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)