Direct Confirmation of Two Bar Pattern Speeds Direct Confirmation of Two Bar Pattern Speeds
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Large-scale bars are present in some two-thirds of all disk galaxies. Secondary stellar bars within large-scale bars are also common, occuring in about one-third of the barred galaxies. Interest in secondary stellar bars is motivated by the hypothesis that they are a mechanism for driving gas to small radii to feed the supermassive black holes powering active galactic nuclei. However, the efficiency of such transport is uncertain because of the lack of knowledge of  the pattern speeds of the primary and secondary bars. Whereas a number of pattern speeds of large-scale bars have been measured, no such measurements in nested systems have been performed yet. The presence of nested bars with different pattern speeds has been inferred largely on the basis of their apparently random relative orientations. Secondary bars can naturally form and survive for more than a few rotation periods in pure stellar disks. However, simulations have also found other possibilities, including cases in which two stellar bars counterrotate.

An ideal target for measuring primary and secondary pattern speeds is the S2B NGC 2950, which is a large and bright early-type barred galaxy. NGC 2950 is classified RSBO(r), and the presence of a secondary stellar bar has been discussed by several authors. The secondary bar of NGC 2950 has an intermediate inclination, and both bars have intermedia position angles between the major and minor axes of the disk and no evidence of spiral arms, patchy dust, or significant companions.

The photometric observations of NGC 2950 were carried out at the Jacobus Kapteyn Telescope. The astronomers took multiple exposures in the Harris B, V, and I bandpasses using the SITE2 CCD, and the spectroscopic observations were carried out at the Telescopio Nazionale Galileo.

Left panel: Large-scale image of NGC 2950 showing the primary bar and disk with I-band contours and slit positions overlaid. Right panel: Zoom into the central region of NGC 2950 showing its secondary bar. [ JPEG | TIFF ]

The astronomers found that the primary bar in NGC 2950 is rapidly rotating. If this is the norm in S2B galaxies, then it guarantees that primary bars are efficient at funnelling gas down to the radius of influence of secondary bars. They also establish that in NGC 2950 the two bars have different pattern speeds, with the secondary bar having a larger pattern speed. This is the first time this is measured directly.

Movie showing the dynamics of the primary and secondary bars of NGC 2950. Credit: IAC. [ High-res movie | Low-res movie ]

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Last modified: 13 December 2010