Diego Paris

We present the first homogeneous study of the radial distribution of the binary fraction across the full extent of six Galactic globular clusters (GCs) spanning a wide range of dynamical ages, from dynamically young systems to core-collapsed clusters. We measured the radial variation of the binary fraction using a combination of deep optical HST observations and wide-field ground-based data. For the first time, we provide evidence that the binary fraction in GCs does not decrease monotonically with radius, as commonly assumed, but instead exhibits a bimodal distribution characterized by an excess in the outer regions. Specifically, the binary fraction displays a central peak, followed by a minimum at intermediate radii and a rising branch beyond approximately 1–2 half-light radii. The position of this minimum correlates with the cluster relaxation timescale, indicating that it is shaped by long-term dynamical effects of two-body relaxation driving binary evolution, segregation, and disruption. The minimum radius also correlates with the A ⁺ _rh parameter derived from the radial distribution of blue straggler stars, an empirical indicator of a cluster’s dynamical age, further supporting the interpretation that this feature is due to internal dynamical processes. Numerical simulations presented in a companion paper show that such bimodal distributions naturally arise from the combined effects of binary disruption and mass segregation of the surviving binaries in clusters.

We present a spectroscopic survey of young stellar objects (YSOs) in Sh2-284, a distant (∼4.5 kpc), low-metallicity ( Z ∼ 1/3 Z _⊙ ) star-forming region (SFR) toward the Galactic anticenter. Candidate YSOs were selected using mid-infrared Spitzer/IRAC data with optical and near-infrared photometry. Follow-up spectroscopy was conducted with the Multi-Object Double Spectrograph (MODS) at the Large Binocular Telescope. We characterize the stellar and accretion properties of the disk-bearing population in a metal-poor environment, probing stellar masses from ∼2.95 to ∼0.35 M _⊙ . This work constitutes the first wide-field ( ∼ 4 5 ′ × 4 5 ′ ) spectroscopic investigation of YSOs in Sh2-284, providing a comprehensive view of star formation in this Galactic environment. We provide tentative first estimates of iron abundance for three low-mass targets and lithium abundance for a limited number of objects, offering an initial characterization of the chemical properties of these stars. We adopt a multidiagnostic approach based on nine tracers, exploiting the spectral coverage of the instrument, using H α , H β , H γ , Ca ii infrared triplet, Pa η , Pa ζ , and Pa ϵ emission lines. We derive key accretion properties and compare them with those measured in nearby, solar-metallicity SFRs to explore potential metallicity-driven differences in accretion behavior and disk evolution. We tentatively find indications of a flattening in the flux–flux relations of the metal-poor YSOs compared to solar-metallicity samples, a behavior that is recovered across diagnostics. Our observations indicate that the selected disk-bearing YSOs in this metal-poor environment exhibit resilient accretion activity, spanning from ∼2.3 × 10 ⁻¹⁰ to ∼1.0 × 10 ⁻⁶ M _⊙ yr ⁻¹ and having a median rate of ∼ 2.2 × 10 ⁻⁸ M _⊙ yr ⁻¹ . Overall, this demonstrates that gas accretion can be efficiently sustained in subsolar-metallicity environments.

The James Webb Space Telescope (JWST) has revealed an unexpected excess of UV-bright galaxies at z > 10, unaccounted for by extrapolations from pre-JWST observations and theoretical models. Understanding the physical properties and star formation histories (SFHs) of high-redshift systems is key to distinguishing between the different proposed scenarios and assessing their implications for the galaxy formation and evolution picture. We identified and analysed a sample of 2420 robust candidates at z ∼ 7 − 14 drawn from the ASTRODEEP-JWST dataset over ∼0.2 deg ² , and modelled their properties with non-parametric SFHs to derive the specific star formation rate (sSFR) and stellar population properties. We find that the median sSFR and M/L remain roughly constant across the probed redshift range. We show that this result is robust against potential systematics unless a hidden population of dust-enshrouded starbursts, undetectable in the current data, exists at these redshifts. In any case, the absence of observed high-sSFR systems at the highest redshifts suggests that any dust-free starburst phase must be short-lived. The observed sSFR evolution is in tension with most (though not all) theoretical models, making it a key quantity for discriminating among competing scenarios. The sample shows a wide range of physical conditions and galaxy classes, including systems with low sSFRs and high mass-to-light ratios (M/L) up to z ∼ 10, indicative of already-evolved galaxies only a few hundred megayears after the Big Bang, and different degrees of dust attenuation. We finally reconstructed the assembly histories of two sub-samples, namely the highest-M/L galaxies at z ∼ 7 − 8, which appear to have formed the bulk of their stars at least 500 Myr before observation, implying progenitors observable as UV-bright sources at z > 20, and the full sample of z > 11 galaxies, which formed through stochastic SFHs, remaining UV-faint for most of their early evolution, before undergoing recent (∼50 Myr old) episodes of major growth.

AT 2022cmc is the first on-axis jetted tidal disruption event (TDE) to be discovered at optical wavelengths. The optically bright nature of AT 2022cmc presents an unprecedented opportunity to place this jetted TDE in the context of the larger optically selected thermal TDE population and explore potential connections to optical TDE subclasses, particularly the class of luminous TDEs that lack optical spectral features. In this work, we present late-time optical observations of AT 2022cmc, both imaging and spectroscopy, that extend the optical dataset to ∼160 days from the first detection in the observed frame. The light curve clearly evolves from red to blue, which we interpret as a transition from a nonthermally dominated spectral energy distribution (SED) to thermally dominated SED. By accounting for the nonthermal emission evident in the optical SED at early times, we extract the properties of the thermal emission and compare to a sample of optically selected thermal TDEs. We find that the properties of AT 2022cmc are consistent with previous correlations found for the evolution and properties of thermal TDEs, with the thermal properties of AT 2022cmc aligning with the class of featureless and luminous TDEs. The confirmation of this similarity motivates the importance of prompt and multiwavelength follow-up of featureless and luminous TDEs in order to further explore the connection they have with jetted TDEs.

Aims. We investigate the galaxy ultraviolet luminosity function (UV LF) at z ≃ 15 − 30 to constrain early galaxy formation scenarios aimed at explaining the mild evolution of the UV LF bright end reported by JWST at z ≈ 10 − 15. Methods. We designed customised Lyman-break colour selection techniques to identify galaxy candidates in the redshift ranges 15 ≤ z ≤ 20 and 20 ≤ z ≤ 28. The selection was performed on the ASTRODEEP-JWST multi-band catalogues of the CEERS, Abell-2744, JADES, NGDEEP, and PRIMER survey fields, covering a total area of ∼0.2 sq. deg. Results. We identified five candidates at 15 ≤ z ≤ 20, whereas no objects were found based on the z ≳ 20 colour selection criteria. Despite exhibiting a > 1.5 mag break, all the objects display multi-modal redshift probability distributions across different SED-fitting codes and methodologies. The alternative solutions correspond to poorly understood populations of low-mass quiescent or dusty galaxies at z ∼ 3 − 7. This conclusion is supported by the analysis of five F200W dropout objects that we found to be interlopers based on NIRSpec PRISM spectra: four dusty star-forming galaxies at z ∼ 2.2 − 6.6 and a passive galaxy at z = 4.91, with log( M _star /M _⊙ )≲9. We measured the UV LF under different assumptions on the contamination level within our sample. We find that if even a fraction of the candidates were indeed found at z ≳ 15, the resulting UV LF would point to a very mild evolution compared to estimates at z < 15, implying a significant tension with existing theoretical models. In particular, confirming our bright ( M _UV < −21) candidates would require substantial revisions to the theoretical framework. In turn, if all these candidates ended up being confirmed as interlopers, we would have to conclude that future surveys might require ten times wider areas to be able to select M _UV ≲ −20 galaxies at z > 15. Observations in the F150W and F200W filters at depths comparable to those in the NIRCam LW bands would also be required to mitigate contamination from rare red objects at z ≲ 8.

KK 153 is a star-forming dwarf galaxy that has been recently proposed as a new member of the sparsely populated class of gas-rich ultra faint dwarfs, lying in the outskirts of the Local Group. We used the Large Binocular Telescope under sub-arcsec seeing conditions to resolve for the first time the outer regions of KK 153 into individual stars, reaching the red giant branch. The magnitude of the red giant branch tip was used to measure a distance of D=3.06 (+0.17/-0.14) Mpc, much more accurate and precise than the estimate previously available in the literature, based on the baryonic Tully-Fisher relation (D=2.0 (+1.7/-0.8) Mpc). The new distance places KK 153 clearly beyond the boundaries of the Local Group, and, together with a new measure of the integrated magnitude, implies a stellar mass of M_*=2.4 \pm 0.2 X 10^6 M_{\sun}. The dwarf populates the extreme low-mass tail of the M_* distribution of gas-rich galaxies but it is significantly more massive than the faintest local gas-rich dwarfs, Leo T and Leo P. In analogy with similar systems, the star formation history of KK 153 may have been impacted by the re-ionisation of the Universe while keeping a sufficient gas reservoir to form new stars several Gyr later.

We present James Webb Space Telescope (JWST)/NIRSpec PRISM spectroscopic characterization of GHZ9 at z = 10.145 ± 0.010, currently the most distant source detected by the Chandra X-ray Observatory. The spectrum reveals several UV high-ionization lines, including C II, Si IV, N IV], C IV, He II, O III], N III], and C III]. The prominent rest-frame equivalent widths (EW(C IV) ≃ 65 Å, EW(O III]) ≃ 28 Å, and EW(C III]) ≃ 48 Å) show the presence of a hard active galactic nucleus (AGN) radiation field, while line ratio diagnostics are consistent with either AGN or star formation as the dominant ionizing source. GHZ9 is nitrogen-enriched (6–9.5 (N/O)_⊙), carbon-poor (0.2–0.65 (C/O)_⊙), metal-poor (Z = 0.01–0.1 Z _⊙), and compact (0.02, implies an accelerated growth of the BH mass with respect to the stellar mass. GHZ9 is the ideal target to constrain the early phases of AGN–galaxy coevolution with future multifrequency observations.

With a small sample of fast X-ray transients (FXTs) with multiwavelength counterparts discovered to date, their progenitors and connections to γ-ray bursts (GRBs) and supernovae (SNe) remain ambiguous. Here, we present photometric and spectroscopic observations of SN 2025kg, the SN counterpart to the FXT EP 250108a. At z = 0.17641, this is the closest known SN discovered following an Einstein Probe (EP) FXT. We show that SN 2025kg’s optical spectra reveal the hallmark features of a broad-lined Type Ic SN. Its light-curve evolution and expansion velocities are comparable to those of GRB-SNe, including SN 1998bw, and two past FXT-SNe. We present JWST/NIRSpec spectroscopy taken around SN 2025kg’s maximum light, and find weak absorption due to He I 1.0830 μm and 2.0581 μm and a broad, unidentified emission feature at ∼4–4.5 μm. Further, we observe broadened Hα in optical data at 42.5 days that is not detected at other epochs, indicating interaction with H-rich material. From its light curve, we derive a ⁵⁶Ni mass of 0.2–0.6 M _⊙. Together with our companion Letter, our broadband data are consistent with a trapped or low-energy (≲10⁵¹ erg) jet-driven explosion from a collapsar with a zero-age main-sequence mass of 15–30 M _⊙. Finally, we show that the sample of EP FXT-SNe supports past estimates that low-luminosity jets seen through FXTs are more common than successful (GRB) jets, and that similar FXT-like signatures are likely present in at least a few percent of the brightest Type Ic-BL SNe.

Context. The detection and follow-up observations of high-redshift (z > 6) gamma-ray bursts (GRBs) provide a unique opportunity to explore the properties of the distant Universe. Unfortunately, they are rather rare, with only a dozen of them identified so far. Aims. We present here the discovery of the GRB with the second highest spectroscopic redshift measured to date, GRB 240218A at z = 6.782, and the broadband analysis of its afterglow. Following the detection by high-energy satellites, we obtained multi-epoch and multi-wavelength photometric follow-up observations, from 68 s to ∼48 d after the detection. These data allow us to perform a comprehensive study of the emission and physical properties of this event. We also compare these properties with GRBs observed at high and low redshift. Methods. We built the X-ray, near-infrared, and radio light curves and studied their temporal evolution. Moreover, we investigated the spectral energy distribution (SED) at different times to trace possible spectral evolution. We also compared the prompt phase properties, X-ray luminosity, and optical extinction of GRB 240218A with those of the long-duration GRB (LGRB) population. Results. The SED analysis reveals a typical afterglow-like behaviour at late times. The origin of the early-time emission is uncertain, with the probable presence of an additional contribution on top of the afterglow emission. From the broadband physical modelling of the afterglow, we identify a narrow Gaussian jet seen slightly off-axis, θ_v = 2.52_−0.29^+0.57 deg, and pinpoint the presence of a possible jet break ∼0.86 d after the trigger. Conclusions. The results of the analysis and the comparison with other high-z GRBs reveal that we can consider GRB 240218A as a ‘standard’ high-redshift LGRB: the prompt phase properties, the X-ray luminosity, and the optical extinction are consistent with the values derived for the LGRB population. The jet opening angle is narrower but compatible with those of high-z bursts, possibly pointing to more collimated jets at high redshift.