The Merger History of Centaurus A:The Merger History of Centaurus A:

New Evidence from Halo and Group New Evidence from Halo and Group DynamicsDynamics

The Merger History of Centaurus A:The Merger History of Centaurus A:

New Evidence from Halo and Group New Evidence from Halo and Group DynamicsDynamics

Kristin A. WoodleyKristin A. WoodleyWilliam E. Harris McMaster UniversityWilliam E. Harris McMaster University

Mike BeasleyMike Beasley Instituto de Astrofisica de CanariasInstituto de Astrofisica de CanariasTerry Bridges Terry Bridges Queen’s UniversityQueen’s UniversityDuncan Forbes Duncan Forbes Swinburne UniversitySwinburne UniversityDoug Geisler, Matias GDoug Geisler, Matias Góómezmez Universidad de ConcepciUniversidad de ConcepcióónnGretchen HarrisGretchen Harris University of WaterlooUniversity of WaterlooEric PengEric Peng Herzberg Institute of AstrophysicsHerzberg Institute of Astrophysics

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 StarsMagellan/IMACS – GC identificationIncreased radial velocity and age distribution samples

• Conclusions

5

4

3

2

1

0

z

Beasley et al. 2002, MNRAS, 333, 383

ESO

Indicators of a Merger History for NGC 5128

• Giant elliptical

• 3.8 ± 0.2 Mpc away

• SN = 2.2 ± 0.6 of the GCS (Harris et al. 2006 astro-ph/0607269) on the low end for giant Elliptical galaxies

Peng et al. 2002, AJ, 124, 3144

CTIO

Major Merger Models

Bekki & Peng 2006, MNRAS, 370, 1737

Model compared with Planetary Nebula Kinematics in NGC 5128

Progenitor masses 2:1

R (kpc)P

Ne

dens

ity (

log 1

0 (

N/k

pc2))

R (kpc)

Vro

t (km

/s)

Harris and Harris 2002, AJ, 120, 2423; Rejkuba et al. 2005, ApJ, 631, 262

How Old is the Bulk of the Galaxy?Halo Field Stars

• 4 fields in NGC 5128 - 8, 21, 31, 38 kpc (HST WFPC2 + ACS)• broad and metal-rich distribution• average age of halo stars ~ Gyr 3

5.38

F606W – F814W

F60

6W

Age (Gyr)

MI

Spectroscopic Metallicities and Ages of GCs from AAT + 2dF

• Spectroscopic Metallicity Distribution is trimodal or multimodal- 50 Milky Way GCs

• Age Distribution - 200 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

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

What can we learn from the Velocity Field of the Globular Cluster and Planetary Nebula Systems?

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

Metal-poor

Metal-rich

Unknown metallicity

Spatial Biases in the Globular Cluster System of NGC 5128

Woodley et al. 2006

Kinematics of the Globular Cluster System of NGC 5128

All GCs – 342

Metal-poor – 179

Metal-rich – 159

)sin( osysr Rvv

vr - projected radial velocityvsys - systemic velocity ΩR - rotation amplitude - azimuthal angle

o - rotation axis

Woodley et al. 2006

ΩRALL = 40 ± 10 km s-1

Rotation Amplitude of the Globular Cluster System

Radial Bins

Equal Number Bins

Exponentially Weighted Bins

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

o ALL = 189 ± 12 o E of N

Rotation Axis of the Globular Cluster System

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

σv ALL= 123 ± 5 km s-1

Velocity Dispersion of the Globular Cluster System

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

Comparison to the Planetary Nebula System

Peng et al. 2004, ApJ, 602, 685

• 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

Kinematics of the Planetary Nebula System

780 PNe from Peng, Ford, & Freeman 2004, ApJ, 602, 685

ΩR = 76 ± 6 km s-1 o = 170 ± 5 ° E of N

• the rotation amplitude and rotation axis are similar to the globular cluster population

σv = 118 ± 13 km s-1

• the velocity dispersion of the PNe decreases with radius

Woodley et al. 2006

Total Mass Estimate of NGC 5128

RotationPressureTotal MMM

i

isysip RvvGN

CM 2)(

G

vRM out

r

2max

Tracer Mass Estimator Sperical Jeans Equation(Evans et al. 2003, ApJ, 583, 752)

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⊙

Woodley et al. 2006

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/0608497

Kinematics of the Halo of NGC 5128 Compared to the Cen Group: Are they Dynamically Connected?

ΩR = 125 ± 50 km s-1

Globular Clusters

Centaurus Group

Radially Binned Galaxies

Successively Binned Galaxies

Woodley 2006 astro-ph/0608497

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

Total Mass of NGC 5128 and the Centaurus Group

Zero Velocity Surface(Karachentsev et al. 2006 astro-ph/0603091)

Crossing Radius

Dynamical Radius

Centaurus Group:

MT = (9.2 ± 3.0) x 1012 M⊙

M/LB = 153 ± 50 M⊙/L⊙

Woodley 2006 astro-ph/0608497

NGC 5128 - 340 GCs - MP and MR systems rotate about a similar axis - velocity dispersion increases with radius - no dispersion variations between MR and MP

M87 - 280 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)

M49 - 260 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 1399 - 460 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)

Kinematic Comparison to Other Giant Galaxies: Is there a Kinematic Trend?

12.026.0)/( MPR 11.036.0)/( MRR

13.011.047.0)/(

MPR 14.012.043.0)/(

MRR

19.011.027.0)/(

MPR 27.025.010.0)/(

MRR

09.005.0)/( MPR 09.003.0)/( MRR

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

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 satellitegalaxies. 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

•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

Serendipitous Finding in IMACS Images

ESO 270-G012