ALMA calibration data toward QSO J1851+0035 (l=33.498°, b=+0.194°) were used to study absorption lines by Galactic molecular gas. A total of 14 species (CO, 13CO, HCO+, H13CO+, HCO, HCN, HNC, CN, C2H, c-C3H2, CS, SO, SiO and C) were detected. No or only marginal absorption was detected for C18O, HC18O+, H13CN, 13CN, ND, N2H+, HCS+, CCS and CH3OH. More than 16 independent velocity components located between 4.7 and 10.2 kpc from the Galactic Center were identified in 12CO, 13CO and HCO+ lines. Their column density, excitation temperature, and the CO intensity distributions obtained from the FUGIN survey indicate that most of them correspond to diffuse clouds or cloud outer edges.
Simultaneous multiple-Gaussian fitting of 13CO J=1?0 and J=2?1 absorption lines show that these are composed of “continuous components with broad line widths and low excitation temperature” and “discrete components with narrow line widths”. The kinetic temperature deduced from the high N(HCN)/N(HNC) ratio (?4) reaches 40 K, and corresponding thermal width accounts for the line widths of the narrow components. We also observed high N(13CO)/N(C18O) ratio (up to > 56), which is significantly higher than those observed in nearby star-forming regions.
We confirmed a very tight and linear correlation of the column densities among the CN group (HCN-HNC-CN). Similar correlation was also found in the hydrocarbon group (C2H-C3H2).
The CO/HCO+ column density ratio showed a dispersion as large as 3 orders of magnitude. Some of the velocity components are detected in single-dish CO emission and in ALMA HCO+ absorption, but without corresponding ALMA CO absorption. This may be explained by the mixture of clumpy CO emitters not resolved with the single-dish beam (~1 pc) and with extended components with very low CO/HCO+ abundance ratio (i.e., CO-dark gas).