Abstract
Nearby dwarf galaxies provide a unique laboratory in which to test stellar population models below Z⊙/2. Such tests are particularly important for interpreting the surprising high-ionization ...ultraviolet (UV) line emission detected at z > 6 in recent years. We present HST/COS UV spectra of 10 nearby metal-poor star-forming galaxies selected to show He ii emission in SDSS optical spectra. The targets span nearly a dex in gas-phase oxygen abundance (7.8 < 12 + log O/H < 8.5) and present uniformly large specific star formation rates (sSFR ∼102 Gyr−1). The UV spectra confirm that metal-poor stellar populations can power extreme nebular emission in high-ionization UV lines, reaching C iii equivalent widths comparable to those seen in systems at z ∼ 6–7. Our data reveal a marked transition in UV spectral properties with decreasing metallicity, with systems below 12 + log O/H ≲ 8.0 (Z/Z⊙ ≲ 1/5) presenting minimal stellar wind features and prominent nebular emission in He ii and C iv. This is consistent with nearly an order of magnitude increase in ionizing photon production beyond the He+-ionizing edge relative to H-ionizing flux as metallicity decreases below a fifth solar, well in excess of standard stellar population synthesis predictions. Our results suggest that often-neglected sources of energetic radiation such as stripped binary products and very massive O-stars produce a sharper change in the ionizing spectrum with decreasing metallicity than expected. Consequently, nebular emission in C iv and He ii powered by these stars may provide useful metallicity constraints in the reionization era.
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.
Measurements of the galaxy UV luminosity function at z 6 suggest that young stars hosted in low-mass star-forming galaxies produced the bulk of hydrogen-ionizing photons necessary to reionize the ...intergalactic medium (IGM) by redshift z ∼ 6. Whether star-forming galaxies dominated cosmic reionization, however, also depends on their stellar populations and interstellar medium properties, which set, among other things, the production rate of H-ionizing photons, ξ ion , and the fraction of these escaping into the IGM. Given the difficulty of constraining with existing observatories the physical properties of z 6 galaxies, in this work we focus on a sample of ten nearby objects showing UV spectral features comparable to those observed at z 6. We use the new-generation tool to model the UV-to-optical photometry and UV/optical emission lines of these Local 'analogues' of high-redshift galaxies, finding that our relatively simple, yet fully self-consistent, physical model can successfully reproduce the different observables considered. Our galaxies span a broad range of metallicities and are characterised by high ionization parameters, low dust attenuation, and very young stellar populations. Through our analysis, we derive a novel diagnostic of the production rate of H-ionizing photons per unit UV luminosity, ξ ion , based on the equivalent width of the bright O λλ4959,5007 line doublet, which does not require measurements of H-recombination lines. This new diagnostic can be used to estimate ξ ion from future direct measurements of the O λλ4959,5007 line using JWST/NIRSpec (out to z ∼ 9.5), and by exploiting the contamination by Hβ + O λλ4959,5007 of photometric observations of distant galaxies, for instance from existing Spitzer/IRAC data and from future ones with JWST/NIRCam.
We present the Magellan/FIRE detection of highly ionized C ivλ1550 and O iiiλ1666 in a deep infrared spectrum of the z = 6.11 gravitationally lensed low-mass galaxy RXC J2248.7-4431-ID3, which has ...previously known Ly . No corresponding emission is detected at the expected location of He iiλ1640. The upper limit on He ii, paired with detection of O iii and C iv, constrains possible ionization scenarios. Production of C iv and O iii requires ionizing photons of 2.5-3.5 Ryd, but once in that state their multiplet emission is powered by collisional excitation at lower energies (∼0.5 Ryd). As a pure recombination line, He ii emission is powered by 4 Ryd ionizing photons. The data therefore require a spectrum with significant power at 3.5 Ryd but a rapid drop toward 4.0 Ryd. This hard spectrum with a steep drop is characteristic of low-metallicity stellar populations, and less consistent with soft AGN excitation, which features more 4 Ryd photons and hence higher He ii flux. The conclusions based on ratios of metal line detections to helium non-detection are strengthened if the gas metallicity is low. RXJ2248-ID3 adds to the growing handful of reionization-era galaxies with UV emission line ratios distinct from the general population in a way that suggests hard ionizing spectra that do not necessarily originate in AGNs.
Abstract
We present constraints on the physical properties (including stellar mass, age, and star formation rate) of 207 6 ≲
z
≲ 8 galaxy candidates from the Reionization Lensing Cluster Survey ...(RELICS) and Spitzer-RELICS surveys. We measure photometry using T-PHOT and perform spectral energy distribution fitting using EA
z
Y and BAGPIPES. Of the 207 candidates for which we could successfully measure (or place limits on) Spitzer fluxes, 23 were demoted to likely
z
< 4. Among the high-
z
candidates, we find intrinsic stellar masses between 1 × 10
6
M
⊙
and 4 × 10
9
M
⊙
, and rest-frame UV absolute magnitudes between −22.6 and −14.5 mag. While our sample is mostly comprised of
L
m
UV
/
L
m
UV
*
<
1
galaxies, it extends to
L
m
UV
/
L
m
UV
*
∼
2
. Our sample spans ∼4 orders of magnitude in stellar mass and star formation rates, and exhibits ages that range from maximally young to maximally old. We highlight 11
z
≥ 6.5 galaxies with detections in Spitzer/IRAC imaging, several of which show evidence for some combination of evolved stellar populations, large contributions of nebular emission lines, and/or dust. Among these is PLCKG287+32-2013, one of the brightest
z
∼ 7 candidates known (AB mag 24.9 at 1.6
μ
m) with a Spitzer 3.6
μ
m flux excess suggesting strong O
iii
+ H-
β
emission (∼1000 Å rest-frame equivalent width). We discuss the possible uses and limits of our sample and present a public catalog of Hubble + Spitzer photometry along with physical property estimates for all objects in the sample. Because of their apparent brightnesses, high redshifts, and variety of stellar populations, these objects are excellent targets for follow-up with the James Webb Space Telescope.
Abstract
Extreme, young stellar populations are considered to be the primary contributor to cosmic reionization. How the Lyman continuum (LyC) escapes these galaxies remains highly elusive, and it is ...challenging to observe this process in actual LyC emitters without resolving the relevant physical scales. We investigate the Sunburst Arc, a strongly lensed LyC emitter at
z
= 2.37 that reveals an exceptionally small-scale (tens of parsecs) region of high LyC escape. The small (<100 pc) LyC-leaking region has extreme properties: a very blue UV slope (
β
= −2.9 ± 0.1), a high ionization state (O
iii
λ
5007/O
ii
λ
3727 = 11 ± 3 and O
iii
λ
5007/H
β
= 6.8 ± 0.4), strong oxygen emission (EW(O
iii
) = 1095 ± 40 Å), and a high Ly
α
escape fraction (0.3 ± 0.03), none of which are found in nonleaking regions of the galaxy. The leaking region’s UV slope is consistent with approximately “pure” stellar light that is minimally contaminated by the surrounding nebular continuum emission or extinguished by dust. These results suggest a highly anisotropic LyC escape process such that LyC is produced and escapes from a small, extreme starburst region where the stellar feedback from an ionizing star cluster creates one or more “pencil-beam” channels in the surrounding gas through which LyC can directly escape. Such anisotropic escape processes imply that random sight-line effects drive the significant scatters between measurements of galaxy properties and LyC escape fraction, and that strong lensing is a critical tool for resolving the processes that regulate the ionizing budget of galaxies for reionization.
Abstract
We report the discovery of a spectroscopically confirmed strong Ly
α
emitter at
z
= 7.0281 ± 0.0003, observed as part of the Reionization Lensing Cluster Survey (RELICS). This galaxy, dubbed ...“Dichromatic Primeval Galaxy” at
z
∼ 7 (DP7), shows two distinct components. While fairly unremarkable in terms of its ultraviolet (UV) luminosity (
∼
0.3
L
UV
*
, where
L
UV
*
is the characteristic luminosity), DP7 has one of the highest observed Ly
α
equivalent widths (EWs) among Ly
α
emitters at
z
> 6 (>200 Å in the rest frame). The strong Ly
α
emission generally suggests a young metal-poor, low-dust galaxy; however, we find that the UV slope
β
of the galaxy as a whole is redder than typical star-forming galaxies at these redshifts, −1.13 ± 0.84, likely indicating, on average, a considerable amount of dust obscuration, or an older stellar population. When we measure
β
for the two components separately, however, we find evidence of differing UV colors, suggesting two separate stellar populations. Also, we find that Ly
α
is spatially extended and likely larger than the galaxy size, hinting to the possible existence of a Ly
α
halo. Rejuvenation or merging events could explain these results. Either scenario requires an extreme stellar population, possibly including a component of Population III stars, or an obscured active galactic nucleus. DP7, with its low UV luminosity and high Ly
α
EW, represents the typical galaxies that are thought to be the major contribution to the reionization of the universe, and for this reason DP7 is an excellent target for follow-up with the James Webb Space Telescope.
We present Keck/MOSFIRE observations of UV metal emission lines in four bright (H = 23.9–25.4) gravitationally lensed z ≃ 6–8 galaxies behind the cluster Abell 1703. The spectrum of A1703-zd6, a ...highly magnified star-forming galaxy with a Lyα redshift of z = 7.045, reveals a confident detection of the nebular C iv λ1548 emission line (unresolved with full width at half-maximum<125 km s−1). UV metal emission lines are not detected in the three other galaxies. At z ≃ 2–3, nebular C iv emission is observed in just 1 per cent of UV-selected galaxies. The presence of strong C iv emission in one of the small sample of galaxies targeted in this paper may indicate that hard ionizing spectra are more common at z ≃ 7. The total estimated rest-frame equivalent width of the C iv doublet and C iv/Lyα flux ratio are comparable to measurements of narrow-lined AGNs. Photoionization models show that the nebular C iv line can also be reproduced by a young stellar population, with very hot metal-poor stars dominating the photon flux responsible for triply ionizing carbon. Regardless of the origin of the C iv, we show that the ionizing spectrum of A1703-zd6 is different from that of typical galaxies at z ≃ 2, producing more H ionizing photons per unit 1500 Å luminosity (log(ξion/erg− 1 Hz) = 25.68) and a larger flux density at 30–50 eV. If such extreme radiation fields are typical in UV-selected systems at z ≳ 7, it would indicate that reionization-era galaxies are more efficient ionizing agents than previously thought. Alternatively, we suggest that the small sample of Lyα emitters at z ≳ 7 may trace a rare population with intense radiation fields capable of ionizing their surrounding hydrogen distribution. Additional constraints on high-ionization emission lines in galaxies with and without Lyα detections will help clarify whether hard ionizing spectra are common in the reionization era.
Recent studies have revealed intense ultraviolet (UV) metal emission lines in a modest sample of z > 7 Lyman-α emitters, indicating a hard ionizing spectrum is present. If such high ionization ...features are shown to be common, it may indicate that extreme radiation fields play a role in regulating the visibility of Lyα in the reionization era. Here, we present deep near-infrared spectra of seven galaxies with Lyα emission at 5.4 < z < 8.7 (including a newly confirmed lensed galaxy at zLyα = 6.031) and three bright z ≃ 7 photometric targets. In nine sources, we do not detect UV metal lines. However in the zLyα = 8.683 galaxy EGSY8p7, we detect a 4.6σ emission line in the narrow spectral window expected for N Vλ1243. The feature is unresolved (FWHM < 90 km s-1) and is likely nebular in origin. A deep H-band spectrum of EGSY8p7 reveals non-detections of C IV, He II, and O III. The presence of N V requires a substantial flux of photons above 77 eV, pointing to a hard ionizing spectrum powered by an active galactic nucleus or fast radiative shocks. Regardless of its origin, the intense radiation field of EGSY8p7 may aid the transmission of Lyα through what is likely a partially neutral intergalactic medium. With this new detection, five of 13 known Lyα emitters at z > 7 have now been shown to have intense UV line emission, suggesting that extreme radiation fields are commonplace among the Lyα population. Future observations with JWST will eventually clarify the origin of these features and explain their role in the visibility of Lyα in the reionization era.
We discuss new Keck/MOSFIRE spectroscopic observations of four luminous galaxies at z ... 7-9 selected to have intense optical line emission by Roberts-Borsani et al. Previous follow-up has revealed ...Ly... in two of the four galaxies. Our new MOSFIRE observations confirm that Ly... is present in the entire sample. We detect Ly... emission in the galaxy COS-zs7-1, confirming its redshift as z sub( Ly...) = 7.154, and we detect Ly... in EGS-zs8-2 at z sub( Ly...) = 7.477, verifying an earlier tentative detection. The ubiquity of Ly... in this sample is puzzling given that the IGM is expected to be significantly neutral over 7 < z < 9. To investigate this result in more detail, we have initiated a campaign to target UV metal lines in the four Ly... emitters as a probe of both the ionizing field and the Ly... velocity offset at early times. Here we present the detection of C iii emission in the z = 7.73 galaxy EGS-zs8-1, requiring an intense radiation field and moderately low metallicity. We argue that the radiation field is likely to affect the local environment, increasing the transmission of Ly... through the galaxy. Moreover, the centroid of C...iii indicates that Ly... is redshifted by 340 km s super( -1). This velocity offset is larger than that seen in less luminous systems, providing an explanation for the transmission of Ly... emission through the IGM. Since the transmission is further enhanced by the likelihood that such systems are also situated in large ionized bubbles, the visibility of Ly... at z > 7 is expected to be strongly luminosity-dependent, with transmission accelerated in systems with intense star formation. (ProQuest: ... denotes formulae/symbols omitted.)
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