Accretion in the Galactic Halo

Abstract

Fig. 1.—The available a-element ratios for each star were combined into the weighted mean and plotted against stellar metallicity. Open squares represent stars with kpc, while solid squares represent stars with apoR ≥ 16 apo galactica smaller than 16 kpc. The Milky Way disk is enveloped in a diffuse, dynamically hot collection of stars and star clusters collectively known as the “stellar halo.” Photometric and chemical analyses suggest that these stars are ancient fossils of the galaxy formation epoch. Yet little is known about the origin of this trace population. Is this system merely a vestige of the initial burst of star formation within the decoupled proto-Galaxy, or is it the detritus of cannibalized satellite galaxies? In an attempt to unravel the history of the Milky Way’s stellar halo, I performed a detailed spectroscopic analysis of 55 metal-poor stars possessing “extreme” kinematic properties. It is thought that stars on orbits that either penetrate the remote halo or exhibit large retrograde velocities could have been associated with assimilated (or “accreted”) dwarf galaxies. The hallmark of an accreted halo star is presumed to be a deficiency (compared with normal stars) of the a-elements (O, Mg, Si, Ca, Ti) with respect to iron, a consequence of sporadic bursts of star formation within the diminutive galaxies. Abundances for a select group of light metals (Li, Na, Mg, Si, Ca, Ti), iron-peak nuclides (Cr, Fe, Ni), and neutron-capture elements (Y, Ba) were calculated using line strengths measured from high-resolution, high signal-to-noise spectral observations collected with the Keck I 10 m and KPNO 4 m telescopes. The abundances extracted from the spectra reveal the following, as seen in Figure 1: (1) The vast majority of outer halo stars possess supersolar ( ) ratios. (2) The [a/Fe] ratio appears to [a/Fe] 1 0.0 decrease with increasing metallicity. (3) The outer halo stars have lower ratios of [a/Fe] than inner halo stars at a given metallicity. (4) At the largest metallicities, there is a large spread in the observed [a/Fe] ratios. (5) [a/Fe] anticorrelates with Rapo. (6) Only one star (BD 80 245) exhibits the peculiar abundances expected of an assimilated star. The general conclusion extracted from these data is that the formation of the nascent Milky Way was not dominated by the late accretion of dwarf galaxies like the ones that currently orbit the Galaxy. However, the assimilation of fragments early in the evolution of the Galaxy is a natural by-product of hierarchical models of structure formation and can explain many properties of the halo population.