Molecules traced in absorption

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Molecules traced in absorption

• SAAS-FEE Lecture 7
• Françoise COMBES

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Advantages of the absorption
Absorption measures are very useful, in
particular in the Galaxy, where both emission
and absorption can be detected along the same
line of sight Obtention of the physical
conditions, T, N Spatial resolution with
absorption (QSO size) However, there is a bias
towards cold gas, for absorption In the
Rayleigh-Jeans domain TA (Tex -Tbg) (1 -
e-t) Emission when Tex gt Tbg
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For the atomic line HI at 21cm for instance large
influence of stimulated emission ("negative
absorption") since the ?T between the two levels
F1, 0 is only 0.7 K
ƒtdv N/T
In emission, N(cm-2) ƒTextdv ƒTadv gt
independent of temperature While the optical
depth of the absorption signal is in
1/T Experiences ON-source, and OFF-source
Ta(ON), Ta(OFF) gives Tex or Tsp
In the millimeter, CO rotation for instance at
2.6mm there exists the whole rotational ladder
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Emission depends on temperature, since Nu/Ntot
gu/Z e-Eu/kT Nu(cm-2) ƒTextdv ƒTadv if t ltlt
1, and Ntot T Nu eEu/kT Absorption
ƒtdv N/T (1- e-h?/kT) strongly weighted by
the temperature Tex Since collisional excitation
requires 4 104 cm-3 for CO, and 1.6 107 cm-3 for
HCN In hot (kinetic temperature) and diffuse
media, the excitation temperature will be very
low, --gt 2.76 K
Absorption is weighted by the diffuse medium
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Molecular absorption in the Galaxy
More difficult to observe, since continuum
sources are weaker (S ?-a) and smaller. Requires
interferometry to resolve and distinguish from
emission gt explains the work is recent (the
last decade) Marscher et al (1991) in front of
BlLac Small filling factor in surface, even of
the diffuse CO medium 9 3C sources/100 have CO
emission (Liszt Wilson 93) Among them, 60 show
absorption Extinction of only Av1 mag, but
already very abundant chemistry (Lucas Liszt,
1994)!
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Liszt Lucas 2001
Survey of 30 l.o.s. (Lucas Liszt 96) HCO
30 as often as HI abs more frequent than CO
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13CO, CN, HCO, HCN, HNC, C2H, N2H (Lucas
Liszt 94-98) with line ratios quite variable
from one l.o.s. to the other Big surprise, the
strength of HCO absorption, in these diffuse
media -- higher critical density, so HCO is
"cold" -- chemistry to be revised in diffuse
medium! Some lines are very optically thick
(13CO is detected) others t ltlt 1 (hyperfine
lines of HCN, in the ratio 531 expected) ?V
0.5 - 1km/s Abundances of CO versus HCO
variable by 20! Bistability? Chaos ? (Le Bourlot
et al 1993)
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Absorptions sometimes variable over a year
time-scale presence of clumpy material, of sizes
10-100 AU in front on the continuum source Also
spatial fluctuations in the chemistry (Liszt
Lucas 2000) CO can form rapidly from HCO in
diffuse clouds H2 can form at relatively low
density whenever H2 is there HCO/H2 2 10-9 and
then CO forms by recombination of HCO (CO turn
on) HCO is linearly correlated with OH X(HCO)
0.03-0.05 X(OH) even at low column density CO
forms later (when C is recombined) Diffuse
clouds have chemical abundances of dark clouds!
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Liszt Lucas, 96, 2000
OH and HCO tightly correlated at low column
density, contrary to CO
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Computed temperatures for gas spheres of N(H) 5
1020cm-2, according to density
CO and C column density for the same models
(Liszt Lucas 2000) H2 formation can occur at
low density, while HCO is present, but not
CO the C is still largely under C
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N(CO) increases abruptly when N(HCO) 1-2 1012
cm-2 slope of the power-law 1.5
CO and H2 column density from the UV (Federman et
al 95) Slope of the power-law is 2.02
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?V(HCO) 15 higher ?V(CO)
Surprisingly large 13CO abundance Fractionation,
much more efficient than selective
photodissociation 12CO 13C --gt 13CO 12C
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Extragalactic molecular absorptions

• When the line of sight of a radio-loud QSO
  crosses a galaxy, and
• also a molecular cloud (quite rare) gt
  absorption in the mm, cm
• Prolongation to the high column density of the
  Lya absorbers
• in particular DLA gt N(NH) power law
• Lya forest N 1013 cm-2 (intergalactic
  filaments)
• HI-21cm 1020 cm-2 (Damped Lya systems) Outer
  parts of galaxies
• CO, HCO.. 1020-24 cm-2 (the center of
  galaxies)
• The number (N) decreases as a power law

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Comparison with emission
The absorption technique is much more sensitive
than emission At high redshift for instance, the
detection limit is 1010Mo While the absorption
limit does not depend on redshift As soon as the
QSO source behind is detected, the
absorption limit is in optical depth t The
source is quasi ponctual at mm, up to
1012K Galactic versus extragalactic for MW
absorption studies, interferometer is required,
since absorption is generally buried among strong
emission of local molecular clouds The nearest
absorption is Centaurus A, where both are of the
same order
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Centaurus A
In CO line emission and absorption are detected
Many other lines are detected in absorption only
(Wiklind Combes 1997)
Eckart et al 90
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No temporal variation (Wiklind Combes 1997) gt
Constraints can be put on the source size, of gt
500 AU Low density gas, low excitation and low
Tkin optically thin lines Wide absorption in
HCO, could correpond to a nuclear disk
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Higher redshift absorptions
First high-z absorption towards the BLLac object
PKS1413135 (Wiklind Combes 1994), after many
unfruitful searches towards DLAs Since then, 4-5
systems are known, but remain rare Half of them
are gravitationally lensed objects PKS1830-211
and B0218357 The absorbing molecular clouds are
in the lensing galaxy gt a way to find molecules
in normal galaxies at high z Redshifts range up
to z1 (the QSO at z2), difficult to find
higher redshifts QSO, that are strong enough in
the mm (steep spectrum)
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In absorption, detected masses can be only 1
Mo Large variety of line widths, optical depths,
sometimes several lines are detected along the
same l.o.s.
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Selection of candidates -- Strong mm source
(0.15 Jy at 3mm) only 100-200 -- already an
absorption detected in HI-21cm, or DLAs, or MgII
or CaII -- absence of previous absorption, but
known gravitational lens (VLBI) (Webster et al
95, Stickel Kuhr 93) -- same as above, without
any known redshift the case of PKS1830-211 The
redshift was discovered in the mm sweeping of the
band (14 GHz 14 tuning, and already 2
lines) --sources where the redshift searched is
that of the QSO Mostly negative results!
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PKS1413135 z0.247
Very narrow absorption lt 1km/s (2 comp) BlLac,
very variable, also in radio optically thin,
N(H2) gt 1022 cm-2, Av gt 30 mag
McHardy et al 94
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Temporal variability, and small-scale structure
The opacity ratio between the two components has
varied by 2.3 over 2 years
Variations due to the l.o.s. change due to the
variability of the continuum source Superluminic
source Core unresolved 2.3mas or 7pc, might be
10µas 0.03pc
250km/s 50AU/yr insufficient (100yrs) gt 25 000
km/s required gt must come from the core
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Compatible with either a multi-component model
with similar filling factors or with dense
clumps embedded in a diffuse medium The diffuse
component accounts for most of the absorption,
while the clumps comprise most of the mass
Because of the very narrow velocity width the
cloud along the l.o.s. must be quite small 1pc
according to size/line-width relation n(H2) 104
cm-3 variability seen in the CO, not in
HCO (more optically thick) HCO more from the
diffuse component
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B3 1504377 z0.672
7 different molecular lines Large separation
330km/s nuclear ring spiral arm absorption
hosted by the source HNC/HCN gt Tkin Tex HCO
enhanced by 10-100 diffuse clumps
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B0218357 z0.685
Gravitational lens (two images A and B) The
largest column density 1024cm-2
Two images separation 335mas (1.8kpc)
All three CO isotopes are optically thick
This was an excellent oppotunity to search for O2
without atmospheric absorption Lines at 368 and
424 GHz O2/CO lt 2 10-3 (Combes et al 97) most of
O in OI??
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LiH tentative detection
HD and LiH cooling lines LiH 21K above ground
H2O detection at 557 GHz, very large t 40 000
H2O ubiquitous and cold T10-15 K H2O/H210-5
444GHz line of LiH, optically thin very narrow,
LiH/H2 3 10-12
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Two images A and B, separated by
HST (335mas) VLBA measurements (Patnaik et al
93, 95) The two images separated in A1, A2, B1,
B2 (lens potential non spherical)
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PKS1830-211 z0.88582
Frye et al 97
2 images, Einstein ring But 2 absorbing
systems, one at z0.19 seen in HI
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Two components, covering each one image of the
source as confirmed by PdB (Wiklind Combes
1998) Slight temporal variability
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Monitoring, measure of H0
The single dish (without resolving the 2 images)
can follow the intensity of the two, since they
are absorbing at two V Monitoring during 3 years
(1h per week) gt delay of 245 days, H0 69 12
km/s/Mpc
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Evolution of chemical conditions?
Various line ratios have been obtained in the
many absorptions at all z There does not seem to
be variations versus z0 (open circles) but large
scatter, even at z0 (Lucas Liszt 94, 06)
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Measure of Tmb (z)
Low excitation (diffuse gas) Tex Tmb The case
for PKS1830-211 Several transitions give the
same result (slightly lower, due to a microlens)
From UV H2 lines Srianand et al 2000
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Variation of constants
Kaluza-Klein theories, Strings and M-theory,
predict variations of a e2/hc fine structure
constant Heterodyne resolution R106
Method of Alkali doublet of many multiplet Webb
et al (2001) Appears to have a variation, at
high z only Murphy et al (2001) ? a / a
(-0.72 0.18 )10-5
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H2 bands in absorption, at high z
UV lines at high z Foltz et al (1988) N(H2)
1018cm-2, Ge Bechtold (97) z1.97 N(H2) 7
1019cm-2, T70K n 300cm-3 total N(H) 1020cm-2,
f(H2) 0.22 dust and strong CI Srianand et al
(2000), Petitjean et al (2000)
LMC
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PKS1232082 z2.3377 Petitjean et al 00
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Conclusion
Absorption is a precious tool to observe cold
gas diffuse, with low excitation Small masses
are detected Chemistry can be investigated Gas
in galaxies that are not ultra-luminous Bias in
the optical/UV towards low column density