A Systematic Study of Glycolaldehyde in Sagittarius B2(N) at 2 and 3 mm: Criteria for Detecting Large Interstellar Molecules

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Abstract
A comprehensive study of glycolaldehyde (CH2OHCHO) has been conducted at 2 and 3 mm toward Sgr B2(N) using the Arizona Radio Observatory 12 m telescope. Forty favorable transitions of this species were observed in the range 68-169 GHz. Emission on the 20-70 mK level was detected at frequencies of 38 of these lines, including all transitions arising from the Ka = 0, 1, and 2 ladders. The two transitions not detected were weak and originate in the less populated Ka = 3 levels. Twenty-one percent of the detected lines are distinct, individual features. The remaining transitions are either contaminated by emission from abundant molecules or blended with equivalently weak features. The unblended transitions indicate VLSR = 62.3 ± 2.4 km s-1 and ΔV1/2 = 8.3 ± 3.4 km s-1, line parameters characteristic of organic species in Sgr B2(N). A rotational diagram yields a column density of 5.9 × 1013 cm-2 for glycolaldehyde, suggesting a fractional abundance of f(H2) = 5.9 × 10-11. Observations of formaldehyde toward Sgr B2(N) suggest that H2CO and CH2OHCHO arise from the same gas with an abundance ratio of ~1/27. H2CO may function as the precursor to glycolaldehyde in a gas-phase "formose" reaction. These observations, combined with past results of Hollis et al., provide convincing evidence for the presence of glycolaldehyde in the ISM. This study suggests that an extensive, self-consistent data set is necessary to identify large organic species in interstellar gas.