Journal Article

Evidence for two phases of galaxy formation from radial trends in the globular cluster system of NGC 1407

Duncan A. Forbes, Lee R. Spitler, Jay Strader, Aaron J. Romanowsky, Jean P. Brodie and Caroline Foster

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 413, issue 4, pages 2943-2949
Published in print June 2011 | ISSN: 0035-8711
Published online May 2011 | e-ISSN: 1365-2966 | DOI:
Evidence for two phases of galaxy formation from radial trends in the globular cluster system of NGC 1407

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Here we present the colours of individual globular clusters (GCs) around the massive elliptical galaxy NGC 1407 out to a projected galactocentric radius of 140 kpc or 17 galaxy effective radii (Re). Such data are a proxy for the halo metallicity. We find steep, and similar, metallicity gradients of ∼−0.4 dex dex−1 for both the blue (metal poor) and red (metal rich) GC subpopulations within 5–8.5 Re (40–70 kpc). At larger radii the mean GC colours (metallicity) are constant. A similar behaviour is seen in a wide-field study of M87's GC system, and in our own Galaxy. We interpret these radial metallicity trends to indicate an inner region formed by early in situ dissipative processes and an outer halo formed by the ongoing accretion of low-mass galaxies and their GCs. These results provide observational support for the model of galaxy formation whereby massive galaxies form inside-out in two phases. We have also searched the literature for other massive early-type galaxies with reported GC metallicity gradients in their inner regions. No obvious correlation with galaxy mass or environment is found but the sample is currently small.

Keywords: globular clusters: general; galaxies: formation; galaxies: individual: NGC 1407; galaxies: star clusters: general

Journal Article.  5778 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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