Stellar populations of ultraluminous infrared galaxies

TL;DR

This study analyzes the stellar populations of 160 ULIRGs to understand their evolution, revealing that older stars and higher stellar mass dominate later in the evolutionary sequence, with gas fraction decreasing as stellar mass increases.

Contribution

It provides a detailed stellar population analysis of a large ULIRG sample, linking stellar age, mass, and gas content to evolutionary stages, which was not extensively studied before.

Findings

Older stellar populations increase along the evolutionary sequence.

Gas fraction is inversely related to stellar mass.

Massive ULIRGs are likely in later evolutionary stages.

Abstract

Ultraluminous infrared galaxies (ULIRGs) have several types according to dominance of starburst or AGN component. We made stellar population analysis for a sample of 160 ULIRGs to study the evolution of ULIRGs. We found that the dominance of intermediate-age and old stellar populations increases along the sequence of HII-like ULIRGs, Seyfert-HII composite ULIRGs, and Seyfert 2 ULIRGs. Consequently the typical mean stellar age and the stellar mass increase along the sequence. Comparing the gas mass estimated from the CO measurements with the stellar mass estimated from the optical spectra, we found that gas fraction is anti-correlated with the stellar mass. HII-like ULIRGs with small stellar masses do not possess enough gas and the total mass, and therefore have no evolution connections with massive Seyfert 2 ULIRGs. Only massive ULIRGs may follow the evolution sequence toward AGNs, and massive HII-like ULIRGs are probably in an earlier stage of the sequence.