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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>m</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>cl</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mi>α</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> with velocities drawn from a turbulent velocity field. The user may specify α , the lower limit of the cloudlet mass distribution ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>m</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>cl</mml:mi> <mml:mo>,</mml:mo> <mml:mi>min</mml:mi> </mml:mrow> </mml:msub> </mml:math> ), 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 &lt; 0.3 Å increases significantly with decreasing <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>m</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>cl</mml:mi> <mml:mo>,</mml:mo> <mml:mi>min</mml:mi> </mml:mrow> </mml:msub> </mml:math> , cloudlet number density ( n cl ), and complex size. We then use this framework to predict the halo-scale W 2796 distribution around ∼ L * galaxies. We show that the observed incidences of W 2796 &gt; 0.3 Å sight lines with impact parameters 10 kpc &lt; R ⊥ &lt; 50 kpc in projected QSO–galaxy studies are consistent with our model over much of parameter space. However, they are underpredicted by models with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>m</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>cl</mml:mi> <mml:mo>,</mml:mo> <mml:mi>min</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≥</mml:mo> <mml:mn>100</mml:mn> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> </mml:math> 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.