We investigate the molecular gas content of z ∼ 6 quasar host galaxies using the Institut de Radioastronomie Millimétrique Northern Extended Millimeter Array. We targeted the 3 mm dust continuum, and the line emission from CO(6–5), CO(7–6), and [C I ] 2−1 in ten infrared–luminous quasars that have been previously studied in their 1 mm dust continuum and [C II ] line emission. We detected CO(7–6) at various degrees of significance in all the targeted sources, thus doubling the number of such detections in z ∼ 6 quasars. The 3 mm to 1 mm flux density ratios are consistent with a modified black body spectrum with a dust temperature T dust ∼ 47 K and an optical depth τ ν = 0.2 at the [C II ] frequency. Our study provides us with four independent ways to estimate the molecular gas mass, M H2 , in the targeted quasars. This allows us to set constraints on various parameters used in the derivation of molecular gas mass estimates, such as the mass per luminosity ratios α CO and α [CII] , the gas-to-dust mass ratio δ g/d , and the carbon abundance [C]/H 2 . Leveraging either on the dust, CO, [C I ], or [C II ] emission yields mass estimates of the entire sample in the range M H2 ∼ 10 10 –10 11 M ⊙ . We compared the observed luminosities of dust, [C II ], [C I ], and CO(7–6) with predictions from photo-dissociation and X-ray dominated regions. We find that the former provide better model fits to our data, assuming that the bulk of the emission arises from dense ( n H > 10 4 cm −3 ) clouds with a column density N H ∼ 10 23 cm −2 , exposed to a radiation field with an intensity of G 0 ∼ 10 3 (in Habing units). Our analysis reiterates the presence of massive reservoirs of molecular gas fueling star formation and nuclear accretion in z ∼ 6 quasar host galaxies. It also highlights the power of combined 3 mm and 1 mm observations for quantitative studies of the dense gas content in massive galaxies at cosmic dawn.
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Reproducing the CO-to-H2 conversion factor in cosmological simulations of Milky-Way-mass galaxies
ABSTRACT We present models of CO(1–0) emission from Milky-Way-mass galaxies at redshift zero in the FIRE-2 cosmological zoom-in simulations. We calculate the molecular abundances by post-processing the simulations with an equilibrium chemistry solver while accounting for the effects of local sources, and determine the emergent CO(1–0) emission using a line radiative transfer code. We find that the results depend strongly on the shielding length assumed, which, in our models, sets the attenuation of the incident UV radiation field. At the resolution of these simulations, commonly used choices for the shielding length, such as the Jeans length, result in CO abundances that are too high at a given H2 abundance. We find that a model with a distribution of shielding lengths, which has a median shielding length of ∼3 pc in cold gas (T < 300 K) for both CO and H2, is able to reproduce both the observed CO(1–0) luminosity and inferred CO-to-H2 conversion factor at a given star formation rate compared with observations. We suggest that this short shielding length can be thought of as a subgrid model, which controls the amount of radiation that penetrates giant molecular clouds.
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- NSF-PAR ID:
- 10208462
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 499
- Issue:
- 1
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- 837 to 850
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/mnras/staa2839
