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Molecules in galaxies How to observe the H 2 component? 2. Molecular component of the Milky Way

Molecules in galaxies How to observe the H 2 component? 2. Molecular component of the Milky Way 3. H 2 in external galaxies 4. Molecules in absorption. How to observe the H 2 component?. IRAM Summer School Lecture 1 Françoise COMBES. The H 2 molecule. Symmetrical, no dipole

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Molecules in galaxies How to observe the H 2 component? 2. Molecular component of the Milky Way

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  1. Molecules in galaxies • How to observe the H2 component? • 2. Molecular component of the Milky Way • 3. H2 in external galaxies • 4. Molecules in absorption

  2. How to observe the H2 component? IRAM Summer School Lecture 1 Françoise COMBES

  3. The H2 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) 512K 170K

  4. H2 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 1020 cm-2 in standard UV field H2 will be present, while other molecules such as CO would be already photo-dissociated

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

  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++H2 • n(O)/n(P) ~ exp(-170/T) • Anormal ratios observed (ISO) • IR lines J=2-1 at 42 μ, 1-0 at 84 μ ? • A = 10-10 cm3/s (Black & Dalgarno 1976) • A=2 10-10 cm3/s (Gerlich 1990) reaction favors o-H2

  7. Infrared Lines of H2 • 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, nH2 > 104cm-3 • exceptional merger N6240: 0.01% of L in the 2.2 μ line (all vib lines 0.1%?)

  8. H2 distribution in NGC891 (Valentijn, van der Werf 1999) S(0) filled; S(1) open – CO profile (full line)

  9. NGC 891, Pure rotational H2 lines S(0) & S(1) S(0) wider: more extended? Derived N(H2)/N(HI) = 20 ; Dark Matter?

  10. H2 v=1-0 S(1) 2.15μ in NGC 6240 van der Werf et al (2000) HST

  11. UV Lines of H2 • Absorption lines with FUSE (Av < 1.5) • Very sensitive technique, down to column densities of NH2 1014 cm-2 • Ubiquitous H2 in our Galaxy (Shull et al 2000, Rachford et al 2001) translucent or diffuse clouds • 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)

  12. Ly 4-0 FUSE Spectrum of the LMC star Sk-67-166 (Tumlinson et al 02) NH2 = 5.5 1015cm-2

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

  14. Detection of H2 in absorption by FUSE in HVCs

  15. Sembach et al 2001

  16. The CO Tracer • In galaxies, H2 is traced by the CO rotational lines • CO/H2 ~10-5 • CO are excited by collision with H2 • The dipole moment of CO is relatively weak •  ~0.1 Debye • Spontaneous de-excitation rate Aul2 • Aul is low, molecules remain excited in low-density region about 300 cm-3

  17. 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 Cul/Aul • varies as n(H2)T1/2 /( 32) • Critical density ncrit for which Cul/Aul = 1 • Molecule CO NH3 CS HCN •  (Debye) 0.1 1.5 2.0 3.0 • ncrit (cm-3) 4E4 1.1E5 1.1E6 1.6E7

  18. 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 H2 column density? Is it proportional? How to calibrate?

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

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

  21. Successive calibrations knowing 13CO/H2 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 103 Mo something like 104 or 105 clouds in the beam No overlap, since they are separated in velocity Filling factor fs fv << 1 (hypothesis) Usually TA* ~ 0.1K for nearby galaxies, 10K for a cloud constant factor between ICO and NH2

  22. 3- More justified method: the virial Each cloud contributes to the same TA* in average reflecting the excitation temperature of the gas the width of the spectrum gives the cloud mass through the virial hypothesis V2 r ~ GM The conversion ratio can then be computed as a function of average brightness TR and average density of clouds n

  23. Milky Way Virial mass versus LCO Mvt=39LCO.81 Slope is not 1 Solomon et al 1987

  24. Area A of the beam A = /4 (D)2 N clouds, of diameter d, projected area a= /4 d2 velocity dispersion V ICO = A-1 N (/4 d2)TR V Mean surface density NH2 = A-1 N (/6 d3) n NH2 / ICO = 2/3 nd/( TR V) from the Virial V ~ n1/2 d and the conversion ratio as n1/2 /TR

  25. This factor is about 2.8 E20 cm-2 /(km/s) for TR ~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

  26. The size of clouds, where  = 1, is varying strongly Models with  ~r-2, NH2 ~ r-1 Diameter of clouds d ~ Z (or O/H) Then filling factor in Z2 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/H2 ~(O/H)2) In external galaxies, the MH2/MHI appears to vary indeed as (O/H)2 (Arnault et al 88, Taylor et al 1998)

  27. Arnault, Kunth, Casoli & Combes 1988 LCO/M(HI) α (O/H)2.2 Confirmed by Taylor & Kobulnicky (98) But see Walter et al (2003)

  28. 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 TR is larger, but nH2 too, and NH2 / ICO varies as n1/2 /TR Possible chemical peculiarities in starbursts 12C primary element, while 13C secondary Isotopic ratios vary Can be seen through C18O

  29. High Density Tracers Nuclei of Galaxies possess denser gas GMC to survive to tidal forces must be denser High-J levels of CO higher critical density to be excited (105cm-3) as well as HCN, HCO+, CS, CH3OH, H2CO, OCS, etc.. SiO traces shocks (for instance supershells in starbursts) Isotopic studies: primary or secondary elements can trace the age of the star formation events

  30. M82 Mao et al (2000) High J-levels of CO Images are roughly similar in morphology although somewhat less extended than CO(1-0) Two hot spots on either side of the nucleus Part of the molecular torus seen edge-on Ring due to the bar (or also void due to starburst?)

  31. High density tracers, at low temperatures CS, HCN The ratios CS/CO and HCN/CO are correlated with LFIR (1/6 in ULIRGs, 1/80 normal, as MW) Starbursts have a larger fraction of dense gas

  32. Isotopic molecules 12C/13C in the MW, from 50-90 at the Sun radius towards 10-20 in the center Tracer of the astration, 13C is secondary In the Galactic Center, also deficiency of deuterium In Starbursts and ULIRGS (Arp220 type), CO/13CO larger Not due to a low optical depth, since C18O is normal with respect to 12CO But 12C is overproduced in the nucleosynthesis of a recent burst (Casoli et al 1992)

  33. 12C/13C ratios determined in M82 and IC342 by Henkel et al 98 from CN, HCN, HCO+ observations Always 12C/13C >40 (not as low as in the Galactic Center) 16O/18O > 100, 14N/15N > 100 HC15N detected in LMC and N4945 (Chin et al 99) 14N/15N = 111 lower than in the Milky Way ==>15N is synthesized by massive stars Controversial about this formation: destruction in H-burning formation in SN-II, 14N more secondary, and the ratio increase with time and astration

  34. 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

  35. 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 H2

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

  37. 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 Tex, could reach 4 But in general < 1 in the disk of galaxies Tex (21) < Tex (10) upper level not populated even if Tkin would have allowed them

  38. M82, CO3-2, 2-1, 1-0 _|¯ __ - - Isotopic ratio of about 10-15 for 12CO/13CO ==> Opticall thick gas TA* = (Tex -Tbg) (1 - e-τ) If optically thin R(21/10) --> 4 Survey of CO(3-2) in 30 spiral galaxies (Mauersberger et al 99) R(32/10)= 0.2-0.7, predicted if Tkin < 50K and n(H2) < 103cm-3

  39. Braine & Combes 1992, IRAM Survey

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

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

  42. CI/CO = 0.2 (in Kkm/s) cooling comparable Normally smaller than C+ (except Arp220 and Mkn231!) Gerin & Phillips 2000

  43. Conclusion The H2 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), and excitation Very important to have other tracers dense core tracers, HCN, HCO+, isotopes.. H2 pure rotational lines, also a tracer of the "warm" H2, always present when cold H2 is there

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