1 Molecules in galaxies 1.How to observe the H 2 component? 2. Molecular component of the Milky Way...

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  • 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

  • How to observe the H2 component?IRAM Summer School Lecture 1Franoise COMBES

  • The H2 moleculeSymmetrical, no dipoleQuadrupolar transitions J = +2

    Light molecule => low inertial moment and high energy levels

    Para (even J) and ortho (odd J) molecules (behave as two different species)170K512K

  • H2 is the most stable form of hydrogen at low Tdominant in planetary atmospheres?

    Formation: on dust grains at 10KHowever 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 densities1020 cm-2 in standard UV field

    H2 will be present, while other molecules such as CO would be already photo-dissociated

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

  • Ortho-Para transitions?Formation in the para state not obviousLarge energy of formation 2.25 eV/atomortho-para conversion in collisions H++H2

    n(O)/n(P) ~ exp(-170/T)Anormal ratios observed (ISO)

    IR lines J=2-1 at 42 , 1-0 at 84 ?A = 10-10 cm3/s (Black & Dalgarno 1976)A=2 10-10 cm3/s (Gerlich 1990) reaction favors o-H2

  • Infrared Lines of H2Ground state, with ISO (28, 17, 12, 9)S(0), S(1), S(2), S(3)From the ground, 2.2 , v=1-0 S(1)

    excitation by shocks, SN, outflowsor UV-pumping in starbursts, X-ray, AGNrequire T > 2000K, nH2 > 104cm-3exceptional merger N6240: 0.01% of L in the 2.2 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)

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

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

  • UV Lines of H2Absorption 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)

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

  • Column densities and molecular fraction compared to modelsR0R0/3IoIo*20

  • Detection of H2 in absorption by FUSE in HVCs

  • Sembach et al 2001

  • The CO TracerIn galaxies, H2 is traced by the CO rotational linesCO/H2 ~10-5 CO are excited by collision with H2The dipole moment of CO is relatively weak ~0.1 Debye

    Spontaneous de-excitation rate Aul 2Aul is low, molecules remain excited in low-density region about 300 cm-3

  • Competition between collisional excitation and radiative transitions, to be excited above the 2.7K backgroundJ=1 level of CO is at 5.2K

    The competition is quantified by the ratio Cul/Aulvaries as n(H2)T1/2 /( 3 2)

    Critical density ncrit for which Cul/Aul = 1Molecule CONH3 CS HCN (Debye) 0.11.5 2.03.0ncrit (cm-3) 4E41.1E5 1.1E61.6E7

  • 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 thickAt least locally every molecular cloud is optically thickAlthough 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?

  • NGC 6946 CO(2-1) map 13" beamIRAM 30mSpectra, Weliachew et al 1988

  • Isotopic molecule 13CO, UV lines Statistics of "standard" clouds The Virial relation

    1- Use the isotope 13CO much less abundantat the solar radius: Ratio ~90therefore 13CO lines more optically thinA standard cloud in the MW has CO ~10and 13 ~ 0.1The average ratio between integrated CO and 13CO intensities is of the order of 10

  • Successive calibrationsknowing 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" cloudsFor extragalactic studies, numerous clouds in the beamTypical mass of a cloud 103 Mosomething like 104 or 105 clouds in the beamNo overlap, since they are separated in velocityFilling factor fs fv

  • 3- More justified method: the virialEach cloud contributes to the same TA* in averagereflecting the excitation temperature of the gasthe width of the spectrum gives the cloud mass through the virial hypothesisV2 r ~ GM

    The conversion ratio can then be computed as a function of average brightness TR and average density of clouds n

  • Solomon et al 1987Milky Way

    Virial massversus LCO


    Slope is not 1

  • Area A of the beam A = /4 (D)2N clouds, of diameter d, projected area a= /4 d2velocity dispersion V

    ICO = A-1 N (/4 d2)TR V

    Mean surface densityNH2 = A-1 N (/6 d3) n

    NH2 / ICO = 2/3 nd/( TR V)from the Virial V ~ n1/2 dand the conversion ratio as n1/2 /TR

  • 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 thicknessand 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

  • The size of clouds, where = 1, is varying strongly

    Models with ~r-2, NH2 ~ r-1Diameter of clouds d ~ Z (or O/H)Then filling factor in Z2The 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)

  • Arnault, Kunth, Casoli & Combes 1988LCO/M(HI) (O/H)2.2

    Confirmed by Taylor & Kobulnicky (98)But see Walter et al (2003)

  • On the contrary, in the very center of starbursts galaxies, an overabundance of CO could overestimate the molecular contentNot clear and definite variations, since TR is larger, but nH2 too, and NH2 / ICO varies as n1/2 /TR

    Possible chemical peculiarities in starbursts12C primary element, while 13C secondaryIsotopic ratios vary Can be seen through C18O

  • High Density TracersNuclei of Galaxies possess denser gasGMC to survive to tidal forces must be denser

    High-J levels of COhigher critical density to be excited (105cm-3)as well asHCN, HCO+, CS, CH3OH, H2CO, OCS, etc..SiO traces shocks (for instance supershells in starbursts)

    Isotopic studies: primary or secondary elements can trace the ageof the star formation events

  • M82Mao et al (2000)High J-levels of COImages are roughly similar in morphologyalthough somewhat less extended than CO(1-0)Two hot spots on either side of the nucleusPart of the molecular torus seen edge-onRing due to the bar (or also void due to starburst?)

  • High density tracers, at low temperaturesCS, 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

  • Isotopic molecules12C/13C in the MW, from 50-90 at the Sun radiustowards 10-20 in the center

    Tracer of the astration, 13C is secondaryIn the Galactic Center, also deficiency of deuterium

    In Starbursts and ULIRGS (Arp220 type), CO/13CO largerNot due to a low optical depth, since C18O is normal withrespect to 12CO

    But 12C is overproduced in the nucleosynthesis of a recent burst(Casoli et al 1992)

  • 12C/13C ratios determined in M82 and IC342 by Henkel et al 98from 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 = 111lower than in the Milky Way

    ==>15N is synthesized by massive starsControversial about this formation: destruction in H-burningformation in SN-II, 14N more secondary, and the ratio increasewith time and astration

  • Another tracer: cold dustAt 1mm, the emission is Rayleigh-JeansB(, T) ~ 2 k T / 2flux quasi-linear in T (between 20 and 40K)In general optically thin emission

    Proportional to metallicity Z Z decreases exponentially with radius

  • 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

  • Radial profiles N891 (Gulin et al 93) & N4565 (Neininger et al 96)

  • The excitation effects combine to metallicityExplains why it drops more rapidly than dust with radius

    CO(2-1) line tells us about excitationBoarder of galaxies, CO subthermally excited

    When optically thick CO21/CO10 ratio ~1If optically thin, and same Tex, could reach 4But in general < 1 in the disk of galaxies

    Tex (21) < Tex (10) upper level not populatedeven if Tkin would have allowed them

  • M82, CO3-2,