Photodissociation of H2 in protogalaxies: modelling self‐shielding in three‐dimensional simulations

By Jemma Wolcott-Green Zolt'an Haiman Greg L. Bryan. 2011

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The ability of primordial gas to cool in protogalactic haloes exposed to ultraviolet (UV) radiation is critically dependent on the self-shielding of H2. We perform radiative transfer calculations of LW line photons, post-processing outputs from three-dimensional adaptive mesh refinement simulations of haloes with Tvir≳ 104 K at z ∼ 10. We calculate the optically thick photodissociation rate numerically, including the effects of density, temperature and velocity gradients in the gas, as well as line overlap and shielding of H2 by H i, over a large number of sightlines. In low-density regions (n ≲ 104 cm−3) the dissociation rates exceed those obtained using most previous approximations by more than an order of magnitude; the correction is smaller at higher densities. We trace the origin of the deviations primarily to inaccuracies of (i) the most common fitting formula for the suppression of the dissociation rate, from Draine and Bertoldi and (ii) estimates for the effective shielding column density from local properties of the gas. The combined effects of gas temperature and velocity gradients are comparatively less important, typically altering the spherically averaged rate only by a factor of ≲2. We present a simple modification to the Draine & Bertoldi fitting formula for the optically thick rate which improves agreement with our numerical results to within ∼15 per cent, and can be adopted in future simulations. We find that estimates for the effective shielding column can be improved by using the local Sobolev length. Our correction to the H2 self-shielding reduces the critical LW flux to suppress H2 cooling in Tvir≳ 104 K haloes by an order of magnitude; this increases the number of such haloes in which supermassive (M ∼ 105 M⊙) black holes may have formed.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/d69e8026a7b113f53645ab487cc203fb5d2abf1e

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In Astronomy and Astrophysics

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Citations in the corpus

• The First Stars: Mass Growth Under Protostellar Feedback (2011, background): “We note a recent update to their fshield fitting formula (Wolcott-Green et al. 2011; Wolcott-Green & Haiman 2011), but do not expect this to significantly affect the results for our particular case.”
• Primordial star formation under the influence of far ultraviolet radiation: 1540 cosmological haloes and the stellar mass distribution (2015, methodology): “We employ the self-shielding functions (Wolcott-Green & Haiman 2011; Wolcott-Green et al. 2011) for H2 and HD molecules.”
• The First Billion Years project: the impact of stellar radiation on the co-evolution of Populations II and III (2012, methodology): “To estimate the self-shielded LW flux we follow the approach suggested by Wolcott-Green et al. (2011).”
• Building up the Population III initial mass function from cosmological initial conditions (2016, methodology): “The value is then used to calculate the shielding factor fshield (Draine & Bertoldi 1996, but see also Wolcott-Green et al. 2011; Wolcott-Green & Haiman 2011).”
• Black hole formation in the early Universe (2013, background): “We also note that for a radiation temperature of 105 K, H2 is mainly dissociated by the Solomon process while for the stellar spectrum of 104 K the main dissociation route is H− (Shang et al. 2010; Wolcott-Green et al. 2011).”