Sara Mascia

Little red dots (LRDs) are a class of sources discovered by the James Webb Space Telescope observationally defined by a V-shaped rest-frame UV-optical spectral energy distribution, a compact or unresolved morphology, and, frequently, broad hydrogen emission lines. How these characteristics translate into physical and structural properties remains unclear. Among various models, those involving a quasi-star interpret LRDs as an intermediate stage in the evolution of a super-massive black hole (SMBH) seed into a classic active galactic nucleus. In this paper, we employ the radiative-transfer code Cloudy to study whether this model is able to reproduce the spectral features commonly observed in LRDs. The model consists of an accreting SMBH ( M _BH ∼ 10 ^5-6 M _⊙ ) surrounded by a convective layer where a black-body (BB) spectrum with T ∼ 5000 K and L ∼ 10 ^44.4 erg s ⁻¹ is produced. This BB spectrum is then reprocessed by a concentric thick (∆ R ∼ 1000 AU) shell of dense ( n _H ∼ 10 ¹¹ cm ⁻³ ) gas partially ionised by thermal collisions. The emerging radiation is further reprocessed by a diffuse clumpy medium surrounding the quasi-star. We fit this model to JWST/NIRSpec spectra of LRDs from the literature, deriving the main physical parameters and the SMBH masses. Once coupled with the UV emission from a host galaxy, this model is able to reproduce the shape of the UV-to-NIR continuum, including the presence of a Balmer break, as well as the luminosity of the hydrogen emission lines. However, this quasi-star model does not natively account for the presence of broad helium lines and for the possible presence of hot dust, needing additional components to match these observables. Our main result is to show how some LRDs can be modelled as quasi-stars, highlighting that a significant degeneracy exists among different LRD models. This has important consequences for our understanding of the mechanisms driving black hole growth in the early Universe.

Recent studies at high redshift have revealed an enigmatic class of little red dots (LRDs) with extreme Balmer breaks, stronger than in any stellar atmosphere. However, it is unclear whether such objects exist at lower redshift, especially given the low number of LRDs reported at z ≲ 2. Here, we report the discovery of PAN-BH*-1, an LRD with an extreme Balmer break at z = 1.73, identified from JWST/NIRCam pure-parallel imaging taken by the PANORAMIC survey, and confirmed by deep VLT/X-Shooter spectroscopy. The rest-optical to near-infrared spectral energy distribution of PAN-BH*-1 is consistent with a photospheric continuum with effective temperature T _eff ≈ 4800 K. The broad H α emission line shows remarkably deep absorption, stronger than previously measured in any LRD. The absorption trough spans from −520 to +267 km s ⁻¹ with respect to the systemic redshift. The presence of blue- and red-shifted absorption suggests complex dynamics of the obscuring gas along the line of sight. We speculate that the absorption trough can be produced by a thick wind launched from a thick, rotating photospheric disk, the latter being the source of the red optical continuum. While the source is unresolved in the rest-optical JWST data ( r _eff < 47 pc), the rest-near-UV Hubble Space Telescope imaging shows an extended morphology with r eff = 1 . 0 − 0.3 + 0.5 kpc, which we interpret as a host galaxy with a stellar mass of ∼10 ⁸ M _⊙ , in line with the narrow H α emission. The discovery of this object at cosmic noon highlights the feasibility of systematic searches for extreme LRDs with wide-area facilities such as Euclid and Roman.

Using deep, medium-resolution, JWST rest-optical spectra of a sample of typical star-forming galaxies (Lyman-break galaxies and Lyman- α emitters) from the LyC22 survey at z ∼ 3, we determined the nebular abundances of N, O, and Ne relative to H for a subsample of 25 objects with a direct method based on auroral [O III ] λ 4363 line detections. Our measurements increased the number of accurate N/O determinations at z ∼ 2 − 4 using a homogeneous approach. We found a mean value of log(N/O) = − ^+0.25 _−0.21 over a metallicity range of 12 + log(O/H) = 7.56 to 8.44. The observed N/O ratio and scatter are indistinguishable from that observed in low- z galaxies and H II regions over the same metallicity range, thus showing no redshift evolution of N/O for typical galaxies over a significant fraction of cosmic time. We also show that typical z ∼ 3 galaxies have a similar offset in the BPT diagram to galaxies from the low- z Lyman Continuum Survey (LzLCS) when compared to the average of SDSS galaxies, and we demonstrate that this offset is not due to enhanced nitrogen abundances. Our results establish a basis for future studies of the evolution of N and O at higher redshifts.

We investigate the properties of extreme emission line galaxies (EELGs) at z = 4 − 9 and their role in reionization. Compact, low-mass galaxies with intense optical emission lines are linked to elevated specific star formation rates (sSFRs) and recent bursts of star formation. Feedback in these systems may enable the leakage of ionizing radiation into the intergalactic medium. Using JWST/NIRSpec spectroscopy from the CAPERS, CEERS, and RUBIES surveys, we compiled 160 NIRCam-selected EELGs in the EGS field. These galaxies show extreme rest-frame equivalent widths (EWs), with a median EW([O III]+H β ) = 1616 Å and EW(H α ) = 763 Å. They are low-mass (median log( M _★ / M _⊙ ) = 8.26) with high sSFRs (median 43 Gyr ⁻¹ ), above the z ∼ 6 main sequence. UV slopes are diverse, with a median value of β = −2.0, and only 7% have extremely blue continua ( β < −2.6). Emission-line diagnostics suggest stellar populations as the primary ionizing source, although an AGN fraction of 14% cannot be entirely ruled out. These galaxies are efficient ionizing photon producers, with median log( ξ _ion [Hz erg ⁻¹ ]) = 25.37, exceeding typical values at similar redshifts. Escape fractions, however, are heterogeneous: 16% of EELGs at z < 7 show escape fractions > 5% for both Ly α and LyC photons, while 82% lack detectable Ly α emission. The median inferred LyC escape fraction is modest (5%) but enhanced in compact super-Eddington systems with sSFR > 25 Gyr ⁻¹ . These results indicate that EELGs contribute approximately 16 − 40% of the total ionizing emissivity required to sustain hydrogen reionization. EELGs are extremely compact, with a median effective radius of 0.49 kpc, and exhibit a recent star formation burst. Our analysis indicates that sSFR and star formation rate surface density are the primary drivers of their extreme emission line strengths.

The resonant scattering nature of Ly α photons interacting with neutral hydrogen makes Ly α emitters (LAEs) robust tracers of the intergalactic neutral hydrogen fraction, and thus sensitive probes of cosmic reionization. We present an extensive study of the Ly α evolution from galaxies at 4.5 ≤ z ≤ 11 in the UDS field, observed as part of the CAPERS survey, and complemented with spectra from the DAWN JWST Archive. The combined sample includes 651 spectroscopically confirmed Ly α -break galaxies, among which we find 73 S/N > 3 LAEs in JWST-NIRSpec PRISM spectra. We trace the redshift evolution of the LAE fraction with EW ₀ > 25 Å (X _{Ly α} ) between z = 5 and z = 9, extending such an analysis to the UDS field for the first time. At z = 5 and 6, the UDS results agree with the average JWST X _{Ly α} values from multiple fields. However, JWST measurements are consistently lower than ground-based results. To investigate this, we compare JWST observations to a population of star-forming galaxies at z ∼ 6 observed with VLT-FORS2. We find that a Ly α slit-loss of 35 ± 10% in JWST spectra accounts for the offset, as the resonant Ly α emission is more spatially extended than the stellar continuum. From z = 6 to 7, the UDS field shows a significant drop in Ly α visibility, from which we infer a neutral hydrogen fraction of X _HI = 0.7–0.9. Finally, we identify two robust ionized bubbles at z = 7.29 and 7.77, with radii of R _ion = 0.6 and 0.5 physical Mpc and photometric overdensities of N/⟨N⟩ = 3 and 4, based on candidate counts down to the photometric completeness limit. Compared to the large ionized region at z ∼ 7 in the EGS field, these results indicate significant field-to-field variation, supporting a patchy, inhomogeneous reionization process.

Early results from JWST suggest that Epoch of Reionization (EoR) galaxies produce copious ionizing photons, which, if they escape efficiently, could cause reionization to occur too early. We study this problem using JWST imaging and prism spectroscopy for 412 galaxies at 4.5 < z < 9.0. We fit these data simultaneously with stellar population and nebular emission models that include a parameter for the fraction of ionizing photons that escape the galaxy, f _esc . We find that the ionization production efficiency, ξ _ion = Q ( H ₀ )/ L _UV , increases with redshift and decreasing UV luminosity, but shows significant scatter, σ ( log ξ ion ∣ z , M UV ) ≃ 0.3 dex. The inferred escape fractions averaged over the population are low, ranging from〈 f _esc 〉 ≃ 2.6% ± 1.4% at 6 < z < 9 to 6.5% ± 2.2% at 4.5 < z < 6, with weak or no indication of evolution with redshift. This implies that in our models most of the ionizing photons need to be absorbed to account for the nebular emission. We compute the impact of our results on reionization, including the distributions for ξ _ion and f _esc , and the evolution and uncertainty of the UV luminosity function. Considering galaxies brighter than M _UV < −16 mag would produce an intergalactic medium hydrogen-ionized fraction of x _e = 0.5 at 5.3 < z < 5.8, possibly too late compared to constraints from from quasi-stellar object (QSO) sight lines. Including fainter galaxies, M _UV < −14 mag, we obtain x _e = 0.5 at 6.0 < z < 8.1, fully consistent with QSO and cosmic microwave background data. This implies that EoR galaxies produce plenty of ionizing photons, but that these do not efficiently escape. This may be a result of high gas column densities combined with burstier star formation histories, which limit the time massive stars are able to clear channels through the gas for ionizing photons to escape.

Galaxies exhibit a tight correlation between their star formation rate (SFR) and stellar mass over a wide redshift range known as the star-forming main sequence (SFMS). With JWST, the SFMS can now be investigated at high redshifts down to masses of ∼10 ⁶ M _⊙ , using sensitive star formation rate tracers such as the H α emission, which allow us to probe the variability in the star formation histories. We present inferences of the SFMS based on 316 H α -selected galaxies at z ∼ 4 − 5 with log( M _★ / M _⊙ ) = 6.4 − 10.6. These galaxies were identified behind the Abell 2744 lensing cluster with NIRCam grism spectroscopy from the survey All the Little Things (ALT). At face value, our data suggest a shallow slope in the SFMS (SFR ∝ M _★ ^α , with α = 0.45). After we corrected this for the H α -flux limited nature of our survey using a Bayesian framework, the slope steepened to α = 0.59 ^+0.10 _−0.09 , whereas current data on their own are inconclusive on the mass dependence of the scatter. These slopes differ significantly from the slope of ∼1 that is expected from the observed evolution of the galaxy stellar mass function and from simulations. When we fixed the slope to α = 1, we found evidence for a decreasing intrinsic scatter with stellar mass (from ∼0.5 dex at M _★ = 10 ⁸ M _⊙ to 0.4 dex at M _★ = 10 ¹⁰ M _⊙ ). This difference might be explained by a (combination of) luminosity-dependent SFR(H α ) calibration, a population of (mini)-quenched low-mass galaxies, or underestimated dust attenuation in high-mass galaxies. Future deep observations with different facilities can quantify these processes, which will enable us to achieve better insights into the variability of the star formation histories.

The population of the little red dots (LRDs) may represent a key phase of supermassive black hole (SMBH) growth. A cocoon of dense excited gas is emerging as a key component to explain the most striking properties of LRDs, such as strong Balmer breaks and Balmer absorption, as well as the weak IR emission. To dissect the structure of LRDs, we analyzed new deep JWST/NIRSpec PRISM and G395H spectra of FRESCO-GN-9771, one of the most luminous known LRDs at z = 5.5. These spectra reveal a strong Balmer break, broad Balmer lines, and very narrow [O III ] emission. We revealed a forest of optical [Fe II ] lines, which we argue are emerging from a dense ( n _H = 10 ^9 − 10 cm ⁻³ ) warm layer with electron temperature T _e ≈ 7000 K. The broad wings of H α and H β have an exponential profile due to electron scattering in this same layer. The high H α : H β : H γ flux ratio of ≈10.4 : 1 : 0.14 is an indicator of collisional excitation and resonant scattering dominating the Balmer line emission. A narrow H γ component, unseen in the other two Balmer lines due to outshining by the broad components, could trace the ISM of a normal host galaxy with a star formation rate of ∼5 M _⊙ yr ⁻¹ . The warm layer is mostly opaque to Balmer transitions, producing a characteristic P Cygni profile in the line centers suggesting outflowing motions. This same layer is responsible for shaping the Balmer break. The broadband spectrum can be reasonably matched by a simple photoionized slab model that dominates the λ > 1500 Å continuum and a low-mass (∼10 ⁸ M _⊙ ) galaxy that could explain the narrow [O III ], with only a subdominant contribution to the UV continuum. Our findings indicate that Balmer lines are not directly tracing the gas kinematics near the SMBH and that the BH mass scale is likely much lower than virial indicators suggest.

We aim to characterise the mass-metallicity relation (MZR) and the 3D correlation between the stellar mass, metallicity, and star formation rate (SFR) known as the fundamental metallicity relation (FMR) for galaxies at 5 < z < 7. Using ∼800 [O III ] selected galaxies from deep NIRCam grism surveys, we present our stacked measurements of direct-T _e metallicities, which we used to test recent strong-line metallicity calibrations. Our measured direct-T _e metallicities (0.1–0.2 Z _⊙ for M _★ ≈ 5 × 10 ^7 − 9 M _⊙ , respectively) match recent JWST/NIRSpec-based results. However, there are significant inconsistencies between observations and hydrodynamical simulations. We observe a flatter MZR slope than the SPHINX ²⁰ and FLARES simulations, which cannot be attributed to selection effects. With simple models, we show that the effect of an [O III ] flux-limited sample on the observed shape of the MZR is strongly dependent on the FMR. If the FMR is similar to the one in the local Universe, the intrinsic high-redshift MZR should be even flatter than is observed. In turn, a 3D relation where SFR correlates positively with metallicity at fixed mass would imply an intrinsically steeper MZR. Our measurements indicate that metallicity variations at fixed mass show little dependence on the SFR, suggesting a flat intrinsic MZR. This could indicate that the low-mass galaxies at these redshifts are out of equilibrium and that metal enrichment occurs rapidly in low-mass galaxies. However, being limited by our stacking analysis, we are yet to probe the scatter in the MZR and its dependence on SFR. Large carefully selected samples of galaxies with robust metallicity measurements can put tight constraints on the high-redshift FMR and help us to understand the interplay between gas flows, star formation, and feedback in early galaxies.

The abundant population of little red dots (LRDs), compact objects with red UV to optical colors and broad Balmer lines at high redshift, is revealing new insights into the properties of early active galactic nuclei (AGN). Perhaps the most surprising features of this population are the presence of Balmer absorption and ubiquitous strong Balmer breaks. Recent models link these features to an active supermassive black hole (SMBH) cocooned in very dense gas ( N _H ∼ 10 ²⁴ cm ⁻² ). We present a stringent test of such models using VLT/MUSE observations of A2744-45924, the most luminous LRD known to date ( L _{H α} ≈ 10 ⁴⁴ erg s ⁻¹ ), located behind the Abell-2744 lensing cluster at z = 4.464 ( μ = 1.8). We detect a moderately extended Ly α nebula ( h ≈ 5.7 pkpc), spatially offset from the point-like H α seen by JWST by ≈1.6 pkpc. The Ly α emission is narrow (FWHM = 270 ± 15 km s ⁻¹ ), and faint (Ly α = 0.07H α ) compared to Ly α nebulae typically observed around quasars of similar luminosity. We detect compact N IV ] λ 1486 emission, spatially aligned with H α , and a spatial shift in the far-UV continuum matching the Ly α offset. We discuss that H α and Ly α have distinct physical origins: H α originates from the AGN, while Ly α is powered by star formation. In the environment of A2744-45924, we identified four extended Ly α halos (Δ z < 0.02, Δ r < 100 pkpc). Their Ly α luminosities match the expectations based on H α emission, and show no evidence for radiation from A2744-45924 affecting its surroundings. The lack of strong, compact, and broad Ly α and the absence of a luminous extended halo, suggest that the UV AGN light is obscured by dense gas cloaking the SMBH with a covering factor close to unity.