We present a systematic study for ionization state of inter-stellar medium in galaxies at z = 0– with ∼140 000 Sloan Digital Sky Survey (SDSS) galaxies and 108 intermediate- to high-redshift galaxies ...from the literature, using an ionization parameter sensitive line ratio of O iiiλ5007/O iiλ3727 and photoionization models. We confirm that z ∼ 2–3 galaxies show an O iii/O ii ratio significantly higher than a typical star-forming galaxy of SDSS by a factor of ≳ 10, and the photoionization models reveal that these high-z galaxies have an ionization parameter of log (q
ion/cm s−1) ∼ 7.6–9.0, a factor of ∼4–10 higher than local galaxies. For galaxies at any redshift, we identify a correlation between the O iii/O ii ratio and galaxy global properties of star formation rate (SFR), stellar mass (M
*), and metallicity (Z). We extend the fundamental metallicity relation (FMR) and develop the fundamental ionization relation (FIR), a four-dimensional relation of ionization parameter, SFR, M
*, and Z. The intermediate- and high-z galaxies up to z ≃ 3 follow the FIR defined with the local galaxies, in contrast with the FMR whose possible evolution from z ∼ 2 to 3 is reported. We find that the FMR evolution of z ∼ 2–3 appears, if one omits ionization parameter differences, and that the FMR evolution does not exist for an average metallicity solution of z ∼ 3 galaxies with a high-ionization parameter. Interestingly, all of two local Lyman-continuum emitting galaxies (LyC leakers) have a high O iii/O ii ratio, indicating a positive correlation between O iii/O ii and ionizing photon escape fraction (f
esc), which is successfully explained by our photoionization models. Because O iii/O ii ratios of z ∼ 2–3 galaxies, especially Lyα emitters (LAEs), are comparable to, or higher than, those of the local LyC leakers, these high-z galaxies are candidates of Lyman-continuum emitting objects. A strong Lyα emission can coexist with a large f
esc of ≲ 0.8, and the increasing fraction of LAEs towards high-z reconciles the picture of cosmic reionization whose major ionizing sources are faint galaxies having intrinsically bright Lyα emission.
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.
We discuss the rest-frame optical emission line spectra of a large (∼50) sample of z ∼ 3.1 Ly emitting galaxies (LAEs) whose physical properties suggest such sources are promising analogs of galaxies ...in the reionization era. Reliable Lyman continuum (LyC) escape fractions have now been determined for a large sample of such LAEs from the LymAn Continuum Escape Survey (LACES) undertaken via deep Hubble Space Telescope imaging in the SSA22 survey area reported in Fletcher et al. Using new measures of O ii emission secured from Keck MOSFIRE spectra we re-examine, for a larger sample, earlier claims that LyC leakages may correlate with the nebular emission line ratio O iii/O ii as expected for density-bound H ii regions. We find that a large O iii/O ii line ratio is indeed a necessary condition for LyC leakage, strengthening earlier claims made using smaller samples at various redshifts. However, not all LAEs with large O iii/O ii line ratios are leakers and leaking radiation appears not to be associated with differences in other spectral diagnostics. This suggests the detection of leaking radiation is modulated by an additional property, possibly the viewing angle for porous H ii regions. We discuss our new results in the context of the striking bimodality of LAE leakers and non-leakers found in the LACES program and the implications for the sources of cosmic reionization.
Abstract
We discuss the rest-frame ultraviolet (UV) emission line spectra of a large (∼100) sample of low luminosity redshift z ∼ 3.1 Lyman alpha emitters (LAEs) drawn from a Subaru imaging survey in ...the SSA22 survey field. Our earlier work based on smaller samples indicated that such sources have high O iii/O ii line ratios possibly arising from a hard ionizing spectrum that may be typical of similar sources in the reionization era. With optical spectra secured from VLT/VIMOS, we re-examine the nature of the ionizing radiation in a larger sample using the strength of the high ionization diagnostic emission lines of Ciiiλ1909, Civλ1549, Heiiλ1640, and O iiiλλ1661, 1666 Å in various stacked subsets. Our analysis confirms earlier suggestions of a correlation between the strength of Ly α and Ciii emission and we find similar trends with broad-band UV luminosity and rest-frame UV colour. Using various diagnostic line ratios and our stellar photoionization models, we determine both the gas phase metallicity and hardness of the ionization spectrum characterized by ξion – the number of Lyman continuum photons per UV luminosity. We confirm our earlier suggestion that ξion is significantly larger for LAEs than for continuum-selected Lyman break galaxies, particularly for those LAEs with the faintest UV luminosities. We briefly discuss the implications for cosmic reionization if the metal-poor intensely star-forming systems studied here are representative examples of those at much higher redshift.
We present results from the LymAn Continuum Escape Survey (LACES), a Hubble Space Telescope (HST) program designed to characterize the ionizing radiation emerging from a sample of Ly -emitting ...galaxies at redshift z 3.1. As many show intense O iii emission characteristic of z > 6.5 star-forming galaxies, they may represent valuable low-redshift analogs of galaxies in the reionization era. Using HST Wide Field Camera 3/UVIS F336W to image Lyman continuum emission, we investigate the escape fraction of ionizing photons in this sample. For 61 sources, of which 77% are spectroscopically confirmed and 53 have measures of O iii emission, we detect Lyman continuum leakage in 20%, a rate significantly higher than is seen in individual continuum-selected Lyman break galaxies. We estimate that there is a 98% probability that ≤2 of our detections could be affected by foreground contamination. Fitting multiband spectral energy distributions (SEDs) to take account of the varying stellar populations, dust extinctions and metallicities, we derive individual Lyman continuum escape fractions corrected for foreground intergalactic absorption. We find escape fractions of 15%-60% for individual objects and infer an average 20% escape fraction by fitting composite SEDs for our detected samples. Surprisingly, however, even a deep stack of those sources with no individual F336W detections provides a stringent upper limit on the average escape fraction of less than 0.5%. We examine various correlations with source properties and discuss the implications in the context of the popular picture that cosmic reionization is driven by such compact, low-metallicity star-forming galaxies.
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.
Abstract We investigate the galaxy properties of ∼400 low-mass (<10 9 M ⊙ ) H α emitters (HAEs) at z ∼ 2.3 in the ZFOURGE survey. The selection of these HAEs is based on the excess in the observed K ...s broadband flux compared to the stellar continuum estimated from the best-fit spectral energy distribution. These low-mass HAEs have elevated SFR(H α ) above the star formation main sequence, making them potential analogs of the galaxies that reionized the Universe during the Epoch of Reionization. The ionizing photon production efficiencies ( ξ ion ) of the low-mass HAEs have a median value of log ( ξ ion / erg − 1 Hz ) = 25.24 − 0.13 + 0.10 ( 25.35 − 0.15 + 0.12 ) , assuming the Calzetti (SMC) curve for the stellar continuum dust correction. This value is higher than that of main-sequence galaxies by ∼0.2 dex at similar redshift, indicating that the low-mass HAEs are more efficient in producing ionizing photons. Our results also consolidate the trend of increasing ξ ion with redshift, but reveal a “downsizing” relationship between ξ ion and stellar mass ( M ⊙ ) with increasing redshift. We further explore the dependence of ξ ion on other galaxy properties, such as the UV spectral slope ( β UV ), the UV magnitude ( M UV ), and the equivalent widths of H α and O iii emission lines. Galaxies with bluer UV slopes, fainter UV luminosities, and higher equivalent widths exhibit elevated ξ ion by a factor of ∼2 compared to the median ξ ion of our sample. JWST data will provide an opportunity to extend our method and further investigate the properties of low-mass galaxies at high redshifts.
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.
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