First Constraints on the ISM Conditions of a Low Mass, Highly Obscured z=4.27 Main Sequence Galaxy
arxiv(2024)
摘要
We present the molecular gas content and ISM conditions of MACSJ0717 Az9, a
strong gravitationally lensed z=4.273, M_*≃ 2×10^9M_⊙
star-forming galaxy with an unusually high (∼ 80%) obscured star
formation fraction. We detect CO(4-3) in two independent lensed images, as well
as [N II]205μm, with ALMA. We derive a molecular gas mass of
log_10[M_H_2 (M_⊙)] = 9.77 making it moderately deficient in
molecular gas compared to the lower redshift gas fraction scaling relation.
Leveraging photodissociation region (PDR) models, we combine our CO(4-3)
measurements with existing measurements of the [C II] 158μm line and total
infrared luminosity to model the PDR conditions. We find PDR conditions similar
to local star-forming galaxies, with a mean hydrogen density log_10[n_H
cm^-3] = 4.80±0.39 and a mean radiation field strength log_10[G_0
Habing] = 2.83±0.26. Based on Band 3 continuum data, we derive an upper
limit on the intrinsic dust mass of log_10[M_ dust (M_⊙)] <
7.73, consistent with existing estimates. We use the 3D tilted-ring model
fitting code 3D-Barolo to determine the kinematic properties of the CO(4-3)
emitting gas. We find that it is rotationally dominated, with a V/σ=4.6
± 1.7, consistent with the kinematics of the [C II]. With PDR conditions
remarkably similar to normal dusty star-forming galaxies at z 0.2 and a
stable molecular disk, our observations of Az9 suggest that the dust-obscured
phase for a low-mass galaxy at z∼4 is relatively long. Thus, Az9 may be
representative of a more widespread population that has been missed due to
insufficiently deep existing millimeter surveys.
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