KES lecture Lyman-alpha FAB

The Hydrogen Lyman-alpha line in emission

Fabrizio Arrigoni Battaia

Lyman 1904

https://arxiv.org/pdf/1704.03416.pdf

Most of these slides use materials from:

Relative contributions to Universal energy/mass density

The Hydrogen Lyman-‐a line

Lya (2p -‐> 1s) à 1215.67 Å or 10.2 eV

Selection rules (electric dipole approx.)

Victor Schumann1896

1892

Chronicle of Lya Observations (extragalactic)

Partridge&Peebles 1967

“Are young galaxies visible?”

Hu&Cowie+ 1987

Lowenthal+ 1990 (and many more)

Heckman+ 1991Partrid

ge 1974

Davis&

Wilkinson1974

Schm

idt 1

963 QSO

’s discovery

1965 Ly

ain 3C 9 (z=2.012)

Spinrad+

1985 Ly

ain HzRGs

IUE satellite 1978

1980s C

CDs a

dven

t low-‐z AGN

(Fosbu

ry+1982;

Ferla

nd&Osterbrock1985)

1993 8-‐m

eter class

telescop

e era be

gins…

Møller&Warren+1993 Lya from DLA

Hu&Cowie+ 1987

Hu&McM

ahon

+1996

LAEs z~

4.5

Steide

l+ 1991

1990s HST time begins…

Steide

l+2000

Lyaim

aging of protocluster

Bergeron+ 1999

e.g., Bahcall+ 1993

Pentericci+2000

Lyaim

aging arou

nd HzRGs


Lya (2p -‐> 1s) à 1215.67 Å or 10.2 eV



Lya (2p -‐> 1s) à 1215.67 Å or 10.2 eV


A(2p-‐>1s) = 6.248e+08 s-‐1


Lya (2p -‐> 1s) à 1215.67 Å or 10.2 eV


A(2p-‐>1s) = 6.248e+08 s-‐1

A(3p-‐>2s) = 4.4101e+07 s-‐1

Lya emission mechanisms: collisions and recombination

Lya emission mechanisms: collisions

Hydrogen atom excited at the expenses of kinetic energy of free electrons.


Hydrogen atom excited at the expenses of kinetic energy of free electrons.

C(T ) / exp

✓�118348

T

◆

Black+1981

Fraction of cooling in Lya


Cooling rate for primordial gas(CIE assumption)

collisional excitation

collisional Ionization of H

collisional excitation of HeII

T sets ionization state


Lya emission mechanisms: recombination

Recombination of free proton and electron + radiative cascades to the ground state à Lya line


Recombination of free proton and electron + radiative cascades to the ground state à Lya line

Case A: medium optically thin at all photon frequencies

Case B: medium opaque to all Lyman series photons and to ionizing photons from direct recombination to ground state. “On the spot approx.”


P(Lya)=0.68 at T=104 K

Lya radiative transfer: the radiative transfer equation


emission


emission absorption

Lya radiative transfer: absorption

Line center/resonance Off line center

⌧0 = �↵NHI


emission absorption ``destruction’’

Lya radiative transfer: ``destruction’’

• Dust can absorb Lya photons. The dust grains increase their temperature and re-‐radiate at longer wavelengths. (dust cross-‐section basically constant)

• Molecular Hydrogen has two vibrational transitions close to Lya resonance (at 99 km/s and 15 km/s)

• 2g-‐photons emission : collisional mixing of 2p and 2s states

2g emission

2g (2s -‐> 1s)


A(2p-‐>1s) = 6.248e+08 s-‐1

A(2s-‐>1s) = 8.23 s-‐1

Lya radiative transfer: ``destruction’’

• Dust can absorb Lya photons. The dust grains increase their temperature and re-‐radiate at longer wavelengths. (dust cross-‐section basically constant)

• Molecular Hydrogen has two vibrational transitions close to Lya resonance (at 99 km/s and 15 km/s)

• 2g-‐photons emission : collisional mixing of 2p and 2s states

• Other minor processes, e.g. Lya photons can photoionize hydrogen atoms not in the ground state; ….


emission absorption ``destruction’’

scattering

Lya radiative transfer: scattering

Following absorption, re-‐emission occurs instantly à ``scattering’’


Following absorption, re-‐emission occurs instantly à ``scattering’’

Lya scattering is a double diffusion process

Lya scattering is a double diffusion process

x=

⌫�⌫

↵

⌫

↵v

th/c


Probability Distribution Function R(xout|xin) or redistribution functions

x =⌫ � ⌫↵

⌫↵vth/c

Lya radiative transfer: through an uniform, static gas cloud

Lya radiative transfer: through an uniform, static gas cloud

Analytic solutions available: Harrington 1973; Neufeld 1990; Dijkstra+2006

Orsi+2012

Lya radiative transfer: through uniform, expanding or contracting

gas cloud

Laursen+2009

Lya radiative transfer: dust…

Laursen+2009

Gronke+2017

You can find the videos at https://www.aanda.org/articles/aa/olm/2017/11/aa31013-‐17/aa31013-‐17.html

KES lecture Lyman-alpha FAB

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