Laura Bisigello

The hotspots of powerful radio galaxies are the sites of high-energy particle acceleration, but radio observations give us limited information on the electron energy distributions that they generate, which affects our overall understanding of their physics and energetics. Optical synchrotron counterparts to radio galaxy hotspots, which probe high electron energies, have been known for many years, but have only been studied in a small number of objects selected as being very bright in the radio. By combining Low-Frequency Array (LOFAR) observations with the Euclid Q1 data release in the Euclid Deep Field North, we identify 77 powerful radio galaxies with hotspots and show that a small but significant fraction of them, 7–15%, may have detected infrared and optical counterparts to their hotspots. The hotspot spectral energy distributions in our clearest cases are consistent with particle acceleration up to (but not beyond) 1 TeV in energy on the assumption of field strengths close to equipartition, whereas non-detected hotspots either do not reach such high electron energies or would have to have very different electron energy spectra or significantly sub-equipartition magnetic field strengths. Two new candidate optical hotspots are clearly detected matching high-resolution LOFAR data; one is remarkable as both the highest-redshift optical hotspot known to date ( z ≈1.6) and also the first optical detection of the hotspot of the inner lobe of a restarting source. We discuss approaches to performing a large-scale study of hotspots with the full wide-area Euclid surveys, particularly when combined with the international baselines of LOFAR, enabling for the first time a statistical analysis of high-energy particle acceleration in hotspots.

We present the discovery of two disky titans in the first data release of the Euclid satellite. These sources are massive ( M _* > 10 ¹¹ M _⊙ ) star-forming (SFR ∼ 20 M _⊙ yr ⁻¹ ) disks located in strong over-dense environments at intermediate redshifts ( z ∼ 0.75). They represent a small fraction of the massive galaxies in over-dense regions (just four candidates in over 20 deg ² analysed in this study). Their existence is puzzling, given the abundance of passive, bulge-dominated sources commonly found at the centre of groups and clusters at low redshifts. First, our analysis shows that these objects are located in massive groups ( M _h ∼ 10 ^13.8 M _⊙ ), where rapid accretion of cold gas should be prevented by the formation of a static hot halo. Despite this, a millimetre follow-up with NOEMA shows significant cold gas reservoirs ( M H ₂ ∼ 10 ^10.3 M _⊙ ) within these sources. Second, our morphological analysis indicates a massive, passive bulge in these galaxies. This bulge is expected to stabilise the disk against fragmentation, thereby suppressing further star formation. However, these sources lie on the Schmidt–Kennicutt relation or even slightly above. Building on these observations, we propose a scenario where these disky titans are the product of a merger-induced rejuvenation episode, in which the most massive galaxy in a group accretes cold gas from another member and briefly restarts star formation. Such a scenario is supported by a comparison with the TNG300 simulation and easily explains the persistence of star formation activity in massive galaxies in over-dense environments as temporary stages of more complex evolution. More generally, our study showcases the ability of Euclid to find rare objects, thanks to the unprecedented statistic offered by its surveys and the scientific potential residing in the synergy between Euclid and other facilities observing at longer wavelengths.

We present and investigate the properties of a sample of radio-selected, Euclid -dark galaxies, identified from LOFAR HBA observations at 144 MHz within the Euclid Deep Field-North (EDF-N), covering approximately 10 deg ² . Starting from 1051 radio sources lacking optical counterparts down to a 5σ depth of 23.9 in the y band in previous surveys (but detected with Spitzer /IRAC), we identified 166 galaxies with no emission at a depth of more than 3σ from Euclid Quick Release 1 (Q1) images, reaching a 5σ depth of 23.9 in the H _E band, and no matches in the Euclid Q1 catalogue. To minimise contamination from nearby sources, we selected a sub-sample of 88 isolated galaxies. By exploiting multi-band images and catalogues available for the EDF-N, we inferred the physical properties of our sample via SED fitting. The resulting redshift distribution spans 0.4 ≤ z _ph ≤ 5.0. We made use of recent sub-arcsecond imaging from the International LOFAR Telescope over an area of 2.5 deg × 2.5 deg to constrain the nature of the compact radio emission in our targets through brightness-temperature estimates. By combining this information with the radio excess relative to the infrared/radio-correlation (IRRC), we searched for possible active galactic nucleus (AGN) activity in our sample. Approximately 30% of our sources show a radio excess relative to star formation. This fraction increases to 40% when including AGNs identified via their brightness temperature. The Euclid -dark sources detected in the far-infrared (FIR) are consistent with a population of heavily obscured (⟨ A _V ^ISM ⟩ = 5.0) massive (⟨log ₁₀ ( M _⋆ / M _⊙ )⟩ = 11.6) star forming galaxies (SFGs) with high star formation rates (⟨log ₁₀ (SFR/ M _⊙ yr ⁻¹ )⟩ = 3.3). Their location above the star-forming main sequence is consistent with the high-mass and high-SFR end of similar near-infrared (NIR)-dark galaxy populations selected at higher radio frequencies reported in the literature. We performed a UV-to-radio median stacking analysis, dividing the sources with LOFAR ILT imaging according to the presence of radio excess. Both sub-samples are detected in the H _E band at 3σ and 9σ for the radio-excess and non–radio-excess sources, respectively. The two stacked populations exhibit similar global physical properties and differ primarily in their radio emission, with values consistent with those observed for individual sources, albeit with less extreme SFRs. These preliminary results indicate that, compared to similar selections carried out over smaller fields, the wide area covered by Euclid enables the identification of a higher fraction of systems in which intense star formation and AGN activity coexist, likely capturing a key phase of galaxy–black hole co-evolution.

The evolution of galaxies in clusters is driven by their interaction with the environment. This deeply affects and alters the properties of the multiphase interstellar medium. We used MeerKAT observations to study the properties of the neutral and nonthermal interstellar medium, traced by the HI 21 cm line and by the radio continuum emission at 1.4 GHz, respectively, out to about twice the virial radius of three local (z ≃ 0.04) galaxy clusters, namely IIZW108, A4059, and A3558, with M ₂₀₀ ≃ 2–9 × 10 ¹⁴ M _⊙ . We assembled a sample of 61 galaxies, including the so-called jellyfish and unwinding galaxies, detected in HI and radio continuum, and derived their optical properties from IR-to-UV photometry using ancillary data. The cluster galaxies in our sample are ~2.7 times more star-forming on average than galaxies with the same stellar and HI mass in the field, indicating that the HI + radio-continuum selection has intercepted galaxies at the very early stages of their environmental processing when the external pressure has not yet removed the gas, but the resulting fast compression has enhanced the star formation and the radio continuum luminosity. Our study also reveals that galaxies in the initial stage can feature a radio luminosity excess because the old relativistic electrons permeate the interstellar medium. We show that unwinding galaxies are characterized by high stellar and HI masses, and we argue that their peculiar morphology might result from their high baryonic mass. Finally, via the stacking analysis of 677 optically selected cluster members, we quantitatively show that cluster galaxies are poorer in HI than in the field and that the deficiency steadily grows toward the cluster center, where the HI mass of galaxies is lower by a factor of about 3 than the HI mass of galaxies in the field.

Dusty star-forming galaxies (DSFGs) dominate the far-infrared (FIR) and sub-millimetre (sub-mm) number counts, but single-dish surveys at these wavelengths suffer from poor angular resolution, making identifications of multi-wavelength counterparts difficult. Prior driven deblending techniques require extensive fine-tuning and struggle to process large fields. This work aims to develop a fast and reliable deep-learning-based deconvolution and denoising super-resolution (SR) technique. We employed a transformer neural network to improve the resolution of the /SPIRE 500 μm observations by a factor of 4.5, with input comprised of /MIPS 24μm and /SPIRE 250, 350, 500μm images. The network was trained on simulations from SIDES and SHARK. To mimic realistic observations, we injected instrumental noise into the input simulated images, while keeping the target images noise-free to enhance the de-noising capabilities of our method. We evaluated the performance of our method on simulated test sets and real JCMT/SCUBA-2 450 μm observations in the COSMOS field that have a superior resolution compared to . Herschel Spitzer Herschel Herschel Our SR method achieves an inference time of sim1s/deg^2 on consumer-grade GPUs, which is much faster than traditional deblending techniques. Using the simulation test sets, we show that fluxes of the extracted sources from the super-resolved image are accurate to within 5% for sources with an intrinsic flux ≳ 8 mJy, which is a substantial improvement compared to blind extraction on the native images. Astrometric error is low, at łesssim 1arcsec compared to the 12arcsec pixel scale. In terms of reliability and completeness, ≳ 90% of the extracted sources brighter than sim3 mJy are reliable and more than 90% of the input sources with intrinsic fluxes gtrsim5 mJy are recovered. When applied to the real 500 μm observations, the fluxes of the extracted sources from the super-resolved map agree well with the SCUBA-2 measured fluxes (after converting the 450 μm fluxes to 500 μm using a correction factor of 0.84) for sources above sim10 mJy. Our 500 μm number counts are also consistent with previous SCUBA-2 measurements. Thanks to its speed, our technique enables SR over hundreds of deg^2 without the need for fine-tuning, facilitating statistical analysis of DSFGs.

We present a detailed visual morphology catalogue for the Euclid Quick Release 1 (Q1). Our catalogue includes galaxy features such as bars, spiral arms, and ongoing mergers, for the 378 000 bright ( I _E < 20.5) or extended (area ≥700 pixels) galaxies in Q1. The catalogue was created by finetuning the Zoobot galaxy foundation models on annotations from an intensive one-month campaign by Galaxy Zoo volunteers. Our measurements are fully automated, and hence fully scaleable. This catalogue is the first 0.4% of the approximately 100 million galaxies where Euclid will ultimately resolve detailed morphology.

The first Euclid Quick Data Release (Q1) contains millions of galaxies with excellent optical and near-infrared (IR) coverage. To complement this dataset, we investigate the average far-IR properties of Euclid -selected main sequence (MS) galaxies using existing Herschel and SCUBA-2 data. We used 17.6 deg ² (2.4 deg ² ) of overlapping Herschel (SCUBA-2) data, containing 2.6 million (240 000) MS galaxies. We binned the Euclid catalogue by stellar mass and photometric redshift and performed a stacking analysis following SimStack , which accounts for galaxy clustering and bin-to-bin correlations. We detected stacked far-IR flux densities across a significant fraction of the bins. We fitted modified blackbody spectral energy distributions in each bin and derived mean dust temperatures ( T _d ), dust masses ( M _d ), and star-formation rates (SFRs). We find similar mean SFRs compared to the Euclid catalogue, and we show that the average dust-to-stellar mass ratios decreased from z ≃ 1 to the present day. Average dust temperatures are largely independent of stellar mass and are well-described by the function T ₂ + ( T ₁ − T ₂ ) e ^{− t / τ} , where t is the age of the Universe, T ₁ = (79.7 ± 7.4) K, T ₂ = (23.2 ± 0.1) K, and τ = (1.6 ± 0.1) Gyr. We argue that since the dust temperatures converge to a non-zero value below z = 1, the dust is now primarily heated by the existing cooler and older stellar population, as opposed to hot young stars in star-forming regions at higher redshifts. We show that since the dust temperatures are independent of stellar mass, the correlation between dust temperature and SFR depends on stellar mass. Lastly, we estimate the contribution of the Euclid catalogue to the cosmic IR background (CIB), finding that it accounts for > 60% of the CIB at 250, 350, and 500 μm. As the Euclid mission progresses, larger catalogues will allow us to probe the far-IR properties of MS galaxies out to higher redshifts and lower stellar masses, potentially recovering the complete CIB.

The well-known bimodality between star-forming discs and quiescent spheroids requires the existence of two main processes: galaxy quenching, causing the strong reduction of star formation, and morphological transformation, causing the transition from disc-dominated structures to bulge-dominated ones. In this paper, we aim to understand the link between these two processes and their relation with the stellar mass of galaxies and their local environment. Taking advantage of the first data released by the Euclid Collaboration, covering more than 60 deg ² with space-based imaging and photometry, we analyse a mass-complete sample of nearly one million galaxies in the range 0.25 < z < 1 with M _* > 10 ^9.5 M _⊙ , using a combination of photometric and spectroscopic redshifts. We divide the sample into four sub-populations of galaxies, based on their star-formation activity (star-forming and quiescent) and morphology (disc-dominated and bulge-dominated). We then analyse the physical properties of these populations and their relative abundances in the stellar mass versus local density plane. Together with confirming the passivity-density relation and the morphology-density relation, we find that quiescent discy galaxies are more abundant in the low-mass regime of high-density environment where log ₁₀ (1 + δ ) > 1.3. At the same time, star-forming bulge-dominated galaxies are more common in field regions with log ₁₀ (1 + δ ) < 0.8, preferentially at high masses. Building on these results and interpreting them through comparison with simulations, we propose a scenario where the evolution of galaxies in the field significantly differs from that in higher-density environments. The morphological transformation in the majority of field galaxies takes place before the onset of quenching and is mainly driven by secular processes taking place within the main sequence, leading to the formation of star-forming bulge-dominated galaxies as intermediate-stage galaxies. Conversely, quenching of star formation precedes morphological transformation for most galaxies in higher-density environments. This causes the formation of quiescent disc-dominated galaxies before their transition into bulge-dominated ones.

Active galactic nuclei (AGN) are an important phase in galaxy evolution. However, they can be difficult to identify due to their varied observational signatures. Furthermore, to understand the impact of an AGN on its host galaxy, it is important to quantify the strength of the AGN with respect to the host galaxy. We developed a deep learning (DL) model to identify AGN in imaging data by deriving the contribution of the central point source. The model was trained with Euclidised mock galaxy images in which we artificially injected different levels of AGN, in the form of varying contributions of the point spread function (PSF). Our DL-based method can precisely and accurately recover the injected AGN contribution fraction, f _PSF , with a mean difference between the predicted and true f _PSF of −0.0078 and an overall root mean square error of 0.051. With this new method, we move beyond the simplistic AGN versus non-AGN classification and are able to precisely quantify the AGN contribution and study galaxy evolution across a continuous spectrum of AGN activity. We applied our method to a stellar-mass-limited sample (with M _* ≥ 10 ^9.8 M _⊙ and 0.5 ≤ z ≤ 2.0 ) selected from the first Euclid quick data release (Q1) and, using a threshold of f _PSF > 0.2, we identified 61 432 ± 70 AGN over 63.1 deg ² (9.8 ± 0.1% of our sample). We compared these DL-selected AGN with AGN selected in the X-ray, mid-infrared (MIR), and via optical spectroscopy and investigated their overlapping fractions depending on different thresholds on the PSF contribution. We find that the overlap increases with increasing X-ray or bolometric AGN luminosity. We observe a positive correlation between the luminosity in the I _E filter of the AGN and the host galaxy stellar mass, suggesting that supermassive black holes (SMBHs) generally grow faster (in absolute terms, i.e. the luminosity of the PSF component is larger) in more massive galaxies. Moreover, the mean relative contribution of the AGN is higher in the quiescent galaxy population than in the star-forming population. In terms of absolute power, starburst galaxies, as well as the most massive galaxies (across the star-formation main sequence), tend to host the most luminous AGN, indicating concomitant assembly of the SMBH and the host galaxy.