RADIAL TRENDS IN IMF-SENSITIVE ABSORPTION FEATURES IN TWO EARLY-TYPE GALAXIES: EVIDENCE FOR ABUNDANCE-DRIVEN GRADIENTS

Abstract

ABSTRACT 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 Na I (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 Na I index and the CN1 index at 4160 Å exhibit significantly steeper gradients, with a break at r ∼ 0.1 r eff ( r ∼ 300 pc). Inside this radius, Na I 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 0.1 r eff if the IMF is a single power law. While gradients in the mass function above ∼ 0.4 M ⊙ may occur, exceptional care is required to disentangle these IMF variations from the extreme variations in individual element abundances near the galaxies’ centers.