We present a survey of spiral arm extinction substructures referred to as feathers in 223 spiral galaxies using Hubble Space Telescope WFPC2 images. The sample includes all galaxies in the RC3 ...catalog with cz < 5000 km s super(-1), B sub(T) < 15, i < 60, 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, 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 spiral galaxies with well-defined primary dust lanes (the lanes that line the inner edge of an arm); well-defined primary dust lanes 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). We find that feathers are coincident with interarm filaments strikingly revealed in Spitzer 8 km images. Comparison with CO (1-0) maps of NGC 0628 and NGC 5194 from BIMA SONG shows that feathers originate at the primary dust lane coincident with gas surface density peaks. Contrary to the appearance at 8 km, the CO maps show that gas surface density in feathers decreases rapidly with distance from the primary dust lane. We also find that the spacing between feathers decreases with increasing gas surface density, consistent with formation via a gravitational instability.
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 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 interarm region and into the next spiral arm passage.
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{sub 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.
Astrophys.J.650:818-834,2006 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.
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.
Spiral arms are the hallmark of spiral galaxies. Spiral density waves gather gas, dust, and stars in the disks of spiral galaxies, forming the arms. Observations show that these large-scale features ...are often composed of smaller scale substructures, commonly referred to as feathers and spurs, which give the arm a patchy and segmented appearance. Within these select galaxies, the substructure appears associated with much of the star formation in the arm. In this thesis, we study the frequency and characteristics of spiral arm substructure in a sample of 233 spiral galaxies; using newly acquired high-resolution (∼4'') CARMA CO(1-0) velocity-integrated intensity maps of five target spiral galaxies, we compare the distribution of star formation tracers along spiral arms and test formation theories for spiral arm substructure. We use a multi-wavelength data set to study the frequency and characteristics of spiral arm feathers and test the predictions for their formation. Using broad-band HST images, we study how common feathers are in our sample of 233 spiral galaxies. With high-resolution CARMA CO(1-0) observations of five target galaxies previously known to have feathers, we study the distribution of the molecular gas relative to the location of neutral gas and star formation tracers. We measure the spacing between the feathers and use the CO(1-0) observations to estimate the molecular gaseous surface density along the spiral arms. We find that feathering substructure is not limited to late type, grand design spiral galaxies and is detected in a range of galaxy types, from early to late type, and spiral arm environments, including barred, ringed, and flocculent. In our target sample, we find that feathers are more than mere extinction features and are associated with a significant amount of the star-forming gas. We further find a relationship between the spacing of feathers and the gaseous surface density along spiral arms, which suggests these features form via a gravitational instability.
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