| |
 |
| Paper: |
Radiative Cooling of the Diffuse Gas |
| Volume: |
348, Astrophysics in the Far Ultraviolet: Five Years of Discovery with FUSE |
| Page: |
448 |
| Authors: |
Lehner, N.; Savage, B.D.; Wakker, B.P. |
| Abstract: |
The heating and cooling of the ISM allow the gas 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 II absorption or emission. In a wide range of environments the dominant cooling takes place through the transition from the excited 2P3/2 state of C II to its 2P1/2 ground state, resulting in the emission of a 157.7 μm photon. The amount of cooling is proportional to the column density of C II in its excited state, which can be measured through the C II* absorption lines at 1037.018 Å and 1335.708 Å, observable with FUSE and STIS, respectively. We present the results of a survey of these absorption lines in the spectra of 43 objects. Combining N(C II*) with the intensity of Hα emission, we derive that on average 50% of the C II radiative cooling is in the warm ionized phase. For galactic gas at z < 500 pc we find, on average, a cooling rate of 10−25.70±0.190.36 erg s−1 per H atom, with a range of a factor ∼2, that compare well with infra-red measurements of the C II 157.7 μm emission line. For an IVC at z ∼ 1 kpc we find that on average the cooling is a factor 2 lower. For Complex C (an HVC at least at z > 5 kpc) and WW187 (an HVC at z ∼ 20–40 kpc) we find the much lower rate of 10−26.99±0.210.53 and < 10−26.84 (3σ) erg s−1 per H atom, respectively, implying the cooling and hence the heating decrease at higher z. The rates observed in the HVCs are similar to those in a sample of damped Lyα absorbers. |
|
|
|
|
|
