The Merger History of Centaurus A: New Evidence from Halo and Group Dynamics

1. 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. 5 4 3 2 1 0 z 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 Beasley et al. 2002, MNRAS, 333, 383

3. ESO CTIO Peng et al. 2002, AJ, 124, 3144 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

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

5. MI F606W Age (Gyr) F606W – F814W 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 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 - 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

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 oALL = 189 ± 12 o E of N oMP = 177 ± 22 ° E of N oMR = 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 σvALL= 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. σv = 118 ± 13 km s-1 ΩR = 76 ± 6 km s-1 o = 170 ± 5 ° E of N • the velocity • dispersion of the • PNe decreases • with radius Kinematics of the Planetary Nebula System 780 PNe from Peng, Ford, & Freeman 2004, ApJ, 602, 685 • the rotation amplitude and rotation axis • are similar to the globular cluster population Woodley et al. 2006

16. 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⊙ Total Mass Estimate of NGC 5128 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/0608497

18. ΩR = 125 ± 50 km s-1 • the rotation amplitude • and rotation axis of the • globular clusters • continue smoothly to • the satellite galaxies σv = 115 ± 25 km s-1 o = 159 ± 23 ° E of N Globular Clusters Centaurus Group Radially Binned Galaxies Successively Binned Galaxies • galaxies beyond • 1.5 Mpc from NGC • 5128 are not yet • virialized NFW with rs = 14 kpc Kinematics of the Halo of NGC 5128 Compared to the Cen Group: Are they Dynamically Connected? Woodley 2006 astro-ph/0608497

19. 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

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

21. 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 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

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