Hunt for cold H2 molecules

Françoise Combes, Observatoire de Paris

20 Septembre 2005

H2 pure rotational lines

Uncertainties in X= H2/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 explainedthrough HI-scaling by a factor 7-10Quite easy to explain rotation curves with dark H2 gas

Only 10-20% of the dark baryons in galaxiesThe rest in cosmic filaments

Dark baryons (90% of them) are not in compact objects (microlensing)

and should be in gas: either hot or cold

Perspectives of detectionH2 is a furtive molecule at low T: symmetric, no dipole

Hyperfine structure? (or ultrafine)Para-H2: I=0, J even -- Ortho-H2: I=1, Jodd

F=I+J F = 0, 1 and 2Interaction between nuclear spin, and B generated by rotationF=1-0: 546.4 kHz ( =0.5km)F=2-1 54.8 kHz (= 5.5km)

Line intensities extremely weak (nuclear magnetic dipole, A = 10-32s-1)

Require 106x surface for HI detection, i.e. 300x300km (beam 5’)Grid at 4, 8 per km (2400x2400 grid on the Moon?)

At the limit of T=0, H2 should be all paraO/P = 9 exp(-170/T)

But out of equilibrium, at low densityProton exchange H+ + H2(J=1) H+ +H2(J=0)

Significant amount of ortho-H2 at 3K

At 3K, the pressure of H2 clumps is 100 times the saturatedvapor pressure (Combes & Pfenniger 1997)

Latent heat = 110K/H2, no time to form much snowBut conditions for dimerisation

Continuum emission of dimers (dipole induced by collisions)Schaefer (1996, 1998)

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, = 112Only 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, H2O absorptionLiH/H2 ~ 10-10

~1 N(LiH) = 1012cm-2 or N(H2) =1022cm-2

H2+:Abundance 10-11-10-10

Hyperfine structure, but not in N=0 state, only N=1, I=1, S=1/2Eu=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 (Av=0.25mag?)

Problem of heating sources, T ~ Tbg

Absorption, bias towards diffuse cloudsSurface filling factor < 1%

Mass ~ 10-3 Modensity ~1010 cm-3

size ~ 20 AU

N(H2) ~ 1025 cm-2

tff ~ 1000 yr

Adiabatic regime:much longer life-time

Fractal: collisionslead to coalescence, heating, and to astatistical equilibrium(Pfenniger & Combes 94)

Hardly visible cold H2 Clouds

Around galaxies, the baryonicmatter dominates

Connection with the filaments

The stability of cold H2 gas is dueto its fractal structure

Fractal M ~ rD

Recursive Jeansfragmentation

Projected mass log scale (15 mag)

The surface fillingfactordepends strongly on D

< 1% for D=1.7

Pfenniger & Combes 1994

-rays: Dark gas in the solar neighborhood

H2 x a factor 2 (or more)Grenier et al (2005)

Dust detected in B-V(by extinction)and in emission at 3mm

-Emission associatedto the dark gas

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 detectedEdge (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 blackHole: schocks, jets, acoustic waves, bubles...

Perseus H (WIYN) and CO (IRAM)

H, Conselice 01 Salome, Combes, Edge et al 05

Perseus ClusterFabian et al 2003

OSER: Optical Scintillation by Extraterrestrial Refractors

Moniez, 2005

ISO –Pure

H2 rotational lines

N(H2)= 1023 cm-2

T = 80 – 90 K

5-15 X N(HI)

NGC 891

Valentijn &

Van der Werf 99

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

ConclusionThe H2/CO conversion ratio is still a big question in many « non-standard » circumstances: high-redshift galaxies, externalregions of galaxies, etc..

There could be large quantities of cold H2 not yet detected

Best future tracers pure rotational H2 lines, with H2* 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)