Properties of HII Regions in Resonance Ring Spiral Galaxies

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Session 105 -- Barred Spirals
Display presentation, Thursday, 12, 1995, 9:20am - 6:30pm

[105.09] Properties of HII Regions in Resonance Ring Spiral Galaxies

D.A. Crocker (U. Ala.), P.D. Baugus (Rhodes Coll.), R. Buta (U. Ala.)

Resonance ring galaxies are barred spiral galaxies which display an inner, outer, or nuclear ring in the light distribution. A study has been made of the HII region properties in 30 resonance ring galaxies selected mainly from the Catalogue of Southern Ringed Galaxies (CSRG; see abstract by Buta, this meeting). The data consist of calibrated H$\alpha$ line and local continuum images obtained in 1993 August with the CTIO 1.5-m telescope and a TEK 1024 CCD array. The sample focusses mainly on early-to-intermediate Hubble type systems having outer pseudorings which belong to the subclasses known as R$_1$, R$_1^{\prime}$, and R$_2^{\prime}$, based on blue-light images. From the spatial distributions we find: 1) a general prominence of the observed rings in H$\alpha$; 2) a double nuclear ring/pseudoring in NGC 1326; 3) a distinctive H$\alpha$ morphology in NGC 6782 consisting of a circular nuclear ring, a pointy oval inner ring, and a partial outer pseudoring; 4) that the most intrinsically oval inner rings in the sample have HII regions more concentrated and more luminous near the intrinsic ring major axis; 5) that the outer HII regions in some galaxies tend to follow an R$_2^{\prime}$ morphology even when the stellar distribution shows a strong R$_1$ morphology; and 6) a distinct lack of HII regions in the bar regions of SB(r) galaxies. The observed HII region luminosity functions of most of the sample galaxies have slopes comparable to nonringed galaxies except for the effect of nuclear rings, which are usually the locus of the brightest HII regions when present. The characteristics we find in these galaxies are consistent with some expectations of resonance theory. The tendency for the rings to be very prominent in the ionized gas is attributable to the compression of gas gathered into resonant orbits. Clumping of HII regions near the major axis of an oval inner ring is probably a result of the slower tangential velocities expected in the bar frame. The appearance of a double nuclear ring feature in NGC 1326 can be explained in terms of an inner Lindblad resonance, and the characteristics of the outer rings fit readily into an evolutionary scenario whereby outer rings develop initially as R$_1$ types and evolve into R$_2^{\prime}$ types (Byrd et al. 1994, AJ, 108, 476).

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