C1 ISM Lecture6

8/3/2019 C1 ISM Lecture6

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The ISM HI and Molecular tracers

The Rosette nebula as

seen by Herschel

90% of the atoms are HydrogenCan be present as Molecular, H2, Atomic, H I,or Ionized H II

Observations of all 3 phases may be required to build acomplete picture. The dominant state of regions, and

indeed of the whole Universe, has changed over time.

Measurement of the 3 different states of hydrogensample different physical conditions and use differenttechniques. Molecular hydrogen can be probed at mid/near-IR (vib-ro) or

UV (electronic) wavelengths. But its lack of dipole momentgives weak emission and so CO is often used as a proxy.

Atomic hydrogen is probed using the 21cm line H II is measured via hydrogen recombination lines.

21cm Hyperfine transition in H

Spin-flip transition in theground state of Hydrogen.1420MHz, 21cm

Very low probability:A ~3 x 10 -15 sec -1

Predicted in 1944, first detectedin 1951

But vast clouds of Hydrogenmake it easily detectable atracer of neutral Hydrogen inour (and other) Galaxy

Potentially very important fortracing the ionizationconditions in the earlyUniverse

The 21cm H lineWith A = 2.85 x 10-15, lifetime of the upper state is ~3 1014

s or 10 Myr.

The critical density is extremely low ncrit ~ 10-5cm-3 and so

collisional excitation ensures that H is in thermalequilibrium throughout the ISM.

The hyperfine levels have F= 1 and 0, giving statisticalweights for of 3 and 1 for the upper and lower statesrespectively.

With = h = 1.4x109 h, h/kT = 0.068/T and so is verysmall everywhere, such that

N1

N0

=

g1

g0

e(E/ kT)

3 and N H = N0 + N1 4N0


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21cm Emission Brightness

The line emissivity ku,l:

So that in the optically thin case, for the 21cm line the intensityper unit solid angle is

So we can determined directly the column density of atomichydrogen along the line of sight by measuring the brightnessof the 21cm line (for an isothermal population) integrated overits width

I=

3

16A

ulh N

H dl

kul =gu

gl

NH

4Aulh

Emission and AbsorptionThe 21cm line can also appear in absorption against a backgroundsource with TB > Ts.

It has been used to map out the distribution of atomic hydrogenthroughout the galaxy.

Because it has such a small transition probability, the natural width is

very small, so velocity structure can be measured in detailThe ISM has cold clouds immersed in a diffuse warmer medium. Thecold clumps produce absorption spectra against a warm background.

From Dickey et al

2000 N157b a

supernova remnant

in the LMCEmission (top) andabsorption spectra towardsN157b in the LMC. Notethe weak absorption atV~ 260 and 285 km/sattributed to coolcondensations (T~30 K)

[from Mebold et al 1997]

The emission spectra widthsare generally broader thanthe absorption spectra,giving a picture of coldclumps within a more diffusewarm medium


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Position_Velocity diagrams

CO

Galactic Structure

Long wavelength lines (CO, 21cm,masers ) can be measured throughoutour Galaxy and used as probes ofGalactic structure

If we start with a simple model of circularorbits, the radial velocities measured

can be interpreted as distances.The Galactic centre is ~8kpc from the

sun, and the sun orbits the Galaxy at220km/s

Radial component of velocities areR sin for the sun and RggSinfor a gas cloud in orbit at radius R

Giving a differential radial velocity of(g ) Rsin

The velocity reaches a maximum at thetangent point, and coherent structurescan be traced as a position of Galacticlatitude and longitude

Galactic Structure

21cm and CO maps have delineated major Galactic features spiral arms,HII regions etc., separating different kinematic structures

For individual objects there can be ambiguity between near- and far-distances

H2 and other molecules

H2 is a symmetric, homonuclear molecule with nodipole moment, so it is a very inefficient radiator.

It can be traced using weak pure rotationaltransitions in the mid-IR, or via ro-vibrationaltransitions in the near-IR in photo- or shock-

excited regions.To trace molecular gas in the ISM, observations ofCO are usually used as a proxy for H2. CO is themost abundant molecule and has bright lines atmicrowave frequencies.

many other molecules can also be used, tracingdifferent environments and different chemistries.


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Molecules in Space

For detected molecules see:http://www.ph1.uni-koeln.de/

vorhersagen/

C detected in the IRD -detected in the UV

E -detected in the visibleG detected in the sub-mm

All others found in the radio(from Snow & Bierbaum 2008)

Molecular

TransitionsElectronic, vibrational and

rotational transitions havelarge energy differences

so that the effects are very

largely decoupled (Born-

Oppenheimer

approximation).These transitions occur in

the UV/optical, near-IRand microwave/radio

spectral regions and have

their own nomenclatureH2 has no dipole moment

so the vib-rotationaltransitions are weak, but

the electronic transitions

can be probed in the UV

along sightlines of low

extinction.

From Osterbrock

H2 along reddened sight lines (Rachford et al)Analysis of H2 line through curve

fitting


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Vibration-Rotation TransitionsLow frequency transitions fromcold gas.

For CO, the main tracer ofmolecular gas, the groundstate rotational transition J=1-0is at 2.6mm, whilst the 1-0vibrational transition is at4.7m.

The low-lying lines can be veryoptically thick, and so theisotopes 13CO or C18O oftenprovide better estimates ofcolumn density, though subjectto assumptions on isotopicratios.

Different moleculespreferentially probe differentphysical and chemicalconditions.

Characteristic diatomic molecular

emissionThe rotational transitions associated with each vibrationaltransition have a characteristic P and R branch appearancegiven byJ=-1 and J=+1

And hence

= v0

+2(J+1)B for the R branch and = v0 -2(J)B for the P branch

where the rotational constant B = h/82Iand Iis the moment of inertia of a rigid rotator I= r0

2

With increasing temperatures, higher rotational and vibrationallevels are populated, giving additional lines at higherfrequencies.

Carbon Monoxide probed through microwave rotational transitions or vib-rotational near-IR transitions. In the cores of dense cold clouds, COcondenses as ice.

Silicon Monoxide v=1 Fundamental BandThev=1 fundamental ro-vibrational band of SiO is at 8.1mEach vibrational level has rotational structure, leading to the P- and R-branches typical of a vib-rot band.With a model anharmonic oscillator the emission from each rotational level,J, is given by

where B and C are molecular constantsThe rotational and vibrational level populations depend on the temperatureand are reflected in the profile of the bandIn SN 1987A, the band profile indicates T~1500K for a thermal population

and with the mass of SiO ~ 4 x10-6 Mo on day 500I =1

4r2

hAijNi dV


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Silicon Monoxide MasersMaser emission can occur when

the population in an excited state

is higher than in the lower state,

and stimulated emission leads toamplification

Silicon Monoxide shows brightlaser emission in the V=1, J=1-0

and V=1, J=2-1 transitions.

The population inversion is caused

by pumping by infrared photons

and/or collisional excitation

SiO Maser emission in TX CamA Mira variable starwith a dense extendeddust shell.

The maser emissionarises predominantlyin the inner regions ofthe stellar shell where

T~1500K and variesas the star pulsates.

Movie from VLBAobservations overseveral months

(Diamond & Kemball2002 ApJ 599, 1372)

70 milli-arcsec ~ 30 AU

CO at high redshift in SDSSJ12143912.04+111740.5

(Srianand et al 2008) Excitation temperature at z= 2.42

The UV electronic transitions canbe used to estimate thetemperature of CO molecules

The CO excitation diagram forSDSS J143912.04+111740.5

A straight line with slope 1/(Tex

ln10) indicates thermalization of thelevels. The diagram is given for themain CO component at zabs =2.41837. The three lines give themean and 1 range obtained fromT01 , T02 , and T12 . The diagramis compatible with thermalizationby a black-body radiation oftemperature 9.15 0.72 K whenTCMBR = 9.315 0.007 K (longdashed line) is expected at zabs =2.4185 from a hot big-bang.

C1 ISM Lecture6

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