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

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.

Figures (13)

• Figure 1: Normalized CO SLEDs predicted by non-LTE models. All curves are normalized to the CO(9--8) luminosity. The calculations were performed with the ndradex package Taniguchi2023, which implements the RADEX formalism 2007AA...468..627V, assuming an LVG geometry with $n_{\rm H_2}=10^{5}\,{\rm cm^{-3}}$, $N_{\rm CO}=10^{18}\,{\rm cm^{-2}}$, $\Delta v = 100~{\rm km~s^{-1}}$, and a background radiation temperature of $T_{\rm bg}=16.35$ K corresponding to the cosmic microwave background at $z=4$.
• Figure 2: Growth-curve test for W2246$-$7143. Integrated CO line flux as a function of synthesized beam size, measured by re-imaging the calibrated visibility data with a range of $uv$ tapers.
• Figure 3: CO(10--9), CO(13--12), and CO(14--13) spectra of BR1202$-$0725 (SMG component). Velocities are referenced to the best-fit CO line center. The gray curves show the best-fit models. The vertical dotted line marks the expected position of H$_2$O. Shaded channels indicate the velocity range adopted for the integrated-flux measurements.
• Figure 4: (Left) Normalized CO SLEDs of individual high-redshift sources: Hot DOGs (pentagons), SMGs (squares), and quasars (circles). Curves show the PCHIP interpolation in $\log L_{\rm CO}$--$J_{\rm up}$ space, which passes through all measured points. (Right) Class-median normalized CO SLEDs for the high-redshift sample. Solid lines show the class median SLED, and shaded regions indicate the 16th--84th percentile range across sources at each $J_{\rm up}$ (Table \ref{['tab:cosled_norm_band']}). For a given source, transitions that are not directly measured are evaluated from the interpolated SLED. Only bins with at least three contributing sources are shown.
• Figure 5: Distribution of the excitation parameter $\alpha'$ for the three high-redshift classes and the three local excitation classes defined by 2015ApJ...801...72R. Points indicate per-object median $\alpha'$ values with 16th--84th percentile error bars from Monte Carlo propagation of measurement uncertainties. Boxes show the interquartile range (25th--75th percentiles) with the median, and whiskers indicate the 16th--84th percentile range.