EVIDENCE FOR PopIII-LIKE STELLAR POPULATIONS IN THE MOST LUMINOUS Lyα EMITTERS AT THE
EPOCH OF REIONIZATION: SPECTROSCOPIC CONFIRMATION*
David Sobral
1,2,3,7
, Jorryt Matthee
3
, Behnam Darvish
4
, Daniel Schaerer
5,6
, Bahram Mobasher
4
,
Huub J. A. Röttgering
3
, Sérgio Santos
1,2,7
, and Shoubaneh Hemmati
4
1
Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa, OAL, Tapada da Ajuda, PT1349-018 Lisbon, Portugal; sobral@iastro.pt
2
Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Edifício C8, Campo Grande, PT1749-016 Lisbon, Portugal
3
Leiden Observatory, Leiden University, P.O. Box 9513, NL-2300 RA Leiden, The Netherlands
4
Department of Physics and Astronomy, University of California, 900 University Avenue, Riverside, CA 92521, USA
5
Observatoire de Genève, Département d’Astronomie, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
6
CNRS, IRAP, 14 Avenue E. Belin, F-31400 Toulouse, France
Received 2015 April 7; accepted 2015 June 4; published 2015 July 28
ABSTRACT
Faint Lyα emitters become increasingly rarer toward the reionization epoch (z ∼ 6–7). However, observations
from a very large (∼5 deg
2
) Lyα narrow-band survey at z = 6.6 show that this is not the case for the most luminous
emitters, capable of ionizing their own local bubbles. Here we present follow-up observations of the two most
luminous Lyα candidates in the COSMOS field: “MASOSA” and “CR7.” We used X-SHOOTER, SINFONI, and
FORS2 on the Very Large Telescope, and DEIMOS on Keck, to confirm both candidates beyond any doubt. We
find redshifts of z = 6.541 and z = 6.604 for “MASOSA” and “CR7,” respectively. MASOSA has a strong
detection in Lyα with a line width of 386 ± 30 km s
−1
(FWHM) and with very high EW
0
(>200 Å), but undetected
in the continuum, implying very low stellar mass and a likely young, metal-poor stellar population. “CR7,” with an
observed Lyα luminosity of 10
43.92±0.05
erg s
−1
is the most luminous Lyα emitter ever found at z > 6 and is
spatially extended (∼16 kpc). “CR7” reveals a narrow Lyα line with 266 ± 15 km s
−1
FWHM, being detected in
the near-infrared (NIR)(rest-frame UV; β = −2.3 ± 0.1) and in IRAC/Spitzer. We detect a narrow He
II
1640 Å emission line (6σ, FWHM = 130 ± 30 km s
−1
) in CR7 which can explain the clear excess seen in the
J-band photometry (EW
0
∼ 80 Å).Wefind no other emission lines from the UV to the NIR in our X-SHOOTER
spectra (He
II/O III] 1663 Å > 3 and He II/C III] 1908 Å > 2.5). We conclude that CR7 is best explained by a
combination of a PopIII-like population, which dominates the rest-frame UV and the nebular emission, and a more
normal stellar population, which presumably dominates the mass. Hubble Space Telescope/WFC3 observations
show that the light is indeed spatially separated between a very blue component, coincident with Lyα and He
II
emission, and two red components (∼5 kpc away), which dominate the mass. Our findings are consistent with
theoretical predictions of a PopIII wave, with PopIII star formation migrating away from the original sites of star
formation.
Key words: dark ages, reionization, first stars – early universe – galaxies: evolution
1. INTRODUCTION
The study of the most distant sources such as galaxies,
quasars, and gamma-ray bursts offers unique constraints on
early galaxy and structure formation. Such observations are
particularly important to test and refine models of galaxy
formation and evolution (e.g., Vogelsberger et al. 2014; Schaye
et al. 2015) and to study the epoch of reionization (e.g.,
Shapiro et al. 1994; Furlanetto et al. 2004; Sokasian et al. 2004;
Iliev et al. 2006; McQuinn et al. 2006). Over the last two
decades, considerable effort has been dedicated toward finding
the most distant sources. More recently, and particularly due to
the upgraded capabilities of the Hubble Space Telescope
(HST), multiple candidate galaxies up to z ∼ 8–11 (e.g.,
Bouwens et al. 2011; Ellis et al. 2013) have been found with
deep broadband photometry. However, spectroscopic confir-
mation is still limited to a handful of galaxies and quasars at
z > 6.5 (e.g., Mortlock et al. 2011; Ono et al. 2012; Finkelstein
et al. 2013; Pentericci et al. 2014; Schenker et al. 2014; Oesch
et al. 2015), for both physical (galaxies becoming increasingly
fainter) and observational reasons (the need for deep near-
infrared (NIR) exposures). At these redshifts (z > 6.5), the Lyα
line is virtually the only line available to confirm sources with
current instruments. However, Lyα is easily attenuated by dust
and neutral hydrogen in the inter-stellar and inter-galactic
medium. Indeed, spectroscopic follow-up of UV-selected
galaxies indicates that Lyα is suppressed at z >7(e.g.,
Caruana et al. 2014 ; Tilvi et al. 2014) and not a single z >8
Lyα emitter candidate has been confirmed yet (e.g., Sobral
et al. 2009; Faisst et al. 2014; Matthee et al. 2014). If the
suppression of Lyα is mostly caused by the increase of neutral
hydrogen fraction toward higher redshifts, it is clear that z ∼
6.5 (just over 0.8 Gyr after the Big Bang) is a crucial period,
because reionization should be close to complete at that redshift
(e.g., Fan et al. 2006).
Narrow-band searches have been successful in detecting and
confirming Lyα emitters at z ∼ 3–7 (e.g., Cowie & Hu 1998;
Malhotra & Rhoads 2004; Iye et al. 2006; Murayama
et al. 2007; Hu et al. 2010; Ouchi et al. 2010). The results
show that the Lyα luminosity function (LF)
is constant from z
∼ 3toz ∼ 6, but there are claims that the number density drops
from z ∼ 6toz ∼ 6.6 (e.g., Ouchi et al. 2010; Kashikawa
et al. 2011) and that it drops at an even faster rate up to z ∼ 7
(e.g., Shibuya et al. 2012; Konno et al. 2014). Moreover, the
The Astrophysical Journal, 808:139 (14pp), 2015 August 1 doi:10.1088/0004-637X/808/2/139
© 2015. The American Astronomical Society. All rights reserved.
* Based on observations obtained with X-SHOOTER, FORS2, and SINFONI
on the VLT, ESO DDT time (294.A-5018, 294.A-5039 ) and with DEIMOS on
Keck II (U082D).
7
FCT-IF/Veni Fellow.
1
fact that the rest-frame UV LF declines from z ∼ 3−6 (e.g.,
Bouwens et al. 2015) while the Lyα LF is roughly constant
over the same redshift range (e.g., Ouchi et al. 2008) implies
that the cosmic average Lyα escape fraction is likely
increasing, from ∼5% at z ∼ 2 (e.g., Hayes et al. 2010;
Ciardullo et al. 2014), to likely ∼20%–30% around z ∼ 6 (e.g.,
Cassata et al. 2015). Surprisingly, it then seems to fall sharply
with increasing redshift beyond z ∼ 6.5. Current results could
be a consequence of reionization not being completed at
z ∼ 6–7, particularly when taken together with the decline in
the fraction of Lyman break selected galaxies with high EW
Lyα emission (e.g., Caruana et al. 2014; Pentericci et al. 2014;
Tilvi et al. 2014). However, it is becoming clear that
reionization by itself is not enough to explain the rapid decline
of the fraction of strong Lyα emitters toward z ∼ 7 (e.g.,
Dijkstra 2014; Mesinger et al. 2015).
It is likely that reionization was very heterogeneous/patchy
(e.g., Pen tericci et al. 2014 ), with the early high density
regions reionizing first, followed by the rest of the Universe.
If that were the c ase, this process could have a distinguish-
able effect on the evolution of the Ly
α LF, and it may be that
the luminous end of the LF evolves differently from the
fainter end, as luminous Lyα emitters should in principle be
capable of ionizing their surroun ding s and thus are easier to
observe. This is exactly what is found by Matthee et al.
(2015 ), in agreement with spectroscopic results from Ono
et al. (2012 ).
In addition to using Lyα emitters to study reionization, they
are also useful for identifying the most extreme, metal-poor, and
young galaxies. Studies of Lyα emitters at z >2–3 show that, on
average, these sources are indeed very metal-poor (Finkelstein
et al. 2011; Nakajima et al. 2012; Guaita et al. 2013), presenting
high ionization parameters (high [O
III]/Hβ line ratios; Nakajima
et al. 2013) and very low typical dust extinctions (e.g., Ono
et al. 2010). Given these observations, Lyα searches should also
be able to find metal-free, PopIII stellar populations (since
galaxies dominated by PopIII emit large amounts of Lyα
photons, e.g., Schaerer 2002, 2003). However, so far, although
some candidates for PopIII stellar populations have been found
(e.g., Jimenez & Haiman 2006; Dijkstra & Wyithe 2007;Nagao
et al. 2008; Kashikawa et al. 2012; Cassata et al. 2013),andsome
metal-poor galaxies have been confirmed (e.g., Prescott
et al. 2009),theyareallsignificantly more metal-rich than the
expected PopIII stars, and show e.g., C
III] and C IV emission. For
example, when there is no evidence for the presence of an active
galactic nucleus (AGN) and no metal lines, the short-lived He
II
1640 Å emission line (the “smoking gun” for PopIII stars in
extremely high EW Lyα emitters without any metal emission
line) was never detected with high enough EW (e.g., Nagao
et al. 2008; Kashikawa et al. 2012).
Until recently, Lyα studies at the epoch of reionization have
been restricted to the more numerous, relatively faint sources of
L
Lyα
∼ 10
42.5
erg s
−1
(with some exceptions, e.g., z = 5.7
follow-up: Westra et al. 2006; Lidman et al. 2012; and
“Himiko”: Ouchi et al. 2009). However, with the wide-field
capabilities of current instruments (including Hyper Suprime-
Cam; Miyazaki et al. 2012), the identification of luminous Lyα
emitters will become increasingly easier. Recently, significant
progress was made toward finding luminous Lyα emitters
at z = 6.6 (Matthee et al. 2015), through a ∼5 deg
2
narrow-
band survey, which resulted in the identification of the most
luminous Lyα emitters at the epoch of reionization. Matthee
et al. (2015) reproduced the Lyα LF of Ouchi et al. (2010) for
relatively faint Lyα emitters at z = 6.6 for the UDS field, who
find a decrease in their number density compared to lower
redshifts. However, Matthee et al. ( 2015) find that the
luminous end of the z = 6.6 LF resembles the z = 3–5.7 LF,
and is thus consistent with no evolution at the bright end since
z ∼ 3. Extremely luminous Lyα emitters at z ∼ 6.6 are thus
found to be much more common than expected, with space
densities of
1.5 10
0.7
1.2
5
´
-
+
-
Mpc
−3
. The results may mean that,
because such bright sources can be observed at z ∼ 6.6, we are
witnessing preferential reionization happening around the most
luminous sources first. Such luminous sources may already be
free (in their immediate surroundings) of a significant amount
of neutral hydrogen, thus making their Lyα emission
observable. Furthermore, these sources open a new window
toward exploring the stellar populations of the most luminous
Lyα emitters at the epoch of reionization even before the James
Webb Space Telescope (JWST) and ∼30–40 m class telescopes
(Extremely Large Telescopes, ELTs) become operational, as
these are bright enough to be studied in unprecedented
detail with e.g., HST, ALMA, the Very Large Telescope
(VLT), Keck.
Here we present spectroscopy of the two most luminous Lyα
emitters found so far at the epoch of reionization (
z
7~
). This
paper is organized in the following way. Section 2 presents the
observations, the Lyα emitter sample, and the data reduction.
Section 3 outlines the details of the optical and NIR spectro-
scopic observations and measurements with the VLT and Keck
data. Section 4 presents the discovery of the most luminous
Lyα emitters and comparison with previous studies. Section 5
discusses spectral energy distribution (SED) fitting, model
assumptions and how HST high spatial resolution data
corroborates our best interpretation of the data. Section 6
presents the discussion of the results. Finally, Section 7 outlines
the conclusions. A H
0
= 70 km s
−1
Mpc
−1
, Ω
M
= 0.3 and
Ω
Λ
= 0.7 cosmology is used. We use a Salpeter (Salpeter 1955)
initial mass function (IMF) and all magnitudes are in the AB
system, unless noted otherwise.
2. SAMPLE AND SPECTROSCOPIC OBSERVATIONS
2.1. The Luminous Lyα Candidates at z = 6.6
Matthee et al. (2015) used the Subaru telescope and the
NB921 filter on Suprime-cam (Miyazaki et al. 2002) to survey
∼3 deg
2
in the SA22 (PI: D. Sobral), ∼1 deg
2
in COSMOS/
UltraVISTA (PI: M. Ouchi) and ∼1 deg
2
in UDS/SXDF (PI:
M. Ouchi) fields in order to obtain the largest sample of
luminous Lyα emitters at the epoch of reionization.
Out of the 135 Ly α candidates found in Matthee et al.
(2015), we discover two very bright Lyα candidates in the
COSMOS/UltraVISTA field: “CR7” (COSMOS Redshift 7)
and MASOSA.
8
MASOSA is particularly compact (0″.7),
while CR7 is extended (∼3″). We show the location of the Lyα
emitters within the COSMOS field footprint in Figure 1,in
which the size of the symbols scales with luminosity. We also
show their properties in Table 2.
8
The nickname MASOSA consists of the initials of the first three authors of
Matthee et al. (2015).
2
The Astrophysical Journal, 808:139 (14pp), 2015 August 1 Sobral et al.
Thumbnails in various wavelengths ranging from observed
optical to observed mid-infrared are shown in Figure 2. Both
candidates show very high rest-frame Lyα EWs
9
in excess of
>200 Å. By taking advantage of the wealth of data in the
COSMOS/UltraVISTA field (e.g., Capak et al. 2007; Scoville
et al. 2007; Ilbert et al. 2009; McCracken et al. 2012),we
obtain multi-band photometry for both sources. The measure-
ments are given in Table 2. We remeasure the 3.6 and 4.5 μm
photometry for CR7, in order to remove contamination from a
nearby source. Such contamination is at the level of 10%–20%,
and is added in quadrature to the photometry errors.
We find that CR7 has very clear detections in the NIR and
mid-infrared (Figure 2), showing a robust Lyman-break
(Steidel et al. 1996). CR7 is detected in IRAC, with colors
as expected for a z ∼ 6.6 source (Smit et al. 2014 ), likely due to
contribution from strong nebular lines with EWs in excess of a
few 100 Å in the rest-frame optical (see Figure 2). Because of
the detections in the NIR and MIR, the rest-frame UV
counterpart of CR7 was already identified as a z ∼ 6− 7
Lyman-break candidate (Bowler et al. 2012, 2014). However,
because of its very uncommon NIR colors (i.e., excess in J
relative to Y, H, and K), the clear IRAC detections, and,
particularly, without the NB921 data (Figure 2), it was classed
as an unreliable candidate, possibly a potential interloper or
cool star. MASOSA has a clear detection in the narrow-band
and is weakly detected in z, but the z-band detection can be
fully explained by Lyα. It is not detected at >1σ in the NIR
(J > 25.7, H > 24.5, K > 24.4), although a very weak signal is
visible in the thumbnails (Figure 2). This indicates that the Lyα
EW is very high and highlights that the Lyman-break selection
can easily miss such sources, even if they are extremely bright
and compact in Lyα, as MASOSA is not detected at the current
depth of the UltraVISTA survey (Bowler et al. 2014).
2.2. Spectroscopic Observations and Data Reduction
Spectroscopic observations were made with the VLT
10
using
X-SHOOTER and SINFONI (for CR7) and FORS2 (for
MASOSA). The choice of X-SHOOTER for CR7 was due to
the fact that it was detected in the NIR and showed evidence for
excess in the J band, likely indicating strong emission lines—
SINFONI was used to confirm the results and avoid potential
biases in the slit choice and inclination. Both sources were
observed with DEIMOS on the Keck II telescope (see Table 1)
as well. Spectra for both sources, obtained with both the VLT
and Keck, are shown in Figure 3, including the spectra obtained
by combining both data sets.
2.2.1. DEIMOS/Keck Observations
DEIMOS/Keck observations targeted both “CR7” and
“MASOSA” in two different masks and two different nights.
Observations were conducted on 2014 December 28 and 29.
The seeing was ∼0″. 5 on the first night, when we observed
“CR7,” and ∼0″. 7 on the second night, when we observed
“MASOSA.” Observations were done under clear conditions
with midpoint airmass of <1.1 for both sources. We used a
central wavelength of 7200 Å and the 600I grating, with a
resolution of 0.65 Å pix
−1
, which allowed us to probe from
4550 to 9850 Å. We used the 0″. 75 slit.
For CR7, we obtained four individual exposures of 1.2 ks
and one exposure of 0.6 ks, resulting in a total of 5.4 ks. For
MASOSA, we obtained a total of 2.7 ks. A strong, extended
and asymmetric line is clearly seen in every individual 1.2 ks
exposure prior to any data reduction.
We reduced the data using the DEIMOS
SPEC2D pipeline
(Cooper et al. 2012; Newman et al. 2013). The observed
spectra were flat-fielded, cosmic-ray-removed, sky-subtracted,
and wavelength-calibrated on a slit-by-slit basis. We used
standard Kr, Xe, Ar, and Ne arc lamps for wavelength solution
and calibration. No dithering pattern was used for sky
subtraction. The pipeline also generates 1D spectrum extraction
from the reduced 2D per slit, and we use the optimal extraction
algorithm (Horne 1986). This extraction creates a one-
dimensional spectrum of the target, containing the summed
flux at each wavelength in an optimized window. We also
extract the spectrum of both sources with varying apertures and
at various positions, in order to take advantage of the fact that
the sources are clearly spatially resolved. The final spectrum is
shown in Figure 3.
2.2.2. FORS2/VLT Observations
FORS2/VLT (Appenzeller et al. 1998) observations targeted
“MASOSA” and were obtained on 2015 January 12 and
February 11. The seeing was 0″. 7 and observations were done
under clear conditions. We obtained individual exposures of
1 ks and applied three different offsets along the slit. In total,
we obtained 6 ks. We used the OG590+32 filter together with
Figure 1. Projected positions on the sky of all Lyα candidates (green circles)
found in the COSMOS/UltraVISTA field. The gray background points
represent all detected sources with the NB921 filter, highlighting the masking
applied (due to the presence of artifacts caused by bright stars and noisy
regions, see Matthee et al. 2015).Lyα candidates are plotted with a symbol
size proportional to their Lyα luminosity. CR7 and MASOSA are highlighted
in red: these are the most luminous sources found in the field. Their coordinates
are given in Table 1.
9
EWs computed by using either z or Y lead to results in excess of 200 Å. We
also present EWs computed based on Y band and from our spectroscopic
follow-up in Table 2.
10
Observations conducted under ESO DDT programs 294.A-5018 and 294.A-
5039; PI: D. Sobral.
3
The Astrophysical Journal, 808:139 (14pp), 2015 August 1 Sobral et al.
the GRIS300I+11 Grism (1.62 Å pix
−1
) with the 1″ slit. Lyα is
clearly seen in each individual exposure of 1 ks.
We use the ESO FORS2 pipeline to reduce the data, along
with a combination of Python scripts to combine the 2D and
extract the 1D. The steps implemented follow a similar
procedure to that used for DEIMOS.
2.2.3. X-SHOOTER/VLT Observations
Our X-SHOOTER/VLT (Vernet et al. 2011) observations
targeted “CR7” and were obtained on 2015 January 22 and
February 15. The seeing varied between 0″. 8 and 0″. 9 and
observations were done under clear conditions. We obtained
individual exposures of 0.27 ks for the optical arm, while for
NIR we used individual exposures of 0.11 ks. We nodded from
an A to a B position, including a small jitter box in order to
always expose on different pixels. We used 0″. 9 slits for both
the optical and NIR arms (resolution of R ∼ 7500 and
R ∼ 5300, for the optical and NIR arms, respectively). In total,
for the X-SHOOTER data, we obtained 8.1 ks in the optical
and 9.9 ks in the NIR. The differences are driven by the slower
read-out time in the optical CCD compared to the NIR detector.
We use the ESO X-SHOOTER pipeline to fully reduce the
visible (optical) and NIR spectra separately. The final spectrum
is shown in Figure 3.
2.2.4. SINFONI/VLT Observations
We have also observed CR7 with the SINFONI (Eisenhauer
et al. 2003; Bonnet et al. 2004) integral field unit on the VLT
on 2015 March 8, 11–13, 17, and April 4. The seeing varied
between 0″. 6 and 0″.9 (median: 0″.77) in the J band and
observations were done under clear conditions. We used the
non-adaptive optics mode (spaxel size: 0″. 25, field of view of
8×8″) with the J-band grism (R ∼ 2000) and individual
exposure times of 0.3 ks. We took advantage of the relatively
large spatial coverage to conduct our observations with a jitter
box of 2″ (nine different positions for each set of 2.7 ks
observations). We obtained 45 exposures of 0.3 ks each,
resulting in a total exposure time of 13.5 ks.
We use the SINFONI pipeline (v2.5.2) in order to reduce the
data. The SINFONI pipeline dark subtracts, extracts the slices,
wavelength calibrates, flat-fields and sky-subtracts the data.
Flux calibration for each observation was carried out using
standard star observations which were taken immediately
before or after the science frames. A final stacked data-cube is
produced by co-adding reduced data from all the observations.
The collapsed data-cube does not result in any continuum
detection, as expected given the faint J flux. We extract the 1D
spectrum using an aperture of 1″.
3. MEASUREMENTS AND SED FITTING
3.1. Redshifts
In both the VLT (FORS2 or X-SHOOTER) and Keck
(DEIMOS) spectra for the two targets, we detect the very
strong Lyα line (Figure 3) in emission, and no continuum
either directly red-ward or blue-ward of Lyα
. The very clear
asymmetric profiles leave no doubts about them being Lyα and
about the secure redshift (Figure 3). Particularly for CR7, the
high signal-to-noise ratio (S/N)
150s>
(combined Keck and
VLT) at Lyα, despite the very modest exposure time for such a
high-redshift galaxy, clearly reveals that this source is unique.
Based on Lyα, we obtain redshifts of z = 6.604 for CR7
11
and z = 6.541 for MASOSA. The redshift determination yields
the same answer for both our data sets: X-SHOOTER and
DEIMOS, for CR7 and FORS2 and DEIMOS, for MASOSA
(see Figure 3, which shows the agreement). It is worth noting
that for CR7 we find that the Lyα emission line is detected in a
lower transmission region of the NB921 filter profile (50% of
peak transmission). Therefore, the Lyα luminosity of CR7 is
higher than estimated from the NB921 photometry, making the
source even more luminous than thought.
3.2. Spectral Line Measurements
By fitting a Gaussian profile to the emission lines, we
measure the EW (lower limits, as no continuum is detected)
and FWHM. Emission line fluxes are obtained by using NB921
and Y photometry (similarly to e.g., Ouchi et al. 2009, 2013),
in combination with the NB921 filter profile and the
appropriate redshift. We also check that the integrated emission
line (without any assumption on the fitting function) provides
results that are fully consistent.
For MASOSA, we find no other line in the optical spectrum,
and also find no continuum at any wavelength probed (see
Figure 3). For CR7, we find no continuum either directly blue-
ward or red-ward of Lyα in the optical spectrum (both in
X-SHOOTER and DEIMOS; Figure 3). However, we make a
continuum detection (spatially very compact) in the rest-frame
Figure 2. Thumbnails of both luminous Lyα emitters in the optical to MIR from left to right. Each thumbnail is 8 × 8″, corresponding to ∼44 × 44 kpc at z ∼ 6.6. Note
that while for MASOSA the Lyα emission line is detected by the NB921 filter at full transmission, for CR7 the Lyα is only detected at ∼50% transmission. Therefore,
the NB921 only captures ∼50% of the Lyα flux: the observed flux coming from the source is ∼2× larger.
11
CR7 has a redshift of z = 6.600 based on He II 1640 Å (see Section 3.3).
4
The Astrophysical Journal, 808:139 (14pp), 2015 August 1 Sobral et al.
916–1017 Å for CR7 (rest-frame Lyman–Werner photons),
with clear absorption features corresponding to the Lyα forest.
The reddest wavelength for which we can see continuum
directly from the spectra corresponds to Lyα at z = 5.3. For
higher redshifts, the flux is consistent with zero for our spectra.
This clear continuum detection at wavelengths slightly redder
than the Lyman-limit, but then disappearing for longer
wavelengths, can be explained by a combination of a very
blue, strong continuum ( intense Lyman–Werner radiation) and
an average increase of the neutral hydrogen fraction along the
line of sight toward higher redshift, similar to the Gunn-
Peterson trough observed in quasar spectra (e.g., Becker
et al. 2001; Meiksin 2005). However, due to the average
transmission of the intergalactic medium (IGM), the fact that
even just a fraction of the light is able to reach us is a unique
finding. These findings will be investigated separately in
greater detail in a future paper, including further follow-up
which will allow an even higher S/N.
3.3. NIR Spectra of CR7: He
II and no Other Lines
We explore our X-SHOOTER NIR spectra to look for any
other emission lines in the spectrum of CR7. The photometry
reveals a clear J-band excess (0.4 ± 0.13 mag brighter than
expected from Y, H and K; see Table 2), which could
potentially be explained by strong emission lines (e.g., C
IV
1549 Å, He II 1640 Å, O III] 1661 Å, O III] 1666 Å, N III]
1750 Å).
We mask all regions for which the error spectrum is too large
(>1.5× the error on OH line free regions), including the
strongest OH lines. We then inspect the spectrum for any
emission lines. We find an emission line at 12464 Å (see
Figure 4).Wefind no other emission lines in the spectrum
Table 1
Sources and Observation Log
Source
a
R.A. Decl. Int. Time VLT
b
[Keck] Dates of Observations Features Detected in Spectrum
c
(J2000)(J2000)(ks/pixel)
CR7 10 00 58.005 +01 48 15.251 VIS: 8.1, [5.4] 2014 Dec 28; 2015 Jan 22, Feb 15 916–1017 Å, Lyα,He
II 1640 Å
CR7 10 00 58.005 +01 48 15.251 NIR: 9.9 2015 Jan 22, Feb 15 He
II 1640 Å
CR7 10 00 58.005 +01 48 15.251 SINFONI: 13.5 2015 Mar 8, 11–13, 17 and 2015 Apr 4 He
II 1640 Å
MASOSA 10 01 24.801 +02 31 45.340 VIS: 6, [2.7] 2014 Dec 29; 2015 Jan 12 Lyα
Notes.
a
Observation log for the two luminous Lyα emitter candidates, observed with both the VLT (using X-SHOOTER and SINFONI: CR7 and FORS2: MASOSA) and
Keck (using DEIMOS for both sources), which we have spectroscopically confirmed.
b
Note that for X-SHOOTER (CR7) the VIS (visible) and NIR (near-infrared) arms provide different total exposure times (due to different read-out times), and we
thus provide them separately in different lines. Keck/DEIMOS exposure times are presented between square brackets: [exposure time].
c
We also note the features and lines detected in the spectra: for CR7 we detect rest-frame UV just redder of the Lyman-limit, with a clear Lyα forest, but such flux
completely disappears toward redder wavelengths, implying a very blue spectra. No continuum is detected apart from that. An extremely luminous, high EW and
narrow Lyα emission line is detected for each of the sources. For CR7, where we have NIR coverage, from both X-SHOOTER and SINFONI, we detect high EW
He
II 1640 Å emission fro both instruments, but no other lines.
Figure 3. Left: “CR7” 1D and 2D optical spectra, showing the strong and clear Lyα emission line. We also show the NB921 filter profile which was used to select the
source. Note that Lyα is detected at the wing of the NB921 filter. Thus, while the NB921 photometry already implied that the source was very luminous, its true
luminosity was still underestimated by a factor of 2. We show both our Keck/DEIMOS and VLT/X-SHOOTER spectra, which show perfect agreement, but with
X-SHOOTER providing an even higher spectral resolution, while the DEIMOS spectrum gives an even higher S/N. Right: “MASOSA” 1D and 2D optical spectra
(FORS2), showing the strong and clear Lyα emission line. We also show the NB921 filter profile which was used to select the source. We show both the VLT/FORS2
and Keck/DEIMOS spectra, showing that they agree very well. The DEIMOS spectrum provides higher resolution, but both clearly reveal the asymmetry of the line,
confirming it as Lyα without any doubt.
5
The Astrophysical Journal, 808:139 (14pp), 2015 August 1 Sobral et al.
![Figure 4. Left: our X-SHOOTER NIR spectrum of CR7, revealing a significant detection of the He II 1640 Å emission line. We show both the data at full resolution and binning in wavelength (with a resolution of 0.4 Å). We also show the combined X-SHOOTER and SINFONI data and also show the sky spectrum. We note that sky lines were explicitly masked, but some residuals are still visible, including those of two OH lines just redder of the He II emission line, which show up as a slight flux increase and another one bluer of He II, which has been slightly over-subtracted. We note that no continuum is found in the NIR spectra, and that we would require significantly deeper observations in order to detect it simply based on the NIR photometry. Right: we investigate the NIR X-SHOOTER spectrum for other emission lines (e.g., Stark et al. 2014). We do not find any other emission line apart from He II, but we show one of the lines that should be stronger in our spectrum (typically ∼2× stronger than He II, thus He II/C III] ∼ 0.5). We use our He II line detection and show it at the position of the C III] doublet for a typical line ratio of He II/C III] of 0.5, but also a line ratio of ∼1 and ∼2. Our data allow us to place a limit of He II/C III] > 2.5. We also investigate the presence of N V 1240, N IV 1487, C IV 1549, O III] 1661, O III] 1666, N III] 1750: all of these are undetected.](/figures/figure-4-left-our-x-shooter-nir-spectrum-of-cr7-revealing-a-36427of5.webp)
