COBE AND THE GALACTIC INTERSTELLAR MEDIUM: GEOMETRY OF THE SPIRAL ARMS FROM FIR COOLING LINES

We present a new model for the spiral structure of the Milky Way based upon the essentially all-sky intensity maps of the [C II] 158 mu m and [N II] 205 mu m lines of the interstellar medium (ISM) obtained by the FIRAS instrument of the Cosmic Background Explorer (COBE), with ancillary data from the Balloon-borne Infrared Carbon Explorer, and Infrared Space Observatory. These lines are important coolants of the ISM and strong tracers of the spiral structure. The model provides the volume emissivity of these species as a function of position within the Galaxy. Two-, three-, and four-arm models are examined, using a number of spiral functional forms. Two-arm models are found to be inconsistent with the COBE/FIRAS data. A three-arm model can be constructed that reproduces the [C II] and [N II] intensity profiles along the Galactic plane. This model, however, is discounted by historical observations of the Perseus and Cygnus (Outer) arms. A four-arm model, with arms defined by logarithmic spiral forms, reproduce the observations extremely well. Models of the Milky Way's spiral geometry proposed from similar to 1980 to the present are examined in light of the COBE data and compared with the model presented herein. The preponderance of the evidence supports the existence of four well-defined logarithmic spiral arms in the gaseous component of the ISM. We note that essentially all two-arm models proposed since the mid-1980s are based upon observations of older evolved stars. We address the question of why studies based upon observations of stellar densities yield two-arm models while models based upon observations of more traditional tracers of spiral arms, i.e., enhanced gas and dust densities, star formation, and young stellar populations, yield four-arm models.