The bulgeless disk galaxies at the end of the Hubble Sequence evolve at a glacial pace relative to their more violent, earlier-type cousins. The causes of their internal, or secular evolution are important because secular evolution represents the future fate of all galaxies in our accelerating Universe and is a key ingredient to understanding galaxy evolution in lower-density environments at present. The rate of secular evolution is largely determined by the stability of the cold ISM against collapse, star formation, and the buildup of a central bulge. Key diagnostics of the ISM's stability are the presence of compact molecular clouds and narrow dust lanes. Surprisingly, edge-on, pure disk galaxies with circular velocities below 120 km/s do not appear to contain such dust lanes. We propose to obtain ACS/WFC F606W images of a well-selected sample of extremely late-type disk galaxies to measure the characteristic scale size of the cold ISM and determine if they possess the unstable, cold ISM necessary to drive secular evolution. Our sample has been carefully constructed to include disk galaxies above and below the critical circular velocity of 120 km/s where the dust properties of edge-on disks change so remarkably. We will then use surface brightness profiles to search for nuclear star clusters and pseudobulges, which are early indicators that secular evolution is at work, as well as measure the pitch angle of the dust lanes as a function of radius to estimate the central mass concentrations.

Observations of 20 low-surface brightness galaxies, one galaxy per orbit. Each orbit should consist of 3 exposures of approximately equal length through the F606W filter. There is a dither between each exposure to remove the gap between the two chips. ISM IN EXTERNAL GALAXIES 12 false 0 0.0 0 false false false false false false SUPPORTED GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 24.6 mag/arcsec2 J2000 01 49 10.3000 -10 03 40.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 22.8 mag/arcsec2 J2000 17 59 25.3000 +06 17 13.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 22.8 mag/arcsec2 J2000 12 49 57.9000 +05 18 41.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 23.3 mag/arcsec2 J2000 12 33 30.3000 +08 39 16.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 23.2 mag/arcsec2 J2000 11 49 40.5000 +48 25 33.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 22.7 mag/arcsec2 J2000 00 59 50.1000 -07 34 41.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 23.5 mag/arcsec2 J2000 18 33 52.6000 +49 16 43.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 23.8 mag/arcsec2 J2000 06 03 36.8000 -20 39 15.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 24.3 mag/arcsec2 J2000 01 05 4.9000 -06 12 45.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 24.8 mag/arcsec2 J2000 15 37 36.3000 +05 58 24.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 24.1 mag/arcsec2 J2000 11 57 17.4000 +49 17 0.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 24.5 mag/arcsec2 J2000 11 26 40.4000 +53 44 51.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 23.2 mag/arcsec2 J2000 13 04 19.1000 -07 38 58.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 21.9 mag/arcsec2 J2000 12 36 8.1000 +19 19 21.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 24.3 mag/arcsec2 J2000 09 20 20.4000 +64 06 10.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 23.1 mag/arcsec2 J2000 02 14 56.9000 -20 12 42.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 24.7 mag/arcsec2 J2000 10 35 23.2000 -24 45 9.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 23.5 mag/arcsec2 J2000 02 24 24.8000 -02 09 44.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 23.8 mag/arcsec2 J2000 11 40 53.4000 +56 12 7.00 J2000 GALAXY DUST LANE SPIRAL Coordinates measured from GSCII with DS9. mean mu_B25 = 23.1 mag/arcsec2 J2000 03 31 30.6000 -30 12 48.00 J2000 ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F606W 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