A Lyman-$\alpha$ Galaxy at Redshift $z=6.944$ in the COSMOS Field^††thanks: This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile

Lyman-$\alpha$ emitting galaxies provide a valuable probe of reionization, because resonant scattering of Lyman-$\alpha$ photons in the intergalactic medium (IGM) can suppress the observed Lyman-$\alpha$ line by a factor of $\gtrsim 3$ for any neutral fraction $\gtrsim 50\%$ (Malhotra & Rhoads, 2004; Santos, 2004). Such strong flux suppression will cause a change in the Lyman-$\alpha$ luminosity function that should be obvious— especially since Lyman-$\alpha$ galaxies show little evolution at $3\lesssim z\lesssim 6$, either in luminosity function (Dawson et al., 2007; Zheng et al., 2012) or physical properties (Malhotra et al., 2012). Early studies concluded that the IGM neutral fraction was already small at $z\approx 6.5$ (Malhotra & Rhoads, 2004; Stern et al., 2005). More recent work has established the Lyman-$\alpha$ luminosity function at both $z\approx 5.7$ and $6.5$ to considerable accuracy (Ouchi et al., 2010; Hu et al., 2010; Kashikawa et al., 2011), showing a modest but statistically significant difference between these observed Lyman-$\alpha$ luminosity functions: The $z=6.5$ luminosity function is below that at $z=5.7$, and the difference can be adequately characterized by a pure luminosity evolution by a factor of $\sim 1.3$ (Ouchi et al., 2010).

Yet, other Lyman-$\alpha$ based tests for reionization— including the apparent spatial clustering of Lyman-$\alpha$ galaxies (McQuinn et al., 2007), the minimum ionized volume around observed Lyman-$\alpha$ sources (Malhotra & Rhoads, 2006), and Lyman-$\alpha$ line profiles (Hu et al., 2010; Ouchi et al., 2010)— show little evidence for neutral gas at $z\approx 6.5$. This leaves an open question— is lower Lyman-$\alpha$ luminosity function at $z=6.5$ due to neutral gas, or is it an intrinsic evolution in the galaxy populations?

To distinguish between these possibilities, we can look to still higher redshifts, where the IGM neutral fraction should be higher and its effects on Lyman-$\alpha$ stronger. The highest redshift readily accessible to Lyman-$\alpha$ searches using CCDs is $z\approx 7.0$, in the 9650Å window in the night sky OH emission spectrum. We are pursuing a 9650Å narrowband survey using the IMACS imaging spectrograph on the 6.5m Magellan I Baade Telescope at Las Campanas Observatory (Hibon et al., 2011). We surveyed 465 square arcminutes, corresponding to $\sim 72000\textrm{Mpc}^{3}$. After a careful selection, we found 6 z$\sim$6.96 LAE candidates (Hibon et al., 2011). To confirm whether these are real LAEs, we obtained multi-object spectra with IMACS. In this Letter, we present the spectrum of LAE J095950.99+021219.1, which was identified as a candidate redshift $z\approx 7$ Lyman-$\alpha$ emitting galaxy (candidate LAE 3) in Hibon et al. (2011). Our spectroscopy reveals a single, isolated Lyman-$\alpha$ line at redshift $z=6.944$.

Throughout the paper, we adopt a $\Lambda$CDM “concordance cosmology” with $\Omega_{M}=0.27$, $\Omega_{\Lambda}=0.73$, and $H_{0}=71{\rm\,km\,s^{-1}\,Mpc^{-1}}$.

We interpret the line at 9657Å in LAE J095950.99+021219.1 as Lyman-$\alpha$ at redshift $z=6.944$, based on non-detections in all filters blueward of this line, and on the absence of other optical lines.

Were the primary line H$\alpha$ (at $z\approx 0.472$), or [O III] $\lambda$5007 (at $z\approx 0.928$), we would expect other prominent emission lines in our IMACS spectrum. Figure 3 shows non-detections in the 1D spectrum at the expected line wavelengths. Corresponding upper limits are summarized in table 2. The “H$\alpha$” case ($z\approx 0.472$) is disfavored by the non-detections of [O III] $\lambda$5007 and [O II] $\lambda$3727, with $3\sigma$ line ratio limits $f($O III$)/f({\rm H}\alpha)<1/2$ and $f($O II$)/f({\rm H}\alpha)<2/3$. If the primary line is [O III] $\lambda$5007, unfortunately placed night-sky line residuals overlap the expected locations of [O III] $\lambda$4959 and H$\beta$, precluding interesting limits. Fortunately, the expected [O II] $\lambda$3727 line location is clean, and gives a tight upper limit $f($O II$)/f($O III$)<0.2$ ($3\sigma$). This provides some evidence against the [O III] $\lambda$5007 interpretation. Ratios of $f($O II$)/f($O III$)$ this small are seen in a significant minority of star forming galaxies (Xu et al., 2007; McLinden et al., 2011; Richardson et al., 2011, in prep; Xia et al., 2012, in prep), but more can be ruled out by this line ratio.

To address the [O II] $\lambda$3727 possibility and further improve our constraints on [O III] $\lambda$5007, we examine equivalent widths. Following Hibon et al. (2011), we combine our spectroscopic line flux with optical magnitude limits of 27.9, 27.6, and 27.3 mag ($5\sigma$, AB) in g^′, r^′, and i^′ filters respectively. Since star-forming galaxies have $f_{\nu}\sim\hbox{constant}$, we have $f_{\lambda}(9657{\rm\AA })\approx 3.6\times 10^{-20}\times 10^{-0.4\times 28}\times c/(9657{\rm\AA })^{2}=7.3\times 10^{-21}{\rm\,erg\,cm^{-2}\,s^{-1}\,\AA ^{-1}}$. The nondetection in these optical images then implies a $5\sigma$ limit $EW\equiv f_{{}_{\rm line}}/f_{\lambda}\approx(8\times 10^{-18}/7.3\times 10^{-21}){\rm\AA }=1100{\rm\AA }$ (observer frame).

While [O III] $\lambda$5007 and H$\alpha$ emission line sources with equivalent widths this large exist (Rhoads et al., 2000; Kakazu et al., 2007; Straughn et al., 2008, 2009; van der Wel et al., 2011; Atek et al., 2011), they are exceptional, rare objects. In Hibon et al. (2011), we estimated the numbers expected in our survey based on published line luminosity functions (Kakazu et al., 2007; Geach et al., 2010) and equivalent width distributions (Straughn et al., 2009). We found that $\lesssim 0.3$ [O III] $\lambda$5007 emitters and $\lesssim 0.6$ H$\alpha$ emitters are expected.

[O II] $\lambda$3727 emitters have generally smaller equivalent widths. We found no [O II] $\lambda$3727-selected objects with $EW\gtrsim 1100{\rm\AA }/(1+z_{[OII]})\approx 425{\rm\AA }$ in the samples from Straughn et al. (2009) (30 objects), Kakazu et al. (2007) (24 objects), Xia et al. (2012, in prep) (11 objects), or Drozdovsky et al. (2005) (400 objects). Thus, $\lesssim 1/465=0.0022$ of [O II] $\lambda$3727 emitters might enter our sample as LAE candidates. The luminosity function from Rigopoulou et al. (2005) suggests that our survey volume should contain $\sim 45$ [O II] $\lambda$3727 galaxies. Among these, $\lesssim 0.1$ object should pass our Lyman-$\alpha$ selection criteria.

Thus, the aggregate sample of foreground emitters expected in our survey is $<1$ galaxy. In contrast, our survey volume at $z\approx 6.95$ should contain between $\sim 2.5$ and $\sim 11$ Lyman-$\alpha$ galaxies with line fluxes $\gtrsim 8\times 10^{-18}{\rm\,erg\,cm^{-2}\,s^{-1}}$, based on the $z\approx 6.5$ luminosity functions of Hu et al. (2010) and Ouchi et al. (2010). We thus regard Lyman-$\alpha$ as the best interpretation of the observed emission line.

Redshift $z\approx 7$ is the current frontier in reionization studies, an area of active exploration where our observational knowledge is growing rapidly.

The recently discovered quasar at $z\approx 7.1$ (Mortlock et al 2011), combined with spectroscopic followup (and occasional confirmation) of $z\sim 7$ galaxy candidates from HST WFC3 surveys, provide an unprecedented look at this epoch. Observations of these objects seem to favor the continued existence of significant neutral intergalactic gas as late as $z\sim 7$. This is surprising, given that microwave background polarization data from WMAP favor a characteristic reionization redshift $z_{r}\sim 11$, and that the IGM at $z\sim 6.2$ is highly ionized, with a neutral fraction of only 1–4% based on quasar spectra (Fan et al 2006). Nonetheless, the ionized bubble around the $z\approx 7.1$ quasar appears too small to be comfortably explained in a fully ionized medium, unless the quasar is itself remarkably young ($\sim 10^{6}$ years) (Bolton et al 2011). Similarly, three independent research groups have argued that the fraction of $z\sim 7$ Lyman break candidates showing Lyman-$\alpha$ emission appears smaller than would be expected in an ionized medium (Vanzella et al 2010; Stark et al 2010; Ono et al 2011; Schenker et al 2011; Pentericci et al 2011). Still, these results depend on the reliability of photometric selection criteria, and a contamination of order 50% could explain the observations without recourse to neutral gas (e.g., Schenker et al 2011).

Lyman-$\alpha$ galaxy surveys offer a complementary approach to studying reionization. The underlying physics is the same as for spectroscopic followup of Lyman break samples, but the survey selection proceeds differently, leading to different potential selection biases. The uncertainties in the method are likewise very different from those associated with the quasar near-zone measurement (Mortlock et al., 2011; Bolton et al., 2011), or the Gunn-Peterson trough (Fan et al., 2006). Because of this, conclusions about cosmological reionization will be strongest when they are based on multiple independent methods.

The work we present here is only the second large-area narrowband survey for for Lyman-$\alpha$ galaxies at redshift $z\approx 7.0$, following on the work of Iye et al. (2006) and Ota et al. (2010). The spectroscopic confirmation of LAE J095950.99+021219.1 demonstrates that such objects can be identified at flux levels considerably fainter than the $2\times 10^{-17}{\rm\,erg\,cm^{-2}\,s^{-1}}$ line of IOK-1 (Iye et al., 2006) or the recently reported $z\approx 7.2$ narrowband-selected Lyman-$\alpha$ galaxy SXDF-NB1006-2 (Shibuya et al., 2011). Our observed line flux, $8.5\times 10^{-18}{\rm\,erg\,cm^{-2}\,s^{-1}}$, corresponds to a rest-frame line luminosity of only $L_{Ly\alpha}=(4.9\pm 1)\times 10^{42}{\rm\,erg\,s^{-1}}$. This is at or below the characteristic line luminosity $L^{*}_{Ly\alpha}$ from Schechter function fits to $z\approx 6.5$ Lyman-$\alpha$ samples (e.g., $L^{*}_{Ly\alpha}=(4.4\pm 0.6)\times 10^{42}{\rm\,erg\,s^{-1}}$ and $\Phi^{*}=8.5\times 10^{-4}{cMpc}^{-3}$, Ouchi et al. (2010); $L^{*}_{Ly\alpha}=1.0\times 10^{43}{\rm\,erg\,s^{-1}}$ and $\Phi^{*}=6\times 10^{-5}{cMpc}^{-3}$,Hu et al. (2010)).

Thus, our survey has achieved sensitivity to typical Lyman-$\alpha$ emitters. Were the $z=7$ luminosity function unchanged from that at $z=6.5$, we would expect our survey volume to contain 11 or 2.6 Lyman-$\alpha$ emitters brighter than LAE J095950.99+021219.1, based on the LF of Ouchi et al. (2010) or Hu et al. (2010) respectively. This reflects large differences in expectations from different published $z=6.5$ luminosity functions. We will present a detailed analysis of our survey’s constraints on the Lyman-$\alpha$ luminosity function in a future paper, after we have more sensitive spectra for our remaining candidates. For now, the galaxy LAE J095950.99+021219.1 is among the few most distant spectroscopically confirmed galaxies known, and the faintest to be discovered through a direct search for Lyman-$\alpha$ line emission.