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Understanding the evolution of carbon and iron in the Milky Way's halo is ofimportance because these two elements play crucial roles constraining starformation, Galactic assembly, and chemical evolution in the early Universe.Here, we explore the spatial distributions of carbonicity, [C/Fe], andmetallicity, [Fe/H], of the halo system based on medium-resolution ($R \sim$1,300) spectroscopy of $\sim$58,000 stars in the Southern Hemisphere from theAAOmega Evolution of Galactic Structure (AEGIS) survey. The AEGIS carbonicitymap exhibits a positive gradient with distance, as similarly found for theSloan Digital Sky Survey (SDSS) carbonicity map of Lee et al. The metallicitymap confirms that [Fe/H] decreases with distance, from the inner halo to theouter halo. We also explore the formation and chemical-evolution history of thehalo by considering the populations of carbon-enhanced metal-poor (CEMP) starspresent in the AEGIS sample. The cumulative and differential frequencies ofCEMP-no stars (as classified by their characteristically lower levels ofabsolute carbon abundance, $A$(C) $\leq$ 7.1 for sub-giants and giants)increases with decreasing metallicity, and is \textit{substantially higher thanprevious determinations} for CEMP stars as a whole. In contrast, that ofCEMP-$s$ stars (with higher $A$(C)), remains almost flat, at a value$\sim$10\%, in the range $-\,4.0 \lesssim$ [Fe/H] $\lesssim-$2.0. Thedistinctly different behaviors of the CEMP-no and CEMP-$s$ stars relieve thetension with population-synthesis models assuming a binary mass-transferorigin, which previously struggled to account for the higher reportedfrequencies of CEMP stars, taken as a whole, at low metallicity.

Galactic Archeology with the AEGIS Survey: The Evolution of Carbon and Iron in the Galactic Halo