The Warm Gas in the Milky Way: The Kinematical Model of C IV and Its Connection to Si IV

Abstract

We compose a 265-sight-line Milky Way C IV line-shape sample using the Hubble Space Telescope/Cosmic Origins Spectrograph 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 are identical to those Si IV within ≈2σ. 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 midplane and the normal-line direction. Both solutions can reproduce the existing sample and suggest a warm gas disk mass of \mathrm{log}M({M}_{\odot })\approx 8$ and an upper limit of \mathrm{log}M({M}_{\odot })\lt 9.3$ within 250 kpc, which is consistent with Si IV. There is a decrease in the C IV/Si IV column density ratio from the Galactic center to the 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°-10° for possible turbulence within individual clouds (≈1 kpc).