Transcript Molecules traced in absorption

Molecules traced in absorption
SAAS-FEE Lecture 7
Françoise COMBES
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-τ)
Emission when Tex > Tbg
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For the atomic line HI at 21cm for instance
large influence of stimulated emission ("negative absorption")
since the ΔT between the two levels F=1, 0 is only ~0.7 K
ƒτdv ~ N/T
In emission, N(cm-2) ~ƒTexτdv ~ƒTadv
==> 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) ~ƒTexτdv ~ƒTadv if τ << 1, and Ntot ~T Nu eEu/kT
Absorption:
ƒτdv ~ N/T (1- e-hν/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, --> 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 ~ν-α) and smaller. Requires interferometry to resolve and
distinguish from emission
==> 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 Av~1 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 τ << 1 (hyperfine lines of HCN, in the ratio 5:3:1 expected)
ΔV = 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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ΔV(HCO+) = 15% higher ΔV(CO)
Surprisingly large 13CO abundance
Fractionation, much more efficient
than selective photodissociation
12CO
+ 13C+ --> 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) ==> absorption in the mm, cm
Prolongation to the high column density of the Lyα absorbers
in particular DLA ==> N(NH) power law
•Lyα forest N ~ 1013 cm-2 (intergalactic filaments)
•HI-21cm 1020 cm-2 (Damped Lyα 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 τ
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)
==> Constraints can be put on the source size,
of > 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 PKS1413+135
(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 B0218+357
The absorbing molecular clouds are in the lensing galaxy
==> a way to find molecules in normal galaxies at high z
Redshifts range up to z~1 (the QSO at z~2), 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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PKS1413+135 z=0.247
Very narrow absorption < 1km/s (2 comp)
BlLac, very variable, also in radio
optically thin, N(H2) > 1022 cm-2, Av > 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)
> 25 000 km/s required ==> must come from the21core
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 1504+377 z=0.672
7 different molecular lines
Large separation 330km/s
nuclear ring + spiral arm
absorption hosted by the source
HNC/HCN => Tkin = Tex
HCO+ enhanced by 10-100
diffuse + clumps
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B0218+357 z=0.685
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 < 2 10-3 (Combes et al 97)
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most of O in OI??
LiH tentative detection
HD and LiH cooling lines
LiH 21K above ground
H2O detection at 557 GHz, very large τ =40 000
H2O ubiquitous and cold
T=10-15 K
H2O/H2=10-5
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 z=0.88582
Frye et al 97
2 images, + Einstein ring
But 2 absorbing systems,
one at z=0.19 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)
==> delay of 24+5 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 z=0 (open circles)
but large scatter, even at z=0 (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 α = e2/hc fine structure constant
Heterodyne resolution R=106
Method of Alkali doublet
of many multiplet
Webb et al (2001)
Appears to have a variation, at
high z only
Murphy et al (2001)
Δ α / α =(-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) z=1.97
N(H2) = 7 1019cm-2, T=70K 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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PKS1232+082
z=2.3377
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
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