The comparison between H2CO and OH in the Milky Way

Abstract

A catalog of 4231 Galactic H2CO absorption lines ( $\sim 2241$ sources) at 6 cm has been collected from the literature published up to 2014. The distribution of these H2CO absorption lines has been compared with the OH masers at 18 cm in the Milky Way. Most of the OH masers are distributed in the inner Galaxy ( $-75^{\circ} < l < 75^{\circ}$ ) with a higher Galactic latitude than the H2CO sources ( $-5^{\circ} < b < 5^{\circ}$ ). The longitude-velocity distributions of H2CO and OH show that the majority of the objects are located in the molecular ring following the CO molecular gas distribution. The range of velocities of the OH masers is larger and distribution more diffuse than the H2CO sources in the Galactic longitude range $30^{\circ} < l < 90^{\circ}$ . The H2CO sources have been matched with the four ground state transitions of OH and a close associations have been found with H2CO clouds with small velocity differences ( $<5~\mbox{km}\,\mbox{s}^{-1}$ ). The H2CO sources observed with a higher resolution telescope have a better correlation with the OH maser, which may imply a common excitation process and may be attributed to shock excitation. The correlation index of the 1720 MHz OH for the relations $L_{\mathrm{OH}}$ – $L_{\mathrm{H}_{2}\mathrm{CO}}^{\alpha}$ and $L_{\mathrm{OH}}$ – $L_{\mathrm{IR}}^{\alpha}$ is much weaker as compared with 1612, 1665, and 1667 MHz OH transitions. The correlation between H2CO and the interstellar OH maser is much stronger than between H2CO and the stellar OH maser. Similarly the correlation is stronger for the presence than for the absence of far-infrared radiation (FIR) sources. This may indicate that both the FIR and the radio continuum connects the OH with H2CO characteristics.