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Session 54 - Spiral Galaxy Kinematics.
Oral session, Tuesday, January 14
It has been suggested that sizeable numbers of counter-rotating stars may lurk undetected in the disks of a significant fraction of galaxies. It has also been suggested that such counter-rotating populations might preferentially be found in early-type disk galaxies. A startling example of this phenomenon can be seen in the S0 galaxy NGC 4550, where 50% of the stars rotate in each direction. We have therefore obtained high quality spectra of 28 S0 galaxies in order to search for other examples of this phenomenon. Our analysis of the Doppler broadening of the spectral lines in these data imply that none of these systems contain more than 5% of their stars on retrograde orbits. We can therefore conclude (at 95% confidence) that no more than 10% of galaxies contain significant counter-rotating stellar populations. This result contrasts with the prevalence of counter-rotating gas in such systems: combining our new observations with existing data, we find that 24 \pm 8% of the gas disks in S0 galaxies counter-rotate relative to their stellar components. In the case of the more massive counter-rotating gaseous systems, it is hard to see why counter-rotating stellar populations have not formed. If there does exist some mechanism for inhibiting star formation in counter-rotating gaseous disks, then the mystery surrounding the rare cases that do contain counter-rotating stellar populations deepens still further.
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