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

Year: 2024 DOI: 10.3847/1538-4357/ad5965
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 ∼10⁴ 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 ∝ m_(cl)−α with 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 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 iiλ2796 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 W₂₇₉₆ < 0.3 Å increases significantly with decreasing m_(cl,min), cloudlet number density (ncl), and complex size. We then use this framework to predict the halo-scale W₂₇₉₆ distribution around ∼L* galaxies. We show that the observed incidences of W₂₇₉₆ > 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_(cl,min) ≥ 100 M⊙ and ncl ≥ 0.03 cm−3, in keeping with a picture in which the inner cool circumgalactic medium (CGM) is dominated by numerous low-mass cloudlets (mcl ≲ 100M⊙) 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.