Mark Sargent

Context. Recent studies show that the radio power of quasars accreting at very high rates can reach surprisingly high values. These studies suggest that this radio emission could originate from star formation (SF); however, the lack of data does not rule out the presence of a jetted active galactic nucleus (AGN). Aims. We investigated the origin of the radio emission of a sample of 18 super-Eddington candidates, over a wide range of redshifts. These sources are expected to have an extreme radiative output per unit black hole mass, show high-velocity outflows, and are therefore thought to be prime mover of galactic evolution via radiative and mechanical feedback. Methods. We present new Karl G. Jansky Very Large Array (VLA) observations at the L , C , and X bands of these sources, which we combined with observations from the LOw-Frequency ARray (LOFAR) Two-meter Sky Survey (LoTSS) and the Very Large Array Sky Survey (VLASS). We also used optical and IR data to derive estimates of accretion and wind parameters, as well as star formation rates (SFRs), which we compared with those derived from the radio emission. Results. Based on radio variability, luminosity, morphology, radio spectral properties, radio versus IR estimates of SFR, and radio-to-mid IR flux ratio, we find that seven of our 18 targets likely have radio emission predominantly arising from SF, six from a combination of SF and AGN-related mechanisms, and only three from a core or jetted AGN origin. This result is consistent with previous studies and supports the prevalence of lower-power radio structures associated with star-forming activity rather than relativistic jets in the high Eddington ratio regime. In the same sample, however, the data suggest that three sources simultaneously exhibit super-Eddington accretion and relativistic ejections.

Context. It is well known that star-forming galaxies (SFGs) exhibit a tight correlation between their radio and infrared emissions, commonly referred to as the infrared-radio correlation (IRRC). Recent empirical studies have reported a dependence of the IRRC on the galaxy stellar mass, in which more massive galaxies tend to show lower infrared-to-radio ratios ( q _IR ) with respect to less massive galaxies. One possible, yet unexplored, explanation is a residual contamination of the radio emission from active galactic nuclei (AGNs), not captured through “radio-excess” diagnostics. Aims. To investigate this hypothesis, we aim to statistically quantify the contribution of AGN emission to the radio luminosities of SFGs located within the scatter of the IRRC. Methods. Our Very Large Baseline Array (VLBA) AGN-sCAN program has targeted 500 galaxies that follow the q _IR distribution of the IRRC, i.e., with no prior evidence for radio-excess AGN emission based on low-resolution (∼arcsec) VLA radio imaging. Our VLBA 1.4 GHz observations reach a 5 σ sensitivity limit of 25 μJy/beam, corresponding to a radio-brightness temperature of T _b ∼ 10 ⁵ K. This classification serves as a robust AGN diagnostic, regardless of the host galaxy’s star formation rate. Results. We detect four VLBA sources in the deepest regions, which are also the faintest VLBI-detected AGNs in SFGs to date. The effective AGN detection rate is 9%, when considering a control sample matched in mass and sensitivity, which is in good agreement with the extrapolation of previous radio AGN number counts. Despite the non-negligible AGN flux contamination (∼30%) in our individual VLBA detections, we find that the peak of the q _IR distribution is completely unaffected by this correction. Although we cannot rule out a high incidence of radio-silent AGNs at (sub)μJy levels among the VLBA non-detections, we derive a conservative upper limit of < 0.1 dex of their cumulative impact on the q _IR distribution. We conclude that residual AGN contamination from non-radio-excess AGNs is unlikely to be the primary driver of the M _★ – dependent IRRC.

We present the first comprehensive study of the structural evolution of brightest group galaxies (BGGs) from redshift z ≃ 0.08 to z = 3.7 using the James Webb Space Telescope ’s 255-hour COSMOS-Web program. This survey provides deep NIRCam imaging in four filters (F115W, F150W, F277W, and F444W) in ∼ 0.54 deg ² , allowing robust size and morphological measurements for ∼ 1700 BGGs spanning ∼ 12 Gyr of cosmic history. High-resolution imaging enables consistent measurement of galaxy sizes in the rest-frame optical (red to near-infrared; ∼6000–8000 Å) across cosmic time through redshift-dependent filter selection. We classified BGGs as star-forming and quiescent using both rest-frame NUV– r – J colors and redshift-dependent specific star formation rate (sSFR) thresholds. Our structural analysis reveals that quiescent BGGs are systematically more compact than their star-forming counterparts across all redshifts, exhibiting steeper size–mass slopes ( α _QG ∼ 0.6–1.2 vs. α _SF ∼ 0.0–0.3). The effective radius evolves as R _e ∝ (1+ z ) ^{− α} , with α = 0.96 ± 0.07 for star-forming BGGs and α = 1.24 ± 0.09 for quiescent BGGs, indicating stronger size growth in quenched systems. The corresponding growth factor at fixed stellar mass (log M _∗ = 10.7) from z = 3.7 to z = 0.08 is ∼ 4.4 for star-forming and ∼ 6.6 for quiescent BGGs. The intrinsic scatter in the size–mass relation increases toward higher redshift for both populations, reaching ∼ 0.3–0.4 dex at z > 2, reflecting greater structural diversity in the early universe. Compared to field galaxies, BGGs show systematically smaller sizes at fixed stellar mass, particularly among quiescent systems, highlighting environmental effects on galaxy structure. We further compare the evolution of the quiescent fraction, the Sérsic index, and ellipticity with those of field galaxies, finding consistent trends that reinforce our main conclusions. These results establish the foundation for understanding how group-scale environments shape the structural evolution of central galaxies and provide crucial constraints for models of galaxy formation in intermediate-mass dark matter halos.

Context. Galaxy groups with total masses below ∼10¹⁴ M_⊙ and up to a few tens of members are the most common galaxy environment, marking the transition between the field and the most massive galaxy clusters. In this framework, identifying and studying groups plays a crucial role in understanding structure formation and galaxy evolution. Despite the challenges in detecting such relatively small structures, modern deep surveys allow us to build well-characterized samples of galaxy groups up to the regime where the structures we observe today were taking shape. Aims. We aim to build the largest deep catalog of galaxy groups to date over the COSMOS-Web field effective area of 0.45 deg². Methods. We leveraged the deep imaging, high resolution, and high-quality photometry from the James Webb Space Telescope observations of the COSMOS-Web field. We used the recent COSMOS-Web photometric catalog with sky position, photometric redshift, and magnitude in a reference band for each selected galaxy. We performed the group search with the Adaptive Matched Identifier of Clustered Objects (AMICO) algorithm, a linear matched filter based on an analytical model for the cluster/group signal. This algorithm has already been tested in wide and deep field surveys, including a successful application to COSMOS data up to z = 2. In this work, we tested the algorithm’s performances at even higher redshift and searched for protocluster cores and groups at z > 2. To benchmark this relatively unexplored regime, we compiled a list of known protoclusters in COSMOS at 2 ≤ z ≤ 3.7 and matched them with our detections. We studied the spatial connection between detected cores through a clustering analysis. We estimated the purity and the completeness of our group sample by creating data-driven mocks via a Monte Carlo approach with the SinFoniA code and linked signal-to-noise to purity levels to define desired purity thresholds. Results. We detected 1678 groups in the COSMOS-Web field up to z = 3.7 with a purity level of ∼77%, providing a deep catalog of galaxy members that extends nearly two magnitudes deeper than the previous application of AMICO to COSMOS. Around 670 groups have been detected with a purity of 90%. Our catalog includes more than 850 groups whose photometric redshift was confirmed by assigning robust spectroscopic counterparts. Conclusions. This catalog of galaxy groups is the largest ultra-deep group sample built on JWST observations so far and offers a unique opportunity to explore several aspects of galaxy evolution in different environments spanning ∼12 Gyr and study groups themselves, from the least rich population of groups to the formation of the most massive clusters.

Exploiting IRAM 30 m CO spectroscopy, we find that SDSS post-merger galaxies display gas fractions and depletion times enhanced by 25-50%, a mildly increased CO excitation, and standard molecular-to-atomic gas ratios, compared to non-interacting galaxies with similar redshift, stellar mass ( M ⋆ ) and star-formation rate (SFR). To place these results in context, we compile further samples of interacting or starbursting galaxies, from pre-coalescence kinematic pairs to post-starbursts, carefully homogenising gas mass, M ⋆ and SFR measurements in the process. We explore systematics by duplicating our analysis for different SFR and M ⋆ estimators, finding good qualitative agreement in general. Molecular gas fractions and depletion times are enhanced in interacting pairs, albeit by less than for post-mergers. Among all samples studied, gas fraction and depletion time enhancements appear largest in young (a few 100 Myr) post-starbursts. While there is only partial overlap between post-mergers and post-starbursts, this suggests that molecular gas reservoirs are boosted throughout most stages of galaxy interactions, plausibly due to torque-driven inflows of halo gas and gas compression. The gas fraction and depletion time offsets of mergers and post-starbursts anti-correlate with their distance from the galaxy main sequence , evidencing the role of star-formation efficiency (SFE) in driving the high SFRs of the strongest starbursts. Post-starbursts display the steepest dependency of gas fraction and SFE-offsets on , with an evolving normalisation that reflects gas reservoir depletion over time. Our multi-sample analysis paints a coherent picture of the starburst-merger connection throughout the low- z merger sequence. It reconciles contradictory literature findings by highlighting that gas fraction enhancements and SFE variations both play their part in merger-driven star formation.

The unique characteristics of the brightest group galaxies (BGGs) serve as a link in the evolutionary continuum between galaxies such as the Milky Way and the more massive brightest cluster galaxies found in dense clusters. This research investigates the evolution of the stellar properties of BGGs over cosmic time (z = 0.08 − 1.30), extending the work from our prior studies. We analyzed the data of 246 BGGs selected from our X-ray galaxy group catalog within the COSMOS field, examining stellar age, mass, star-formation rate (SFR), specific SFR, and halo mass. We compared observations with the Millennium and Magneticum simulations. Additionally, we investigated whether stellar properties vary with the projected offset from the X-ray peak or the hosting halo center. We evaluated the accuracy of SED-derived stellar ages using a mock galaxy catalog, finding a mean absolute error of around 1 Gyr. Interestingly, the observed BGG age distributions exhibit a bias toward younger intermediate ages compared to both semi-analytical models and the Magneticum simulation. Our analysis of stellar age versus mass unveils intriguing trends with a positive slope, hinting at complex evolutionary pathways across redshifts. We observed a negative correlation between stellar age and SFR across all redshift ranges. We employed a cosmic time dependent main sequence framework to identify star forming BGGs and find that approximately 20% of BGGs in the local universe continue to exhibit characteristics typical of star forming galaxies, with this proportion increasing to 50% at z = 1.0. Our findings support an inside-out formation scenario for BGGs, where older stellar populations reside near the X-ray peak and younger populations at larger offsets indicate ongoing star-formation. The observed distribution of stellar ages, particularly for lower-mass BGGs in the range of 10¹⁰⁻¹¹ M_⊙, deviates from the constant ages predicted by the models across all stellar mass ranges and redshifts. This discrepancy aligns with the current models’ known limitations in accurately capturing galaxies’ complex star-formation histories.

Context. Sources that leak Lyman continuum (LyC) photons and lead to the reionisation of the universe are an object of intense study using multiple observing facilities. Recently, the Low-redshift LyC Survey (LzLCS) has presented the first large sample of LyC emitting galaxies at low redshift (z ∼ 0.3) with the Hubble Space Telescope Cosmic Origins Spectrograph. The LzLCS sample contains a robust estimate of the LyC escape fraction (f_esc^LyC) for 66 galaxies, spanning a wide range of f_esc^LyC values. Aims. Here, we aim to study the dependence of f_esc^LyC on the radio continuum (RC) properties of LzLCS sources. Overall, RC emission can provide unique insights into the role of supernova feedback, cosmic rays (CRs), and magnetic fields from its non-thermal emission component. RC emission is also a dust-free tracer of the star formation rate (SFR) in galaxies. Methods. In this study, we present Karl G. Jansky Very Large Array (VLA) RC observations of the LzLCS sources at gigahertz (GHz) frequencies. We performed VLA C (4−8 GHz) and S (2−4 GHz) band observations for a sample of 53 LzLCS sources. We also observed a sub-sample of 17 LzLCS sources in the L (1−2 GHz) band. We detected RC from both C- and S-bands in 24 sources for which we are able to estimate their radio spectral index across 3−6 GHz, denoted as α_6 GHz^3 GHz. We also used the RC luminosity to estimate their SFRs. Results. The radio spectral index of LzLCS sources spans a wide range, from flat (≥ − 0.1) to very steep (≤ − 1.0). They have a steeper mean α_6 GHz^3 GHz (≈ − 0.92) compared to that expected for normal star-forming galaxies (α_6 GHz^3 GHz ≈ −0.64). They also show a larger scatter in α_6 GHz^3 GHz (∼0.71) compared to that of normal star-forming galaxies (∼0.15). The strongest leakers in our sample show flat α_6 GHz^3 GHz, weak leakers have α_6 GHz^3 GHz close to normal star-forming galaxies and non-leakers are characterized by steep α_6 GHz^3 GHz. We argue that a combination of young ages, free-free absorption, and a flat cosmic-ray energy spectrum can altogether lead to a flat α_6 GHz^3 GHz for strong leakers. Non-leakers are characterized by steep spectra which can arise due to break or cutoff at high frequencies. Such a cutoff in the spectrum can arise in a single injection model of CRs characteristic of galaxies which have recently stopped star-formation. The dependence of f_esc^LyC on α_6 GHz^3 GHz (which is orientation-independent) suggests that the escape of LyC photons is not highly direction-dependent at least to the first order. The radio-based SFRs (SFR^RC) of LzLCS sources show a large offset (∼0.59 dex) from the standard SFR^RC calibration. We find that adding α_6 GHz^3 GHz as a second parameter helps us to calibrate the SFR^RC with SFR_UV and SFR_Hβ within a scatter of ∼0.21 dex. Conclusions. For the first time, we have found a relation between α_6 GHz^3 GHz and f_esc^LyC. This hints at the interesting role of supernovae feedback, CRs, and magnetic fields in facilitating the escape (alternatively, and/or the lack) of LyC photons.

Context. Radio-excess active galactic nuclei (radio-AGNs) are essential to our understanding of both the physics of black hole (BH) accretion and the interaction between BHs and host galaxies. Recent deep and wide radio continuum surveys have made it possible to study radio-AGNs down to lower luminosities and up to higher redshifts than previous studies, and are providing new insights into the abundance and physical origin of radio-AGNs. Aims. Here we focus on the cosmic evolution, physical properties, and AGN-host galaxy connections of radio-AGNs selected from a total sample of ∼400 000 galaxies at 0 < z < 4 in the GOODS-N and COSMOS fields. Methods. Combining the deep radio continuum data with multi-band, de-blended far-infrared, and submillimeter data, we were able to identify 983 radio-AGNs out of the entire galaxy sample through radio excess relative to the far-infrared–radio relation. Results. We studied the cosmic evolution of 1.4 GHz radio luminosity functions (RLFs) for both star-forming galaxies (SFGs) and radio-AGNs, which can be well described by a pure luminosity evolution of L_⋆ ∝ (1 + z)^{−0.34 × z + 3.57} and a pure density evolution of Φ_⋆ ∝ (1 + z)^{−0.77 × z + 2.69}, respectively. We derived the turnover luminosity, above which the number density of radio-AGNs surpasses that of SFGs. We show that this crossover luminosity increases with increasing redshifts, from 10^22.9 W Hz⁻¹ at z ∼ 0 to 10^25.2 W Hz⁻¹ at z ∼ 4. At the full redshift range of 0 < z < 4, we further derive the probability (p_radio) of SFGs and quiescent galaxies (QGs) hosting a radio-AGN, as a function of stellar mass (M_⋆), radio luminosity (L_R), and redshift (z), which yields p_radio ∝ (1+z)^3.08 M_⋆^1.06 L_R^−0.77 for SFGs, and p_radio ∝ (1+z)^2.47 M_⋆^1.41 L_R^−0.60 for QGs, respectively. Conclusions. The quantitative relation for the probabilities of galaxies hosting a radio-AGN indicates that radio-AGNs in QGs prefer to reside in more massive galaxies with higher L_R than those in SFGs. The fraction of radio-AGN increases toward higher redshift in both SFGs and QGs, with a more rapid increase in SFGs.

Context. The infrared-radio correlation (IRRC) of star-forming galaxies can be used to estimate their star formation rate (SFR) based on the radio continuum luminosity at MHz–GHz frequencies. For its practical application in future deep radio surveys, it is crucial to know whether the IRRC persists at high redshift z. Aims. Previous works have reported that the 1.4 GHz IRRC correlation of star-forming galaxies is nearly z-invariant up to z ≈ 4, but depends strongly on the stellar mass M_⋆. This should be taken into account for SFR calibrations based on radio luminosity. Methods. To understand the physical cause behind the M_⋆ dependence of the IRRC and its properties at higher z, we constructed a phenomenological model for galactic radio emission. Our model is based on a dynamo-generated magnetic field and a steady-state cosmic ray population. It includes a number of free parameters that determine the galaxy properties. To reduce the overall number of model parameters, we also employed observed scaling relations. Results. We find that the resulting spread of the infrared-to-radio luminosity ratio, q(z, M_⋆), with respect to M_⋆ is mostly determined by the scaling of the galactic radius with M_⋆, while the absolute value of the q(z, M_⋆) curves decreases with more efficient conversion of supernova energy to magnetic fields and cosmic rays. Additionally, decreasing the slope of the cosmic ray injection spectrum, α_CR, results in higher radio luminosity, decreasing the absolute values of the q(z, M_⋆) curves. Within the uncertainty range of our model, the observed dependence of the IRRC on M_⋆ and z can be reproduced when the efficiency of supernova-driven turbulence is 5%, 10% of the kinetic energy is converted into magnetic energy, and α_CR ≈ 3.0. Conclusions. For galaxies with intermediate to high (M_⋆ ≈ 10^9.5 − 10¹¹ M_⊙) stellar masses, our model results in an IRRC that is nearly independent of z. For galaxies with lower masses (M_⋆ ≈ 10^8.5 M_⊙), we find that the IR-to-radio flux ratio increases with increasing redshift. This matches the observational data in that mass bin which, however, only extends to z ≈ 1.5. The increase in the IR-to-radio flux ratio for low-mass galaxies at z ≳ 1.5 that is predicted by our model could be tested with future deep radio observations.