Mapping the high ionization rate of the GC starburst Sgr B2 through low HCO+ /N2H+ J=1-0 intensity ratios

Abstract

We still do not understand which mechanisms dominate the heating and ionization of the extended molecular gas in galactic nuclei. The starburst Sgr B2, in the Galactic Center (GC), is an excellent template to spatially resolve the high-mass star-forming cores from the extended cloud environment, and to study the properties of the warm neutral gas in conditions likely prevalent in star-forming galaxies. We mapped ~1000 pc2 of Sgr B2 complex, using the IRAM 30m telescope, in the N2H+, HCO+ J=1-0 and SiO J=2-1 line emission. The extended nature of the N2H+ J=1-0 emission is remarkable. Compared to molecular clouds in the disk of the galaxy, the N2H+ J=1-0 emission is not confined to cold and dense cores and filaments. This can be explained by the high ionization rate (ζ ≳10−15 s−1), leading to overabundant H+3, He+, and N2H+. The enhanced ionization rate is likely responsible of the much lower line intensity ratio RI =HCO+/N2H+ J=1-0 observed in Sgr B2 (RI ≈ 2 ± 2), Arp 220 (RI ≈ 2), and NGC 253 (RI ≈ 5), compared to disk clouds such as Orion B (RI ≈ 24) and starburst galaxies such as M82 (RI ≈ 21).