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Register a new userMMT Spectroscopy of Lyman-alpha at z$simeq$7: Evidence for Accelerated Reionization Around Massive Galaxies
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Reionization-era galaxies tend to exhibit weak Ly$alpha$ emission, likely reflecting attenuation from an increasingly neutral IGM. Recent observations have begun to reveal exceptions to this picture, with strong Ly$alpha$ emission now known in four of the most massive z=7$-$9 galaxies in the CANDELS fields, all of which also exhibit intense [OIII]$+$H$beta$ emission (EW$>$800 $mathrm{mathring{A}}$). To better understand why Ly$alpha$ is anonymously strong in a subset of massive z$simeq$7$-$9 galaxies, we have initiated an MMT/Binospec survey targeting a larger sample (N=22) of similarly luminous ($simeq$1$-$6 L$^{ast}_{mathrm{UV}}$) z$simeq$7 galaxies selected over very wide-area fields ($sim$3 deg$^2$). We confidently ($>$7$sigma$) detect Ly$alpha$ in 78% (7/9) of galaxies with strong [OIII]$+$H$beta$ emission (EW$>$800 $mathrm{mathring{A}}$) as opposed to only 8% (1/12) of galaxies with more moderate (EW=200$-$800 $mathrm{mathring{A}}$) [OIII]$+$H$beta$. We argue that the higher Ly$alpha$ EWs of the strong [OIII]$+$H$beta$ population likely reflect enhanced ionizing photon production efficiency owing to their large sSFRs ($gtrsim$30 Gyr$^{-1}$). We also find evidence that Ly$alpha$ 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 Ly$alpha$ transmission, consistent with a picture wherein massive z$simeq$7 galaxies often reside in large ionized regions. We detect three closely-separated ($R$ = 1.7 physical Mpc) z$simeq$7 Ly$alpha$ 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.
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We discuss new Keck/MOSFIRE spectroscopic observations of four luminous galaxies at z~7-9 selected to have intense optical line emission by Roberts-Borsani et al. (2016). Previous follow-up has revealed Lyman-alpha in two of the four galaxies. Our new MOSFIRE observations confirm that Lyman-alpha is present in the entire sample. We detect Lyman-alpha emission in COS-zs7-1, confirming its redshift as z=7.154, and we detect Lyman-alpha in EGS-zs8-2 at z=7.477, verifying a tentative detection presented in an earlier study. The ubiquity of Lyman-alpha in this sample is puzzling given that the IGM is likely significantly neutral over 7<z<9. To investigate this result in more detail, we have initiated a campaign to target UV metal emission in the four Lyman-alpha emitters as a probe of both the radiation field and the velocity offset of Lyman-alpha. Here we present the detection of intense CIII] emission in EGS-zs8-1, a galaxy from this sample previously shown to have Lyman-alpha at z=7.73. Photoionization models indicate that an intense radiation field and low metallicity are required to reproduce the intense CIII] and optical line emission. We argue that this extreme radiation field is likely to affect the local environment, increasing the transmission of Lyman-alpha through the galaxy. Moreover, the centroid of CIII] indicates that Lyman-alpha is redshifted from the systemic value by 340 km/s. This velocity offset is larger than that seen in less luminous systems, providing an additional explanation for the transmission of Lyman-alpha emission through the IGM. Since the transmission is further enhanced by the likelihood that such systems are also situated in the densest regions with the largest ionized bubbles, the visibility of Lyman-alpha at z>7 is expected to be strongly luminosity-dependent, with the most effective transmission occurring in systems with intense star formation.
We present Gemini/GNIRS spectroscopic observations of 4 z-band (z~7) dropout galaxies and VLT/XSHOOTER observations of one z-band dropout and 3 Y-band (z~8-9) dropout galaxies in the Hubble Ultra Deep Field, which were selected with Wide Field Camera 3 imaging on the Hubble Space Telescope. We find no evidence of Lyman-alpha emission with a typical 5-sigma sensitivity of 5X10^-18erg/cm^2/s, and we use the upper limits on Lyman-alpha flux and the broad-band magnitudes to constrain the rest-frame equivalent widths for this line emission. Accounting for incomplete spectral coverage, we survey 3.0 z-band dropouts and 2.9 Y-band dropouts to a Lyman-alpha rest-frame equivalent width limit > 120Ang (for an unresolved emission line); for an equivalent width limit of 50Ang the effective numbers of drop-outs surveyed fall to 1.2 z-band drop-outs and 1.5 Y-band drop-outs. A simple model where the fraction of high rest-frame equivalent width emitters follows the trend seen at z=3-6.5 is inconsistent with our non-detections at z=7-9 at the ~ 1-sigma level for spectrally unresolved lines, which may indicate that a significant neutral HI fraction in the intergalactic medium suppresses the Lyman-alpha line in z-drop and Y-drop galaxies at z > 7.
Ly$alpha$-emitting galaxies (LAEs) are easily detectable in the high-redshift Universe and are potentially efficient tracers of large scale structure at early epochs, as long as their observed properties do not strongly depend on environment. We investigate the luminosity and equivalent width functions of LAEs in the overdense field of a protocluster at redshift $z simeq 3.78$. Using a large sample of LAEs (many spectroscopically confirmed), we find that the Ly$alpha$ luminosity distribution is well-represented by a Schechter (1976) function with $log(L^{ast}/{rm erg s^{-1}}) = 43.26^{+0.20}_{-0.22}$ and $log(phi^{ast}/{rm Mpc^{-3}})=-3.40^{+0.03}_{-0.04}$ with $alpha=-1.5$. Fitting the equivalent width distribution as an exponential, we find a scale factor of $omega=79^{+15}_{-15}$ Angstroms. We also measured the Ly$alpha$ luminosity and equivalent width functions using the subset of LAEs lying within the densest cores of the protocluster, finding similar values for $L^*$ and $omega$. Hence, despite having a mean overdensity more than 2$times$ that of the general field, the shape of the Ly$alpha$ luminosity function and equivalent width distributions in the protocluster region are comparable to those measured in the field LAE population by other studies at similar redshift. While the observed Ly$alpha$ luminosities and equivalent widths show correlations with the UV continuum luminosity in this LAE sample, we find that these are likely due to selection biases and are consistent with no intrinsic correlations within the sample. This protocluster sample supports the strong evolutionary trend observed in the Ly$alpha$ escape fraction and suggest that lower redshift LAEs can be on average significantly more dusty that their counterparts at higher redshift.
We quantify the distribution of [OIII]+H$beta$ line strengths at z$simeq$7 using a sample of 20 bright (M$_{mathrm{UV}}$ $lesssim$ $-$21) galaxies. We select these systems over wide-area fields (2.3 deg$^2$ total) using a new colour-selection which precisely selects galaxies at z$simeq$6.63$-$6.83, a redshift range where blue Spitzer/IRAC [3.6]$-$[4.5] colours unambiguously indicate strong [OIII]$+$H$beta$ emission. These 20 galaxies suggest a log-normal [OIII]$+$H$beta$ EW distribution with median EW = 759$^{+112}_{-113}$ $mathrm{mathring{A}}$ and standard deviation = 0.26$^{+0.06}_{-0.05}$ dex. We find no evidence for strong variation in this EW distribution with UV luminosity. The typical [OIII]+H$beta$ EW at z$simeq$7 implied by our sample is considerably larger than that in massive star forming galaxies at z$simeq$2, consistent with a shift toward larger average sSFR (4.4 Gyr$^{-1}$) and lower metallicities (0.16 Z$_odot$). We also find evidence for the emergence of a population with yet more extreme nebular emission ([OIII]+H$beta$ EW$>$1200 $mathrm{mathring{A}}$) that is rarely seen at lower redshifts. These objects have extremely large sSFR ($>$30 Gyr$^{-1}$), as would be expected for systems undergoing a burst or upturn in star formation. While this may be a short-lived phase, our results suggest that 20% of the z$simeq$7 population has such extreme nebular emission, implying that galaxies likely undergo intense star formation episodes regularly at z$>$6. We argue that this population may be among the most effective ionizing agents in the reionization era, both in terms of photon production efficiency and escape fraction. We furthermore suggest that galaxies passing through this large sSFR phase are likely to be very efficient in forming bound star clusters.
We present diffuse Lyman-alpha halos (LAHs) identified in the composite Subaru narrowband images of 100-3600 Lyman-alpha emitters (LAEs) at z=2.2, 3.1, 3.7, 5.7, and 6.6. First, we carefully examine potential artifacts mimicking LAHs that include a large-scale point-spread function (PSF) made by instrumental and atmospheric effects. Based on our critical test with composite images of non-LAE samples whose narrowband-magnitude and source-size distributions are the same as our LAE samples, we confirm that no artifacts can produce a diffuse extended feature similar to our LAHs. After this test, we measure the scale lengths of exponential profile for the LAHs estimated from our z=2.2-6.6 LAE samples of L(Lyman-alpha) > 2 x 10^42 erg s^-1. We obtain the scale lengths of ~ 5-10 kpc at z=2.2-5.7, and find no evolution of scale lengths in this redshift range beyond our measurement uncertainties. Combining this result and the previously-known UV-continuum size evolution, we infer that the ratio of LAH to UV-continuum sizes is nearly constant at z=2.2-5.7. The scale length of our z=6.6 LAH is larger than 5-10 kpc just beyond the error bar, which is a hint that the scale lengths of LAHs would increase from z=5.7 to 6.6. If this increase is confirmed by future large surveys with significant improvements of statistical and systematical errors, this scale length change at z > 6 would be a signature of increasing fraction of neutral hydrogen scattering Lyman-alpha photons, due to cosmic reionization.
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