A Dynamical Miss: A Study of the Discrepancy Between Optical and Infrared Kinematics in Mergers

Recently, controversy has erupted over whether gas-rich spiralspiral mergers are capable of forming m∗ ellipticals. Measurements of σ◦ from the 2.29μm CO band-head for local LIRG/ULIRGs, suggest they are not. IRbright mergers are often cited as the best candidates for forming massive ellipticals, so the recent observations have raised doubts about both the Toomre Merger Hypothesis and the fundamental assumptions of Λ-CDM galaxy formation models. However, kinematics obtained with the Calcium II Triplet at 8500 Å suggest mergers are forming m ≥ m∗ ellipticals. In this work, we show that kinematics derived from the CO stellar absorption band-head leads to a significant underestimation of the masses of LIRGs/ULIRGs. This is primarily due to the presence of a young population affecting CO band-head measurements.