Improved sky subtraction for galaxy kinematics: application to Magellan spectroscopy of NGC 7727

The Earth's atmosphere contributes light to galaxy spectra taken from the ground and this contamination can corrupt measurements of absorption line strengths and stellar velocity dispersions. We develop a method of scaling a non-contemporary sky exposure for use with galaxy spectra for which the galaxy light fills the entire slit. The method relies on the comparison of the spectral light profile (SLP) with a light profile taken from an independent, sky-subtracted image (ILP). The method is applied to longslit spectra of NGC 7727 (Arp 222) taken with the Magellan I (Baade) 6.5-m telescope. We demonstrate the improvement of stellar kinematical measurements with the new method. NGC 7727 is particularly interesting because it is undergoing a merger and our slit crosses the remnant of the secondary nucleus.