584 research outputs found
Long-Lived Double-Barred Galaxies From Pseudo-Bulges
A large fraction of barred galaxies host secondary bars that are embedded in
their large-scale primary counterparts. These are common also in gas poor
early-type barred galaxies. The evolution of such double-barred galaxies is
still not well understood, partly because of a lack of realistic -body
models with which to study them. Here we report a new mechanism for generating
such systems, namely the presence of rotating pseudo-bulges. We demonstate with
high mass and force resolution collisionless -body simulations that
long-lived secondary bars can form spontaneously without requiring gas,
contrary to previous claims. We find that secondary bars rotate faster than
primary ones. The rotation is not, however, rigid: the secondary bars pulsate,
with their amplitude and pattern speed oscillating as they rotate through the
primary bars. This self-consistent study supports previous work based on
orbital analysis in the potential of two rigidly rotating bars. The pulsating
nature of secondary bars may have important implications for understanding the
central region of double-barred galaxies.Comment: Paper submitted to ApJ
Disc galaxies with multiple triaxial structures. II. JHK surface photometry and numerical simulations
We present detailed JHK surface photometry with ellipse fits of 13 galaxies
selected from previous optical observations as likely candidates for having a
secondary bar or a triaxial bulge within the primary bar. We have found 7
double-barred galaxies, 3 double-barred galaxies with an additional
intermediate structure with twisted isophotes, and 3 galaxies with a bar and
central twisted isophotes. A global analysis of the structural parameter
characteristics in the I- and K-bands is presented. Various numerical models of
galaxies with bars within bars are also analysed using the ellipse fitting
technique and compared to the observations. A thorough review of the possible
hypotheses able to explain this phenomenon is given with emphasis on the most
likely ones.Comment: 12 pages, AATEX. Accepted for publication in A&A. Large color
postscript figures omitted (Figs. 1), figures 2-9 included; gzip'ed
postscript files of the paper and Figs. 1 available via anonymous ftp at
ftp://obsftp.unige.ch/pub/fri/aasjhk/ , files fri_aasjhk.ps.gz and ngc*.ps.g
Direct Confirmation of Two Pattern Speeds in the Double Barred Galaxy NGC 2950
We present surface photometry and stellar kinematics of NGC 2950, which is a
nearby and undisturbed SB0 galaxy hosting two nested stellar bars. We use the
Tremaine-Weinberg method to measure the pattern speed of the primary bar. This
also permits us to establish directly and for the first time that the two
nested bars are rotating with different pattern speeds, and in particular that
the rotation frequency of the secondary bar is higher than that of the primary
one.Comment: 12 pages, 4 figures. To appear in ApJ Letter
Double Bars, Inner Disks, and Nuclear Rings in Early-Type Disk Galaxies
We present results from an imaging survey of an unbiased sample of
thirty-eight early-type (S0--Sa), low-inclination, optically barred galaxies in
the field. Our goal was to find and characterize central stellar and gaseous
structures: secondary bars, inner disks, and nuclear rings. Bars inside bars
are surprisingly common: at least one quarter of the sample galaxies (possibly
as many as 40%) are double-barred, with no preference for Hubble type or the
strength of the primary bar. A typical secondary bar is ~12% of the size of its
primary bar and 240--750 pc in radius. We see no significant effect of
secondary bars on nuclear activity. We also find kiloparsec-scale inner disks
in at least 20% of our sample, almost exclusively in S0 galaxies. These disks
are on average 20% the size of their host bar, and show a wider range of
relative sizes than do secondary bars. Nuclear rings are present in about a
third of our sample. Most are dusty, sites of current or recent star formation,
or both; such rings are preferentially found in Sa galaxies. Three S0 galaxies
(15% of the S0's) appear to have purely stellar nuclear rings, with no evidence
for dust or recent star formation. The fact that these central stellar
structures are so common indicates that the inner regions of early-type barred
galaxies typically contain dynamically cool and disklike structures. This is
especially true for S0 galaxies, where secondary bars, inner disks, and/or
stellar nuclear rings are present at least two thirds of the time. (abridged)Comment: LaTeX, 15 pages, 7 EPS figures; to appear in The Astronomical Journal
(July 2002
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