The Interstellar Medium in Double Barred Galaxies I. Morphology and Dynamics

Facilities: OVRO
Collaborators: Dr. C. D. Wilson
Status: Preliminary results are published in Petitpas & Wilson 2000. To appear the August 20 2002 issue of th Astrophysical Journal.

Summary: We have used high resolution CO maps to determine the molecular gas properties in a sample of galaxies which appear double barred in near infrared images. We find that in general NIR images are not a good way to search for double barred galaxies. Two out of five exhibit `true' double barred structures, while the isophote twists seen in the other three are likely the result of a triaxial stellar bulge seen in projection. In the cases where double bars are seen in the CO maps, the small scale nuclear bars very closely resemble existing simulations and observations of large scale bars seen in the disks of regular barred galaxies.

Computer simulations suggest that once molecular gas gets trapped at an Inner Lindblad Resonance (ILR) and forms a ring, the ring may become unstable. It can collapse and form a miniature bar interior to the ILR, giving the galaxy essentially a nested set of bars that can transport molecular material inward in turn; first through the large scale galactic bar down to the ILR radius, then in through the miniature nuclear bar (Shlosman et al. 1989; Friedli & Martinet 1997; Shaw et al. 1997; see Fig. 1).

Fig 1: Schematic diagram of how a large scale bar (Main Bar) passing though a disk can transport material towards the nucleus of a double barred galaxy. Once at the ILR, gas can be transferred to the inner bar (or nuclear bar) and transported deeper into the nucleus.

In order to determine how molecular gas gets transported interior to the ILR, we studied a small set of galaxies that contain isophote twists in the nucleus. These isophote twists are thought to be the signature of a double barred galaxy. There are at least three competing models that explain these twists: two of them claim that the nuclear bar is real and the third claims it is a triaxial stellar bulge seen in projection (Shaw et al. 1997; Friedli & Martinet 1997; Kormendy 1979). In an attempt to rule out at least one of the three competing models of these isophote twists, we obtained high resolution CO J=1-0 observations of a sample of galactic nuclei containing isophote twists. We can use molecular gas observations to at rule out the triaxial stellar bulge theory, since there is very little CO emission coming from the bulge of a galaxy. Using the molecular gas dynamics, we may be able to distinguish between the other two models that claimed the nuclear bars are real. We found that the CO maps exhibited a rather wide variety of nuclear morphologies, ranging from nuclear spirals, nuclear rings, random blobs, to miniature nuclear bars.

This result immediately suggests that a triaxial stellar bulge may be responsible for the isophote twists in at least some galaxies and thus isophote twists do not necessarily indicate the presence of nuclear bars. There are a few instances (namely NGC 470 and NGC 2273) where the isophote twists do seem to be the result of nuclear bar. In the case of NGC 2273, we see that the nuclear bar (as seen in the CO maps) actually breaks down into three separate emitting regions: one in the middle and two brighter ones at the ends of the nuclear bar. This is similar in structure to the large scale bars seen in other galaxies, where there is a pile up of molecular gas at the ends of the large scale bar (near the Corotation Resonance radius), as well as a central concentration that is likely the result of inflow along the bar. This is important because is suggests that both large scale bars and nuclear bars behave in similar ways, except that the outer Corotation Resonance of the nuclear bar, happens to be at the same radius as the Inner Lindblad Resonance of the large scale bar.


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