Molecules in Starbursts and Active Galactic Nuclei
Amiel Sternberg School of Physics & Astronomy Tel Aviv University
NGC 1068
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.
Photon-Dominated Regions (PDRs) X-ray Dominated Regions (XDRs)
Neutral hydrogen clouds where (keV) X-raysdominate 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.
Seyfert-2 Galaxy NGC 1068Orion Bar
PAH
COH2
PDRs: Basic Structure: 1-D steady-state model, escape probability method.
FUV6-13.6 eV
(“non-ionizing”)100 to 2000 K 10 to 100 K
Grain surface formation,H2 self-shielding.
Controlling parameters:
1) cloud density, pressure2) FUV intensity3) grain scattering properties4) H2 formation rate coefficient5) geometry/clumpiness6) gas phase abundances7) magnetic field...
Starburst Galaxy M82 (optical)
distance = 3.3 Mpc
vib
rotH2
infrared/submillimeter/millimeter spectroscopy (e.g. Genzel & Cesarsky 2000)
PDR emission lines
Dense PDRs:
Dense PDRs exposed to intense FUV fields in star-forming molecular cloudsare important as sources of luminous atomic fine-structure and molecularline (cooling) radiation, and far-IR thermal dust continuum emission.
Line to continuum ratiotypically 0.1 – 1%.
Far-infrared dust continuumis reradiated ultravioletstellar radiation.
Heating Mechanisms:
FUV photoelectric heating,beginning with Spitzer 1948 ApJ 107, 6
FUV-pumping of H2 in dense PDRs.
Sternberg & Dalgarno 1989 ApJ 338, 197
Leads to high gastemperatures
T > 1000 K
in the H/H2 layers.
CII 157.7 m Fine-Structure Line Cooling:
Expected theoretically- e.g. Dalgarno & McCray 1972 ARAA 10, 375
First far-IR (Lear jet) detections were inin NGC 2024 and Orion.Russell et al. 1980 ApJ 240, L99
Since then, widely detected withfar-infrared spectrometers, KAO, ISO...
…and most recently in the millimeter-waves, in a high redshift quasar. Maiolino et al. 2005 AA 440 L51
Tielens & Hollenbach 1985 ApJ 291, 722
CNM
WNM
Wolfire et al. 2003
densePDRs
T (K)
CII
157.7
m
(e
rg s
-1 p
art
icle
-1)
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 eVtypical 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).
Fine-Structure Emission Lines in Star-Forming Molecular Clouds:
FUV intensity G0
line inte
nsi
ty (
erg
cm
-2 s
-1 s
r-1)
line inte
nsi
ty (
erg
cm
-2 s
-1 s
r-1)
]CII [157.7 m
[OI] 63.2 m
Fine-Structure Line Emissions from PDRs:
Hollenbach & Tielens 1997 ARAA, 35, 179Hollenbach & Tielens 1999 Rev. Mod. Phys., 71, 173
0.05
x f
ar-IR
0.05
x f
ar-IR
Molecular Diagnostics: CN / HCN in NGC 7023 (Reflection Nebula):
Reflection Nebula NGC 7023 (optical)
Molecular Diagnostics: CN / HCN in NGC 7023 (Reflection Nebula):
At interface:G0 = 2.4 x 103
n = 1.0 x 104 cm-3
Plateau de Bure & 30m
illuminating star(Herbig B3Ve)
Fuente et al. 1993 AA 276, 473 2003 AA 899, 913
Molecular Peak CN / HCN small
PDR PeakCN / HCN large
Distance from star (arcseconds)
Free Carbon and CN / HCN:
Boger & Sternberg 2005 ApJ 632, 302
G0 = 103
n = 104 cm-3
Free Carbon and CN / HCN:
Boger & Sternberg 2005 ApJ 632 302
G0 = 103
n = 104 cm-3
CN / HCN in the “Radical Region”:
CN peak forms where the C+ density is still high, butwhere the CN photodissociation rate is attenuated.
XDR hi
ghly
ioni
zed
regi
on
H H/H2 0.01 H2
xe 10-2 – 10-1 xe 10-3 – 10-2 xe < 10-3
High HXray / n ……………………..…… Low HXray / nkeVX-rays
HXray = X-ray photon energy deposition rate
n = gas density
X-ray Dominated Regions (XDRs):
Maloney, Hollenbach & Tielens 1996Sternberg, Yan & Dalgarno 1996Yan & Dalgarno 1997Meijerink & Spaans 2005
T ~ 104 K T ~ 2000 K T < 200 K
C+, C C, C+ CO, C, C+
O O O, OH, O 2, H2O
large penetrationdepth
ionization
excitation
Coulomb scattering
X-ray Photon Energy
Secondary Electron Energy Deposition:
H2 + Xray H2+ + e*
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
Energy lost to e* per ion-pair produced = 37.7 eVDalgarno, Yan & Liu 1999 ApJS, 125 237
So, for a molecular XDR, theX-ray heating efficiency
15.4 / 37.7 = 0.4
…much higher than in PDRs.
UV decay:
photo-dissociation
and
dust heating, far-IR
“fast electron”
n = 103 cm-3
Fine-Structure Line Emissions from XDRs:
far-IR continuum
Maloney, Hollenbach & Tielens 1996 ApJ 466, 571
Extragalactic PDRs and XDRs
“The Antennae”NGC 4038/4039 Interacting Galaxies
Hubble Space Telescope (optical)
Infrared Space Observatory (10 m)
Keck
FUV-Pumped Molecular Hydrogen in the Antennae:
Mid-IR Cluster Gilbert et al. 2000 ApJ 533, L57
H2 vibrationalemission lines
FUV Pumping and Photodissociation of Molecular Hydrogen:
Near IR
FUV
Pumping
Dissociation
Effe
ctiv
e P
ote
ntia
l En
erg
y
v=0
12
3
Internuclear Separation
En
erg
y
Black & Dalgarno 1976Black & van Dishoeck 1987Sternberg 1988Sternberg & Dalgarno 1989Draine & Bertoldi 1996Sternberg & Neufeld 1999 Shaw et al. 2005 CLOUDY
NGC 4038/4039 interacting galaxies CII emission from NGC 4038/4039 Nikola et al. 1998 ApJ 504, 749
Dense PDRs dominate:
G0 = 500nH = 105 cm-3
(CNM a small fraction.)
High Redshift Detection of the CII 157.7 m Line:
Maiolino et al. 2005 AA 440 L51IRAM 30-meter
z = 6.42 quasar J1148+5251
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)
Starburst Galaxy M82 (optical)
distance = 3.3 Mpc
CN / HCN in M82
Fuente et al. 2005 ApJ 619 L155
CN/HCN ~ 5across 650 pcstar-forming disk.
PDR signature.
XDRs and the Active Nucleus in NGC 4258:
Central luminosity providedby an accreting, super-massive, black hole.
X-rays.
distance = 7.3 Mpc
NGC 4258:Water Masers Around the Supermassive Black Hole in NGC 4258:
red-shifted maser spots
blue-shifted maser spots
In the galaxy center:A resolved Keplerian diskwith orbiting water maser spots.
v2 = GM / R
M = 3 x 107 M
ortho - H2O
22 GHzEn
erg
y (c
m-1)
rotational quantum number”
XDR Production of H2O:
MASING
Neufeld, Maloney & Conger 1994 ApJ 436, L127
H2O Formation:
OH H2O
ee
H3O+H2O+OH+
H3+
H2 H2
O
ion-molecule (cold)
Herbst & Klemperer, 1973, ApJ, 185, 505
Prasad & Huntress, 1980, ApJS, 43, 1
H2 H2
neutral-neutral (warm)
Neufeld & Dalgarno 1989, ApJ, 340 869 (J-shocks)
Sternberg & Dalgarno 1995, ApJS, 99, 565 (PDRs)
Maloney, Hollenbach & Tielens 1996, ApJ, 466, 571 (XDRs)
NGC 1068 (optical)
Molecules in the Seyfert-2 Galaxy NGC 1068 (M77):
1 kpc
distance = 15.5 Mpc
Molecules in the Seyfert-2 Galaxy NGC 1068:
100 pcIRAM 30m + PdB
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
XDRs in NGC 1068 ?
In the nucleus, HCN/CO 10-3 (large!)……possible signature of enhanced ionization rate in molecular gasexposed to X-rays.
Lepp & Dalgarno 1996, AA, 306, L21
Usero et al. 2004, AA, 419, 897
Mgas 5 x 107 M
LXray 2 x 1011 L
fract
ional abundance
n / -17 (cm-3)
gas-density / ionization-rate
“high-ionization phase”
“low-ionization phase”
Boger & Sternberg 2005 ApJ 632, 302 2006 ApJ in press
High-ionization rategives large HCN / CO
…and also large
CN / HCN
as observed in thenucleus of NGC 1068.Usero et al. 2004
Note:
C / CO
is large in the high-ionizationphase.
XDRs versus PDRs?
In XDRs multiply charged atomic ions co-exist with neutral H, H2 and He.
Produced via inner-shellphotoionization,& 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+.
A Golden Age!
Space Infrared Telescope Facility (SIRTF)
Stratospheric Observatory for Infrared Astronomy
Atacama Large Millimeter Array (ALMA)
Herschel Space Observatory (submillimeter)