DIRECT CONFIRMATION
OF TWO PATTERN SPEEDS IN THE DOUBLE-BARRED GALAXY NGC 2950
JKT+CCD
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.
Some references:
|
