DUAL STELLAR HALOS IN THE STANDARD ELLIPTICAL GALAXY M105 AND FORMATION OF MASSIVE EARLY-TYPE GALAXIES

Abstract

ABSTRACT M105 is a standard elliptical galaxy, located in the Leo I Group. We present photometry of the resolved stars in its inner region at R ≈ 4′ ≈ 4R eff, obtained from F606W and F814W images in the Hubble Space Telescope archive. We combine this with photometry of the outer region at R ≈ 12′ ≈ 12R eff from archival imaging data. Color–magnitude diagrams of the resolved stars in the inner region show a prominent red giant branch (RGB) with a large color range, while those for the outer region show better a narrow blue RGB. The metallicity distribution function (MDF) of the RGB stars shows the existence of two distinct subpopulations: a dominant metal-rich population (with a peak at [M/H] ≈ 0.0) and a much weaker metal-poor population (with a peak at [M/H] ≈ −1.1). The radial number density profiles of the metal-rich and metal-poor RGB stars are fit well by a Sérsic law with n = 2.75 ± 0.10 and n = 6.89 ± 0.94, and by a single power law (σ ∝ R −3.8 and σ ∝ R −2.6), respectively. The MDFs of the inner and outer regions can be described well by accretion gas models of chemical evolution with two components. These provide strong evidence that there are two distinct stellar halos in this galaxy, blue metal-poor and red metal-rich halos, consistent with the results based on globular cluster systems in bright early-type galaxies (ETGs). We discuss the implications of these results with regard to the formation of massive ETGs in the dual halo mode formation scenario.