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abstract
Buta and Crocker (1991, AJ 102, 1715) found an interesting correlation between the morphology (R$_1$ or R$_2$) of an outer ring and the presence of nuclear star formation in a sample of 22 galaxies. Is the correlation real, and if so, how do such different parts of the galaxy know about each other? We have addressed this through a study of the IR properties of outer-ringed galaxies. This study has confirmed the correlation, and simple considerations suggest that the pattern speed of the bar is the controlling element. The idea is that outer rings, through their morphology and other characteristics, can be identified with the outer Lindblad resonance (OLR), one of the major resonances in galaxy structure. The work of Schwarz (1981, ApJ 247, 77) suggested that whenever there was enough gas beyond the OLR, then an R$_2$ outer ring would be favored. This could effectively be done by having a high pattern speed, thus moving the OLR inward within the gas disk. The bar pattern speed would then more likely be high enough to avoid an inner Lindblad resonance (ILR) and such objects would not contain nuclear or circumnuclear star formation.
Here we present the IR study which focused on outer-ringed galaxies where we knew precisely the nucleus characteristics, as well as a large database of ringed galaxies with unknown nucleus characteristics. For the sample with known nuclear characteristics we found a correlation between the far-IR-to-blue light ratio (FIR/L(B)) and the type of nuclear structure in the galaxy (red nucleus vs. blue nuclear ring vs. blue nucleus). In the sample with unknown nucleus characteristics we found a correlation between this ratio and the type of outer ring. Outer ring type R$_1$ galaxies correlated well with higher values of FIR/L(B) as did blue nucleus and blue nuclear ring galaxies, indicating a preference for blue nuclear rings or blue nuclei. Outer ring type R$_2$ galaxies correlated well with low values of FIR/L(B), as did red nuclei galaxies, indicating a preference for red nuclei.
The samples will be presented and the implications of pattern speed will be more thoroughly discussed.
