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Lyα Emission: Physics, Astrophysics, and Observational Challenges

This lecture provides an overview of Lyα physics and astrophysics, including the observations and theoretical challenges in studying Lyα emission from distant and primeval galaxies.

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Lyα Emission: Physics, Astrophysics, and Observational Challenges

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  1. Outline of the lectures 1a. Introduction 1b. PopIII stars and galaxies --> « top down » theoretical approach 2.,3a. Ly physics and astrophysics 3.b,4. Distant/primeval galaxies: - observational searches - current knowledge about high-z galaxies --> « bottom up » observational approach and confrontation with theory

  2. Outline of Part 2+3a Ly physics and astrophysics ISM emission Ly: the observational « problem » Lessons from local starbursts Ly radiation transfer (+dust) Lessons from Lyman Break Galaxies Ly trough the InterGalactic Medium Ly from sources prior to reionisation Ly Luminosity Function and reionisation

  3. Ly Emission • Galaxies with intense star formation (starbursts): • Intense UV radiation, ionising flux (>13.6 eV), and • emission lines from HII regions and diffuse ionised ISM • H, He recombination lines, [semi-]forbidden metal lines … • case B: L(Ly, H, …) = cl * QH and I(Ly)/I(Hn) = c(T,ne) 2/3 of recombinations lead to emission of 1 Lya photon (cf. lectures G. Stasinska)

  4. Ly Emission At (very) low metallicity: strong/dominant Ly ! since • increased ionising flux from stellar pops. • dominant cooling line (few metals) • emissivity increased by collisional excitation • (higher nebular temperature, Te) • --> up to ~10% of Lbol emitted in Ly! • ==> potentially detectable out to highest redshifts!! • …searches unsuccessful • until 1990ies --> Part 3 Partridge & Peebles (1967)

  5. Ly escape fraction 0 E_B-V 0.1 Ly TRANSFER Ly - the «problem » • Observable UV (>912 Ang): galaxies optically thin • However, very rapidly optically thick in Ly line (NHI >~ 1013 cm-2 ) --> Radiation transfer within the galaxy determines the emergent line profile and Ly « transmission » ! • Furthermore: dust may destroy Ly photons GENERAL: fate of Ly photons scattering until escape --> Ly halo Ly destruction by dust destruction through 2 photon emission (only in HII region) {

  6. Ly: THE « OBSERVATIONAL » PROBLEM The Ly puzzle(s) in nearby starbursts • 1980-90ies: several searches for Ly emission from z~2-3 primordial galaxies unsuccesful --> 1 or 2 puzzles: small number of galaxies and/or lower Ly emission? • IUE satellite: UV spectra of nearby starbursts (Ly) + optical spectra (H,H) ==> 1) extinction corrected I(Ly)/I(H) << case B (Meier & Terlevich 1981, Hartmann et al. 1984, Deharveng et al. 1986,… Giavalisco et al. 1996) Valls-Gabaud (1993) Terlevich et al. (1993)

  7. Ly: THE « OBSERVATIONAL » PROBLEM The Ly puzzle(s) in nearby starbursts • 1980-90ies: several searches for Ly emission from z~2-3 primordial galaxies unsuccesful --> 1 or 2 puzzles: small number of galaxies and/or lower Ly emission? • IUE satellite: UV spectra of nearby starbursts (Ly) + optical spectra (H,H) ==> 1) extinction corrected I(Ly)/I(H) << case B and W(Ly) smaller than expected (synthesis models) ==> 2) no trend with metallicity (O/H) • Possible explanations: • dust (Charlot & Fall 1993) (but 2!) • With « appropriate » (metallicity-dependent) extinction law no problem. Also underlying stellar Ly absorption (Valls-Gabaud 1993) • Inhomogeneous ISM geometry primarily determining factor, not dust (Giavalisco et al. 1996) • Short « duty cycle » of SF may explain small number of Ly emitters

  8. Ly: THE « OBSERVATIONAL » PROBLEM The Ly puzzle(s) in nearby starbursts Possible explanations for individual objects: • dust ? • With « appropriate » (metallicity-dependent) extinction law no problem. Also underlying stellar Ly absorption RULED out as SOLE explanations by IZw18, SBS 0335-052 (most metal poor stabursts known) which show no Ly emission !! • Inhomogeneous ISM geometry primarily determining factor, not dust OK, but quantitatively ? Kunth et al. (1994)

  9. Ly:LESSONS FROM LOCAL STARBURST The Ly puzzle(s) in nearby starbursts Detection of (neutral gas) outflows in 4 starbursts with Ly in emission • metallicities 12+log(O/H)~8.0…8.4..solar • EB-V ~ 0.1 - 0.55 ==> outflows, superwinds main crucial/determining factor for Ly escape!? Kunth et al. (1998)

  10. Ly:LESSONS FROM LOCAL STARBURST 2-3 D studies of Ly in nearby starbursts ACS/HST imaging in Ly + narrow continuum filter WFPC2/HST images in 5 other filters --> stellar population, UV slope … ==> Diffuse Ly emission seen ! Contains 2/3 of total flux in large aperture (IUE…) --> confirmation of Ly resonant scattering halo * different regions: different H kinematics --> but no constraint on HI kinematics at this spatial scale (requires SKA)! Hayes et al. (2005) Ly line image (cont.subtracted)

  11. Ly:LESSONS FROM LOCAL STARBURST 2-3 D studies of Ly in nearby starbursts Imaging (ACS)+ kinematics (H Integral Field, Ly long-slit STIS) ESO 350-IG038: knots B + C: similar, high extinction • one shows emission other not. Kinematics, NOT DUST, dominant SBS 0335-052: only absorption seen. If dust affects Ly, it must do so at even small scale (1 pixel ~ 6-9 pc!) Kunth et al. (1998) Kunth et al. (2003)

  12. Ly:LESSONS FROM LOCAL STARBURST 2 2-3 D studies of Ly in nearby starbursts Diversity of line profiles explained by evolutionary sequence of staburst driven supershells / superwind? 4 2 1 1 5,6 3, 4 M82 Tenorio-Tagle et al. (1999) Mas-Hesse et al. (2003)

  13. Ly:LESSONS FROM LOCAL STARBURST Lessons from nearby starbursts • W(Ly) and Ly/Hb < case B prediction ! • No clear correlation of Ly with metallicity, dust, other parameters found. • Strong variation of Ly observed within a galaxy • Ly scattering « halo » observed • Starbursts show complex structure (super star clusters + diffuse ISM); outflows ubiquitous Ly affected by: • ISM kinematics • ISM (HI) geometry • Dust Precise order of importance unclear! • Quantitative modeling including known constraints (stars, emitting gas, HI, dust + kinematics) with 3D radiation transfer model remains to be done

  14. Ly TRANSFER: THE ESSENTIALS Verhamme, Schaerer, Masseli (2006) Ly transfer: basics Cross section in atoms frame Optical depth taking Maxwellian velocity distr. into account Ly optical depth (in convenient units) <==> ~1 at line center for NH=3.1013 cm-2(and T=104K) Line absorption profile (Voigt)

  15. Ly TRANSFER: THE ESSENTIALS •  >> 1 at line center for NH >3.1013 cm-2 • (and T=104K) • Very large number of scatterings required to escape. • E.g. NH=1020 --> Nscatt ~ 107 for static slab • BUT:velocity fields or inhomogeneous medium can ease escape • (Ly) line scattering NOT a random walk: • walk in coupled spatial and frequency space • transport dominated by excursions to line wing! --> lower opacity --> longer mean free path Ly transfer: basics From Hubeny

  16. Ly TRANSFER: THE ESSENTIALS Ly: not simple - coherent and isotropic - scattering Absorption probability (=profile): Voigt/Hjertig function Ly transfer: basics T decrease x=frequency shift from line center (in Doppler width units) Angle averaged frequency redistribution functions RII (Hummer 1962) ==> Close to core: redistribution over ~[-xin,+xin] ==> Sufficiently far in wing:photon re-emitted close to initial frequency (~coherent) (in comoving frame) wing core scattering

  17. Ly TRANSFER: THE ESSENTIALS Ly transfer: Example Source inside homogeneous static slab emitting monochromatic line at line center Static case + symmetric Ly emission profile ==> double-peaked profile Separation increases with column density (opt.depth) Emission frequency shifting from line center to wing - Equivalent to approaching/receeding screen --> blue/red-shifted peak Neufeld (1990)

  18. Ly TRANSFER: THE ESSENTIALS Ly transfer: Example Ly emission inside expanding shell with velocity vexp ==> asymmetric redshifted line (single or double-peaked) profile + faint blue part ==> Main peak measures « in general » 2*vexp ! -vexp Verhamme et al. (2006)

  19. Ly TRANSFER: THE ESSENTIALS Ly transfer: Example Ly emission inside expanding shell with vexp Emission line + continuum for varying EW(Ly): --> from P-Cygni to asymmetric line profiles Dependence on vexp Dependence on NH Verhamme et al. (2006)

  20. Ly escape fraction 0 E_B-V 0.1 Ly TRANSFER WITH DUST Ly transfer with dust Dust scattering and absorption Within Ly line: interaction with HI or dust? Interaction with dust: negligible at line center (H >> d!) possible in wings due to multiple scattering ==> Efficient destruction of Ly photons by dust! NOTE depends also on HI kinematics!

  21. Ly TRANSFER: ISM GEOMETRY?! BUT: Ly transfer depends strongly on geometry --> photons follow « path of least resistance » In reality: • Inhomogeneous ISM: UV continuum photons penetrate more than Lya photons --> higher EW(Lya) (Neufeld 1991, Hansen & Oh 2006) • Outflows & galactic winds ubiquitous in starburst galaxies --> complex geometries and velocity structures with « open » directions … ==> Orientation effects expected…

  22. Ly: LESSONS FROM LBGs z~3 Lyman Break Galaxies (LBG) Galaxies with ongoing SF selected from their UV (restframe) emission >~1000 LBG with spectroscopic redshift (in 2003, now larger surveys) --> stellar, interstellar and nebular lines ==> show presence of massive stars, diversity of Ly line profiles (emission, P-Cygni … absorption) and strengths Composite spectrum W(Ly) distribution Spectral groups… Shapley et al. (2003)

  23. Ly-: LESSONS FROM LBGS z~3 Lyman Break Galaxies (LBG) • InterStellar lines blueshifted wrt stellar lines (v(abs-*)=-15060 km/s) • shift between IS absorption lines and Ly observed (v(em-abs)~450-650 km/s) • Correlations between extinction/UV slope, W(Ly), W(IS), SFR not or poorly understood (but cf. Ferrara & Ricotti 2007) Shapley et al. (2003)

  24. Ly: LESSONS FROM LBGs IS abs. Example: Fitting Ly emission in z~3 Lyman Break Galaxies (LBG) • Shift between *, IS and Ly naturally understood if ~global shell geometry: v(em-abs)~ 3* v(abs-*), i.e. Ly at 2~vexp • Variety of Ly line profiles understood from radiation transfer models and in agreement with observational constraints (v, extinction, …) Verhamme et al. (2007) FDF 4691: ~ static FDF 4454:vexp ~220 km/s, low extinction cB58:vexp ~255 km/s, EB-V=0.3

  25. density Lya map Profile along disk Profile along outflow

  26. Exemple:“Blob” Lyman-alpha a z=4.8 (Wilman et al. 20005, Nature) • Observation of a large structure of Lyman-a (~100 kpc side) • Ionisation source ? Galaxy or AGN ? • Lya profiles observed in different regions: • Strong emission + absorption ? • Can be interpreted as emission from different moving clouds and absorption in an expanding shell (signature of a strong galactic wind!) BUT: other interpretations possible! a) ~static cloud in front of Ly source ?! b) Collapsing protogalaxy !? (Dijkstra & Haiman 05) Simulated MC spectrum Verhamme et al. (2006)

  27. Summary of Part 2 Ly physics and astrophysics ISM emission Ly: the observational « problem » Ly emission fainter than expected from rec. theory Lessons from local starbursts ISM kinematics and geometry and dust all play a role. But any one dominant?! Ly radiation transfer (+dust) Scattering of Ly photons in geometrical and frequence space. Destruction by dust easy when number of scatterings large. Lessons from Lyman Break Galaxies Superwind shell geometry is a good description of ISM. Variety of observed Ly profiles can be understood quantitatively… More work needed for « complete » understanding of Ly and to establish Lya as reliable quantitative diagnostic

  28. Outline of Part 2+3a Ly physics and astrophysics ISM emission Ly: the observational « problem » Lessons from local starbursts Ly radiation transfer (+dust) Lessons from Lyman Break Galaxies Ly through the InterGalactic Medium Ly from sources prior to reionisation Ly Luminosity Function and reionisation

  29. neutral hydrogen redshift observer source Observed Spectrum flux observed wavelength Lyman-alpha at source redshift Ly and the IGM Ly « transfer » through the InterGalacticMedium (IGM) Or: how the Ly profile emerging from galaxies is transformed/transmitted on its way to the observer? • Radiation from distant background source is scattered out of line of sight • (+evtl. absorbed by dust) • Voigt absorption line profile - no real transfer effect in this geometry Observed spectrum = emergent spectrum attenuated by superposition of Voigt(zi,NHi,bi) ==> In principle: computation of attenuated spectrum trivial for given density & velocity distribution along line of sight Observations: Lyman forest Emergent galactic Ly profile altered and line flux reduced if neutral H present close in velocity/redshift to source

  30. Ly and the IGM Ly « transfer » through the InterGalacticMedium (IGM) Approaching or beyond reionisation: Lyman forest --> Gunn-Peterson trough for high z

  31. Ly and the IGM Ly « transfer » through the InterGalacticMedium (IGM) 1) Gunn-Peterson trough observed in z~6 QSOs from Sloan survey Fan et al. (2001, 2003), Becker et al. (2001)

  32. Ly and the IGM Ly « transfer » through the InterGalacticMedium (IGM) 2) QSO/galactic Ly profile altered and observed Ly line flux reduced! Absorption from red damping wing Fan et al. (2003) Miralda-Escude (1998) • Weak, but significant flux inside the trough •  highly ionized bubble by intervening galaxy or local void? • z=6.37 by fitting weak metal lines •  presence of neutral gas in the quasar vicinity?

  33. Ly and the IGM Ly « transfer » through the InterGalacticMedium (IGM) • Implications: • Ly(+) forest attenuation of SED at <1216 Ang (e.g. Madau 1995) • For z>~4-5: spectral break at Ly not at Lyc (=912 Ang) • photometric redshift estimates • Ly flux reduced • SFR(Ly) underestimates true SFR • LF(Ly) modified • Detectability of high-z galaxies affected! z > zreionis sources detectable ? • Ly line profile, Ly transmission, LF(Ly) contain information on hydrogen ionisation fraction! --> constraint on cosmic reionisation(z)

  34. HII HI cosmol. HII IGM observer reionisation z~6.5 redshift, wavelength Ly FROM SOURCES PRIOR TO REIONISATION • Galaxy/QSO with intrinsic Lya emission and a fraction fesc of ionising photons escaping the galaxy: • Temporal evolution of cosmological HII sphere: • ==> Neglecting recombinations (since timescale >> tHubble) and • assuming source turned on (and constant) during tQ the Stroemgren radius becomes: Proximity effect • Inside the HII region one has a residual HI fraction x (given by photoionisation equilibrium) of: • Then: attenuation given by with (e.g. Shapiro & Giroux 1987, Cen & Haiman 2000)

  35. HII HI cosmol. HII IGM observer reionisation z~6.5 redshift, wavelength • Observability of Ly emission from objects prior to reionisation. Example: • source z=6.556 • SFR = 9 Msun/yr • fesc = 25 % • age of source ~108 yr • --> HII region of 0.45 (3) Mpc proper (comoving) radius • Intrinsic Lya profile: FWHM=300 km/s • ==> transmission of ~16% Ly flux • ==> asymmetric line profile residual HI within HII region Haiman (2002)

  36. asymmetry • Transmission increases with: • SFR • escape fraction • source lifetime • intrinsic line width • Other factors affecting transmission and line profile: • IGM infall • outflows (galactic winds) • peculiar velocity of emitting gas within halo • halo mass • … transmission intrinsic asymmetry Static IGM IGM infall transmission Haiman (2002) Santos (2004)

  37. Ly FROM SOURCES PRIOR TO REIONISATION « Complications » arising from spherical symmetry to more realistic structures • clustering of sources helps create larger HII region • clustering probability increases with z and for fainter galaxies • ionised regions extend towards directions with lower density IGM • ==> strong variations depending on object and direction expected • ==> simple scaling properties of spherical model may not apply! Gnedin & Prada (2004), Furlanetto et al. (2004), Wyithe & Loeb (2004), Cen et al. (2004), ...

  38. Ly FROM SOURCES PRIOR TO REIONISATION Q: Ly emission from z > zreionis sources detectable ? A: yes - but transmission depends on many factors…! ==> observational approach! (cf. Lecture 3)

  39. USING THE Ly LF TO PROBE REIONISATION Lya luminosity function (LF) to probe cosmic reionisation: • absolute LF in principle sensitive to ionisation fraction xHI • evolution of LF with z --> constraint on reionisation • -> expect rapid decline of LF approaching end of reionisation (i.e. xHI >0) • E.g. Haiman & Spaans (1999) … • Observations + interpretation: Malhotra & Rhoads (2004), Le Delliou et al. (2005, 2006), Furlanetto et al. (2006) Haiman & Cen (2005)

  40. USING THE Ly LF TO PROBE REIONISATION Predicting the Lya LF Press-Schechter formalism Lya luminosity Transmission model ==> LF determined by 2 main parameters: Results: • transmission ~unchanged between z=5.7 and 6.5 (reionisation close to complete) • LF evolution due to evolution of halo Mass • Function Djikstra, Whyithe & Haiman (2006)

  41. Summary of Part 3a Ly physics and astrophysics Ly trough the InterGalactic Medium From Ly forest to trough… Attenuation of Ly line Ly from sources prior to reionisation Detection in principle possible. Transmission depends on many factors (geometry, superwinds, proximity effect…) Ly Luminosity Function and reionisation Change of Ly LF observed from z=4.5 to 6.5. Interpretation in terms of IGM ionisation fraction uncertain

  42. Outline of the lectures 1a. Introduction 1b. PopIII stars and galaxies --> « top down » theoretical approach 2.,3a. Ly physics and astrophysics 3.b,4. Distant/primeval galaxies: - observational searches - current knowledge about high-z galaxies --> « bottom up » observational approach and confrontation with theory

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