Extended Cold Molecular Gas Reservoirs in z ≃ 3.4 Submillimeter Galaxies

Abstract

We report the detection of spatially resolved CO(J = 1→0) emission in the z ~ 3.4 submillimeter galaxies (SMGs) SMM J09431+4700 and SMM J13120+4242, using the Expanded Very Large Array (EVLA). SMM J09431+4700 is resolved into the two previously reported millimeter sources H6 and H7, separated by ~30 kpc in projection. We derive CO(J = 1→0) line luminosities of L'_(CO(1-0)) = (2.49 ± 0.86) and (5.82 ± 1.22) × 10^(10) K km s^–1 pc^2 for H6 and H7, and L'_(CO(1-0)) = (23.4 ± 4.1) × 10^(10) K km s^(–1) pc^2 for SMM J13120+4242. These are ~1.5-4.5 times higher than what is expected from simple excitation modeling of higher-J CO lines, suggesting the presence of copious amounts of low-excitation gas. This is supported by the finding that the CO(J = 1→0) line in SMM J13120+4242, the system with the lowest CO excitation, appears to have a broader profile and more extended spatial structure than seen in higher-J CO lines (which is less prominently seen in SMM J09431+4700). Based on L'_(CO(1-0)) and excitation modeling, we find M_(gas) = 2.0-4.3 and 4.7-12.7×10^(10) M _☉ for H6 and H7, and M_(gas) = 18.7-69.4×10^(10) M_☉ for SMM J13120+4242. The observed CO(J = 1→0) properties are consistent with the picture that SMM J09431+4700 represents an early-stage, gas-rich major merger and that SMM J13120+4242 represents such a system in an advanced stage. This study thus highlights the importance of spatially and dynamically resolved CO(J = 1→0) observations of SMGs to further understand the gas physics that drive star formation in these distant galaxies, which is possible only now that the EVLA is rising to its full capabilities.

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