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Register a new userLyman-alpha and CIII] Emission in z=7-9 Galaxies: Accelerated Reionization Around Luminous Star Forming Systems?
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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.
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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.
Lyman-alpha (Lya) photons that escape the interstellar medium of star-forming galaxies may be resonantly scattered by neutral hydrogen atoms in the circumgalactic and intergalactic media, thereby increasing the angular extent of the galaxys Lya emission. We present predictions of this extended, low surface brightness Lya emission based on radiative transfer modeling in a cosmological reionization simulation. The extended emission can be detected from stacked narrowband images of Lya emitters (LAEs) or of Lyman break galaxies (LBGs). Its average surface brightness profile has a central cusp, then flattens to an approximate plateau beginning at an inner characteristic scale below ~0.2 Mpc (comoving), then steepens again beyond an outer characteristic scale of ~1 Mpc. The inner scale marks the transition from scattered light of the central source to emission from clustered sources, while the outer scale marks the spatial extent of scattered emission from these clustered sources. Both scales tend to increase with halo mass, UV luminosity, and observed Lya luminosity. The extended emission predicted by our simulation is already within reach of deep narrowband photometry using large ground-based telescopes. Such observations would test radiative transfer models of emission from LAEs and LBGs, and they would open a new window on the circumgalactic environment of high-redshift star-forming galaxies.
We present statistical properties of diffuse Lyman-alpha halos (LAHs) around high-$z$ star-forming galaxies with large Subaru samples of Lyman-alpha emitters (LAEs) at $z=2.2$. We make subsamples defined by the physical quantities of LAEs central Lyman-alpha luminosities, UV magnitudes, Lyman-alpha equivalent widths, and UV slopes, and investigate LAHs radial surface brightness (SB) profiles and scale lengths $r_n$ as a function of these physical quantities. We find that there exist prominent LAHs around LAEs with faint Lyman-alpha luminosities, bright UV luminosities, and small Lyman-alpha equivalent widths in cumulative radial Lyman-alpha SB profiles. We confirm this trend with the anti-correlation between $r_n$ and Lyman-alpha luminosities (equivalent widths) based on the Spearmans rank correlation coefficient that is $rho=-0.9$ ($-0.7$) corresponding to the $96%$ ($93%$) confidence level, although the correlation between $r_n$ and UV magnitudes is not clearly found in the rank correlation coefficient. Our results suggest that LAEs with properties similar to typical Lyman-break galaxies (with faint Lyman-alpha luminosities and small equivalent widths) possess more prominent LAHs. We investigate scenarios for the major physical origins of LAHs with our results, and find that the cold stream scenario is not preferred, due to the relatively small equivalent widths up to $77$AA in LAHs that include LAEs central components. There remain two possible scenarios of Lyman-alpha scattering in circum-galactic medium and satellite galaxies that cannot be tested with our observational data.
We make use of ALMA continuum observations of $15$ luminous Lyman-break galaxies at $z$$sim$$7$$-$$8$ to probe their dust-obscured star-formation. These observations are sensitive enough to probe to obscured SFRs of $20$ $M_{odot}$$/$$yr$ ($3sigma$). Six of the targeted galaxies show significant ($geq$$3$$sigma$) dust continuum detections, more than doubling the number of known dust-detected galaxies at $z$$>$$6.5$. Their IR luminosities range from $2.7$$times$$10^{11}$ $L_{odot}$ to $1.1$$times$$10^{12}$ $L_{odot}$, equivalent to obscured SFRs of $20$ to $105$ $M_{odot}$$/$$yr$. We use our results to quantify the correlation of the infrared excess IRX on the UV-continuum slope $beta_{UV}$ and stellar mass. Our results are most consistent with an SMC attenuation curve for intrinsic $UV$-slopes $beta_{UV,intr}$ of $-2.63$ and most consistent with an attenuation curve in-between SMC and Calzetti for $beta_{UV,intr}$ slopes of $-2.23$, assuming a dust temperature $T_d$ of $50$ K. Our fiducial IRX-stellar mass results at $z$$sim$$7$$-$$8$ are consistent with marginal evolution from $z$$sim$$0$. We then show how both results depend on $T_d$. For our six dust-detected sources, we estimate their dust masses and find that they are consistent with dust production from SNe if the dust destruction is low ($<$$90$%). Finally we determine the contribution of dust-obscured star formation to the star formation rate density for $UV$ luminous ($<$$-$$21.5$ mag: $gtrsim$$1.7$$L_{UV} ^*$) $z$$sim$$7$$-$$8$ galaxies, finding that the total SFR density at $z$$sim$$7$ and $z$$sim$$8$ from bright galaxies is $0.18_{-0.10}^{+0.08}$ dex and $0.20_{-0.09}^{+0.05}$ dex higher, respectively, i.e. $sim$$frac{1}{3}$ of the star formation in $gtrsim$$1.7$$L_{UV} ^*$ galaxies at $z$$sim$$7$$-$$8$ is obscured by dust.
Deep spectroscopic observations of z~6.5 galaxies have revealed a marked decline with increasing redshift in the detectability of Lyman-alpha emission. While this may offer valuable insight into the end of the reionisation process, it presents a fundamental challenge to the detailed spectroscopic study of the many hundreds of photometrically-selected distant sources now being found via deep HST imaging, and particularly those bright sources viewed through foreground lensing clusters. In this paper we demonstrate the validity of a new way forward via the convincing detection of an alternative diagnostic line, CIII]1909, seen in spectroscopic exposures of two star forming galaxies at z=6.029 and 7.213. The former detection is based on a 3.5 hour X-shooter spectrum of a bright (J=25.2) gravitationally-lensed galaxy behind the cluster Abell 383. The latter detection is based on a 4.2 hour MOSFIRE spectra of one of the most distant spectroscopically confirmed galaxies, GN-108036, with J=25.2. Both targets were chosen for their continuum brightness and previously-known redshift (based on Lyman-alpha), ensuring that any CIII] emission would be located in a favorable portion of the near-infrared sky spectrum. We compare our CIII] and Lyman-alpha equivalent widths in the context of those found at z~2 from earlier work and discuss the motivation for using lines other than Lyman-alpha to study galaxies in the reionisation era.
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