Kinematics of the Magellanic Stream and Implications for its Ionization
  The Magellanic Stream and the Leading Arm form a massive, filamentary system
of gas clouds surrounding the Large and Small Magellanic Clouds. Here we
present a new component-level analysis of their ultraviolet (UV) kinematic
properties using a sample of 31 sightlines through the Magellanic System
observed with the Hubble Space Telescope/Cosmic Origins Spectrograph. Using
Voigt profile fits to UV metal-line absorption, we quantify the kinematic
differences between the low-ion (Si II and C II), intermediate-ion (Si III),
and high-ion (Si IV and C IV) absorption lines and compare the kinematics
between the Stream and Leading Arm. We find that the Stream shows generally
simple, single-phase kinematics, with statistically indistinguishable b-value
distributions for the low-, intermediate-, and high-ion components, all
dominated by narrow (b<25 km/s) components that are well aligned in velocity.
In contrast, we find tentative evidence that the Leading Arm shows complex,
multi-phase kinematics, with broader high ions than low ions. These results
suggest that the Stream is photoionized up to C IV by a hard ionizing radiation
field. This can be naturally explained by the Seyfert-flare model of
Bland-Hawthorn et al. (2013, 2019), in which a burst of ionizing radiation from
the Galactic Center photoionized the Stream as it passed below the south
Galactic pole. The Seyfert flare is the only known source of radiation that is
both powerful enough to explain the H-alpha intensity of the Stream and hard
enough to photoionize Si IV and C IV to the observed levels. The flare's
timescale of a few Myr suggests it is the same event that created the giant
X-ray/gamma-ray Fermi Bubbles at the Galactic Center.
