June 5, 2006 AAS/Calgary Stellar Populations: Old Stars in the Nearest E Galaxy From Field Stars to Globular Clusters

June 5, 2006 AAS/Calgary 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!?

June 5, 2006 AAS/Calgary 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

June 5, 2006 AAS/Calgary 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?

June 5, 2006 AAS/Calgary some collaborators  Bill Harris – McMaster  Doug Geisler – Concepcion  Kristin Woodley – McMaster  Marina Rejkuba – ESO/Garching  Eric Peng – NRC/HIA  Laura Greggio – ESO/Garching

June 5, 2006 AAS/Calgary 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

June 5, 2006 AAS/Calgary 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

June 5, 2006 AAS/Calgary HST photometry in NGC 5128 halo fields  halo field image with >10 4 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?

June 5, 2006 AAS/Calgary CMDs and GC giant branches  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 M I = -4  bright end of MS much bluer & fainter – not on these diagrams GC giant branches and [Fe/H] Saviane et al

June 5, 2006 AAS/Calgary NGC 5128 halo is a metal-rich population F1~20kpc F3~30kpc F2~8kpc data from Harris&Harris 2002, 2000, 1999  broad color range  RGB tip well defined => mainly old stars  compared with GC giant branch or model tracks CMD colour range => broad metallicity range

June 5, 2006 AAS/Calgary 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 r gc = 8-40 kpc  natural consequence of continuous gas infall during star formation Rejkuba et al. 2005

June 5, 2006 AAS/Calgary 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

June 5, 2006 AAS/Calgary 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

June 5, 2006 AAS/Calgary 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?

June 5, 2006 AAS/Calgary 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