Cosmological simulations of merging galaxies have demonstrated that mergers lead to triggering AGN (active galactic nuclei) and enhanced star-formation. One of the expected outputs of galaxy mergers is pairs of AGN known as dual AGN (DAGN). These systems are important to understand the evolution of galaxy mergers, AGN feedback etc. We have compiled a sample of galaxy merger systems candidates from the literature and observed them at multiple frequencies with VLA, UVIT, and GMRT to confirm the nature of the nuclei and star-formation. In this presentation, I will show the results from our multi-wavelength study of these systems in detail.

Galaxy clusters are the largest gravitationally bound structures of the Universe. At the center of these clusters reside the most massive galaxies observable today, Brightest Cluster Galaxies (BCGs). BCGs are the final products of 10 Gyrs of consecutive galactic mergers in the densest regions of the cosmic web. They are excellent tracers of cluster formation, and ideal to study the impact of environmental processes on galactic formation.

In one commonly accepted paradigm of massive galaxy evolution, the massive galaxy undergoes several transitions prompted by interactions with other galaxies, and feedback from the active galactic nucleus (AGN). However, the impact and frequency of minor mergers and feedback is unconstrained, and the direct effect of AGN on the ISM of their host galaxies remains to be determined in the high-redshift Universe. In this talk, I will showcase the importance of using multiple gas tracers to determine the ISM properties, gas kinematics, and surrounding environment of high-redshift active galaxies. To do this I concentrate on three galaxies at high redshift: AzTEC-3 at z=5.3, BRI0952-0115 at z=4.432, and G09v1.97 at z = 3.63. Through high-resolution ALMA data covering multiple atomic and molecular lines, including [CII], [CI], CO, H2O, OH, and OH+, we have probed the multi-component ISM of these galaxies and studied their environments. BRI0952-0115 and AzTEC-3 are situated in complex environments, with multiple nearby faint companion sources detected in [CII] emission showing evidence of ongoing interactions classified as minor mergers based on dynamical mass estimates.The [CII] line profiles of these massive sources and their companions indicate complex kinematics including showing evidence of outflows. Radiative transfer modeling of the detected lines in the quasar BRI0952-0115 indicates the need for a multi-component model of the ISM while also demonstrating the necessity of additional lines to distinguish between stellar heating and the influence of the AGN on the gas properties. Unlike BRI0952-0115 and AzTEC-3, new high-resolution ALMA data of G09v1.97 suggests a very different picture of a single, bright disk galaxy without clear indications of companions or merger activity, pointing to a different evolutionary scenario for this galaxy. Through our detailed studies of these galaxies, we have found that even within a small but targeted sample, we are able to examine multiple evolutionary scenarios for massive galaxies at high redshift, while simultaneously probing their ISM properties and environments.

We here present 0.02-0.04'' resolution ALMA observation of the compact obscured nucleus (CON) of IRAS17578-0400. A dusty torus within the nucleus, approximately 4 pc in radius, has been uncovered, exhibiting a usually flat spectral index at ALMA band 3, likely due to the millimeter corona emission from the central supermassive black hole (SMBH). The dense gas disk, traced by 13CO(1-0), spans 7 pc in radius and suggests an outflow driven by a disk wind due to its asymmetrical structure along the minor axis. Collimated molecular outflows (CMO), traced by the low-velocity components of the HCN(3-2) and HCO+(3-2) lines, align with the minor axis gas disk. Examination of position-velocity plots of HCN(3-2) and HCO+(3-2) reveals a flared dense gas disk extended a radius of ∼ 60 pc, infalling and rotating at speeds of about 200 km/s and 300 km/s, respectively. A centrifugal barrier, located around 4 pc from the dynamical center, implies an SMBH mass of approximately 108 M⊙, consistent with millimeter corona emission estimates. The CMO maintains a steady rotation speed of 200 km/s over the 100 pc scale along the minor axis. The projected speed of the CMO is about 80 km/s, corresponding to around ∼ 500 km/s, assuming an inclination angle of 80∘. Such a kinematics structure of disk-driven collimated rotating molecular outflow with gas supplies from a falling rotating disk indicates that the feedback of the compact obscured nucleus is likely regulated by the momentum transfer of the molecular gas that connects to both

The most intense star-forming events in the Universe remain hidden behind dusty birthclouds, and can only be identified through mm/sub-mm mapping observations. The Herschel revolution increased the number of known dusty star-forming galaxies from hundreds to hundreds of thousands. Like the other mm/submm mapping observations, Herschel suffers from a lack of spectral and spatial resolution. In order to overcome the spectral limitations, we used ALMA and NOEMA to identify the redshifts spectroscopically, producing a sample of 300 of the brightest Herschel sources with large molecular reservoirs and enabling further near- and far-infrared spectroscopic follow-up (e.g., [Hɑ], [CII], [OIII]). HST and ALMA observations further show a remarkably-diverse galaxy population consisting of compact dusty systems, extended galaxies and mergers. However, this diversity is not mimicked in models, which disagree between each other on the nature of these dusty star-bursts. Since no model to date can accurately represent these galaxies, resolved spectroscopic studies of the obscured and unobscured stars and interstellar medium are now appear crucial to establish the origins of the discrepancy between the most intensely star-forming events in models and reality.

Understanding the relationship between the gas reservoir of galaxies and the change in their star formation rate with cosmic time is a major goal of galaxy evolution studies which (will) combine observations from ALMA with the SKA and its precursors to study the total gas content beyond the local Universe. I will present results from the highest redshift sample of galaxies with both spatially resolved molecular and atomic gas measurements to date, derived from the full CHILES 1000-hour HI survey with the Very Large Array, and targeted follow-up of HI detected galaxies with ALMA in CO (1-0) at z > 0.1. The HI detected parent sample span a range of stellar masses, star formation rates, and environments while we only directly detect CO in the highest stellar mass galaxies. The CO distributions are centrally concentrated, and spatially coincident with the highest stellar mass surface density and star forming regions of the galaxies. In addition we stack the CO non-detections, and we combine the total gas fraction and H_2-to-HI ratios as a function of redshift for CHILES with other samples. We find modest evidence for evolution in the gas reservoirs with redshift. This study provides the first resolved look at atomic and molecular hydrogen beyond the local Universe, and a view of what is to come with SKA, its precursors, and ALMA in its next decade. It also illustrates the short comings of our existing knowledge: direct measurements over a limited redshift range and limited to high stellar mass galaxies. I will discuss where we go from here.

Studying the stability of cold gas discs in star-forming galaxies (SFGs) is crucial to understand their formation and evolution. Indeed, local gravitational instabilities (LGI) are expected to 1) cause the fragmentation of gas discs into clumps, that can eventually collapse and form stars, and 2) trigger radial flows that can increase the gas turbulence. There is a lively debate on the increasing number of unstable discs at high redshift, as powerful facilities like JWST and ALMA are now available to investigate LGI in detail. My talk focuses on a key but often neglected property of galactic gas discs, this is their thickness, and the stabilising effect of their vertical stratification. We apply a new 3D stability criterion (Q_3D) that self-consistently takes into account the vertical structure of gas discs in the galactic potential, with significant improvement with respect to the classical 2D Toomre criterion (Q_2D) for infinitesimally-thin and self-gravitating discs. We apply this approach to 44 SFGs at 02. We find that fewer gas discs are unstable using Q_3D (0 vs 3 at z~0 and 7 vs 10 at z>0) and that the unstable regions are ~30% smaller on average. We estimate the expected properties of clumps formed in the unstable regions, offering testable predictions for simulations and for ALMA and JWST observations. Finally, I will discuss the possible implications of these results for the role of LGIs in regulating star formation and turbulence in galaxies.

The line emissions from the fine structure FIR lines, [CII]158μm and [OIII]88μm, are used to probe the physical conditions of the Interstellar Medium (ISM), as they trace cold neutral and warm ionized gas. Observations from galaxies at redshift z > 6 show an enhanced [OIII]88μm/[CII]158μm luminosity ratio when compared to their local analogs. While observations from the Atacama Large Millimeter Array (ALMA) has previously shown a deficit in [CII] at high redshifts, the exact origin of the enhanced luminosity ratio is still uncertain. Different analytical approaches has been used to try recreate the observed ratio, assuming core-collapse supernova yields and starburst galaxies, but so far no simulation has successfully managed to do this. Here we use the photoionization code CLOUDY (Ferland et al., 2017) to model the emission lines of a simulated galaxy merger at z = 6.5, to investigate the origin of the high [OIII]88μm/[CII]158μm ratios observed at high z. We compare different CLOUDY models by changing the abundances based on observed C/O abundance ratios from high-z galaxies and their local analogs. From our tests, using a lower C/O ratio compared to Solar, we see an enhancement in the total luminosity ratio for [OIII]88μm/[CII]158μm. In future work, we will test our C/O abundance assumptions against observational constraints, including new JWST data of high-z galaxies.

I will present the results of 60-hour multi-band ALMA observations of a lensed sub-L* galaxy at z=6. This object is a normal main-sequence star-forming galaxy belonging to the numerous population that should have contributed to the Reionization of the Universe. We sample the dust continuum emission from rest-frame 90 to 370um with 6 bands (+Herschel) and constrain the molecular gas content via CO J=1,7, [CI](2-1), [CII], and dust for 2 lensed images (magnification~20). The lensing effect allows for probing luminosity and mass regimes two orders of magnitude lower than what explored so far in the field. We complement ALMA with resolved photometry and spectroscopy with JWST/NIRCam+NIRSpec, providing key properties such as the gas-phase metallicity. We explore the efficiency at which the cold gas is converted into stars, cross-check 5 different gas mass tracers, and witness how quickly dust is produced in a <1e9 Mstar galaxy. The dust-to-stellar and dust-to-metals mass ratios are consistent with recent models and simulations including SNae/destruction/ISM growth, which we can finally test at crucially low-mass and luminosity regimes. We find dust temperatures consistent with those of brighter SMGs and LBGs. The wealth of data across wavelengths - and especially in the FIR/sub-mm, where the SED of high-redshift galaxies is typically sampled by one or two data points - makes this galaxy the perfect template to propel future multi-facility studies of normal objects at z>6.

Luminous infrared starburst galaxies in the early universe are thought to be the progenitors of massive quiescent galaxies observed at redshift z= 2-4. So far our knowledge about these extreme galaxies comes from mm- continuum and [CII] observations, revealing only the neutral and molecular medium in these galaxies. These observations reveal star formation rates of ~1000 Msun/year coming from a compact starburst (~2kpc). Due to their extreme faintness at rest-frame optical wavelengths, their stellar structure is mostly unknown; HST observations only reveals the most unobscured regions in these galaxies. As part of the European MIRI GTO program the z=4.05 dusty star forming galaxy GN20 was observed with MIRI in spectroscopy and imaging. The observations trace the rest frame near-infrared of the galaxy, revealing for the first time the ionized gas emission (Paschen alpha) and an unobscured view of the stellar distribution of the galaxy. We find that the star formation in GN20 is concentrated in massive clumps. We find large differences between the star formation rates derived from mm continuum observations and the ionized gas tracers, suggesting that extinctions are high in those galaxies and that most of the star formation is very deeply buried.

The Sunburst Arc is a gravitationally lensed, Lyman-Continuum leaking starburst galaxy at z=2.37. It is observed in at least 12 full or partial lensed images along an approximately circular arc. One unique clump, however, does not appear to have a counterpart anywhere else in the arc. It has been suggested that the clump could be a supernova, in which case we can expect delayed counter-images to show up in the future. An almost perfect Einstein ring-like structure of the Sunburst Arc and the continuous observation of the peculiar clump over 7 years raise questions about its transitory nature, leading to the proposition that it could be a highly magnified, extremely bright star such as a luminous blue variable (LBV). We investigate this mysterious clump using the high-resolution JWST/NIRSpec IFU data, which reveals unparalleled details that were beyond the reach of previous ground-based telescopes. The lack of He II line detection undermines the supernova hypothesis, whereas the presence of the robust [Ne III] line contradicts the 'LBV at the eruption phase' scenario, as erupting LBVs typically lack the temperature required to produce [Ne III] lines. The electron density derived from emission lines, along with the interesting behavior of Bowen fluorescent lines, provides us with further insights into the intrinsic character of the clump. In my talk, I will discuss our attempts to unveil the true identity of the distinctive clump by integrating all of these findings.

Stellar feedback plays an important role in the baryon cycle of galaxies. Understood as the balance between gas accretion, star-formation and material ejecta via galactic winds, the baryon cycle regulates galaxy growth at all epochs, while naturally explains many empirical findings such as the mass-metallicity relation (MZR). Until recently, measurements of feedback at early stages of galaxy evolution were unachievable, and simulations relied on recipes calibrated based upon other observables of the Early Universe, such as Luminosity Functions. To tackle this problem, in this work we search for signatures of outflows over a sample of 15 typically-luminous galaxies at z > 6 selected from HST deep-fields. Our high-resolution JWST/NIRSpec follow-up spectroscopy shows an incidence fraction of ionized outflows over 30 per cent. By modelling the broad-wings of the strongest rest-optical emission lines, we report outflow velocities and mass-outflow rates, and compare these values against calibrations commonly used in cosmological simulations. Together with the star-formation rate, our mass-outflow rates lead to one of the first measurements of the mass-loading factor at z > 7, a quantity which ultimately sets the slope and normalization of the MZR. Thanks to the high-resolving power of NIRSpec, we also estimate the typical velocity dispersion of ionised gas at z > 7 as well as dynamical masses, disentangling between rotation- and dispersion-dominated structures for one of the first times at the Dawn of cosmic star-formation.

The JWST discovery of the first z > 5 quiescent galaxies (JADES-GS-z7-01-QU and MACS0417-z5BBG) provides a unique opportunity to study the imprint of feedback processes on early galaxy evolution. We build a sample of 130 low-mass (log M*/Msun < 9.5) galaxies from the SERRA cosmological zoom-in simulations, revealing a feedback-regulated, bursty star formation history (SFH). The fraction of time spent in an active phase increases with the stellar mass from f_duty ~ 0.6 at logM*/Msun = 7.5 to ~ 0.99 at logM*/Msun > 9. On average, 30% of the galaxies are quiescent in the range 6 < z < 8.4; they become the dominant population at log M*/Msun < 8.3. However, none of these quiescent systems matches the Spectral Energy Distributions of the observed quiescent galaxies, unless their SFH is artificially truncated a few Myr after the main star formation peak. As supernova (SN) feedback can only act on a longer timescale (> 30 Myr), the observed quenching cannot be produced by SNe alone. To further investigate the impact of SNe feedback, we developed a simple physical model describing the minimum star formation rate (SFR) required to quench a galaxy at a given redshift, halo mass, and metallicity. The model reveals that not only does the quenching of JADES-GS-z7-01-QU and MACS0417-z5BBG appear to be too swift to be produced by SNe, but also that the SFR level sustained in the observed bursts cannot produce enough energy to completely suppress star formation. These findings imply that an additional faster physical mechanism must be at play, such as radiation-driven winds from young massive stars and/or an active galactic nucleus.

The details of how the large-scale structure influences the local star-forming regions, and viceversa, remains an open question in the theory of galaxy formation. With observations of massive star-forming galaxies at extremely high redshift, this problem has become all the more pressing. Multi-scale simulations are now required to pin down these observations on well-calibrated models of star formation. I will use VINTERGATAN, a high-resolution cosmological zoom-in simulation, to describe how star formation histories of Milky Way-like galaxies are driven by their cosmological context and the local properties of their ISM at different cosmic epochs. The assembly of a galactic disc in combination with galaxy mergers paints a picture at high redshift where order-of-magnitude changes in gas depetion timescales cause shifts from quiescence to starburstiness. At the level of individual clouds, the galactic disc enables gravitational interactions to tidally compress large volumes of gas into stars. Thus, the density and turbulent structures of the ISM react to the galactic environment and yield a star formation efficiency per free-fall time distribution that spans several orders of magnitude, but does not change significantly between 0.2 < z < 9. This indicates that turbulence is not the main driver of star formation variability. Our implemented efficiency model does not emphasise the role of turbulence in comparison to other models, yet the star forming porperties of galaxies in the Local Universe are reproduced, with outliers explained by rapid transitions between star formation modes. My results set the groundings towards understanding Milky Way-like progentoris at high redshift.

The first observations of the James Webb Space Telescope have unveiled an unexpectedly high abundance of early (z>10) luminous galaxy candidates. Several explanations of this high abundance have been proposed, including feedback-free starbursts, radiation-driven outflows removing dust from star-forming regions, or a top-heavy initial mass function (IMF). We have investigated how an IMF that becomes more top-heavy towards higher redshifts and lower gas-phase metallicities, as suggested by simulations of star-forming clouds (Chon et al. 2022), affects the properties of galaxies in the first billion years and the reionization of the intergalactic hydrogen gas. For this purpose, we have built a theoretical model that follows both galaxy evolution and reionization, and accounts for such an evolving IMF. Specifically, we have updated the ASTRAEUS (semi-numerical rAdiative tranSfer coupling of galaxy formaTion and Reionization in N-body dArk mattEr simUlationS) framework to account for an evolving, top-heavy IMF in supernova feedback, metal enrichment and ionising and UV radiation emissions. In this talk, I will discuss how assuming such an evolving IMF changes the predicted abundance of early bright galaxies and the properties of galaxies when compared to a constant Salpeter IMF. In particular, I will elaborate on how the evolving IMF changes the mass-to-light ratio of galaxies and why it leaves the galaxy star-forming main sequence mainly unchanged.

The James Webb Space Telescope (JWST) has opened up a new frontier in astrophysics, providing us with unprecedented amount of data on high-redshift galaxies. However, the true value of this data lies in our ability to properly interpret it. To prepare for this challenge, we have developed the First Light and Reionisation Epoch Simulations (FLARES), targeting massive overdensities in the EoR, to gain insight into the plethora of galaxies detected by JWST as well as Euclid. In this talk, I will introduce the FLARES model and describe our forward modelling approach for generating the galaxy photometry. I will highlight the model's numerous successes, including predictions for the evolution of the UV-slope, line luminosity function and equivalent widths, and areas for improvement. Specifically in this talk, I will explore the impact of the complex star-dust geometry on nebular line ratios. The clumpy nature of these nascent galaxies can complicate the interpretation of the data, and I will discuss the reliability of Balmer decrements for dust corrections. Furthermore, I will emphasise the need for caution when interpreting data from line ratios, as conclusions drawn from this data should be treated with care unless the effective attenuation curve of the galaxies are known a-priori.

I model the interstellar dust content of the reionization era with a suite of cosmological, fluid-dynamical simulations of galaxies with stellar masses ranging from $\sim 10^5 - 10^9 M_{\odot}$ in the first $1.2$ billion years of the universe. I use a post-processing method that accounts for dust creation and destruction processes, allowing me to systematically vary the parameters of these processes to test whether dust-dependent observable quantities of galaxies at these epochs could be useful for placing constraints on dust physics. I then forward model observable properties of these galaxies to compare to existing data. I find that I am unable to simultaneously match existing observational constraints with any one set of model parameters. Specifically, the models which predict the largest dust masses $D/Z \gtrsim 0.1$ at $z = 5$ -- because of high assumed production yields and/or efficient growth via accretion in the interstellar medium -- are preferred by constraints on total dust mass and infrared luminosities, but these models produce far too much extinction in the ultraviolet, preventing them from matching observations of $\beta_{\rm UV}$. To investigate this discrepancy, I analyze the relative spatial distribution of stars and dust as probed by infrared and ultraviolet emission, which appear to exhibit overly symmetric morphologies compared to existing data, likely due to the limitations of the stellar feedback model used in the simulations. My results indicate that the observable properties of the dust distribution in high redshift galaxies are a particularly strong test of stellar feedback and therefore merit further theoretical modeling efforts, which I will outline at the end of my talk.