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H 2 in external galaxies and baryonic dark matter. London March 2007 Françoise COMBES. Hypothesis for dark baryons. W b ~ 5%  90% of baryons are dark Baryons in compact objects (brown dwarfs, white dwarfs, black holes) are either not favored by micro-lensing experiments

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h 2 in external galaxies and baryonic dark matter

H2 in external galaxies and baryonic dark matter

London March 2007

Françoise COMBES

hypothesis for dark baryons
Hypothesis for dark baryons

Wb ~ 5%  90% of baryons are dark

Baryons in compact objects(brown dwarfs, white dwarfs,

black holes) are either not favored by micro-lensing experiments

or suffer major problems

(Alcock et al 2001, Lasserre et al 2000, Tisserand et al 2004)

Best hypothesis is gas,

Either hot gas in the intergalactic and inter-cluster medium

(Nicastro et al 2005)

Or cold gasin the vicinity of galaxies and cosmic

filaments (Pfenniger & Combes 94)

dark gas in the solar neighborhood
Dark gas in the solar neighborhood

Dust detected in B-V

(by extinction)

and in emission at 3mm

Emission Gamma associated

To the dark gas

By a factor 2 (or more)

Grenier et al (2005)


CO as a tracer of H2

Arnault et al 1988


Dwarfs and low

metallicity environments

LCO/M(HI) α (O/H)2.2

Confirmed by Taylor & Kobulnicky (98)

But see Walter et al (2003) Leroy et al (2005)

hi as a tracer of dm
HI as a tracer of DM

HI gas is the interface with the extragalactic radiation field

Beyond the HI disk, truncature due to ionisation

the interface is ionized

Explains the correlation sDM/sHI

(Bosma 1981, Freeman 1994, Carignan 1997)

The observed ratio sDM/sHI ~10 for spiral galaxies, varies slightly

with morphological type, decreases for dwarfs and LSB

Mass profiles for dwarf Irr galaxies dominated by DM

stringent test that constrain CDM (Burkert & Silk 1997)

even collisional (Spergel & Steinhardt 99)

extension in uv galex xuv disks m83 and others
Extension in UV (GALEX) XUV disks, M83 and others

M83, Galex, +HI contours (red)

Thilker et al 2005

Yellow line RHII, 10Mo/pc2 in HI

Bluer regions outside

Younger SF + scattered light

extension of galaxies in hi

M83: optical

Extension of galaxies in HI

Dark halo exploration


NGC 5055 Sbc Milky Way-like spiral (109 M of HI): M83


Hoekstra et al (2001)


In average ~10

rotation curves of dwarfs
Rotation Curves of dwarfs

DM has a radial distribution identical to that of HI gas

The ratio DM/HI depends slightly on type

(larger for early-types)


HI x 6.2

From Combes 2000

combination with mond
Combination with MOND

NGC 1560 Tiret & Combes 2007, variation of a0 ~ 1/(gas/HI)

V4 = a0 GM


Baryonic dark matter in

cold H2 clouds

Mass ~ 10-3 Mo

density ~1010 cm-3

size ~ 20 AU

N(H2) ~ 1025 cm-2

tff ~ 1000 yr

Adiabatic regime:

much longer life-time

Fractal: collisions

lead to coalescence,

heating, and to a

statistical equilibrium

(Pfenniger & Combes 94)

Around galaxies, the baryonic

matter may dominate

The stability of cold H2 gas is due

to its fractal structure

first structures
First structures

After recombinaison, GMCs of 105-6 Mo collapse and fragment

down to 10-3 Mo, H2 cooling efficient

The bulk of the gas does not form stars

but a fractal structure, in statistical equilibrium with TCMB

Sporadic star formation

 after the first stars, Re-ionisation

The cold gas survives and will be assembled in more large scale

structures to form galaxies

A way to solve the « cooling catastrophy »

Regulates the consumption of gas into stars (reservoir)

where are the baryons


Where are the baryons?
  • 6% in galaxies ; 3% in galaxy clusters (X-ray gas)
  • <18% in Lyman-alpha forest of cosmic filaments
  • 5-10% in the Warm-Hot WHIM 105-106K
  • 65% are not yet identified!

The majority of baryons are

not in galaxies



ly alpha forest
Ly-alpha forest

W(Lya) = 0.008[N14J-23 R100 4.8/(a+3)]1/2 h70

= 18% of baryons

N14 = typical Lya column density

J = J-23n-a Extragalactic background radiation field

R100 = assumed radius of absorber

Could be lower by a factor 3, if R100 = 0.1

Broad to narrow Lya ratio is 3 times larger at low redshift

Lehner et al (2006)

whim from ovi absorptions
WHIM from OVI absorptions

Stocke et al (2006) FUSE

The WHIM is observed at

350kpc from large galaxies

At 100 kpc from dwarf


Certainly due to SN and

superbubbles outflow

AGN feedback, or

Intergalactic accretion


(Shull 2006)

Multiphase gas

HI and OVI not


whim 10 5 6 k ovi 5 10
WHIM 105-6K (OVI) 5-10%

Danforth & Shull 2005

Wb(OVI) = 0.002-0.004 (0.2/f)(0.1/Z) = 5-10%

f(OVI) assumed ionisation fraction 20%

Z metallicity, assumed 0.1 solar

Ionisation (photo) and metallicity quite uncertain

NeVIII more difficult to find, but photoionisation less uncertain

F(NeVIII) < 15%

Wb(NeVIII) < Wb(OVI)

Assuming IGM, but if only around galaxies?

10 6 k whim observations ovii oviii
>106K WHIM observations?OVII, OVIII

Detection of 2 filaments at z=0.011 and z=0.027 with Chandra

In front of the los of Mk421 blazar, during an outburst (ToO)

n = 10-6 cm-3, N ~10 15cm-2 (d ~5-100)

X-ray absorption lines OVII, NVII +FUSE OVI

OVII, and individual lines at 2-4 s (Nicastro et al 2005)

Not confirmed by XMM summary of observations of Mk421

Williams et al (2006)

May be 40% of the missing baryons, as predicted

by CDM simulations (Cen et al 1999)

nicastro et al 2005
Nicastro et al 2005

3 lines fitted

at the same time






uv lines of h 2
UV Lines of H2
  • Absorption lines with FUSE (Av < 1.5)
  • Ubiquitous H2 in our Galaxy (Shull et al 2000, Rachford et al 2001) translucent or diffuse clouds, from 1014cm-2
  • Absorption in LMC/SMC reduced H2 abundances, high UV field (Tumlinson et al 2002)
  • High Velocity Clouds detected (Richter et al 2001) in H2

(not in CO)

  • 16/35 IVCs detected, while 1/19 HVC detected in H2

Wakker et al 2006


Ly 4-0

FUSE Spectrum of the LMC star Sk-67-166 (Tumlinson et al 2002)

NH2 = 5.5 1015cm-2

infrared lines of h 2
Infrared Lines of H2
  • Ground state, with ISO & Spitzer (28, 17, 12, 9μ)
  • From the ground, 2.2 μ, v=1-0 S(1)
  • excitation by shocks, SN, outflows, UV pumping, X
  • require T > 2000K, nH2 > 104cm-3
  • exceptional merger N6240: 0.01% of L in the 2.2 μ line (all vib lines 0.1%?)

H2 distribution in NGC891 (Valentijn, van der Werf 1999)

S(0) filled; S(1) open – CO profile (full line)


Large quantities

of H2 revealed by ISO

N(H2) = 1023 cm-2

T = 80 – 90 K

5-15 X HI

NGC 891, Pure rotational H2 lines S(0) & S(1)

S(0) wider: more extended

Derived N(H2)/N(HI) = 20 ; Dark Matter?

spitzer h 2 results
Spitzer H2 results
  • H2 line survey for 77 ULIRGs z=0.02-0.93 (Higdon S. et al 2006)
  • H2 mass (warm)= 107 to 109 Mo
  • Warm H2 is 1% of all H2 (CO)
  • H2 in Tidal Dwarf Galaxies :NGC5291 N/S: 460, 400 K
  • MH2 (warm) =1-1.5105 Mo; if colder (150 K): 106 Mo
  • H2 in Stephan’s quintet: large-scale shock (Appleton et al 06)
  • H2 in the nascent starburst N1377 (Roussel et al 2006)
  • H2 in Cooling flows filaments (Egami et al 2006)

High Velocity Clouds (HVC) infalling onto the Galaxy

Spitzer and

IRAS Images

+HI spectra (GBT)

infrared hi correlation
Infrared-HI correlation

In (x,y) = Si ani NHIi (x,y) + Cn (x,y)

  • First detection of dust emission in the HVC
    • HVC Emissivity at 100 mm ~ 10 times smaller than local gas, but only a factor 2 smaller at 160mm
    • Colder dust

Miville-Deschênes et al 2005

h 2 in stephan s quintet
H2 in Stephan’s quintet

Appleton et al 2006

broad (870 km/s) bright H2

group-wide shock wave

typical H2 excitation diagram:

T01=185K at 51018T35=675K

No PAH features,

very low excitation ionized gas

Shocks when the high-V intruder

collides with gas filaments in the group

perseus cluster
Perseus Cluster

Fabian et al 2003

Salome, Combes, Edge et al 06

  • Dark baryons should in the form of gas
  • A significant part could be cold molecular gas
  • The best tracer: pure rotational lines:
  • Observations of excited warm H2 as a tracer
  • H2 in the outer parts of galaxies: H2* is a tracer of the bulk of
  • molecular gas, which is invisible; In the main disk CO is a tracer,
  • but it fails in the outer parts
  • Goals of the H2EX mission:
  • Distribution of the warm H2 with respect to the underlying SF
  • Relation between the HI and H2 in galaxies; the detailed kinematics

will help to associate the various gas phases

  • 4 lines
  • 1000 x more sensitive ISO-SWS
  • L2
  • Soyuz
  • 100-200 Meuro

Survey integration 5s 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  Cosmic Vision ESA