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Title: Spectral imaging of the Central Molecular Zone in multiple 7-mm molecular lines
Authors: P.A. Jones, M.G. Burton, M.R. Cunningham, N.F.H. Tothill, A.J. Walsh

We have imaged 24 spectral lines in the Central Molecular Zone (CMZ) around the Galactic Centre, in the range 42 to 50 GHz. The lines include emission from the CS, CH3OH, HC3N, SiO, HNCO, HOCO+, NH2CHO, OCS, HCS+, CCS, C34S, 13CS, 29SiO, H13CCCN, HCC13CN and HC5}N molecules, and three hydrogen recombination lines. The area covered is Galactic longitude -0.7 to 1.8 deg. and latitude -0.3 to 0.2 deg., including the bright cores around Sgr A, SgrB2, SgrC and G1.6-0.025. This work used the 22-m Mopra radio telescope in Australia, obtaining ~ 1.8 km/s spectral and ~ 65 arcsec spatial resolution. We present peak images from this study and conduct a principal component analysis on the integrated emission from the brightest 10 lines, to study similarities and differences in the line distribution. We examine the integrated line intensities and line ratios in selected apertures around the bright cores, as well as for the complete mapped region of the CMZ. We compare these 7-mm lines to the corresponding lines in the 3-mm band, for five molecules, to study the excitation. There is a variation in 3-mm to 7-mm line ratio across the CMZ, with relatively higher ratio in the centre around Sgr B2 and Sgr A. We find that the lines are sub-thermally excited, and from modelling with RADEX find that non-LTE conditions apply, with densities of order 10^4 cm^{-3}.

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Title: What controls star formation in the central 500 pc of the Galaxy?
Authors: J. M. Diederik Kruijssen (1), Steven N. Longmore (2,3), Bruce G. Elmegreen (4), Norman Murray (5), John Bally (6), Leonardo Testi (2,7), Robert C. Kennicutt Jr. (8) ((1) MPA Garching, (2) ESO Garching, (3) LJMU Liverpool, (4) IBM T.J. Watson Research Center, (5) CITA Toronto, (6) University of Colorado, (7) INAF Firenze, (8) IoA Cambridge)

The star formation rate (SFR) in the Central Molecular Zone (CMZ, i.e. the central 500 pc) of the Milky Way is lower by a factor of >10 than expected for the substantial amount of dense gas it contains, which challenges current star formation theories. In this paper, we quantify which physical mechanisms could be causing this observation. On scales larger than the disc scale height, the low SFR is found to be consistent with episodic star formation due to secular instabilities or variations of the gas inflow along the Galactic bar. The CMZ is marginally Toomre-stable when including gas and stars, but highly Toomre-stable when only accounting for the gas, indicating that the condensation of self-gravitating clouds may be limited. On small scales, we find that the SFR in the CMZ is consistent with an elevated critical density for star formation due to the high turbulent pressure - potentially aided by weak magnetic effects and an underproduction of massive stars due to a bottom-heavy IMF. The existence of a universal density threshold for star formation is ruled out, as well as the importance of the HI-H_2 phase transition of hydrogen, the tidal field, the magnetic field, radiation pressure, and cosmic ray heating. We propose observational and numerical tests to distinguish between the remaining candidate star formation inhibitors, in which ALMA will play a key role. We conclude the paper by proposing a self-consistent cycle of star formation in the CMZ, in which the plausible star formation inhibitors are combined. Their ubiquity suggests that the perception of a lowered central SFR should be a common phenomenon in other galaxies. We discuss the implications for galactic star formation and supermassive black hole growth, including a prediction that the recently reported bimodality of star formation in high-redshift galaxies may emanate from a difference in the gas inflow rates.

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The Central Molecular Zone or CMZ is a region of the Milky Way rich in molecular gas. It is found near the centre of the Milkyway, and as such is in Sagittarius, between galactic longitude 1.7 and -0.7, and latitudes -0.2 and +0.2.
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