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1 Molecules in galaxies at all redshifts 1. How to observe the H 2 component? 2. Molecular component of the Milky Way 3. Fractal Structure 4. Formation.

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Presentation on theme: "1 Molecules in galaxies at all redshifts 1. How to observe the H 2 component? 2. Molecular component of the Milky Way 3. Fractal Structure 4. Formation."— Presentation transcript:

1 1 Molecules in galaxies at all redshifts 1. How to observe the H 2 component? 2. Molecular component of the Milky Way 3. Fractal Structure 4. Formation of the fractal, shear, turbulence 5. H 2 in external galaxies 6. H 2 in ULIRGs, Dense tracers 7. Molecules in absorption 8. CO at high redshift 9. Primordial H 2, history

2 How to observe the H 2 component? SAAS-FEE Lecture 1 Françoise COMBES

3 3 The H 2 molecule Symmetrical, no dipole Quadrupolar transitions ΔJ = +2 Light molecule => low inertial moment and high energy levels Para (even J) and ortho (odd J) molecules (behave as two different species)

4 4 H 2 is the most stable form of hydrogen at low T dominant in planetary atmospheres? Formation: on dust grains at 10K However formation still possible in primordial gas (H + H - Palla et al 1983) Destruction: through UV photons (Ly band) Shielded by HI, since the photodissociation continuum starts at 14.7eV, and photo-ionization at 15.6 eV (HI ionization at 13.6 eV) Self-shielding from low column densities cm -2 in standard UV field H 2 will be present, while other molecules such as CO would be already photo-dissociated

5 5 Potential curves involved in the Lyman and Werner bands (Roueff 00)

6 6 Ortho-Para transitions? Formation in the para state not obvious Large energy of formation 2.25 eV/atom ortho-para conversion in collisions H + +H 2 n(O)/n(P) = 9.35 exp(-170/T) Anormal ratios observed (ISO) IR lines J=2-1 at 42 μ, 1-0 at 84 μ ? A = cm 3 /s (Black & Dalgarno 1976)

7 7 Infrared Lines of H 2 Ground state, with ISO (28, 17, 12, 9μ) S(0), S(1), S(2), S(3) From the ground, 2.2 μ, v=1-0 S(1) excitation by shocks, SN, outflows or UV-pumping in starbursts, X-ray, AGN require T > 2000K, nH 2 > 10 4 cm -3 exceptional merger N6240: 0.01% of L in the 2.2 μ line (all vib lines 0.1%?)

8 8 H 2 distribution in NGC891 (Valentijn, van der Werf 1999)

9 9 NGC 891, Pure rotational H2 lines S(0) & S(1)

10 10 H 2 v=1-0 S(1) 2.15μ in NGC 6240 van der Werf et al (2000) HST

11 11

12 12 UV Lines of H 2 Absorption lines with FUSE Very sensitive technique, down to column densities of NH cm-2 Ubiquitous H 2 in our Galaxy (Shull et al 2000, Rachford et al 2001) translucent or diffuse clouds Absorption in LMC/SMC reduced H 2 abundances, high UV field (Tumlinson et al 2002) High Velocity Clouds detected (Richter et al 2001)

13 13 FUSE Spectrum of the LMC star Sk (Tumlinson et al 02) NH 2 = cm -2 Ly 4-0

14 14 Column densities and molecular fraction compared to models R0 R0/3 Io Io*20

15 15 Detection of H 2 in absorption by FUSE in HVCs

16 16 Sembach et al 2001

17 17 The CO Tracer In galaxies, H 2 is traced by the CO rotational lines CO/H 2 ~10 -5 CO are excited by collision with H 2 The dipole moment of CO is relatively weak  ~0.1 Debye Spontaneous de-excitation rate A ul   2 A ul is low, molecules remain excited in low-density region about 300 cm -3

18 18 Competition between collisional excitation and radiative transitions, to be excited above the 2.7K background J=1 level of CO is at 5.2K The competition is quantified by the ratio C ul /A ul varies as n(H 2 )T 1/2 /( 3  2 ) Critical density n crit for which C ul /A ul = 1 Molecule CONH3 CS HCN  (Debye) n crit (cm -3 ) 4E41.1E5 1.1E61.6E7

19 19 Various tracers can be used, CO for the wide scale more diffuse and extended medium, the dense cores by HCN, CS, etc.. The CO lines (J=1-0 at 2.6mm, J=2-1 at 1.3mm) are most often optically thick At least locally every molecular cloud is optically thick Although the "macroscopic" depth is not realised in general, due to velocity gradients Relation between CO integrated emission and H 2 column density? Is it proportional? How to calibrate?

20 20 NGC 6946 CO(2-1) map 13" beam IRAM 30m Spectra, Weliachew et al 1988

21 21  Isotopic molecule 13 CO, UV lines  Statistics of "standard" clouds  The Virial relation 1- Use the isotope 13 CO much less abundant at the solar radius: Ratio ~90 therefore 13 CO lines more optically thin A standard cloud in the MW has  CO ~10 and  13 ~ 0.1 The average ratio between integrated CO and 13 CO intensities is of the order of 10

22 22 Successive calibrations knowing 13 CO/H 2 ratio in the solar neighbourhood (direct observations of these lines in UV absorption in front of stars, with diffuse gas on the line of sight) 2- Statistically "standard" clouds For extragalactic studies, numerous clouds in the beam Typical mass of a cloud 10 3 Mo something like 10 4 or 10 5 clouds in the beam No overlap, since they are separated in velocity Filling factor f s f v << 1 (hypothesis) Usually T A * ~ 0.1K for nearby galaxies, 10K for a cloud constant factor between I CO and NH 2

23 23 3- More justified method: the virial Each cloud contributes to the same T A * in average reflecting the excitation temperature of the gas the width of the spectrum gives the cloud mass through the virial hypothesis V 2 r ~ GM The conversion ratio can then be computed as a function of average brightness T R and average density of clouds n

24 24 Solomon et al 1987 Milky Way Virial mass versus L CO M vt =39L CO.81 Slope is not 1

25 25 Area A of the beam A =  /4 (  D) 2 N clouds, of diameter d, projected area a=  /4 d 2 velocity dispersion  V I CO = A -1 N (  /4 d 2 )T R  V Mean surface density NH 2 = A -1 N (  /6 d 3 ) n NH 2 / I CO = 2/3 nd/( T R  V) from the Virial  V ~ n 1/2 d and the conversion ratio as n 1/2 /T R

26 26 This factor is about 2.8 E20 cm -2 /(km/s) for T R ~10K and n~200cm-3 This simple model expects a low dependence on metallicity, since the clouds have high optical thickness and are considered to have top-hat profiles (no changes of sizes with metallicity) However, for deficient galaxies such as LMC, SMC, where clouds can be resolved, and the virial individually applied, the conversion factor appears very dependent on metallicity

27 27 The size of clouds, where  = 1, is varying strongly Models with  ~r -2, NH 2 ~ r -1 Diameter of clouds d ~ Z (or O/H) Then filling factor in Z 2 The dependence of the conversion ratio on metallicity could be more rapid than linear (the more so that C/O ~O/H in galaxies, and CO/H 2 ~(O/H) 2 ) In external galaxies, the MH 2 /MHI appears to vary indeed as (O/H) 2 (Arnault et al 88, Taylor et al 1998)

28 28 Arnault, Kunth, Casoli & Combes 1988 L CO /M(HI) α (O/H) 2.2

29 29 On the contrary, in the very center of starbursts galaxies, an overabundance of CO could overestimate the molecular content Not clear and definite variations, since T R is larger, but nH 2 too, and NH 2 / I CO varies as n 1/2 /T R Possible chemical peculiarities in starbursts 12 C primary element, while 13 C secondary Isotopic ratios vary Can be seen through C 18 O

30 30 Another tracer: cold dust At 1mm, the emission is Rayleigh-Jeans B(, T) ~ 2 k T / 2 flux quasi-linear in T (between 20 and 40K) In general optically thin emission Proportional to metallicity Z Z decreases exponentially with radius

31 31 When the molecular component dominates in galaxies, the CO emission profile follows the dust profile (example NGC 891) When the HI dominates, on the contrary, the dust does not fall as rapidly as CO with radius, but follows more the HI (example NGC 4565) CO might be a poor tracer of H 2

32 32 Radial profiles N891 (Guélin et al 93) & N4565 (Neininger et al 96)

33 33 The excitation effects combine to metallicity Explains why it drops more rapidly than dust with radius CO(2-1) line tells us about excitation Boarder of galaxies, CO subthermally excited When optically thick CO21/CO10 ratio ~1 If optically thin, and same T ex, could reach 4 But in general < 1 in the disk of galaxies T ex (21) < T ex (10) upper level not populated even if T kin would have allowed them

34 34 Braine & Combes 1992, IRAM Survey

35 35 Gradient of excitation in the LMC vs MW Sorai et al (2001) Average value of 0.6 for MW from Sakamoto et al 1995

36 36 CO(2-1)/CO(1-0) vs IRAS, and vs CII in LMC (grey band = MW)

37 37 Conclusion The H 2 molecule is invisible, in cold molecular clouds (the bulk of the mass!) CO is not a good tracer, both because metallicity effect (non -linear, since depending on UV flux, self-shielding, etc. Very important to have other tracers dense core tracers, HCN, HCO+, isotopes.. H 2 pure rotational lines, also a tracer of the "warm" H 2, always present when cold H 2 is there


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