tokens: of the total CGM column density within ±600 km s −1 of the galaxy systemic velocity was found at ∣ v ∣ 100 km s −1 (Tumlinson et al. 2013 ; Werk et al. 2014 ). Kinematic analysis of warm gas signatures in the quasar absorption-line survey COS-GAL also shows that a substantial portion of the CGM is likely hidden at ∣ v ∣ 100 km s −1 (Zheng et al. 2019 ). In the recent CGM 2 survey, the majority of the H i column density detected in absorption around z 0.48 galaxies lies within ±250 km s −1 of the galaxy systemic velocity, despite a velocity window of 1000 km s −1 (Wilde et al. 2020 ). Additionally, Martin et al. ( 2019 ) find that the velocity of low-ionization gas around nearby galaxies is tied to the rotation of the disk out to radii of ∼70 kpc. If the Milky Way is similar to other star-forming galaxies at low redshift, these observations further reinforce the evidence suggesting that a significant fraction of Galactic baryons are yet to be detected in the low-velocity Galactic CGM. Simulations of Milky Way analogs also broadly agree that a significant fraction of circumgalactic baryons are moving at low velocities. The FOGGIE simulations, which focus on resolving the CGM around galaxies with extreme spatial and mass resolution, show with mock observations that the strongest absorption is confined to within 200 km s −1 of the systemic velocity (Peeples et al. 2019 ; Zheng et al. 2020 ). Other synthetic observations of a Milky Way−mass galaxy generated from the high-resolution cosmological simulation Enzo also predict that ∼ 65% of the Milky Way's CGM mass at Galactic latitudes ∣ b ∣ > 20° is hidden at low to intermediate velocities (∣ v ∣ 100 km s −1 ) for all gas phases and that the warm−hot component (10 5 K T 10 6 K) is primarily moving at ∣ v ∣ 150 km s −1 (Zheng et al. 2015 ; Joung et al. 2012 ). The C iv doublet is a useful probe of warm ionized gas in galaxy halos because of its strong oscillator strength (Savage Wakker 2009 ) and high covering fraction in the CGM of external galaxies (Keeney et al. 2013 ; Liang Chen 2014 ). In low-redshift galaxies that span a wide range of stellar mass (9.5 log ( M * / M ⊙ ) 11), estimates of log N C IV (cm −2 ) range from 13.5 to 14.5, with a covering fraction of ∼50% out to half the virial radius for log N C IV > 13.5 cm −2 (Bordoloi et al. 2014 ; Burchett et al. 2016 ). Using conservative ionization fraction arguments that take the maximum fraction of carbon that presents as C iv , ∼30%, for both photoionized and collisionally ionized gas, such measurements imply that > 10 6 M ⊙ of carbon sits in the CGM of external galaxies. For sub- L * galaxies with stellar masses 10 8.5 ≲ M * ( M ⊙ ) ≲ 10 10 , this estimated CGM carbon mass amounts to 50%–80% of the total ISM carbon mass (Bordoloi et al. 2014 ). Measurements of the Milky Way's CGM are needed at all velocities in order to directly probe the baryonic mass of the Milky Way, test the expectations from cosmological simulations, and compare the
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