The Nearest Giant Ellipticals: Rosetta Stones or Fool s Gold?

Transcription

1 The Nearest Giant Ellipticals: Rosetta Stones or Fool s Gold? A hypothesis or theory is clear, decisive, and positive, but it is believed by no one but the man who created it. Experimental findings, on the other hand, are messy, inexact things which are believed by everyone except the man who did that work. Harlow Shapley

2 NGC 5128 (Centaurus group) W.E.H. SDSS NGC 3377 (Leo group) NGC 3379 (Leo group)

3 In these three galaxies. Old halo and bulge RGB stars are readily accessible with HST imaging (V,I) photometry works well (high metallicity sensitivity and takes full advantage of the optical cameras) Interpolate within RGB tracks (calibrated onto the Milky Way globular cluster grid) Fast, efficient way to derive first-order Metallicity Distribution Function

4 NGC 5128 Unique chance to study a halo population in an E/S0 giant at close range d = 3.8 Mpc from several standard candles: TRGB, PNLF, SBF, Cepheids Metallicity Age

5 Previous work: 20 kpc field Allocated in Cycle 5, mid-1994 Exposures taken August 1997! Harris, Harris, & Poole 1999, AJ 117, 855 Harris, Poole, & Harris 1998, AJ 116, 2866

6 But are these really mostly former disk stars from the merged progenitors?

7 More fields kpc = 1.4 R eff kpc = 3.7 R eff kpc = 5.4 R eff kpc = 6.7 R eff deepest, with ACS

8 More fields kpc = 1.4 R eff kpc = 3.7 R eff kpc = 5.4 R eff kpc = 6.7 R eff Peng, Ford & Freeman 2002 Malin 1983

9 Harris & Harris 2000, 2002 Rejkuba, Greggio, Harris, Harris & Peng 2005 Metal-rich all the way out!

10 Mean age = 8.5 Gy

11 Leo Group ellipticals: d = 10.5 Mpc NGC 3377 M V = ksec V, 22.3 ksec I field center at 12 kpc ( R eff ) NGC 3379 M V = ksec V, 22.3 ksec I field center at 33 kpc ( R eff ) Harris, Harris, Layden & Stetson 2007, AJ in press Harris, Harris, Layden & Wehner 2007, submitted

12 NGC 3377

13 NGC 3377 No trace of metallicity gradient! Entirely old, simple formation history, intermediate metallicity

14 NGC 3379 ACS Textbook standard giant elliptical! A walking advertisement for the devaucouleurs law (Statler & Smecker-Hane 1999) NICMOS fields Gregg et al. (2004) old and high-z DeVaucouleurs & Capaccioli 1979 ( note close agreement with r 1/4 law )

15 ACS NGC 3379

16 V filter cutoff NGC stage chemical evolution Are we seeing the true metal-poor halo for Z < 0.2?

17 Are we seeing the region of transition to the classic metal-poor halo?

18 Are we seeing the region of transition to the classic metal-poor halo? Why didn t we see it in the others? Are we looking at two distinct components?? (bulge + halo) NGC R eff NGC R eff Should we expect to find the transition starting routinely around 12 R eff? NGC R eff Kalirai et al && M31 halo Z-gradient Metal-poor past R > 10 R eff

19 Williams et al Virgo ICM stars Kalirai et al && Metal-poor past R > 10 R eff

20 Increasing luminosity of host galaxy NGC 5128 Cycle 15 ACS target fields at R = 70, 110, 140 kpc (13, 20, 25 R eff )

21 What about the globular cluster population? N5128 permits detailed comparison VERY DIFFICULT TO WORK WITH: - Finding them in the first place is hard. Field is at intermediate latitude (b = 19 o ), thus field contamination by both foreground stars and faint background galaxies is very significant - Low S N = 2 (1500 GC s total), thus signal is low - Nearby (D = 4 Mpc), thus GCS is very dilute against the field - Observing runs are always plagued by bad weather, bad seeing, or instrument failure [this includes HST]

22 ~450 clusters now known, 340 with radial velocities Woodley et al. 2007, AJ, in press: kinematics & dynamics Radial velocity measurements Hi-res imaging (HST, Gemini, VLT, Magellan) G.Harris et al CMT 1 database

23 (C-T 1 ) 0 metallicity Metallicity distribution function

24 Metallicity distribution function S S N N ( MP) ( MR) 5 The new specific frequency problem! S N = S N =

25 Probably a common feature of ge galaxies. Assuming the low-metallicity clusters formed in massive pregalactic dwarfs, did they form -preferentially early relative to the field stars, followed by truncation of star formation? -at ~5 x higher efficiency? Harris & Harris 2002 Beasley et al Jordan et al Rhode et al S N = S N = Major star formation phase of the ge

26 Age estimates from Lick indices: Beasley et al courtesy Thomas Puzia Highest-metallicity GCs appear 6-8 Gy old. cf. Rejkuba && halo-star mean age of 8.5 Gy But -- Great majority > 10 Gy

27 Gemini GMOS study of inner GC population Preliminary!! Woodley, Harris, Harris, Geisler, Gomez, & Puzia 2007

28 N5128: GC kinematics Woodley et al. 2007, astro-ph/ Blue, MP Red, MR

29 GCS NGC 5128 NGC 3379 PNe Woodley et al Romanowsky et al Bergond et al. 2006

30 Dynamical connection of a giant central galaxy with its surroundings NGC 5128: Woodley 2006, AJ 132, 2424 M87: Cote et al. 2001

31 There appears to be good reason to think that the metal-rich globular clusters and the main population of stars in E galaxies formed together and We may now have reason to think that the metal-poor globular clusters and the (more elusive) metal-poor halo stars formed together as well (but in a different ratio). But are these generalizations risky? Prospects for continuing the halo-star studies are great if we can get HST back with WFC3 (or ACS) more extended coverage of N5128, 3379, plus Virgo, Fornax E s and many others. Stellar populations are what happen when galaxy populations run out!

32 There appears to be good reason to think that the metal-rich globular clusters and the main population of stars in E galaxies formed together and We may now have reason to think that the metal-poor globular clusters and the (more elusive) metal-poor halo stars formed together as well (but in a different ratio). All depends on keeping the eye steadily fixed on the facts of nature, and so receiving their images as they are. For God forbid that we should give out a dream of our own imagination for a pattern of the world. Francis Bacon (1623)