CO emission in distant galaxies on and above the main sequence

Abstract

We present the detection of multiple carbon monoxide CO line transitions with ALMA in a few tens of infrared-selected galaxies on and above the main sequence at z = 1.1−1.7. We reliably detected the emission of CO (5 − 4), CO (2 − 1), and CO (7 − 6)+[C I](3P2 − 3P1) in 50, 33, and 13 galaxies, respectively, and we complemented this information with available CO (4 − 3) and [C I](3P1 − 3P0) fluxes for part of the sample, and by modeling of the optical-to-millimeter spectral energy distribution. We retrieve a quasi-linear relation between LIR and CO (5 − 4) or CO (7 − 6) for main-sequence galaxies and starbursts, corroborating the hypothesis that these transitions can be used as star formation rate (SFR) tracers. We find the CO excitation to steadily increase as a function of the star formation efficiency, the mean intensity of the radiation field warming the dust (⟨U⟩), the surface density of SFR (ΣSFR), and, less distinctly, with the distance from the main sequence (ΔMS). This adds to the tentative evidence for higher excitation of the CO+[C I] spectral line energy distribution (SLED) of starburst galaxies relative to that for main-sequence objects, where the dust opacities play a minor role in shaping the high-J CO transitions in our sample. However, the distinction between the average SLED of upper main-sequence and starburst galaxies is blurred, driven by a wide variety of intrinsic shapes. Large velocity gradient radiative transfer modeling demonstrates the existence of a highly excited component that elevates the CO SLED of high-redshift main-sequence and starbursting galaxies above the typical values observed in the disk of the Milky Way. This excited component is dense and it encloses ∼50% of the total molecular gas mass in main-sequence objects. We interpret the observed trends involving the CO excitation as to be mainly determined by a combination of large SFRs and compact sizes, as a large ΣSFR is naturally connected with enhanced dense molecular gas fractions and higher dust and gas temperatures, due to increasing ultraviolet radiation fields, cosmic ray rates, as well as dust and gas coupling. We release the full data compilation and the ancillary information to the community.

The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: CO Excitation and Atomic Carbon in Star-forming Galaxies at z = 1–3

“…They found all sources were significantly excited in their CO(5-4) transition, compared to the lower-J transitions, indicating the presence of both a cold and a denser, possibly warmer gas component. Very recently, Valentino et al (2020a) expanded these results with observations of a larger sample of similarly IR-bright SFGs at z=1.25. However, all these samples were preselected based on their SFR, and are still among the most massive and IR luminous main-sequence galaxies at these redshifts, with only specific sources selected for multiline follow-up.…”

“…The connection between L IR and excitation is less direct, however, because the total SFR does not correlate with the density and temperature of the clouds as S SFR does (see Narayanan & Krumholz 2014). This conclusion is also reached by Valentino et al (2020a), who show that the intrinsic scatter in ther L 52 IR relation is greater than that in the -S r 52 SFR relation. Note that in the case of equally sloped -¢ L L CO IR relations for different J (for example, the linear relations found by Liu et al 2015), a correlation between L IR and excitation would also not be expected.…”

“…The solid line (and shaded region) show the mean stack of all sources. The literature sample shown here is the same as shown in Figure 5, with the addition of the recently observed main sequence and (extreme) starburst galaxies of Valentino et al (2020a), shown by the brown, solid, and (dotted) dashed lines. For all ladders, we propagate the uncertainty on the transition to which the ladders are normalized to the higher-J lines.…”

“…The lower excitation of the ASPECS galaxies can be naturally explained by their lower surface density of star formation, as the excitation correlates with the radiative energy input into the gas. The CO excitation is sensitive to the gas density and temperature and is known to correlate with the dust temperature (Rosenberg et al 2015) and radiation field strength, star formation efficiency and SFR surface density (Daddi et al 2015;Valentino et al 2020a). The excitation has Figure 12.…”

“…This is on average lower than BzK-selected galaxies, in particular in CO(5-4) transition (r 52 =0.30± 0.06; Daddi et al 2015). 22 We now also add the recently published CO ladders for SFGs at z=1.25 from Valentino et al (2020a), who separate their sample in main-sequence galaxies and (extreme) starbursts (the latter being defined as lying a factor SFR/SFR MS 3.5×and 7×above the main sequence of Sargent et al 2014). Their main-sequence galaxies show excitation intermediate between the BzK galaxies and ASPECS, with r 42 =0.36±0.06 and r 52 = 0.28±0.05.…”