ALMA High-J CO Spectroscopy of High-Redshift Galaxies. I. An Archive-based Catalog of CO Spectral Line Energy Distributions

TL;DR

This paper compiles and analyzes high-J CO spectral line energy distributions for high-redshift galaxies using ALMA archival data, revealing excitation differences among galaxy types and potential AGN influence.

Contribution

It provides the first comprehensive catalog of high-J CO SLEDs for a diverse sample of high-redshift galaxies, enabling population-level insights.

Findings

SMGs show stronger low- to mid-J CO emission compared to Hot DOGs.

Hot DOGs exhibit higher excitation possibly due to AGN-related XDR heating.

An anti-correlation suggests AGN influence enhances CO excitation in Hot DOGs.

Abstract

High-J CO emission in high-redshift galaxies has been studied primarily on an individual-source basis, limiting our ability to draw population-level conclusions about molecular-gas excitation. To address this limitation, we present a catalog of CO spectroscopy based on archival data from the Atacama Large Millimeter/submillimeter Array (ALMA) for a sample of galaxies at z > 3, focusing on high-J transitions (Jup = 9-17). Combining ALMA archival data with published measurements, we compile CO spectral line energy distributions (SLEDs) for 38 well-studied systems spanning z ~ 3.1-6.9, including 5 hot dust-obscured galaxies (Hot DOGs), 17 submillimeter-bright galaxies (SMGs), and 16 optically selected quasars. The class-median SLEDs rise steeply to Jup = 9 and remain approximately flat through Jup ~ 11-12. SMGs show relatively stronger low- to mid-J emission relative to CO J=9-8, while Hot DOGs exhibit tentative evidence for higher excitation. Comparison with simple excitation models suggests that X-ray dominated region (XDR) heating or dense, shock-heated gas can account for the extended high-J CO SLEDs. A tentative anti-correlation between the CO(9-8)-to-infrared luminosity ratio and excitation among the dusty galaxy populations suggests that the enhanced excitation in Hot DOGs may be driven by XDR heating from obscured AGN activity rather than by shocks.