5 research outputs found
An HST Archival Survey of Feathers in Spiral Galaxies
We present a survey of spiral arm extinction substructure referred to as
feathers in 223 spiral galaxies using HST WFPC2 images. The sample includes all
galaxies in the RC3 catalog with cz < 5000 km/s, B_T < 15, i < 60 degrees, and
types Sa--Sd with well-exposed broadband WFPC2 images. The detection frequency
of delineated, periodic feathers in this sample is 20% (45 of 223). This work
is consistent with Lynds (1970), who concluded that feathers are common in
prototypical Sc galaxies; we find that feathers are equally common in Sb
galaxies. Sb--Sc galaxies without clear evidence for feathers either had poorer
quality images, or flocculent or complex structure. We did not find clearly
defined feathers in any Scd--Sd galaxy. The probability of detecting feathers
was highest (83%) for spirals with well-defined primary dust lanes (PDLs; the
lanes which line the inner edge of an arm); well-defined PDLs were only noted
in Sab--Sc galaxies. Consistent with earlier work, we find that neighboring
feathers tend to have similar shapes and pitch angles. OB associations are
often found lining feathers, and many feathers transition to the stellar
substructures known as spurs (Elmegreen 1980). We find that feathers are
coincident with interarm filaments strikingly revealed in Spitzer 8 micron
images. Comparison with CO 1-0 maps of NGC 0628 and NGC 5194 from BIMA SONG
shows that feathers originate at the PDL coincident with gas surface density
peaks. Contrary to the appearance at 8 microns, the CO maps show that gas
surface density in feathers decreases rapidly with distance from the PDL. Also,
we find that the spacing between feathers decreases with increasing gas surface
density, consistent with formation via a gravitational instability.Comment: 47 pages, 22 figures (Figures 1-16,18 are in JPEG format, figures
17,19-22 are embedded postscript files; full resolution images at
http://www.astro.umd.edu/~mlavigne/research/hst-survey-06-2006/). Accepted
for publication in the Ap
Dynamically Driven Evolution of the Interstellar Medium in M51
We report the highest-fidelity observations of the spiral galaxy M51 in CO
emission, revealing the evolution of giant molecular clouds (GMCs) vis-a-vis
the large-scale galactic structure and dynamics. The most massive GMCs
(so-called GMAs) are first assembled and then broken up as the gas flow through
the spiral arms. The GMAs and their H2 molecules are not fully dissociated into
atomic gas as predicted in stellar feedback scenarios, but are fragmented into
smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as
the chains of GMCs that emerge from the spiral arms into interarm regions. The
kinematic shear within the spiral arms is sufficient to unbind the GMAs against
self-gravity. We conclude that the evolution of GMCs is driven by large-scale
galactic dynamics --their coagulation into GMAs is due to spiral arm streaming
motions upon entering the arms, followed by fragmentation due to shear as they
leave the arms on the downstream side. In M51, the majority of the gas remains
molecular from arm entry through the inter-arm region and into the next spiral
arm passage.Comment: 6 pages, including 3 figures. Accepted, ApJ
Dynamically Driven Evolution of the Interstellar Medium in M51
Massive star formation occurs in giant molecular clouds (GMCs); an understanding of the evolution of GMCs is a prerequisite to develop theories of star formation and galaxy evolution. We report the highest-fidelity observations of the grand-design spiral galaxy M51 in carbon monoxide (CO) emission, revealing the evolution of GMCs vis-a-vis the large-scale galactic structure and dynamics. The most massive GMCs (giant molecular associations (GMAs)) are first assembled and then broken up as the gas flow through the spiral arms. The GMAs and their H_2 molecules are not fully dissociated into atomic gas as predicted in stellar feedback scenarios, but are fragmented into smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as the chains of GMCs that emerge from the spiral arms into interarm regions. The kinematic shear within the spiral arms is sufficient to unbind the GMAs against self-gravity. We conclude that the evolution of GMCs is driven by large-scale galactic dynamics—their coagulation into GMAs is due to spiral arm streaming motions upon entering the arms, followed by fragmentation due to shear as they leave the arms on the downstream side. In M51, the majority of the gas remains molecular from arm entry through the interarm region and into the next spiral arm passage