Molecular component in the Milky Way

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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 lt 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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Comparison with HI and 100µ IRAS maps
CO smoothed to 36' ICOgt1Kkm/s blanked
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Comparison of ICO with a prediction from the FIR
and HI emission Take the ratio Ngas/IRAS when gas
HI only, then from IRAS derive gas map, subtract
the observed HI gt H2 ?WCO/WCO 50 average up
to gt 100
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The predicted H2 map can be used to estimate the
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
models Determination 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
spectrum or by absorption (in front or behind)
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Locus
Ambiguity of distances

V_rad (r,l) Rsun sinl (O(R)-Osun)
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Radial Distribution
gt Large concentration in the center gt Hole
around 2kpc gt Galactic Molecular ring between 4
and 8 kpc gt 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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Observed l-V diagram
Original
Retrieved
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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 potential parallel x1 orbits,
perpendicular x2 orbits (Binney et al
97) Characteristic parallelogram Nuclear disk
decoupled from the main disk
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Expanding molecular ring EMR
-0.6 lt b lt 0.6 13CO Bally et al (1988)
Clump 2
Clump 1
3kpc arm
parallelogram
-0.1 lt b lt 0.1 12CO Bally et al 87
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From Fux (1999) N-body simulationsSPH
Bar taken from DIRBE The center of the bar
wanders
Gas flow asymmetric non-stationary
Transient


3kpc arm is a spiral round the bar Parallelogram
interpreted as leading dust-lanes
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Bania's clump and V-elongated features near l55
are gas lumps crossing 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 m1
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Fux (1999) Velocities above the circular model
The region around 3kpc arm is subject to strong
non-circular motions
Strong asymmetries
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Interpretation of the central l-V diagram from
Fux (1999) x2 orbits are almost circular x1
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 (gt 104 cm-3)
• revealed by high density tracers, HCN, CS
• Tidal forces larger differentiating V2/R, if
  Vcst
• V2/R2 (d/2) 4 GMc/d2, gives the minimum
  density of clouds
• ?c 3/(4pG) 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 sg

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Vertical equilibrium
?Isothermal disk model, self-gravitating hg(r)
?2(r) /(2??g(r)) The density profile is then a
sech2 law ?If the gas is considered as test
particles in a potential of larger scale height
Kz z The density has then a gaussian profile ?g
?0 exp(-Kzz2/(2?g2)) with a characteristic
height hg(r) ?(r) /(Kz)1/2 with Kz 4?G?t
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?Observed in the HI gas, the velocity dispersion
is constant with 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)
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NGC 628 face-on CO(1-0) dispersion
Soustraction of the expected linewidth due to
the systematic gradient (rotation)
Combes Becquaert 97
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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) gt an
idea of the heating processes? Large uncertainty
in the literature, from 4 to 11 km/s clumpiness
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 over 200-400pc), scale-height of
50-75pc Scale-height slightly increasing with
radius The shape of the density law not
gaussian, but tails of small clouds
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Sodroski et al. (1987)
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Corrugation and warp The centroid of the plane
departs from z0 more than the scale
height ?Phase transition HI --gt H2 Could
explain that the velocity dispersions of atomic
and molecular gas are close The 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 areas Toomre criterion for
stability self-regulating Flaring of the
plane thickness increasing linearly with
radius visible in HI, and also in the molecular
plane The total density in the plane is
decreasing Less restoring force, same velocity
dispersion gt increased thickness
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Flaring of the HI plane almost 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
HI Asymmetrical (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 of the warped plane
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Models of PV diagrams 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 Stream of 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
Z0.1 solar required 1Mo/yr to solve the G-dwarf
problem External galaxies dwarf companions, Lya
forest, ...
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Gamma-ray surveys In the Milky Way, the
detection of gamma-rays of high energy (gt
100Mev) is a tracer of all matter Nucleons (HI,
H2, HII..) interact with cosmic rays to
produce pions, that disintegrate in
gamma-rays Early surveys showed that the CO/H2
conversion ratio must not 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
extent from their sources (the CR, Supernovae),
and the gas extent Diffusion of CR? Today, the
lack of gamma-rays in the center is confirmed by
EGRET on GRO Excess towards high latitude, above
the plane Interpretation in terms of nucleons?
(de Paolis et 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 profile at high latitudes, for E
70-100 Mev horizontal line isotropic background
Gamma-ray spectrum for the inner galaxy, Models
for "conventional" CR spectra
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Gamma-ray spectrum of inner Galaxy Models for a
hard electron injection spectrum Data 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 center
• and much more radially extended
• Repartition in clouds, GMC of 106Mo --
  clumpiness
• Thinner plane than HI, about the same sg
• same flare and warping