COUNTER-WINDING SPIRAL STRUCTURE IN ESO 297-27

Abstract

ESO 297-27, an intermediate-to-late-type spiral galaxy, exhibits a rare counter-winding spiral structure in which two sets of nonoverlapping arms open in opposite senses. We imaged ESO 297-27 along with 41 other galaxies for a survey of the measurable properties of nonbarred ringed galaxies, which is the category to which the galaxy is assigned in the Catalogue of Southern Ringed Galaxies. We analyzed broadband, narrowband, and spectroscopic data of ESO 297-27 to measure and update its physical parameters. Using a rotation curve obtained from slit spectra of the galaxy's Hα emission in combination with the Hubble-de Vaucouleurs dust asymmetry method, we determined that a single inner spiral arm is leading while at least three weak outer arms are trailing. We believe that none of the galaxy's catalogued companions are strong candidates for distorting the galaxy gravitationally. However, an unclassified companion may have perturbed ESO 297-27 in the past, but velocity data are needed to confirm this. To date, we have identified ESO 297-27 as only the second clear-cut example of a counter-winding spiral pattern, suggesting either that such patterns are very transient or that extraordinary circumstances are needed to produce them. It is important to emphasize that counter-rotation is not necessarily synonymous with a counter-winding spiral structure. Currently, there are no known cases of counter-rotation that show counter-winding spirals as strong as what is seen in ESO 297-27.