MMT spectroscopy of Lyman-alpha at z ≃ 7: evidence for accelerated reionization around massive galaxies

Abstract

Reionization-era galaxies tend to exhibit weak Lyα emission, likely reflecting attenuation from an increasingly neutral IGM. Recent observations have begun to reveal exceptions to this picture, with strong Lyα emission now known in four of the most massive z=7–9 galaxies in the CANDELS fields, all of which also exhibit intense [OIII]+Hβ emission (EW>800 Å). To better understand why Lyα is anomalously strong in a subset of massive z ≃ 7 − 9 galaxies, we have initiated an MMT/Binospec survey targeting a larger sample (N=22) of similarly luminous (≃1–6 L$^{\ast }_{\mathrm{UV}}$) z≃7 galaxies selected over very wide-area fields (∼3 deg2). We confidently (>7σ) detect Lyα in 78 per cent (7/9) of galaxies with strong [OIII]+Hβ emission (EW>800 Å) as opposed to only 8 per cent (1/12) of galaxies with more moderate (EW=200-800 Å) [OIII]+Hβ. We argue that the higher Lyα EWs of the strong [OIII]+Hβ population likely reflect enhanced ionizing photon production efficiency owing to their large sSFRs (≳30 Gyr−1). We also find evidence that Lyα transmission from massive galaxies declines less rapidly over 6 < z < 7 than in low-mass lensed systems. In particular, our data suggest no strong evolution in Lyα transmission, consistent with a picture wherein massive z≃7 galaxies often reside in large ionized regions. We detect three closely-separated (R = 1.7 physical Mpc) z≃7 Lyα emitters in our sample, conceivably tracing a large ionized structure that is consistent with this picture. We detect tentative evidence for an overdensity in this region, implying a large ionizing photon budget in the surrounding volume.

CLEAR: Boosted Ly$α$ Transmission of the Intergalactic Medium in UV bright Galaxies

“…This is explained as UV bright galaxies are likely located in highly ionized bubbles which were created by a wealth of ionizing photons produced by those galaxies as well as a potentially larger number of nearby fainter galaxies (Finkelstein et al 2019b, Larson et al 2021. Endsley et al (2021a) provide additional evidence for accelerated reionization around massive galaxies. Particularly, they find a higher Lyα detection rate at z 7 from massive galaxies with strong [O III]+Hβ emission, which reflects enhanced ionizing photo production rates (e.g., Roberts-Borsani et al 2016;Tang et al 2019Tang et al , 2021aEndsley et al 2021b), arguing for higher Lyα equivalentwidths (EWs) from the strong [O III]+Hβ emitting population as similar as shown at lower redshifts of z 2 -3 (Tang et al 2021b).…”

“…These UV bright galaxies are likely to be centered in overdense regions where they cluster with fainter galaxies (and these may be beyond current detection limits). The combination of ionizing photons from the galaxies in the overdense regions may provide significant ionizing photon budget around UV bright galaxies (Finkelstein et al 2019b;Endsley et al 2021a, Larson et al 2021. In contrast, UV-fainter galaxies have more neutral gas around them (i.e., they sit in smaller ionized bubbles), thus the Lyα transmission to the IGM is suppressed.…”

“…Although it is slightly increased from our fiducial value listed in the table, the change is insignificant and does not alter our conclusion. galaxies becomes ionized earlier than that of fainter objects, allowing for more efficient Lyα-photon escape (e.g., Zheng et al 2017;Mason et al 2018a;Endsley et al 2021a).…”

Subaru/HSC z-Broadband Excess Selection of Extreme Emission Line Galaxies at z<1

“…EELGs are easy to identify in grism observations (Atek et al 2011;Maseda et al 2018;Boyett et al 2021), which have confirmed a steep positive evolution in their number density from the local value to higher redshifts (Noeske et al 2006). More recent studies have used slit-based spectroscopy to push these measurements to redshift z ≈ 7 and beyond (Endsley et al 2021). The upcoming Near-Infrared (NIR) grism missions Euclid and the Nancy Grace Roman Space Telescope (RST ) should detect the full population of these galaxies selected by Hα emission at 0.7 < z < 2.05 and by [OIII]λ5007 emission at 1.2 < z < 3 over tens of thousands of square degrees.…”

Exploration of the high-redshift universe enabled by THESEUS

“…These studies yield a drop in the fraction of Ly α emitting galaxies at z > 6 (e.g. [10][11][12][13][14][15][16][17][18][19][20][21][22][23]), although there exist exceptions such as in observations of UV-faint galaxies behind galaxy cluster lenses [24], massive galaxies [25], and possibly in cases of ionised bubbles around bright galaxies [26]. There is also some evidence for a late reionization scenario (e.g.…”