The ubiquity of Lyman alpha (Ly ) emission in a sample of four bright O iii-strong star-forming galaxies with redshifts above seven has led to the suggestion that such luminous sources represent a ...distinct population compared with their fainter, more numerous counterparts. The presence of Ly emission within the reionization era could indicate that these sources created early ionized bubbles due to their unusually strong radiation, possibly because of the presence of active galactic nuclei. To test this hypothesis, we secured long integration spectra with XSHOOTER on the VLT for three sources selected to have similar luminosities and prominent excess fluxes in the IRAC 3.6 or 4.5 m band, usually attributed to strong O iii emission. We secured additional spectroscopy for one of these galaxies at z = 7.15 using MOSFIRE at the Keck telescope. For the most well-studied source in our sample with the strongest IRAC excess, we detect significant nebular emission from He ii and N v indicative of a non-thermal source. For the other two sources at z = 6.81 and z = 6.85, for which no previous optical/near-infrared spectroscopy was available, Ly is seen in one and C iii emission in the other. Although based on a modest sample, our results further support the hypothesis that the phenomenon of intense O iii emission is associated preferentially with sources lying in early ionized bubbles. However, even though one of our sources at z = 7.15 suggests the presence of non-thermal radiation, such ionized bubbles may not uniquely arise in this manner. We discuss the unique advantages of extending such challenging diagnostic studies with JWST.
A fundamental quest of modern astronomy is to locate the earliest galaxies and study how they influenced the intergalactic medium a few hundred million years after the Big Bang
. The abundance of ...star-forming galaxies is known to decline
from redshifts of about 6 to 10, but a key question is the extent of star formation at even earlier times, corresponding to the period when the first galaxies might have emerged. Here we report spectroscopic observations of MACS1149-JD1
, a gravitationally lensed galaxy observed when the Universe was less than four per cent of its present age. We detect an emission line of doubly ionized oxygen at a redshift of 9.1096 ± 0.0006, with an uncertainty of one standard deviation. This precisely determined redshift indicates that the red rest-frame optical colour arises from a dominant stellar component that formed about 250 million years after the Big Bang, corresponding to a redshift of about 15. Our results indicate that it may be possible to detect such early episodes of star formation in similar galaxies with future telescopes.
Abstract
Although low-mass star-forming galaxies are the leading candidates of the reionization process, we cannot conclusively rule out high-mass star-forming galaxies as candidates. While most ...simulations indicate the former is the best candidate, some models suggest that at
z
≥ 6 massive, UV-bright galaxies – “oligarchs” – account for at least 80% of the ionizing budget. To test this hypothesis, we target massive (
log
10
(
M
⋆
M
⊙
)
>
10
), UV-bright (
M
UV
∼ −22) Ly
α
emitters at
z
> 7 in archival data, observed with similar resolution spectrographs (Very Large Telescope/X-shooter and Keck/MOSFIRE). To increase the reliability of our conclusions, we stack all spectra and obtain a deep-stacked spectrum of 24.75 hr. The stacked Ly
α
profile displays a clear asymmetric red peak and an absence of a blue peak. We additionally estimate the intrinsic stacked Ly
α
profile of our targets by correcting for intergalactic medium (IGM) transmission using a range of neutral hydrogen fractions, finding no significant change in the profile. We measure a velocity offset
V
red
> 300 km s
−1
and an asymmetry in our red peak
A
∼ 3. Using various models and estimators, such as the peak separation, the asymmetry of the red peak, the ratio between Ly
α
and H
β,
and the
β
slope, we conclude that the escape fraction in these three UV-bright, massive (∼10
10
M
⊙
),
z
≥ 7 galaxies is
f
esc
(LyC) ≤ 10%.
Abstract
We examine the absorption-line spectra of a sample of 31 luminous (
) Lyman break galaxies at redshift
z
≃ 6 using data taken with the FOCAS and OSIRIS spectrographs on the Subaru and GTC ...telescopes. For two of these sources we present longer exposure data taken at higher spectral resolutions from ESO’s X-shooter spectrograph. Using these data, we demonstrate the practicality of stacking our lower-resolution data to measure the depth of various interstellar and stellar absorption lines to probe the covering fraction of low-ionization gas and the gas phase and stellar metallicities near the end of the era of cosmic reionization. From maximum absorption-line depths of Si
ii
λ
1260 and C
ii
λ
1334, we infer a mean covering fraction of ≥0.85 ± 0.16 for our sample. This is larger than that determined using similar methods for lower-luminosity galaxies at slightly lower redshifts, suggesting that the most luminous galaxies appear to have a lower escape fraction than fainter galaxies, and therefore may not play a prominent role in concluding reionization. Using various interstellar absorption lines we deduce gas-phase metallicities close to solar, indicative of substantial early enrichment. Using selected stellar absorption lines, we model our spectra with a range of metallicities using techniques successfully employed at lower redshift and deduce a stellar metallicity of
solar, consistent with the stellar mass—stellar metallicity relation recently found at
z
∼ 3–5. We discuss the implications of these metallicity estimates for the typical ages of our luminous galaxies and conclude our results imply initial star formation at redshifts
z
∼ 10, consistent with independent analyses of earlier objects.
Abstract
Physical properties of galaxies at
z
> 7 are of interest for understanding both the early phases of star formation and the process of cosmic reionization. Chemical abundance measurements ...offer valuable information on the integrated star formation history, and hence ionizing photon production, as well as the rapid gas accretion expected at such high redshifts. We use reported measurements of O
iii
88
μ
m emission and star formation rate to estimate gas-phase oxygen abundances in five galaxies at
z
= 7.1–9.1 using the direct
method. We find typical abundances
= 7.9 (∼0.2 times the solar value) and an evolution of 0.9 ± 0.5 dex in oxygen abundance at fixed stellar mass from
z
≃ 8 to 0. These results are compatible with theoretical predictions, albeit with large (conservative) uncertainties in both mass and metallicity. We assess both statistical and systematic uncertainties to identify promising means of improvement with the Atacama Large Millimeter/submillimeter Array (ALMA) and the James Webb Space Telescope (JWST). In particular we highlight O
iii
52
μ
m as a valuable feature for robust metallicity measurements. Precision of 0.1–0.2 dex in
-based O/H abundance can be reasonably achieved for galaxies at
z
≈ 5–8 by combining O
iii
52
μ
m with rest-frame optical strong lines. It will also be possible to probe gas mixing and mergers via resolved
-based abundances on kiloparsec scales. With ALMA and JWST, direct metallicity measurements will thus be remarkably accessible in the reionization epoch.
Abstract
The 100% detection rate of Ly
α
emission in a sample of four luminous
z
∼ 8 galaxies with red Spitzer/IRAC colors suggests objects with unusual ionizing capabilities that created early ...ionized bubbles in a neutral era. Whether such bubbles reflect enhanced ionizing properties (nature) or an overdense environment (nurture), however, remains unclear. Here we aim to distinguish between these hypotheses via a search for Ly
α
emission in five fainter galaxies drawn from the CANDELS-GOODS fields using a similar IRAC excess and UV magnitudes that should reflect reduced clustering effects. Using Keck/MOSFIRE we tentatively detect >4
σ
line emission in only two targets at redshifts
z
Ly
α
= 7.1081 and 7.9622 with rest-frame EWs of 16–17 Å, ∼1.5× weaker compared to their brighter counterparts. Thus, we find a reduced rate for Ly
α
emission of
0.40
−
0.25
+
0.30
compared to
1.00
−
0.44
+
0.00
for more luminous examples. The lower rate agrees with predictions from simulations of a mostly neutral intergalactic medium and an intrinsic EW
0,Ly
α
distribution for
z
∼ 6 galaxies. However, even with an extreme EW
0,Ly
α
model, it is challenging to match the detection rate for the luminous objects. Spectral energy distribution fitting of our fainter sample indicates young and star-forming systems, albeit with less extreme star formation rates and ionization parameters compared to their luminous counterparts. The enhanced Ly
α
rate in luminous galaxies is thus likely a byproduct of both extreme ionizing properties as well as environmental effects. Further studies with JWST may be required to resolve the physical nature of this puzzling population.
Abstract
We present new observations with the Atacama Large Millimeter/submillimeter Array for a gravitationally lensed galaxy at
z
= 9.1, MACS1149-JD1. O
iii
88
μ
m emission is detected at 10
σ
...with a spatial resolution of ∼0.3 kpc in the source plane, enabling the most distant morphokinematic study of a galaxy. The O
iii
emission is distributed smoothly without any resolved clumps and shows a clear velocity gradient with Δ
V
obs
/2
σ
tot
= 0.84 ± 0.23, where Δ
V
obs
is the observed maximum velocity difference and
σ
tot
is the velocity dispersion measured in the spatially integrated line profile, suggesting a rotating system. Assuming a geometrically thin self-gravitating rotation disk model, we obtain
V
rot
/
σ
V
=
0.67
−
0.26
+
0.73
, where
V
rot
and
σ
V
are the rotation velocity and velocity dispersion, respectively, still consistent with rotation. The resulting disk mass of
0.65
−
0.40
+
1.37
×
10
9
M
⊙
is consistent with being associated with the stellar mass identified with a 300 Myr old stellar population independently indicated by a Balmer break in the spectral energy distribution. We conclude that the most of the dynamical mass is associated with the previously identified mature stellar population that formed at
z
∼ 15.
We search for high-redshift dropout galaxies behind the Hubble Frontier Fields (HFF) galaxy cluster MACS J1149.5+2223, a powerful cosmic lens that has revealed a number of unique objects in its ...field. Using the deep images from the Hubble and Spitzer space telescopes, we find 11 galaxies at z > 7 in the MACS J1149.5+2223 cluster field, and 11 in its parallel field. The high-redshift nature of the bright z 9.6 galaxy MACS1149-JD, previously reported by Zheng et al., is further supported by non-detection in the extremely deep optical images from the HFF campaign. With the new photometry, the best photometric redshift solution for MACS1149-JD reduces slightly to z = 9.44 0.12. The young galaxy has an estimated stellar mass of ( 7 2 ) × 10 8 M , and was formed at z = 13.2 − 1.6 + 1.9 when the universe was 300 Myr old. Data available for the first four HFF clusters have already enabled us to find faint galaxies to an intrinsic magnitude of M UV − 15.5 , approximately a factor of 10 deeper than the parallel fields.
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