Search

We have conducted a sensitive search for the redshifted HCN(1−0) line (v_(rest) = 88.6304 GHz) towards three luminous submillimeter galaxies at z ∼ 2.5 − 2.8 using the K-band receiver (17.6-27.1GHz) on the 110m Robert C. Byrd Telescope (also known as the Green Bank Telescope). Aided by gravitational lensing, we place strong upper limits on the intrinsic line luminosity of all three sources (L′_(HCN(1−0)) ≤ 2×10^(10) K kms^(−1) pc^2). We conclude that all four submillimeter galaxies that have been observed in HCN to date are consistent with the tight relation between FIR and HCN luminosity observed in a variety of local star-forming systems, ranging from Galactic dense cores to (ultra)luminous infrared galaxies at z ≤ 0.1.

Current evidence suggests that the epoch of disk formation occurred between 1 < z < 3. What were the properties of galaxy disks at the epoch of their formation? How did they evolve to their present state, and how was the Hubble sequence assembled? Although large and comprehensive datasets such as COSMOS, GEMS, and GOODS are now becoming available, it is possible that these questions will remain unanswered because of the difficulty in obtaining redshifts from optical spectroscopy as emission lines are redshifted into the infrared. This historical shortcoming has also hampered millimeter and submillimeter studies where the limited bandwidth and sensitivity of current telescopes have restricted studies to only a handful of bright galaxies with spectroscopic redshifts. With the future generation of z-machines, we can overcome the current obstacles and combine optical, infrared, millimeter, and submillimeter observations to trace the evolution of disk galaxies. In this contribution, we describe a research strategy to study the assembly of disk galaxies using space- and ground-based telescopes at multiple wavelengths. In particular, we emphasize the critical role of z-machines and millimeter/submillimeter interferometers.

We report the first detection of a submillimeter galaxy (SMG) in CO(1 →0) emission using the GBT. We identify a line with Δv_(FWHM) ~1000 kms^(−1) in the 1 cm spectrum of SMM J13120+4242 at z = 3.408, which is significantly greater than the width of the previously detected CO(4→3) line. If the observed CO(1→0) line profile arises from a single object and not several merging objects, the CO(4 →3)/CO(1→0) brightness temperature ratio of ~0.26 suggests n(H_2) > 10^3 cm^(−3) and the presence of sub-thermally excited gas. The 10σ integrated line flux implies a cold molecular gas mass M(H2) ~10^(11)M_⊙, comparable to the dynamical mass estimate and four times larger than the H_2 mass found from the CO(4 →3) line. While our observations confirm that this SMG is massive and highly gas-rich, they also suggest that J_(upper) > 3 transitions of CO may not accurately trace cold, diffuse molecular gas in SMGs.