The Interstellar Medium in Double Barred Galaxies III. A 12CO J=2-1 Survey
Facilities: JCMT, NRAO 12-m
Collaborators: Dr. C. D. Wilson
Status: Preprint is ready. Results will be published in an upcoming journal article.
Summary: To enlarge the sample of galaxies used in Papers I and II, we have performed a CO survey of all northern hemisphere double barred galaxy candidates. We find that many of these galaxies seem to be weak in CO emission. This suggests that these galaxies are either deficient in molecular gas or they are hot enough to emit predominantly at higher J transitions of CO.
Papers I and II present a complete set of molecular gas properties for a sample of five galaxies that were previously thought to contain a double bar in the nucleus. In order to enlarge our sample, we have been granted time to sample the nuclei of all galaxies thought to contain nuclear bars that are visible from the northern hemisphere. We have sampled twelve barred galaxies with nuclear NIR isophote twists (which are though to indicate the existence of a nuclear bar; Shaw et al. 1997, Mulchaey et al. 1997). All galaxies sampled are within approximately 30 Mpc. Of these, we tend to detect stronger emission in galaxies that exhibit some form of nuclear activity (Liner or Seyfert; see Fig 1).
Click the image to enlarge.
Fig 1: Sample 12CO J=2-1 spectra taken at the NRAO 12-m for galaxies thought to contain nuclear bars. Notice how galaxies with nuclear activity are preferentially detected (`L' indicates Liner galaxies; `S2' indicates Seyfert 2 activity).
Computer simulations of galaxies with double bars require large amounts (at least 10% by mass) of molecular gas in order to create and maintain the double barred structure for reasonable timescales. The weak CO line strength found in this survey suggests that there may be a deficiency in molecular gas in these galaxies. Our survey reaches a sensitivity limit of 8 mK, which corresponds to a few x107 solar masses of molecular gas for galaxies near 30 Mpc. If these galaxies are typically-sized galaxies, this translates to a molecular gas content of ~0.1% (by mass), much less than is required by the simulations. There are two possibilities that may explain why we do not see strong CO emission. Either the models are wrong and these galaxies are gas deficient, or the molecular gas is there, and just isn't emitting at the J=2-1 transition. If the first scenario is correct, the models will need to be modified so they can explain double bars in gas-poor environments. If the second scenario is correct, we should search for CO emission at other J transitions. Previous studies have shown that CO J=1-0 emission is not a good tracer of molecular gas in starburst galaxies (Petitpas & Wilson 1998; Kohno et al. 1998). Perhaps these galaxies are hot enough that most of the emission comes from higher J transitions of CO. While this may explain why the galaxies that are detected seem to be deficient, it does not explain why the non-active galaxies tend not to be detected at all.
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