Unveiling the Sigma-Discrepancy in IR-Luminous Mergers I: Dust & Dynamics

TL;DR

This paper reveals how dust and stellar populations bias dynamical mass estimates in IR-luminous mergers, showing that optical measurements better reflect their true mass and that these mergers resemble elliptical galaxies.

Contribution

It demonstrates the impact of dust and stellar populations on mass measurements and establishes that IR-bright mergers are consistent with forming massive ellipticals.

Findings

Infrared-based dynamical masses underestimate galaxy masses.

Optical measurements reveal merger remnants as massive ellipticals.

Merger remnants align with the I-band Fundamental Plane.

Abstract

Mergers in the local universe present a unique opportunity for studying the transformations of galaxies in detail. Presented here are recent results, based on multi-wavelength, high-resolution imaging and medium resolution spectroscopy, which demonstrate how star-formation and the presence of Red Supergiants and/or Asymptotic Giant Branch stars has lead to a serious underestimation of the dynamical masses of infrared-bright galaxies. The dominance of a nuclear disk of young stars in the near-infrared bands, where dust obscuration does not block their signatures, can severely bias the global properties measured in a galaxy, including mass. This explains why past studies of gas-rich Luminous & Ultraluminous Infrared Galaxies, which have measured dynamical masses using the 1.62 or 2.29 micron CO band-heads, have found that these galaxies are forming m < m* ellipticals. On the other hand, precisely because of dust obscuration, I-band photometry and central velocity dispersions obtained with the Calcium II triplet at 0.85 microns reflect the global properties of the mergers and suggest that all types of merger remnants, including infrared-bright ones, will form m > m* ellipticals. Moreover, merger remnants, including LIRGs, are placed on the I-band Fundamental Plane for the first time and appear to be virtually indistinguishable from elliptical galaxies.