Clumpiness of Observed and Simulated Cold Circumgalactic Gas

This figure shows a comparison (from Augustin et al. 2021 [1]) between the observed absorption variation on kpc scales in clumpy galactic atmospheres (left and right panels) and a high-resolution simulation of such an atmosphere (center panel).

Determining the clumpiness of matter around galaxies is pivotal to a full understanding of the spatially inhomogeneous, multi-phase gas in the circumgalactic medium (CGM). In Augustin et al. (2021), we combine high spatial resolution 3D observations with hydrodynamical cosmological simulations to measure the clumpiness of cold circumgalactic gas. We also present new adaptive-optics-assisted VLT/MUSE observations of a quadruply lensed quasar, targeting the CGM of two foreground 𝑧 ~ 1 galaxies observed in absorption.

Through a joint approach combining 3D-spectroscopy observations of lensed systems and simulations with extreme resolution in the CGM, we can put new constraints on the clumpiness of cold CGM gas, which is a key diagnostic of the baryon cycle. From FOGGIE simulations [2] with exquisite spatial resolution (∼0.1 kpc scales) in the CGM of galaxies we compute the physical properties of cold gas traced by Mg II absorbers. By contrasting mock-observables from those simulations with the VLT/MUSE observations, we find a large spread of fractional variations of Mg II equivalent width with physical separation, both in observations and simulations. The simulations indicate that the Mg II coherence length depends on the underlying gas morphology—inflowing filaments have a long coherence length and outflowing clumps have a short coherence length.

The 𝑧_abs=1.168 MgII system in the right-hand panels above shows coherence over ~ 6 kpc and is associated with an [OII] emitting galaxy (stellar mass $10^{9.6 \pm 0.2} \, M_\odot$ ) situated 89 kpc away, with a star formation rate $\ge 4.6 \pm 1.5 \, M_\odot \, {\rm yr}^{-1}$. Based on our combined analysis, the absorber is consistent with being an inflowing filament.

The 𝑧_abs=1.393 Mg II system in the left-hand panels traces dense CGM gas clumps varying in strength over physical scales < 2 kpc . Our findings therefore suggest that this absorber is related to an outflowing clump.