1. What we look for when we look for the dark gas * John Dickey Wentworth Falls 26 Nov 2013 *Wordplay on a title by Raymond Carver, "What we talk about, when we talk about love"

2. Summary: The Dark Gas is a mixture of atomic and molecular hydrogen that is not traced by either 21-cm emission or CO emission. The Dark Gas shows the continuity between the CNM and the diffuse molecular medium. The large amount of dark gas indicated by the dust emission shows that the effects of 21- cm absorption are significant and must be included to interpret low latitude surveys. The C+ line probes this medium; it will be excellent for comparison with 21-cm in both emission and absorption!

4. Boulanger & Perrault 1988 (see also Heiles, Reach, & Koo 1988)

5. For 20 years we thought we understood this! Why was the dark gas a surprise in 2008/10 ? The physics is similar to what we see in photo-dissociation regions.

6. Lee et al. 2012 GALFA + IRAS + COMPLETE study of the Perseus molecular cloud and its surroundings

7. Lee et al. 2012 Arecibo map of Perseus, cf. Planck consortium XXIV GBT maps of 14 high latitude fields. Note that R mol drops suddenly at a threshold radius.

8. Lee et al. 2012 compare with theoretical models by Wolfire, Hollenbach & McKee 2010 and Krumholz, McKee &Tumlinson 2008, 2009. They find a clear threshold for the molecular/atomic transition, but they don't see the dark molecular gas – there is still a factor of 3 disagreement between the observations and the models. The equilibrium between H 2 formation and destruction is very complicated!

9. A quick review of PDR physics, following Draine & Bertoldi 1996a,b and Draine 2011 book.

10. from The transitions are compressed in a PDR :

11. A view of the far uv, from 10.4 eV to 13.6 eV photon energy:  (Angstroms) 912 1101 C – C + 1216 Ly  1110 Lyman-Werner Bands 1026 Ly  The photon energy for ionization of Carbon is at the lower edge of the Lyman-Werner band where photons can photodissociate H 2, and H 2 self-shields.

12. f mol time From Draine and Bertoldi 1996

13. interstellar radiation field in the neutral medium near an HII region

14. Interstellar radiation field in the neutral medium in a typical region (near the sun)

17. 10% 50% 90%

18. So there must be a range of at least an order of magnitude in shielding column density over which the molecular fraction of H changes smoothly from 0.1 to 0.9. So is there much molecular H in classical CNM regions? Yes, it was seen in far-uv absorption by FUSE and Copernicus.

19. 1977 molecular fraction f

21. Rachford et al. 2009 Summary of FUSE and Copernicus results for the excitation temperature of the H 2 molecules and their fractional abundance.

23. Correcting 21-cm Measurements of H I Column Density for self-absorption

35. Conclusion: No wonder there's "dark gas". The dust emission shows that we have underestimated the correction factor due to the optical depth of the CNM acting on the background continuum as well as the background line emission.

36. What about the optical depths of the fine structure lines? Could an absorption-emission experiment be done to measure the optical depth?

37. WNM CNM The atomic ISM cools through the 158  C II and the 63  O I lines. Tracing these lines traces the energy flow through the ISM.

38. The fine structure lines have sub-thermal excitation in diffuse ISM conditions: n T = 2000 K cm -3 n T = 4000 K cm -3

39. In diffuse atomic clouds every collisional excitation is followed by a radiative de-excitation.

40. What does Herschel see? Langer et al. 2010

41. Nearby SGPS HI emission and absorption spectra (Strasser 2006) note the HISA features

42. Comparing the integrated line fluxes of the C II and H I lines... (Langer et al. 2010)

43. Gerin et al. 2013 A&A submitted

45. Slide from Gerin et al. 2013

47. Conclusions: The dark gas must be partly CNM We can trace the missing gas by combining continuum and 21-cm emission surveys with a densely sampled absorption survey. The 158  C + line will be a great tracer of the CNM

48. Questions: Theory: could we please have a dynamical model of the molecule formation vs. photodissociation that includes time dependent changes in the radiation field? Observations: Let's make a well sampled map of the 21-cm optical depth along a large number of lines of sight at low latitudes, plus a continuum map to use with them to correct the HI column density maps.