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, NSpatial resolution with absorption (QSO size)

However, there is a bias towards cold gas, for absorption

In the Rayleigh-Jeans domainTA* = (Tex -Tbg) (1 - e-τ)

Emission when Tex > Tbg

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For the atomic line HI at 21cm for instancelarge 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 2.6mmthere 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 medium9 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 absmore 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-scalepresence of clumpy material, of sizes 10-100 AU in front on thecontinuum sourceAlso spatial fluctuations in the chemistry (Liszt & Lucas 2000)CO can form rapidly from HCO+ in diffuse clouds

H2 can form at relatively low densitywhenever H2 is there HCO+/H2 = 2 10-9

and then CO forms by recombination of HCO+ (CO turn on)

HCO+ is linearly correlated with OHX(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 correlatedat low column density,contrary to CO

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Computed temperatures for gas spheresof 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 COthe C is still largely under C+

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N(CO) increases abruptlywhen N(HCO+) = 1-2 1012 cm-2

slope of the power-law: 1.5

CO and H2 column density from theUV (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 abundanceFractionation, much more efficientthan 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, andalso a molecular cloud (quite rare) ==> absorption in the mm, cm

Prolongation to the high column density of the Lyα absorbersin 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 1010MoWhile the absorption limit does not depend on redshiftAs soon as the QSO source behind is detected, the absorptionlimit is in optical depth τThe source is quasi ponctual at mm, up to 1012K

Galactic versus extragalactic:for MW absorption studies, interferometer is required, since absorptionis 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

Eckart et al 90

In CO line emission and absorption aredetected

Many other lines are detected in absorptiononly (Wiklind & Combes 1997)

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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 anuclear 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 objectsPKS1830-211 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 higherredshifts QSO, that are strong enough in the mm (steep spectrum)

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In absorption, detected masses can be only 1 MoLarge variety of line widths, optical depths, sometimesseveral 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 PKS1830-211The redshift was discovered in the mmsweeping 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

McHardy et al 94

Very narrow absorption < 1km/s (2 comp)BlLac, very variable, also in radio

optically thin, N(H2) > 1022 cm-2, Av > 30 mag

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Temporal variability, and small-scale structure

The opacity ratio between the two components has varied by 2.3over 2 years

Variations due to thel.o.s. change due to the variability of the continuum source

Superluminic sourceCore unresolved 2.3masor 7pc, might be 10μas = 0.03pc

250km/s = 50AU/yr insufficient (100yrs)> 25 000 km/s required ==> 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, whilethe clumps comprise most of the mass

Because of the very narrow velocity widththe cloud along the l.o.s. must be quite small1pc according to size/line-width relationn(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 linesLarge separation 330km/snuclear ring + spiral armabsorption hosted by the sourceHNC/HCN => Tkin = Tex

HCO+ enhanced by 10-100diffuse + 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 isotopesare optically thick

This was an excellent oppotunity to searchfor O2 without atmospheric absorptionLines at 368 and 424 GHzO2/CO < 2 10-3 (Combes et al 97)most of O in OI??

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H2O detection at 557 GHz, very large τ =40 000

LiH tentative detection

H2O ubiquitous and coldT=10-15 KH2O/H2=10-5

HD and LiH cooling linesLiH 21K above ground

444GHz line of LiH, optically thinvery 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

2 images, + Einstein ring

But 2 absorbing systems,one at z=0.19 seen in HI

Frye et al 97

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Two components, covering each one image of the sourceas 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 followthe 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 ~TmbThe case for PKS1830-211

Several transitions give the sameresult (slightly lower, due to a microlens)

From UV H2 linesSrianand et al 2000

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Variation of constants

Kaluza-Klein theories, Strings and M-theory, predictvariations of α = e2/hc fine structure constantHeterodyne resolution R=106

Method of Alkali doubletof many multipletWebb 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=1.97N(H2) = 7 1019cm-2, T=70K n =300cm-3

total N(H) = 1020cm-2, f(H2) = 0.22 dust and strong CISrianand et al (2000), Petitjean et al (2000)

LMC

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PKS1232+082z=2.3377

Petitjean et al 00

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Conclusion

Absorption is a precious tool to observe cold gasdiffuse, 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