%0 Generic %1 hassan2020galaxy %A Hassan, Sultan %A Gronke, Max %D 2020 %K library %T Can galaxy evolution mimic cosmic reionization? %U http://arxiv.org/abs/2010.00023 %X Lyman-$\alpha$ (Ly$\alpha$) emitting galaxies are powerful tools to probe the late stages of cosmic reionization. The observed sudden drop in Ly$\alpha$ fraction at $z>6$ is often interpreted as a sign of reionization, since the intergalactic medium (IGM) is more neutral and opaque to Ly$\alpha$ photons. Crucially, this interpretation of the observations is only valid under the assumption that galaxies themselves experience a minimal evolution at these epochs. By modelling Ly$\alpha$ radiative transfer effects in and around galaxies, we examine whether a change in the galactic properties can reproduce the observed drop in the Ly$\alpha$ fraction. We find that an increase in the galactic neutral hydrogen content or a reduction in the outflow velocity toward higher redshift both lead to a lower Ly$\alpha$ escape fraction, and can thus mimic an increasing neutral fraction of the IGM. We furthermore find that this change in galactic properties leads to systematically different Ly$\alpha$ spectra which can be used to differentiate the two competing effects. Using the CANDELSz7 survey measurements which indicate slightly broader lines at $z\sim 6$, we can rule out the scenario of a mere increase in the galactic column density towards higher $z$. We also show that a decrease in outflow velocity is not ruled out by existing data but leads to more prominent blue peaks at $z>6$. Our results caution the use of Ly$\alpha$ observations to estimate the IGM neutral fraction without accounting for the potential change in the galactic properties, e.g., by mapping out the evolution of Ly$\alpha$ spectral characteristics.

@misc{hassan2020galaxy, abstract = {Lyman-$\alpha$ (Ly$\alpha$) emitting galaxies are powerful tools to probe the late stages of cosmic reionization. The observed sudden drop in Ly$\alpha$ fraction at $z>6$ is often interpreted as a sign of reionization, since the intergalactic medium (IGM) is more neutral and opaque to Ly$\alpha$ photons. Crucially, this interpretation of the observations is only valid under the assumption that galaxies themselves experience a minimal evolution at these epochs. By modelling Ly$\alpha$ radiative transfer effects in and around galaxies, we examine whether a change in the galactic properties can reproduce the observed drop in the Ly$\alpha$ fraction. We find that an increase in the galactic neutral hydrogen content or a reduction in the outflow velocity toward higher redshift both lead to a lower Ly$\alpha$ escape fraction, and can thus mimic an increasing neutral fraction of the IGM. We furthermore find that this change in galactic properties leads to systematically different Ly$\alpha$ spectra which can be used to differentiate the two competing effects. Using the CANDELSz7 survey measurements which indicate slightly broader lines at $z\sim 6$, we can rule out the scenario of a mere increase in the galactic column density towards higher $z$. We also show that a decrease in outflow velocity is not ruled out by existing data but leads to more prominent blue peaks at $z>6$. Our results caution the use of Ly$\alpha$ observations to estimate the IGM neutral fraction without accounting for the potential change in the galactic properties, e.g., by mapping out the evolution of Ly$\alpha$ spectral characteristics.}, added-at = {2020-10-02T05:12:37.000+0200}, author = {Hassan, Sultan and Gronke, Max}, biburl = {https://www.bibsonomy.org/bibtex/2112b38be0353311e347d8e4367b3955e/gpkulkarni}, description = {Can galaxy evolution mimic cosmic reionization?}, interhash = {b14daba78aca37b66af99d872f778483}, intrahash = {112b38be0353311e347d8e4367b3955e}, keywords = {library}, note = {cite arxiv:2010.00023Comment: to be submitted to ApJ, comments are welcome}, timestamp = {2020-10-02T05:12:37.000+0200}, title = {Can galaxy evolution mimic cosmic reionization?}, url = {http://arxiv.org/abs/2010.00023}, year = 2020 }