2
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
3
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?
4
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
5
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)
7
Excellent correlationRadio -FIR
q=log(FIR/Radio21cm)
some exceptions arethe radio-loud AGN
Origin of the correlationstarburst, SN
ULIRGS have very high SF efficiency
8
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
9
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
10
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
2
M~σ4/ Σg will condense on the same time-scale
11
Tight correlation CO-100μ, supportsBlack-Body model
Small sizes, N(H2) > 1024 cm-2, ==> τ ~1 at 100μ for the dust
Slope 1
12
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
13
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
14
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)
15
Without bulge, disk more unstable
At the end, the same SFR
Several burst of SFaccording to the pericenters
Star formation can be delayed
17
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?)
19
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?
20
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
21
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
22
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
23
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)
25
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
26
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
27
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)
29
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?
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