# Optical/NIR stellar absorption and emission-line indices from luminous infrared galaxies

## TL;DR

This study provides a comprehensive set of optical and near-infrared stellar absorption and emission-line indices for luminous infrared galaxies, highlighting correlations and model comparisons to improve stellar population understanding.

## Contribution

It offers a homogeneous dataset of 45 spectral indices for 16 infrared-luminous and 19 early-type galaxies, facilitating testing of stellar population models across optical and NIR wavelengths.

## Key findings

- Optical indices agree well with models.
- NIR indices are challenging to interpret.
- Models match early-type spirals but not star-forming galaxies.

## Abstract

We analyze a set of optical-to-near-infrared long-slit nuclear spectra of 16 infrared-luminous spiral galaxies. All of the studied sources present H$_2$ emission, which reflects the star-forming nature of our sample, and they clearly display H I emission lines in the optical. Their continua contain many strong stellar absorption lines, with the most common features due to Ca I, Ca II, Fe I, Na I, Mg I, in addition to prominent absorption bands of TiO, VO, ZrO, CN and CO. We report a homogeneous set of equivalent width (EW) measurements for 45 indices, from optical to NIR species for the 16 star-forming galaxies as well as for 19 early type galaxies where we collected the data from the literature. This selected set of emission and absorption-feature measurements can be used to test predictions of the forthcoming generations of stellar population models. We find correlations among the different absorption features and propose here correlations between optical and NIR indices, as well as among different NIR indices, and compare them with model predictions. While for the optical absorption features the models consistently agree with the observations,the NIR indices are much harder to interpret. For early-type spirals the measurements agree roughly with the models, while for star-forming objects they fail to predict the strengths of these indices.