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CloudFlex: A Flexible Parametric Model for the Small-Scale Structure of the Circumgalactic Medium
Cameron B. Hummels, Kate H. R. Rubin, Evan E. Schneider, Drummond B. Fielding
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We present CloudFlex, a new open-source tool for predicting the absorption-line signatures of cool gas in galaxy halos with complex small-scale structure. Motivated by analyses of cool material in hydrodynamical simulations of turbulent, multiphase media, we model individual cool gas structures as assemblies of cloudlets with a power-law distribution of cloudlet mass ∝m_(cl)^(-α) and relative velocities drawn from a turbulent velocity field. The user may specify α, the lower limit of the cloudlet mass distribution (m_(cl,min)), and several other parameters that set the total mass, size, and velocity distribution of the complex. We then calculate the MgII 2796 absorption profiles induced by the cloudlets along pencil-beam lines of sight. We demonstrate that at fixed metallicity, the covering fraction of sightlines with equivalent widths W₂₇₉₆ < 0.3 Ang increases significantly with decreasing m_(cl,min), cool cloudlet number density (n_(cl)), and cloudlet complex size. We then present a first application, using this framework to predict the projected W₂₇₉₆ distribution around ∼L^* galaxies. We show that the observed incidences of W₂₇₉₆>0.3 Ang sightlines within 10 kpc < R_⊥ < 50 kpc are consistent with our model over much of parameter space. However, they are underpredicted by models with m_(cl,min)≥100M_⊙ and n_(cl)≥0.03 cm⁻³, in keeping with a picture in which the inner cool circumgalactic medium (CGM) is dominated by numerous low-mass cloudlets (m_(cl)≲100M_⊙) with a volume filling factor ≲1%. When used to simultaneously model absorption-line datasets built from multi-sightline and/or spatially-extended background probes, CloudFlex will enable detailed constraints on the size and velocity distributions of structures comprising the photoionized CGM.
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