The Warm Gas in the MW: the Kinematical Model of C IV and its Connection with Si IV

Abstract

We compose a 265-sight line MW C IV line shape sample using the HST/COS archive, which is complementary to the existing Si IV samples. C IV has a higher ionization potential ($47 - 64$ eV) than Si IV ($33 - 45$ eV), so it also traces warm gas, which is roughly cospatial with Si IV. The spatial density distribution and kinematics of C IV is identical with Si IV within $\approx 2 \sigma$. C IV is more sensitive to the warm gas density distribution at large radii with a higher element abundance. Applying the kinematical model to the C IV sample, we find two possible solutions of the density distribution, which are distinguished by the relative extension along the disk mid-plane and the normal-line direction. Both two solutions can reproduce the existing sample, and suggest a warm gas disk mass of $\log M(M_\odot) \approx 8$ and an upper limit of $\log M(M_\odot) < 9.3$ within 250 kpc, which is consistent with Si IV. There is a decrease of the C IV/Si IV column density ratio from the Galactic center to outskirts by $0.2-0.3$ dex, which may suggest a phase transition or different ionization mechanisms for C IV and Si IV. Also, we find that the difference between C IV and Si IV is an excellent tracer of small-scale features, and we find a typical size of $5^\circ-10^\circ$ for possible turbulence within individual clouds ($\approx 1\rm~kpc$).