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

• Published in: The Astrophysical journal Vol. 972; no. 2; pp. 148 - 166
• Main Authors: Hummels, Cameron B., Rubin, Kate H. R., Schneider, Evan E., Fielding, Drummond B.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01-09-2024; IOP Publishing
• Subjects: Absorption; Astronomical models; Astrophysical fluid dynamics; Circumgalactic medium; Galactic halos; Galaxies; Galaxy distribution; Galaxy evolution; Legal issues; Line spectra; Mass distribution; Metallicity; Parameters; Photoionization; Quasars; Velocity; Velocity distribution; Visual perception
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ad5965

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 ∝ 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 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 increases significantly with decreasing m cl , min , 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 > 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 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.