arXiv subjects

Roberta Tripodi

Publications and source records attributed to Roberta Tripodi.

At least 19 recordsLinked to original sources

arXiv2026

A New Window on the H{\alpha} Luminosity Function and Star Formation Rate Density from 1.2 < z < 6.6 from JWST Medium-Band Photometry

We present the first self-consistent measurement of the H{\alpha} luminosity function over a wide redshift range, covering cosmic noon into the epoch of reionization. Our analysis utilizes a novel method based on James Webb Space Telescope (JWST) NIRCam medium-band imaging. We combine data from the CANUCS, JWST in Technicolor, and JUMPS surveys which offer deep, uniform imaging (29.5-30 AB, 3{\sigma}) with extensive NIRCam medium-band coverage, reaching up to 29 total filters (up to 20 JWST) when including ancillary Hubble Space Telescope (HST) ACS and WFC3/UVIS data. The superb spectral energy distribution (SED) sampling enables precise, reliable photometric redshift estimation (outlier fraction 1.7\%, {\sigma} N_MAD = 0.039 for this sample) as well as accurate continuum subtraction and line flux measurement verified by spectroscopic follow-up (no systematic offset, 0.23 dex scatter). We measure the H{\alpha} luminosity function (LF) from 1.25 < z < 6.6 by tracing the H{\alpha} emission line in 11 medium-band filters. The combination of depth, redshift coverage, and statistical power is unique, providing strong constraints on the shape of the LF over almost three orders of magnitude in luminosity. Our dense SED sampling enables us to reliably correct for dust attenuation and derive dust-corrected star formation rate functions as well as the evolution of the cosmic star formation rate density over the full redshift range. We recover the peak at z ~ 2 and a decrease toward z = 6, though with a higher normalization more in line with recent IR measurements than UV, though eclipsing both. This potential tension will be addressed in future work utilizing larger surveys with MIR coverage to better constrain the bright end of the luminosity function and the effects of dust.

astro-ph.GA↗

arXiv2026

Little Red and Blue Dots: AGN-excited narrow lines, Lyman-$\alpha$ emission, and resemblance to standard quasars

We present an analysis of a sample of 36 Little Red and Blue Dots (LRDs and LBDs) at $2.26<z<7.89$, identified by JWST in the GOODS fields. While both categories are selected to have broad Balmer lines, both of them are extremely X-ray weak. Both classes share the same location on various diagnostic diagrams, consistent with AGN excitation (with some deviations which can be ascribed to low metallicity), although their weak HeII emission suggests a generally softer ionizing spectrum than ordinary AGN. LRDs display Ly$\alpha$ emission stronger than normal star-forming galaxies, and with a broad component consistent with the broad component of H$\alpha$. Overall, these findings indicate that LRDs and LBDs are both powered by growing black holes and their ionizing radiation escapes to ionize the surrounding interstellar medium (ISM). The broad Balmer lines ($H\alpha_b$ and $H\beta_b$) have different apparent properties: LBDs have EW(H$\alpha _b$) and $H\alpha_b/H\beta_b$ broadly consistent with normal AGN, while LRDs have higher values of both quantities, although still in the tail of the quasars distribution. LRD models in which a gas envelope completely encases the black hole, are inconsistent with these results -- these scenarios need modification to include clumpiness, or a (classical) equatorial geometry, letting ionizing photons reach the ISM. The different broad Balmer properties imply that LBDs cannot simply be LRDs with more galaxy contribution. Scenarios in which LRDs are simply dust-obscured LBDs seem broadly consistent with the observations. Finally, these results indicate that LRDs' bolometric luminosities estimated assuming isotropic emission and complete covering by the absorber are inadequate. The few X-ray-detected LRDs suggest no deviation from the standard AGN bolometric corrections, once absorption is accounted for.

astro-ph.GA↗

arXiv2026

HYPERION. The cold ISM of rapidly growing $z>6$ quasars: diverse gas reservoirs, dust enrichment, and feedback signatures

Luminous QSOs at $z>6$ host some of the most rapidly assembled SMBHs in the early Universe. Characterizing their cold ISM is essential to determine their evolution. We investigate the molecular gas, cold dust, star formation, gas-to-dust ratio, and ionized ISM of ten HYPERION QSOs using new ALMA Band 3 observations targeting CO(6-5) and the underlying $\sim100$ GHz continuum, complemented by archival and literature ALMA/NOEMA data. We detect $\sim100$ GHz continuum emission in eight targets and CO(6-5) emission in four QSO hosts, J025-33, J083+11, J231-20, and J0252-0503, as well as in the companion of J231-20. The inferred molecular gas masses are of order $10^{10}~M_\odot$, while the non-detections imply upper limits of a few $10^9~M_\odot$, indicating a broad range of molecular reservoirs within the HYPERION population. For J025-33 and J083+11, the FIR SEDs are well sampled and yield low dust temperatures, $T_{\rm dust}=36^{+13}_{-7}$ K and $32^{+4}_{-3}$ K, respectively, well below the average value for $z>6$ QSOs. Combining gas and dust masses, we find a gas-to-dust ratio for J083+11, ${\rm GDR}=16^{+5}_{-4}$, among the lowest measured in a high-redshift QSO host. We also detect [NII]$\lambda 205\,\mu$m emission in J025-33 and tentatively in J083+11, suggesting dense or highly structured ionized gas. Finally, we identify a tentative connection among $T_{\rm dust}$, the X-ray photon index $\Gamma$, and the C IV velocity shift. These trends may indicate that more powerful winds redistribute dust away from the central AGN heating source, lowering its temperature and weakening the connection between the large-scale dust reservoir and the X-ray corona. Overall, HYPERION QSOs emerge as a heterogeneous population in which SMBH growth, star formation, gas consumption, enrichment, and feedback are not necessarily synchronized.

astro-ph.GA↗

arXiv2026

A Population of Red Galaxies with Very Strong Emission Lines at $z > 5$ Revealed by the NIRCam Medium Bands: ''Classic'' LRDs, Dusty Star-Forming Galaxies, and a Missing Population of LRDs

The NIRCam medium-bands have proven to be efficient at identifying Emission Line Galaxies (ELGs) with high equivalent width (EW) H$\alpha$ and [OIII]+H$\beta$ emission lines. In this paper we exploit this efficiency to identify a sample of ELGs at $4.9 \lesssim z \lesssim 8.9$ using medium-band imaging from the CANUCS, Technicolor, and JUMPS surveys. We find that the ELGs exhibit a strong correlation between continuum color and emission line strength, such that galaxies with bluer UV/optical continua have stronger H$\alpha$ and [OIII]+H$\beta$ emission lines. We identify 26 galaxies that are outliers from this relation, which we call the Red Emission line Galaxies (REGs), because of their red continuum color and strong emission lines. We classify the REGs into three categories: 1) ''classic'' Little Red Dots (LRDs) selected with common literature criteria, 2) extended REGs, resolved in F444W and consistent with being Dusty Star Forming Galaxies (DSFGs), and 3) compact REGs, unresolved in F444W but not classified as LRDs. The compact REGs fail common LRD selections for several reasons, including faint continuua, contamination from emission lines (very strong [OIII]+H$\beta$), and UV/optical colors that are flatter than those of LRDs. We conclude that the compact REGs are likely LRDs that ''classic'' selection criteria miss, and are therefore missing from existing samples. Our results suggest that medium-band selection can provide more complete samples of these objects.

astro-ph.GA↗

arXiv2026

GREX-PLUS Science Book v2

GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) is a mission candidate for a JAXA strategic L-class mission to be launched in the 2030s. Its primary science goals are two-fold: galaxy formation and evolution, and planetary system formation and evolution. The GREX-PLUS spacecraft will carry a telescope with a 1 m primary mirror aperture cooled down to 50 K. The two science instruments will be onboard: a wide-field camera in the 2--8 $\mu$m wavelength band and a high-resolution spectrometer with a wavelength resolution of 30,000 in the 10--18 $\mu$m band. The GREX-PLUS wide-field camera aims to detect the first generation of galaxies at redshift $z>15$. The GREX-PLUS high-resolution spectrometer aims to identify the location of the water ``snowline'' in protoplanetary disks. Both instruments will provide unique datasets for a broad range of scientific topics, including galaxy mass assembly, the origin of supermassive blackholes, infrared background radiation, molecular spectroscopy in the interstellar medium, transit spectroscopy of exoplanet atmospheres, planetary atmospheres in the Solar System, and so on. This document is the second version of a collection of scientific themes that can be achieved with GREX-PLUS. Each section in Chapters~2 and 3 is based on presentations at several GREX-PLUS Science Workshops.

astro-ph.IM↗

arXiv2026

Beyond the [OIII]$\lambda$4363 auroral line: [NeIII]$\lambda$3868 as a direct tracer of electron temperature

Auroral lines enable accurate measurements of chemical abundances in ionized gaseous nebulae thanks to their sensitivity to electron temperature. However, metal-enriched systems remain a challenge, as even deep observations cannot retrieve auroral lines due to their intrinsic faintness. To overcome this limitation, we present a novel approach to estimate electron temperatures in the conditions where the [OIII]$\lambda$4363 auroral line is barely detectable ($T_{e} < 11,000$ K). This approach relies on the detection of [NeIII]$\lambda$3868 and [OIII]$\lambda$4959,5007, which are among the brightest rest-frame optical emission lines. By means of detailed photoionization models, we derive a tight relation between the O3Ne3$\equiv$[OIII]$\lambda$4959,5007/[NeIII]$\lambda$3868 ratio and the electron temperature weighted in the O$^{++}$ dominated region. We test the validity of this relation in a large sample of galaxies that cover a wide range of redshifts z$\sim$0-9 and extragalactic HII regions. Our results show that the O3Ne3 ratio, in combination with the O3O2 ratio (tracer of ionization), yields electron temperature estimates consistent within the uncertainties with those based on [OIII]$\lambda$4363. The proposed relation can be used to estimate electron temperature in the cool (equivalently high-metal) regime [6,000, 13,500 K] where the emissivity of [OIII]$\lambda$4363 drops drastically.

astro-ph.GA↗

arXiv2026

Two Exciting High-redshift Galaxy Candidates Turn Out to Be Two Exciting Ultra-cool Brown Dwarfs

From the onset of observations of JWST we have discovered unexpectedly luminous galaxies at redshifts $z>10$ and as high as $z=14$. With their discovery, the question immediately followed as to where their progenitors are, since such progenitors should be within reach of existing surveys. However, the discovery of several bright candidates at $z>15$ may indicate further discrepancies between pre-JWST model predictions and current observations. Progenitors of the bright $z\sim 14$ galaxies should be visible at redshifts as high as $z\sim 20-30$, showing in the data as F277W and F356W dropouts. We identify two such candidates in the Bullet Cluster JWST data; however, subsequent NIRSpec follow-up data show spectra that can be well fit with Y dwarf templates with temperatures ${272\mbox{--}351\mbox{K}}$ and ${445\mbox{--}525\mbox{K}}$ (using ATMO2020 and Sonora Elf Owl models) and distances of $\sim 150\mbox{--}650\mbox{pc}$. The first is one of the lowest-temperature brown dwarfs known, and the lowest-temperature brown dwarf detected spectroscopically outside the solar neighborhood. With additional NIRCam imaging taken $\sim 1$ year later, we also detect their proper motions of $(49 \pm 8)\,\mbox{mas/yr}$ and $(24 \pm 3)\,\mbox{mas/yr}$, further indicating that at least some F277W and F356W dropouts are sub-stellar cold Milky Way objects such as brown dwarfs.

astro-ph.GA↗

arXiv2026

The Drivers of Cosmic Dust Temperature Evolution

Observations of the rest-frame far-infrared (far-IR) emission of galaxies suggest a mild increase of dust temperature $T_{\rm dust}$ with redshift, although constraining $T_{\rm dust}$ in high-redshift systems remains challenging due to limited sampling of the far-IR spectral energy distribution (SED). We present and discuss the redshift evolution of $T_{\rm dust}$ predicted by a cosmological galaxy evolution simulation with dust treatment, and interpret its dependence on other galaxy physical properties. We use a semi-analytic model of galaxy formation that includes an explicit treatment of dust, post-processed with radiative transfer. Dust temperatures are derived by applying modified blackbody SED fitting to the simulated galaxies, mirroring the methodology adopted in most observational studies. The dust temperature of simulated galaxies increases with redshift, in broad agreement with observational results. A feature-importance analysis reveals that the star formation rate surface density $\Sigma_{\rm SFR}$ and the dust-to-gas ratio (DTG) are the main drivers of dust temperature, tracing the intensity of the interstellar radiation field and the optical depth of warm molecular clouds, respectively. Galaxies with higher star formation rate surface density and lower DTGs -- common conditions at high$-z$ -- are associated with warmer dust. We provide a simple relation to estimate DTG from $\Sigma_{\rm SFR}$, $T_{\rm dust}$, and redshift. Variations in dust grain size and chemical composition have a negligible impact on $T_{\rm dust}$. Our results are particularly relevant to the study of dust properties with observations of high-z galaxies, where far-IR dust emission is not fully sampled.

astro-ph.GA↗

arXiv2026

Little Red Dots: One Photometric Tag Concealing Diverse Spectroscopic Flavors of Massive Star Formation and Black Hole Activity

We compile JWST/NIRSpec prism and MIRI data for 249 Little Red Dots (LRDs) at 2.3<z<9.3, forming a representative spectroscopic subset of NIRCam-selected LRDs. We derive a median stacked spectrum covering rest-frame 0.09-1.2 $\mu$m, with MIRI photometry extending the spectral energy distribution to 4 $\mu$m. Four additional stacks for subsamples defined by optical-to-UV luminosity ratios show that LRDs form a heterogeneous population spanning diverse continuum slopes and line properties. Assuming LRDs host super-massive black holes (BHs) surrounded by dense gas clouds, and stars accompany this core, we infer masses of $M_{BH}\sim10^{6.0-6.5}$ M$_\odot$ and $M_\bigstar\sim10^{8.3}$ M$_\odot$, corresponding to BH-to-stellar mass ratios of 1-2%. The stacks show ubiquitous UV and optical FeII emission, indicating a direct view of the broad-line region and high (but sub-Eddington) accretion ($\lambda_{Edd}=0.6\pm0.2$). We find a significant stellar contribution in the far-UV, reaching $\sim80$% in the bluest systems. Possible Wolf-Rayet features (HeII$\lambda$4687, nitrogen lines) are identified, tracing a young (3-7 Myr) compact starburst event. We also detect strong Balmer breaks and atypical Balmer, Paschen, [OIII], and optical and near-infrared HeI line ratios, and an absorption at $\sim4550$ Angstrom (probably linked to FeII), all consistent with radiative-transfer effects in high-density gas with warm temperatures (4000-7000 K). We find a diversity of LRD flavors modulated by the luminosity ratio between between a short ($\lesssim20$ Myr) and intense phase of BH activity, the most extreme stage lasting $\sim3-7$ Myr, characterized by near-Eddington-limit radiation, and a nuclear and compact starburst dominated by massive stars (even super-massive, $\mathrm{M}_\mathrm{SMS}\sim10^{5}$ M$_\odot$), all embedded in dense gas with modest dust content producing a variety of optical depths.

astro-ph.GA↗

arXiv2026

UV slopes of Starforming Galaxies in Strong Lensing fields at the Epoch of Reionization with JWST

UV slopes ($\beta$) are a powerful diagnostics for galaxies at the Epoch of Reionization, tracing star formation, ISM ionization, and the escape fraction $f_{esc}$ of ionizing photons. Studies at low and intermediate z find a gradual $\beta$ reddening with time and steeper slopes for fainter galaxies, however recent JWST studies reveal a flattening of this trend at $z>7$. We want measure $\beta$ for galaxies at $z>7.5$ using the strong lensing around massive galaxy clusters to observe high-redshift and faint galaxies. The low-brightness regime is of particular interest for reionization, as most of the recent models of this process posit that numerous faint galaxies are the prime drivers of reionization. We use NIRCam and NIRSpec data from CANUCS, Technicolor, JUMPS, Silver Bullet, UNCOVER and MEGASCIENCE across 7 strong lensing fields. We find galaxies down to $M_{UV}\sim-16$ and 7.5 7.5$ suggests similar properties between $300$ and $600 Myr$ after the Big Bang. The weak trend between $\beta$ and $M_{UV}$ suggests an analogous composition for low- and high-mass galaxies' ISM, likely due to a lack of time for dust buildup. While average $f_{esc}$ is higher than necessary to ionize the IGM by z~6, the model extrapolated at low-z may overestimate its value.

astro-ph.GA↗

arXiv2026

Mapping dark matter in the Bullet Cluster using JWST imaging and spectroscopy

We present an updated gravitational lens model of the Bullet cluster (1E 0657-56) by combining JWST NIRCam imaging and NIRSpec spectroscopy. Although previous lens models relied on many multiply imaged galaxies, only six systems had spectroscopic redshifts prior to this work. Our lens model is constrained by a catalogue of 135 secure multiple images from 27 background galaxies with spectroscopic redshifts, uniformly covering both subclusters and a wide redshift range of 0.9 - 6.7. We also provide a catalogue of 199 multiple image candidates. We modelled the cluster with Lenstool and incorporated several large-scale haloes, cluster members, the intracluster gas, and group-scale haloes surrounding the cluster core, motivated by spectroscopic studies of cluster member kinematics. We describe the main cluster component with a complex, elongated double-peaked distribution, and the subcluster with a single large-scale halo aligning closely with the brightest cluster galaxy ($4_{-2}^{+3}$ kpc). The uncertainty of the displacement has been improved threefold thanks to the addition of JWST systems. The addition of group-scale substructures, roughly following the two axes of cluster assembly, improves the fit to the multiple image positions and provides a physically motivated alternative to constant shear. Our lens model shows the closest agreement with previous studies in aperture mass profiles at $\sim60$ kpc from the brightest cluster galaxies (BCGs), but exhibits significant differences in the detailed mass distribution as a result of different lens-modelling strategies and adopted constraints. The differences are reflected in small but spatially coherent deviations between the new spectroscopic redshifts and redshifts predicted by earlier lens models.

astro-ph.CO↗

arXiv2025

From "The Cliff" to "Virgil": Mapping the Spectral Diversity of Little Red Dots with JWST/NIRSpec

One of JWST's most unexpected discoveries is the emergence of "Little Red Dots'' (LRDs): compact sources at $z \gtrsim 3$ with blue rest-frame UV continua, red optical slopes, and broad Balmer emission lines that challenge standard models and suggest a population of early, unusual active galactic nuclei (AGNs). Using a comprehensive photometric selection and public NIRSpec/PRISM spectroscopy across six JWST deep fields, we identify a large sample of 118 LRDs with high-S/N spectra, enabling a population-wide analysis of their UV-optical continuum and emission lines. We find clear correlations between rest-frame color ([0.3-0.9\,$\mu$m]) and slopes: bluer LRDs have blue UV slopes ($\beta_{\nu,\mathrm{UV}} \sim 0.3$) and red optical slopes, while redder LRDs exhibit redder UV slopes ($\beta_{\nu,\mathrm{UV}} \sim 1.1$). The continuum shape shows a similar trend: redder LRDs display prominent Balmer breaks and curvature, while bluer LRDs follow power-law-like optical SEDs. From literature compilations, $\sim$60% of known broad-line AGNs satisfy our LRD criteria, and up to 90% of LRDs show broad Balmer lines. Emission-line diagnostics reveal a shift from high H$_{\alpha}$/H$_{\beta}$ and low [OIII]$\lambda5007$/H$_{\beta}$ in redder LRDs to the opposite in bluer ones, along with stronger narrow-line equivalent widths, suggesting a transition from AGN- to host-dominated emission. We fit the spectra with a two-component model combining a gas-enshrouded black hole (BH) and a galaxy host. Redder LRDs require higher-luminosity, unreddened BHs and modestly reddened hosts; bluer LRDs require lower-luminosity, reddened BHs and dust-free galaxies. This framework reproduces the diversity in colors and spectral shape by varying BH luminosity, obscuration, and host-to-BH luminosity ratio.

astro-ph.GA↗

arXiv2025

Little red dot variability over a century reveals black hole envelope via a giant Einstein cross

"Little red dots" (LRDs) represent a new population of astronomical objects uncovered by JWST whose nature remains debated. Although many LRDs are suspected as active galactic nuclei (AGN), they show little variability on days-years timescales. We report the discovery of two gravitationally lensed LRDs at redshift $\sim$4.3 behind the cluster RXCJ2211-0350, one of which (RX1) is quadruply imaged with time delays spanning $\sim$130 years. RX1 exhibits intrinsic color and brightness variations of up to 0.7 magnitude among its images. These changes are consistent with blackbody-temperature variations of a photosphere, indicating long-term variability analogous to Cepheid-like pulsations but in a far more extended ($R \sim 2000$ AU) and massive ($M \gtrsim 10^6 \, M_{\odot}$) systems. These results suggest LRDs as a distinct class of AGN with stellar-like envelopes.

astro-ph.GA↗

arXiv2025

VENUS: When Red meets Blue -- A multiply imaged Little Red Dot with an apparent blue companion behind the galaxy cluster Abell 383

We report the discovery of a doubly-imaged Little Red Dot (LRD) candidate behind the galaxy cluster Abell 383, which we dub A383-LRD1. Initially classified as a dropout galaxy in HST imaging with several ground-based emission line detections placing it at $z_{\mathrm{spec}}=6.027$, new JWST/NIRCam observations taken as part of the cycle 4 VENUS survey now reveal that the source consists of two underlying components: A red point-source with a V-shaped SED consistent with LRD selection criteria, and a nearby ($\sim 380$ pc) compact blue companion which was the main contributor to the previous rest-frame UV detections. Based on lensing symmetry and its SED, the LRD appears to lie at a similar redshift as well. The magnification of the two images of A383-LRD1 is $\mu_{\mathrm{A}}=16.2\pm1.2$ and $\mu_\mathrm{B}=9.0\pm0.6$, respectively, and the predicted time delay between them is $\Delta t_{\mathrm{grav}}=5.20\pm0.14$ yr ($\sim0.7$ yr in the rest-frame). After correcting for the lensing magnification, we derive an absolute magnitude of $M_{\mathrm{UV,LRD}}=-16.8\pm 0.3$ for the LRD, and $M_{\mathrm{UV,BC}}=-18.2\pm 0.2$ for the blue companion. We perform SED fits to both components, revealing the LRD to be best fitted with a black hole star (BH*) model and a substantial host galaxy, and the blue companion with an extremely young, emission-line dominated star-forming nebula. A383-LRD1 represents the second known multiply-imaged LRD detected to date, following A2744-QSO1, and to our knowledge, the first LRD system with a confirmed detection of [C $_{II}$]$\lambda158 \ \mu$m emission from ALMA observations. Thanks to lensing magnification, this system opens a unique door to study the relation between a LRD, its host galaxy, and its environment, and represents a prime candidate for deep JWST spectroscopy and high-resolution ALMA follow-up observations.

astro-ph.GA↗

arXiv2025

VENUS: A Strongly Lensed Clumpy Galaxy at $z\sim11-12$ behind the Galaxy Cluster MACS J0257.1-2325

We present the discovery of a strongly lensed galaxy at $z\sim11-12$, dubbed the ``Misty Moons'', identified in the JWST Treasury Survey, Vast Exploration for Nascent, Unexplored Sources (VENUS). The Misty Moons is gravitationally lensed by the galaxy cluster MACS J0257.1-2325 at $z=0.505$, and has five multiple images suggested by two independent lensing models. Two of the five images, ID1 and ID2 ($\mu\sim 20-30$), are very bright (F200W$\sim26$ AB mag) and exhibit blue SEDs with prominent Ly$\alpha$ breaks. In the source plane, the Misty Moons is a sub-$L^*$ galaxy ($M_{\rm UV}\sim-18.0$ mag) resolved into multiple stellar clumps, each of which has an effective radius of $r_\mathrm{eff}\sim 10-70$ pc and a stellar mass of $\sim10^7\ M_\odot$. These clumps dominate the stellar mass budget of the Misty Moons ($\gtrsim80\%$), similar to other high-$z$ clumps, which suggests a highly clustered mode of star formation in the early Universe, unlike seen in local dwarf galaxies. We convolve the source-plane image with the JWST/NIRCam point-spread function to produce a mock NIRCam image of the Misty Moons without lensing magnification, and find that the intrinsic galaxy has a radial surface-brightness profile comparable to those of $z\gtrsim10$ faint galaxies, such as JADES-GS-z13-0 and JADES-GS-z14-1, indicating that the Misty Moons represents a typical $z\gtrsim10$ faint galaxy. The Misty Moons, a lensed galaxy with resolved internal structures, provides an ideal laboratory for exploring the early stages of galaxy formation at $z\gtrsim10$.

astro-ph.GA↗

arXiv2025

High$-z$ [OI] emission lines: ColdSIM simulations and ALMA observations

Neutral-oxygen [OI] far-infrared emission lines at $63\mu$m and $145\mu$m are powerful probes of the physical conditions in the interstellar medium, although they have not been fully exploited in high-redshift studies. We investigate the connection between [OI] emission lines and key galaxy properties, such as star formation rate (SFR) and H$_2$ content. Our predictions are compared with existing observations and new data analysed in this work. We post-process the outputs of the ColdSIM cosmological simulations with the DESPOTIC model, taking into account [OI]$63\mu$m self-absorption by cold foreground material. A Random Forest algorithm is employed to accelerate computations and new observational ALMA data for galaxies at redshift $z\simeq 5-7$ are used to validate our model. Our predictions show significant [OI]$63\mu$m luminosities ($\approx 10^8\,\rm L_\odot$) for galaxies with SFRs of $\approx 10^2\,\rm M_\odot\,{\rm yr}^{-1}$. The $145\mu$m line luminosity is typically $15 \%$ the [OI]$63\mu$m one and is a factor $\approx 2-20$ below high-$z$ observations. Both [OI] lines correlate with SFR and molecular mass, but exhibit flattening in scaling relations with metallicity and stellar mass. Foreground self-absorption reduces the [OI] flux by a factor of $2-4$, consistent with empirical corrections in observational studies. We find typical line ratios of [OI]$63\mu$m / [CII]$158\mu$m $\approx 1$ and [OI]$145\mu$m / [CII]$158\mu$m $ \approx 0.2 \, -$ consistent with $z\gtrsim 6$ observations, but only when [OI]$63\mu$m self-absorption is included. Both [OI]$63\mu$m and [OI]$145\mu$m lines serve as tracers of star formation and molecular gas at high redshift. Their joint detection can provide constraints on the properties of the early interstellar medium and self-absorption of the [OI]$63\mu$m line.

astro-ph.GA↗

arXiv2025

The rise and fall of Little Red Dots could be driven by the environment

The Little Red Dot (LRD) paradigm comprises three main unknowns that are intrinsically connected: (1) What is the nature of these sources? (2) How do they form? (3) How do they evolve? Larger spectroscopic samples and high-resolution data are needed to delve deeper into the mechanisms ruling these sources. Understanding their formation and evolution requires identifying the rise and fall of the key features that characterize these systems, such as their compactness and ''V''-shaped spectral energy distributions. In this work, we present a galaxy system nicknamed The Stingray that was identified in the Canadian NIRISS Unbiased Cluster Survey (CANUCS). This group contains three sources at $z_{\mathrm{spec}} = 5.12$, including an active galactic nucleus (AGN), a Balmer break galaxy, and a star-forming satellite. The latter resembles a Building Block System in which interactions boost stellar mass and black hole mass growth beyond what is expected from secular processes alone. The AGN in this system exhibits features indicative of a transitional object, bridging a normal AGN and an LRD phase. These are a blue rest-frame ultraviolet slope, compact size, and a broad H$\alpha$ line (all of which are characteristic of LRDs), but a flatter rest-frame optical slope compared to that observed in LRDs. The features in this source point to the emergence or fading of an LRD, potentially triggered by environmental effects.

astro-ph.GA↗

arXiv2025

Star Formation under a Cosmic Microscope: Highly magnified z = 11 galaxy behind the Bullet Cluster

We present measurements of stellar population properties of a newly discovered spectroscopically confirmed $z=11.10^{+0.11}_{-0.26}$, gravitationally lensed galaxy, using JWST NIRSpec PRISM spectroscopy and NIRCam imaging. The arc is highly magnified by the Bullet Cluster (magnification factor ${\mu}=14.0^{+6.2}_{-0.3}$. It contains three star-forming components of which one is barely resolved and two are unresolved, giving intrinsic sizes of $\lesssim 10pc$. The clumps also contain ~50% of the total stellar mass. The galaxy formed the majority of its stars ~150Myr ago (by z~14). The spectrum shows a pronounced damping wing, typical for galaxies deep in the reionisation era and indicating a neutral IGM at this line of sight. The intrinsic luminosity of the galaxy is $0.086^{+0.008}_{-0.030} L^*$ (with $L^*$ being the characteristic luminosity for this redshift), making it the lowest luminosity spectroscopically confirmed galaxy at $z>10$ discovered to date.

astro-ph.GA↗