2020MNRAS.492..556L

We have performed high-sensitivity mapping observations of several complex organic molecules around Sagittarius B2 with the ARO 12 m telescope at 3 mm wavelength. Based on their spatial distribution, molecules can be classified as either `extended', those detected not only in Sgr B2(N) and Sgr B2(M), or `compact', those only detected toward or near Sgr B2(N) and Sgr B2(M). The `extended' molecules include glycolaldehyde (CH2OHCHO), methyl formate (CH3OCHO), formic acid (t-HCOOH), ethanol (C2H5OH) and methyl amine (CH3NH2), while the `compact' molecules include dimethyl ether (CH3OCH3), ethyl cyanide (C2H5CN), and amino acetonitrile (H2NCH2CN). These `compact' molecules are likely produced under strong UV radiation, while the `extended' molecules are likely formed at low temperatures, via gas-phase or grain-surface reactions. The spatial distribution of `warm' CH2OHCHO at 89 GHz differs from the spatial distribution of `cold' CH2OHCHO observed at 13 GHz. We found evidence for an overabundance of CH2OHCHO compared to that expected from the gas-phase model, which indicates that grain-surface reactions are necessary to explain the origin of CH2OHCHO in Sagittarius B2. Grain-surface reactions are also needed to explain the correlation between the abundances of `cold' CH2OHCHO and C2H5OH. These results demonstrate the importance of grain-surface chemistry in the production of complex organic molecules.