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