Stellar Populations: Old Stars in the Nearest E Galaxy

1. Stellar Populations:Old Stars in the Nearest E Galaxy From Field Stars to Globular Clusters AAS/Calgary

2. Women & CASCA • CASCA is 35 yr old • 2/19 women presidents…so far • young women at CASCA 2006: => doctoral prize => 3/6 student prizes for posters & talks • women are on the move!? AAS/Calgary

3. Women and Lobbying • Coalition has been talking to government for 6yr to fund Canada’s LRP • broad parameter space (age, gender, “style”) for those who speak to government is good • helps reduce stereotype of scientist as old male in a lab coat AAS/Calgary

4. The stars in a galaxy tell its story • how did star formation and chemical enrichment occur in early galaxies? • do globular cluster (GC) stars and halo field stars tell the same story? • in the Milky Way… ● field halo stars predominantly metal-poor; [Fe/H] << solar ● majority of GCs the same with [Fe/H] ~ -1.6 • what is the story for other galaxies? AAS/Calgary

5. some collaborators • Bill Harris – McMaster • Doug Geisler – Concepcion • Kristin Woodley – McMaster • Marina Rejkuba – ESO/Garching • Eric Peng – NRC/HIA • Laura Greggio – ESO/Garching AAS/Calgary

6. NGC 5128 • nearest large elliptical • d ~ 4Mpc • 1’ ~1200pc; 1” ~20pc • brightest field stars resolved by HST • GC Luminosity Function turnover at V~21 • look at both GCs and field stars -> compare AAS/Calgary

7. Field stars in the NGC 5128 halo • 4 HST fields studied so far (Harris & Harris et al.) • VI photometry ≥ 4 mag fainter than giant branch tip • more to come! 40 AAS/Calgary

8. HST photometry in NGC 5128 halo fields • halo field image with >104 stars • also one globular • sample = brightest stars: giant branch • halo stars here are metal-rich! • what is the balance: metal-poor vs. metal-rich? • how can we tell? AAS/Calgary

9. CMDs and GC giant branches - 2.1 • when studying stars in distant galaxies, samples are magnitude-limited • GC giant branch different mainly due to chemical composition effects • more metal-rich => redder • GB tip ~constant at MI= -4 • bright end of MS much bluer & fainter – not on these diagrams - 0.7 GC giant branches and [Fe/H] Saviane et al. 2000 AAS/Calgary

10. NGC 5128 halo is a metal-rich population • broad color range • RGB tip well defined => mainly old stars • compared with GC giant branch or model tracks CMD colour range => broad metallicity range F1~20kpc F3~30kpc data from Harris&Harris 2002, 2000, 1999 F2~8kpc AAS/Calgary

11. Metallicity Distribution Function (MDF) • convert V-I colour to metallicity via model giant branches • very metal-rich, not a total surprise • very few metal-poor stars, a big surprise • MDF peak at [Fe/H] ~-0.7 for all 4 fields, from rgc = 8-40 kpc • natural consequence of continuous gas infall during star formation Rejkuba et al. 2005 AAS/Calgary

12. What About Age? • MDF result assumes old stellar population – is this valid? • effect of age on giant branch colour is << than for metallicity • deep photometry to HB => mean age ~8Gyr • for a given metallicity, younger = bluer; and for a younger population. MDF peak more metal-rich • can’t avoid it – NGC 5128 halo = metal-rich population Rejkuba et al. 2005 AAS/Calgary

13. MDF for Globular Clusters • NGC 5128 has ~ same % of metal-poor and metal-rich clusters • different from MW but not uncommon for extragalactic GCSs Woodley et al. 2005 AAS/Calgary

14. MDF comparison: GCs vs. halo field stars • halo stars and metal-rich GCs have ~same metallicity • but metal-poor GCs have almost no counterpart in field • why? AAS/Calgary

15. Summary • NGC 5128 halo is dominated by metal-rich stars • relatively easy to explain via continuous star formation with simultaneous gas infall • MW halo the “opposite” • both galaxies have metal-rich and metal-poor GCs but proportions very different • hard to explain so many metal-poor GCs in gE • need to understand both field stars and GCs to understand a galaxy’s history AAS/Calgary