Probability Distribution Function of Gas Surface Density in M33
Journal
Date Issued
2018
Author(s)
Abstract
The probability distribution functions (PDFs) for atomic, molecular, and
total gas surface densities of M33 are determined at a resolution of about
50~pc over regions that share coherent morphological properties to unveil
fingerprints of self-gravity across the star-forming disk. Most of the total
gas PDFs from the central region to the edge of the star-forming disk are
well-fitted by log-normal functions whose width decreases radially outwards.
Because the HI velocity dispersion is approximately constant across the disk,
the decrease of the PDF width is consistent with a lower Mach number for the
turbulent ISM at large galactocentric radii where a higher fraction of HI is in
the warm phase. The atomic gas is found mostly at face-on column densities
below N$_{H}^{lim}$=2.5 10$^{21}$~cm$^{-2}$, with small radial variations of
N$_{H}^{lim}$. The molecular gas PDFs do not show strong deviations from
log-normal functions in the central region where molecular fractions are high.
Here the high pressure and rate of star formation shapes the PDF as a
log-normal function dispersing self-gravitating complexes with intense feedback
at all column densities that are spatially resolved. Power law PDFs for the
molecules are found near and above N$_H^{lim}$, in the well defined southern
spiral arm and in a continuous dense filament extending at larger
galactocentric radii; this is evident in cloud samples at different
evolutionary stages along the star formation cycle. In the filament nearly half
of the molecular gas departs from a log-normal PDF and power laws are also
observed in pre-star forming molecular complexes. The slope of the power law is
between -1 and -2. This slope, combined with maps showing where the different
parts of the power law PDFs come from, suggest a power-law stratification of
density within molecular cloud complexes, which is consistent with the
dominance of self-gravity.
Volume
617
Start page
A125
Issn Identifier
0004-6361
Ads BibCode
2018A&A...617A.125C
Rights
open.access
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