CiiRadiative Cooling of the Diffuse Gas in the Milky Way

Abstract

The heating and cooling of the interstellar medium (ISM) allow the gas in the ISM to coexist at very different temperatures in thermal pressure equilibrium. The rate at which the gas cools or heats is therefore a fundamental ingredient for any theory of the ISM. The heating cannot be directly determined, but the cooling can be inferred from observations of ·C *, which is an important coolant in different environments. The amount of cooling can be measured through either the intensity of the 157.7 μm [C II] emission line or the C * absorption lines at 1037.018 and 1335.708 A, observable with the Far Ultraviolet Spectroscopic Explorer and the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, respectively. We present the results of a survey of these far-UV absorption lines in 43 objects situated at b 30°. Measured column densities of C *, S II, P II, and Fe II are combined with H I 21 cm emission measurements to derive the cooling rates (per H atom using H I and per nucleon using S II) and to analyze the ionization structure, depletion, and metallicity content of the low-, intermediate-, and high-velocity clouds (LVCs, IVCs, and HVCs) along the different sight lines. Based on the depletion and the ionization structure, the LVCs, IVCs, and HVCs consist mostly of warm neutral and ionized clouds. For the LVCs, the mean cooling rate in ergs s-1 per H atom is -25.70 dex (1 σ dispersion). With a smaller sample and a bias toward high H I column density, the cooling rate per nucleon is similar. The corresponding total Galactic C II luminosity in the 157.7 μm emission line is L ~ 2.6 × 107 L☉. Combining N(C *) with the intensity of Hα emission, we derive that ~50% of the C * radiative cooling comes from the warm ionized medium (WIM). The large dispersion in the cooling rates is certainly due to a combination of differences in the ionization fraction, in the dust-to-gas fraction, and physical conditions between sight lines. For the IVC Intermediate-Velocity (IV) Arch at z ~ 1 kpc we find that on average the cooling is a factor of 2 lower than in the LVCs that probe gas at lower z. For an HVC (complex C, at z > 6 kpc) we find the much lower rate of -26.99 dex, similar to the rates observed in a sample of damped Lyα absorber systems (DLAs). The fact that in the Milky Way a substantial fraction of the C II cooling comes from the WIM implies that this is probably also true in the DLAs. We also derive the electron density, assuming a typical temperature of the warm gas of 6000 K: for the LVCs, ne = 0.08 ± 0.04 cm-3, and for the IV Arch, ne = 0.03 ± 0.01 cm-3 (1 σ dispersion). Finally, we measured the column densities N(S II) and N(P II) in many sight lines and confirm that sulphur appears undepleted in the ISM. Phosphorus becomes progressively more deficient when log N(H ) > 19.7 dex, which can mean that either P becomes more depleted into dust as more neutral gas is present or P is always depleted by about -0.3 dex, but the higher value of P II at lower H I column density indicates the need for an ionization correction.