The Detectability of Lyα Emission from Galaxies during

the Epoch of Reionization

Dijkstra, Mesinger, Wyithe. JC 2.11.4042 Alex Fry

• 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)

Today’s question brought to you by Philosoraptor:

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

• 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).

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

I. Source clustering on 1 Mpc scale.

II. Patchiness/clumpiness.III. Radiative transfer effects

through outflows of interstellar HI gas which imparts redshift to Lyα photons before they leave local halo

Considerations

Evidence for Winds‘P-Cygni type 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 2003Winds appear present in all galaxies and affect the Lyα spectrum (Steidel et al. 2010)

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.

Model• Slab in front of monochromatic point

source

Verhamme, et al. 2006

Model• Shell around monochromatic point

source

Verhamme, et al. 2006

Model• Why?

Verhamme, et al. 2006

‘Why are single peaks formed in expanding/infalling media 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 halocontains 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.’

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.

The probability density for the fraction of Lyα photons that get to the observer, TIGM

& the cumulative distribution function of TIGM

Enhanced column density

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

ObservationsObservations, 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.

Results Equivalent Widths• Solid line, exponential Lyα REW

distribution for z=6 drop out population

• Dashed line, z=7 where neutral volume fraction changed

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.

The fraction of dropout galaxies with REW > 75 A is f~0.1 at z=7.

Z=6, IGM ionized

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.

The fraction of dropout galaxies with REW > 75 A is f~0.1 at z=7.

Z=7, IGM half ionized

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.

The fraction of dropout galaxies with REW > 75 A is f~0.1 at z=7.

Z=7, IGM half ionized

ConclusionsLyα 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.

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

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, 35 http://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

• Rest Frame Equivalent Width (REW)

Γ - Ionization rate (units of inverse seconds)

T IGM or τ IGM – 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)

Variables

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-α line 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.

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.