaxies, and as a star formation rate (SFR) indicator at z ≥ 4. In this paper, we have used a semi-analytical model of galaxy evolution (G.A.S.) combined with the photoionisation code CLOUDY to predict the [CII] luminosity of a large number of galaxies (25 000 at z ≃ 5) at 4 ≤ z ≤ 8. We assumed that the [CII]-line emission originates from photo-dominated regions. At such high redshift, the CMB represents a strong background and we discuss its effects on the luminosity of the [CII] line. We studied the L[CII]–SFR and L[CII]–Zg relations and show that they do not strongly evolve with redshift from z = 4 and to z = 8. Galaxies with higher [CII] luminosities tend to have higher metallicities and higher SFRs but the correlations are very broad, with a scatter of about 0.5 and 0.8 dex for L[CII]–SFR and L[CII]–Zg, respectively. Our model reproduces the L[CII]–SFR relations observed in high-redshift star-forming galaxies, with [CII] luminosities lower than expected from local L[CII]–SFR relations. Accordingly, the local observed L[CII]–SFR relation does not apply at high-z (z ≳ 5), even when CMB effects are ignored. Our model naturally produces the [CII] deficit (i.e. the decrease of L[CII]/LIR with LIR), which appears to be strongly correlated with the intensity of the radiation field in our simulated galaxies. We then predict the [CII] luminosity function, and show that it has a power law form in the range of L[ CII] probed by the model (1 × 107–2 × 109 L⊙ at z = 6) with a slope α = −1. The slope is not evolving from z = 4 to z = 8 but the number density of [CII]-emitters decreases by a factor of 20×. We discuss our predictions in the context of current observational estimates on both the differential and cumulative luminosity functions

Chandra Observation of Abell 2065: An Unequal Mass Merger?

We present an analysis of a 41 ks Chandra observation of the merging cluster Abell 2065 with the ACIS-I detector. Previous observations with ROSAT and ASCA provided evidence for an ongoing merger, but also suggested that there were two surviving cooling cores, which were associated with the two cD galaxies in the center of the cluster. The Chandra observation reveals only one X-ray surface brightness peak, which is associated with the more luminous, southern cD galaxy. The gas related with that peak is cool and displaced slightly from the position of the cD. The data suggest that this cool material has formed a cold front. On the other hand, in the higher spatial resolution Chandra image, the second feature to the north is not associated with the northern cD; rather, it appears to be a trail of gas behind the main cD. We argue that only one of the two cooling cores has survived the merger, although it is possible that the northern cD may not have possessed a cool core prior to the merger. We use the cool core survival to constrain the kinematics of the merger and we find an upper limit of ~< 1900 km/s for the merger relative velocity. A surface brightness discontinuity is found at ~140 kpc from the southern cD; the Mach number for this feature is ${\cal M} = 1.66^{+0.24}_{-0.32}$, although its nature (shock or cold front) is not clear from the data. We argue that Abell 2065 is an example of an unequal mass merger. The more massive southern cluster has driven a shock into the ICM of the infalling northern cluster, which has disrupted the cool core of the latter, if one existed originally. We estimate that core crossing occurred a few hundred Myr ago, probably for the first time.Comment: 15 pages, 10 figures, ApJ in pres

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 (CMB), as well as by a nearby active galactic nucleus (AGN). 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 percent 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.Comment: 12 pages, 7 figures, accepted for publication to Ap

Self-Consistent Grain Depletions and Abundances II: Effects on strong-line diagnostics of extragalactic H II regions

The depletion of elements onto dust grains is characterized using a generalized depletion strength $F_*$ for any sightline, and trend-line parameters $A_X, B_X$ and $z_X$. The parameters $A_X, B_X$ and $z_X$ define the relative depletion pattern, for which values are published in previous works. The present study uses these parameters to calculate post-depleted gas-phase abundances of 15 different elements while varying $F_*$ from 0 to 1. An analysis of emergent strong spectral line intensities, obtained by inputting the calculated abundances into a cloudy model, shows that the depletion strength has a non-trivial effect on predicted emission lines and the thermal balance of the ionized cloud. The amount by which elements deplete also affects the coolant abundances in the gas. Furthermore, it was found that each of the parameters - metallicity, ionization parameter U and depletion strength $F_*$ have degenerate effects on the emission-line strengths, and thermal balance of the interstellar medium (ISM). Finally, comparing our results to a sample of H II regions using data obtained from the Mapping Nearby Galaxies at Apache Point Observatory survey (MaNGA) revealed that the best-fit $F_*$ was approximately 0.5. However, this best-fit value does not work well for all metallicities. Removing the sulfur depletion and changing the nitrogen abundance pattern can improve the fit. As a result, extra observational evidence is required to verify the choices of parameters and better constrain the typical depletion strength in galaxies.Comment: 11 pages, 5 figure

The 23.01 release of Cloudy

We announce the C23.01 update of Cloudy. This corrects a simple coding error, present since $\sim$ 1990, in one routine that required a conversion from the line-center to the mean normalization of the Ly$\alpha$ optical depth. This affects the destruction of H I Ly$\alpha$ by background opacities. Its largest effect is upon the Ly$\alpha$ intensity in high-ionization dusty clouds, where the predicted intensity is now up to three times stronger. Other properties that depend on Ly$\alpha$ destruction, such as grain infrared emission, change in response.Comment: 4 pages, 1 figur

Thermodynamically-Consistent Semi-Classical \u3cem\u3eℓ\u3c/em\u3e-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 in a series of papers by Vrinceanu et al, most recently Vrinceanu et al (2012 Astrophys. J. 747 56), 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 modeling recombination through Rydberg cascades, an important process in understanding the state of material in the early universe. The rates for Δℓ = ±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 and Seaton (1964 Mon. Not. R. Astron. Soc. 127 165), 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

H, He-Like Recombination Spectra – I. \u3cem\u3el\u3c/em\u3e-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 → nl′ = l ± 1) between energy-degenerate states within an n-shell. The work of Pengelly & Seaton has, for half-a-century, been considered the definitive study which ‘solved’ the problem. Recent work by Vrinceanu et al. recommended the use of rate coefficients from a semiclassical approximation which are nearly an order of magnitude smaller than those of Pengelly & Seaton, 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 per cent. This far exceeds the 1 per cent 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 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. \u3cem\u3el\u3c/em\u3e-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 \u3c 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 ι, 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 ι 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 ι emissivities in simulations of H ιι regions using spectral code CLOUDY