1. William E. Harris McMaster University
The Merger History of Centaurus A: New Evidence from Halo and Group Dynamics
Kristin A. Woodley
William E. Harris McMaster University
Mike Beasley Instituto de Astrofisica de Canarias
Terry Bridges Queen’s University
Duncan Forbes Swinburne University
Doug Geisler, Matias Gómez Universidad de Concepción
Gretchen Harris University of Waterloo
Eric Peng Herzberg Institute of Astrophysics

2. - What are the properties of the oldest galaxy components?
Talk Outline
Introduction of NGC 5128 – How much of this galaxy is old?
- What are the properties of the oldest galaxy components?
… Halo Stars, Globular Clusters, Planetary Nebulae
Kinematics and Dynamics:
Globular Cluster (GC)
Planetary Nebula (PNe)
Satellite Group Connection
Upcoming work:
HST/ACS - Halo Field Stars
Magellan/IMACS – GC identification
Increased radial velocity and age distribution samples
Conclusions 5 4 3 2 1 z
Beasley et al. 2002, MNRAS, 333, 383

3. Indicators of a Merger History for NGC 5128
Peng et al. 2002, AJ, 124, 3144
CTIO
ESO
Giant elliptical
3.8 ± 0.2 Mpc away
SN = 2.2 ± 0.6 of the GCS
(Harris et al. 2006
astro-ph/ )
on the low end for
giant Elliptical galaxies

4. Major Merger Models Model compared with Planetary Nebula Kinematics in
R (kpc)
PNe density (log10 (N/kpc2))
Model compared
with Planetary Nebula
Kinematics in
NGC 5128
Progenitor masses 2:1
R (kpc)
Vrot (km/s)
Bekki & Peng 2006, MNRAS, 370, 1737

5. How Old is the Bulk of the Galaxy?
Halo Field Stars
4 fields in NGC , 21, 31, 38 kpc (HST WFPC2 + ACS)
broad and metal-rich distribution
average age of halo stars ~ Gyr
Age (Gyr) MI
F606W – F814W
F606W
Harris and Harris 2002, AJ, 120, 2423; Rejkuba et al. 2005, ApJ, 631, 262

6. Spectroscopic Metallicities and Ages of GCs from AAT + 2dF
Spectroscopic Metallicity Distribution is trimodal or multimodal
- 50 Milky Way GCs
Age Distribution NGC 5128 GCs
all metal-poor GCs are old
50% of metal-rich GCs
are 6 – 8 Gyr
handful of metal-rich are 1-3 Gyr
Beasley et al. 2006

7. What can we learn from the Velocity Field of the Globular Cluster and Planetary Nebula Systems?
Foreground Stars:
vr < 200 km s-1
Globular Clusters:
250 km s-1 ≤ vr ≤ 1000 km s-1
Background Galaxies:
vr > 1000 km s-1
Woodley, Harris, & Harris, 2005, AJ, 129, 2654

8. Velocity Field of the Globular Clusters
possible contamination
of metal-poor halo field
stars in the Milky Way
at low radial velocities
Isotropic distribution
Woodley et al. 2006

9. Spatial Biases in the Globular Cluster System of NGC 5128
Metal-poor
Metal-rich
Unknown metallicity
Woodley et al. 2006

10. Kinematics of the Globular Cluster System of NGC 5128
All GCs – 342
Metal-poor – 179
Metal-rich – 159
vr projected radial velocity
vsys - systemic velocity
ΩR - rotation amplitude
- azimuthal angle
o - rotation axis
Woodley et al. 2006

11. Rotation Amplitude of the Globular Cluster System
Radial Bins
Equal Number Bins
Exponentially
Weighted Bins
ΩRALL = 40 ± 10 km s-1
ΩRMP = 31 ± 14 km s-1
ΩRMR = 47 ± 15 km s-1
similar rotation
amplitude between
the metal-poor and
metal-rich
subpopulations
Woodley et al. 2006

12. Rotation Axis of the Globular Cluster System
o ALL = 189 ± 12 o E of N
o MP = 177 ± 22 ° E of N
o MR = 202 ± 15 ° E of N
the rotation axis of
both the metal-poor
and the metal-rich
subpopulations are
similar
Woodley et al. 2006

13. Velocity Dispersion of the Globular Cluster System
σv ALL= 123 ± 5 km s-1
σv MP = 117 ± 7 km s-1
σv MR = 129 ± 9 km s-1
the velocity dispersion
of the metal-poor and
metal-rich sub-
populations are near
constant in the inner
regions and increases
with radius beyond
12 kpc
Woodley et al. 2006

14. Comparison to the Planetary Nebula System
2° x 2° DSS image
Total 780 PNe confirmed by radial
velocity extending out to 90 kpc
PNe are the most direct look at the
kinematics of the field stars
Peng et al. 2004, ApJ, 602, 685

15. Kinematics of the Planetary Nebula System
σv = 118 ± 13 km s-1
the velocity
dispersion of the
PNe decreases
with radius
780 PNe from Peng, Ford, & Freeman
2004, ApJ, 602, 685
the rotation amplitude and rotation axis
are similar to the globular cluster population
ΩR = 76 ± 6 km s-1
o = 170 ± 5 ° E of N
Woodley et al. 2006

16. Total Mass Estimate of NGC 5128
Globular Cluster: MT = (1.3 ± 0.5) x 1012 M⊙ M/LB = 52 ± 22 M⊙/L⊙
Planetary Nebula: MT = (1.0 ± 0.2) x 1012 M⊙
Tracer Mass Estimator Sperical Jeans Equation
(Evans et al. 2003, ApJ, 583, 752)
Woodley et al. 2006

17. Is NGC 5128 Connected with the Surrounding Group?
Extending the Velocity Field Outward
Centaurus Galaxies
NGC 5128
M83 Galaxies
Surrounding Galaxies
Woodley 2006 astro-ph/

18. Kinematics of the Halo of NGC 5128 Compared to the Cen Group: Are they Dynamically Connected?
o = 159 ± 23 ° E of N
the rotation amplitude
and rotation axis of the
globular clusters
continue smoothly to
the satellite galaxies
σv = 115 ± 25 km s-1
galaxies beyond
1.5 Mpc from NGC
5128 are not yet
virialized
NFW with rs = 14 kpc
ΩR = 125 ± 50 km s-1
Globular Clusters
Centaurus Group
Radially Binned
Galaxies
Successively
Binned Galaxies
Woodley 2006 astro-ph/

19. Total Mass of NGC 5128 and the Centaurus Group
Zero Velocity Surface
(Karachentsev et al.
2006 astro-ph/ )
Crossing Radius
Dynamical Radius
Centaurus Group:
MT = (9.2 ± 3.0) x 1012 M⊙
M/LB = 153 ± 50 M⊙/L⊙
Woodley 2006 astro-ph/

20. Kinematic Comparison to Other Giant Galaxies:
Is there a Kinematic Trend?
NGC GCs - MP and MR systems rotate about a similar axis
- velocity dispersion increases with radius
- no dispersion variations between MR and MP
M GCs - both MR and outer MP rotate about the photometric minor axis
- inner MP population rotate about the major axis
- increase in velocity dispersion with radius
(Côté et al. 2001, ApJ, 559, 828)
M GCs - MR shows no strong evidence for rotation
- MP rotates about the galaxy’s photometric minor axis
- MP has an overall higher velocity dispersion than MR
(Côté et al. 2003, ApJ, 591, 850)
NGC GCs - no rotation for MR population
- marginal rotation for the outer MP population
- MP group had a higher dispersion than MR clusters
(Richtler et al. 2004, AJ, 127, 2094)

21. Upcoming Work
HST ACS+WFC – trace halo of NGC 5128 out to R ~ 140 kpc and measure the stellar metallicity distribution function
PROBLEM: Finding GCs in heavy field contamination!
Magellan/IMACS – 25 fields (1.4 deg2) around NGC 5128 with 0.45’’ seeing
5000 objects identified as candidate GCs - follow up major radial velocity
program is underway ~ 450 new GCs => 800 GCs total
800 GCs will
Remove spatial biases beyond 12 kpc
Better than the PNe sample (no spatial bias)
Better look at the metallicity trends
Look for higher orders of velocity disperison

22. Summary NGC 5128 has a substantial old component!
Halo field stars of NGC 5128 are old ~ 8 Gyr or more
Globular Clusters in NGC 5128 are predominantly old
– develop the age distribution further with an increased sample
Kinematics of the NGC 5128 halo extends smoothly out to the Centaurus satellite
galaxies. Is the central galaxy an inward extension of the
Centaurus group?
Small number of major mergers vs. many satellite accretions
New sample of 800 Globular Clusters and outer halo field stars will help
further constrain its history

23. Serendipitous Finding in IMACS Images
B, R images with
0.45’’ seeing
30’ southwest of
NGC 5128
field shown is
1.7’ = 74 kpc
150 Mpc away
length of filaments
~ 50 kpc
ESO 270-G012