The Metallicity Distribution in the Halo Stars of NGC 5128: Implications for Galaxy Formation

Details The Metallicity Distribution in the Halo Stars of NGC 5128: Implications for Galaxy Formation The Metallicity Distribution in the Halo Stars of NGC 5128: Implications for Galaxy Formation

1998-10-08

arxiv.org/abs/astro-ph/9810134v1

Astronomy and Astrophysics

Astrophysics

23 pages, LaTeX, plus 13 figures in separate files; submitted to Astronomical Journal. Complete preprint, with better quality

Scientific paper

10.1086/300749

The Hubble Space Telescope has been used to obtain WFPC2 (V,I) photometry for a sample of 10,000 red giant stars in the outer halo of the giant elliptical NGC 5128. Comparison with the fiducial RGBs of Milky Way globular clusters and model isochrones demonstrates that this outer-halo population is completely dominated by old stars, with an extremely broad metallicity range extending from the most metal-poor Galactic globulars at [Fe/H] = -2 up to Solar abundance. By interpolation within the fiducial lines we derive the metallicity distribution function (MDF) for the halo, which turns out to have a distinct two-component structure: about 2/3 of the stars belong to a sharply peaked metal-richer component ([Fe/H] = -0.3, sigma = 0.22 dex), while the remaining one-third belong to a more extended metal-poor distribution sloping gradually down to low metallicity. The MDF can be remarkably well matched by a simple closed-box model of chemical enrichment, where the first component starts with an initial abundance Z_0 =0 and the second component with Z_0 = 0.18 Z_Sun. A basic "in situ" formation picture with two distinct epochs of star formation fits the observations better than other models involving major contributions from accretions or mergers.

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