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

TL;DR

This study reveals that CO band-head measurements underestimate the masses of LIRGs/ULIRGs due to young stellar populations, impacting our understanding of galaxy merger outcomes and elliptical galaxy formation.

Contribution

It demonstrates that CO-based kinematic measurements can significantly underestimate galaxy masses in mergers, highlighting the importance of stellar population effects.

Findings

CO band-head measurements underestimate galaxy masses

Young stellar populations affect kinematic measurements

Implications for galaxy formation models

Abstract

Recently, controversy has erupted over whether gas-rich spiral-spiral mergers are capable of forming {\it m$^{*}$} ellipticals. Measurements of $\sigma$$_{\circ}$ from the 2.29$\micron$ CO band-head for local LIRG/ULIRGs, suggest they are not. IR-bright 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 $\Lambda$-CDM galaxy formation models. However, kinematics obtained with the Calcium II Triplet at 8500 {\AA} suggest mergers are forming {\it m} $\ge$ {\it 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.