We make radial measurements of stellar initial mass function ( IMF ) sensitive absorption features in the two massive early-type galaxies NGC 1277 and IC 843 .
Using the Oxford Short Wavelength Integral Field SpecTrogaph ( SWIFT ) , we obtain resolved measurements of the NaI0.82 and FeH0.99 indices , among others , finding both galaxies show strong gradients in NaI absorption combined with flat FeH profiles at \sim 0.4 Å .
We find these measurements may be explained by radial gradients in the IMF , appropriate abundance gradients in [ Na/Fe ] and [ Fe/H ] , or a combination of the two , and our data is unable to break this degeneracy .
We also use full spectral fitting to infer global properties from an integrated spectrum of each object , deriving a unimodal IMF slope consistent with Salpeter in IC 843 ( x = 2.27 \pm 0.17 ) but steeper than Salpeter in NGC 1277 ( x = 2.69 \pm 0.11 ) , despite their similar FeH equivalent widths .
Independently , we fit the strength of the FeH feature and compare to the E-MILES and CvD12 stellar population libraries , finding agreement between the models .
The IMF values derived in this way are in close agreement with those from spectral fitting in NGC 1277 ( x _ { \mathrm { CvD } } = 2.59 ^ { +0.25 } _ { -0.48 } , x _ { \mathrm { E - MILES } } = 2.77 \pm 0.31 ) , but are less consistent in IC 843 , with the IMF derived from FeH alone leading to steeper slopes than when fitting the full spectrum ( x _ { \mathrm { CvD } } = 2.57 ^ { +0.30 } _ { -0.41 } , x _ { \mathrm { E - MILES } } = 2.72 \pm 0.25 ) .
This work highlights the importance of a large wavelength coverage for breaking the degeneracy between abundance and IMF variations , and may bring into doubt the use of the Wing-Ford band as an IMF index if used without other spectral information .