A Fractal Analysis of the H I Emission from the Large Magellanic Cloud
B. G. Elmegreen, S. Kim, L. Staveley-Smith;
ApJ, 2001, 548, 749
ABSTRACT:Fourier transform power spectra of the distribution of neutral hydrogen emission in the Large Magellanic Cloud is approximately a power law
over ~2 decades in length.
Power spectra in the azimuthal direction look
about the same as the rectilinear spectra.
No difference is seen between
power spectra of single-channel maps and power spectra of either the peak
emission map or the integrated emission map at the same location.
There is a
slight steepening of the average one-dimensional and two-dimensional LMC
power spectra at high spatial frequencies.
Delta-variance methods also
show the same power-law structure.
These results suggest that most of the
interstellar medium in the LMC is fractal, presumably the result of pervasive
turbulence, self-gravity, and self-similar stirring.
The similarity between
the channel and integrated maps suggests they cover about the same
line-of-sight depth.
The slight steepening of the power spectra at high spatial
frequency, corresponding to wavelengths smaller than ~100 pc, could mark the
transition from large-scale emission that is relatively shallow on the line of
sight to small-scale emission that is relatively thick on the line of
sight.
Such a transition, if real, would provide a method to obtain the thickness of
a face-on galactic gas layer.
To check this possibility,
three-dimensional fractal models are made from the inverse Fourier transform of noise
with a power-law cutoff.
The models are viewed in projection with a
Gaussian density distribution on the line of sight to represent a face-on galaxy
disk with finite disk thickness.
The density structure from turbulence is
simulated in the models by using a log-normal density distribution function with
a scale factor dependent on the Mach number.
Additional density
structure from simulated H I phase transitions is included in some models.
After
tuning the Mach number, galaxy thickness, and mathematical form of the phase
transition, the models can be made to reproduce the observed LMC power spectra, the
amplitude of the H I brightness fluctuations, and the probability distribution
function for brightness.
In all cases, the H I structure arises from a
relatively thin layer in the LMC; the thick part of the H I disk has little spatial
structure.
The large amplitude of the observed intensity variations cannot be
achieved by turbulence alone; phase transitions are required.
The character
of the fractal H I structure in the LMC is viewed in another way by comparing
positive and negative images of the integrated emission.
For the isotropic
fractal models, these two images have the same general appearance, but for the
LMC they differ markedly.
The H I is much more filamentary in the LMC than in
an isotropic fractal, making the geometric structure of the
high-emission regions qualitatively different than the geometric structure of the
low-emission (intercloud) regions.
The high-emission regions are also more
sharply peaked than the low-emission regions, suggesting that compressive
events formed the high-emission regions, and expansion events, whether from
explosions or turbulence, formed the low-emission regions.
The character of the
structure is also investigated as a function of scale using unsharp masks and
enlargements with four different resolutions.
The circular quality of the
low-emission regions and the filamentary quality of the high-emission regions is
preserved on scales ranging from several tens to several hundreds of
parsecs.
The spatial scales for sources of turbulent energy input may be
illustrated by rms variations in the power spectra with position in the
galaxy.
This rms decreases from ~0.6 at kpc scales to ~0.25 on ~20 pc scales.
The
large-scale variations are probably from known supershells.
The smaller scale
variations could be the result of a combination of turbulent cascades from these
large-scale energy inputs and additional energy sources with smaller sizes.
KEYWORDS: galaxies: ism, ism: clouds, ism: structure, galaxies: magellanic clouds, turbulence
PERSOKEY:turbulence, ,
CODE: elmegreen2001