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H 2 in External Galaxies. SAAS-FEE Lecture 5 Françoise COMBES. H 2 content and morphological types. Several surveys of CO in galaxies, Young & Knezek (1989) more than 300 galaxies in the FCRAO survey Review by Young & Scoville (1991, ARAA)

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h 2 in external galaxies

H2 in External Galaxies

SAAS-FEE Lecture 5

Françoise COMBES

h 2 content and morphological types
H2 content and morphological types

Several surveys of CO in galaxies, Young & Knezek (1989)

more than 300 galaxies in the FCRAO survey

Review by Young & Scoville (1991, ARAA)

The H2 mass is comparable in average to the HI mass in spiral


But this could be due to the IRAS-selection for many of them

Recent survey by Casoli et al (1998)

M(H2)/M(HI) in average = 0.2

Varies with morphological type, by a factor ~ 10


H2 content, normalised by

surface or dynamical mass

From Casoli et al (1998)


When taken into account the

mass of galaxies

and not only types

Mass is related to metallicity

Z ~ M1/2


Tamura et al (2001)

dwarf spheroidals

line = model from

Yoshii & Arimoto 87

Winds and SN ejections in

small potentials

Zaritsky (1993)

dE, Irr, and giant spiral galaxies

abundances measured at 0.4r0


For galaxies of high masses, there is no trend of decreasing H2

with type

The dependence on type could be entirely due to metallicity

The conversion factor X can vary linearly (or more) with Z

Dust depleted by 20 ==> only 10% less H2 but 95% less CO

(Maloney & Black 1988)

There is no CO deficiency in galaxy clusters, as there is HI

According to the FIR bias, M(H2)/M(HI) can vary from 0.2 to 1

co in dwarf galaxies
CO in Dwarf Galaxies

Difficult to detect, because of sizes, and low Z (Arnault et al 88)

DE easier to detect than Dirr (Sage et al 1992)

The more recent observations by Barone et al (98, 00), Gondhalekar

et al (98), Taylor et al (98) confirm a steep dependence on Z

even higher below 1/10 of solar

Dwarfs have not deep enough potentials to retain their metals

During a starbursts, large gas ejections (Lyα at large V)


Only detections are on the right

O/H is the main factor (below 7.9, galaxies are undetectable)

But other factors, too; like the SFR (UV)

Barone et al (2000)

low surface brightness lsb
Low Surface Brightness LSB

Large characteristic radii, large gas fraction

(up to fg=95% LSB dwarfs Shombert et al 2001)

and dark matter dominated ==> unevolved objects

Same total gas content as HSB (McGaugh & de Blok 97)

Low surface density of HI, too, although large sizes


Resemble the outer parts of normal HSB galaxies

No CO? (de Blok & van der Hulst 1998), but H2?

Some weak bursts of SF, traveling over the galaxy


Bothun et al 97

LSB are a reservoir

of baryons

Unevolved, since less



Poor environments?

High Spin?


LSB dwarfs

compared to others

low masses, small sizes

Schombert et al. 2001


Gas fraction vs SB

in LSB dwarfs

compared to models

by Boissier & Prantzos 00


CO in LSB, de Blok & van der Hulst 98

But, detection by Matthews & Gao (2001) in edge-on LSB galaxies

M(H2)/M(HI) ~1-5 10-2


Same Tully-Fisher relation

(for the same V, galaxies twice as large) M ~V2R

M/L increases as surface density decreases

Low efficiency of star formation (Van Zee et al 1997)

Gas Σg below critical

A gas rich galaxy is stable only at very low Σg

(cf Malin 1, Impey & Bothun 1989)

Galaxy interaction, by driving a high amount of gas

==> trigger star formation

LSB have no companions (Zaritsky & Lorrimer 1993)


Tully-Fisher relation

for gaseous galaxies

works much better in

adding gas mass

Relation Mbaryons

with Rotational V

Mb ~ Vc4

McGaugh et al (2000) => Baryonic TF

radial distribution in spirals
Radial Distribution in Spirals

HI versus H2

The H2 is restricted to the optical disk

while the HI extends 2-4 x optical radius

HI hole or depression in the centers, sometimes compensated by H2






Often exponential disks

similar to optical

Radial distribution in

NGC 6946

The HI is the only component

not following star formation


Bima-SONG, radial


spiral structure
Spiral Structure
  • The H2 component participates even better than the HI and
  • stellar component to the density waves
  • due to its low velocity dispersion
  • Larger contrast than other components
  • streaming motions, due to the spiral density wave
  • excitation different in arms? Density
  • Star formation, heating?
  • Formation of GMC in arms
  • Formation of H2? Chemical time-scale 105 yrs
  • HI is formed out of photo-dissociation of H2
  • CO exist also in the interarms

M51 spiral

+ nuclear ring

Tilanus & Allen


Pearls on string

GMC complexes


On the Fly map of M31

at IRAM 30m, Neininger et

al (1998)

h 2 in barred galaxies
H2 in Barred Galaxies

H2 is particularly useful to map the bar and rings

since HI is in general depleted in central regions

Hα is often obscured

Barred galaxies have more CO emission, and the H2 gas is more

concentrated (Sakamoto et al 1999)

This confirms dynamical theories of transport of the gas by bars

More than half of the gas in the central kpc comes from outside

and is too high (and recent) to have been consumed through SF

Rate of 0.5-1 Mo/yr

No relation, however, to the AGN activity


Sakamoto et al (99)

CO Survey of barred


All kinds of morphologies

Rings, bars,

spiral structure

Twin Peaks

(leading offset dust lanes)


Reynaud & Downes


CO trace the

dust lanes

NGC 1530


In barred galaxies, star formation is influenced by the gas flow

the resonances, the accumulation in rings

Offset of 320 pc in average between Hα arms and CO arms (Sheth 01)

The gas flows can inhibit star formation, when too fast

(Reynaud & Downes, NGC 1530, 1999)

Favors SF when accumulation in rings, nuclear bars

Two patterns are sometimes required by observations of morphology

and dynamics (cf M100, method of gas response in the potential derived

from the NIR images)

Counter-rotating gas (NGC 3593), or gas outside the plane in galaxy



Simulations of 2 bars in M100

(Garcia-Burillo et al 1998)

Ω = 23 and 160km/s

1 pattern

2 patterns


Molecular gas in counter

rotation with respect to

the stellar component

Simulations explain the

m=1 leading arm

Garcia-Burillo et al 00)


NGC4631/56 interacting

HI Rand & van der Hulst 93

dust IRAM Neininger 00

Molecular gas out of the planes

also NGC4438 in Virgo

(Combes et al 1988)

molecular gas in polar rings
Molecular gas in Polar Rings

PRG are due either to accreted gas from a companion

or are formed in a merger of tho galaxies with orthogonal disk

orientations (Bekki 1998, Bournaud & Combes 2002)

The polar ring is due to gas resettling in the polar plane, but

stars are dispersed

The ages of the stars in the polar ring date the event

CO detected in polar rings (Taniguchi et al 90, Combes

et al 92, Watson et al 1994)

can give insight in the event: metallicity formed from the

recent star formation, or the polar galaxy non destroyed?

Used to determine the flattening of the dark matter

(3D potential probed)


NGC4650a, PRG sen with HST

Simulations by Bekki (1998)

Self-gravity of the PR?

(more mass in the PR system)

molecular gas in ellipticals
Molecular gas in Ellipticals

Most E-galaxies possess accreted gas, already detected in HI

(Knapp et al 1979, van Gorkom et al 1997)

Either the remnant of the merger event at their birth, or

accretion of small gas-rich companions

Dust through IRAS (Knapp et al 1989), et CO molecules

are also detected (Lees et al 1991, Wiklind et al 1995)

M(H2)/M(HI) 2-5 times lower than in spiral galaxies

more in field ellipticals

Low excitation temperature

Small gas-to-dust ratio (but correlated to low Tdust)

No correlation with the stellar component ==> accretion


D> 30Mpc

Comparison between H2 mass obtained

from CO and FIR (dust)

Wiklind et al (1995)

Lines are for g/d = 700

Dash g/d = 50

shells around ellipticals
Shells around ellipticals

The merging events giving birth to ellipticals are also forming shells

Stellar shells known from Malin & Carter (1983) unsharp masking

Schweizer (1983) remark that they accompany mergers and interactions

Simulations confirm the scenario (Quinn 84, Dupraz & Combes 87)

The stars of the small companion, disrupted in the interaction

phase wrap in the E-galaxy potential

Recently, HI gas detected in shells (Schiminovich, 1994, 95)

Normally, the diffuse gas condenses to the center in the phase-wrap


But CO is now also detected in shells (Charmandaris et al 2000)


Phase wrapping Formation mechanism for shells

Period increasing function of radius

Accumulation at apocenter


Star shells

in yellow

HI white


CO points

in red

Radio jets

in blue

Charmandaris, Combes, van der Hulst 2000

co dragged outside galaxies
CO dragged outside galaxies

Interactions of galaxies, formation of tidal dwarfs

CO detected in these small dwarfs, supposed to be formed in the


Braine et al (2000, 01)

Is the molecular gas dragged with the tidal tail gas and reclump in the

tidal dwarf, or the molecular gas re-formed in the collapse?

Trigger some star formation, but in general insufficient to have

solar metallicity

More likely that the gas and metals come from the main galaxies

Fate of these tidal dwarfs? In general, they are re-accreted

and merge

  • The CO emission depends on type, relative to HI,
  • but could be only a metallicity effect
  • Galaxies can have large gas mass fractions, when they have low
  • surface brightness, and are therefore stable LSB
  • Unevolved, extended, un-concentrated systems, may contain H2
  • but have low CO emission
  • No companion, large spin
  • CO is a good tracer of density waves, spirals, bars, rings
  • Radial distribution overall exponential, following the optical
  • But large departures, contrary to stars

Elliptical galaxies contain H2, with lower M(H2)/M(HI)

  • either due to excitation? Different conversion?
  • Gas coming from accretion
  • No correlation with stellar component
  • CO emission very useful to trace density, star formation
  • perturbations like warps, polar rings, gas dragged out of the
  • spiral planes