H2 in External Galaxies

SAAS-FEE Lecture 5

Françoise COMBES

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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 spiralgalaxies

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

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From Young & Scoville 91

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H2 content, normalised bysurface or dynamical mass

From Casoli et al (1998)

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H2 to HI mass ratiosversus type

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When taken into account the mass of galaxiesand not only types

Mass is related to metallicity

Z ~ M1/2

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Tamura et al (2001)dwarf spheroidalsline = model from Yoshii & Arimoto 87

Winds and SN ejections in small potentials

Zaritsky (1993)dE, Irr, and giant spiral galaxiesabundances measured at 0.4r0

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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

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

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Taylor et al 98

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O/H is the main factor (below 7.9, galaxies are undetectable)But other factors, too; like the SFR (UV)Barone et al (2000)

Only detections are on the right

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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 sizesUn-compact

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

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Bothun et al 97

LSB are a reservoir of baryons

Unevolved, since lessgravitationnallyunstable

Poor environments?High Spin?

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Schombert et al. 2001

LSB dwarfscompared to otherslow masses, small sizes

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Gas fraction vs SB

in LSB dwarfscompared to modelsby Boissier & Prantzos 00

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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

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Same Tully-Fisher relation(for the same V, galaxies twice as large) M ~V2RM/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 formationLSB have no companions (Zaritsky & Lorrimer 1993)

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McGaugh et al (2000) => Baryonic TF

Tully-Fisher relationfor gaseous galaxiesworks much better inadding gas mass

Relation Mbaryons

with Rotational V

Mb ~ Vc4

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Radial Distribution in Spirals

HI versus H2

The H2 is restricted to the optical diskwhile the HI extends 2-4 x optical radius

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

HI

CO

CO

HI

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Radial distribution inNGC 6946

The HI is the only componentnot following star formation

Often exponential disks

similar to optical

21Bima SONG (Regan et al 2001)

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Bima-SONG, radialdistributions

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Spiral Structure

The H2 component participates even better than the HI andstellar component to the density waves

due to its low velocity dispersion

Larger contrast than other componentsstreaming 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

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M51 spiral+ nuclear ringTilanus & Allen1991

Pearls on string

GMC complexes

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On the Fly map of M31at IRAM 30m, Neininger etal (1998)

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H2 in Barred Galaxies

H2 is particularly useful to map the bar and ringssince HI is in general depleted in central regionsHα is often obscured

Barred galaxies have more CO emission, and the H2 gas is moreconcentrated (Sakamoto et al 1999)

This confirms dynamical theories of transport of the gas by barsMore than half of the gas in the central kpc comes from outsideand is too high (and recent) to have been consumed through SFRate of 0.5-1 Mo/yr

No relation, however, to the AGN activity

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Sakamoto et al (99)

CO Survey of barredgalaxies

All kinds of morphologiesRings, bars, spiral structure

Twin Peaks (leading offset dust lanes)

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Barred galaxies have more concentrated CO emissionand more gas in the central parts

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Reynaud & Downes1997

CO trace the dust lanes

NGC 1530

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In barred galaxies, star formation is influenced by the gas flowthe 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 morphologyand dynamics (cf M100, method of gas response in the potential derivedfrom the NIR images)

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

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Simulations of 2 bars in M100(Garcia-Burillo et al 1998)

Ω = 23 and 160km/s

1 pattern

2 patterns

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Molecular gas in counterrotation with respect tothe stellar component

Simulations explain them=1 leading arm Garcia-Burillo et al 00)

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NGC4631/56 interactingHI Rand & van der Hulst 93dust IRAM Neininger 00

Molecular gas out of the planesalso NGC4438 in Virgo(Combes et al 1988)

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Molecular gas in Polar RingsPRG are due either to accreted gas from a companionor 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, butstars are dispersed The ages of the stars in the polar ring date the event

CO detected in polar rings (Taniguchi et al 90, Combeset al 92, Watson et al 1994)can give insight in the event: metallicity formed from therecent star formation, or the polar galaxy non destroyed?

Used to determine the flattening of the dark matter (3D potential probed)

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NGC4650a, PRG sen with HST

Simulations by Bekki (1998)

Self-gravity of the PR?(more mass in the PR system)

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Molecular gas in EllipticalsMost 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 moleculesare also detected (Lees et al 1991, Wiklind et al 1995)

M(H2)/M(HI) 2-5 times lower than in spiral galaxiesmore in field ellipticalsLow excitation temperatureSmall gas-to-dust ratio (but correlated to low Tdust)

No correlation with the stellar component ==> accretion

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Comparison between H2 mass obtainedfrom CO and FIR (dust)Wiklind et al (1995)Lines are for g/d = 700Dash g/d = 50

D> 30Mpc

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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 interactionsSimulations confirm the scenario (Quinn 84, Dupraz & Combes 87)The stars of the small companion, disrupted in the interactionphase 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-wrapprocess

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

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Phase wrapping Formation mechanism for shellsPeriod increasing function of radiusAccumulation at apocenter

40Charmandaris, Combes, van der Hulst 2000

Star shellsin yellow

HI whitecontours

CO pointsin red

Radio jetsin blue

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CO dragged outside galaxies

Interactions of galaxies, formation of tidal dwarfsCO detected in these small dwarfs, supposed to be formed in the interactionBraine et al (2000, 01)

Is the molecular gas dragged with the tidal tail gas and reclump in thetidal dwarf, or the molecular gas re-formed in the collapse?Trigger some star formation, but in general insufficient to havesolar metallicityMore likely that the gas and metals come from the main galaxies

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

43Braine et al 00, 01

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Conclusions•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 lowsurface brightness, and are therefore stable LSBUnevolved, extended, un-concentrated systems, may contain H2

but have low CO emissionNo companion, large spin

•CO is a good tracer of density waves, spirals, bars, rings

•Radial distribution overall exponential, following the opticalBut large departures, contrary to stars

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

either due to excitation? Different conversion?

•Gas coming from accretionNo correlation with stellar component

•CO emission very useful to trace density, star formationperturbations like warps, polar rings, gas dragged out of thespiral planes