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Molecules in galaxies

1. 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 1

Françoise COMBES

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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)

170K

512K

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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

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

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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

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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%?)

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

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

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

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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)

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

Ly 4-0

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Column densities and molecular fraction compared to models

R0 R0/3

Io

Io*20

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Detection of H2 in absorption by FUSE in HVCs

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Sembach et al 2001

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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

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• 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

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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?

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

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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

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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

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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

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Solomon et al 1987

Milky Way

Virial massversus LCO

Mvt=39LCO.81

Slope is not 1

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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

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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

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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)

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Arnault, Kunth, Casoli & Combes 1988

LCO/M(HI) α (O/H)2.2

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

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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 starbursts12C primary element, while 13C secondary

Isotopic ratios vary

Can be seen through C18O

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High Density Tracers

Nuclei 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

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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?)

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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

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Isotopic molecules

12C/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)

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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

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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

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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

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

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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

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M82, CO3-2, 2-1, 1-0 _|¯ __ - -

Isotopic ratio of about10-15 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

40Braine & Combes 1992, IRAM Survey

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Gradient of excitationin the LMC vs MWSorai et al (2001)Average value of 0.6 for MW fromSakamoto et al 1995

42CO(2-1)/CO(1-0) vs IRAS, and vs CII in LMC (grey band = MW)

43Gerin & Phillips 2000

CI/CO = 0.2 (in Kkm/s)cooling comparableNormally smaller than C+(except Arp220 and Mkn231!)

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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 tracersdense core tracers, HCN, HCO+, isotopes..

H2 pure rotational lines, also a tracer of the "warm" H2, alwayspresent when cold H2 is there