CloudFlex: A Flexible Parametric Model for the Small-scale Structure of the Circumgalactic Medium

Abstract

We present CloudFlex , an open-source tool for predicting absorption-line signatures of cool gas in galaxy halos with small-scale structure. Motivated by analyses of ∼10 ^4 K material in hydrodynamical simulations of turbulent, multiphase media, we model cool gas structures as complexes of cloudlets sampled from a power-law distribution of mass $\propto {m}_{\mathrm{cl}}^{-\alpha }$ with velocities drawn from a turbulent velocity field. The user may specify α , the lower limit of the cloudlet mass distribution ( ${m}_{\mathrm{cl},\min }$ ), and several other parameters that set the mass, size, and velocity distribution of the complex. This permits investigation of the relation between these parameters and absorption-line observables. As a proof-of-concept, we calculate the Mg ii λ 2796 absorption induced by the cloudlets in background quasi-stellar object (QSO) spectra. We demonstrate that, at fixed metallicity, the covering fraction of sight lines with equivalent widths W _2796 0.3 Å sight lines with impact parameters 10 kpc < R _⊥ < 50 kpc in projected QSO–galaxy studies are consistent with our model over much of parameter space. However, they are underpredicted by models with ${m}_{\mathrm{cl},\min }\geqslant 100{M}_{\odot }$ 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 ≲ 100 M _⊙ ) with a volume filling factor ≲1%. When used to model absorption-line data sets built from multi-sight line and/or spatially extended background probes, CloudFlex enables detailed constraints on the size and velocity distributions of structures comprising the photoionized CGM.

Keywords

• Circumgalactic medium
• Galaxy evolution
• Astrophysical fluid dynamics