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J. Cabanela (Haverford College), M. Womack (St. Cloud State University), J. M. Dickey (University of Minnesota)
Low surface brightness galaxies (LSBs) are an important species of extragalactic object, differing from ``normal'' HSB galaxies in that their stellar disks are very diffuse (see Bothun, Impey, and McGaugh 1997). LSBs are typically {H{\tt I}}-rich galaxies, ranging in size from giants (e.g. Malin 1) to dwarfs. One explanation for their LSB nature has been that LSBs could have negligible current star formation rates and probably negligible past star formation. This interpretation is supported by the fact that while {H{\tt I}}-rich, the gas surface densities of LSBs are observed to be below the Toomre threshold for star formation over most of their gas disks (van der Hulst etal 1993). However, Martin & Kennicutt (2001) note there are examples of globally subcritical galaxies in which star formation is actually quite vigorous (e.g. NGC 2403 and M33). This star formation likely reflects sporadic star formation in regions of these galaxies that have had their gas densities increase above the Toomre threshold. Could such sporadic star formation been characteristic of LSBs and could some LSBs show evidence for this?
We used the University of Arizona Steward Observatory (formerly NRAO) 12m to perform deep CO(1--0) observations of three LSBs (47-211, N09-2, and N10-4) selected on the basis of their O-E colors as measured off the POSS I. By picking LSBs with redder O-E colors, we hoped to select galaxies with an older population of stars and possibly a history of previous star formation, despite their globally subcritical nature. In addition to these three galaxies, we also observed the LSB P06-1 which was the only unambiguous LSB galaxy previously detected in CO(1--0) (O'Neil, Hofner, and Schinnerer 2000).
Our observations had single channel 3{\sigma} detection limits of TMB \approx 1 - 4 mK (the deepest observations were on P06-1, the shallowest on N09-2). No CO was detected in any of the targeted LSBs. These null detections can be used to establish lower limits on the molecular gas content of these LSBs and rough limits on the level of previous star formation. The non-detection of P06-1 suggests the previous CO detection in P06-1 may have been spurious, although the former detection was in both CO(1--0) and CO(2--1).