Lyα Emission Line Profiles of Extreme [OIII] Emitting Galaxies at z≳2: Implications for Lyα Visibility in the Reionization Era
arxiv(2024)
摘要
JWST observations have recently begun delivering the first samples of Ly
velocity profile measurements at z>6, opening a new window on the reionization
process. Interpretation of z>6 line profiles is currently stunted by
limitations in our knowledge of the intrinsic Lya profile (before encountering
the IGM) of the galaxies that are common at z>6. To overcome this shortcoming,
we have obtained resolved (R 3900) Lya spectroscopy of 42 galaxies at z=2.1-3.4
with similar properties as are seen at z>6. We quantify a variety of Lya
profile statistics as a function of [OIII]+Hβ EW. Our spectra reveal a
new population of z 2-3 galaxies with large [OIII]+Hβ EWs (>1200A) and a
large fraction of Lya flux emerging near the systemic redshift (peak velocity
0km/s). These spectra indicate that low density neutral hydrogen channels are
able to form in a subset of low mass galaxies that experience a burst of star
formation (sSFR >100/Gyr). Other extreme [OIII] emitters show weaker Lya that
is shifted to higher velocities ( 240km/s) with little emission near line
center. We investigate the impact the IGM is likely to have on these intrinsic
line profiles in the reionization era, finding that the centrally peaked Lya
emitters should be strongly attenuated at z>5. We show that these line profiles
are particularly sensitive to the impact of resonant scattering from infalling
IGM and can be strongly attenuated even when the IGM is highly ionized at z 5.
We compare these expectations against a database of z>6.5 galaxies with robust
velocity profiles measured with JWST/NIRSpec in its grating mode. We find one
z=8.28 source with strong Lya (EW=137A) and very small velocity offset
(32km/s), likely implying a rare ionized sightline (>0.5pMpc) at z>8. But
overall the results confirm the prediction of our simulations, revealing
significantly larger peak velocities than are seen in our z 2-3 sample.
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