1. Hunt for cold H 2 molecules Françoise Combes, Observatoire de Paris 20 Septembre 2005

2. H 2 pure rotational lines

3. Uncertainties in X= H 2 /CO  Metallicity gradients (factor 10)  Density structure (factor 10, across the mass spectrum)  Excitation conditions: High-z SB galaxies, X divided by 5 In spiral and dwarf galaxies, rotation curves could be explained through HI-scaling by a factor 7-10 Quite easy to explain rotation curves with dark H 2 gas Only 10-20% of the dark baryons in galaxies The rest in cosmic filaments Dark baryons (90% of them) are not in compact objects ( microlensing) and should be in gas: either hot or cold

4. Perspectives of detection H 2 is a furtive molecule at low T: symmetric, no dipole  Hyperfine structure? (or ultrafine) Para-H 2 : I=0, J even -- Ortho-H 2 : I=1, Jodd F=I+J  F = 0, 1 and 2 Interaction between nuclear spin, and B generated by rotation F=1-0: 546.4 kHz ( =0.5km) F=2-1 54.8 kHz ( = 5.5km) Line intensities extremely weak (nuclear magnetic dipole, A = 10 -32 s -1 ) Require 10 6 x surface for HI detection, i.e. 300x300km (beam 5’) Grid at  4, 8 per km (2400x2400 grid on the Moon?)

5. At the limit of T=0, H 2 should be all para O/P = 9 exp(-170/T) But out of equilibrium, at low density Proton exchange H + + H 2 (J=1)  H + +H 2 (J=0)  Significant amount of ortho-H 2 at 3K At 3K, the pressure of H 2 clumps is 100 times the saturated vapor pressure (Combes & Pfenniger 1997) Latent heat = 110K/H 2, no time to form much snow But conditions for dimerisation  Continuum emission of dimers (dipole induced by collisions) Schaefer (1996, 1998)

6. Primordial molecules HD: weak dipole moment (proton more mobile than deuteron)  = 5.8 10 -4 Debye (Trefler & Gush 1968) J=1-0, 130 K above ground state, = 112  Only from excited regions  not a good tracer LiH :  = 5.9 Debye (Laurence et al 1963) J=1-0, 21 K above ground state, = 0.67mm (450GHz) Impossible from the ground, H 2 O absorption LiH/H 2 ~ 10 -10  ~1 N(LiH) = 10 12 cm -2 or N(H 2 ) =10 22 cm -2

7. H2+: Abundance 10 -11 -10 -10 Hyperfine structure, but not in N=0 state, only N=1, I=1, S=1/2 E u =110K, the more likely at 1343 MHz Traces of C and O in « primordial » gas A residual 10 -3 solar abundance in Ly  clouds CO emission very sensitive to Z (  =1 cloud size varies) Threshold of photo-dissociation (A v =0.25mag?) Problem of heating sources, T ~ T bg Absorption, bias towards diffuse clouds Surface filling factor < 1%

8. Mass ~ 10 -3 Mo density ~10 10 cm -3 size ~ 20 AU N(H 2 ) ~ 10 25 cm -2 t ff ~ 1000 yr Adiabatic regime: much longer life-time Fractal: collisions lead to coalescence, heating, and to a statistical equilibrium (Pfenniger & Combes 94) Hardly visible cold H 2 Clouds Around galaxies, the baryonic matter dominates Connection with the filaments The stability of cold H 2 gas is due to its fractal structure

9. Fractal M ~ r D Recursive Jeans fragmentation Projected mass log scale (15 mag) The surface filling factor depends strongly on D < 1% for D=1.7 Pfenniger & Combes 1994

10.  -rays: Dark gas in the solar neighborhood H 2 x a factor 2 (or more) Grenier et al (2005) Dust detected in B-V (by extinction) and in emission at 3mm  -Emission associated to the dark gas

11. Cooling flows in galaxy clusters Cooling time < Hubble time at the center of clusters  Gas Flow, 100 to 1000 Mo/yr Mystery: cold gas or stars formed are not detected? Today, the amplitude of the flow has been reduced by 10 and the cold gas is detected Edge (2001) Salomé & Combes (2003) 23 detected galaxies in CO Results from Chandra & XMM: cooling flow self-regulated Re-heating process, feedback due to the active nucleus or black Hole: schocks, jets, acoustic waves, bubles...

12. Perseus H  (WIYN) and CO (IRAM) H , Conselice 01 Salome, Combes, Edge et al 05

13. Perseus Cluster Fabian et al 2003

14. OSER: Optical Scintillation by Extraterrestrial Refractors Moniez, 2005

15. ISO –Pure H 2 rotational lines N(H 2 )= 10 23 cm -2 T = 80 – 90 K 5-15 X N(HI) NGC 891 Valentijn & Van der Werf 99

16. H2EXplorer Survey integration 5  limit total area [sec] [erg s -1 cm -2 sr -1 ] [degrees] Milky Way 100 10-6 110 ISM SF 100 10-6 55 Nearby Galaxies 200 7 10-7 55 Deep Extra-Galactic 1000 3 10-7 5   CNES  Spitzer  Milky Way, NGC 1560 4 lines 1000 x more sensitive ISO-SWS L2 Soyuz 99 Meuro

17. Conclusion The H 2 /CO conversion ratio is still a big question in many « non-standard » circumstances: high-redshift galaxies, external regions of galaxies, etc.. There could be large quantities of cold H 2 not yet detected Best future tracers  pure rotational H 2 lines, with H 2 * excited as tracer (H2EX)  gamma rays (but cosmic rays?)  primordial molecules, HD, LiH (high z, ALMA)  absorption lines (UV, mm..)  continuum dust emission (e.g. Miville-Deschênes et al 2005)