Exploring the central molecular zone of the Galaxy using spectroscopy of H ₃ ⁺ and CO

Abstract

The central 400 parsecs of the Milky Way, a region known as the central molecular zone (CMZ), contains interstellar gas in a wide range of physical environments, from ultra-hot, rarified and highly ionized to warm, dense and molecular. The combination of infrared spectroscopy of H + 3 and CO is a powerful way to determine the basic properties of molecular interstellar gas, because the abundance ratio of H + 3 to CO in 'dense' clouds is quite different from that in 'diffuse' clouds. Moreover, the energy-level structure and the radiative properties of H + 3 combined with the unusually warm temperatures of molecular gas in the CMZ make H + 3 a unique probe of the physical conditions there. This paper describes how, using infrared absorption spectroscopy of H + 3 and CO, it has been discovered that a large fraction of the volume of the CMZ is filled with warm, diffuse and partially molecular gas moving at speeds of up to approximately 200 km sand that the mean cosmic ray ionization rate in the CMZ exceeds by roughly an order of magnitude values found in diffuse molecular clouds elsewhere in the Galaxy.

THE ORIGIN OF THE 6.4 keV LINE EMISSION AND H 2 IONIZATION IN THE DIFFUSE MOLECULAR GAS OF THE GALACTIC CENTER REGION

“…3 that even for ∆t = 500 yr, the model slightly underestimates the measured 6.4 keV line flux at longitudinal distances from Sgr A * of ∼ 1 • . This suggests that the warm and diffuse gas extends beyond the CMZ, which might be already suggested by the measured outwards movement of this gas component (see Geballe 2012).…”

“…These authors also suggested that the remaining 90% of the X-ray emission is produced by interactions of electrons with a denser (n ∼ 10 3 cm −3 ) molecular gas. But this would imply the existence of a massive (∼ 10 7 M ⊙ ) molecular gas component with a high ionization rate (ζ 2 > ∼ 10 −14 s −1 ), which should have been detected with H + 3 observations (see, e.g., Geballe 2012). Independent of the exact density of the diffuse molecular gas, the comparison of the cross sections σ Kα iFe and σ ioni iH2 (Fig.…”

The H+ + H2reaction

“…Since the first report by Geballe and Oka [71] of detected H + 3 absorption from measurements of the stellar objects GL2136 and W33A, the ISM has also been the goal of different investigations [72][73][74]. The spectroscopic study of this species has been employed as a way to explore central regions of the Galaxy [75][76][77], to analyse cosmic ray ionization rates [78][79][80][81][82] and to investigate star formation regions [83][84][85][86][87].…”

Physical conditions in the central molecular zone inferred by H3+

“…Geballe (2012) stated that N(3, 3)/N(1, 1) is a good thermometer for any temperature and N(3, 3)/N(2, 2) is a good densimeter for high temperature; see discussion in Oka (2013). Oka & Epp (2004) computed maps of these ratios in the plane n(H 2 )-T and deduced that the gas probed by H + 3 in the CMZ is diffuse (n(H 2 ) ≤ 70 cm −3 ) and warm (T ≥ 300 K).…”