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Abstract. A large scale survey of the Galactic center region in the C18 O(J = 1 → 0) transition is presented. This survey was obtained with the 1.2 m Southern Millimeter-Wave Telescope (SMWT) at the Cerro Tololo Interamerican Observatory (CTIO) near La Serena, Chile. It covers the region −1.• 05 ≤ l ≤ +3.• 6 and −0.• 9 ≤ b ≤ +0.• 75 with a grid spacing of 9 , i.e. the sampling is at full FWHP beamwidth. 357 positions were in total observed. After reviewing the instrumentation of the 1.2 m SMWT, the observing techniques, and the methods used in the data reduction, the data of the survey are presented and morphologically described. In addition, data of the HNCO(5 0,5 − 4 0,4 ) line are presented, which was also included in the large bandwidth of the spectrometer.12 CO(1 − 0) measurements performed for comparison purposes are presented and compared with other 12 CO results. The maps of the C 18 O(1 − 0) survey demonstrate that there are great differences between the distribution of the optically thin C 18 O(1 − 0) emission and the usually optically thick 12 CO(1 − 0) emission.

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Abstract. We present the data from a CO(J = 1 → 0) survey of the central region of our Galaxy. The observations were obtained with the 1.2-m Southern Millimeter-wave Telescope at Cerro Tololo Interamerican Observatory, Chile. This instrument has a full beam-width at half maximum (FWHM) of 8.8 at 115 GHz, the frequency of the observed transition. The survey covers a strip 4• wide in latitude along the galactic equator in the range −12• ≤ l ≤ +13• . The spatial sampling interval was 7.5 (0.85 beam-widths) for |b| ≤ 1• , and 15 (1.7 beamwidths) for 1• < |b| ≤ 2• . The velocity coverage allowed by the filterbank was of 333 km s −1 with a resolution of 1.3 km s −1 at 115 GHz. For the central few degrees (−4.5• < l ≤ 5 • ) where this velocity span was not wide enough, two spectra, taken with different velocities with respect to the local standard of rest (VLSR), were combined in order to cover the full range of the galactic center CO emission. The antenna temperature noise level of the spectra was ≤ 0.10 K (rms). The data are presented in a latitude-longitude spatial map and in sets of longitudevelocity and latitude-velocity diagrams. This survey combines the characteristics of being very sensitive and well sampled, and of having one of the widest and more homogeneous spatial coverage of the region within ∼ 2 kpc from the galactic center.

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Observations of interstellar dust are often used as a proxy for total gas column density N H . By comparing Planck thermal dust data (Release 1.2) and new dust reddening maps from Pan-STARRS 1 and 2MASS (Green et al. 2018), with accurate (opacity-corrected) Hi column densities and newlypublished OH data from the Arecibo Millennium survey and 21-SPONGE, we confirm linear correlations between dust optical depth τ 353 , reddening E(B−V )and the total proton column density N H in the range (1-30)×10 20 cm −2 , along sightlines with no molecular gas detections in emission. We derive an N H /E(B−V ) ratio of (9.4±1.6)×10 21 cm −2 mag −1 for purely atomic sightlines at |b|>5 • , which is 60% higher than the canonical value of Bohlin et al. (1978). We report a ∼40% increase in opacity σ 353 =τ 353 /N H , when moving from the low column density (N H <5×10 20 cm −2 ) to moderate column density (N H >5×10 20 cm −2 ) regime, and suggest that this rise is due to the evolution of dust grains in the atomic ISM. Failure to account for Hi opacity can cause an additional apparent rise in σ 353 , of the order of a further ∼20%. We estimate molecular hydrogen column densities N H2 from our derived linear relations, and hence derive the OH/H 2 abundance ratio of X OH ∼1×10 −7 for all molecular sightlines. Our results show no evidence of systematic trends in OH abundance with N H2 in the range N H2 ∼(0.1−10)×10 21 cm −2 . This suggests that OH may be used as a reliable proxy for H 2 in this range, which includes sightlines with both CO-dark and CO-bright gas.* Estimated from OH(1667) 3σ detection limits using Tex=3.5 K, FWHM=1 km/s and N OH /N H 2 =10 −7 (see Section 5).