A systematic study of glycolaldehyde in sagittarius B2(N) at 2 and 3 mm: Criteria for detecting large interstellar molecules

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 K_a = 0, 1, and 2 ladders. The two transitions not detected were weak and originate in the less populated K_a = 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 V_LSR = 62.3 ± 2.4 km s^-1 and ΔV_1/2 = 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 cm₂ for glycolaldehyde, suggesting a fractional abundance of f(H ₂) = 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.