Molecules in ULIRGS, and high density tracers

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Molecules in ULIRGS, and high density tracers

• SSAS-FEE Lecture 6
• Françoise COMBES

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Ultra-Luminous Galaxies
Interacting galaxies appear to have more H2
content or at least much more CO emission The 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 interactions driving gas very
quickly to the centers
triggers a starburst The condition of starburst
accumulating gas in a time short enough that
feedback mechanisms have no time to regulate
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More M(H2) in proportion for disturbedinteracting
More star-formation, too
Would it be the conversion X? Problems, since
high density X n1/2/Tr Star formation
efficiency SFELFIR/M(H2) SFE too large?
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High end of the luminosity function
At L gt 1011 Lo infrared galaxies are the dominant
population zlt0.3 more abundant than QSOs Energy
from starbursts at Lgt 1012Lo, all major
mergers In some cases, an AGN is superposed
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Spectral Energy Distribution SED The ratio
LIR/LB varies considerably and is an indication
of starbursts The brightest objects are the
more obscured ones The ratio F60/F100 also
increases with LIR the brightest objects
are hotter (more star formation)
Sanders Mirabel 96
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Borne et al 99 HST WFPC2 ULIRGS
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Excellent correlation Radio -FIR qlog(FIR/Radio2
1cm) some exceptions are the radio-loud
AGN Origin of the correlation starburst,
SN ULIRGS have very high SF efficiency
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Molecular gas in ULIRGS
These ultra-luminous galaxies have huge
quantities of H2 gas The gas is dense and hot
105 to 107 cm-3, 60-80K similar to the star
forming regions in GMC Large sample observed in
Solomon et al (97) Tight correlation between the
CO and 100µ luminosity gt black body
emission Small sizes of the emission, example
Arp200, 300pc justifies the optical
thickness usually 100µ emission is thin t
?ß with ß 2, but begins at 60µ
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The molecular emission is highly concentrated
within 1kpc or even smaller, cf Arp220 Two disks
are merging, as seen in the dispersion, and mm
continuum
CO on the optical HST image Downes Solomon 98
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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 gasstars (Downes et al
98) gt formation of giant clusters If the
dispersion is larger, the Jeans mass is
larger Jeans length ?J s2/ Sg tff s/ Sg
instability as soon as Q s?/ Sg 1 For the same
ratio s/ Sg, complexes of masses M Sg ?J2 Ms4/
Sg will condense on the same time-scale
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Slope 1
Tight correlation CO-100µ, supports Black-Body
model Small sizes, N(H2) gt 1024 cm-2, gt t 1
at 100µ for the dust
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Black- Body model
LFIR 4 p R2 s Td4 (no
term in t ?ß) LCO 4 p2 R2 (2k/?2) sƒ
Tbd? LFIR/LCO Td3/(fv ?V) 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
Myr LFIR 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 gt
stars M/LFIR 5 10-3 Mo/Lo (L/M 200) If
1012Lo comes from accretion onto a black hole,
at the efficiency of L 0.1 dm/dt c2, the
accretion must be only 1 Mo/yr, and therefore
only 1 of the gas would be 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 and torques that drive the
gas towards the center, with the help of a small
rate of dissipation This depends essentially on
the stability of the disk prior the interaction,
therefore on the bulge-to-disk ratio Finally the
role of the geometry of the interaction is
secondary direct 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 SF according to the pericenters
Star formation can be delayed
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Mihos Hernquist 96
Simulations of disk/halo galaxies
Gas and young stars are plotted
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High Density Tracers
Nuclei of Galaxies possess denser gas GMC to
survive to tidal forces must be denser High-J
levels of CO higher critical density to be
excited (gt105cm-3) as well as HCN, HCO, CS,
CH3OH, H2CO, OCS, etc.. SiO traces shocks (for
instance supershells in starbursts) Isotopic
studies primary or secondary elements can trace
the age of the star formation events
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M82 Mao et al (2000)
High J-levels of CO Images are roughly similar in
morphology although somewhat less extended than
CO(1-0)
Two hot spots on either side of the nucleus Part
of the molecular torus seen edge-on Ring due to
the bar (or also void due to starburst?)
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LVG model N(H2) 1023cm-2 M(H2) a few
108Mo n(H2) 104cm-3 close to the tidal limit
Emission comes primarily from PDR
photon-dominated regions quite different from
the other high density tracers
Two components in the molecular gas dense cores,
intercloud A diffuse component intervenes in
the CO emission, also CI/CO is high Is this
representative of starburst at high z?
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Kinetic temperatures derived are 20-60K, rather
low Heating 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 supershells not the
star forming regions Vertical filament SiO
chimney coincident with radio cm emission Gas
ejected by the starburst Shock chemistry
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M82, CO3-2, 2-1, 1-0 _ __
- -
Isotopic ratio of about 10-15 for 12CO/13CO gt
Opticall thick gas TA (Tex -Tbg) (1 - e-t)
If optically thin R(21/10) --gt 4
Survey of CO(3-2) in 30 spiral galaxies
(Mauersberger et al 99) R(32/10) 0.2-0.7,
predicted if Tkin lt 50K and n(H2) lt 103cm-3
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High density tracers, at low temperatures CS,
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 92 HCN in IC342 Same morphology
than in CO 2 spiral arms winding up in a ring
CO/HCN ratio from 7 to 14 going outwards The 3mm
continuum is free-free, not thermal 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
radius towards 10-20 in the center Tracer of the
astration, 13C is secondary In the Galactic
Center, also deficiency of deuterium In
Starbursts and ULIRGS (Arp220 type), CO/13CO
larger Not due to a low optical depth, since C18O
is normal with respect 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 98 from CN, HCN, HCO
observations Always 12C/13C gt40 (not as low as
in the Galactic Center) 16O/18O gt 100, 14N/15N gt
100 HC15N detected in LMC and N4945 (Chin et al
99) 14N/15N 111 lower than in the Milky
Way gt15N is synthesized by massive
stars Controversial about this formation
destruction in H-burning formation in SN-II, 14N
more secondary, and the ratio increase with time
and astration
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Deuterated species LMC DCN, DCO Ratios about
20 strong fractionation
D/H 2 10-5, but the deuterated molecules have
lower energies At low temperature HD HCN --gt 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 temperature HNC (hydrogen isocyanide)
is a high density tracer as well HNC is weaker
than HCN, except in ULIRGS such as Arp 220 where
it is gt 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-density subthermally
excited, clumpy structure gt point out very
different physical conditions and various
chemistry, from one galaxy to the next
(Huttemeister et al 97)
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Methanol asymmetric top A-type, E-type lines
blended n(H2) gt 105cm-3
Atomic Carbon CI fine structure line 3P1-3P0 at
492 GHz important 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 line and dust opacity
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CI/CO 0.2 (in Kkm/s) cooling comparable Normally
smaller than C (except Arp220 and Mkn231!)
Gerin Phillips 2000
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Conclusions
The molecular component is much more important in
starbursts and 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
flow Explains the transformation of HI --gt H2
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Only in strong starbursts is the H2 gas dense
enough to emit 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?