Molecules in ULIRGS, and high density tracers

SSAS-FEE Lecture 6

Françoise COMBES

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Ultra-Luminous GalaxiesInteracting galaxies appear to have more H2 contentor at least much more CO emissionThe H2 gas is also more concentrated

In average, the H2 content is multiplied by 4-5(Braine & Combes 1993)

This can be explained by the gravitational torques of the interactionsdriving gas very quickly to the centers

triggers a starburst

The condition of starburst: accumulating gas in a time short enoughthat feedback mechanisms have no time to regulate

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More M(H2) in proportionfor disturbed=interacting

More star-formation, too

Would it be the conversion X?Problems, since high density

X ~n1/2/Tr

Star formation efficiencySFE=LFIR/M(H2)SFE too large?

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High end of the luminosity function

At L > 1011 Lo infrared galaxiesare the dominant population z<0.3

more abundant than QSOs

Energy from starburstsat L> 1012Lo, all major mergers

In some cases, an AGN is superposed

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Sanders & Mirabel 96

Spectral Energy DistributionSED

The ratio LIR/LB variesconsiderably

and is an indication of starbursts

The brightest objects are the more obscured ones

The ratio F60/F100 also increaseswith LIR: the brightest objects arehotter (more star formation)

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Borne et al 99

HST WFPC2

ULIRGS

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Excellent correlationRadio -FIR

q=log(FIR/Radio21cm)

some exceptions arethe radio-loud AGN

Origin of the correlationstarburst, SN

ULIRGS have very high SF efficiency

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Molecular gas in ULIRGS

These ultra-luminous galaxies have huge quantities of H2 gasThe gas is dense and hot 105 to 107 cm-3, 60-80Ksimilar to the star forming regions in GMC

Large sample observed in Solomon et al (97)Tight correlation between the CO and 100μ luminosity==> black body emission

Small sizes of the emission, example Arp200, 300pcjustifies the optical thickness

usually 100μ emission is thin τ ~ νβ

with β ~2, but begins at 60μ

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CO on the optical HST image

Downes & Solomon 98

The molecular emission is highlyconcentrated within 1kpc oreven smaller, cf Arp220

Two disks are merging, as seen inthe dispersion, and mm continuum

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Gas is concentrated in central nuclear disks or rings

Stability in these central disks?

Q = 2.2 for gas only, Q= 1 for gas+stars (Downes et al 98)

==> formation of giant clustersIf the dispersion is larger, the Jeans mass is larger

Jeans length λJ ~σ2/ Σg

τff ~ σ/ Σg instability as soon as Q ~σκ/ Σg =1

For the same ratio σ/ Σg, complexes of massesM ~ Σg λJ

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M~σ4/ Σg will condense on the same time-scale

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Tight correlation CO-100μ, supportsBlack-Body model

Small sizes, N(H2) > 1024 cm-2, ==> τ ~1 at 100μ for the dust

Slope 1

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Black- Body model

LFIR = 4 π R2 σ Td4 (no term in τ ~νβ)

LCO = 4 π2 R2 (2k/λ2) σƒ Tbdν

LFIR/LCO ~ Td3/(fv ΔV)

predicted curve

fv filling factor in velocity

The relation departs slightly, because of Td different than Tb

CO and FIR not exacty the same regions (CO size larger?)filling factor not unity

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AGN or starburst?Molecular disks of 500pc, V = 300km/s, Periods 10 MyrLFIR = 1012Lo

50 Mo/yr formation rate, in 100 Myr (or 10 rotations)half of the gas is turned in new stars,5 109Mo of H2 gas => stars

M*/LFIR= 5 10-3 Mo/Lo (L/M ~200)

If 1012Lo comes from accretion onto a black hole, atthe efficiency of L = 0.1 dm/dt c2, the accretion must beonly 1 Mo/yr, and therefore only 1% of the gas wouldbe accreted on the same time-scale

The gas would remain available at 99% to form stars

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Dynamical Triggering

Numerical simulations (Barnes & Hernquist 92)Mihos & Hernquist 1994 including star formation recipes

Galaxy interactions produce strong non-axisymmetry andtorques that drive the gas towards the center, with the helpof a small rate of dissipation

This depends essentially on the stability of the disk prior theinteraction, therefore on the bulge-to-disk ratio

Finally the role of the geometry of the interaction is secondarydirect or retrograde (provided there is a merger)

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Without bulge, disk more unstable

At the end, the same SFR

Several burst of SFaccording to the pericenters

Star formation can be delayed

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Mihos & Hernquist 96

Simulations of disk/halo galaxies

Gas and young starsare plotted

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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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LVG modelN(H2) ~1023cm-2

M(H2) a few 108Mo

n(H2) ~104cm-3

close to the tidal limit

Emission comes primarily from PDR photon-dominated regionsquite different from the other highdensity tracers

Two components in the molecular gas: dense cores, + intercloudA diffuse component intervenes in the CO emission, also CI/CO is highIs this representative of starburst at high z?

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Kinetic temperatures derived are 20-60K, rather lowHeating: star formation, cosmic rays, turbulence

Consistent with the weakness of CH3OH or SiO high temp tracers

SiO mapped by Garcia-Burillo et al 01

SiO traces the walls of the supershellsnot the star forming regions

Vertical filament: SiO chimneycoincident with radio cm emissionGas ejected by the starburstShock chemistry

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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

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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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Downes et al 92HCN in IC342

Same morphology than in CO2 spiral arms winding up in a ring

CO/HCN ratio from 7 to 14 goingoutwards

The 3mm continuum is free-free, notthermal dust emission(no starburst emission)

Not very high density (except dense cores, high resolution)

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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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Deuterated speciesLMC: DCN, DCO+

Ratios about 20

strong fractionation

D/H = 2 10-5, but the deuterated molecules have lower energiesAt low temperature HD +HCN --> H2 +DCN

Here temperature is 20K

Chin et al 1996

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The HNC/HCN ratio

Useful to disentangle abundances, excitation, density or temperatureHNC (hydrogen isocyanide) is a high density tracer as well

HNC is weaker than HCN, exceptin ULIRGS such as Arp 220where it is > 1

Not very clear however, since in NGC 6240, it is 10 times lower

Huttemeister et al 95

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Other molecules

Other molecules, which trace different physical conditions

OCS in NGC 253, M82 (Mauersberger et al 95)NH3 in Maffei 2 and others (Henkel et al 00)rotational temperatures of 85K

H2CO and CH3OH tracing high-densitysubthermally excited,clumpy structure

=> point out very different physical conditionsand various chemistry, from one galaxy to the next (Huttemeister et al 97)

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Methanolasymmetric top A-type, E-type

lines blendedn(H2) > 105cm-3

Atomic Carbon CI fine structure line 3P1-3P0 at 492 GHzimportant tracer of non-ionising radiation

In Arp 220 CI is strong, as predicted from its FIR flux,while CII emission is depleted

This could be due to higher density, optical thickness of the C+ lineand dust opacity

30Gerin & Phillips 2000

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

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Conclusions

The molecular component is much more important in starburstsand ULIRGs; it is not the case for the HI component

It explains the considerable enhancement in star forming efficiency

Not only a problem of gas excitation, density or temperature, since all gas density tracers confirm the large H2 abundance and density

Dynamical origin of the gas flowExplains the transformation of HI --> H2

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Only in strong starbursts is the H2 gas dense enough toemit sufficiently high-J CO lines

this has important consequence for high-z galaxies

Various molecules help to constrain the physical conditions(density, temperature, excitation, clumpiness, chemical abundances)

At least two components: hot dense cores where stars form+ intercloud, more diffuse medium, subthermal?