Velocity Modification of H I Power Spectrum
A. Lazarian, D. Pogosyan;
ApJ, 2000, 537, 720
ABSTRACT:The distribution of atomic hydrogen in the Galactic plane is usually mapped using the Doppler shift of 21 cm emission line, and this causes the
modification of the observed emission spectrum.
We calculate the emission
spectrum in velocity slices of data (channel maps) and derive its dependence on
the statistics of velocity and density fields.
We find that, (1) if the
density spectrum is steep, i.e., n<-3, the large k asymptotics of the
emissivity spectrum are dominated by the velocity fluctuations; and (2) the
velocity fluctuations make the emission spectra shallower, provided that the
data slices are sufficiently thin.
In other words, turbulent velocity
creates small-scale structure that can erroneously be identified as
clouds.
The effect of thermal velocity is very similar to the change of the
effective slice thickness, but the difference is that, while an increase of the
slice thickness increases the amplitude of the signal, the increase of the
turbulent velocity leaves the measured intensities intact while washing out
fluctuations.
The contribution of fluctuations in warm H I is suppressed relative to
those in the cold component when the velocity channels used are narrower than
the warm H I thermal velocity and small angular scale fluctuations are
measured.
We calculate how the spectra vary with the change of velocity slice
thickness and show that the observational 21 cm data is consistent with the
explanation that the intensity fluctuations within individual channel maps are
generated by turbulent velocity fields.
As the thickness of velocity slices
increases, density fluctuations begin to dominate emissivity.
This allows us to
disentangle velocity and density statistics.
The application of our technique to
Galactic and SMC data reveals spectra of density and velocity with power law
indexes close to -11/3.
This is a Kolmogorov index, but the explanation of the
spectrum as due to the Kolmogorov-type cascade faces substantial
difficulties.
We generalize our treatment for the case of a statistical study of
turbulence inside individual clouds.
The mathematical machinery developed is
applicable to other emission lines.
KEYWORDS: galaxy: structure, ism: kinematics and dynamics, radio lines: ism
PERSOKEY:turbulence, ,
CODE: lazarian2000