The Detectability of Lyα Emission from Galaxies during the Epoch of Reionization

1. The Detectability of Lyα Emission from Galaxies during the Epoch of Reionization Dijkstra, Mesinger, Wyithe. JC 2.11.4042 Alex Fry

2. The physics of reionization (co-moving star formation rate, number of ionizing photons above 91.2 nm or 13.7 eV, fraction of escaping photons) • Observational probes (Lyα, Lyman break, quasars, radio) • The discovery and study of high redshift, z>7, galaxies (Hubble upgrades make it possible)

3. Today’s question brought to you by Philosoraptor:

4. outline • Lyα emission spectrum from ‘first’ galaxies • Redshift of Lyα line from winds • Observations • Conclusions • IGM is opaque to the UV spectrum • Radiative transfer models • Z=6 to 7 evolution & Equivalent Widths

5. The first galaxies formed just before the Epoch of Reionization (EOR) when the Universe was dominated by neutral hydrogen. • First generation of galaxies were metal poor. Thus hotter stars. Thus more ionizing radiation. Thus stronger nebular emission. Thus more Lyα (and large equivalent widths of 1500 A).

6. Lyα emission line may be difficult to observe, due to the large opacity of the intervening neutral intergalactic medium: for example, a source needs to be embedded in a >∽1 Mpc HII region to allow Lyα photons to redshift far away from the line center before they reach the IGM

7. Considerations Source clustering on 1 Mpc scale. Patchiness/clumpiness. Radiative transfer effects through outflows of interstellar HI gas which imparts redshift to Lyα photons before they leave local halo

8. Evidence for Winds ‘P-Cygnitype Lyα profiles exhibited in nearly half of starburst galaxies, both nearby and high-z, are believed to be formed by an expanding supershell surrounding a star-forming region.’ Ahn 2003 Winds appear present in all galaxies and affect the Lyα spectrum (Steidel et al. 2010)

9. Model • Galaxy emission: The authors use a radiative transfer code from their previous work with winds modeled by spherically symmetric wind shell of HI gas. Photons are emitted from center and Monte-Carlo propagated. • IGM opacity: Publicly available DexM3 code to generate evolved density, velocity, halo, and ionization field at z=8.5 (Mesinger & Furlanetto 2007) for 250 Mpc size box. • Does not include dust.

10. Model • Slab in front of monochromatic point source Verhamme, et al. 2006

11. Model • Shell around monochromatic point source Verhamme, et al. 2006

12. Model ‘Why are single peaks formed in expanding/infallingmedia with a central point source emitting monochromatic radiation at the Lyα line center? The reason is simple. The probability to escape the medium for a photon at line center is e−τ0 , i.e. close to zero for both cases shown here. As an expanding halo contains atoms with velocities v(r) from 0 to Vmax, all photons in the frequency range x = [0, Vmax] will be seen in the line center by atoms of the corresponding velocity, and are thus “blocked”. Therefore the only possibility to escape is to be shifted to the red side.’ • Why? Verhamme, et al. 2006

14. The Doppler-shifting from the winds means that by the time the Lyα photons reach a neutral path of the IGM, their absorption cross-sections are further out on the damping wing tail.

16. The probability density for the fraction of Lyα photons that get to the observer, TIGM & the cumulative distribution function of TIGM Enhanced column density

17. The probability density for the fraction of Lyα photons that get to the observer, TIGM & the cumulative distribution function of TIGM Decreased neutral hydrogen fraction

18. Observations Observations, although preliminary, indicate the fraction of drop-out galaxies with strong Lyα emission decreases strongly from z=6 to z=7(Stark t al. 2010a). • The fraction of galaxies with a Rest Frame Equivalent width (REW) >75 A decreased by a factor of 2 between z=6 & 7.

19. Results Equivalent Widths • Solid line, exponential Lyα REW distribution for z=6 drop out population • Dashed line, z=7 where neutral volume fraction changed

20. Results Equivalent Widths • The fraction of dropout galaxies with REW > 75 A is f~0.2 at z=6. Assuming IGM at z=6 was transparent to Lyα then only the evolution of ionization in IGM changed to z=7. This would require the ionization rate to change to x_HI=0.51 to explain the observations. Z=6, IGM ionized The fraction of dropout galaxies with REW > 75 A is f~0.1 at z=7.

21. Results Equivalent Widths • The fraction of dropout galaxies with REW > 75 A is f~0.2 at z=6. Assuming IGM at z=6 was transparent to Lyα then only the evolution of ionization in IGM changed to z=7. This would require the ionization rate to change to x_HI=0.51 to explain the observations. Z=7, IGM half ionized The fraction of dropout galaxies with REW > 75 A is f~0.1 at z=7.

22. Results Equivalent Widths • The fraction of dropout galaxies with REW > 75 A is f~0.2 at z=6. However, this rapid ionization fraction with redshift is unrealistic, furthermore sinks of ionizing photons are likely to slow the final stages of reionization. Z=7, IGM half ionized The fraction of dropout galaxies with REW > 75 A is f~0.1 at z=7.

23. Conclusions Lyα is visible during the epoch of reionization. Observations suggest that I) current observations of drop-out galaxies at z=7 are flawed or populate a more neutral than average space in the Unvierse II) winds lose strength towards higher redshifts or III) The Universe at z=6 contained a non-negligible volume fraction of neutral hydrogen. The visibility of Lyα means that the presence or absence of Lyα emitters alone does not uniquely determine the state of the IGM (originally one might think the highest z Lyα emitter visible is when reionizaiton was complete), however the redshift evolution of properties such as REW and the UV luminosity function already provide interesting and useful constrains on models of reionizaiton.

24. tl;dr Winds mean Lyα emission can theoretically be seen even when the Universe is mostly ionized.

25. References Ahn, S.-H., Lee, H.-W., & Lee, H. M. 2003, MNRAS, 340, 863 http://arxiv.org/abs/astro-ph/0204004 Dijkstra, M., & Wyithe, J. S. B. 2010, MNRAS, 408, 35http://arxiv.org/abs/1004.2490 Robertson, B. E., Ellis, R. S., Dunlop, J. S., McLure, R. J., & Stark, D. P. 2010, Nature, 468, 49 http://arxiv.org/abs/1011.0727 Verhamme, A., Schaerer, D., & Maselli, A. 2006, A&A, 460, 397 http://arxiv.org/abs/astro-ph/0608075

26. Rest Frame Equivalent Width (REW)

27. Variables Γ - Ionization rate (units of inverse seconds) TIGM or τIGM – Fraction of Lyα photons transmitted through IGM to observer (pure number) xHI or xbar – neutral HI volume fraction (pure number) Jυ – normalized Lyα spectrum that emerges from galaxy (pure number)

28. Early galaxies (z~7 or 800 Myr after Big Bang) provide detailed constraints on the amount of ultraviolet radiation available. Quasar Lyman α forest measures the line of site amount of Hydrogen. Lyman-αline emission from a galaxy indicates that neutral gas outside the galaxy is not present. Radio interferometry at 21-cm will ultimately map the epoch of reionization large scale structure.

29. The Detectability of Lyα Emission from Galaxies during the Epoch of Reionization: Early galaxies were metal poor bright Lyα emitting sources embedded in a neutral hydrogen IGM. Lyα photons are absorbed my neutral hydrogen readily and thus observing these galaxies is difficult before reionization is complete, however because of radiative effects (specifically wind outflows create a P-Cygni type profile) a significant fraction Lyα photons can escape. Thus early Lyα galaxies are seen at very high redshifts before reionization is complete and provide additional constraints on reionization.