# SPIRE Spectroscopy of Early Type Galaxies

## TL;DR

This study uses SPIRE spectroscopy to analyze the molecular and atomic gas properties of CO-rich early-type galaxies, revealing low excitation conditions and promising methods for high-redshift galaxy studies.

## Contribution

First application of SPIRE spectroscopy to CO-rich early-type galaxies, demonstrating the use of [CI] lines for gas mass estimation and insights into interstellar medium conditions.

## Key findings

- Low gas excitation in most ETGs except NGC 1266
- [CI] lines provide reliable molecular gas mass estimates
- [NII] 205/CO(6-5) ratio correlates with IRAS colors

## Abstract

We present SPIRE spectroscopy for 9 early-type galaxies (ETGs) representing the most CO-rich and far-infrared (FIR) bright galaxies of the volume-limited Atlas3D sample. Our data include detections of mid to high J CO transitions (J=4-3 to J=13-12) and the [CI] (1-0) and (2-1) emission lines. CO spectral line energy distributions (SLEDs) for our ETGs indicate low gas excitation, barring NGC 1266. We use the [CI] emission lines to determine the excitation temperature of the neutral gas, as well as estimate the mass of molecular hydrogen. The masses agree well with masses derived from CO, making this technique very promising for high redshift galaxies. We do not find a trend between the [NII] 205 flux and the infrared luminosity, but we do find that the [NII] 205/CO(6-5) line ratio is correlated with the 60/100 $\mu$m Infrared Astronomical Satellite (IRAS) colors. Thus the [NII] 205/CO(6-5) ratio can be used to infer a dust temperature, and hence the intensity of the interstellar radiation field (ISRF). Photodissociation region (PDR) models show that use of [CI] and CO lines in addition to the typical [CII], [OI], and FIR fluxes drive the model solutions to higher densities and lower values of G$_0$. In short, the SPIRE lines indicate that the atomic and molecular gas in the CO-rich ETGs have similar properties to other galaxies. As might be expected from their low levels of star formation activity, the ETGs have rather low excitation CO SLEDs, low temperatures inferred from the [CI] lines, and modestly lower [CI]/CO ratios.