arXiv subjects

A. Zanella

Publications and source records attributed to A. Zanella.

At least 19 recordsLinked to original sources

arXiv2026

A Cosmic Archipelago of lensed metal-poor galaxies at $z\sim6$

The Cosmic Archipelago is an ensemble of galaxies, strongly lensed by the cluster MACSJ0416, showing extreme physical properties at $z\sim6.14$. We combine JWST/NIRCam with deep VLT/X-Shooter and JWST/NIRSpec IFU to perform a joint spectrophotometric analysis from the far ultraviolet to red optical rest-frame. We focus on CA4, a UV-faint ($M_{UV}=-17.7$), compact ($r_e=81\pm11$ pc) galaxy at $z=6.1446$, magnified by a factor $\mu=3.73$. CA4 is a young, low-mass ($M_\star =4.3\times10^6$ M$\odot$), star-forming (${\rm SFR}=0.46$ M$\odot$/yr), and metal-poor ($Z\sim0.02$ Z$\odot$) galaxy, and an efficient producer of ionizing photons ($\log(\xi_{ion}/{\rm erg^{-1} Hz})\sim25.5$). Its properties place CA4 at the poorly explored interface between massive stellar clusters and dwarf galaxies during the epoch of reionization. Moreover, CA4 shows large Ly$\alpha$ ($f_{esc}^{\rm Ly\alpha}\sim43\%$) and Lyman-continuum ($f_{esc}\sim47\%$) escape fractions, consistent with its small Ly$\alpha$ velocity offset ($\Delta v\sim100$ km/s) and extremely blue UV-continuum slope ($\beta=-3.10$). These characteristics suggest that such UV-faint, metal-poor galaxies may contribute significantly to cosmic reionization. We also confirm five additional systems at the redshift of the Cosmic Archipelago, magnified by factors up to 12.5. They are all young (mass-weighted ages $<11$ Myr) and metal-poor ($Z<0.05$ Z$_\odot$), spanning a wide range of stellar masses and SFRs. Given the large number of these bursty star-forming galaxies in a small cosmic volume, we estimate that the currently known members of the Cosmic Archipelago result in a significant overdensity at $z\sim6$ ($\Delta z\sim0.08$), with $\delta_{gal}=12.3^{+6.6}_{-4.6}$. These results highlight the Cosmic Archipelago as an unprecedented laboratory for studying the earliest groups of low-mass, low-metallicity galaxies during the epoch of reionization.

astro-ph.GA↗

arXiv2026

A first [CII] view of high-z quiescent galaxies

We present ALMA detections (or stringent upper limits) of the [CII] 158 $\mu m$ emission line and underlying dust continuum from five massive quenched galaxies (QGs) at 2<z<4.7. We find extreme variations in the molecular gas fractions ($\rm{f_g=M_{mol}/M_{\star}}$), spanning 0.1%-25%, if a standard $\rm{\alpha_{[CII]}}$ applies. We attempt a first empirical calibration of $\rm{\alpha_{[CII]}}$ with respect to dust continuum in a $z=2$ lensed QG and with respect to CO(3-2) in a $z=3.1$ QG, finding no evidence of strong deviations from the standard value. Dust continuum measurements, coupled with JWST/MIRI fluxes, suggest higher dust temperatures compared to expectations from $z<2$ QGs, reaching $T_{d}\sim40-50 \,K$ in two galaxies. Coupled with remarkably high total infrared luminosities (LIR) not explained by observed JWST colors not by energy balance based on literature dust extinction measurements, and with [CII] deficits down to $\rm{[CII]/LIR\sim 2\times10^{-4}}$ typical of (Ultra)Luminous Infrared Galaxies, our findings point to additional dust-heating mechanisms other than dust-absorbed stellar radiation. Surprisingly, JWST/NIRCam and ALMA imaging reveal widespread disturbed stellar morphologies and offsets/tails in dust and gas, indicative of ongoing interactions. While larger samples are needed to assess how common these features are in high-z QGs, these findings support a merger-driven origin for the phenomenology observed in these systems, with key similarities with respect to local post-starburst galaxies where low-velocity shocks and turbulence also inject energy into the residual ISM.

astro-ph.GA↗

arXiv2026

Spatially Resolved Star Formation relations in local LIRGs along the complete merger sequence

We investigate the properties of the interstellar medium (ISM) at ~100 pc scales in a sample of 27 nearby luminous infrared galaxies (LIRGs) spanning the entire merger sequence. In particular, we study the relations between star-formation (SF) and molecular gas surface density as a function of the interaction stage using two complementary approaches: beam-sized (unresolved, line-of-sight) regions and physically identified molecular gas clumps. To map the distribution of molecular gas we use ALMA CO(2-1) observations, while SF is traced using HST Pa-alpha or Pa-beta images. We derive spatially resolved Kennicutt-Schmidt (KS) relations for each galaxy. When using beam-sized regions, we find that 67% of galaxies follow a single relation between Sigma_SFR and Sigma_H2. However, in the remaining galaxies, the relation splits into two branches, indicating the presence of a duality in this relation. In contrast, when using physical gas clumps, the duality disappears and all galaxies show a single trend. We also study other ISM/clump properties as a function of the merger stage. We find that isolated galaxies and systems in early stages of interaction exhibit lower amounts of gas and SF. As the merger progresses, however, the amount of gas in the central kpc of the galaxy undergoing the merger increases, along with the SFR, and the slope of the KS relation becomes steeper, indicating an increase in SF efficiency of the gas clumps. Clumps in late-stage mergers are predominantly located at small distances from the nucleus, confirming that most of the activity is concentrated in the central regions. Finally, the relation between the SF efficiency and the boundedness parameter evolves from being roughly flat in the early stages of the merger to becoming positive in the final phases, indicating that clump self-gravity only starts to regulate the SF process between the early- and mid-merger stages.

astro-ph.GA↗

arXiv2025

A faint M$_{\rm UV} = -14.5$ Lyman-continuum leaker candidate in the epoch of reionization: Unprecedented Ly$\alpha$ properties at z=5.725

We report the unprecedented Ly$\alpha$ properties of AMORE6, an extremely metal-poor ($12+\log({\rm O/H}) < 6$), low-mass ($M_\star = 4.4\times10^{5}\,M_\odot$), and ultracompact (effective radius $\sim30$ pc) dwarf galaxy at $z=5.7253$, which is gravitationally lensed by the cluster A2744. A prominent, narrow, and nearly symmetric Ly$\alpha$ emission line is detected at the systemic redshift (the latter traced by H$\beta$, from JWST/NIRCam slitless spectroscopy), with a rest-frame equivalent width of $150 \pm 10$ \AA, a full width at half maximum of $\rm 58\pm1$ km s$^{-1}$, and a slight asymmetry, resulting in a flux excess of $\rm \sim10\%$ in the red wing of the line. The negligible velocity offset from systemic ($dv = 4\pm67$ km s$^{-1}$, $3\sigma$ uncertainty), together with the sharpness and symmetry of the profile, indicates minimum radiative transfer effects, which implies a neutral hydrogen column density consistent with an optically thin medium that in turn is compatible with a nonzero ionizing photon escape fraction. If indirect spectral diagnostics calibrated at $z<4.5$ remain the only viable tools for identifying LyC leakers during reionization, then based on its strongest indicator (Ly$\alpha$), AMORE6 stands out as one of the most compelling LyC-leaking candidates yet discovered in the epoch of reionization.

astro-ph.GA↗

arXiv2025

Gas excitation of post-starburst galaxies at 0.6 < z < 1.3

Molecular gas traces the fuel for star formation and the processes that regulate it. Observing its physical state (e.g. excitation) reveals when and why galaxies quench. We observed the CO(5-4) emission of 8 post-starburst (SB) galaxies at z~0.6-1.3. To our knowledge, this is the first time that high-J transitions are probed for quiescent galaxies beyond the local Universe. All targets are detected in CO(2-1) or CO(3-2) and have gas fractions up to 20%. Using the ratio R52=L'CO(5-4)/L'CO(2-1) as a proxy for gas excitation, we distinguish among mechanisms responsible for the low SFE of post-SBs. In the first scenario, the molecular gas is predominantly diffuse and cold, implying a low fraction of dense star-forming gas and low R52 values. In the second scenario, elevated gas temperatures at moderate densities, e.g. due to AGN activity, shocks, or turbulence, produce high R52 values. On average our post-SBs have R52=0.28, comparable to high-redshift galaxies. However, CO(5-4) non-detections, corresponding to galaxies without signs of interaction, yield R52<0.10, 2 times lower than local star-forming galaxies. The average CO Spectral Line Energy Distribution (SLED) peaks at J=3, similar to the Milky Way. Three galaxies show signs of ongoing mergers and have R52 = 0.40 and CO SLEDs peaking at J > 4-5, similar to high-redshift galaxies. At least one requires additional mechanisms (AGN, shocks) to explain the rise of the SLED up to J=5. Our results favor a scenario in which most systems are dominated by low-density molecular gas with low excitation, consistent with quenching driven by gas stabilization, feedback regulation, or stripping. In interacting systems instead, enhanced excitation is likely driven by heating processes not related to star-formation (e.g., AGN, turbulence, shocks). Residual star formation is insufficient to exhaust the remaining molecular gas in the majority of post-SBs.

astro-ph.GA↗

arXiv2025

A Pristine Star-Forming Complex at z=4.19

We report the discovery of a faint (M_1700 ~ -12.2), oxygen-deficient strongly-lensed ionizing source -- dubbed LAP2 (Lensed And Pristine 2) -- at a spectroscopic redshift of z=4.19. LAP2 appears to be isolated and lies very close to the caustic produced by the lensing galaxy cluster Abell 2744. It was observed with the James Webb Space Telescope (JWST) NIRSpec MSA in prism mode as part of the UNCOVER program. The NIRSpec spectrum reveals prominent Lya emission (7.1 sigma), clear Ha emission (6.2 sigma), tentative Hb emission (2.8 sigma), and no detectable [OIII]4959,5007 (~ 7 times fainter than Ha). The inferred [OIII] 2 sigma upper limit corresponds to an R3 index <0.85 (assuming the Ha/Hb ~ 2.86 case~B recombination ratio), which, under high-ionization conditions, implies a metallicity of Z < 6 X 10^(-3) Z_sun. The combination of faint ultraviolet luminosity, large rest-frame Ha equivalent width (~ 650 A), and extremely compact size (< 10 pc) suggests that LAP2 is being caught in an early, pristine formation phase consistent with an instantaneous-burst scenario, with an estimated stellar mass of at most a few 10^4 Msun. Deep VLT/MUSE observations further reveal copious Lya emission forming an arclet that straddles the critical line. LAP2 thus joins the rare class of extremely metal-poor star-forming complexes that JWST has started to unveil at redshifts 3 - 7, and provides a rare glimpse into a still very poorly explored faint-luminosity regime.

astro-ph.GA↗

arXiv2025

The z = 9.625 Cosmic Gems Galaxy was a "Compact Blue Monster" Propelled by Massive Star Clusters

The recent discovery of five massive stellar clusters at z=9.625 in the Cosmic Gems has raised the question about the formation mechanism of star clusters in the first half Gyr after the Big-Bang. We infer the total stellar mass in clusters by normalizing and integrating the stellar cluster mass function (SCMF, dn(M)/dM ~ (n$_0$) $M^\beta$), assuming three different slopes $\beta$ = -1.5, -2.0 and -2.5 and different lower-mass limits between $10^2$ and $10^5$ Msun. The total integrated cluster stellar mass is compared to the stellar mass inferred from the counter-image of the Cosmic Gems, which provides the best, modestly magnified ($\mu$ = 1.84$\pm$0.05) representation of the entire galaxy. The delensed stellar mass of the Cosmic Gems galaxy is estimated as 3.5$_{-1.8}^{+3.3}$ x$10^7$ Msun, with an effective radius of Reff = 103$_{-15}^{+13}$ parsec and a stellar surface mass density of $\Sigma$mass = 520$_{-225}^{+340}$ Msun pc$^{-2}$. Accounting for normalization uncertainties - including different lensing magnification scenarios for the arc - a modified SCMF, combined with a significantly high star cluster formation efficiency (approaching 100%), appears to be a necessary condition to explain the relatively short formation timescale of both the star clusters and the counter-image, without exceeding the galaxy's stellar mass. By extrapolating the physical properties at the peak of the burst we find that in its recent past (<~ 30 Myr) the Cosmic Gems galaxy has likely experienced a specific star formation rate (sSFR) exceeding 25 Gyr$^{-1}$ and luminosity approaching the ``blue monster'' regime (M$_{UV}$ < -20). Our study provides insights into the extreme clustered nature of star formation in early galaxies and shed light into the formation of bound star clusters that might survive to z = 0 as globular clusters, older than 13 Gyr.

astro-ph.GA↗

arXiv2025

JWST spectroscopic confirmation of the Cosmic Gems arc at z=9.625 -- Insights into the small scale structure of a post-burst system

We present JWST/NIRSpec integral field spectroscopy of the Cosmic Gems arc, strongly magnified by the galaxy cluster SPT-CL J0615$-$5746. Six-hour integration using NIRSpec prism spectroscopy (resolution $\rm R\simeq 30-300$), covering the spectral range $0.8-5.3~\mu m$, reveals a pronounced $\rm Ly\alpha$-continuum break at $\lambda \simeq 1.3~\mu m$, as well as weak optical $\rm H\beta$ and $\rm [OIII]\lambda4959$ emission lines at $z=9.625\pm0.002$, located in the reddest part of the spectrum ($\lambda > 5.1~\mu m$). No additional ultraviolet or optical emission lines are reliably detected. A weak Balmer break is measured alongside a very blue ultraviolet slope ($\beta \leq-2.5$, $\rm F_{\lambda} \sim \lambda^{\beta}$). Spectral fitting with $\tt Bagpipes$ suggests that the Cosmic Gems galaxy is in a post-starburst phase, making it the highest-redshift system currently observed in a mini-quenched state. Spatially resolved spectroscopy at tens of parsecs shows relatively uniform features across subcomponents of the arc. These findings align well with the physical properties previously derived from JWST/NIRCam photometry of the stellar clusters, now corroborated by spectroscopic evidence. In particular, five observed star clusters exhibit ages of $\rm 7-30~Myr$. An updated lens model constrains the intrinsic sizes and masses of these clusters, confirming they are extremely compact and denser than typical star clusters in local star-forming galaxies. Additionally, four compact stellar systems consistent with star clusters ($\lesssim10$ pc) are identified along the extended tail of the arc. A sub-parsec line-emitting HII region straddling the critical line, lacking a NIRCam counterpart, is also serendipitously detected.

astro-ph.GA↗

arXiv2025

SIEGE IV: compact star clusters in cosmological simulations with high star formation efficiency and sub-parsec resolution

The formation of compact high-redshift star-forming clumps, the physical processes driving their evolution and their potential connection to present-day Globular Clusters are key open questions in galaxy formation. In this work, we aim to shed light on these aspects using the SImulating the Environment where Globular clusters Emerged (SIEGE) project, a suite of cosmological zoom-in simulations with sub-parsec resolution specifically designed to investigate the physical conditions behind the origin of compact stellar systems in high-redshift environments. The simulation object of this study focuses on a dwarf galaxy with a virial mass of a few $10^9$ $M_\odot$ at $z=6.14$, where the spatial resolution reaches 0.3 pc $h^{-1}$. Individual stars are formed directly by sampling the initial mass function with a 100\% star formation efficiency, a setup designed to explore the impact of a high star formation efficiency under high-redshift conditions. The simulation reveals the emergence of numerous stellar clumps with sizes of 1-3 pc, stellar surface densities up to almost $10^4$ $M_\odot$ pc$^{-2}$, and masses predominantly spanning from $10^3$ $M_\odot$ to several $10^4$ $M_\odot$, with a few reaching $10^5$ $M_\odot$ and up to $10^6$ $M_\odot$. All clumps form during intense, short bursts of star formation lasting less than a Myr, often with negligible dark matter content (dark-to-stellar mass ratios below 1 within three times their effective radii). We measure a clear correlation between mass and size, and a clump mass function described by a power-law with a slope of -2. Star formation conditions in the simulation behave similarly to those of a feedback-free starburst scenario, where dense clumps form due to inefficient stellar feedback over small timescales. Notably, some clumps exhibit properties closely resembling those of present-day globular clusters.

astro-ph.GA↗

arXiv2025

SIEGE III: The formation of dense stellar clusters in sub-parsec resolution cosmological simulations with individual star feedback

Star clusters stand at the crossroads between galaxies and single stars. Resolving the formation of star clusters in cosmological simulations represents an ambitious and challenging goal, since modelling their internal properties requires very high resolution. This paper is the third of a series within the SImulating the Environment where Globular clusters Emerged (SIEGE) project, where we conduct zoom-in cosmological simulations with sub-parsec resolution that include the feedback of individual stars, aimed to model the formation of star clusters in high-redshift proto-galaxies. We investigate the role of three fundamental quantities in shaping the intrinsic properties of star clusters, i. e., i) pre-supernova stellar feedback (continuous or instantaneous ejection of mass and energy through stellar winds); ii) star formation efficiency, defined as the fraction of gas converted into stars per freefall time, for which we test 2 different values (epsi_ff=0.1 and 1), and iii) stellar initial mass function (IMF, standard vs top-heavy). All our simulations are run down to z=10.5, which is sufficient for investigating some structural properties of the emerging clumps and clusters. [Abridged] The prescription for a continuous, low-intensity feedback, along with the adoption of epsi_ff=1, produces star clusters with maximum stellar density values up to 10^4 M_sun pc^(-2), in good agreement with the surface density-size relation observed in local young star clusters (YSCs). Therefore, a realistic stellar wind description and a high star formation effiency are the key ingredients that allow us to achieve realistic star clusters characterised by properties comparable to those of local YSCs. In contrast, the other models produce too diffuse clusters, in particular the one with a top-heavy IMF.

astro-ph.GA↗

arXiv2025

Unraveling the Lyman Continuum Emission of Ion3: Insights from HST multi-band imaging and X-Shooter spectroscopy

We provide a comprehensive analysis of Ion3, the most distant LyC leaker at $z=3.999$, using multi-band HST photometry and X-Shooter spectroscopy. Deep HST F390W imaging probe uncontaminated LyC flux blueward $\sim$880Å, while the non-ionizing UV 1500Å/2800Å~flux is probed with the F814W/F140W band. High angular resolution allows us to properly mask low-$z$ interlopers and prevent contamination of measured LyC radiation. We confirm the detection of LyC flux at SNR $\sim$3.5 and estimate the escape fraction of ionizing photons to be in the range $f_{\rm esc, rel}$ = 0.06 -- 1, depending on the adopted IGM attenuation. Morphological analysis reveals a clumpy structure made of two main components, with effective radii of R$_{\rm eff}$ $\sim$180 pc and R$_{\rm eff}$ < 100 pc, and a total estimated de-lensed area in the rest-frame 1600Å~of 4.2~kpc$^{2}$. We confirm the presence of faint ultraviolet spectral features HeII$λ$1640, CIII]$λ$1907,1909 and [NeIII]$λ$3968, with rest-frame EW(HeII) = (1.6$\pm$0.7)Å and EW(CIII]) = (6.5$\pm$3)Å. From [OII]$λ$$λ$3726,3729 and [CIII]$λ$1909/CIII]$λ$1906 we derive electron densities $n_{\rm e}^{\rm [OII]}$ = 2300$\pm$1900 cm$^{-3}$ and $n_{\rm e}^{\rm CIII]}$ > 10$^{4}$ cm$^{-3}$, corresponding to an ISM pressure log(P/k) > 7.90. Furthermore, we derive an intrinsic SFR(H$α$) $\approx$ 77 M$_{\odot}$ yr$^{-1}$ (corresponding to $Σ_{\rm SFR} = 20$~M$_{\odot}$~yr$^{-1}$~kpc$^{-2}$ for the entire galaxy) and sub-solar metallicity $12+\rm log(O/H)$ = 8.02$\pm$0.20 using the EW(CIII]) as a diagnostic. The detection of [NeIII]$λ$3968 line and [OII]$λ$$λ$3726,3729, provide an estimate of the ratio [OIII]$λ$5007/[OII]$λ$$λ$3727,29 of O32 > 50 and high ionization parameter log$U$ > $-$1.5 using empirical and theoretical correlations.

astro-ph.GA↗

arXiv2025

Spatially resolved [CII]-gas conversion factor in early galaxies

Determining how efficiently gas collapses into stars at high-redshift is key to understanding galaxy evolution in the Epoch of Reionization (EoR). Globally, this process is quantified by the gas depletion time ($t_{dep}$); on resolved scales, by the slope and normalization of the Kennicutt-Schmidt (KS) relation. This work explores the global ($\alpha_{[CII]}$) and spatially resolved ($W_{[CII]}$) [CII]-to-gas conversion factors at high-$z$ and their role in inferring reliable gas masses, surface densities, and $t_{dep}$ in the EoR. We select galaxies at 4<z<9 from the SERRA cosmological zoom-in simulation, that features on-the-fly radiative transfer and resolves interstellar medium properties down to $\approx$30 pc. The [CII] emission modelling from photodissociation regions allow us to derive global $\alpha_{ [CII]}$, and maps of $W_{[CII]}$. We study their dependence on gas metallicity (Z), density (n), Mach number (M), and burstiness parameter ($k_s$), and provide best fit relations. The $\alpha_{[CII]}$ decreases with increasing $Z$ and galaxy compactness, while the resolved $W_{[CII]}$ shows two regimes: at $Z< 0.2 Z_\odot$, it anticorrelates with n and Z, but not with $k_s$; above this threshold, it also depends on $k_s$, with more bursty regions showing lower conversion factors. This implies $W_{[CII]}\propto \Sigma_{[CII]}^{-0.5}$, as dense, metal-rich, and bursty regions exhibit higher [CII] surface brightness. Applying a constant $\alpha_{[CII]}$ overestimates $\Sigma_{gas}$ in bright $\Sigma_{[CII]}$ patches, thus flattening the KS slope and overestimating $t_{dep}$ by a factor of $\approx$4.

astro-ph.GA↗

arXiv2025

Anatomy of a z=6 Lyman-α emitter down to parsec scales: extreme UV slopes, metal-poor regions and possibly leaking star clusters

We present a detailed JWST/NIRSpec and NIRCam analysis of a gravitationally-lensed galaxy ($\rm μ=17-21$) at redshift 6.14 magnified by the Hubble Frontier Field galaxy cluster MACS J0416. The target galaxy is overall a typical compact and UV-faint ($\rm M_{UV}=-17.8$) Lyman-$α$ emitter, yet the large magnification allows the detailed characterization of structures on sub-galactic scales (down to a few parsecs). Prominent optical $\rm Hα$, $\rm Hβ$ and [OIII]$λ\lambda4959,5007$ lines are spatially resolved with the high spectral resolution grating (G395H, R~2700), with large equivalent widths, EW($\rm Hβ$+[OIII])$\gtrsim1000$ Å, and elevated ionising photon production efficiencies $\rm log(ξ_{ion}/erg^{-1}Hz)=25.2-25.7$. NIRCam deep imaging reveals the presence of compact rest-UV bright regions along with individual star clusters of $\rm R_{eff}=3-8~pc$ in size and $\rm M\sim2\cdot10^5-5\cdot10^{6}~M_\odot$ in mass. These clusters are characterised by steep UV slopes, $\rmβ_{UV}\lesssim-2.5$, which in some cases are associated with a dearth of line emission, indicating possible leaking of the ionizing radiation, as also supported by a Lyman-$\rm α$ emission peaking at $\rm \sim100~km~s^{-1}$ from the systemic redshift. While the entire system is characterised by low-metallicity, $\sim0.1~Z_\odot$, the NIRSpec-IFU map also reveals the presence of a low-luminosity, metal-poor region with $\rm Z\lesssim2\%~Z_\odot$, barely detected in NIRCam imaging; this region is displaced by $\rm >200~pc$ from one of the brightest structures of the system in UV, and would have been too faint to detect if not for the large magnification of the system.

astro-ph.GA↗

arXiv2025

Constraining the geometry of the gas surrounding a typical galaxy at $z = 3.4$ with Ly$α$ polarization

Ly$α$ emission is the strongest tracer of recombining ionized hydrogen in young, star-forming galaxies, but its origin is still debated. Ly$α$ arises when emitted photons scatter in neutral hydrogen and, so far, observational efforts have mostly focused on the Ly$α$ surface brightness and spectral profile, which depend on the neutral hydrogen column density, geometry, kinematics, powering mechanism and on the region from which the photons are emitted. Different processes produce similar spectra, but have different degrees of polarization, that we can use to discriminate between them. In this paper, we present the first spectropolarimetric observations of a typical star-forming galaxy at $z\sim 3.4$, strongly lensed by the cluster of galaxies Abell 2895, taken with the PMOS mode of the VLT/FORS2 instrument. We measure a Ly$α$ degree of polarization $1σ$ upper limit of $4.6\%$. We develop new Ly$α$ radiative transfer models to reproduce the observations, that can be explained by assuming the star-forming galaxy being embedded in a CGM with a biconical outflow geometry, with an opening angle of the wind $θ_{o,Wind}\sim 30^\circ$ for line-of-sight angles $θ_{LOS} \leq 20^\circ$, $θ_{o,Wind}\sim 45^\circ$ for $θ_{LOS}\leq 20^\circ$, $θ_{o,Wind}\sim 60^\circ$ for $θ_{LOS}\leq 20^\circ$, and $θ_{o,Wind}\sim 75^\circ$ for $θ_{LOS}\leq 40^\circ$, where $θ_{LOS}=0^\circ$ means observing in the direction of the outflow. We notice that the constraints from polarization are complementary to those from the spectral line profile. This study shows the potential of adding measurements of the Ly$α$ degree of polarization to constrain the geometry of the gas surrounding typical star-forming galaxies and paves the way to spatially resolved studies that will allow us to disentangle between different Ly$α$ origin mechanisms.

astro-ph.GA↗

arXiv2024

Cosmography from accurate mass modeling of the lens group SDSS J0100+1818: five sources at three different redshifts

Systems where multiple sources at different redshifts are strongly lensed by the same deflector allow one to directly investigate the evolution of the angular diameter distances with redshift, and thus to learn about the geometry of the Universe. We present measurements of the values of the total matter density, $Ω_m$, and of the dark energy equation of state parameter, $w$, through a strong lensing analysis of SDSSJ0100+1818, a group-scale system at $z=0.581$ with five lensed sources, from $z=1.698$ to $4.95$. We use new MUSE data to securely measure the redshift of 65 sources, including the five multiply imaged background sources (lensed into a total of 18 multiple images) and 19 galaxies on the deflector plane (the brightest group galaxy, BGG, and 18 fainter members), all employed to build robust strong lensing models with the software GLEE. We measure $Ω_m = 0.14^{+0.16}_{-0.09}$ in a flat $Λ$ cold dark matter (CDM) model, and $Ω_m = 0.19^{+0.17}_{-0.10}$ and $w=-1.27_{-0.48}^{+0.43}$ in a flat $w$CDM model. We quantify, through a multi-plane approach, the impact of different sources angularly close in projection on the inferred values of the cosmological parameters. We obtain consistent median values, with uncertainties for only $Ω_m$ increasing by a factor of 1.5. We accurately measure a total mass of $(1.55 \pm 0.01) \times 10^{13}$ M$_\odot$ within 50 kpc and a stellar over total mass profile decreasing from $45.6^{+8.7}_{-8.3}\%$ at the BGG effective radius to $(6.6\pm 1.1)\%$ at $R\approx 77$ kpc. Our results confirm that SDSSJ0100+1818 is one of the most massive (lens) galaxies known at intermediate redshift and that group-scale systems that act as lenses for $\geq 3$ background sources at different redshifts enable to estimate the values of the cosmological parameters with an accuracy that is competitive with that obtained from lens galaxy clusters.

astro-ph.CO↗

arXiv2024

Extreme Ionizing Properties of Metal-Poor, Muv ~ -12 Star Complex in the first Gyr

We report the serendipitous discovery of a faint (M_UV > -12.2), low-metallicity (Z ~ 0.02 Zsun), ionizing source (dubbed T2c) with a spectroscopic redshift of z=6.146. T2c is part of a larger structure amplified by the Hubble Frontier Field galaxy cluster MACSJ0416, and was observed with JWST/NIRSpec IFU. Stacking the short-wavelength NIRCam data reveals no stellar continuum detection down to a magnitude limit of m_UV ~ 31.0 (3 sigma). However, prominent Hb, [OIII]4959,5007, and Ha emissions are detected, with equivalent widths exceeding 200A, 800A, and 1300A (3 sigma), respectively. The corresponding intrinsic (magnification-corrected x23 +/- 3) ultraviolet and optical rest-frame magnitudes exceed 34.4 and 33.9 (corresponding to M_uv and M_opt fainter than -12.2 and -12.8, at lambda_rest ~ 2000A and ~5000A, respectively), suggesting a stellar mass lower than a few 10^4 Msun under an instantaneous burst scenario. The inferred ionizing photon production efficiency (xi_ion) is high, xi_ion >~ 26.08(25.86) 3(5)sigma, assuming no dust attenuation and no Lyman continuum leakage, indicating the presence of massive stars despite the low mass of the object. The very poor sampling of the initial mass function at such low mass star-forming complex suggests that the formation of very massive stars might be favored in very low metallicity environments. T2c is surrounded by Balmer and weak oxygen emission on a spatial scale of a few hundred parsecs after correcting for lensing effects. This system resembles an HII region potentially powered by currently undetected, extremely efficient, low-metallicity star complexes or clusters. We propose that massive O-type stars populate this low-mass and metallicity high-redshift satellites, likely caught in an early and short formation phase, contributing to the ionization of the surrounding medium.

astro-ph.GA↗

arXiv2024

The ALMA-ALPAKA survey II. Evolution of turbulence in galaxy disks across cosmic time: difference between cold and warm gas

The gas in the interstellar medium (ISM) of galaxies is supersonically turbulent. Measurements of turbulence typically rely on cold gas emission lines for low-z galaxies and warm ionized gas observations for z>0 galaxies. Studies of warm gas kinematics at z>0 conclude that the turbulence strongly evolves as a function of redshift, due to the increasing impact of gas accretion and mergers in the early Universe. However, recent findings suggest potential biases in turbulence measurements derived from ionized gas at high-z, impacting our understanding of turbulence origin, ISM physics and disk formation. We investigate the evolution of turbulence using velocity dispersion ($σ$) measurements from cold gas tracers (i.e., CO, [CI], [CII]) derived from a sample of 57 galaxy disks spanning the redshift range z=0-5. This sample consists of main-sequence and starburst galaxies with stellar masses $\gtrsim 10^{10} M_{\odot}$. The comparison with current H$α$ kinematic observations and existing models demonstrates that the velocity dispersion inferred from cold gas tracers differ by a factor of $\approx 3$ from those obtained using emission lines tracing warm gas. We show that stellar feedback is the main driver of turbulence measured from cold gas tracers. This is fundamentally different from the conclusions of studies based on warm gas, which had to consider additional turbulence drivers to explain the high values of $σ$. We present a model predicting the redshift evolution of turbulence in galaxy disks, attributing the increase of $σ$ with redshift to the higher energy injected by supernovae due to the elevated star-formation rate in high-z galaxies. This supernova-driven model suggests that turbulence is lower in galaxies with lower stellar mass compared to those with higher stellar mass. Additionally, it forecasts the evolution of $σ$ in Milky-Way like progenitors.

astro-ph.GA↗

arXiv2024

A 3D view on the local gravitational instability of cold gas discs in star-forming galaxies at $0 \lesssim \mathrm{z} \lesssim 5$

Local gravitational instability (LGI) is considered crucial for regulating star formation and gas turbulence in galaxy discs, especially at high redshift. Instability criteria usually assume infinitesimally thin discs or rely on approximations to include the stabilising effect of the gas disc thickness. We test a new 3D instability criterion for rotating gas discs that are vertically stratified in an external potential. This criterion reads $Q_{\rm3D}<1$, where $Q_{\rm3D}$ is the 3D analogue of the Toomre parameter $Q$. The advantage of $Q_{\rm3D}$ is that it allows us to study LGI in and above the galaxy midplane in a rigorous and self-consistent way. We apply the criterion to a sample of 44 star-forming galaxies at $0\lesssim\mathrm{z}\lesssim5$ hosting rotating discs of cold gas. The sample is representative of galaxies on the main sequence at $\mathrm{z}\approx 0$ and includes massive star-forming and starburst galaxies at $1\lesssim\mathrm{z}\lesssim5$. For each galaxy, we first apply the Toomre criterion for infinitesimally thin discs, finding 10 unstable systems. We then obtain maps of $Q_{\rm 3D}$ from a 3D model of the gas disc derived in the combined potential of dark matter, stars and the gas itself. According to the 3D criterion, two galaxies with $Q<1$ show no evidence of instability and the unstable regions that are 20% smaller than those where $Q<1$. No unstable disc is found at $0\lesssim\mathrm{z}\lesssim 1$, while $\approx 60$% of the systems at $2\lesssim\mathrm{z}\lesssim5$ are locally unstable. In these latter, a relatively small fraction of the total gas ($\approx 30$%) is potentially affected by the instability. Our results disfavour LGI as the main regulator of star formation and turbulence in moderately star-forming galaxies in the present-day Universe. LGI likely becomes important at high redshift, but the input by other mechanisms seems required [abridged]

astro-ph.GA↗