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 presidentsso 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 Canadas 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] 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 mean age ~8Gyr for a given metallicity, younger = bluer; and for a younger population. MDF peak more metal-rich cant 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 galaxys history