PHOSPHORUS AT THE EDGE OF THE GALAXY: OBSERVATIONS OF PN AND PO AT RGC ~23 kPC

The Galactic Habitable Zone (GHZ) is characterized by metallicity, orbital kinematics, star formation rates, supernovae explosions, and time, among other quantities. Previously, the Outer Galaxy (R_GC > 16 kpc) was not included in the GHZ due to low abundances of the NCHOPS elements – those required for life. Recent observations of dense clouds in the Outer Galaxy, however, have identified various molecules that contain all of the NCHOPS elements, except phosphorus. Because this element is essential to prebiotic chemistry, a search was conducted for phosphorus-bearing molecules in these Galactic edge clouds using the 12m telescope of the Arizona Radio Observatory (ARO) at 2mm in wavelength and the 30m telescope of the Institut de radioastronomie millimétrique (IRAM) at 3mm. From these observations, the J = 2→1 and J = 3→2 rotational transitions of PN, as well as four hyperfine components of the J = 2.5→1.5 transition of PO, were detected in edge cloud WB89-621, located 22.6 kpc from the Galactic Center. The fractional abundances, relative to H₂, of PN and PO were determined to be f~3.0(±1.6)×10^-12 and 2.0(±1.1)×10^-11, respectively. This discovery is the furthest identification of phosphorus in any form at such large distances from the Galactic Center. These results complete the inventory of the NCHOPS elements in the Outer Galaxy, suggesting that habitable exoplanets may exist out to R_GC~23 kpc, redefining the boundaries of the GHZ.