36x48 vertical poster template

1
THE DISTRIBUTION OF BAR STRENGTHS AND ITS
IMPLICATIONS FOR THE EVOLUTION OF GALAXIES R. J.
Buta (U. Alabama), E. Laurikainen and H. Salo (U.
Oulu), J. H. Knapen (U. Hertfordshire), and D. L.
Block (U. Witwatersrand)
BACKGROUND
RESULTS

• Why is the distribution of bar strengths in
  galaxies important?
• Bars are the most important internal features
  that redistribute angular momentum in galaxies
• Knowledge of the distribution of bar strengths
  could tell us about the robustness of bars over a
  Hubble time
• How is the distribution of bar strengths
  measured?
• Tangential forces inferred from near-infrared
  images in conjunction with a well-defined
  statistical sample of galaxies is one way of
  doing this
• For what galaxies has the distribution of bar
  strengths been measured in this manner?
• The distribution of bar strengths in spirals
  (Fig. 1) has been measured by Buta et al. (2005),
  based on the Ohio State University Bright Galaxy
  Survey (OSUBGS, Eskridge et al. 2002).
• It is also currently being measured for
  early-type galaxies (S0- to Sa) using the
  Near-Infrared S0 Survey (Laurikainen et al. 2005,
  2006 Buta et al. 2006)

The distribution of bar strengths in spirals is
shown in Figure 1. This is based on 147 OSUBGS
galaxies and the BBK method of bar-spiral
separation (Buta et al. 2005). Thus, it is the
distribution of Qb, not Qg. The plot shows 1. an
approximately exponentially declining number of
galaxies with increasing bar strength.. 2. At
least 40 of the sample galaxies have Qb lt
0.1. Impact of uncertainties Fig. 5 highlights
some of this with the distribution of Qg from
Buta, Laurikainen, and Salo (2004). The results
also apply to Fig. 1. Vertical Scale Height The
vertical scale height hz is not measured directly
but is scaled from a radial scale length.
Uncertainties in hz tend to broaden the
distribution of bar strengths and enhance the
extended tail, but do not cause the tail
entirely. Fig. 6 shows the impact of a gaussian
hz error function on Qb values, using the galaxy
NGC 4548 as a reference. Orientation and Bulge
Parameters Figure 5 shows that uncertainties in
bulge and orientation parameters have little
impact on the distribution of bar strengths, but
would be expected to affect individual galaxies
especially if a bar is end-on or
broadside-on. Dark Matter Ignoring dark matter
causes us to overestimate all values of Qb,
unless the dark matter is distributed like the
visible matter. Buta, Laurikainen, and Salo
(2004) showed that on average, ignoring dark
matter causes gravitational torque strengths to
be overestimated by 5. The tendency would be to
skew the distribution of bar strengths slightly
towards larger values (Fig. 7). Relative Fourier
Intensity Amplitudes Figure 8 shows that the
strong bar in NGC 1452 can be represented in
terms of a double-Gaussian. The weaker bar in NGC
1533 can be represented as a single Gaussian.
OBJECTIVES OF THIS STUDY
- Evaluate uncertainties in the currently known
distribution of bar strengths in spirals,
considering vertical scale heights
orientation parameters
dark matter - Examine relative Fourier intensity
amplitudes for early-type galaxy bars - What
factors might explain the observed distribution
of bar strengths?
Fig. 6 Frequency distribution of Qb for galaxy
NGC 4548 as estimated with a Gaussian
distribution of vertical scale heights having
hr11.9/-5 arcsec.
Fig. 5 Plots showing how uncertainties in bulge
treatment, vertical scale height, orientation
parameters, and other factors impact a
distribution of torque strengths.
Fig. 1 The distribution of bar strengths Qb for
147 OSUBGS spiral galaxies (Buta et al. 2005).
Fig. 2 Deprojected Ks image of NGC 4596.
MATERIALS AND METHODS
Fig. 7 Impact of dark matter corrections (dashed
histogram) on the distribution of torque
strengths.
-For spirals, we have used the H-band (1.65
micron) images from the OSUBGS - For S0s, we use
K-short (Ks) images obtained for the
Near-Infrared S0 Survey (NIRS0S, Laurikainen et
al. 2005, 2006 Buta et al. 2006) Decompositions
and Deprojected Images (Laurikainen, Salo, and
Buta 2005 Laurikainen et al. 2006) - bulges
modeled by Sersic r1/n law - disks modeled as
exponentials - bars modeled as Ferrers or Sersic
functions - images deprojected by subtracting
bulge model, deprojecting disk light, and adding
back bulge as spherical (Fig. 2) Bar-Spiral
Separation (Buta, Block, and Knapen 2003 (BBK)
Buta et al. 2005) - light distributions analyzed
as Fourier series I(r,theta)I0( r) I1c( r)
cos(theta) I1s( r)sin(theta) I2c cos(2theta)
I2s sin(2theta) . - radial profiles of Im/I0
examined for multiple symmetric or Gaussian
components - a symmetry assumption is used to
separate bars from spirals Bar Strength
Calculation (Laurikainen, Salo, and Buta
2004) - two-dimensional map of ratio of
tangential force FT to mean radial force F0R
computed from Ks image assuming constant M/L
(Fig. 3) - derive radial variation of maximum
tangential to radial force QT(r)FT/F0Rmax(r)
- total nonaxisymmetry strength computed as Qg
QTmax (Combes and Sanders 1981) (Fig. 4)
similarly for bar strength Qb and spiral
strength Qs (Buta, Block, and Knapen 2003)
Fig. 8 Gaussian fits to the relative Fourier
intensity amplitudes (m2,4,6) for NGC 1452 and
NGC 1533.
INTERPRETATIONS
- If bars dissolve and reform regularly, then the
distribution of observed bar strengths suggests
that spiral galaxies spend most of their time in
a nonbarred or weakly barred state (Bournaud and
Combes 2002 Block et al. 2002). - If bars are
more robust, and do not evolve much over a Hubble
time, then the observed distribution of bar
strengths suggests that cold, unstable disks
preferentially form weaker bars (Jogee et al.
2004). - Single and double Gaussian
representations of the relative Fourier intensity
profiles of bars seem to be in accordance with
bar strength being affected by both the halo
angular momentum transfer process and the effects
of central mass concentrations (Athanassoula
2003 Athanassoula et al. 2004). This work was
supported by NSF grants AST 0205143 and AST
0507140 to the University of Alabama, the Academy
of Finland and Magnus Ehrnrooth Foundation, the
Leverhulme Trust Fund, and the Anglo-American
Chairmans Fund.
BIBLIOGRAPHY

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Fig. 4 Example plot s of QT(r)FT/F0Rmax(r)
versus radius for NGC 6951 showing how maximum
relative torque parameters are defined. Bar
strength Qb and spiral strength Qs are based on
a bar-spiral separation.
Fig. 3 Force ratio map (false colors) for NGC
4596. The four islands are areas of maximum
tangential forcing.