Out of oxygen: Extremely metal-poor galaxy candidates identified at $2.5 < z < 6.5$ with deep JADES medium-band imaging

abstract · abstract

JWST is beginning to uncover a population of extremely metal-poor galaxies (EMPGs, $Z < 1\%~\mathrm{Z}_\odot$) at $z > 3$, mostly through serendipitous NIRSpec discoveries and blind slitless spectroscopy. To accelerate our understanding of pristine star formation, we further develop a methodology to identify EMPG candidates from photometry, using the extensive deep medium-band imaging from JADES. Our EMPG candidates at $2.5 < z < 6.5$ exhibit strong photometric boosts by H$α$, yet correspondingly weak boosts by [O III] + H$β$, likely indicating extremely low metallicity to explain their lack of [O III] emission. We further demand our EMPG candidates to have strong Balmer jumps, as revealed by medium-band imaging, to ensure that they are young starbursts, as opposed to broad-line AGN/LRDs, though contamination by dusty/dense-gas starbursts and highly-obscured AGN remains a concern. SED-fitting with near-pristine models (${\sim}0.1$-$1\%~\mathrm{Z}_\odot$) indicates that our 22 EMPG candidates are low-mass (median $M_* \approx 10^{6.7}~\mathrm{M}_\odot$), faint dwarf galaxies ($M_\mathrm{UV} \approx -16.6$), with high ionizing photon production efficiencies ($\log\, (ξ_\mathrm{ion, obs}/\mathrm{(Hz\ erg^{-1})}) \approx 26.0$). Hence these are plausible sites of near-pristine star formation, comprising ${\sim}0.04$-$0.6\%$ of $2.5 < z < 6.5$ galaxies at $-19 < M_\mathrm{UV} < -16$. We discuss this extremely metal-poor extension to the mass-metallicity relation. We forecast that deep (${\sim}28$ h) NIRCam slitless spectroscopy can identify bright EMPGs through strong H$β$ but lack of [O III] emission, or secure the redshifts of fainter systems through H$α$ detections. Highly-multiplexed NIRSpec spectroscopy offers an alternate route to discovering the faintest pristine galaxies out to $z=10$, without requiring deep medium-band/MIRI imaging to identify secure candidates.

S5: 5 Conclusions / block 1 · html

We photometrically identify 22 extremely metal-poor galaxy candidates with Z < 1 % ​ Z ⊙ Z<1\%~\mathrm{Z}_{\odot} at 2.5 < z < 6.5 2.5<z<6.5 , made possible by the extensive deep JADES NIRCam medium-band imaging. EMPGs are identified based off strong photometric boosts by H α \alpha , yet correspondingly weak boosts by [ O iii ] + H β \beta , likely requiring extremely low metallicity to explain the lack of [ O iii ] emission, resulting in characteristically red m [ O ​ III ] − m H ​ α m_{\mathrm{[O\ III]}}-m_{\mathrm{H\alpha}} colours in the wide-band filters covering [ O iii ] and H α \alpha . Medium bands greatly aid in the confident identification of EMPG candidates, also yielding tighter constraints on the inferred gas-phase metallicities. The medium bands disentangle the line and continuum emission, provide constraints on the rest-frame optical/NIR continuum level, establish the presence of Balmer jumps, and tighten the photometric redshift constraints.

S5: 5 Conclusions / block 1 · html

From SED-fitting with close-to-pristine models, we infer our EMPG candidates to be very low-mass dwarfs (median M ∗ = 10 6.7 ​ M ⊙ M_{*}=10^{6.7}~\mathrm{M}_{\odot} ), constituting the extreme low metallicity tail of the mass–metallicity relation. The EMPGs are starburst galaxies, characterised by strong Balmer jumps and high H α \alpha equivalent widths, placed well above the star-forming main sequence. They are > 1 >1 dex more metal-poor than would be expected given their stellar mass and star formation rate, being near-pristine young starburst galaxies, representing an extreme departure from the slowly-evolving equilibrium framework of the fundamental metallicity relation. The EMPG candidates are generally very UV-faint, with median M UV = − 16.6 M_{\mathrm{UV}}=-16.6 , brought into view thanks to the exceptionally deep JADES imaging, though the rare, brighter tail would be detectable with shallower imaging from wider programmes. The EMPG candidates are extremely efficient producers of ionising photons, with median log ⁡ ( ξ ion , obs / ( Hz ​ erg − 1 ) ) = 26.0 \log\,(\xi_{\mathrm{ion,obs}}/\mathrm{(Hz\ erg^{-1})})=26.0 . To power this extreme emission, they possibly harbour top-heavy stellar populations, their exceptionally high ξ ion , obs \xi_{\mathrm{ion,obs}} compatible with being nebular-dominated, indicating a possibly dominant nebular continuum in the rest-frame UV. Despite their UV faintness, the EMPG candidates have intermediate H α \alpha luminosities (median L H ​ α = 10 41.4 L_{\mathrm{H\alpha}}=10^{41.4} erg s -1 ), favouring their detection in deep JWST spectroscopy. EMPGs are currently rare even in the deep JADES imaging, constituting ∼ 0.04 ​ – ​ 0.6 % {\sim}0.04\textrm{--}0.6\% of 2.5 < z < 6.5 2.5<z<6.5 galaxies at − 19 < M UV < − 16 -19<M_{\mathrm{UV}}<-16 . We anticipate that even deeper imaging (likely boosted by strong gravitational lensing) will reveal progressively more EMPG candidates.

S1: 1 Introduction / block 2 · html

ic feature ( Trussler et al., 2023 ; Nishigaki et al., 2023 ; Fujimoto et al., 2025a ; Fujimoto et al., 2025b , e.g.) . Finally, the extremely hot Pop III stars power a hard ionising spectrum that is very effective at doubly ionising helium, resulting in characteristically strong He ii recombination emission, the definitive Pop III indicator ( Schaerer, 2002 ; Schaerer, 2003 ; Katz et al., 2023a ; Trussler et al., 2023 , e.g.) . Observations of Pop III galaxies and near-pristine extremely metal-poor galaxies (EMPGs, here defined as Z gas < 1 % ​ Z ⊙ Z_{\mathrm{gas}}<1\%~\mathrm{Z}_{\odot} ) should provide unique insights on the nature of the first stars. Measurements of the He ii λ ​ 1640 \lambda 1640 equivalent width can in principle help distinguish between different Pop III initial mass functions ( Trussler et al., 2023 , IMFs,) . Additionally, the shape of the nebular continuum can constrain primordial H ii region temperatures ( Cameron et al., 2024 ; Katz et al., 2025 ; Trussler et al., 2025a ) , which together with the luminosity of the Balmer lines can give further indirect insights on the ionising spectrum. Moreover, metal–metal line ratios in EMPGs can deliver elemental abundance ratios, yielding the chemistry of the primordial nucleosynthesis pattern ( D’Eugenio et al., 2024 ; Nakajima et al., 2025 ; Scholtz et al., 2025b , see e.g.) .

S4.SS3.SSS4: 4.3.4 Summary / block 1 · html

To summarise, EMPGs exhibit characteristically red m [ O ​ III ] − m H ​ α m_{\mathrm{[O\ III]}}-m_{\mathrm{H\alpha}} colours, due to their strong H α \alpha but relatively weak [ O iii ] emission. Their otherwise relatively flat wide-band SED helps to discriminate them from LRDs and most AGN, with medium bands further revealing their Balmer jumps. Hence we assess that EMPGs likely can be identified using wide-band photometry alone ( Trussler et al., 2023 , as suggested by) , albeit with looser gas-phase metallicity constraints and at lower confidence due to the lack of additional medium-band constraints on line strengths and photometric redshift.

S4.SS4.SSS2: 4.4.2 EMPG comoving number density / block 1 · html

Our observational results combined with the simulation predictions suggest that EMPGs may become increasingly common provided one pushes to sufficiently faint UV magnitudes and low stellar masses, only accessible observationally by strong gravitational lensing from galaxy cluster fields. Indeed, if the mass–metallicity relation continues to decline with decreasing stellar mass, then the EMPG fraction continues to rise with decreasing stellar mass. Combined with the increasing comoving number density of galaxies at lower stellar masses, the EMPG comoving number density would be expected to strongly rise as one progressively pushes further into the dwarf galaxy regime.

S5: 5 Conclusions / block 1 · html

We utilise the medium-band optical/NIR continuum measurements to empirically determine emission line equivalent widths from photometric boosts in wide-band filters. Preliminary EMPG candidates are identified based off low [ O iii ] + H β \beta equivalent widths relative to H α \alpha . This represents an extension from the wide-band Pop III selection of Trussler et al. (2023) , which was restricted to the youngest pristine starbursts, to older non-pristine EMPG starbursts. In cases where the empirical estimates of line equivalent widths are deemed unreliable, we select EMPG candidates based off their red m [ O ​ III ] − m H ​ α m_{\mathrm{[O\ III]}}-m_{\mathrm{H\alpha}} wide-band colours. We further require our EMPG candidates to exhibit strong Balmer jumps, likely indicating that they are young starbursts, rather than LRDs which have Balmer breaks or broad-line AGN which tend to have redder UV–optical colours due to their red continuum slopes. We then establish the most secure EMPG candidates by fitting their full NIRCam + HST/ACS photometry with Bagpipes, utilising low metallicity ( OPEN < 0.03 ​ Z ⊙ ) <0.03~\mathrm{Z}_{\odot}) , intermediate metallicity ( 0.03 < Z / Z ⊙ < 0.20 0.03<Z/\mathrm{Z}_{\odot}<0.20 ), and in some cases, edge-case models (to account for [ O iii ] falling in the NIRCam dichroic gap between F200W and F277W). Our final 22 EMPG candidates are inferred to have close-to-pristine metallicities ( ∼ 0.001 ​ – ​ 0.01 ​ Z ⊙ {\sim}0.001\textrm{--}0.01~\mathrm{Z}_{\odot} ), strongly favouring the low metallicity fit ( Δ ​ χ 2 > 9 \Delta\chi^{2}>9 ) over the intermediate metallicity and edge-case models. However, motivated by the NIRSpec PRISM spectrum of EMPG candidate ID 123650, which exhibits a substantial Balmer decrement H ​ α / H ​ β > 5.33 \mathrm{H}\alpha/\mathrm{H}\beta>5.33 , we discuss how dusty starbursts and starbursts radiating into dense gas remain a contaminant concern, as well as extremely dust-obscured AGN for candidate ID 1008821.

S5: 5 Conclusions / block 1 · html

Owing to their faintness, it is challenging to detect the optical/NIR continua of EMPGs with medium bands. Hence we explore the prospects of identifying EMPG candidates using wide-band imaging only. In principle, EMPGs can be selected based off their red m [ O ​ III ] − m H ​ α m_{\mathrm{[O\ III]}}-m_{\mathrm{H\alpha}} colours and otherwise relatively flat SEDs. This sets them apart from contaminant LRDs and most BLR AGN which generally exhibit redder SEDs. However, such wide-band-only EMPG selections will likely come at the cost of lower confidence (smaller Δ ​ χ 2 \Delta\chi^{2} with standard models), as well as looser constraints on the inferred gas-phase metallicity.

S5: 5 Conclusions / block 1 · html

The photometric identification of EMPGs based off their strong H α \alpha but weak [ O iii ] emission will be difficult beyond z > 6.6 z>6.6 . Due to the lower MIRI imaging sensitivity (due to the higher background), exposure times will be extraordinarily long ( ∼ 40 × {\sim}40\times longer than NIRCam), not to mention the fainter source brightness due to the greater redshift and the 4 × 4\times smaller MIRI field of view. Hence EMPG detections will be limited, unless MIRI pointings are optimised. This can be achieved by targeting highly-magnified galaxies and/or promising EMPG candidates which appear to exhibit weak [ O iii ] emission through flat m [ O ​ II ] − m [ O ​ III ] m_{\mathrm{[O\ II]}}-m_{\mathrm{[O\ III]}} colours together with a Balmer jump / line boost in medium bands.

S5: 5 Conclusions / block 1 · html

From the line luminosities inferred for our EMPG candidates, we forecast future spectroscopic prospects with JWST . Deep ( ∼ 28 {\sim}28 h) NIRCam slitless spectroscopy can blindly identify the brightest EMPGs through strong H β \beta but weak [ O iii ] emission. It can also yield spectroscopic redshifts for the faintest EMPGs through H α \alpha detections. Efficient follow-up of EMPG candidates demands the line sensitivity of NIRSpec grating spectroscopy, being able to deliver H β \beta detections for the faintest EMPGs, also providing the most stringent upper limits on gas-phase metallicities. Massively-multiplexed, highly-complete dense-shutter spectroscopy with the NIRSpec MSA provides an alternative route to discovering the faintest pristine galaxies out to z = 10 z=10 , without requiring deep existing medium-band / MIRI imaging to identify secure candidates.

S2.SS2: II.2 Star Formation and Stellar Feedback / block 1 · html

Table 2: Summary of supernova (SN) feedback parameters from stellar particles. We show values for a single stellar particle of m ⋆ ≃ 600 ​ M ⊙ m_{\star}\simeq 600~\mathrm{M}_{\odot} , based on the average over the stellar IMF. More specifically, we assume that all Pop II stellar particles are identical, whereas Pop III particles contain individual stars sampled from the input IMF [ 47 , 46 ] . Here, E SN E_{\rm SN} is the total SN energy, M Z M_{Z} the mass of the metals released, t ⋆ t_{\star} the typical life time of massive stars, and r final r_{\rm final} the final radius of the SN bubble. One can refer to table 1 in Liu and Bromm [62] for a more detailed description.

S3: III Results / block 1 · html

With the stellar feedback recipes described in Section II , we now analyze the resulting PBH simulation runs, comparing to previous work 4 4 4 Also see the appendix in [ 73 ] . that did not include the full feedback physics. To study the synergy between stellar and BH feedback, metal enrichment and gas outflow, we first track the star formation history (SFH) and the assembly of galaxies in Section III.1 , followed by a discussion of the metal enrichment history in Section III.2 .

S4: IV Results: Simulated NIRCam Images / block 1 · html

Before continuing the analysis of the NIRCam observations, we turn back to the simulated images to predict how instrumental effects and cosmological dimming influence the observations of NLRs at cosmic noon. For illustration purposes, we explore whether NIRCam is able to resolve the morphologies of [O III] cones at z = 2 − 3 z=2-3 through simulated NIRCam narrow- and medium-band images based on MUSE datacubes of low-z AGN. In this section, we present the results derived from our simulated images. We use the nearby galaxy, NGC 2992, as a reference object in Sections IV.1 - IV.4 due to the clarity and visibility of its [O III] bicone. NGC 2992 ( z = 0.007296 z=0.007296 ) is spiral galaxy, optically classified as a Seyfert 1.9 with an AGN bolometric luminosity of 10 43.3 10^{43.3} erg s -1 [ 86 , 67 ] . NGC 2992 has an ionization bicone extending approximately 4 4 kpc SE and NW of the central engine, detectable in [O III] and H α \alpha , and is also reported to host a double-lobed radio structure [ 41 , 135 ] .

S4.SS5: IV.5 Simulating the NLR Size-AGN Luminosity Relation at Cosmic Noon / block 1 · html

We find that the addition of instrument noise results in smaller observed NLR radii across the entire sample. The median difference between the measured average NLR radii for the the sample without simulated noise (SB = 2.4 × 10 − 17 =2.4\times 10^{-17} erg s -1 cm -2 arcsec -2 ) and with simulated noise (SB = 1.4 × 10 − 16 =1.4\times 10^{-16} erg s -1 cm -2 arcsec -2 ) is 2.5 2.5 kpc ( 0.48 0.48 dex) with a standard deviation of 0.5 0.5 kpc ( 0.11 0.11 dex). For reference, when the same limiting surface brightness level (SB = 1.4 × 10 − 16 =1.4\times 10^{-16} erg s -1 cm -2 arcsec -2 ) is applied to both sample types, we find that the noiseless NLR radii are − 0.01 -0.01 dex smaller than the NLR radii of the noisy sample. However, these NLR radii (black squares with yellow outlines) do not reflect accurate NLR measurements for the noiseless [O III] maps of our MUSE AGN sample (as evidenced by the disparities between these plotted size measurements and the white dashed outlines in the fourth column of Figures 12 and 13 in Appendix A . These results indicate that NIRCam is able to detect ionization cones in cosmic noon galaxies above the instrumental noise level, but the measured sizes of these NLRs may be biased lower by instrumental noise and cosmological surface brightness dimming.

S5.SS2: V.2 Results from SED fitting / block 1 · html

When including an AGN component, both Synthesizer-AGN and Prospector consistently yield log 10 ⁡ ( M ⋆ / M ⊙ ) ≈ 9 \log_{10}(M_{\star}/M_{\odot})\approx 9 , although Bagpipes retrieves a significantly lower stellar mass by ≈ 1 \approx 1 dex: log 10 ⁡ ( M ⋆ / M ⊙ ) = 7.92 \log_{10}(M_{\star}/M_{\odot})=7.92 . Similarly, CIGALE finds log 10 ⁡ ( M ⋆ / M ⊙ ) = 8.50 \log_{10}(M_{\star}/M_{\odot})=8.50 . Notably, Bagpipes predicts a stellar age of t age = 400 − 260 + 260 t_{\mathrm{age}}=400_{-260}^{+260} Myr, consistent with both CIGALE and Prospector within the uncertainties ( t age ≈ 400 − 600 t_{\mathrm{age}}\approx 400-600 Myr). However, its mass-weighted age, from Bagpipes , is only 4 − 3 + 2 4_{-3}^{+2} Myr, indicating that a substantial fraction of the stellar mass formed recently, which is consistent with a very recent burst of star formation.

S5.SS2: V.2 Results from SED fitting / block 1 · html

Interestingly, both Synthesizer-AGN and Prospector , when invoking an AGN, predict A V ≈ 2 − 4 A_{V}\approx 2-4 . In particular, Synthesizer-AGN predicts A V ≈ 0.74 A_{V}\approx 0.74 mag for the young population ( t age ≈ 10 t_{\mathrm{age}}\approx 10 Myr), in line with our estimate from NIRSpec/PRISM data, and A V ≈ 2.40 A_{V}\approx 2.40 for the older population ( t age ≈ 32 t_{\mathrm{age}}\approx 32 Myr). Similarly, Bagpipes and CIGALE yield A V A_{V} values consistent with those inferred from Balmer lines tracing the young population. Across all fits, the metallicity remains consistent at 10 10 – 20 % , Z ⊙ 20\%,Z_{\odot} , in agreement with our spectroscopic constraints.

S6: VI Summary and Conclusions / block 1 · html

Based on the Chandra X-ray data and the BPT diagram, nearly 70% (11/16) of the UDF+ASPECS sources are classified as AGNs. We find that only one source shows prominent broad Balmer lines (H α \alpha and H β \beta , FWHM H ​ α ∼ 6500 ​ km ​ s − 1 \mathrm{FWHM_{H\alpha}\sim 6500~km~s^{-1}} ) with narrow [O iii ] lines, indicating type-1 AGN. These results suggest that the faint SMGs preferentially host obscured AGNs.

S5.SS3: V.3 Interpretations / block 1 · html

Here, we discuss possible physical interpretations of faint SMGs. Figure 17 summarizes the results of our analysis and previous studies, then presents a possible interpretation as a schematic illustration. As discussed in Section V.1 , the UDF+ASPECS sources largely follow the star-forming main sequence (Figure 4 ) and mass-metallicity relation (Figure 12 ). In addition, their electron densities are comparable to those of other star-forming galaxies with similar stellar mass and SFR (Figure 13 ). These results suggest that the global ISM conditions are not exceptionally extreme, and the UDF+ASPECS sources represent typical massive star-forming galaxies with AGN fraction of ∼ 70 % \sim 70\% (Section IV.2 and Figure 10 ).

S3: III Results / block 1 · html

By co-adding the X-ray counts across all LRDs, we obtained 1444 (3730) total counts in the soft ( 0.3 − 2 0.3-2 keV) and hard ( 2 − 7 2-7 keV) band and, after normalizing the background to the same extraction area, we got 1409 (3697) background counts in the soft and hard bands, respectively. This yields 35 ± 41 35\pm 41 net counts in the soft band and 33 ± 66 33\pm 66 in the hard band, corresponding to non-detections. In the absence of detections, we derive upper limits on the flux of the stacked LRD population. To this end, we adopt the exposure maps and convert the count rates into fluxes, obtaining 3 ​ σ 3\sigma upper limits of < 4.7 × 10 − 18 <4.7\times 10^{-18} erg/s/cm 2 in the soft and < 1.3 × 10 − 17 <1.3\times 10^{-17} erg/s/cm 2 in the hard band. We note that the choice of photon index adopted to convert the count rates into fluxes affects these latter by only ≈ 20 % \approx 20\% , leaving our main results unchanged.

S1: I Introduction / block 1 · html

This paper is organized as follows. In Section II we describe our procedure to analyze (stack) the X-ray data; in Section III we present our results; in Section IV we discuss the implications of our X-ray non-detections and discuss the compatibility of our results with current models of X-ray weakness, and we finally draw our conclusions in Section V . In this work we adopt a flat Λ \Lambda CDM cosmology with H 0 = 70 H_{0}=70 km/s/Mpc and Ω M = 0.3 \Omega_{M}=0.3 .

article / block 1 · html

Metal absorbers represent a powerful probe of galaxy feedback and reionization, as highlighted by both observational and theoretical results showing an increased abundance of low-ionised metal species at higher redshifts. The origin of such absorbers is currently largely unknown because of the low number of galaxy counterparts detected, suggesting that they might be surrounded by low-mass faint sources that fall below the detection threshold of current instruments. We use the thesan-zoom radiation hydrodynamic simulations to investigate the connection between properties of neutral oxygen ( O i ) absorbers and galaxies within the redshift range z = 5 − 8 z=5-8 . We find that the circumgalactic medium of galaxies becomes progressively ionised with cosmic time, leading to a decrease of ≈ 0.2 \approx 0.2 in the covering fraction of neutral oxygen, while the total oxygen covering fraction remains constant. The observable absorbers ( N O i ≳ 10 13 ​ cm − 2 N_{\text{O\,{i}}}\gtrsim 10^{13}\,\text{cm}^{-2} ) are not confined to haloes: at z ≥ 5 z\geq 5 the majority ( ≳ 60 % \gtrsim 60\% ) arise beyond R vir R_{\mathrm{vir}} , and including these systems is essential to reproduce the observed increase in absorber incidence with redshift. The simulated absorbers preferentially reside in overdensities rich in low-mass galaxies ( M ⋆ ≤ 10 8 ​ M ⊙ M_{\star}\leq 10^{8}\,\rm{M}_{\odot} ), explaining the scarcity of detected counterparts, while not excluding the possibility of nearby star-forming sources ( ≥ 5 ​ M ⊙ ​ yr − 1 \geq 5\,\text{M}_{\odot}\,\text{yr}^{-1} ) similar to those suggested by the latest ALMA observations and, at larger distances, by the JWST . These results establish O i absorbers as sensitive tracers of the evolving ionisation structure around faint galaxies to be probed by forthcoming deep spectroscopic surveys.

S5.SS2: 5.2 O i comoving line density / block 1 · html

Following a galaxy-centric method, the recent observational results of Higginson et al. (2025) also show the high incidence of neutral oxygen absorbers outside the virial radius of high-redshift galaxies. In their work, the O i covering fraction around [ O iii ]-emitting galaxies is used to evaluate the line density of neutral oxygen absorbers at z = 6 z=6 , with an approach similar to the one used in our work. Their result, shown as a light gray diamond in Fig. 11 , is lower than what blind quasar surveys suggest, implying that most of the neutral oxygen ( ≈ 65 % \approx 65\% ) should reside at larger distances from massive detectable galaxies ( M ⋆ ≳ 10 7.5 ​ M ⊙ M_{\star}\gtrsim 10^{7.5}\,\rm{M}_{\odot} ) at z = 6 z=6 , close to low-mass sources or in dense neutral IGM pockets that survived reionization. Based on both our results and these recent observations, we investigate in the next section the role of neutral oxygen outside the CGM of haloes considered in our study.

abstract · abstract

JWST/NIRCam slitless spectroscopy enables dynamical mass measurements for typical star-forming galaxies only a billion years after the Big Bang. We model the H$α$ morpho-kinematics of 163 galaxies at redshift $z\approx4$-6 from FRESCO and CONGRESS (with JADES imaging), using the $\texttt{geko}$ code, and infer rotational velocities and dispersions within $r_{\rm e}$. Our sample spans $\log M_{\star}\approx7$-10 and $\log M_{\rm dyn}\approx9$-11. Gas masses are estimated via scaling relations, yielding baryonic masses and dark-matter (DM) fractions $f_{\rm DM}(r<r_{\rm e})$ within the H$α$ half-light radius. We find high median fractions of $\langle f_{\rm gas}\rangle=0.77$ and $\langle f_{\rm DM}\rangle=0.73$, where $f_{\rm gas}$ is measured with respect to the baryonic mass and $f_{\rm DM}$ with respect to the DM+baryonic mass. About two-thirds of systems are DM-dominated within $r_{\rm e}\sim0.5-1$ kpc. Both $f_{\rm gas}$ and $f_{\rm DM}$ decrease with stellar mass, consistent with simulations. The stellar Tully-Fisher relation shows a tentative offset to higher $v_{\rm circ}$ at fixed $M_{\star}$ and substantial intrinsic scatter, suggesting that the relation is only beginning to emerge at $z\sim5$. We measure a negative correlation between $f_{\rm DM}$ and baryonic surface density $Σ_{\rm bar}$, weaker but broadly consistent with trends at cosmic noon and at $z\sim0$. Qualitatively comparing with modified NFW profiles coupled to an empirical stellar-to-halo mass relation suggests that the lowest $f_{\rm DM}$ ($\lesssim0.4$) require cored inner DM profiles, while the highest fractions favour cuspier profiles, potentially reflecting adiabatic contraction. Overall, the elevated $f_{\rm gas}$ and $f_{\rm DM}$ at $z\gtrsim4$ are compatible with progenitors of baryon-dominated systems at $z\sim2$ and naturally anticipate overmassive black holes at fixed $M_{\star}$.

S6: 6 Summary and outlook / block 1 · html

We have demonstrated that Euclid slitless infrared spectroscopy is a powerful tool to identify quasars and to eliminate confusion with ultracool dwarfs by cross-correlating NISP RG E {\rm RG}_{\scriptscriptstyle\rm E} spectra with templates. The NISP RG E {\rm RG}_{\scriptscriptstyle\rm E} spectral coverage is particularly well-matched to strong spectral features in L and T dwarfs (Fig. 3 , and Mohandasan et al. 2025 ), but it can also help with the classifications for objects with less prominent spectral features in the 1206– 1892 1892\, nm spectral range, such as late M dwarfs (Fig. 4 ). Without the spectroscopic information, the ultracool dwarfs discussed here could mistakenly have been selected as high-redshift quasar candidates. Similar but more distant (thus fainter) brown dwarfs could incorrectly have been selected as high-redshift galaxies ( Roberts-Borsani et al., 2025 , e.g.,) . Atek et al. (2025) argue that requiring I E − Y E > 3 I_{\scriptscriptstyle\rm E}-Y_{\scriptscriptstyle\rm E}>3 reduces contamination by brown dwarfs. However, as shown in Fig. 2 , late L- and T-dwarfs with such a significant colour break do exist and, therefore, brown dwarfs can still be a substantial contaminant to the z > 6 z>6 galaxy candidates presented in Weaver et al. (2025) .

S6: 6 Summary and outlook / block 1 · html

In this paper, we focus on the highest redshift quasars to explore how efficiently Euclid can help to fill the quasar redshift gaps at z ≈ 5.5 z\approx 5.5 and z ≳ 7 z\gtrsim 7 , where NISP RG E {\rm RG}_{\scriptscriptstyle\rm E} spectra allow us to identify two emission lines (Fig. 1 ). In the future, we will combine the grism data with photometric information (see Fig. 2 ). In that case, having even only one (or zero) strong emission line in the NISP RG E {\rm RG}_{\scriptscriptstyle\rm E} spectra will help constrain the source redshift. Additionally, in the EDS, the expanded wavelength coverage provided by the NISP BG E {\rm BG}_{\scriptscriptstyle\rm E} spectra can be effectively utilised for the reliable identification of sources (see Fig. 1 and Appendix C ).