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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
The Hydrogen Lyman-‐a line
Lya (2p -‐> 1s) à 1215.67 Å or 10.2 eV
Selection rules (electric dipole approx.)
The Hydrogen Lyman-‐a line
Lya (2p -‐> 1s) à 1215.67 Å or 10.2 eV
Selection rules (electric dipole approx.)
A(2p-‐>1s) = 6.248e+08 s-‐1
The Hydrogen Lyman-‐a line
Lya (2p -‐> 1s) à 1215.67 Å or 10.2 eV
Selection rules (electric dipole approx.)
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.
Lya emission mechanisms: collisions
Hydrogen atom excited at the expenses of kinetic energy of free electrons.
Lya emission mechanisms: collisions
Hydrogen atom excited at the expenses of kinetic energy of free electrons.
C(T ) / exp
✓�118348
T
◆
Black+1981
Fraction of cooling in Lya
Lya emission mechanisms: collisions
Cooling rate for primordial gas(CIE assumption)
collisional excitation
collisional Ionization of H
collisional excitation of HeII
T sets ionization state
Lya emission mechanisms: collisions
Lya emission mechanisms: recombination
Recombination of free proton and electron + radiative cascades to the ground state à Lya line
Lya emission mechanisms: recombination
Recombination of free proton and electron + radiative cascades to the ground state à Lya line
Lya emission mechanisms: recombination
Recombination of free proton and electron + radiative cascades to the ground state à Lya line
Lya emission mechanisms: recombination
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.”
Lya emission mechanisms: recombination
Lya emission mechanisms: recombination
P(Lya)=0.68 at T=104 K
Lya radiative transfer: the radiative transfer equation
Lya radiative transfer: the radiative transfer equation
emission
Lya radiative transfer: the radiative transfer equation
emission absorption
Lya radiative transfer: absorption
Line center/resonance Off line center
⌧0 = �↵NHI
Lya radiative transfer: the radiative transfer equation
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)
Selection rules (electric dipole approx.)
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; ….
Lya radiative transfer: the radiative transfer equation
emission absorption ``destruction’’
scattering
Lya radiative transfer: scattering
Following absorption, re-‐emission occurs instantly à ``scattering’’
Lya radiative transfer: 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
Lya radiative transfer: scattering
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: 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