Molecules in Starbursts and

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Molecules in Starbursts and Active Galactic
Nuclei Amiel Sternberg School of Physics
Astronomy Tel Aviv University
NGC 1068
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Photon-Dominated Regions (PDRs)
X-ray Dominated Regions (XDRs)

• Neutral interstellar hydrogen clouds where
• 6-13.6 eV far-ultraviolet (FUV) radiation
• dominates the chemistry and gas heating.
• FUV photon penetration limited by
• dust absorption.
• Heating by photo-electric emission of
• electrons from dust grains.
• Chemistry driven by FUV ionization.

• Neutral hydrogen clouds where (keV) X-rays
• dominate the chemistry and gas heating.
• X-ray penetration limited by photoionization
• of the heavy elements. (Greater photon
• penetration lengths than in PDRs).
• Heating by X-ray photoionization of H and H2.
• (More efficient gas heating than in PDRs).
• Chemistry driven by X-ray ionization.

Orion Bar
Seyfert-2 Galaxy NGC 1068
PAH
CO
H2
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Photon-Dominated Regions
Broadly defined, PDRs include
(8000? K)
(100? K) Diffuse warm neutral
medium (WNM) and cold neutral medium (CNM).
Translucent clouds moderate optical
extinction (AV lt 5),
low gas densities, nH lt 1000 cm-3
weak FUV
fields. Dense molecular clouds high
extinction (AV 10), nH gt 1000 cm-3

intense FUV fields near massive and hot
OB-type
stars. 90 of the Galactic
molecular ISM may be photon-dominated.


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PDRs Basic Structure
1-D steady-state model, escape probability
method.
Grain surface formation, H2 self-shielding.
Controlling parameters 1) cloud density,
pressure 2) FUV intensity 3) grain scattering
properties 4) H2 formation rate coefficient 5)
geometry/clumpiness 6) gas phase abundances 7)
magnetic field . . .
FUV 6-13.6 eV (non-ionizing)
100? to 2000? K
10? to 100? K
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Dense PDRs
Dense PDRs exposed to intense FUV fields in
star-forming molecular clouds are important as
sources of luminous atomic fine-structure and
molecular line (cooling) radiation, and far-IR
thermal dust continuum emission.
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Far-Ultraviolet (FUV) Radiation Field (6 - 13.6
eV 912 - 2070 Å)
Empirically based for solar neighborhood. Habing
1968 BAN 19, 421 Draine 1978 ApJS 36, 595
Produced by massive and hot 3-120 M? OB type
stars, in associations and clusters. Parravano,
Hollenbach McKee 2003 ApJ 584, 797
G0 ? 1
interstellar FUV field
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Radiation Fields of Evolving OB Clusters
Sternberg, Hoffmann Pauldrach 2003, ApJ, 599,
1333
G0 gtgt 1
FUV EUV
Intensity and spectral shape of FUV field depend
on cluster mass and age.
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Heating Mechanisms
FUV photoelectric heating, beginning with
Spitzer 1948 ApJ 107, 6
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CII 157.7 ?m Fine-Structure Line Cooling
Tielens Hollenbach 1985 ApJ 291, 722
Expected theoretically - e.g. Dalgarno McCray
1972 ARAA 10, 375 First far-IR (Lear jet)
detections were in in NGC 2024 and Orion. Russell
et al. 1980 ApJ 240, L99 Since then, widely
detected with far-infrared spectrometers, KAO,
ISO... and most recently in the
millimeter- waves, in a high redshift quasar.
Maiolino et al. 2005 AA 440 L51
CII 157.7 ?m (erg s-1 particle-1)
T (?K)
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Fine-Structure Emission Lines in Star-Forming
Molecular Clouds
Accounting for CII 157.7 ?m
OI 63.2 ?m 145.6 ?m

CI 609.2 ?m 229.9 ?m
and other low-ionization
fine-structure emission lines
observed in star-forming molecular clouds.
Line strengths
0.1 - 1 of far-IR dust continuum.
FUV (6-13.6 eV) heating via photoelectric
emission from dust grains mean photon energy
10 eV typical grain work function 6
eV (for neutral grains). photoelectric
yield 0.1 Thus, heating efficiency
(4/10) x 0.1 0.04 (smaller for positively
charged grains).
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Fine-Structure Line Emissions from PDRs
CII 157.7 ?m
Hollenbach Tielens 1997 ARAA, 35,
179 Hollenbach Tielens 1999 Rev. Mod. Phys.,
71, 173
line intensity (erg cm-2 s-1 sr-1)
OI 63.2 ?m
line intensity (erg cm-2 s-1 sr-1)
FUV intensity G0
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Neutral Atomic Carbon
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Polycyclic Aromatic Hydrocarbons (PAHs)
Herbig B3Ve star
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PAHs in PDRs
Bakes Tielens 1998 ApJ 499, 258
mutual neutralization C PAH- ? C
PAH competes with or dominates radiative
recombination C e ? C
photon Lepp Dalgarno 1988 ApJ 335, 769
dashed without PAHs solid with PAHs
(2 x 10-7)
Similarly, grain-assisted recombination may
become important for fractional ionizations
xe lt 10-3 . Draine Sutin 1987
ApJ 320, 803 Weingartner Draine 2001 ApJ 563,
842
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Molecular Diagnostics CN / HCN in NGC 7023
(Reflection Nebula)
Reflection Nebula NGC 7023 (optical)
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Molecular Diagnostics CN / HCN in NGC 7023
(Reflection Nebula)
Fuente et al. 1993 AA 276, 473
2003 AA 899, 913
At interface G0 2.4 x 103 n 1.0 x 104
cm-3
Plateau de Bure 30m
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Free Carbon and CN / HCN
Boger Sternberg 2005 ApJ 632, 302
G0 103 n 104 cm-3
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Free Carbon and CN / HCN
Boger Sternberg 2005 ApJ 632 302
G0 103 n 104 cm-3
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CN / HCN in the Radical Region
CN peak forms where the C density is still high,
but where the CN photodissociation rate is
attenuated.
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X-ray Dominated Regions (XDRs)
XDR
large penetration depth
H H/H2 ? 0.01
H2
T 104 K T 2000 K
T lt 200 K C, C
C, C
CO, C, C O
O
O, OH, O2, H2O
highly ionized region
xe ? 10-2 10-1 xe ? 10-3 10-2
xe lt 10-3
High HXray / n .. Low HXray / n
keV X-rays
HXray X-ray photon energy deposition rate n
gas density
Maloney, Hollenbach Tielens 1996 Sternberg, Yan
Dalgarno 1996 Yan Dalgarno 1997 Meijerink
Spaans 2005
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Secondary Electron Energy Deposition
fast electron
X-ray Photon Energy
H2 Xray ? H2 e
Coulomb scattering
Energy lost to e per ion-pair produced 37.7
eV Dalgarno, Yan Liu 1999 ApJS, 125 237 So,
for a molecular XDR, the X-ray heating efficiency
? 15.4 / 37.7 0.4 much higher than in
PDRs.
ionization
e H2 ? H2 e e H2 H2 ?
H3 H H3 e ? H2 H So almost all
of the ionization energy (15.4 eV) is available
for gas heating. Glassgold Langer 1974 ApJ
186, 159
excitation
UV decay photo- dissociation and dust
heating, ? far-IR
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Fine-Structure Line Emissions from XDRs
Maloney, Hollenbach Tielens 1996 ApJ 466, 571
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Extragalactic PDRs and XDRs
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The Antennae NGC 4038/4039
Interacting Galaxies Hubble Space Telescope
(optical)
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FUV-Pumped Molecular Hydrogen in the Antennae
Gilbert et al. 2000 ApJ 533, L57
Mid-IR Cluster
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FUV Pumping and Photodissociation of Molecular
Hydrogen
Effective Potential Energy
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NGC 4038/4039 interacting galaxies
Dense PDRs dominate G0 500 nH 105
cm-3 (CNM a small fraction.)
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High Redshift Detection of the CII 157.7 ?m Line
z 6.42 quasar J11485251 LFIR 2.2 x
1013 L? LCII 4.4 x 109 L?
Star-formation rate ? 3000 M ? yr-1 LCII
/ LFIR 2 x 10-4 small! (similar
deficit observed in ULIRGs)
CII
CO(6-5)
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Luhman et al. 2003 ApJ 594, 758

• Possible Explanations
• G0 / n gtgt 1 cm3 in ULIRGs
• inefficient photoelectric gas
  heating.
• Underabundance of OB stars (unlikely).
• 3) Self absorption.
• 4) Absorption of UV photons in dusty
• HII regions. Requires high ionization-
• parameters (U ? ?dust).
• Is this consistent with nebular
• emission-line excitation? TBD

(basic assumption FIR is reradiated FUV energy.)
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CN / HCN in M82
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XDRs and the Active Nucleus in NGC 4258
distance 7.3 Mpc
Central luminosity provided by an accreting,
super- massive, black hole. X-rays.
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Water Masers Around the Supermassive Black Hole
in NGC 4258
NGC 4258
In the galaxy center A resolved Keplerian
disk with orbiting water maser spots. v2 GM
/ R M 3 x 107 M?
Energy (cm-1)

rotational quantum number
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XDR Production of H2O
Neufeld, Maloney Conger 1994 ApJ 436, L127
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H2O Formation
Herbst Klemperer, 1973, ApJ, 185, 505 Prasad
Huntress, 1980, ApJS, 43, 1
ion-molecule (cold)
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Molecules in the Seyfert-2 Galaxy NGC 1068 (M77)
distance 15.5 Mpc
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Molecules in the Seyfert-2 Galaxy NGC 1068
IRAM 30m PdB
100 pc
Tacconi et al. 1994, ApJ, 426, L77 Sternberg,
Genzel Tacconi 1994,
ApJ, 436, L131 Helfer Blitz 1995, ApJ,
450, 90 Usero et al. 2004, AA, 419, 897
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XDRs in NGC 1068 ?
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Boger Sternberg 2005 ApJ 632, 302
2006 ApJ in press
High-ionization rate gives large HCN /
CO and also large CN / HCN as observed
in the nucleus of NGC 1068. Usero et al. 2004
Note C / CO is large in the
high-ionization phase.
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XDRs versus PDRs?
In XDRs multiply charged atomic ions co-exist
with neutral H, H2 and He.
Produced via inner-shell photoionization,
Auger decay.
Multiply charged species may persist when
charge-transfer neutralization is slow, e.g.,
C2 or S2
S III 33.5 ?m 18.7 ?m
fine-structure emissions as
XDR diagnostics. or if react with H2
lead to enhanced CH or SH.
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A Golden Age!