Infrared Spectroscopy of Nearby Radio Active Early-Type Galaxies -- II: Spectral Atlas

TL;DR

This paper provides a comprehensive near-infrared spectral atlas of 163 nearby radio-emitting early-type galaxies, analyzing emission lines, excitation mechanisms, and spectral features to understand nuclear activity and its relation to radio emission.

Contribution

It offers the first extensive spectral atlas of such galaxies, including emission line fluxes, excitation analysis, and spectral indices, revealing insights into nuclear activity and excitation mechanisms.

Findings

H2 excitation temperature of 1957±182 K

Dominant excitation mechanism is thermal and shock heating

Weak correlation between radio power and emission-line luminosity

Abstract

We present a near infrared spectroscopic atlas of nearby, bright early-type galaxies with radio emission, containing 163 galaxies observed by the Palomar 200" TripleSpec instrument, measuring the emission line fluxes for H, He, [Fe II] and H$_2$ and determined the nuclear excitation mechanisms. By stacking spectra, we deduced the H$_2$ excitation temperature ($1957\pm182$ K) and dominant excitation mechanism (thermal and shock heating combined) from the $\textit{K}$-band emission line sequence. Stacking also produces an "average" spectrum of absorption features and spectral indices from the literature; the CO12 absorption line index vs. $\textit{J-K}$ colour shows a trend of stronger nuclear activity producing a weaker CO12 index and a redder (flatter) continuum. The correlations between the radio and the emission-line luminosities finds a trend with radio power; however, the large scatter in the upper limits shows that the two are not directly coupled and the duty cycles of SF and AGN activity are not synchronised.