L'CO/LFIR Relations with CO Rotational Ladders of Galaxies Across the Herschel SPIRE Archive

TL;DR

This study analyzes the relationships between far-infrared luminosity and CO line emissions across multiple rotational transitions in galaxies, introducing a new method for spectral analysis and highlighting the complexity of molecular gas excitation.

Contribution

It provides a comprehensive catalog of CO and [CI], [NII] lines from Herschel SPIRE data, and develops a novel PDF estimation method accounting for spectral correlations.

Findings

Linear correlation between LFIR and L'CO for mid- to high-J lines.

Mid- to high-J CO luminosity ratios increase with LFIR, indicating higher gas excitation.

Single PDR models cannot fully explain observed SLED shapes, suggesting complex excitation mechanisms.

Abstract

We present a catalog of all CO (J=4-3 through J=13-12)), [CI], [NII] lines available from extragalactic spectra from the Herschel SPIRE Fourier Transform Spectrometer (FTS) archive combined with observations of the low-J CO lines from the literature and from the Arizona Radio Observatory. This work examines the relationships between LFIR, L'CO, and LCO/LCO(1-0). We also present a new method for estimating probability distribution functions (PDFs) from marginal signal-to-noise ratio Herschel} FTS spectra, which takes into account the instrumental "ringing" and the resulting highly correlated nature of the spectra. The slopes of log(LFIR) vs. log(L'CO) are linear for all mid- to high-J CO lines and slightly sublinear if restricted to (U)LIRGs. The mid- to high-J CO luminosity relative to CO J=1-0 increases with increasing LFIR, indicating higher excitement of the molecular gas, though these ratios do not exceed ~ 180. For a given bin in LFIR, the luminosities relative to CO J=1-0 remain relatively flat from J=6-5 through J=13-12, across three orders of magnitude of LFIR. A single component theoretical photon-dominated region (PDR) model cannot match these flat SLED shapes, though combinations of PDR models with mechanical heating added qualitatively match the shapes, indicating the need for further comprehensive modeling of the excitation processes of warm molecular gas in nearby galaxies.