Context. The Galactic center is the closest region where we can study star formation under extreme physical conditions like those in high-redshift galaxies. Aims. We measure the temperature of the dense gas in the central molecular zone (CMZ) and examine what drives it. Methods. We mapped the inner 300 pc of the CMZ in the temperature-sensitive J = 3-2 para-formaldehyde (p-H 2 CO) transitions. We used the 3 2,1 −2 2,0 / 3 0,3 −2 0,2 line ratio to determine the gas temperature in n ∼ 10 4 −10 5 cm −3 gas. We have produced temperature maps and cubes with 30 and 1 km s −1 resolution and published all data in FITS form. Results. Dense gas temperatures in the Galactic center range from ∼60 K to >100 K in selected regions. The highest gas temperatures T G > 100 K are observed around the Sgr B2 cores, in the extended Sgr B2 cloud, the 20 km s −1 and 50 km s −1 clouds, and in "The Brick" (G0.253+0.016). We infer an upper limit on the cosmic ray ionization rate ζ CR < 10 −14 s −1 . Conclusions. The dense molecular gas temperature of the region around our Galactic center is similar to values found in the central regions of other galaxies, in particular starburst systems. The gas temperature is uniformly higher than the dust temperature, confirming that dust is a coolant in the dense gas. Turbulent heating can readily explain the observed temperatures given the observed line widths. Cosmic rays cannot explain the observed variation in gas temperatures, so CMZ dense gas temperatures are not dominated by cosmic ray heating. The gas temperatures previously observed to be high in the inner ∼75 pc are confirmed to be high in the entire CMZ.

Aims. We study the origin of large abundances of complex organic molecules in the Galactic center (GC). Methods. We carried out a systematic study of the complex organic molecules CH 3 OH, C 2 H 5 OH, (CH 3 ) 2 O, HCOOCH 3 , HCOOH, CH 3 COOH, H 2 CO, and CS toward 40 GC molecular clouds. Using the LTE approximation, we derived the physical properties of GC molecular clouds and the abundances of the complex molecules. The abundances of complex organic molecules in the GC are compared with those measured in hot cores and hot corinos, in which these complex molecules are also abundant. Results. The CH 3 OH abundance between clouds varies by nearly two orders of magnitude from 2.4×10 −8 to 1.1×10 −6 . The abundance of the other complex organic molecules relative to that of CH 3 OH is basically independent of the CH 3 OH abundance, with variations of only a factor 4-8. We find that both the abundance and the abundance ratios of the complex molecules relative to CH 3 OH in hot cores are similar to those found in the GC clouds. However, hot corinos show different abundance ratios than observed in hot cores and in GC clouds. The rather constant abundance of all the complex molecules relative to CH 3 OH suggests that all complex molecules are ejected from grain mantles by shocks. Frequent (∼10 5 years) shocks with velocities >6 km s −1 are required to explain the high abundances in gas phase of complex organic molecules in the GC molecular clouds. The rather uniform abundance ratios in the GC clouds and in Galactic hot cores indicate a similar average composition of grain mantles in both kinds of regions. The Sickle and the Thermal Radio Arches, affected by UV radiation, show different relative abundances in the complex organic molecules due to the differentially photodissociation of these molecules.

We used the Atacama Pathfinder Experiment (APEX) 12 m telescope to observe the J K A Kc = 3 03 → 2 02 , 3 22 → 2 21 , and 3 21 → 2 20 transitions of para-H 2 CO at 218 GHz simultaneously to determine kinetic temperatures of the dense gas in the central molecular zone (CMZ) of our Galaxy. The map extends over approximately 40 × 8 (∼100 × 20 pc 2 ) along the Galactic plane with a linear resolution of 1.2 pc. The strongest of the three lines, the H 2 CO (3 03 → 2 02 ) transition, is found to be widespread, and its emission shows a spatial distribution similar to ammonia. The relative abundance of para-H 2 CO is 0.5−1.2 × 10 −9 , which is consistent with results from lower frequency H 2 CO absorption lines. Derived gas kinetic temperatures for individual molecular clouds range from 50 K to values in excess of 100 K. While a systematic trend toward (decreasing) kinetic temperature versus (increasing) angular distance from the Galactic center (GC) is not found, the clouds with highest temperature (T kin > 100 K) are all located near the nucleus. For the molecular gas outside the dense clouds, the average kinetic temperature is 65 ± 10 K. The high temperatures of molecular clouds on large scales in the GC region may be driven by turbulent energy dissipation and/or cosmic-rays instead of photons. Such a non-photon-driven thermal state of the molecular gas provides an excellent template for the more distant vigorous starbursts found in ultraluminous infrared galaxies (ULIRGs).

We present an unbiased spectral line survey toward the Galactic Centre (GC) quiescent giant molecular cloud (QGMC), G+0.693 using the GBT and IRAM 30 telescopes. Our study highlights an extremely rich organic inventory of abundant amounts of nitrogen (N)-bearing species in a source without signatures of star formation. We report the detection of 17 N-bearing species in this source, of which 8 are complex organic molecules (COMs). A comparison of the derived abundances relative to H 2 is made across various galactic and extragalactic environments. We conclude that the unique chemistry in this source is likely to be dominated by low-velocity shocks with X-rays/cosmic rays also playing an important role in the chemistry. Like previous findings obtained for O-bearing molecules, our results for N-bearing species suggest a more efficient hydrogenation of these species on dust grains in G+0.693 than in hot cores in the Galactic disk, as a consequence of the low dust temperatures coupled with energetic processing by X-ray/cosmic ray radiation in the GC.

We present a systematic study of the HNCO, C 18 O, 13 CS, and C 34 S emission towards 13 selected molecular clouds in the Galactic center region 1 . The molecular emission in these positions are used as templates of the different physical and chemical processes claimed to be dominant in the circumnuclear molecular gas of galaxies. The relative abundance of HNCO shows a variation of more than a factor of 20 among the observed sources. The HNCO/ 13 CS abundance ratio is highly contrasted (up to a factor of 30) between the shielded molecular clouds mostly affected by shocks, where HNCO is released to gas-phase from grain mantles, and those pervaded by an intense UV radiation field, where HNCO is photo-dissociated and CS production favored via ion reactions. We propose the relative HNCO to CS abundance ratio as a highly contrasted diagnostic tool to distinguish between the influence of shocks and/or the radiation field in the nuclear regions of galaxies and their relation to the evolutionary state of their nuclear star formation bursts.