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A671: Lecture - 1

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Ionized Gas: Line Emission

Lecture 1

Terry Herter

A671 - 1Ionized Gas Emission 2

TopicsIntroduction

HII regions & Planetary Nebulae

Recombination Line EmissionHI, HeI

Collisionally Excited Line EmissionAbundancesAtomic StructureDensity limits / Critical density

Some sample Spitzer galaxy spectra

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ExamplesGas ionized by an energetic radiation field

Lots of photons with hν > 13.6 eVExamples -

Tstar NH Telectron

(104 K) (cm-3) (104 K)HII regions 30 - 50 10 - 105 0.5 - 1.0PNe 80 - 600 103 - 107 1.0 - 1.5AGNs ν-1 - ν-2 ~5 × 103 1.0 - 2.0


HII Regions

Molecular cloud collapses, forming stars.

Ionized Hydrogen (HII) regions surrounding newly formed stars.

Note – these regions have lots of structure and are very clumpy (see images that follow).

A671: Lecture - 1

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M8 - Lagoon Neb.

Trifid (M20

NGC 6559

M16 -Eagle

Nebula

NOAO Image

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Pillars in M16

HST Image

M16: Close-up

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M16

10 ly

Orion Close-up

APOD: 01/20/06

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Planetary Nebulae

Outer layers of red giantare ejected into ISM. Hot core ionized gas

surrounding star..

Again – clump structure is evident (see images that follow).

Helix Nebula (NGC 7293)

APOD: 01/12/2006Spitzer

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Helix Nebula (NGC 7293)

APOD: 12/29/2004


Some handy numbersIonization thresholds:

HI 13.6 eV 912 A 3.29 × 1015 HzHeI 24.5 eV 506 A 5.92 × 1015 HzHeII 54.4 eV 228 A 13.2 × 1015 Hz

For peak of BB to be at ~912 AT ~ 32,000 KImplies a rough lower mass cut-off for HII regions

Conversions:λ(A) = 12398 / E(eV) (1 eV = 1.602 × 10-12 ergs)ν(1015Hz) = 0.2418 eV

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Photoionization cross-sections

0.001

0.01

0.1

1

10

0 10 20 30 40 50 60

HIHeII

ν (1015 Hz)

a ν(1

0-18

cm2 )


Kurucz model atmospheres

1.E-11

1.E-10

1.E-091.E-08

1.E-07

1.E-06

1.E-051.E-04

1.E-03

1.E-02

10 100 1000

T = 50000T = 45000T = 40000T = 35000T = 30000

log g = 4.5

λ (nm)

Hν

(erg

s/cm

2 /s/H

z)

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Sizes of HII regionsrS(pc)

Type T*(K) log(NLyc) N=1 N=104 cm-3

O5 48,000 49.67 108 0.23O6 40,000 49.23 74 0.16O7 35,000 48.84 56 0.12

O9.5 31,000 47.95 29 0.06B0.5 26,200 46.83 12 0.03

( )0

23

34

HQ

NrN BHSLyc

≡

= απNLyc = the number of Lyman continuum photons emitted per second by the star, rS ≡ rHII is called the Strömgren radius, αB = αA - α1 is the recombination coefficient to n = 2


Schematic HII/HeII regions

H+, He0

H+, He+

H+, He+

H0 H0

Hot Star Hotter Star

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Hydrogen Line

Emission Lyα Lyβ Lyγ

Hα Hβ Hγ

Lyman Lines

Balmer Lines2

34

n=1Ground State = Lowest Energy Level

∞

0 eV

10.20 eV

12.09 eV12.75 eV

Wavelength (µm)To nSeries

7.5014.6532.6251.2820.48611.026β

5.9073.7402.1661.0940.43400.973γ

5.12912.373.2826Humphreys3.2967.4582.2795Pfund1.9454.0511.4594Brackett1.0051.8750.82063Paschen0.41020.65630.36472Balmer0.9491.2150.09121LymanδαLimit


Effects of HeHe competes for ionizing photons with H.

aν(HeI) >> aν(HI) for ν > 24.6 eVOpacity is increased.Supplies electrons

~10% contributionsRecombinations that for HeI contribution to the diffuse radiation field.

Recombinations to n=1 can ionized both He and H.Recombinations that populate n=2 can result in photons that can ionize H. See (AGN)2 for further details on including He.

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HeI energy level diagram1S 1P 1D 3S 3P 3D

Ener

gy (e

V)

628 A

10830 A584 A

202.058 µm21


What ions have lines?What ions are present?

The most abundant species are most importantExpect ionic states of C, N, O

Ionization potentials (in eV):

Ion I -> II II -> III III-> IV IV -> VC 11.256 24.376 47.871 64.476N 14.53 29.593 47.426 77.45O 13.614 35.108 54.886 77.394

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aν: multiple electron atoms

ν1ν2

ν3ν4

log ν

log σ b

f

For multi-electron atoms, transition can occur from various energy levels to the continuum.


Elemental AbundancesAbundances of the Chemical Elements

1

10100

1000

10000

1000001E+06

1E+07

1E+08

1E+091E+10

1E+11

1E+12

He

Be C O Ne

Mg Si S Ar Ca Ti Cr

Fe Ni

Zn Ge

Chemical Element

Rel

ativ

e A

bund

ance

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Periodic TableH1

1s

Li3

2s

Na11

3s

K19

4s

Rb37

5s

Cs55

6s

Be4

2s2

Mg12

3s2

Ca20

4s2

Sr38

5s2

Ba56

6s2

C6

2s22p2

N7

2s22p3

O8

2s22p4

F9

2s22p5

Ne10

2s22p6

B5

2s22p

He2

1s2

Si14

3s23p2

P15

3s23p3

S16

3s23p4

Cl17

3s23p5

Ar18

3s23p6

Al13

3s23p

Ge32

4s24p2

As33

4s24p3

Se34

4s24p4

Br35

4s24p5

Kr36

4s24p6

Ga31

4s24p

Sn50

5s25p2

Sb51

5s25p3

Te52

5s25p4

I53

5s25p5

Xe54

5s25p6

In49

5s25p

Pb82

6s26p2

Bi83

6s26p3

Po84

6s26p4

At85

6s26p5

Rn86

6s26p6

Tl81

6s26p

Zn30

3d10

4s2

Cu29

3d9

4s2

Ni28

3d8

4s2

Co27

3d7

4s2

Fe26

3d6

4s2

Mn25

3d5

4s2

Cr24

3d4

4s2

V23

3d3

4s2

Ti22

3d2

4s2

Sc21

3d4s2

Outer electron configurations of neutral atoms in their ground states are shown.

Cd48

4d10

5s2

Ag47

4d9

5s2

Pd46

4d8

5s2

Rh45

4d7

5s2

Ru44

4d6

5s2

Tc43

4d5

5s2

Mo42

4d4

5s2

Nb41

4d3

5s2

Zr40

4d2

5s2

Y39

4d5s2

Hg80

5d10

6s2

Au79

5d9

6s2

Pt78

5d8

6s2

Ir77

5d7

6s2

Os76

5d6

6s2

Re75

5d5

6s2

W74

5d4

6s2

Ta73

5d3

6s2

Hf72

5d2

6s2

La57

5d6s2

IIb

Ib

VIIIb

VIIb

VIb

Vb

IVb

IIIb

Ia

IIa VIIaVIaVaIVaIIIa


Electronic configurationsWhat do these ions look like?What transitions are expected?For the most abundant ions, the outer shell electrons will be in a p configuration, i.e. p1, p2, .. p6

For example: OI -> p4, OII -> p3, OI -> p2, etc.Residual interactions split these states

giving “fine-structure”Transitions between these states are “forbidden” because electric dipole decay can not occur

magnet dipole or electric quadruple decay

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LS fine-structure spitting (p2)

npn’p

S = 0

S = 1

1S0

1P11D2

3S1

3P0,1,2

3D1,2,3

210

31

2Unperturbed

State Spin-Spin

ResidualElectrostatic

Spin-Orbit

AllowedNot Allowed

(see R&L, Ch. 9.4)


p-electronic configurations

2P3/2

3P23P13P0

Ener

gy

2P1/24P3/2

1D2

1S0 2P

2D

3P03P13P2

1D2

1S0

2P1/22P3/2

p1 p2 p3 p4 p5

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p-electron ionsp1 p2 p3 p4 p5

CII CINIII NII NIOIV OIII OII OISIV SIII SII SINeVI NeV NeIV NeIII NeIIArVI ArV ArIV ArIII ArII


OIII exampleTransition A(s-1) Wavelength1D2 - 1S0 1.8 4363.2 A3P2 - 1S0 7.8×10-4 2331.4 A3P1 - 1S0 2.2×10-1 2321.0 A3P2 - 1D2 2.0×10-2 5006.9 A3P1 - 1D2 6.7×10-3 4958.9 A3P0 - 1D2 2.7×10-6 4931.0 A3P1 - 3P2 9.8×10-5 51.8 µm3P0 - 3P2 3.0×10-11 32.7 µm3P0 - 3P1 2.6×10-5 88.4 µm

3P23P13P0

1D2

1S0

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0 20 40 60 80 100 120 140Ionization Potential (eV)

BeBCNOF

NeNaMgAlSiPS

ClArK

CaScTiV

CrMnFe

IR transitions for different ions

609, 370 157

12.8 15.6, 36.0

63.2, 145.5 88.4, 51.8 25.9

24.3, 14.5

205, 122

11.3

57.3

656

14.4, 33 20.4, 11.8 6.7

7.32 9.04, 21.34.48 5.61, 13.5

2.9189.2130,69 34.8

61, 33 17.925.2, 56 33.5, 18.7 10.5

6.99 8.99, 21.8 13.1, 7.90 4.534.62 5.98, 15.4 8.82, 5.58 3.19

3.21 4.16, 11.5 6.15, 4.09

Transition wavelengths in µm.

24.9…17.9… 20.8, 26, 36.5 12.3, 14.7, …22.9, 33… 9.55, 7.81


Level populationsIn order to estimate the emission, we need to know the populations of the various levels in each ion.Population to a given level can occur via

collisional transfer of an electron from another levela radiative transition into the state

Depopulation can occur in the same mannercollisional transfer of the electron out of the levela radiative transition into another state

In equilibrium, population and depopulation of a level must be the same.

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Two level atom

q21 is the collision coefficient for moving an electron from level 2 to 1 (see Osterbrock, p50-51).

2

1

hν 121212212 qNNqNNAN ee =+

iNNN =+ 21

q12q21A21

22/32

22/1

02121

)21(4

2vvvvg

,m

hkT

d))f((q Ω⎟⎠⎞

⎜⎝⎛== ∫

∞

ππσ

( )∫∞

−Ω=Ω0

/);2,1()2,1( kTEkTE deE

Ignoring stimulated processes


Two level atom (cont’d)Ω(1,2) is the collision strength (of order unity, 0.2-7), calculated quantum mechanically.Via detailed balance arguments

kThegg

qq /

1

2

21

12 ν−=

Solving for the population level 2

211221

122 qNqNA

qNNNee

ei

++=

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j for 2-level atomWhich yields for the emissivity

211221

1221

21

21221

4

4

qNqNAqNNAh

ANhj

ee

ei

++=

=

πνπν

Limit for Ne → 0.

1221

4qNNhj eiπ

ν→

So that every collision results in a photon.


j for 2-level atom (Ne limits)Limit for Ne →∞.

kTh

kTh

i

i

eggegNAh

qqqNAhj

/21

/2

2121

1221

1221

21

4

4

ν

ν

πν

πν

−

−

+=

+=

Note that the levels are Boltzmann populated in this case, and j ∝ Ni (no Ne !).At high densities cooling (per ion per electron) is not as efficient because of collisional de-excitation.

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Critical densityCritical density -

The rough transition density from the low density to the high density regime.Collisional depopulations equal radiative depopulations

21

21212212 q

ANqNNAN ce =⇒=

If there are multiple paths we must sum over all of them.


Critical Densities

From Osterbrock & Ferland

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Multiple level atomThe previous discussion is easily extended to atoms with more than two levels.For any level m in an ion

∑∑∑∑≠<≠>

+=+mk

mkemmk

mkmmk

kmekmk

kmk qNNANqNNAN

We have the constraint that

ik

k NN =∑

Where Ni is the total number of ion of type i.


[SIII] 33.5/18.7 µm ratio

Similar plot for [OIII] 88/52 µm ratio

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Densities and Temperatures

Figure from Osterbrock & Ferland

3P23P13P0

1D2

1S0

88 µm

52 µm

5007 A


5 6 7 8 9 10 15 20 25 30 35λrest [µm]

0.01

0.10

1.00

f ν [

Jy]

NGC7714TotalColdWarmStarLinePAH

NGC 7714: Pure Starburst?Part of interacting system, z = 0.00933L = 5.6x1010 LsunThis is a relatively low extinction galaxy.Note dominance of PAH emission features in the spectrumAnd presence of numerous emission lines

7’

6450ANGC 7715z = 0.00924

ArII

SIV

NeII NeIII

SIII SIII

SiII

ArIII

20 kpc

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5 6 7 8 9 10 15 20 25 30 35λrest [µm]

0.1

1.0

f ν [

Jy]

Mrk463TotalColdWarmHotLine

Mrk 463 = UGC 8850Galaxy at z = 0.050L = 6.0x1011 LsunMerging, twin Seyfert 2 nuclei. Mrk 463e dominates in IRSilicate absorption feature and line emission prominent.

2.3’

R (6930A)

287 kpc

SIVNeII

NeV

NeIII

NeV

OIVSIII

Ne VI


5 6 7 8 9 10 15 20 25 30λrest [µm]

0.01

0.10

1.00

f ν [

Jy]

Mrk1014TotalColdWarmHotLinePAH

Mrk 1014Quasar at z = 0.163L = 4.1x1012 LsunRadio quiet, broad-line, dusty QSO with twin tidal tails.No obvious silicate emission, but ..Numerous emission lines and a number of PAH features seen.

4680A

2’

SIVNeII

NeV

NeIII

NeV OIV

PAHPAH

PAH

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