The Interstellar Medium in Double Barred Galaxies II. Physical Conditions

Facilities: JCMT
Collaborators: Dr. C. D. Wilson
Status: Submitted to the Astrophysical Journal.

Summary: CO line ratios suggest that the true double bars tend to occur in galaxies that contain cooler molecular gas.

Near infrared galaxy surveys have revealed the existence of nuclear bars in a large number of barred or lenticular galaxies. High resolution CO maps of these galaxies exhibit a wide range of morphologies. The similarity in near infrared images combined with the wide variety of CO morphologies suggests that the molecular gas may have different properties that allow it to respond differently to similar gravitational potentials. We have performed a multi-transition CO study of the nuclei of seven double barred galaxies that exhibit a variety of molecular gas morphologies in order to determine if the molecular gas properties are correlated with the nuclear structure and activity. We find that the 12CO J=3-2/J=2-1 line ratio (R) is lower in galaxies with molecular bars in the nucleus and higher in galaxies with CO emission dispersed around the galactic center in rings and peaks (see Figure 1).

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Fig 1: Radial CO integrated intensity profile for double barred galaxy candidates (left panels) compare to the gas properties determine using the Local Thermodynamic Equilibrium approximation. Notice how the gas is cooler in galaxies with more centrally concentrated gas distributions.
The 13CO/12CO J=2-1 line ratios are similar for all galaxies, which indicates that the J=3-2/J=2-1 line ratio is tracing variations in gas temperature and density, rather than variation in optical depth (tau). The molecular gas in galaxies with nuclear molecular bars is cooler (and perhaps more viscous) enhancing the gas's ability to clump together and flow inwards. The molecular gas conditions, combined with the star formation activity and models of double barred galaxies suggest that the varation in molecular gas distributions and physical conditions may be the result of galaxy evolution though the double barred phase.