The 2D Wavelet Transform Modulus Maxima (WTMM) method is used to obtain quantitative information on the fractal/multifractal nature and anisotropic
structure of Galactic atomic hydrogen (H i) from the Canadian Galactic Plane Survey. Five mosaics were analyzed in the second quadrant of the Milky Way, corresponding to the Local, Perseus, and Outer spiral arms, as well as two “interarm”
regions located between these three spiral arms. A monofractal signature is found for all five mosaics. An anisotropic signature is detected: the root mean square (rms) roughness fluctuations of the mosaics in the latitude direction differ
from those in the longitude direction. This anisotropy is scale-independent
for the inter-arm regions while it is scale-dependent for the spiral arms. The
longitudinal matter distribution of H i structure is similar for all five mosaics
while the latitudinal distribution is smoother in the spiral arms. These results
hold for all physical length scales studied, from ∼ 2 pc in the Local arm to ∼ 44
pc in the Outer arm. Several hypotheses are investigated to provide a physical
explanation: the scale-height gradient, the density wave, star formation activity,
photo-levitation of dusty clouds, random motion of H i clouds, corrugation, and
turbulence. |
