Molecular gas properties of Q1700-MD94: a massive, main-sequence galaxy at $z\approx2$

TL;DR

This study combines multi-line observations to characterize the molecular gas in a massive galaxy at z≈2, revealing warm, dense gas distributed across the disk, consistent with high star formation activity.

Contribution

It provides a comprehensive analysis of molecular gas properties in a high-redshift galaxy using multiple tracers and LVG modeling, highlighting the distribution and excitation conditions of the gas.

Findings

Molecular gas masses are consistent within a factor of 2 across different tracers.

The gas is warm ($T_{kin}=41$K) and dense ($n_{H_2}=8×10^3$ cm$^{-3}$).

The molecular gas is distributed throughout the galaxy disk.

Abstract

We use a combination of new NOrthern Extended Millimeter Array (NOEMA) observations of the pair of [CI] transitions, the CO(7-6) line, and the dust continuum, in addition to ancillary CO(1-0) and CO(3-2) data, to study the molecular gas properties of Q1700-MD94, a massive, main-sequence galaxy at $z\approx2$. We find that for a reasonable set of assumptions for a typical massive star-forming galaxy, the CO(1-0), the [CI](1-0) and the dust continuum yield molecular gas masses that are consistent within a factor of $\sim2$. The global excitation properties of the molecular gas as traced by the [CI] and CO transitions are similar to those observed in other massive, star-forming galaxies at $z\sim2$. Our large velocity gradient (LVG) modeling using RADEX of the CO and [CI] spectral line energy distributions (SLEDs) suggests the presence of relatively warm ($T_{\rm kin}=41$K), dense ($n_{\rm H_2}=8\times10^{3}~{\rm cm}^{-3}$) molecular gas, comparable to the high-excitation molecular gas component observed in main-sequence, star-forming galaxies at $z\sim1$. The galaxy size in the CO(1-0) and CO(7-6) line emission are comparable, which suggests that the highly-excited molecular gas is distributed throughout the disk powered by intense star formation activity. To confirm this scenario will require spatially resolved observations of the CO and [CI] lines which can now be obtained with NOEMA upgraded capabilities.