Abstract
We conduct a comprehensive study on dropout galaxy candidates at
z
∼ 9–16 using the first 90 arcmin
2
James Webb Space Telescope (JWST) Near Infrared Camera images taken by the early release ...observations (ERO) and early release science programs. With the JWST simulation images, we find that a number of foreground interlopers are selected with a weak photo-
z
determination (Δ
χ
2
> 4). We thus carefully apply a secure photo
-z
selection criterion (Δ
χ
2
> 9) and conventional color criteria with confirmations of the ERO Near Infrared Spectrograph spectroscopic redshifts, and obtain a total of 23 dropout galaxies at
z
∼ 9–16, including two candidates at
z
phot
=
16.25
−
0.46
+
0.24
and
16.41
−
0.55
+
0.66
. We perform thorough comparisons of dropout galaxies found in our work with recent JWST studies, and conclude that our galaxy sample is reliable enough for statistical analyses. We derive the UV luminosity functions at
z
∼ 9–16, and confirm that our UV luminosity functions at
z
∼ 9 and 12 agree with those determined by other Hubble Space Telescope and JWST studies. The cosmic star formation rate (SFR) density decreases from
z
∼ 9 to 12, and perhaps to 16, but the densities at
z
∼ 12–16 are higher than the constant star formation efficiency model. Interestingly, there are six bright galaxy candidates at
z
∼ 10–16 with
M
UV
< −19.5 mag and
M
*
∼ 10
8−9
M
⊙
. Because a majority (∼80%) of these galaxies show no signatures of active galactic nuclei in their morphologies, the high cosmic SFR densities and the existence of these UV-luminous galaxies are explained by the lack of suppression of star formation by the UV background radiation at the pre-reionization epoch and/or an efficient UV radiation production by a top-heavy initial mass function with Population III–like star formation.
We present the first statistical sample of faint type 1 AGNs at z > 4 identified by JWST/NIRSpec deep spectroscopy. Among the 185 galaxies at z _spec = 3.8–8.9 confirmed with NIRSpec, our systematic ...search for broad-line emission reveals 10 type 1 AGNs at z = 4.015–6.936 whose broad component is only seen in the permitted H α line and not in the forbidden O iii λ 5007 line that is detected with greater significance than H α . The broad H α line widths of FWHM ≃ 1000–6000 km s ^−1 suggest that the AGNs have low-mass black holes with M _BH ∼ 10 ^6 –10 ^8 M _⊙ , remarkably lower than those of low-luminosity quasars previously identified at z > 4 with ground-based telescopes. JWST and Hubble Space Telescope high-resolution images reveal that the majority of them show extended morphologies indicating significant contribution to the total lights from their host galaxies, except for three compact objects two of which show red spectral energy distributions, probably in a transition phase from faint AGNs to low luminosity quasars. Careful AGN-host decomposition analyses show that their host’s stellar masses are systematically lower than the local relation between the black hole mass and the stellar mass, implying a fast black hole growth consistent with predictions from theoretical simulations. A high fraction of the broad-line AGNs (∼5%), higher than z ∼ 0, indicates that the number density of such faint AGNs is higher than an extrapolation of the quasar luminosity function, implying a large population of AGNs in the early universe. Such faint AGNs contribute to cosmic reionization, while the total contribution is not large, up to ∼50% at z ∼ 6, because of their faint nature.
Abstract
We present the evolution of the mass–metallicity (MZ) relation at
z
= 4–10 derived with 135 galaxies identified in JWST/NIRSpec data taken from the three major public spectroscopy programs ...of ERO, GLASS, and CEERS. Because there are many discrepancies between the flux measurements reported by the early ERO studies, we first establish our NIRSpec data reduction procedure for reliable emission-line flux measurements and errors, successfully explaining Balmer decrements with no statistical tensions thorough comparisons with the early ERO studies. Applying the reduction procedure to the 135 galaxies, we obtain emission-line fluxes for physical property measurements. We confirm that 10 out of the 135 galaxies with O
iii
λ
4363 lines have electron temperatures of ≃(1.1–2.3) × 10
4
K, similar to lower-
z
star-forming galaxies, which can be explained by heating by young massive stars. We derive the metallicities of the 10 galaxies by a direct method and the rest of the galaxies with strong lines using the metallicity calibrations of Nakajima et al. applicable for these low-mass metal-poor galaxies, anchoring the metallicities with the direct-method measurements. We thus obtain the MZ relation and star formation rate (SFR)–MZ relation over
z
= 4–10. We find that there is a small evolution of the MZ relation from
z
∼ 2–3 to
z
= 4–10, while interestingly the SFR–MZ relation shows no evolution up to
z
∼ 8 but a significant decrease at
z
> 8 beyond the errors This SFR–MZ relation decrease at
z
> 8 may suggest a break of the metallicity equilibrium state via star formation, inflow, and outflow, while further statistical and local-baseline studies are needed for a conclusion.
Abstract
Strong high-ionization lines such as He
ii
of young galaxies are puzzling at high and low redshift. Although recent studies suggest the existence of nonthermal sources, whether their ...ionizing spectra can consistently explain multiple major emission lines remains a question. Here we derive the general shapes of the ionizing spectra for three local extremely metal-poor galaxies (EMPGs) that show strong He
ii
λ
4686. We parameterize the ionizing spectra composed of a blackbody and power-law radiation mimicking various stellar and nonthermal sources. We use photoionization models for nebulae and determine seven parameters of the ionizing spectra and nebulae by Markov Chain Monte Carlo methods, carefully avoiding systematics of abundance ratios. We obtain the general shapes of ionizing spectra explaining ∼10 major emission lines within observational errors with smooth connections from observed X-ray and optical continua. We find that an ionizing spectrum of one EMPG has a blackbody-dominated shape, while the others have convex downward shapes at >13.6 eV, which indicate a diversity of the ionizing spectrum shapes. We confirm that the convex downward shapes are fundamentally different from ordinary stellar spectrum shapes, and that the spectrum shapes of these galaxies are generally explained by the combination of the stellar and ultraluminous X-ray sources. Comparisons with stellar synthesis models suggest that the diversity of the spectrum shapes arises from differences in the stellar age. If galaxies at
z
≳ 6 are similar to the EMPGs, high-energy (>54.4 eV) photons of the nonstellar sources negligibly contribute to cosmic reionization due to relatively weak radiation.
Abstract
We present morphologies of galaxies at
z
≳ 9 resolved by JWST/NIRCam 2–5
μ
m imaging. Our sample consists of 22 galaxy candidates identified by stringent dropout and photo-
z
criteria in ...GLASS, CEERS, SMACS J0723, and Stephan’s Quintet flanking fields, one of which has been spectroscopically identified at
z
= 11.44. We perform surface brightness (SB) profile fitting with GALFIT for six bright galaxies with a signal-to-noise ratio = 10–40 on an individual basis and for stacked faint galaxies with secure point-spread functions (PSFs) of the NIRCam real data, carefully evaluating systematics by Monte Carlo simulations. We compare our results with those of previous JWST studies, and confirm that the effective radii
r
e
of our measurements are consistent with those of previous measurements at
z
∼ 9. We obtain
r
e
≃ 200–300 pc with the exponential-like profiles, Sérsic indexes of
n
≃ 1–1.5, for galaxies at
z
∼ 12–16, indicating that the relation of
r
e
∝ (1 +
z
)
s
for
s
=
−
1.22
−
0.16
+
0.17
explains cosmic evolution over
z
∼ 0–16 for
∼
L
z
=
3
*
galaxies. One bright (
M
UV
= −21 mag) galaxy at
z
∼ 12, GL-z12-1, has an extremely compact profile with
r
e
= 39 ± 11 pc that is surely extended over the PSF. Even in the case that the GL-z12-1 SB is fit by active galactic nuclei + galaxy composite profiles, the best-fit galaxy component is again compact,
r
e
=
48
−
15
+
38
pc, which is significantly (>5
σ
) smaller than the typical
r
e
value at
z
∼ 12. Compared with numerical simulations, we find that such a compact galaxy naturally forms at
z
≳ 10, and that frequent mergers at the early epoch produce more extended galaxies following the
r
e
∝ (1 +
z
)
s
relation.
Abstract
We present electron densities
n
e
in the interstellar medium (ISM) of star-forming galaxies at
z
= 4–9 observed by the JWST/NIRSpec GLASS, Early Release Observations, and CEERS programs. We ...carefully evaluate the line-spread functions of the NIRSpec instrument as a function of wavelength with the calibration data of a planetary nebula taken on board, and obtain secure O
ii
λ
λ
3726, 3729 doublet fluxes for 14 galaxies at
z
= 4.02–8.68 falling on the star formation main sequence with the NIRSpec high- and medium-resolution spectra. We thus derive the electron densities of singly ionized oxygen nebulae with the standard
n
e
indicator of the O
ii
doublet, and find that the electron densities of the
z
= 4–9 galaxies are
n
e
≳ 300 cm
−3
significantly higher than those of low-
z
galaxies at a given stellar mass, star formation rate (SFR), and specific SFR. Interestingly, the typical electron densities of the singly ionized nebulae increase from
z
= 0 to
z
= 1−3 and
z
= 4–9, which is approximated by the evolutionary relation of
n
e
∝ (1 +
z
)
p
with
p
∼ 1–2. Although it is not obvious that the ISM property of
n
e
is influenced by global galaxy properties, these results may suggest that the nebula densities of high-
z
galaxies are generally high due to the compact morphologies of high-
z
galaxies evolving by
r
e
∝
∼
(
1
+
z
)
−
1
(
r
vir
∝ (1 +
z
)
−1
) for a given stellar (halo) mass whose inverse square corresponds to the
p
∼ 2 evolutionary relation. The
p
∼ 1−2 evolutionary relation can be explained by a combination of the compact morphology and the reduction of
n
e
due to the high electron temperature of high-
z
metal-poor nebulae.
Abstract
We present chemical abundance ratios of 70 star-forming galaxies at
z
∼ 4–10 observed by the JWST/NIRSpec Early Release Observations, GLASS, and CEERS programs. Among the 70 galaxies, we ...have pinpointed two galaxies, CEERS_01019 at
z
= 8.68 and GLASS_150008 at
z
= 6.23, with extremely low C/N (C/N ≲ −1), evidenced with C
iii
λλ
1907,1909, N
iii
λ
1750, and N
iv
λλ
1483,1486, which show high N/O ratios (N/O ≳ 0.5) comparable with the one of GN-z11, regardless of whether stellar or active galactic nucleus radiation is assumed. Such low C/N and high N/O ratios found in CEERS_01019 and GLASS_150008 (additionally identified in GN-z11) are largely biased toward the equilibrium of the CNO cycle, suggesting that these three galaxies are enriched by metals processed by the CNO cycle. On the C/N versus O/H plane, these three galaxies do not coincide with Galactic H
ii
regions, normal star-forming galaxies, and nitrogen-loud quasars with asymptotic giant branch stars, but with globular-cluster (GC) stars, indicating a connection with GC formation. We compare the C/O and N/O of these three galaxies with those of theoretical models and find that these three galaxies are explained by scenarios with dominant CNO-cycle materials, i.e., Wolf–Rayet stars, supermassive (10
3
–10
5
M
⊙
) stars, and tidal disruption events, interestingly with a requirement of frequent direct collapses. For all the 70 galaxies, we present measurements of Ne/O, S/O, and Ar/O, together with C/O and N/O. We identify four galaxies with very low Ne/O, log(Ne/O) < −1.0, indicating abundant massive (≳30
M
⊙
) stars.
Abstract We present the evolution of Ly α emission derived from 53 galaxies at z = 6.6–13.2, which have been identified by multiple JWST/NIRSpec spectroscopy programs of Early Release Science, ...General Observer, Director's Discretionary Time, and Guaranteed Time Observations. These galaxies fall on the star formation main sequence and are typical star-forming galaxies with UV magnitudes of −22.5 ≤ M UV ≤ −17.0. We find that 15 out of 53 galaxies show Ly α emission at the >3 σ level, and we obtain Ly α equivalent width (EW) measurements and stringent 3 σ upper limits for the 15 and 38 galaxies, respectively. Confirming that Ly α velocity offsets and line widths of our galaxies are comparable to those of low-redshift Ly α emitters, we investigate the redshift evolution of the Ly α EW. We find that Ly α EWs statistically decrease toward high redshifts on the Ly α EW versus the M UV plane for various probability distributions of the uncertainties. We then evaluate neutral hydrogen fractions x H I with the redshift evolution of the Ly α EW and the cosmic reionization simulation results on the basis of a Bayesian inference framework, and obtain x H I < 0.79, , and at z ∼ 7, 8, and 9–13, respectively. These moderately large x H I values are consistent with the Planck cosmic microwave background optical depth measurement and previous x H I constraints from galaxy and QSO Ly α damping wing absorption and strongly indicate a late reionization history. Such a late reionization history suggests that major sources of reionization would emerge late and be hosted by moderately massive halos compared with the widely accepted picture of abundant low-mass objects for the sources of reionization.
Abstract We present statistics of z ∼ 6–9 galaxy outflows indicated by spatially extended gas emission and broad lines. With a total of 61 spectroscopically confirmed galaxies at z ∼ 6–9 in the JWST ...CEERS, GLASS, and ERO data, we find four galaxies with O iii + H β ionized gas emission significantly extended beyond the kiloparsec-scale stellar components on the basis of the emission line images constructed by the subtraction of NIRCam broadband (line on/off-band) images. By comparison with low- z galaxies, the fraction of galaxies with spatially extended gas, 4/18, at z ∼ 6–9 is an order of magnitude higher than that at z ∼ 0–1, which can be explained by events triggered by frequent major mergers at high redshift. We also investigate medium- and high-resolution NIRSpec spectra of 30 galaxies at z ∼ 6–9, and identify five galaxies with broad (140−800 km s −1 ) lines in the O iii forbidden line emission, suggestive of galaxy outflows. One galaxy at z = 6.38 shows both spatially extended gas emission and the broad lines, while none of the galaxies with spatially extended gas emission or broad lines present a clear signature of active galactic nuclei (AGN) in either the line diagnostics or type 1 AGN line broadening (>1000 km s −1 ), which suggests that outflows are mainly driven by stellar feedback. The existence of galaxies with/without spatially extended gas emission or broad lines may be explained by different viewing angles toward outflows or the fact that these are galaxies in the early, late, or post phases of galaxy outflows at high redshift, where relatively large fractions of such galaxies indicate longer-duration and/or more frequent outflows at early cosmic epochs.
Abstract
We present pure spectroscopic constraints on the UV luminosity functions and cosmic star formation rate (SFR) densities from 25 galaxies at
z
spec
= 8.61–13.20. By reducing the JWST/NIRSpec ...spectra taken in multiple programs of Early Release Observation, Early Release Science, General Observer, and Director’s Discretionary Time observations with our analysis technique, we independently confirm 16 galaxies at
z
spec
= 8.61–11.40, including new redshift determinations, and a bright interloper at
z
spec
= 4.91 that was claimed as a photometric candidate at
z
∼ 16. In conjunction with nine galaxies at redshifts up to
z
spec
= 13.20 in the literature, we make a sample of 25 spectroscopically confirmed galaxies in total and carefully derive the best estimates and lower limits of the UV luminosity functions. These UV luminosity function constraints are consistent with the previous photometric estimates within the uncertainties and indicate mild redshift evolution toward
z
∼ 12, showing tensions with some theoretical models of rapid evolution. With these spectroscopic constraints, we obtain firm lower limits of the cosmic SFR densities and spectroscopically confirm a high SFR density at
z
∼ 12 beyond the constant star formation efficiency models, which supports earlier claims from the photometric studies. While there are no spectroscopically confirmed galaxies with very large stellar masses violating the ΛCDM model due to the removal of the bright interloper, we confirm star-forming galaxies at
z
spec
= 11–13 with stellar masses much higher than model predictions. Our results indicate possibilities of high star formation efficiency (>5%), a hidden active galactic nucleus, a top-heavy initial mass function (possibly with Population III), and large scatter/variance. Having these successful and unsuccessful spectroscopy results, we suggest observational strategies for efficiently removing low-redshift interlopers for future JWST programs.
