Evidence for a Rotational Component in the Circumgalactic Medium of Nearby Galaxies

We present results of a study comparing the relative velocity of $\rm Ly\alpha$ absorbers to the rotation velocity of nearby galaxy disks in the local universe ($z \leq 0.03$). We have obtained rotation curves via long-slit spectroscopy of eight galaxies with the Southern African Large Telescope, and combine this dataset with an additional 16 galaxies with data from the literature. Each galaxy appears within $3R_{\rm vir}$ of a QSO sightline with archival Cosmic Origin Spectrograph (COS) spectra. We study the velocity orientation of absorbers with respect to nearby galaxy's rotation, and compare with results from both the Steidel et al. (2002) monolithic halo model and a new cylindrical Navarro-Frenk-White galaxy halo model to interpret these data in the context of probing 3D galaxy halos via 1D QSO absorption-line spectroscopy. Relative to these models we find that up to $59\pm5\%$ of $\rm Ly\alpha$ absorbers have velocities consistent with co-rotation. We find the $\rm Ly\alpha$ co-rotation fraction to decrease with galaxy luminosity ($L^*$) and impact parameter in a model-independent fashion. We report that both anti-rotating absorbers and those found near luminous galaxies ($L \gtrsim 0.5 L^*$) mostly have low Doppler $b$-parameters ($b \lesssim 50$ km $\rm s^{-1}$). Absorbers consistent with co-rotation show a wide range of Doppler $b$-parameters. Finally, we find a strong anticorrelation between co-rotation fraction and galaxy inclination, which is at odds with recent metal-line kinematic studies and suggests the kinematic and geometric distribution of the circumgalactic medium is complex and multiphase.