arXiv subjects

T. A. Bell

Publications and source records attributed to T. A. Bell.

41 records · Page 3Linked to original sources

arXiv2007

Molecular line intensities as measures of cloud masses - II. Conversion factors for specific galaxy types

We present theoretically-established values of the CO-to-H2 and C-to-H2 conversion factors that may be used to estimate the gas masses of external galaxies. We consider four distinct galaxy types, represented by M51, NGC 6946, M82 and SMC N27. The physical parameters that best represent the conditions within the molecular clouds in each of the galaxy types are estimated using a chi^2 analysis of several observed atomic fine structure and CO rotational lines. This analysis is explored over a wide range of density, radiation field, extinction, and other relevant parameters. Using these estimated physical conditions in methods that we have previously established, CO-to-H2 conversion factors are then computed for CO transitions up to J=9-8. For the conventional CO(1-0) transition, the computed conversion factor varies significantly below and above the canonical value for the Milky Way in the four galaxy types considered. Since atomic carbon emission is now frequently used as a probe of external galaxies, we also present, for the first time, the C-to-H2 conversion factor for this emission in the four galaxy types considered.

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arXiv2006

Molecular dark matter in galaxies

Clouds containing molecular dark matter in quantities relevant for star formation may exist in minihaloes of the type of cold dark matter included in many cosmological simulations or in the regions of some galaxies extending far beyond their currently known boundaries. We have systematically explored parameter space to identify conditions under which plane-parallel clouds contain sufficient column densities of molecular dark matter that they could be reservoirs for future star formation. Such clouds would be undetected or at least appear by current observational criteria to be uninteresting from the perspective of star formation. We use a time-dependent PDR code to produce theoretical models of the chemistry and emission arising in clouds for our chosen region of parameter space. We then select a subset of model clouds with levels of emission that are low enough to be undetectable or at least overlooked by current surveys. The existence of significant column densities of cold molecular dark matter requires that the background radiation field be several or more orders of magnitude weaker than that in the solar neighbourhood. Lower turbulent velocities and cosmic ray induced ionization rates than typically associated with molecular material within a kpc of the Sun are also required for the molecular matter to be dark. We find that there is a large region within the parameter space that results in clouds that might contain a significant mass of molecular gas whilst remaining effectively undetectable or at least not particularly noticeable in surveys. We note briefly conditions under which molecular dark matter may contain a dynamically interesting mass.

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arXiv2006

Molecular line intensities as measures of cloud masses - I. Sensitivity of CO emissions to physical parameter variations

A reliable estimate of the molecular gas content in galaxies plays a crucial role in determining their dynamical and star-forming properties. However, H2, the dominant molecular species, is difficult to observe directly, particularly in the regions where most molecular gas is thought to reside. Its mass is therefore commonly inferred by assuming a direct proportionality with the integrated intensity of the CO(J=1-0) emission line, using a CO-to-H2 conversion factor, X. Although a canonical value for X is used extensively in such estimates, there is increasing evidence, both theoretical and observational, that the conversion factor may vary by over an order of magnitude under conditions different to those of the local neighbourhood. In an effort to understand the influence of changing environmental conditions on the conversion factor, we derive theoretical estimates of X for a wide range of physical parameters using a photon-dominated region (PDR) time-dependent chemical model, benchmarking key results against those of an independent PDR code to ensure reliability. Based on these results, the sensitivity of the X factor to change in each physical parameter is interpreted in terms of the chemistry and physical processes within the cloud. In addition to confirming previous observationally derived trends, we find that the time-dependence of the chemistry, often neglected in such models, has a considerable influence on the value of the conversion factor.

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arXiv2005

Starburst Models For FIR/sub-mm/mm Line Emission. I. An Expanding Supershell Surrounding A Massive Star Cluster

The effect of a newly born star cluster inside a giant molecular cloud (GMC) is to produce a hot bubble and a thin, dense shell of interstellar gas and dust swept up by the H II expansion, strong stellar winds, and repeated supernova explosions. Lying at the inner side of the shell is the photodissociation region (PDR), the origin of much of the far-infrared/sub-millimeter/millimeter (FIR/sub-mm/mm) radiation from the interstellar medium (ISM). We present a model for the expanding shell at different stages of its expansion which predict mm/sub-mm and far-IR emission line intensities from a series of key molecular and atomic constituents in the shell. The kinematic properties of the swept-up shell predicted by our model are in very good agreement with the measurements of the supershell detected in the nearby starburst galaxy M 82. We compare the modeling results with the ratio-ratio plots of the FIR/sub-mm/mm line emission in the central 1.0 kpc region to investigate the mechanism of star forming activity in M 82. Our model has yielded appropriate gas densities, temperatures, and structure scales compared to those measured in M 82, and the total H2 content is compatible with the observations. This implies that the neutral ISM of the central star-forming region is a product of fragments of the evolving shells.

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arXiv2004

The chemistry of transient microstructure in the diffuse interstellar medium

Transient microstructure in the diffuse interstellar medium has been observed towards galactic and extragalactic sources for decades, usually in lines of atoms and ions, and, more recently, in molecular lines. Evidently, there is a molecular component to the transient microstructure. In this paper we explore the chemistry that may arise in such microstructure. We use a PDR code to model the conditions of relatively high density, low temperature, very low visual extinction and very short elapsed time that are appropriate for these objects. We find that there is a well defined region of parameter space where detectable abundances of molecular species might be found. The best matching models are those where the interstellar microstructure is young (<100 yr), small (~100 AU), and dense (>10^4 cm^-3).

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