Yang et al. present a statistical study of the neutral atomic hydrogen (HI) gas extending into the circumgalactic medium perpendicular to the disk for 7 edge-on galaxies with inclinations above  85° from the FEASTS program with a 3σ (20kms⁻¹) column density (N_HI) depth of 5×10¹⁷cm⁻². We develop two photometric methods to separate the extraplanar HI from the disk component, based on existing interferometric data and parametric modeling of the disk flux distribution respectively. With both methods, the FEASTS data exhibit clear extended wings beyond the disk along the minor axis. The extraplanar HI accounts for 5% to 20% of the total HI mass and extends to 20-50 kpc at N_HI=10¹⁸cm⁻². We find a tight positive correlation between vertical extensions of the extraplanar HI and total HI mass M_HI. The iso-density shape of HI at N_HI=10¹⁸cm⁻² has an average axis ratio of 0.56±0.11. The off-disk N_HI profiles of these edge-on galaxies well represent the lower envelop of previous Lyman-α absorption measurements at low-redshift. Our results suggest that at N_HI=5×10¹⁷cm⁻², the HI extends considerably further than the known thin and thick disks in the vertical direction, but still remains much flattener than a spherical distribution, consistent with theoretical expectations that outflow, circulation, and accretion should have different impacts in these two directions. We show the tension of our results with Illustris and TNG predictions, highlighting the constraining power of our results for future simulations.

The false-color atlas of their sample showing the FEASTS H_I clouds in blue overlaid on the optical images. The small circle and horizontal bar at the lower left corner correspond to the FEASTS beam and a length of 10 kpc, respectively. The optical background images are from the Legacy Survey (Dey et al. 2019).