Browsing by Author "Shaw, Gargi"

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Molecular hydrogen in the diffuse interstellar medium at high redshift
(2005-06-01) Srianand, R.; Shaw, Gargi; Ferland, G. J.
The physical conditions within damped Lyα systems (DLAs) can reveal the star formation history, determine the chemical composition of the associated ISM, and hence document the ﬁrst steps in the formation of present day galaxies. Here we present calculations that self-consistently determine the gas ionization, level populations (atomic ﬁne-structure levels and rotational levels of H2), grain physics, and chemistry. We show that for a low-density gas (nH6 0.1 cm−3) the meta-galactic UV background due to quasars is suﬃcient to maintain H2 column densities below the detection limit (i.e N(H2)6 1014 cm−2) irrespective of the metallicity and dust content in the gas. Such a gas will have a 21 cm spin temperature in excess of 7000 K and very low C i and C ii ∗ column densities for H i column densities typically observed 50 per cent in DLAs.We show that the observed properties of the ∼ 15 per cent of the DLAs that do show detectable H2 absorption cannot be reproduced with only the quasar dominated meta-galactic UV radiation ﬁeld. Gas with higher densities (nH> 10 cm−3), a moder- ate radiation ﬁeld (ﬂux density one to ten times that of the background radiation of the Galactic ISM), the observed range of metallicity and dust-to-gas ratio reproduce all the observed properties of the DLAs that show H2 absorption lines. This favors the presence of ongoing star formation in DLAs with H2. The absence of detectable H2 and C i absorption in a large fraction of DLAs can be explained if they originate either in a low-density gas or in a high-density gas with a large ambient radiation ﬁeld. The absence of 21 cm absorption and C ii ∗ absorption will be consistent with the ﬁrst possibility. The presence of 21 cm absorption and strong C ii ∗ without H2 and C i absorption will suggest the second alternative. The N(Al ii)/N(Al iii) ratio can be used to understand the physical properties when only C ii ∗ absorption is present. We ﬁnd nH in components that show C ii ∗ (without H2) is less than that typically inferred from the components with H2 absorption. We also calculate the column density of various atoms in the excited ﬁne-structure levels. The expected column densities of O i ∗, O i ∗∗, and Si ii ∗ in a high-density cold gas is in the range of 1011−1012 cm−2 for log N(H i)> 20 and the observed range of metallicities. It will be possible to conﬁrm whether DLAs that do not show H2 originate predominantly in a high-density gas by detecting these lines in very high S/N ratio spectra.
On the enhanced cosmic-ray ionization rate in the diffuse cloud towards Zeta Persei
(2007-10-27) Shaw, Gargi; Stancil, P. C.; Ferland, G. J.; et al.
The spatial distribution of the cosmic-ray flux is important in understanding the Interstellar Medium (ISM) of the Galaxy. This distribution can be analyzed by studying different molecular species along different sight lines whose abundances are sensitive to the cosmic-ray ionization rate. Recently several groups have reported an enhanced cosmic-ray ionization rate (ζ=χCRζstandard) in diffuse clouds compared to the standard value, ζstandard (=2.5x10-17 s-1), measured toward dense molecular clouds. In an earlier work we reported an enhancement χCR =20 towards HD185418. McCall et al. have reported χCR =48 towards ζ Persei based on the observed abundance of H3+ while Le Petit et al. found χCR ≈ 10 to be consistent with their models for this same sight line. Here we revisit ζ Persei and perform a detailed calculation using a self-consistent treatment of the hydrogen chemistry, grain physics, energy and ionization balance, and excitation physics. We show that the value of χCR deduced from the H3 + column density, N(H3 +), in the diffuse region of the sightline depends strongly on the properties of the grains because they remove free electrons and change the hydrogen chemistry. The observations are largely consistent with χCR ≈ 40, with several diagnostics indicating higher values. This underscores the importance of a full treatment of grain physics in studies of interstellar chemistry.
Physical conditions in the ISM towards HD185418
(2005-06-01) Shaw, Gargi; Srianand, R.
We have developed a complete model of the hydrogen molecule as part of the spectral simulation code Cloudy. Our goal is to apply this to spectra of high-redshift star-forming regions where H2 absorption is seen, but where few other details are known, to understand its implication for star formation. The microphysics of H2 is intricate, and it is important to validate these numerical simulations in better-understood environments. This paper studies a well-defined line-of-sight through the Galactic interstellar medium (ISM) as a test of the microphysics and methods we use. We present a self-consistent calculation of the observed absorption-line spectrum to derive the physical conditions in the ISM towards HD185418, a ine-of-sight with many observables. We deduce density, temperature, local radiation field, cosmic ray ionization rate, chemical composition and compare these conclusions with conditions deduced from analytical calculations. We find a higher density, similar abundances, and require a cosmic ray flux enhanced over the Galactic background value, consistent with enhancements redicted by MHD simulations.