Chemical Cartography. II. The Assembly History of the Galactic Stellar Halo Traced by Carbon-Enhanced Metal-Poor Stars

TL;DR

This study analyzes the kinematic and chemical properties of over 100,000 stars in the Galactic halo, revealing distinct origins and assembly histories for the inner and outer halo regions based on stellar motions and carbon-enhanced star classifications.

Contribution

It provides new insights into the assembly history of the Galactic halo by linking stellar kinematics and chemical signatures to their progenitor satellite galaxies.

Findings

Outer halo stars show retrograde motion and spherical orbits.

Inner halo stars have near-zero net rotation with radial orbits.

CEMP-no stars are more common in the outer halo and exhibit distinct orbital characteristics.

Abstract

We present an analysis of the kinematic properties of stellar populations in the Galactic halo, making use of over 100,000 main sequence turnoff (MSTO) stars observed in the Sloan Digital Sky Survey. After dividing the Galactic halo into an inner-halo region (IHR) and outer-halo region (OHR), based on the spatial variation of carbon-to-iron ratios in the sample, we find that stars in the OHR exhibit a clear retrograde motion of $-$49 $\pm$ 4 km s$^{-1}$ and a more spherical distribution of stellar orbits, while stars in the IHR have zero net rotation ($-$3 $\pm$ 1 km s$^{-1}$) with a much more radially biased distribution of stellar orbits. Furthermore, we classify the carbon-enhanced metal-poor (CEMP) stars among the MSTO sample in each halo component into CEMP-no and CEMP-$s$ sub-classes, based on their absolute carbon abundances, $A$(C), and examine the spatial distributions and kinematics associated with each sub-class. The CEMP-no stars are the majority sub-class of CEMP stars in the OHR ($\sim$ 65%), and the minority sub-class in the IHR ($\sim$ 44%), similar to the results of several previous analyses. The CEMP-no stars in each halo region exhibit slightly higher counter-rotation than the CEMP-$s$ stars, but within statistical errors. The CEMP-no stars also show a more spherical distribution of orbits than the CEMP-$s$ stars in each halo region. These distinct characteristics provide strong evidence that numerous low-mass satellite galaxies (similar to the ultra-faint dwarf galaxies) have donated stars to the OHR, while more-massive dwarf galaxies provided the dominant contribution to the IHR.