arXiv subjects

C. Bacchini

Publications and source records attributed to C. Bacchini.

15 recordsLinked to original sources

arXiv2026

The galaxy-halo connection and the dynamical evolution of a giant disc in a massive node of the Cosmic Web at z~3

Recent JWST observations revealed the surprising presence of a giant and massive disc galaxy in a Cosmic Web node at z$\sim3$. This galaxy, named the Big Wheel, has a size almost three times larger than expected for typical disc galaxies at the same redshift and similar stellar masses. Constraining the origin and formation history of the Big Wheel requires knowledge of its dark matter halo properties, which are difficult to derive from JWST observations alone. Here, we investigate the dark matter halo of the Big Wheel and provide further constraints on the galaxy baryonic content, combining a physically motivated dynamical model with deep ALMA kinematical data. By using priors based on JWST photometric data and CO kinematics, we infer a dark matter halo mass of $\log (M_{h}/M_{\odot})= 12.11^{+0.29}_{-0.17}$ and a stellar mass of $\log(M_{\star}/M_{\odot})=11.00^{+0.11}_{-0.12}$, leading to a stellar-to-halo mass (SHM) ratio of $M_\star/M_h=0.06^{+0.04}_{-0.03}$. This value is significantly higher than expected from state-of-the-art empirical SHM relations. This implies that the Big Wheel may have assembled its stellar content in a much more efficient way with respect to the general galaxy population at z$\sim3$. Combined with its morphological properties, our results suggest that the Big Wheel had a tranquil recent formation history, with probably no major mergers, violent disc instabilities, or strong ejective feedback. We perform a numerical simulation of an idealised galaxy and let it evolve adiabatically for $2.5$ Gyr to demonstrate that it does not develop gravitational instabilities during its evolution that could alter its resemblance to the observed one. Although systems alike the Big Wheel are arguably rare, our results offer new constraints on the contribution of accretion and feedback to the formation history of the most massive discs within high-redshift Cosmic Web nodes.

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arXiv2026

Molecular gas and star formation in GASP jellyfish galaxies

Several studies have reported a nearly linear correlation between the molecular gas and star formation rate surface density, the so-called Kennicutt-Schmidt (KS) law. We aim to retrieve the KS relation for a sample of four star-forming galaxies located in nearby clusters, disturbed by the effects of the ram pressure stripping, as testing this law in galaxies subject to different environmental conditions can provide key information on the physics of star formation. To perform our analysis, we used ALMA band 6 and band 3 data coupled with MUSE data at spatial resolution of ~1 kpc. Moreover, we analyzed data of star-forming complexes detected through their H\alpha ionized gas emission. We also derived the star formation efficiencies of the star-forming regions nested in these big complexes using the star formation rates derived from spatially resolved HST images and various recipes for the corresponding cold gas phase. We find that ram-pressure-stripped galaxies show normal-to-low star formation efficiencies, depending on the position within the galaxy and on the local gas density: the inner dense regions in the disk show higher efficiencies with respect to the outer regions, including the gaseous tails. The global relation between the star formation rate density and the molecular gas surface density is superlinear, likely suggesting the shortening of the depletion times at high gas mass densities caused by the ram pressure. Within the star-forming complexes, the star formation efficiency is very similar to the one observed at 1 kpc scale in undisturbed star-forming disks. Interestingly, this result holds also for the star-forming complexes located in the stripped gas tails. The analysis of HST resolved clumps suggests that the molecular gas is not uniformly distributed within the star-forming complexes, but its density distribution follows a steeper profile.

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arXiv2025

ALMA survey of a massive node of the Cosmic Web at $z\sim 3$. II. A dynamically cold and massive disk galaxy in the proximity of a hyperluminous quasar

Advancing our understanding of the formation and evolution of early massive galaxies and black holes requires detailed studies of dense structures in the high-redshift Universe. In this work, we present high-angular resolution ($\simeq0.3''$) ALMA observations targeting the CO(4--3) line and the underlying 3-mm dust continuum toward the Cosmic Web node MQN01, a region identified through deep multiwavelength surveys as one of the densest concentrations of galaxies and AGN at cosmic noon. At the center of this structure, we identify a massive, rotationally supported disk galaxy located approximately at $\sim10\,{\rm kpc}$ projected-distance and $\sim-300\,{\rm km\,s^{-1}}$ from a hyperluminous quasar at $z=3.2510$. By accurately modeling the cold gas kinematics, we determine a galaxy dynamical mass of $2.5\times10^{11}\,{M_{\odot}}$ within the inner $\simeq 4\,{\rm kpc}$, and a high degree of rotational support of $V_{\rm rot}/\sigma \approx 11$. This makes it the first quasar companion galaxy confirmed as a massive, dynamically cold rotating disk at such an early cosmic epoch. Despite the small projected separation from the quasar host, we find no clear evidence of strong tidal interactions affecting the galaxy disk. This might suggest that the quasar is a satellite galaxy in the early stages of a merger. Furthermore, our spectroscopic analysis reveals a broad, blueshifted component in the CO(4--3) line profile of the quasar host, which may trace a powerful molecular outflow or kinematic disturbances induced by its interaction with the massive companion galaxy. Our findings show that rotationally supported cold disks are able to survive even in high-density environments of the early Universe.

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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 ($\sigma$) 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$\alpha$ 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 $\sigma$. We present a model predicting the redshift evolution of turbulence in galaxy disks, attributing the increase of $\sigma$ 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 $\sigma$ in Milky-Way like progenitors.

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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]

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arXiv2023

The history of star-forming regions in the tails of 6 GASP jellyfish galaxies observed with the Hubble Space Telescope

Using images collected with the WFC3 camera on board of the Hubble Space Telescope, we detect stellar clumps in continuum-subtracted $H\alpha$ and ultraviolet (F275W filter), such clumps are often embedded in larger regions (star-forming complexes) detected in the optical (F606W filter). We model the photometry of these objects using BAGPIPES to obtain their stellar population parameters. The median mass-weighted stellar ages are 27 Myr for $H\alpha$ clumps and 39 Myr for F275W clumps and star-forming complexes, the oldest stars in the complexes can be older than $\sim$300 Myr which indicates that star-formation is sustained for long periods of time. Stellar masses vary from 10$^{3.5}$ to 10$^{7.1}$ $M_\odot$, with star-forming complexes being more massive objects in the sample. Clumps and complexes found further away from the host galaxy are younger, less massive and less obscured by dust. We interpret these trends as due to the effect of ram-pressure in different phases of the interstellar medium. $H\alpha$ clumps form a well-defined sequence in the stellar mass--SFR plane with slope 0.73. Some F275W clumps and star-forming complexes follow the same sequence while others stray away from it and passively age. The difference in stellar age between a complex and its youngest embedded clump scales with the distance between the clump and the center of the complex, with the most displaced clumps being hosted by the most elongated complexes. This is consistent with a fireball-like morphology, where star-formation proceeds in a small portion of the complex while older stars are left behind producing a linear stellar population gradient. The stellar masses of star-forming complexes are consistent with the ones of globular clusters, but stellar mass surface densities are lower by 2 dex, and their properties are more consistent with the population of dwarf galaxies in clusters.

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arXiv2023

3D modeling of the molecular gas kinematics in optically-selected jellyfish galaxies

Cluster galaxies are subject to the ram pressure exerted by the intracluster medium, which can perturb or even strip away their gas while leaving the stars undisturbed. We model the distribution and kinematics of the stars and the molecular gas in four late-type cluster galaxies (JO201, JO204, JO206, and JW100), which show tails of atomic and ionized gas indicative of ongoing ram pressure stripping. We analyze MUSE@VLT data and CO data from ALMA searching for signatures of radial gas flows, ram pressure stripping, and other perturbations. We find that all galaxies, with the possible exception of JW100, host stellar bars. Signatures of ram pressure are found in JO201 and JO206, which also shows clear indications of ongoing stripping in the molecular disk outskirts. The stripping affects the whole molecular gas disk of JW100. The molecular gas kinematics in JO204 is instead dominated by rotation rather than ram pressure. We also find indications of enhanced turbulence of the molecular gas compared to field galaxies. Large-scale radial flows of molecular gas are present in JO204 and JW100, but more uncertain in JO201 and JO206. We show that our sample follows the molecular gas mass-size relation, confirming that it is essentially independent of environment even for the most extreme cases of stripping. Our findings are consistent with the molecular gas being affected by the ram pressure on different timescales and less severely than the atomic and ionized gas phases, likely because the molecular gas is denser and more gravitationally bound to the galaxy.

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arXiv2022

The volumetric star formation law in nearby galaxies

Star formation laws are empirical relations between the cold gas (HI+H$_2$) content of a galaxy and its star formation rate (SFR), being crucial for any model of galaxy formation and evolution. A well known example of such laws is the Schmidt-Kennicutt law, which is based on the projected surface densities. However, it has been long unclear whether a more fundamental relation exists between the intrinsic volume densities. By assuming the vertical hydrostatic equilibrium, we infer radial profiles for the thickness of gaseous discs in a sample of 23 local galaxies, and use these measurements to convert the observed surface densities of the gas and the SFR into the de-projected volume densities. We find a tight correlation linking these quantities, that we call the volumetric star formation law. This relation and its properties have crucial implications for our understanding of the physics of star formation.

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arXiv2022

Evidence for supernova feedback sustaining gas turbulence in nearby star-forming galaxies

HI and CO observations indicate that the cold gas in galaxies is very turbulent. However, the turbulent energy is expected to be quickly dissipated, implying that some energy source is needed to explain the observations. The nature of such turbulence was long unclear, as even the main candidate, supernova (SN) feedback, seemed insufficient. Other mechanisms have been proposed, but without reaching a general consensus. The key novelty of our work is considering that the gas disc thickness and flaring increase the dissipation timescale of turbulence, thus reducing the energy injection rate required to sustain it. In excellent agreement with the theoretical expectations, we found that the fraction of the SN energy (a.k.a. SN coupling efficiency) needed to maintain the cold gas turbulence is $\sim 1$%, solving a long-standing conundrum.

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arXiv2022

Locations and Morphologies of Jellyfish Galaxies in A2744 and A370

We present a study of the orbits, environments and morphologies of 13 ram-pressure stripped galaxies in the massive, intermediate redshift (z$\sim0.3-0.4$) galaxy clusters A2744 and A370, using MUSE integral-field spectroscopy and HST imaging from the Frontier Fields Program. We compare different measures of the locations and morphologies of the stripped sample with a sample of 6 poststarburst galaxies identified within the same clusters, as well as the general cluster population. We calculate the phase space locations of all cluster galaxies and carry out a substructure analysis, finding that the ram-pressure stripped galaxies in A370 are not associated with any substructures, but are likely isolated infalling galaxies. In contrast, the ram-pressure stripped galaxies in A2744 are strictly located within a high-velocity substructure, moving through a region of dense X-ray emitting gas. We conclude that their ram-pressure interactions are likely to be the direct result of the merger between two components of the cluster. Finally, we study the morphologies of the stripped and poststarburst galaxies, using numerical measures to quantify the level of visual disturbances. We explore any morphological deviations of these galaxies from the cluster population, particularly the weaker cases which have been confirmed via the presence of ionised gas tails to be undergoing ram-pressure stripping, but are not strongly visually disturbed in the broad-band data. We find that the stripped sample galaxies are generally divergent from the general cluster sample, with poststarburst galaxies being intermediary in morphology between stripped galaxies and red passive cluster members.

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arXiv2022

The Smallest Scale of Hierarchy Survey (SSH). II. Extended star formation and bar-like features in the dwarf galaxy NGC 3741: recent merger or ongoing gas accretion?

Using Large Binocular Telescope deep imaging data from the Smallest Scale of Hierarchy Survey (SSH) and archival Hubble Space Telescope data, we reveal the presence of two elongated stellar features contiguous to a bar-like stellar structure in the inner regions of the dwarf irregular galaxy NGC 3741. These structures are dominated by stars younger than a few hundred Myr and collectively are about twice as extended as the old stellar component. These properties are very unusual for dwarf galaxies in the nearby Universe and difficult to explain by hydro-dynamical simulations. From the analysis of archival 21-cm observations, we find that the young stellar "bar" coincides with an HI high-density region proposed by previous studies to be a purely gaseous bar; we furthermore confirm radial motions of a few km/s, compatible with an inflow/outflow, and derive a steeply-rising rotation curve and high HI surface density at the center, indicating a very concentrated mass distribution. We propose that the peculiar properties of the stellar and gaseous components of NGC 3741 may be explained by a recent merger or ongoing gas accretion from the intergalactic medium, which caused gas inflows towards the galaxy center and triggered star formation a few hundred Myr ago. This event may explain the young and extended stellar features, the bar-like structure, the very extended HI disc and the central HI spiral arms. The high central HI density and the steeply rising rotation curve suggest that NGC 3741 may be the progenitor or the descendant of a starburst dwarf.

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arXiv2022

GASP XXXVII: The Most Extreme Jellyfish Galaxies Compared to Other Disk Galaxies in Clusters, an HI Study

We present the results of a VLA HI imaging survey aimed at understanding why some galaxies develop long extraplanar H$\alpha$ tails, becoming extreme jellyfish galaxies. The observations are centered on five extreme jellyfish galaxies, optically selected from the WINGS and OmegaWINGS surveys and confirmed to have long H$\alpha$ tails through MUSE observations. Each galaxy is located in a different cluster. In the observations there are in total 88 other spiral galaxies within the field of view (40'x40') and observed bandwidth (6500 km s$^{-1}$). We detect 13 of these 88 spirals, plus one uncatalogued spiral, with HI masses ranging from 1 to 7 $\times$ 10${^9}$ M$_{\odot}$. Many of these detections have extended HI disks, two show direct evidence for ram pressure stripping, while others are possibly affected by tidal forces and/or ram-pressure stripping. We stack the 75 non-detected spiral galaxies and find an average HI mass of 1.9 $\times$ 10$^{8}$ M$_{\odot}$, which given their average stellar mass, implies they are very HI deficient. Comparing the extreme jellyfish galaxies to the other disk galaxies, we find that they have a larger stellar mass than almost all disk galaxies and than all HI detected galaxies, they are at smaller projected distance from the cluster center and at higher relative velocity to the cluster mean than all HI detections and most non-detections. We conclude that the high stellar mass allows extreme jellyfish galaxies to fall deeply into the cluster before being stripped and the surrounding ICM pressure gives rise to their spectacular star-forming tails.

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arXiv2021

Observing ram pressure at work in intermediate redshift clusters with MUSE: the case of Abell 2744and Abell 370

Ram pressure stripping has been proven to be effective in shaping galaxy properties in dense environments at low redshift. The availability of MUSE observations of a sample of distant (z$\sim 0.3-0.5$) clusters has allowed to search for galaxies subject to this phenomenon at significant lookback times. In this paper we describe how we discovered and characterized 13 ram-pressure stripped galaxies in the central regions of two intermediate redshift (z$\sim$0.3-0.4) clusters, A2744 and A370, using the MUSE spectrograph. Emission line properties as well as stellar features have been analyzed to infer the presence of this gas--only stripping mechanism, that produces spectacular ionized gas tails (H$_{\alpha}$ and even more astonishing [OII](3727,3729) departing from the main galaxy body. The inner regions of these two clusters reveal the predominance of such galaxies among blue star-forming cluster members, suggesting that ram-pressure stripping was even more effective at intermediate redshift than in today's Universe. Interestingly, the resolved [OII]/H$_{\alpha}$ line ratio in the stripped tails is exceptionally high compared to that in the disks of these galaxies, (which is comparable to that in normal low-z galaxies), suggesting lower gas densities and/or an interaction with the hot surrounding ICM.

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arXiv2021

GASP XXXIII. The ability of spatially resolved data to distinguish among the different physical mechanisms affecting galaxies in low-density environments

Galaxies inhabit a wide range of environments and therefore are affected by different physical mechanisms. Spatially resolved maps combined with the knowledge of the hosting environment are very powerful to classify galaxies by physical process. In the context of the GAs Stripping Phenomena in galaxies (GASP), we present a study of 27 non-cluster galaxies: 24 of them were selected for showing asymmetries and disturbances in the optical morphology, suggestive of gas stripping, three of them are passive galaxies and were included to characterize the final stages of galaxy evolution. We therefore provide a panorama of the different processes taking place in low-density environments. The analysis of VLT/MUSE data allows us to separate galaxies into the following categories: Galaxy-galaxy interactions (2 galaxies), mergers (6), ram pressure stripping (4), cosmic web stripping (2), cosmic web enhancement (5), gas accretion (3), starvation (3). In one galaxy we identify the combination of merger and ram pressure stripping. Only 6/27 of these galaxies have just a tentative classification. We then investigate where these galaxies are located on scaling relations determined for a sample of undisturbed galaxies. Our analysis shows the successes and limitations of a visual optical selection in identifying the processes that deplete galaxies of their gas content and probes the power of IFU data in pinning down the acting mechanism.

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arXiv2019

The volumetric star formation law in the Milky Way

Several open questions on galaxy formation and evolution have their roots in the lack of a universal star formation law, that could univocally link the gas properties, e.g. its density, to the star formation rate (SFR) density. In a recent paper, we used a sample of nearby disc galaxies to infer the volumetric star formation (VSF) law, a tight correlation between the gas and the SFR volume densities derived under the assumption of hydrostatic equilibrium for the gas disc. However, due to the dearth of information about the vertical distribution of the SFR in these galaxies, we could not find a unique slope for the VSF law, but two alternative values. In this paper, we use the scale height of the SFR density distribution in our Galaxy adopting classical Cepheids (age$\lesssim 200$ Myr) as tracers of star formation. We show that this latter is fully compatible with the flaring scale height expected from gas in hydrostatic equilibrium. These scale heights allowed us to convert the observed surface densities of gas and SFR into the corresponding volume densities. Our results indicate that the VSF law $\rho_\mathrm{SFR} \propto \rho_\mathrm{gas}^\alpha$ with $\alpha \approx 2$ is valid in the Milky Way as well as in nearby disc galaxies.

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