On Interstellar Neutral Hydrogen Associated With an Hα Filament, the Critical Ionization Velocity Effect and the Origin of Filaments

Gaussian analysis of neutral hydrogen
$\lambda 21$
-cm line (HI) profiles associated with an
$\text{H}\alpha $
filament reveal a broad-line underlying component similar to the
$\text{H}\alpha $
structure as well as a narrow-line thread along the backbone of the filament. Both the
$\text{H}\alpha $
and the broad-line HI have a linewidth of about 25 km s
⁻¹
. The narrow-line components are of order 7 and 9 km s
⁻¹
. The filament appears to be part a complex gas structure flowing out of the Perseus arm and is located at an order of 1 kpc from the Galactic plane. Neither traditional photoionization nor shocks seem to be responsible for the energy required to explain the presence of
$\text{H}\alpha $
at this location. We investigate two other possibilities. The first is similar to the solar wind-ionized material that is created near the large numbers of O associations in the Perseus arm, and then streams out into the halo where far lower densities limit recombination. The other is the critical ionization velocity (CIV) where the kinetic energy of the neutral particles relative to the plasma equals the ionization potential of the atoms. This velocity for hydrogen atoms in the
$n=2$
level, the state after the emission of an
$\text{H}\alpha $
photon, is 25 km s
⁻¹
, the measured linewidth for both the
$\text{H}\alpha $
and the broad-line HI. Marklund convection may be responsible for the formation of interstellar filaments. In other astrophysical regimes filaments with thread-like substructure appear to be common. Although different jargon—strands, spines, tendrils, threads, etc.—is used in different disciplines, these may be revealing the same underlying physics.