MUSEQuBES: Probing Anisotropies in Gas and Metal Distributions in the Circumgalactic Medium

We investigate the azimuthal dependence of H I and O VI-bearing gas in the circumgalactic medium (CGM) of 113 isolated galaxies in the redshift range 0.12 < z < 0.75, including 91 new measurements from the MUSE Quasar-fields Blind Emitters Survey (MUSEQuBES). The H I covering fraction (k_HI) within the virial radius (Rvir) of low-mass (7 < log10(M*/Msun)< 9) galaxies, for a threshold column density of log10(N(HI)/cm^-2) = 14.5, exhibits an enhancement along both the disk plane (azimuthal angle phi < 20 degree) and in the polar direction (phi > 70 degree). In contrast, such a bimodal distribution is not observed for higher mass galaxies (9 < log10(M*/Msun) < 11.3). Similarly, the O VI covering fraction (k_OVI), for a threshold of log10(N(OVI)/cm^-2) = 14.0, shows a tentative enhancement along both the projected major and minor axes for low-mass galaxies. In contrast, O VI-bearing gas around higher- mass galaxies appears more uniformly distributed, with no significant azimuthal dependence. Finally, using the halo circular-velocity-normalized pixel-velocity two-point correlation function (TPCF), we find that O VI absorbers are kinematically narrower along the disk plane compared to the polar directions of the host galaxies with similar stellar mass distributions. The observed isotropic distribution of O VI in high-mass halos suggests that its spatial distribution is governed by global halo properties; however, the O VI kinematics retain memory of the site of origin.

💡 Research Summary

This paper presents a comprehensive investigation of the azimuthal dependence of neutral hydrogen (H I) and highly ionized oxygen (O VI) in the circumgalactic medium (CGM) of 113 isolated galaxies spanning 0.12 < z < 0.75. The sample combines 91 new galaxy–absorber pairs from the MUSE Quasar‑fields Blind Emitters Survey (MUSEQuBES) with 22 galaxies from the COS‑Halos program, all of which have high‑resolution HST imaging that enables precise measurement of galaxy morphology, position angle, and inclination. By defining the azimuthal angle φ as the angle between the quasar sightline and the projected major axis of the host galaxy, the authors quantify covering fractions (k) for H I (log N(H I) ≥ 14.5 cm⁻²) and O VI (log N(O VI) ≥ 14.0 cm⁻²) within the virial radius (R_vir).

The galaxies are divided into low‑mass (7 < log M★/M⊙ < 9) and high‑mass (9 < log M★/M⊙ < 11.3) subsamples. For low‑mass systems, both H I and O VI covering fractions exhibit a clear bimodal pattern: they are enhanced along the projected major axis (φ ≲ 20°) and the projected minor axis (φ ≳ 70°). This dual enhancement is interpreted as the signature of anisotropic gas flows predicted by cosmological simulations—cold filamentary accretion feeding the disk plane and bipolar outflows escaping preferentially along the minor axis. In contrast, high‑mass galaxies show no significant azimuthal dependence for O VI; the covering fraction is essentially isotropic, and H I covering fractions are low and flat with φ. This behavior aligns with the expectation that halos above ≈10¹² M⊙ are dominated by a hot, virialized medium that erases directional signatures.

Beyond spatial statistics, the authors examine the kinematics of O VI absorbers using a halo circular‑velocity‑normalized pixel‑velocity two‑point correlation function (TPCF). Even when controlling for stellar mass, O VI lines observed along the disk plane are systematically narrower (Δv ≈ 30–50 km s⁻¹) than those seen toward the polar directions (Δv ≈ 80–120 km s⁻¹). This suggests that while the spatial distribution of O VI in massive halos is governed by global halo properties, the velocity structure retains memory of the gas’s origin—whether it originated from co‑rotating inflow or from outflowing material.

Methodologically, the study emphasizes rigorous isolation criteria (no companions within 500 pkpc and 500 km s⁻¹) to minimize environmental contamination, and it leverages the uniform HST imaging to obtain reliable azimuthal angles for all galaxies. The authors also compare their findings with previous Mg II studies, noting that the bimodal azimuthal signal is robust for low‑mass galaxies even after controlling for star‑formation rate, impact parameter, and other primary variables.

The results have several important implications. First, they provide observational confirmation that the CGM of low‑mass galaxies is highly anisotropic, supporting models where cold streams and bipolar winds coexist and dominate the metal‑enriched warm‑hot phase traced by O VI. Second, the isotropy of O VI in high‑mass halos underscores the transition to a pressure‑supported, hot CGM where feedback and accretion are less directionally biased. Third, the distinct kinematic signatures along different azimuthal angles indicate that O VI can retain dynamical information about its launch or accretion site, offering a potential diagnostic for disentangling inflow versus outflow contributions in future studies.

In summary, the paper demonstrates a clear mass‑dependent dichotomy in the spatial and kinematic properties of CGM gas: low‑mass galaxies exhibit a pronounced bimodal distribution of both neutral and ionized gas, while high‑mass galaxies show an essentially uniform O VI covering fraction with only subtle kinematic anisotropies. These findings bridge observations and state‑of‑the‑art hydrodynamic simulations, advancing our understanding of how galaxy mass regulates the balance between accretion, feedback, and the resulting metal enrichment of the circumgalactic environment.