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D. Schaerer

Publications and source records attributed to D. Schaerer.

At least 19 recordsLinked to original sources
arXiv2026

JWST Reveals Compact Nuclear Starbursts Masquerading as AGNs in Metal-Poor Dwarfs: Where Are the Accreting Intermediate-Mass Black Holes?

We present JWST/NIRSpec spectroscopy of the low-mass, metal-poor galaxy SDSS~J160135.95+311353.7 (J1601), selected for its extreme mid-infrared colors and compact nuclear emission, placing it within widely used WISE color diagnostics for active galactic nuclei (AGNs). Despite this selection, we find no evidence for coronal lines, X-ray emission, or variability typically associated with accretion activity. We compare J1601 to SDSS~J120122.30+021108.3 (J1201), a similar but lower-mass, more metal-poor system studied previously (Doan, 2025). Both galaxies host compact nuclear starbursts but differ in their stellar populations and dust properties: J1601 shows CO bandhead absorption indicative of red supergiants, weak nuclear Wolf--Rayet features, and a circumnuclear PAH ring, consistent with a more developed recent starburst, while J1201 is more dust-enshrouded and chemically primitive. Despite these differences, neither system shows evidence for AGN activity, indicating that the absence of accretion is not simply due to evolutionary timing. Photoionization models show that the weakness of high-ionization emission cannot be explained by low metallicity alone, implying a genuine deficit of hard ionizing photons. Crucially, the red mid-infrared colors in both systems originate from compact, unresolved nuclear emission confined to the nuclear star cluster. These results demonstrate that compact nuclear starbursts can mimic AGN-like mid-infrared colors without accretion, and that commonly used AGN diagnostics may not uniquely identify accreting black holes in metal-poor dwarf galaxies. Our findings suggest that such systems may not provide the conditions required for efficient black hole growth and/or may lie near or below the regime where black hole seeds can form.

astro-ph.GA↗
arXiv2026

Chemical composition and kinematics of ionised gas in low-mass star-forming galaxies with extremely high [OIII]/[OII] ratios

We present Very Large Telescope/Xshooter spectrophotometric observations of eleven low-redshift (z<0.085) compact star-forming galaxies (`high O32 sample'). These galaxies are characterised by extremely high emission-line ratios [OIII]$\lambda$5007/[OII]3727, ranging from 11 to 42. Galaxies with such high ratios are thought to be promising candidates for leaking large amounts of Lyman continuum radiation. They are characterized by low oxygen abundances 12+log(O/H)\,=7.5-8.0 and low stellar masses M*~10^6-10^8 Msun. Strong emission lines of various ions in all spectra are used to derive helium and oxygen abundances, and N/O, Ne/O, S/O, Cl/O, Ar/O and Fe/O abundance ratios. We also derived macroscopic velocity dispersions sigma(lambda) from various emission lines of different ions. We find that sigma(4861) of the Hbeta emission line is increased with increasing stellar mass and decreasing O32 ratio. On the other hand, sigma(lambda)/sigma(4861) ratios for various lines are close to 1. Exceptions are sigma(lambda)/sigma(4861) of two lines, HeII 4686 and HeI 10830, which are considerably higher than unity and of four lines, [OII] 3726,3729, [SII] 6717,6731, with sigma(lambda)/sigma(4861) lower than unity. The two former lines are likely produced in the inner parts of HII regions and are broadened by dynamical processes generated by massive stars, and by radiative scattering in the case of the HeI 10830 emission line. Emission in the four latter lines is produced mainly in the outer and likely more quiet parts of HII regions.

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arXiv2026

A quasi-star is born: formation and evolution of accreting quasi-stars as a pathway to Little Red Dots at non-zero metallicity

The recently discovered Little Red Dots identified by the James Webb Space Telescope are compact high-redshift sources whose properties have motivated models involving black holes embedded within optically thick gaseous envelopes. We investigate their rest-frame optical emission by modeling quasi-stars, i.e. stellar envelopes powered by accretion onto a central black hole, formed from rapidly accreting proto-stars that reach the supermassive star regime ($>10^4$~\Msun) before undergoing general relativistic instability. We compute stellar evolution models with mass gain rates of 0.01, 0.1, and 1~\Msun/yr and metallicities $Z=0$-$0.01$. For accretion rates $\ge0.1$~\Msun/yr, stars remain nearly fully convective with $T_\mathrm{eff}\sim4000$-$9000$~K. General relativistic instability occurs at $M_\star\sim3.5\times10^4$~\Msun\ ($6.8\times10^4$~\Msun) for $\dot{m}=0.1$~\Msun/yr (1~\Msun/yr), at $L\sim10^9$~\Lsun. Assuming the black hole supports the envelope until complete accretion ($M_{\rm BH,max}/M_{\rm QS}=1$), quasi-stars reach maximum lifetimes of $10^7$-$10^8$~yr, $\sim100$-$1000$ times longer than their progenitors. Their formation and evolution are nearly independent of metallicity. Matching our models to Little Red Dots at $z<4.5$ ($L_\mathrm{bol}\sim10^{9.5}$-$10^{11.5}$~\Lsun) implies quasi-star masses of $10^{4.5}$-$10^{6.5}$~\Msun, while the minimum observed luminosity requires progenitor accretion rates $\gtrsim0.1$~\Msun/yr. Our models support quasi-stars as the origin of Little Red Dot optical emission and constrain their masses, lifetimes, progenitor environments, and luminosities. Our models offer a framework supporting quasi-stars as the source of Little Red Dot optical emission, and provide insights into their lifetimes, composition, progenitor's environment as well on their minimum and maximum observed luminosities.

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arXiv2026

Nitrogen abundances in star-forming galaxies 2.2 Gyr after the Big Bang are not elevated

Using deep medium-resolution JWST rest-optical spectra of a sample of typical star-forming galaxies (Lyman break galaxies and Lyman-$\alpha$ emitters) from the LyC22 survey at $z \sim 3$, we determined the nebular abundances of N, O, and Ne relative to H for a subsample of 25 objects with the direct method, based on auroral [OIII]4363 line detections. Our measurements increases the number of accurate N/O determinations at $z \sim 2-4$ using a homogeneous approach. We found a mean value of $\log({\rm N/O})=-1.29^{+0.25}_{-0.21} $ over a metallicity range 12+log(O/H)=7.5 to 8.44. The observed N/O ratio and scatter are indistinguishable from that observed in low-z galaxies and HII regions over the same metallicity range, showing thus no redshift evolution of N/O for typical galaxies over a significant fraction of cosmic time. We also show that typical $z \sim 3$ galaxies show a similar offset in the BPT diagram as galaxies from the low-z Lyman Continuum Survey (LzLCS), when compared to the average of SDSS galaxies, and show 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.

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arXiv2025

N-emitters as possible sign-posts of GC formation

Based on the finding of unusual chemical abundance ratios of N-emitters, which resemble those of globular cluster (GC) stars, their compactness, high ISM densities and other properties, it has been suggested that N-emitters could indicate the formation sites of globulars. A recent statistical study of the N-emitter population has quantified the frequency $f_N$ of these rare objects and their redshift evolution (Morel et al. 2025). Using these results we here test if N-emitters trace the formation of GCs and use the observed cosmic star-formation rate density evolution to predict the cosmological evolution of the GC population with time, their age distribution, and the total present-day stellar mass density formed in globulars. The predicted age distribution of GCs strongly resembles the typical asymmetric observed distributions in the Galaxy and ellipiticals, with a peak at $\sim 11.5-12$ Gyr and a longer tail extending to younger ages. We derive a total stellar mass density formed in N-emitters down to redshift zero of $(2-7) \times 10^5$ M$_{\odot}$ Mpc$^{-3}$, which matches within a factor $\sim 2$ the observed fraction of stellar mass found in the GC population at $z=0$. These results provide additional indirect arguments supporting the hypothesis that N-emitters could represent sign-posts of a short phase of GC formation.

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arXiv2025

Discovery of new N-emitters over a wide redshift range

JWST observations have revealed rare galaxies with UV spectra exhibiting intense lines of nitrogen, indicative of super-solar N/O abundances at low metallicity. To better understand these enigmatic objects and provide new constraints on proposed scenarios, we have undertaken a systematic search for galaxies with UV emission lines of nitrogen. Using public JWST NIRSpec data, we have identified 45 N-emitters with robust NIII] or NIV] detections, including 4 previously known objects. We find N-emitters from redshift $z\sim 3-11$ among a broad diversity of galaxies, in terms of morphology, UV magnitude, stellar mass, SFR, metallicity, and rest-optical line strengths. The UV nitrogen lines show typical equivalent widths between $\sim 5-50$ \AA. Carbon lines are generally fainter than the N lines. Using strong line calibrations established at high-redshift, we find metallicities $12+log(O/H)\sim 7.15-8.5$, including thus also high metallicities. The H$\beta$ equivalent width of N-emitters varies strongly, and sources with low EWs show clear signs of a Balmer break, indicative of composite stellar populations combining both young (< 10 Myr) stars responsible of the UV emission lines and an older population contributing to the rest-optical spectrum. Supersolar N/O ratios are found in all N-emitters. C/O abundances are comparable to those of galaxies at the same metallicity, and all N-emitters show high N/C ratios or lower limits ($\log(N/C) > 0.5$), independently of metallicity. The observed abundance ratios are compatible with ejecta from H-burning and do not show signs of Carbon enhancements, even at higher metallicities. Finally, we find that the fraction of N-emitters increases with redshift, and we quantify this evolution. Our study increases the sample of known N-emitters by a factor $\sim 3$, reveals a diversity of properties among N-emitters, and provides new constraints on their nature.

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arXiv2025

A great diversity of spectral shapes in the ionising spectra of z ~ 0.6-1 galaxies revealed by HST/COS and possible detection of nebular LyC emission

We present observations of eleven compact star-forming galaxies in the redshift range z = 0.6145 - 1.0053, with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST). We aim to spectroscopically measure for the first time the Lyman continuum (LyC) over a wider rest-frame wavelength range of ~ 600 - 900A compared to ~ 850 - 900A in previous studies of galaxies at z ~ 0.3 - 0.4. The HST data are supplemented by SDSS spectra of all galaxies and by a VLT/Xshooter spectrum of one galaxy, J0232+0025. These data are used to derive the spectral energy distribution in the entire UV and optical range, the stellar mass, and the chemical composition from the nebular emission lines. We detect stellar LyC emission in seven out of eleven galaxies with escape fractions, f_esc(LyC), in the range of ~ 2 - 60%, and establish upper limits for f_esc(LyC) in the remaining galaxies. We discover for the first time nebular LyC emission as a bump just bluewards of the LyC limit at 912A in two galaxies, J0232+0025 and J1021+0436. We find a similar bump among our earlier studies in a less distant galaxy J1243+4646 with z = 0.4317. We conclude that the use of the LyC continuum in the wavelength range close to the LyC limit, which contains both the stellar and nebular continua, requires special consideration to not overestimate the observed f_esc(LyC).

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arXiv2025

The ALPINE-CRISTAL-JWST Survey: JWST/IFU Optical Observations for 18 Main-Sequence Galaxies at z=4-6

To fully characterize the formation and evolution of galaxies, we need to observe their stars, gas, and dust on resolved spatial scales. We present the ALPINE-CRISTAL-JWST survey, which combines kpc-resolved imaging and spectroscopy from HST, JWST, and ALMA for 18 representative main-sequence galaxies at z=4-6 and log(M/$M_\odot$) > 9.5 to study their star formation, chemical properties, and extended gas reservoirs. The co-spatial measurements resolving the ionized gas, molecular gas, stars, and dust on 1-2 kpc scales make this a unique benchmark sample for the study of galaxy formation and evolution at $z\sim5$, connecting the Epoch of Reionization with the cosmic noon. In this paper, we outline the survey goals and sample selection, and present a summary of the available data for the 18 galaxies. In addition, we measure spatially integrated quantities (such as global gas metallicity), test different star formation rate indicators, and quantify the presence of H$\alpha$ halos. Our targeted galaxies are relatively metal rich (10-70% solar), complementary to JWST samples at lower stellar mass, and there is broad agreement between different star formation indicators. One galaxy has the signature of an active galactic nuclei (AGN) based on its emission line ratios. Six show broad H$\alpha$~emission suggesting type 1 AGN candidates. We conclude with an outlook on the exciting science that will be pursued with this unique sample in forthcoming papers.

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arXiv2025

Very massive stars at low metallicity: evolution, synthetic spectroscopy, and impact on the integrated light of starbursts

We study the spectroscopic appearance of very massive stars and their effect on the integrated light of starbursts at low metallicity (Z). We adopt two frameworks for the mass loss rates of VMS: in one case we assume no Z dependence, in the other case we assume a linear scaling with Z. We compute evolutionary models for masses 150, 200, 250 and 300 Msun at Z=0.2, 0.1 and 0.01 Zsun. We compute the associated synthetic spectra at selected points along the evolutionary tracks. Finally we build population synthesis models including VMS. We find that the evolution of VMS critically depends on the assumptions regarding mass loss rates. In case of no Z dependence VMS remain hot for all their lifetime. Conversely when mass loss rates are reduced because of lower Z VMS follow a classical evolution towards the red part of the HR diagram. VMS display HeII 1640 emission in most phases of their evolution, except when they become too cool. This line is present in the integrated light of population synthesis models down to 0.1 Zsun whatever the star formation history, and is also sometimes seen at Z=0.01 Zsun. HeII 1640 is weaker in models that include a Z scaling of the mass loss rates. The optical spectra of starbursts, especially the Wolf-Rayet bumps, sometimes display VMS signatures when these stars are present. At low Z, adding VMS to population synthesis models produces more ionising photons down to 45 eV. At higher energy the ionising flux depends on age, Z, assumption regarding VMS mass loss rates, and on the very short phases at the end of VMS evolution. HeII ionising fluxes large enough to produce some amount of nebular HeII 4686 emission can be produced under specific circumstances. Our models are able to reproduce qualitatively and sometimes also quantitatively the UV spectra of star-forming regions. However we are not able to clearly identify which mass loss framework is favoured.

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arXiv2025

The Lyman-alpha and Continuum Origins Survey II: the connection between the escape of ionizing radiation and Lyman-alpha halos in star-forming galaxies

One of the current challenges in galaxy evolution studies is to establish the mechanisms that govern the escape of ionizing radiation from galaxies. Here, we investigate the connection between Lyman Continuum (LyC) escape and the conditions of the Circumgalactic Medium (CGM), as probed by Ly$\alpha$ halos (LAHs) in emission. We use Ly$\alpha$ and UV continuum imaging data from the Lyman alpha and Continuum Origins Survey (LaCOS), targeting 42 nearby ($z \simeq 0.3$), star-forming galaxies with LyC observations (escape fractions of $f_{\rm esc}^{\rm LyC} \simeq 0.01-0.49$). LaCOS galaxies show extended Ly$\alpha$ emission ubiquitously, with LyC emitters (LCEs) having more compact Ly$\alpha$ morphologies than non-LCEs, and Ly$\alpha$ spatial offsets that do not exceed the extent of the UV continuum. We model the diffuse LAHs using a combined S\'ersic plus exponential 2D profile, and find that the characteristic scale length of the Ly$\alpha$ halo is ten times larger than the UV, on average. We unveil a significant anti-correlation between $f_{\rm esc}^{\rm LyC}$ and the Ly$\alpha$ Halo Fraction (HF, or contribution of the halo to the total Ly$\alpha$ luminosity), that we propose as a new LyC indicator. Our observations show that halo scale lengths and HFs both scale positively with the optical depth of the neutral gas in the ISM, revealing a picture in which Ly$\alpha$ and LyC photons in LCEs either emerge directly from the central starbursts or escape isotropically and, in the case of Ly$\alpha$, minimize the number of scattering interactions in a less-extended CGM.

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arXiv2024

The Lyman Alpha Reference Sample. XVI. Global 21cm HI properties of Lyman-$\alpha$ emitting galaxies

The Lyman-$\alpha$ (Lya) line of hydrogen is a well-known tracer of galaxies at high-z. However, the connection between Lya observables and galaxy properties has not fully been established, limiting the use of the line to probe the physics of galaxies. Here, we derive global neutral hydrogen gas (HI) properties of nearby Lya-emitting galaxies to assess the impact of HI on the Lya output of galaxies. We observed 21cm line emission using the VLA in D-array configuration (~55" resolution, ~38 kpc) for 37 star-forming galaxies with available Lya imaging from the Lyman Alpha Reference Samples (LARS and eLARS). We detect 21cm emission for 33/37 galaxies observed. We find no significant correlation of global HI properties with Lya luminosity, escape fraction or equivalent width derived with HST photometry. Additionally, both Lya-emitters and weak or non-emitters are distributed evenly along the HI parameter space of optically-selected z=0 galaxies. Around 74% of the sample is undergoing galaxy interaction, this fraction is higher for Lya-emitters (83% for galaxies with EW$\geq$20\r{A}) than for non or weak emitters (70%). Nevertheless, galaxies identified as interacting have Lya and HI properties statistically consistent with those of non-interacting galaxies. Our results show that global HI properties (on scales > 30kpc) have little direct impact on the Lya output from galaxies. Instead, HI likely regulates Lya emission on small scales: statistical comparisons of Lya and high angular resolution 21cm observations are required to fully assess the role of HI in Lya radiative transfer. While our study indicates that galaxy mergers could play a role in the emission of Lya photons in the local universe, especially for galaxies with high HI fractions, the line-of-sight through which a system is observed ultimately determines Lya observables.

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arXiv2024

Unveiling dust, molecular gas, and high star formation efficiency in extremely UV bright star-forming galaxies at $z\sim 2.1-3.6$

We analysed ALMA FIR (1.3 mm) dust continuum and CO emission of 12 starburst galaxies at $z\sim 2.1-3.6$, selected for their extreme brightness in the rest-UV with $M_{\rm UV} = -23.4$ to $-24.7$. We also analysed VLT HAWK-I $H$- and $K_{\rm s}$-band images. The galaxies are characterised by negligible dust attenuations with blue UV spectral slopes ($-2.62$ to $-1.84$), very young stellar populations of $\sim 10$ Myr, and powerful starbursts with a high mean specific star formation rate of $\rm 112~Gyr^{-1}$, placing them $\sim 1.5$~dex above the main sequence at similar redshifts and stellar masses ($M_{\rm stars} \sim (1.5-4.6)\times 10^9~M_{\odot}$). The FIR dust continuum emission revealed in 9 galaxies yields IR luminosities of $(5.9-28.3)\times 10^{11}~L_{\odot}$ and large dust masses barely produced by SNe within the 10~Myr timescale. The CO emission detected in 8 galaxies evidence large molecular gas masses with a mean molecular gas fraction of 82%. The corresponding star formation efficiencies reach $\gtrsim 40$%, with amazingly short molecular gas depletion timescales between <13 Myr and 71 Myr. These unique properties never reported in previously studied galaxies highlight that these galaxies are likely caught at the very beginning of their stellar mass build-up and undergo a very efficient and fast conversion of gas into stars that can only result from the gas collapse within very short free-fall times. We find that the feedback-free starburst model seems to be able to explain the formation of these galaxies. To reconcile the co-spatial FIR dust emission with the UV-bright unattenuated emission, we speculate about radiation-driven outflows that can temporarily remove dust at the location of the starburst and expel dust at large distances in line with the measured large FIR effective radii ($\rm 1.7~kpc - 5~kpc$) in comparison to very compact stellar radii.

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arXiv2024

A negligible contribution of two luminous $z$ ~ 7.5 galaxies to the ionizing photon budget of reionization

We present indirect constraints on the absolute escape fraction of ionizing photons ($f_{\rm esc}^{\rm LyC}$) of the system GN 42912 which comprises two luminous galaxies ($M_{\rm UV}$ magnitudes of -20.89 and -20.37) at $z\sim7.5$, GN 42912-NE and GN 42912-SW, to determine their contribution to the ionizing photon budget of the Epoch of Reionization (EoR). The high-resolution James Webb Space Telescope NIRSpec and NIRCam observations reveal the two galaxies are separated by only ~0.1$"$ (0.5 kpc) on the sky and have a 358 km s$^{-1}$ velocity separation. GN 42912-NE and GN 42912-SW are relatively massive for this redshift (log($M_\ast/M_\odot$) $\sim$ 8.4 and 8.9, respectively), with gas-phase metallicities of 18 per cent and 23 per cent solar, O$_{32}$ ratios of 5.3 and $>5.8$, and $\beta$ slopes of $-1.92$ and $-1.51$, respectively. We use the Mg II$\lambda\lambda$2796,2803 doublet to constrain $f_{\rm esc}^{\rm LyC}$. Mg II has an ionization potential close to that of neutral hydrogen and, in the optically thin regime, can be used as an indirect tracer of the LyC leakage. We establish realistic conservative upper limits on $f_{\rm esc}^{\rm LyC}$ of 8.5 per cent for GN 42912-NE and 14 per cent for GN 42912-SW. These estimates align with $f_{\rm esc}^{\rm LyC}$ trends observed with $\beta$, O$_{32}$, and the H$\beta$ equivalent width at $z<4$. The small inferred ionized region sizes ($<0.3$ pMpc) around both galaxies indicate they have not ionized a significant fraction of the surrounding neutral gas. While these $z>7$ $f_{\rm esc}^{\rm LyC}$ constraints do not decisively determine a specific reionization model, they support a minor contribution from these two relatively luminous galaxies to the EoR.

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arXiv2024

Bridging the Gap: Examining Vision Foundation Models for Optical and Radio Astronomy Applications

Vision foundation models, which have demonstrated significant potential in many multimedia applications, are often underutilized in the natural sciences. This is primarily due to mismatches between the nature of domain-specific scientific data and the typical training data used for foundation models, leading to distribution shifts. Scientific data often differ substantially in structure and characteristics, and researchers frequently face the challenge of optimizing model performance with limited labeled data of only a few hundred or thousand images. This work evaluates the performance of vision foundation models in astrophysics, with a focus on identifying the best practices for adapting them to domain-specific datasets. We aim to establish a framework for selecting, fine-tuning, and optimizing these models for common tasks in optical and radio astronomy. We compared multiple foundation models, including self-supervised, weakly supervised, and distillation-based architectures, across two representative optical and radio datasets. Experiments involved different fine-tuning strategies, projector heads, and data preprocessing techniques, with performance evaluated on classification and detection metrics. Features extracted by specific foundation models improved classification accuracy for optical galaxy images compared to conventional supervised training. Similarly, these models achieved equivalent or superior performance in object detection tasks with radio images. However, classification performance for radio galaxy images was generally poor, often falling short of supervised approaches. These findings demonstrate that vision foundation models can be effectively adapted to astrophysical applications, provided practitioners iterate on model selection, training strategies, and data handling.

astro-ph.IM↗
arXiv2024

Local Analogs of Primordial Galaxies: In Search of Intermediate Mass Black Holes with JWST NIRSpec

Local low metallicity galaxies with signatures of possible accretion activity are ideal laboratories in which to search for the lowest mass black holes and study their impact on the host galaxy. Here we present the first JWST NIRSpec IFS observations of SDSS J120122.30+021108.3, a nearby ($z=0.00354$) extremely metal poor dwarf galaxy with no optical signatures of accretion activity but identified by WISE to have extremely red mid-infrared colors consistent with AGNs. We identify over one hundred lines between $\sim$ 1.7-5.2 microns, an unresolved nuclear continuum source with an extremely steep spectral slope consistent with hot dust from an AGN ($F_\nu \approx\nu^{-1.5}$), and a plethora of H I, He I, and H$_2$ lines, with no lines from heavier elements, CO or ice absorption features, or PAHs.Our observations reveal that the red WISE source arises exclusively from a bright central unresolved source ($<$ 3pc) suggestive of an AGN, yet there are no He II lines or coronal lines identified in the spectrum, and, importantly, there is no evidence that the radiation field is harder in the nuclear source compared with surrounding regions. These observations can be explained with a young ($<$ 5 Myr) nuclear star cluster with stellar mass $\sim3\times 10^4$ M$_\odot$ and a deeply embedded AGN with bolometric luminosity $\sim$ $2\times10^{41}$ ergs $^{-1}$. The implied black hole mass is $\sim$ 1450 M$_\odot$, based on the Eddington limit, roughly consistent with that expected based on extrapolations of black hole galaxy scaling relations derived for more massive black holes. Longer wavelength observations are crucial to confirm this scenario.

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arXiv2024

Witnessing an extreme, highly efficient galaxy formation mode with resolved Ly$\alpha$ and LyC emission

J1316+2614 at z=3.613 is the UV-brightest ($M_{UV}$ = -24.7) and strongest Lyman continuum (LyC, $f_{esc}^{LyC} \approx$ 90%) emitting star-forming galaxy known, showing also signatures of inflowing gas from its blue-dominated Ly$\alpha$ profile. Here, we present high-resolution imaging with the HST and VLT of the LyC, Ly$\alpha$, rest-UV, and optical emission of J1316+2614. Detailed analysis of the LyC and UV light distributions reveals compact yet resolved profiles, with LyC and UV morphologies showing identical half-light radii of $\simeq$ 220 pc. The continuum-subtracted Ly$\alpha$ emission reveals an extended filamentary structure of $\simeq$ 6.0 kpc oriented south-north with only weak/residual flux within the stellar core, suggesting a Ly$\alpha$ "hole". J1316+2614 presents remarkably high SFR and stellar mass surface densities of log($\Sigma_{SFR}$ [$M_{\odot}$/yr/kpc^2]) = 3.47$\pm$0.11 and log($\Sigma_{M}$ [$M_{\odot}$/pc^2]) = 4.20$\pm$0.06, respectively, which are among the highest observed in star-forming galaxies. Our findings indicate that J1316+2614 is a powerful, young, and compact starburst, leaking significant LyC photons due to the lack of gas and dust within the starburst. We explore the conditions for gas expulsion using a simple energetic balance and find that, given the strong binding force in J1316+2614, a high star formation efficiency ($\epsilon_{SF} \geq 0.7$) is necessary to remove the gas and explain its exposed nature. Our results thus suggest a close link between high $\epsilon_{SF}$ and high $f_{esc}^{LyC}$. This high efficiency can also naturally explain the remarkably high SFR, UV-luminosity, and efficient mass growth of J1316+2614, where at least 62% of its mass formed in the last 6 Myr. J1316+2614 may exemplify an intense, feedback-free starburst with a high $\epsilon_{SF}$, similar to those proposed for UV-bright galaxies at high redshifts. (ABRIDGED)

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arXiv2024

Strong Balmer break objects at z ~ 7-10 uncovered with JWST

We report the discovery of robust spectroscopically confirmed Balmer break (BB) galaxies and candidates, with secure spectroscopic redshifts $7.1 \le z \le 9.6$ from publicly available JWST extra-galactic photometric and spectroscopic surveys. To do so, we used dedicated filters probing the Balmer break and inspected the objects with NIRSpec spectroscopy. We recover the previously known objects with strong Balmer breaks and reveal 10-11 new objects with clear BBs, thus tripling the number of spectroscopically confirmed galaxies with a BB at z >7. Approximately half of them show a pure BB and no signs of recent star formation, whereas the other half shows BB and emission lines, indicating most likely galaxies whose star formation ceased earlier and has restarted recently. Overall we find that ~10-20% of all galaxies from our sample show signatures of an evolved stellar population. Furthermore, we find that the strength of the BB does not significantly depend on the rest-UV and rest-optical brightness of these sources. In short, our work confirms that photometry alone has the potential to measure BB strengths and to identify evolved stellar populations at high redshift and that such objects may be more frequent than previously thought. The presence of galaxies with a range of break strengths and the joint presence of BB and emission lines indicate a bursty nature of the star formation in the early Universe.

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