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Radial Trends in IMF-Sensitive Absorption Features in Two Early-Type Galaxies: Evidence for Abundance-Driven Gradients

, , and . (2015)cite arxiv:1506.07880Comment: 16 page body + 7 page appendix + references. Includes 18 figures. Submitted to ApJ.

Abstract

We have observed two massive early-type galaxies with Keck/LRIS and measured radial gradients in the strengths of stellar absorption features from 4000-5500 \AA$\,$ and 8000-10,000 \AA. We present spatially resolved measurements of the dwarf-sensitive spectral indices NaI (8190 \AA) and Wing-Ford FeH (9915 \AA), as well as indices for species of H, C$_2$, CN, Mg, Ca, TiO, and Fe. Our measurements show a metallicity gradient in both objects, and Mg/Fe consistent with uniform $\alpha$-enhancement, matching widely observed trends for massive early-type galaxies. The NaI index and the CN$_1$ index at 4160 \AA$\,$ exhibit significantly steeper gradients, with a break at $r 0.1 r_eff$ ($r 300$ pc). Inside this radius NaI and CN$_1$ increase sharply toward the galaxy center, relative to other indices. We interpret this trend as a rapid central rise in Na/Fe and N/Fe. In contrast, the FeH index exhibits a marginal decrease toward the galaxy center, relative to Fe. Our investigation is among the first to track FeH as a function of radius, and to demonstrate discrepant behavior between NaI and FeH. We suggest that a shallow gradient in FeH and steep, broken NaI profile reflect unique abundance patterns rather than a gradient in the stellar initial mass function.

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[1506.07880] Radial Trends in IMF-Sensitive Absorption Features in Two Early-Type Galaxies: Evidence for Abundance-Driven Gradients

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