Radial trends in imf-sensitive absorption features in two early-type galaxies: Evidence for abundance-driven gradients

Samples of early-type galaxies show a correlation between stellar velocity dispersion and the stellar initial mass function (IMF) as inferred from gravity-sensitive absorption lines in the galaxies' central regions. To search for spatial variations in the IMF, we have observed two early-type galaxies with Keck/LRIS and measured radial gradients in the strengths of absorption features from 4000–5500 Å and 8000–10000 Å. We present spatially resolved measurements of the dwarf-sensitive spectral indices $NaI$ (8190 Å) and Wing-Ford $FeH$ (9915 Å), as well as indices for species of H, C₂, CN, Mg, Ca, $TiO$, and Fe. Our measurements show a metallicity gradient in both objects, and Mg/Fe consistent with a shallow gradient in α-enhancement, matching widely observed trends for massive early-type galaxies. The $NaI$ index and the CN₁ index at 4160 Å exhibit significantly steeper gradients, with a break at $r\sim 0.1r_{eff}$ ($r\sim 300$ pc). Inside this radius, $NaI$ strength increases sharply toward the galaxy center, consistent with a rapid central rise in [Na/Fe]. In contrast, the ratio of the $FeH$ to Fe index strength decreases toward the galaxy center. This behavior cannot be reproduced by a steepening IMF inside of $0.1r_{eff}$ if the IMF is a single power law. While gradients in the mass function above $\sim 0.4M_{\odot }$ may occur, exceptional care is required to disentangle these IMF variations from the extreme variations in individual element abundances near the galaxies' centers.