Excitation Conditions in the Multi-component Submillimeter Galaxy SMM J00266+1708

TL;DR

This study uses multi-line CO observations to analyze the excitation conditions and merger status of the high-redshift submillimeter galaxy SMM J00266+1708, revealing it as an early-stage merger with complex molecular gas properties.

Contribution

First precise spectroscopic redshift measurement and detailed CO line analysis of SMM J00266+1708, demonstrating its nature as an early-stage merger with distinct components.

Findings

SMM J00266+1708 is a merger with two components separated by ~500 km/s.

Each component's molecular gas is consistent with a single-phase model.

Spectral line data reveal complex kinematics not distinguishable by continuum imaging.

Abstract

We present multiline CO observations of the complex submillimeter galaxy SMM J00266+1708. Using the Zpectrometer on the Green Bank Telescope, we provide the first precise spectroscopic measurement of its redshift (z=2.742). Based on followup CO(1-0), CO(3-2), and CO(5-4) mapping, SMM J00266+1708 appears to have two distinct components separated by ~500 km/s that are nearly coincident along our line of sight. The two components show hints of different kinematics, with the blue-shifted component dispersion-dominated and the red-shifted component showing a clear velocity gradient. CO line ratios differ slightly between the two components, indicating that the physical conditions in their molecular gas may not be alike. We tentatively infer that SMM J00266+1708 is an ongoing merger with a mass ratio of (7.8+/-4.0)/sin^2(i), with its overall size and surface brightness closely resembling that of other merging systems. We perform large velocity gradient modeling of the CO emission from both components and find that each component's properties are consistent with a single phase of molecular gas (i.e., a single temperatures and density); additional multi-phase modeling of the red-shifted component, although motivated by a CO(1-0) size larger than the CO(3-2) size, is inconclusive. SMM J00266+1708 provides evidence of early stage mergers within the submillimeter galaxy population. Continuum observations of J00266 at the ~1" resolution of our observations could not have distinguished between the two components due to their separation (0.73" +/- 0.06"), illustrating that the additional velocity information provided by spectral line studies is important for addressing the prevalence of unresolved galaxy pairs in low-resolution submillimeter surveys.