Molecular component in the Milky Way

IRAM Summer School Lecture 2

Françoise COMBES

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CO surveys of the Milky Way

• CfA-Harvard Survey, 1.2m diameter, beam 9', sampling 0.12°, North and South (Dame et al 1987, 2001), sky coverage ~0.5 until |b| < 32°

• Bell Labs 7m, beam 1.7' (Bally et al 87, 88)

• NRAO-Kitt Peak, 12m, beam 60", sky covered 10-3

• Mass-SB, 14m, beam 45", sky covered 10-2 (Solomon, Scoville, Sanders et al) FCRAO

• CO(2-1) Sakamoto (1995) R21/10 = 0.66

• 13CO Bell Labs , Bordeaux, Columbia…

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Dame et al. 2001

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Comparison with an optical image, of the CO clouds within 2.5kpc distance (within 10 to 35km/s)Dame et al (2001)

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Comparisonwith HI and 100μIRAS maps

CO smoothed to 36'ICO>1Kkm/s blanked

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Comparison of ICO with a prediction from the FIR and HI emissionTake the ratio Ngas/IRAS when gas =HI only, then from IRASderive gas map, subtract the observed HI => H2

ΔWCO/WCO =50% average up to > 100%

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The predicted H2 mapcan be used to estimatethe CO-H2 conversion ratio

As a function of b

The drop with z is 6 times steeper than for a plane // layer

Dame et al (2001)

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CO Distribution and Spiral structure of the Milky Way

How to obtain distances ?

Kinematic modelsDetermination of the rotation curve, from terminal velocities

Assumption of circular velocity for the gas

Ambiguities of distance, for material at longitudes below 90deg

To remove the degeneracy: the latitude or height above the plane Can play a role statistically

Also the distance of the near stars, determined by their spectrumor by absorption (in front or behind)

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Locus

Ambiguity of distances

V_rad (r,l) = Rsun sinl (Ω(R)-Ωsun)

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Radial Distribution

> Large concentration in the center

> Hole around 2kpc

> Galactic Molecular ring between 4 and 8 kpc

> Exponential radial decrease in average

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Radial distribution of CO in the MW, from Bronfman et al 1988

Uncertainties in correcting for the 3kpc arm, calibrations, etc..

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HI and H2 Comparison

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Spiral Structure

•Evidence of a spiral structure, through the l-V diagram

•Very difficult to deproject

•Barred structure (through the orbits, parallelogram..)

•Best is to build N-body models (cf Mulder & Liem 86, Fux 99)

•Second (nuclear) bar? (visible with 2MASS, Alard 2001)

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Original Retrieved

Observed l-V diagram

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Model Mulder & Liem 1986 HI PV diagram

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The inner Galaxy

Always a big puzzle: forbidden velocities in the center

Expansion (Oort 77)? Explosion from the center? (Sanders 76)Bar potential (Peters 1975)

Bar directly seen in COBE-DIRBE (Dwek et al 95)Interpretation in terms of periodic orbits in a bar potentialparallel x1 orbits, perpendicular x2 orbits (Binney et al 97)

Characteristic parallelogramNuclear disk decoupled from the main disk

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-0.6° < b < 0.6° 13CO Bally et al (1988)

-0.1° < b < 0.1°

12CO Bally et al 87

3kpc arm

Expanding molecular ring EMR

Clump 1Clump 2

parallelogram

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From Fux (1999)N-body simulations+SPH

Bar taken from DIRBEThe center of the bar wanders

Gas flow asymmetricnon-stationary

Transient

3kpc arm is a spiralround the bar

Parallelogram interpretedas leading dust-lanes

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Bania's clump and V-elongatedfeatures near l=55° are gas lumpscrossing the dust-lane shocks

Inclination of the bar 25°

Corotation radius 4.5 kpc

b/a = 0.6

Other features:inclined plane in the center

strong m=1

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Fux (1999)

Velocities abovethe circular model

The region around3kpc arm is subjectto strong non-circularmotions

Strong asymmetries

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Interpretation of the central l-V diagramfrom Fux (1999)x2 orbits are almost circularx1 cusped orbits produce the parallelogram

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Nature of molecular clouds in the inner galaxy

•Distinct physical parameters from those of the disk

•Denser, by 2 orders of magnitude (> 104 cm-3)revealed by high density tracers, HCN, CS

•Tidal forces larger: differentiating V2/R, if V~cst

•V2/R2 (d/2) ~ 4 GMc/d2, gives the minimum density of clouds

ρc =3/(4πG) V2/R2 = 103 cm-3 (200pc/R)2

Below this, clouds are sheared off to the diffuse medium

•High velocity dispersion in the center, due to the Toomre criterion

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Vertical Distribution

•Comparison of the molecular and atomic thicknesses

Difficult to obtain, although the (l,b) map is much thinner than HI

Projection effects local gas, and warped outer gas for the HI

•Obtained in the Milky Way, at the tangent points (Malhotra et al 1994, 95)

Obtained in external galaxies in face-on objects,or edge-on systems

Thickness hg and vertical velocity dispersion σg

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Vertical equilibriumIsothermal disk model, self-gravitating

hg(r) = 2(r) /(2g(r))

The density profile is then a sech2 law

If the gas is considered as test particles in a potentialof larger scale height Kz z

The density has then a gaussian profileg = 0 exp(-Kzz2/(2g

2))

with a characteristic height hg(r) = (r) /(Kz)1/2

with Kz= 4Gt

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Observed in the HI gas, the velocity dispersion is constantwith radius, and equal to 10km/s (12km/s in the center)

This is best seen in face-on galaxies

In the molecular component, also g = cste

Face-on galaxies (NGC 628, NGC 3938..)

The surprising observation is that both dispersion (atomic and molecular) are about equal(Combes & Becquaert 1997)

Not compatible with so different thicknesses?(60 and 220 pc)

26Combes & Becquaert 97

NGC 628 face-onCO(1-0) dispersion

Soustraction of the expected linewidthdue to the systematicgradient (rotation)

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In the Milky Way, modelisation of hg and g at the tangential points (Malhotra 1994) azimuthal velocity dispersion

Gives almost no variation with radius (except the galactic center) => an idea of the heating processes?

Large uncertainty in the literature, from 4 to 11 km/sclumpiness of molecular clouds

Scale-height of the gas expected to be higher than that of the cloud centers

In average, in the MW, dispersion of 8km/s (averages over200-400pc), scale-height of 50-75pc Scale-height slightly increasing with radius

The shape of the density law: not gaussian, but tails of small clouds

28Sodroski et al. (1987)

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Corrugation and warp:The centroid of the plane departs from z=0 more than thescale height

Phase transition HI --> H2

Could explain that the velocity dispersions of atomic and molecular gas are closeThe gas changes phase, but follows its dynamics. CO is observed more in the plane than the more diffuse HI, but the dispersion is about the same

(Imamura & Sofue 1997)Sudden transition, depending on P, UV radiation, density

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The HI thickness 200pc is not explained through the tubulent velocity (g = 9km/s) The HI needs extra support to keep its height (Malhotra 1995)

The deduced mid-plane mass density is exponential(constant mass-to-light ratio)

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Origin of the heating?

Star formation in the center of the optical disk

Gravitational instabilities in quiet areasToomre criterion for stability self-regulating

Flaring of the plane: thickness increasing linearly with radius

visible in HI, and also in the molecular planeThe total density in the plane is decreasing

Less restoring force, same velocity dispersion==> increased thickness

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Flaring of the HI planealmost linear

hg = h0 + 0.045 * R

Merrifield (1992)

The CO/H2 follows the flare, and also the warp

Grabelsky et al (1987)

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HI and H2 Flaring

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Warping of the plane

Spectacular in HIAsymmetrical (only one side)

Corrugations (larger amplitude than hg)

The CO follows the warp

Also observed in external galaxies, in particular M31

CO observed with 2 velocities, at each crossing ofthe warped plane

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Models of PVdiagrams for M31

Warped thin plane

from Henderson (79)

Characteristic figure-8 shape (see also Brinks & Burton 84)

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High Velocity Clouds

HI mainly, no CO detected until now

consistent with their belonging to the Magellanic Streamof low metallicity

H2 detected through UV absorption lines(Richter et al 2001, Tumlinson et al 2002)

Very low metallicity gas 0.09 solar (Wakker et al 1999)Infall of gas at Z=0.1 solar required1Mo/yr to solve the G-dwarf problem

External galaxies: dwarf companions, Lyα forest, ...

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Gamma-ray surveys

In the Milky Way, the detection of gamma-rays of high energy (> 100Mev) is a tracer of all matter

Nucleons (HI, H2, HII..) interact with cosmic rays to producepions, that disintegrate in gamma-rays

Early surveys showed that the CO/H2 conversion ratio mustnot be constant throughout the Galaxy(Wolfendale et al 1977)

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Bloemen (1989) Strong et al (1988)

Gamma-rays extend radially much more than the expected extentfrom their sources (the CR, Supernovae), and the gas extent

Diffusion of CR?

Today, the lack of gamma-rays in the center is confirmedby EGRET on GRO

Excess towards high latitude, above the planeInterpretation in terms of nucleons? (de Paoliset al 1999) or inverse-Compton, etc..(Strong & Mattox 1996, Strong et al 1999)

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Dixon et al 98

Galactic diffuse emission model

Halo of MW:residual

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Gamma-ray spectrum for the innergalaxy,Models for "conventional" CR spectra

Gamma-ray profile at high latitudes,for E = 70-100 Mevhorizontal line=isotropic background

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Gamma-ray spectrum of innerGalaxyModels for a hard electron injectionspectrumData from OSSE, COMPTEL, EGRET

Same for high latitudes

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Conclusion

•About comparable amounts of H2 and HI gas in the MW•M(H2) ~ 2-3 109 Mo

Very different radial repartition

H2 is centrally concentrated, then in a molecular ring 4-8kpc

HI depleted in the centerand much more radially extended

Repartition in clouds, GMC of 106Mo -- clumpiness

Thinner plane than HI, about the same σg

same flare and warping