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Stars form and live in hierarchical structures such as star clusters on the smallest scales up to groups of galaxies on the largest scales. The role of young and massive star clusters in the evolution of the host galaxies is one of the most active fields of research in modern astronomy. The most massive stars are able to inject into the interstellar medium large amounts of energy and momentum through their radiation, their stellar winds and ultimately by exploding as supernovae. Also because of their clustered configuration, massive stars have an impact on the host galaxy with their energetic output, which is often referred to as a whole with the term feedback. Feedback processes are invoked by numerical simulations in order to reproduce galaxies with realistic properties. In the most extreme scenarios stellar feedback can launch galactic winds that may suppress the star formation and mix the gas content not only within the galaxy but also with the intergalactic medium. In this thesis I describe the scientific background of my doctoral studies and present my research project on the local starburst galaxy Haro 11 (z=0.021). Together with my supervisors, we use multi-band photometry from the Hubble Space Telescope for resolving the nuclear starburst into tens of star clusters with masses between 105 and 107 solar masses and ages younger than 20 Myr. We observe that star-formation is propagating across the galaxy from the eastern side to the western side. Additionally, we model aperture-matched ultraviolet and optical spectroscopy and derive the physical properties of the stellar populations in the three starburst knots (A, B, C). This also allow us to estimate the following stellar feedback quantities: photo-ionisation rate, energy of stellar winds and supernovae, and their power. We compare the above measurements with the information on the kinematics of the ionised gas within the three knots, extracted from optical spectroscopy. Combining our analysis with previous works, we find a multi-phase medium with maximum outflow velocities up to 400 km/s. The three knots have at different times all hosted powerful outflows, whose energetics is consistent with the quantified stellar feedback. This indicates that the stellar winds and SN explosions have driven the detected outflows. Our work is important not only for shedding light on the physics of feedback, but also because it combines and compares different methodologies (i.e. photometry and spectroscopy, ultraviolet and optical wavelengths). I conclude with an outlook on the upcoming projects of my doctoral studies, which will exploit the methodology that I developed during the first two years of my PhD and extend it to a whole sample of star-forming nearby galaxies. Such projects also connect with the science that will be enabled by the future astronomical observatories. The work on Haro 11 is the subject of a paper recently submitted to the journal MNRAS. A copy of the paper can be found at the end of this thesis.

The PHANGS collaboration has been building a reference data set for the multiscale, multiphase study of star formation and the interstellar medium (ISM) in nearby galaxies. With the successful launch and commissioning of JWST, we can now obtain high-resolution infrared imaging to probe the youngest stellar populations and dust emission on the scales of star clusters and molecular clouds (∼5–50 pc). In Cycle 1, PHANGS is conducting an eight-band imaging survey from 2 to 21 μm of 19 nearby spiral galaxies. Optical integral field spectroscopy, CO(2–1) mapping, and UV-optical imaging for all 19 galaxies have been obtained through large programs with ALMA, VLT-MUSE, and Hubble. PHANGS–JWST enables a full inventory of star formation, accurate measurement of the mass and age of star clusters, identification of the youngest embedded stellar populations, and characterization of the physical state of small dust grains. When combined with Hubble catalogs of ∼10,000 star clusters, MUSE spectroscopic mapping of ∼20,000 H II regions, and ∼12,000 ALMA-identified molecular clouds, it becomes possible to measure the timescales and efficiencies of the earliest phases of star formation and feedback, build an empirical model of the dependence of small dust grain properties on local ISM conditions, and test our understanding of how dust-reprocessed starlight traces star formation activity, all across a diversity of galactic environments. Here we describe the PHANGS–JWST Treasury survey, present the remarkable imaging obtained in the first few months of science operations, and provide context for the initial results presented in the first series of PHANGS–JWST publications., La lista completa de autores que integran el documento puede consultarse en el archivo., Instituto de Astrofísica de La Plata

The arrival of new telescopes and the improvement of the techniques of numerical simulations allow important progress to be made in the understanding of the mechanisms of star formation in the Universe. It's a major challenge, because star formation is determined by events at very large scales (a galaxy) and very small scales (the kernel of the atoms) and all the scales in the middle. This thesis aims at determining what are the main processes that may explain the star formation rate, that is the speed at which the gas from galaxies is converted into stars. In this work I tackle several scales and environments, but I focus on disk galaxies with a gas mass equal or up to fifteen times the gas mass in the Milky Way. The heart of the thesis is the numerical simulation of kiloparsec regions of galactic disk to study the role of stellar feedback, magnetic field and large scale turbulence on the star formation rate. The latter is found to be crucial in gas-rich galaxies and that prompted me to study more precisely the role of turbulence, in particular how it is generated in the galactic disk., Le développement de nouveaux télescopes spatiaux et terrestres, ainsi que l'amélioration des capacités et des techniques de simulations promettent des progrès important dans la compréhension des mécanismes de la formation stellaire. C'est un défi colossal, car la formation stellaire est un processus qui se joue à la fois à l'échelle très vaste d'une galaxie et à l'échelle plus petite d'un système stellaire (voire de l'atome), sans oublier toutes les échelles intermédiaires. Le travail de la thèse que vous vous apprêtez à lire a pour objectif principal de comprendre quels sont les processus physiques majeurs qui expliquent le taux de formation des étoiles, c'est-à-dire la vitesse à laquelle le gaz qui compose en partie les galaxies s'effondre pour former des étoiles. C'est un travail multifacette et multiéchelle, mais qui se focalise sur les disques galactiques, de masse similaire ou jusqu'à quinze fois plus massives que la nôtre. Le cœur de la thèse porte sur la régulation du taux de formation d'étoiles à l'échelle du kiloparsec. À l'aide de simulations numériques, il s'agit de voir quels sont les processus physiques qui permettent d'expliquer les relations observées avec la quantité de gaz, en quantifiant notamment l'importance de la rétroaction stellaire et du champ magnétique. J'y mets en évidence le rôle essentiel joué par la turbulence à grande échelle dans les galaxies riches en gaz. Cela m'a poussé à l'étudier plus précisément, notamment pour comprendre sa génération dans le disque galactique.

Feedback from stars is essential for the formation and evolution of galaxies. It is an energy source that drives gas motions and chemically enriches the galaxy by supplying metals. Without this stellar feedback, numerical galaxy simulations result in galaxies with little resemblance to those observed in our Universe. Modern galaxy simulations frequently reach a mass resolution of a few tens of solar masses. Such high-resolution warrants models incorporating individual stars. These models enable a detailed treatment of when and where stars inject feedback. In this thesis, I present such a model and provide a series of papers exploring physical mechanisms unlocked by this model.In paper I, we investigate how runaway stars affect the galactic winds driven by stellar feedback in Milky Way-like galaxies. Massive runaway stars can venture to places where these short-lived stars are otherwise not found (e.g., between spiral arms). In these regions of diffuse gas, supernovae can efficiently incorporate energy into large volumes of gas, thereby boosting the gas outflow rate of the galaxy. Furthermore, the star formation rate is not significantly affected since parts of the feedback budget move away from star-forming gas. The result is a ten-fold boost in the mass loading factor.Paper II is a follow-up investigation of a surprising signal of star formation in spiral galaxies with runaway stars, found in Paper I. The signal is produced by the rapid migration of runaway stars to the galaxy's outskirts. Via direct comparison to observational data, we find that this explains faint far-ultraviolet radiation detected outside the optical radius of nearby spiral galaxies. This radiation manifests as a trend in the star formation relation with a slope similar to one produced by runaway stars escaping to these regions.In paper III, the star-by-star model is upgraded with a more advanced model for feedback and runaway stars. We showcase this model with a suit of simulations of isolated dwarf galaxies, testing a range of parameters for the natal velocity model of individual stars responsible for incorporating runaway stars. In stark contrast to the Milky Way-like galaxy, we find runaway stars play little to no role in determining outflows in dwarf galaxies. We discuss several possible reasons for the different effects in small and large galaxies.

We quantified the effects of stellar feedback in RCW 49 by determining the physical conditions in different regions using the [CII] 158 $\mu$m and [OI] 63 $\mu$m observations from SOFIA, the $^{12}$CO (3-2) observations from APEX and the H$_2$ line observations from Spitzer telescopes. Large maps of RCW 49 were observed with the SOFIA and APEX telescopes, while the Spitzer observations were only available towards three small areas. From our qualitative analysis, we found that the H$_2$ 0-0 S(2) emission line probes denser gas compared to the H$_2$ 0-0 S(1) line. In four regions ("northern cloud", "pillar", "ridge", and "shell"), we compared our observations with the updated PDR Toolbox models and derived the integrated far-ultraviolet flux between 6-13.6 eV ($G_{\rm 0}$), H nucleus density ($n$), temperatures and pressures. We found the ridge to have the highest $G_{\rm 0}$ (2.4 $\times$ 10$^3$ Habing units), while the northern cloud has the lowest $G_{\rm 0}$ (5 $\times$ 10$^2$ Habing units). This is a direct consequence of the location of these regions with respect to the Wd2 cluster. The ridge also has a high density (6.4 $\times$ 10$^3$ cm$^{-3}$), which is consistent with its ongoing star formation. Among the Spitzer positions, we found the one closest to the Wd2 cluster to be the densest, suggesting an early phase of star formation. Furthermore, the Spitzer position that overlaps with the shell was found to have the highest $G_{\rm 0}$ and we expect this to be a result of its proximity to an O9V star., Comment: 18 pages, 12 figures

Many stages of the stellar life cycle release energy and momentum into the surrounding interstellar medium within a galaxy. This feedback can have profound effects on the host galaxy. This thesis investigates the role of stellar feedback in star-forming galaxies in the local Universe through multiwavelength observations of gas kinematics. First, I study extraplanar diffuse ionized gas (eDIG) which is thought to be produced by gas ejected from the midplane by repeated supernova explosions. By comparing molecular and ionized gas rotation curves derived from a sub-sample of intermediate inclination star-forming galaxies from the EDGE-CALIFA Survey, I find that ~75% of my sample galaxies have smaller ionized gas rotation velocities than the molecular gas. I suggest and show that the lower ionized gas rotation velocity can be attributed to a significant contribution from eDIG in a thick disk which rotates more slowly than gas in the midplane. As a direct follow up to this study, I use a sample of edge-on galaxies selected from the CALIFA survey to directly investigate the prevalence, properties, and kinematics of eDIG. I find that 60% of these galaxies show a decrease in the ionized gas rotation velocity as a function of height above the midplane. The ionization of the eDIG is dominated by star-forming complexes. These studies reveal the pervasiveness and importance of this phase in local star-forming galaxies. Next, I study stellar feedback in the extreme environment of the nuclear starburst in the nearby galaxy NGC 253. Using ALMA observations with 0.5 pc resolution, I detect blueshifted absorption and redshifted emission (P-Cygni profiles) in multiple spectral lines towards three of the super star clusters (SSCs). This is direct evidence for outflows of dense molecular gas from these SSCs. Through a comparison of the outflow properties with predictions from simulations, I find that the outflows are most likely powered by dust-reprocessed radiation pressure or O-star stellar winds. The observed outflows will have very substantial effects on the clusters' evolution. Finally, I find that the arrangement of the SSCs may be morpho-kinematically consistent with a ring or crossing streams from the larger-scale gas flows which fuel the starburst.

Measurements of the rotation curves of dwarf galaxies are often interpreted as requiring a constant density core at the centre, at odds with the "cuspy" inner profiles predicted by $N$-body simulations of cold dark matter (CDM) haloes. It has been suggested that this conflict could be resolved by fluctuations in the inner gravitational potential caused by the periodic removal of gas following bursts of star formation. Earlier work has suggested that core formation requires a bursty and extended star formation history (SFH). Here we investigate the structure of CDM haloes of dwarf galaxies ($M_{{\rm DM}} \sim 10^9-5\times10^{10}\,{\rm M}_\odot$) formed in the APOSTLE ('A Project of Simulating the Local Environment') and AURIGA cosmological hydrodynamic simulations. Our simulations have comparable or better resolution than others that make cores ($M_{{\rm gas}} \sim 10^4\,{\rm M}_\odot$, gravitational softening $\sim 150$ pc). Yet, we do not find evidence of core formation at {\it any} mass or any correlation between the inner slope of the DM density profile and temporal variations in the SFH. APOSTLE and AURIGA dwarfs display a similar diversity in their cumulative SFHs to available data for Local Group dwarfs. Dwarfs in both simulations are DM-dominated on all resolved scales at all times, likely limiting the ability of gas outflows to alter significantly the central density profiles of their haloes. We conclude that recurrent bursts of star formation are not sufficient to cause the formation of cores, and that other conditions must also be met for baryons to be able to modify the central DM cusp., Comment: 15 pages, 9 figures, 2 tables. Updated version accepted for publication in MNRAS

The origin and evolution of galaxies are closely tied to the cyclic feedback processes between stars and the interstellar medium (ISM). The aim of this thesis is to explore characteristics of the ISM, on global (galactic) scales down to sub-cloud (pc) scales. We explore new methods to investigate the ISM in external galaxies, through radio recombination line observations, and develop the tools and strategies needed to process new low-frequency observations with the Low Frequency Array. We also infer the presence of massive stars and characterize their properties and influence on the ISM. This thesis addresses the questions:- How does low-density ionized gas affect the evolution of the massive, galactic star-forming region, Cygnus X? Are the same fingerprints present in surveys of low-density ionized gas in our Galaxy?- What are the properties of star formation (star clusters) in the central starburst of the galaxy NGC 4945?- Can the ISM be explored outside of the local universe through radio recombination line observations? What are the ISM properties of a dwarf-like galaxy at z=1.1?- What techniques are best suited to detect faint radio recombination lines (at a previously unknown redshift) in extragalactic sources?

We unveil the stellar wind driven shell of the luminous massive star-forming region of RCW 49 using SOFIA FEEDBACK observations of the [CII] 158 $\mu$m line. The complementary dataset of the $^{12}$CO and $^{13}$CO J = 3 - 2 transitions is observed by the APEX telescope and probes the dense gas toward RCW 49. Using the spatial and spectral resolution provided by the SOFIA and APEX telescopes, we disentangle the shell from a complex set of individual components of gas centered around RCW 49. We find that the shell of radius ~ 6 pc is expanding at a velocity of 13 km s$^{-1}$ toward the observer. Comparing our observed data with the ancillary data at X-Ray, infrared, sub-millimeter and radio wavelengths, we investigate the morphology of the region. The shell has a well defined eastern arc, while the western side is blown open and is venting plasma further into the west. Though the stellar cluster, which is ~ 2 Myr old gave rise to the shell, it only gained momentum relatively recently as we calculate the shell's expansion lifetime ~ 0.27 Myr, making the Wolf-Rayet star WR20a a likely candidate responsible for the shell's re-acceleration., Comment: 31 pages, 17 figures

The feedback from young stars (i.e. pre-supernova) is thought to play a crucial role in molecular cloud destruction. In this paper, we assess the feedback mechanisms acting within a sample of 5810 HII regions identified from the PHANGS-MUSE survey of 19 nearby ($ 1$, and expanding, yet there is a small sample of compact HII regions with $P_\mathrm{tot,max}/P_\mathrm{de} < 1$ ($\sim$1% of the sample). These mostly reside in galaxy centres ($R_\mathrm{gal}, Comment: 28 pages total, 16+3 figures, and 3 tables. Accepted for publication in MNRAS

Feedback from young, massive stars plays an essential role in the self-regulation of star formation in galaxies, and in shaping the galaxies' global properties. This phenomenon originates at small scales, surrounding the stars, but has been observed to be effective up to galactic-wide scales. The exact mechanism which allows the ionising radiation to escape the star-forming regions (HII regions), initially still embedded in their natal molecular hydrogen gas, is still unknown. Constraining the escape of ionising photons from HII regions is also relevant in order to explain the origin of the diffuse ionised gas (DIG) that is observed to contribute up to 50% to the Ha luminosity of nearby galaxies. I present the results of the study of stellar feedback in two nearby galaxies (NGC 7793 and M83), at spatial scales that critically connect the sources of ionisation with their immediate surroundings. We determine the fraction of DIG and study its properties and origin. We find that in NGC 7793 ionising sources located in the DIG are producing a sufficient amount of hydrogen-ionising (LyC) photons to explain the diffuse gas emission. In M83, on the other hand, the DIG is ionised by a mixed contribution of photoionisation and shocks. We investigate the link between LyC leakage from HII regions and their stellar and gas properties. We find that the age spread of the stellar population in the region does not seem to imply a higher leakage. Also the ionisation structure of the regions (e.g. the presence of "channels" that are transparent to the LyC photons) appears to be uncorrelated with escape in our sample. In M83, we also study the relative importance of different types of stellar feedback. We find that the pressure exerted by the ionised gas is always dominant over the direct radiation pressure. When the total HII region pressure is compared to the environmental pressure, we observe that regions near the galactic centre are in equilibrium with the surroundings, whereas regions in the disk are overpressured and are therefore expanding. We also find that changes in the local environmental conditions are the dominant factor in setting the ionised gas pressure, and that the pressure terms are linked to the physical properties (age and mass) of the young star clusters powering the regions. In the near future, observations from the James Webb Space Telescope will allow us to study the most embedded star-forming regions with a resolution comparable to the present one. At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Submitted, Paper 5: Manuscript.

We present a new implementation of the GAlaxy Evolution and Assembly (GAEA) semi-analytic model, that features an improved modelling of the process of cold gas accretion onto supermassive black hole (SMBHs), derived from both analytic arguments and high-resolution simulations. We consider different scenarios for the loss of angular momentum required for the available cold gas to be accreted onto the central SMBHs, and we compare different combinations of triggering mechanisms, including galaxy mergers and disc instabilities in star forming discs. We compare our predictions with the luminosity function (LF) observed for Active Galactic Nuclei (AGN) and we confirm that a non-instantaneous accretion timescale (either in the form of a low-angular momentum reservoir or as an assumed light curve evolution) is needed in order to reproduce the measured evolution of the AGN-LF and the so-called AGN-downsizing trend. Moreover, we also study the impact of AGN feedback, in the form of AGN-driven outflows, on the SF properties of model galaxies, using prescriptions derived both from empirical studies or from numerical experiments. We show that AGN-driven outflows are effective in suppressing the residual star formation rate in massive galaxies ($> 10^{11} M_\odot$) without changing their overall assembly history. These winds also affect the SFR of lower mass galaxies, resulting in a too large fraction of passive galaxies at $< 10^{10} M_\odot$. Finally, we study the Eddington ratio distribution as a function of SMBH mass, showing that only objects more massive than $10^8 M_\odot$ are already in a self-regulated state as inferred from observations., 19 pages, 12 figures, 1 table, replaced with MNRAS accepted version

The PHANGS program is building the first dataset to enable the multi-phase, multi-scale study of star formation across the nearby spiral galaxy population. This effort is enabled by large survey programs with ALMA, VLT/MUSE, and HST, with which we have obtained CO(2-1) imaging, optical spectroscopic mapping, and high resolution UV-optical imaging, respectively. Here, we present PHANGS-HST, which has obtained five band NUV-U-B-V-I imaging of the disks of 38 spiral galaxies at distances of 4-23 Mpc, and parallel V and I band imaging of their halos, to provide a census of tens of thousands of compact star clusters, and multi-scale stellar associations. The combination of HST, ALMA, and VLT/MUSE observations will yield an unprecedented joint catalog of the observed and physical properties of ~100,000 star clusters, associations, HII regions, and molecular clouds. With these basic units of star formation, PHANGS will systematically chart the evolutionary cycling between gas and stars, across a diversity of galactic environments found in nearby galaxies. We discuss the design of the PHANGS-HST survey, and provide an overview of the HST data processing pipeline and first results, highlighting new methods for selecting star cluster candidates, morphological classification of candidates with convolutional neural networks, and identification of stellar associations over a range of physical scales with a watershed algorithm. We describe the cross-observatory imaging, catalogs, and software products to be released. These high-level science products will seed a broad range of investigations, in particular, the study of embedded stellar populations and dust with JWST, for which a PHANGS Cycle 1 Treasury program to obtain eight band 2-21 $\mu$m imaging has been approved., Comment: ApJS in press. Accepted draft includes moderate revision and updates to the originally posted submitted version. 27 pages, 11 figures, 1 table. Data products released at https://archive.stsci.edu/hlsp/phangs-hst/

It is a major open question which physical processes stop the accretion of gas onto giant molecular clouds (GMCs) and limit the efficiency at which gas is converted into stars within these GMCs. While feedback from supernova explosions has been the popular feedback mechanism included in simulations of galaxy formation and evolution, `early' feedback mechanisms such as stellar winds, photoionisation and radiation pressure are expected to play an important role in dispersing the gas after the onset of star formation. These feedback processes typically take place on small scales ($\sim 10-100$ pc) and their effects have therefore been difficult to constrain in environments other than the Milky Way. We apply a novel statistical method to $\sim 1$" resolution maps of CO and Ha emission across a sample of nine nearby disc galaxies, in order to measure the time over which GMCs are dispersed by feedback from young, high-mass stars, as a function of the galactic environment. We find that GMCs are typically dispersed within $\sim$ 3 Myr after the emergence of unembedded high-mass stars, showing no significant trend with galactocentric radius. Comparison with analytical predictions demonstrates that, independently of the environment, early feedback mechanisms (particularly photoionisation and stellar winds) play a crucial role in dispersing GMCs and limiting their star formation efficiency in nearby galaxies. Finally, we show that the efficiency at which the energy injected by these early feedback mechanisms couples with the parent GMC is relatively low (a few tens of per cent), such that the vast majority of momentum and energy emitted by the young stellar populations escapes the parent GMC., Comment: 17 pages, 8 figures, 3 tables; submitted to MNRAS (October 23, 2020)

Reproducing the observed quenched fraction of satellite galaxies has been a long-standing issue for galaxy formation models. We modify the treatment of environmental effects in our state-of-the-art GAlaxy Evolution and Assembly (GAEA) semi-analytic model to improve our modelling of satellite galaxies. Specifically, we implement gradual stripping of hot gas, ram-pressure stripping of cold gas, and an updated algorithm to account for angular momentum exchanges between the gaseous and stellar disc components of model galaxies. Our updated model is in good agreement with the observed quenched fractions for central and satellite galaxies, and their dependencies on stellar mass and halo mass. We also find consistency between model predictions and observational estimates of quenching time for satellite galaxies; HI, H$_2$ fractions of central galaxies; and deficiencies of HI, H$_2$, SFR of galaxies in cluster haloes. Analysing results from our updated model, we find that the dominant quenching mechanisms are hot-gas stripping for low-mass galaxies, and AGN feedback for massive galaxies. The Ram-pressure stripping of cold gas has a limited impact on the predicted quenched fractions, nonetheless, is needed to reproduce the observed HI deficiencies., Comment: 21 pages, 14 figures; accepted for publication in MNRAS

Young massive clusters play an important role in the evolution of their host galaxies, and feedback from the high-mass stars in these clusters can have profound effects on the surrounding interstellar medium. The nuclear starburst in the nearby galaxy NGC253 at a distance of 3.5 Mpc is a key laboratory in which to study star formation in an extreme environment. Previous high resolution (1.9 pc) dust continuum observations from ALMA discovered 14 compact, massive super star clusters (SSCs) still in formation. We present here ALMA data at 350 GHz with 28 milliarcsecond (0.5 pc) resolution. We detect blueshifted absorption and redshifted emission (P-Cygni profiles) towards three of these SSCs in multiple lines, including CS 7$-$6 and H$^{13}$CN 4$-$3, which represents direct evidence for previously unobserved outflows. The mass contained in these outflows is a significant fraction of the cluster gas masses, which suggests we are witnessing a short but important phase. Further evidence of this is the finding of a molecular shell around the only SSC visible at near-IR wavelengths. We model the P-Cygni line profiles to constrain the outflow geometry, finding that the outflows must be nearly spherical. Through a comparison of the outflow properties with predictions from simulations, we find that none of the available mechanisms completely explains the observations, although dust-reprocessed radiation pressure and O star stellar winds are the most likely candidates. The observed outflows will have a very substantial effect on the clusters' evolution and star formation efficiency., Comment: Accepted to ApJ

In spite of decades of theoretical efforts, the physical origin of the stellar initial mass function (IMF) is still debated. Particularly crucial is the question of what sets the peak of the distribution. To investigate this issue we perform high resolution numerical simulations with radiative feedback exploring in particular the role of the stellar and accretion luminosities. We also perform simulations with a simple effective equation of state (eos) and we investigate 1000 solar mass clumps having respectively 0.1 and 0.4 pc of initial radii. We found that most runs, both with radiative transfer or an eos, present similar mass spectra with a peak broadly located around 0.3-0.5 M$_\odot$ and a powerlaw-like mass distribution at higher masses. However, when accretion luminosity is accounted for, the resulting mass spectrum of the most compact clump tends to be moderately top-heavy. The effect remains limited for the less compact one, which overall remains colder. Our results support the idea that rather than the radiative stellar feedback, this is the transition from the isothermal to the adiabatic regime, which occurs at a gas density of about 10$^{10}$ cm$^{-3}$, that is responsible for setting the peak of the initial mass function. This stems for the fact that $i)$ extremely compact clumps for which the accretion luminosity has a significant influence are very rare and $ii)$ because of the luminosity problem, which indicates that the effective accretion luminosity is likely weaker than expected., Comment: accepted for publication in ApJ

A methodology for numerical magnetohydrodynamics simulations of star cluster formation, accounting for all mechanisms of stellar feedback from massive stars, is developed and used to address a range of problems regarding the formation of stars and star clusters in giant molecular clouds (GMCs). These studies culminate in a new theoretical framework that connects properties of GMCs to those of the star clusters that form in them. The simulation methodology is established and tested, and the problem of the star formation efficiency (SFE) of molecular clouds is addressed. It is found that SFE is set by the balance of feedback and gravity, with very weak dependence upon other factors. A simple dimensional scaling law with cloud surface density emerges from the complex interplay of different feedback physics. Parameter space is found where feedback must fail, and the SFE is high, and the implications of this prediction are explored. The star clusters formed in the simulations are found to resemble observed young, massive star clusters in the form of their surface brightness profiles, leading to the hypothesis that this structure is a result of the star formation process. It is shown that the shallow, power-law density profiles characteristic of young clusters is predicted by the hierarchical star formation scenario. It is shown that the SFE law, when coupled to an analytic cloud collapse model, predicts that gas should be exhausted by highly-efficient star formation at a stellar surface density of ∼ 105 − 106 Msun pc-2, consistent with the maximum observed. A new suite of simulations is developed to specifically model GMCs in the Milky Way. It is found that the picture of feedback-disrupted star formation is able to account for both the normalization and the scatter in the measured SFE of GMCs in the Milky Way, the first theory to do so. The uncertainty in the simulated SFE due to the choice of feedback prescription is quantified, by running a controlled methods study of several different prescriptions in the literature. In the cloud model simulated, the choice of prescription affects the simulated SFE at the factor of ∼ 3 level, explaining discrepancies in the literature and identifying the small-scale details of massive star formation as the main uncertainty in cluster formation simulations. Finally, the simulation suite is extended to model massive GMCs in local spiral galaxies, and to simulate 10 random realizations at each point in parameter space, mapping out the stochastic nature of star cluster formation in GMCs. A model is calibrated to the simulation results, taking the cloud bulk properties as input parameters, and predicting the detailed properties of the star clusters formed in it. A star cluster catalogue is synthesized from observed GMCs in M83, and good agreement is found with observed star cluster properties, including the fraction of stars in bound clusters, the maximum cluster mass, and the distribution of cluster sizes.

A suite of idealised, global, gravitationally unstable, star-forming galactic disc simulations with 2 pc spatial resolution, performed with the adaptive mesh refinement code {\sc ramses} is used in this paper to predict the emergent effects of supernova feedback. The simulations include a simplified prescriptions for formation of single stellar populations of mass $\sim 100 \, M_{\odot}$, radiative cooling, photoelectric heating, an external gravitational potential for a dark matter halo and an old stellar disc, self-gravity, and a novel implementation of supernova feedback. The results of these simulations show that gravitationally unstable discs can generate violent supersonic winds with mass loading factors $\eta \gtrsim 10$, followed by a galactic fountain phase. These violent winds are generated by highly clustered supernovae exploding in dense environments created by gravitational instability, and they are not produced in simulation without self-gravity. The violent winds significantly perturb the vertical structure of the disc, which is later re-established during the galactic fountain phase. Gas resettles into a quasi-steady, highly turbulent disc with volume-weighted velocity dispersion $\sigma > 50 \, {\rm km/s}$. The new configuration drives weaker galactic winds with mass loading factor $\eta \leq 0.1$. The whole cycle takes place in $\leq 10$ dynamical times. Such high time variability needs to be taken into account when interpreting observations of galactic winds from starburst and post-starburst galaxies., Comment: 17 pages, accepted by MNRAS

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of (CO)-C-12(1-0) and (CO)-C-12(2-1) in the central 40 '' (680 pc) of the nuclear starburst galaxy NGC 253, including its molecular outflow. We measure the ratio of brightness temperature for CO(2-1)/CO(1-0), r(21), in the central starburst and outflow-related features. We discuss how r(21) can be used to constrain the optical depth of the CO emission, which impacts the inferred mass of the outflow and consequently the molecular mass outflow rate. We find r(21) less than or similar to 1 throughout, consistent with a majority of the CO emission being optically thick in the outflow, as it is in the starburst. This suggests that the molecular outflow mass is 3-6 times larger than the lower limit reported for optically thin CO emission from warm molecular gas. The implied molecular mass outflow rate is 25-50 M-circle dot yr(-1), assuming that the conversion factor for the outflowing gas is similar to our best estimates for the bulk of the starburst. This is a factor of 9-19 times larger than the star formation rate in NGC 253. We see tentative evidence for an extended, diffuse CO(2-1) component.

Remarkable progress has been made over the last few decades in furthering our understanding of the growth of cosmic structure. Nonetheless, there remains a great deal of uncertainty regarding the precise details of the complex baryonic physics that regulate galaxy formation. Any theory of star formation in galaxies must encompass the radiative cooling of gas into dark matter haloes, the formation of a turbulent, multiphase interstellar medium (ISM), the efficiency with which molecular gas is able to collapse into cores and ultimately stars, and the subsequent interaction of those stars with the gas through ionizing radiation, winds and supernova (SN) explosions. Given the highly non-linear nature of the problem, numerical simulations provide an invaluable tool with which to study galaxy formation. Yet, even with contemporary computational resources, the inherently large dynamical range of spatial scales that must be tackled makes the development of such models extremely challenging, inevitably leading to the adoption of `subgrid' approximations at some scale. In this thesis, I explore new methods of incorporating the physics of star formation and stellar feedback into high resolution hydrodynamic simulations of galaxies. I first describe a new implementation of star formation and SN feedback that I have developed for the state-of-the-art moving mesh code Arepo. I carry out a detailed study into various classes of subgrid SN feedback schemes commonly adopted in the literature, including injections of thermal and/or kinetic energy, two parametrizations of delayed cooling feedback and a `mechanical' feedback scheme that injects the appropriate amount of momentum depending on the relevant scale of the SN remnant (SNR) resolved. All schemes make use of individually time-resolved SN events. Adopting isolated disk galaxy setups at different resolutions, with the highest resolution runs reasonably resolving the Sedov-Taylor phase of the SNR, I demonstrate that the mechanical scheme is the only physically well-posed method of those examined, is efficient at suppressing star formation, agrees well with the Kennicutt-Schmidt relation and leads to converged star formation rates and galaxy morphologies with increasing resolution without fine tuning any parameters. However, I find that it is difficult to produce outflows with high enough mass loading factors at all but the highest resolution. I discuss the various possible solutions to this effect, including improved modelling of star formation. Moving on to a more self-consistent setup, I carry out a suite of cosmological zoom-in simulations of low mass haloes at very high resolution, performed to z = 4, to investigate the ability of SN feedback models to produce realistic galaxies. The haloes are selected in a variety of environments, ranging from voids to crowded locations. In the majority of cases, SN feedback alone has little impact at early times even in low mass haloes ($\sim10^{10}\,\mathrm{M_\odot}$ at z = 0). This appears to be due largely to the build up of very dense gas prior to SN events, suggesting that other mechanisms (such as other stellar feedback processes) are required to regulate ISM properties before SNe occur. The effectiveness of the feedback also appears to be strongly dependent on the merger history of the halo. Finally, I describe a new scheme to drive turbulence in isolated galaxy setups. The turbulent structure of the ISM very likely regulates star formation efficiencies on small scales, as well as affecting the clustering of SNe. The large range of potential drivers of ISM turbulence are not fully understood and are, in any case, unlikely to arise ab initio in a whole galaxy simulation. I therefore neglect these details and adopt a highly idealised approach, artificially driving turbulence to produce an ISM structure of my choice. This enables me to study the effects of a given level of ISM turbulence on global galaxy properties, such as the fragmentation scale of the disk and the impact on SN feedback efficiencies. I demonstrate this technique in the context of simulations of isolated dwarfs, finding that moderate levels of turbulent driving in combination with SN feedback can produce a steady-state of star formation rates and global galaxy properties, rather than the extremely violent SN feedback that is produced by a rapidly fragmenting disk.

Stellar-feedback-driven outflows are thought to be a crucial process in galaxy formation, but our physical understanding of them is still rudimentary. I will present an analytical model for how stellar feedback simultaneously regulates star formation and drives outflows. One interesting prediction of the model is that although stellar feedback can efficiently generate outflows in all galaxies at high redshift, it is unable to drive outflows in massive galaxies below z ~ 1. This prediction agrees with the results of state-of-the-art cosmological zoom simulations that include explicit stellar feedback. This model, combined with the aforementioned simulations, suggests that high-redshift galaxies are highly turbulent, disordered and characterized by strong starbursts and subsequent violent outflows, and the suppression of outflows in massive galaxies at z ~ 1 is crucial for the emergence of well-ordered, steadily star-forming disk galaxies.

I discuss how hydrodynamical simulations enable exploring nonlinear couplings amongst different physical processes in galaxy formation. I focus on three topics: the effect of baryonic processes on dark matter halos, how stellar feedback leads to 'self-regulated' galaxy formation, and the interplay amongst stellar feedback, magnetic fields, and cosmic rays. Invited talk presented at the conference Galaxy Evolution Across Time, 12-16 June 2017, Paris, France, The Flatiron Institute is supported by the Simons Foundation.

The injection of energy and momentum into the interstellar medium by young massive stars’ intense radiation fields and their fast, radiatively driven winds can have a profound influence on their formation and environment. Massive star forming regions are rare and highly obscured, making the early moments of their formation difficult to observe. Instead, we must turn to theory to elucidate the physics involved in the formation of massive stars and massive star clusters (MSCs), which can host thousands of massive stars. In my thesis, I developed analytical and numerical techniques to study the formation of massive stars and how stellar wind feedback affects the dynamics of gas that surrounds MSCs. To estimate the initial rotation rates of massive stars at birth, I developed a protostellar angular momentum evolution model for accreting protostars to determine if magnetic torques can spin down massive stars during their formation. I found that magnetic torques are insufficient to spin down massive stars due to their short formation times and high accretion rates. Radiation pressure is likely the dominate feedback mechanism regulating massive star formation. Therefore detailed simulation of the formation of massive stars requires an accurate treatment of radiation. For this purpose, I developed a new, highly accurate radiation algorithm that properly treats the absorption of the direct radiation field from stars and the re-emission and processing by interstellar dust. With this new tool, I performed a suite of three-dimensional adaptive mesh refinement radiation-hydrodynamic simulations of the formation of massive stars from collapsing massive pre-stellar cores. I found that mass is channeled to the massive star via dense infalling filaments that are uninhibited by radiation pressure and gravitational and Rayleigh-Taylor instabilities. To determine the importance of stellar wind feedback in young MSCs, I used observations to constrain a range of kinetic energy loss channels for the hot gas produced by the shock-heating of stellar winds to explain the low X-ray luminosities observed in Hii regions. I demonstrated that the energy injected by stellar winds is not a significant contributor to stellar feedback in young MSCs.

The majority of recent hydrodynamical simulations indicate the creation of central cores in the mass profiles of low-mass halos, a process that is attributed to star formation-related baryonic feedback. Core creation is regarded as one of the most promising solutions to potential issues faced by the lambda cold dark matter (LambdaCDM) cosmology on small scales. For example, the reduced dynamical mass enclosed by cores can explain the low rotational velocities measured for nearby dwarf galaxies, thus possibly lifting the seeming contradiction with the LambdaCDM expectations (the so-called "too big to fail" problem). Here we test core creation as a solution of cosmological issues by using a sample of dwarfs with measurements of their atomic hydrogen (HI) kinematics extending to large radii. Using the NIHAO hydrodynamical simulation as an example, we show that core creation can successfully reproduce the kinematics of dwarfs with small kinematic radii, R, Comment: v2 matches version accepted by A&A. About 5 pages, 1 figure

In this thesis we test and expand our current knowledge of Star Formation Laws (SF laws) in the extreme environment of dwarf irregular galaxies. We focus on the SF characteristics of our 18 galaxies sample, extending current investigations of the Schmidt-Kennicutt law to the low luminosity, low metallicity regime. The Hi data used in this project have been observed, calibrated and imaged according to the LITTLE THINGS Survey prescription to which I brought my own contribution as a member of the team. Apart from high resolution, VLA data in B, C and D array con gurations, this project makes use of an extensive set of multi- wavelength data (H , FUV, 24 m, 3.6 m, V-band and K-band). Molecular gas in dwarfs is very di fficult to observe, mainly because due to the low metallicity environment, we lose our only molecular tracer, the CO which becomes under luminous. Therefore the gas distribution is represented by Hi gas only. We create our Star Formation Rate (SFR) maps mainly based on FUV maps because our analysis shows that FUV is the SF tracer that allows us the most extensive sampling of the SFR surface density (SFRD) and Hi surface density relation. The main results of our study are: Whereas in spiral galaxies Bigiel et al. (2008) have found a one to one relation between star formation rate and molecular gas and no relation between the SFR and the neutral gas, in a small sample of dwarfs as well as in the outskirts of spiral galaxies Bigiel et al. (2010b) has found that SFRD does correlate with Hi surface density. We con rm the existence of the SFRD vs. Hi surface density relation in dwarf irregular galaxies and a linear fitting through all our data (all 18 galaxies combined) yields a power law relation SFR / 1:87 0:3 HI . We find that the interiors of Hi shells, at 400 pc scales, become resolved and show up in SFRD versus Hi surface density plots although within the shell interior we have SFRD values but no Hi surface density related to them. Thus, the points originating from those regions contribute significantly to the increase of the scatter in the plot. We show that by excluding those points the correlation between SFRD and Hi surface density improves between 10% and 20%. Eight of the 18 galaxies in our sample have Hi maxima higher than the 10M pc-2 value found by Bigiel et al. (2008) for spiral galaxies. Krumholz et al. (2011) predicted that the 10M pc-2 threshold is metallicity dependent in galaxies with sub-solar metallicity, however the theoretically predicted values for our galaxies only match the observed Hi maxima in one case (DDO168). We find that metallicity cannot be the only factor setting the Hi to H2 transition. In fact, we find evidence that the higher the interstellar radiation field (ISRF), the higher the Hi maximum is, hence we suggest that the ISRF should also be taken into consideration in predicting the Hi to H2 transition threshold. We find that even tighter than the SFRD vs. Hi surface density relation is the SFRD vs. V-band surface density relation. Unlike the SFRD vs. Hi surface density relation the SFRD vs. V-band surface density relation follows a power law and can be written as follows: SFR / (10 V )􀀀0:43 0:03. The SFRD vs. V-band surface density relation suggests that the existing stars also play a role in the formation of the next generation of stars. Within our sample of dwarf galaxies the average pressure per resolution element and the SFRD are in a 1:1 linear relation: SFR / P1:02 0:05. A similar relation has been found by Blitz & Rosolowsky (2006) for the low-pressure regimes of spiral galaxies. In conclusion we find that in the extreme environments of dwarf galaxies the metal deficiency and the lack of the classic SF stimulators (spiral arms, shear motions) do not impede the star forming process. In these galaxies, dust-shielding becomes predominantly self-shielding and there is plenty of Hi available to achieve this additional task. Existing stars assume the role of pressure enhancers, which in turn will stimulate SF without the need of spiral arms or shear motion.

Supernova (SN) Refsdal is the first multiply-imaged, highly-magnified, and spatially-resolved SN ever observed. The SN exploded in a highly-magnified spiral galaxy at z=1.49 behind the Frontier Fields Cluster MACS1149, and provides a unique opportunity to study the environment of SNe at high z. We exploit the time delay between multiple images to determine the properties of the SN and its environment, before, during, and after the SN exploded. We use the integral-field spectrograph MUSE on the VLT to simultaneously target all observed and model-predicted positions of SN Refsdal. We find MgII emission at all positions of SN Refsdal, accompanied by weak FeII* emission at two positions. The measured ratios of [OII] to MgII emission of 10-20 indicate a high degree of ionization with low metallicity. Because the same high degree of ionization is found in all images, and our spatial resolution is too coarse to resolve the region of influence of SN Refsdal, we conclude that this high degree of ionization has been produced by previous SNe or a young and hot stellar population. We find no variability of the [OII] line over a period of 57 days. This suggests that there is no variation in the [OII] luminosity of the SN over this period, or that the SN has a small contribution to the integrated [OII] emission over the scale resolved by our observations., Comment: 5 pages, 4 figures, accepted for publication in A&A

The space between stars is filled with a dilute mixture of atoms, molecules, and dust grains, which we call the interstellar medium (ISM). The physics of the ISM is a crucial part in many areas of astronomy, such as the formation and evolution of stars and entire galaxies. It regulates molecule- and dust grain synthesis, which together constitute the very building blocks of planetesimals required to form planetary systems and, ultimately, life itself. Recent observations have revealed that a significant fraction of the ISM is dynamic and filamentary, likely caused by radiation, winds, and supernova explosions from massive stars that constantly stir the material that resides in the ISM. However, the exact mechanisms and contributions of these interactions remain poorly understood. To advance our knowledge of the ISM of galaxies, in first principle, we need to acquire a deep understanding of the interplay between stars and their surroundings. In this thesis, I investigate the interactions between gas, dust, and stars in the ISM, by using the Orion region as a benchmark model.

We analyse the distribution of CO brightness temperature and integrated intensity in M51 at ~40 pc resolution using new CO data from the Plateau de Bure Arcsecond Whirlpool Survey (PAWS). We present probability distribution functions (PDFs) of the CO emission within the PAWS field, which covers the inner 11 x 7 kpc of M51. We find variations in the shape of CO PDFs within different M51 environments, and between M51 and M33 and the Large Magellanic Cloud (LMC). Globally, the PDFs for the inner disk of M51 can be represented by narrow lognormal functions that cover 1 to 2 orders of magnitude in CO brightness and integrated intensity. The PDFs for M33 and the LMC are narrower and peak at lower CO intensities. However, the CO PDFs for different dynamical environments within the PAWS field depart from the shape of the global distribution. The PDFs for the interarm region are approximately lognormal, but in the spiral arms and central region of M51, they exhibit diverse shapes with a significant excess of bright CO emission. The observed environmental dependence of the shape of the CO PDFs is qualitatively consistent with changes that would be expected if molecular gas in the spiral arms has a larger range of average densities, gas temperatures and velocity fluctuations, though further work is required to disentangle the importance of large-scale dynamical effects versus star formation feedback in regulating these properties. We show that the shape of the CO PDFs for different M51 environments is only weakly related to global properties of the CO emission, but is strongly correlated with some properties of the local giant molecular cloud (GMC) and young stellar cluster populations. For galaxies with strong spiral structure such as M51, our results indicate that galactic-scale dynamical processes play a significant role in the formation and evolution of GMCs and stellar clusters.(abridged), 30 pages, 18 figures, accepted to ApJ PAWS special issue

The effect of stellar feedback on the interstellar medium is investigated using numerical simulation. In particular, the roles of supernova feedback and ionization feedback on the star formation rate and structure of the interstellar medium are compared. We use Enzo, an adaptive mesh code, and employ the MUSCL-Hancock hydrodynamics scheme to run simulations of a section of a stratified galactic disk. A turbulent velocity field is imposed in the central region of the disk and self-gravity is applied. Star clusters are formed when density and temperature conditions are met, which, in turn, provide ionization and supernova feedback into the interstellar medium. Simulations were run with and without supernova and ionization feedback and the runs are compared. Ionization feedback is found to dominate over supernova feedback in regulating star formation rates. With no feedback, all the gas is converted to stars by 200 Myr. When supernova feedback is added, 98% of the gas is used to create stars by 300 Myr. With ionization feedback instead, at 1 Gyr into the run, only 30% of the gas is in stars. Even with supernova feedback added to ionization feedback, the gas converted to stars is just 29% at 1 Gyr. Very strong supernovae take this fraction down to 25%. The star formation rates in the runs with supernova feedback are consistent with the low end of the Kennicutt-Schmidt relation, while the runs without ionization feedback have star formation rates that are an order of magnitude larger. Gas phase masses and volumes produced in the ionization runs are broadly consistent with observations. Master of Science (MSc)

We present results based on a set of N-Body/SPH simulations of isolated dwarf galaxies. The simulations take into account star formation, stellar feedback, radiative cooling and metal enrichment. The dark matter halo initially has a cusped profile, but, at least in these simulations, starting from idealised, spherically symmetric initial conditions, a natural conversion to a core is observed due to gas dynamics and stellar feedback. A degeneracy between the efficiency with which the interstellar medium absorbs energy feedback from supernovae and stellar winds on the one hand, and the density threshold for star formation on the other, is found. We performed a parameter survey to determine, with the aid of the observed kinematic and photometric scaling relations, which combinations of these two parameters produce simulated galaxies that are in agreement with the observations. With the implemented physics we are unable to reproduce the relation between the stellar mass and the halo mass as determined by Guo et al. (2010), however we do reproduce the slope of this relation., Comment: Accepted for publication in MNRAS | 12 pages, 8 figures

There is a large consensus that gas in high-$z$ galaxies is highly turbulent, because of a combination of stellar feedback processes and gravitational instabilities driven by mergers and gas accretion. In this paper, we present the analysis of a sample of five Dusty Star Forming Galaxies (DSFGs) at $4 \lesssim z\lesssim 5$. Taking advantage of the magnifying power of strong gravitational lensing, we quantified their kinematic and dynamical properties from ALMA observations of their [CII] emission line. We combined the dynamical measurements obtained for these galaxies with those obtained from previous studies to build the largest sample of $z \sim 4.5$ galaxies with high-quality data and sub-kpc spatial resolutions, so far. We found that all galaxies in the sample are dynamically cold, with rotation-to-random motion ratios, $V/\sigma$, between 7 to 15. The relation between their velocity dispersions and their star-formation rates indicates that stellar feedback is sufficient to sustain the turbulence within these galaxies and no further mechanisms are needed. In addition, we performed a rotation curve decomposition to infer the relative contribution of the baryonic (gas, stars) and dark matter components to the total gravitational potentials. This analysis allowed us to compare the structural properties of the studied DSFGs with those of their descendants, the local early type galaxies. In particular, we found that five out of six galaxies of the sample show the dynamical signature of a bulge, indicating that the spheroidal component is already in place at $z \sim 4.5$., Comment: 34 pages (including appendices), 10 figures, 8 tables; accepted for publication in MNRAS

We present a suite of high-resolution cosmological zoom-in simulations to $z=4$ of a $10^{12}\,{\rm M}_{\odot}$ halo at $z=0$, obtained using seven contemporary astrophysical simulation codes widely used in the numerical galaxy formation community. Physics prescriptions for gas cooling, heating, and star formation, are similar to the ones used in our previous {\it AGORA} disk comparison but now account for the effects of cosmological processes. In this work, we introduce the most careful comparison yet of galaxy formation simulations run by different code groups, together with a series of four calibration steps each of which is designed to reduce the number of tunable simulation parameters adopted in the final run. After all the participating code groups successfully completed the calibration steps, we reach a suite of cosmological simulations with similar mass assembly histories down to $z=4$. With numerical accuracy that resolves the internal structure of a target halo, we find that the codes overall agree well with one another in e.g., gas and stellar properties, but also show differences in e.g., circumgalactic medium properties. We argue that, if adequately tested in accordance with our proposed calibration steps and common parameters, the results of high-resolution cosmological zoom-in simulations can be robust and reproducible. New code groups are invited to join this comparison by generating equivalent models by adopting the common initial conditions, the common easy-to-implement physics package, and the proposed calibration steps. Further analyses of the simulations presented here will be in forthcoming reports from our Collaboration., Accepted for publication to the ApJ. The simulation snapshots used in this paper will be made publicly available in a user-friendly interface. The announcement of the data release will be posted on the AGORA Project webpage (www.agorasimulations.org). Early access to the data can be provided upon request to the authors, or to the project coordinator (sroca01@ucm.es, santacruzgalaxy@gmail.com)

We present a high-resolution view of bubbles within The Phantom Galaxy (NGC 628); a nearby (~10Mpc), star-forming (~2Msun/yr), face-on (i~9deg) grand-design spiral galaxy. With new data obtained as part of the PHANGS-JWST treasury program, we perform a detailed case-study of two regions of interest, one of which contains the largest and most prominent bubble in the galaxy (The Phantom Void; over 1kpc in diameter), and the other being a smaller region that may be the precursor to such a large bubble (The Precursor Phantom Void). When comparing to matched resolution Halpha observations from the Hubble Space Telescope (HST), we see that the ionized gas is brightest in the shells of both bubbles, and is coincident with the youngest (~1Myr) and most massive (~100,000Msun) stellar associations. We also find an older generation (~20Myr) of stellar associations is present within the bubble of The Phantom Void. From our kinematic analysis of the HI, H2 (CO) and HII gas across The Phantom Void, we infer a high expansion speed of around 15 to 50km/s. The large size and high expansion speed of The Phantom Void suggest that the driving mechanism is sustained stellar feedback due to multiple mechanisms, where early feedback first cleared a bubble (as we observe now in The Precursor Phantom Void), and since then SNe have been exploding within the cavity, and have accelerated the shell. Finally, comparison to simulations shows a striking resemblance to our JWST observations, and suggests that such large-scale stellar feedback-driven bubbles should be common within other galaxies., Comment: 12 pages total, 8 figures, and 1 table. Accepted as part of a PHANGS-JWST Focus Issue to appear in ApJ

We present an analysis of the $R\lesssim 1.5$ kpc core regions of seven simulated Milky Way mass galaxies, from the FIRE-2 (Feedback in Realistic Environments) cosmological zoom-in simulation suite, for a finely sampled period ($\Delta t = 2.2$ Myr) of 22 Myr at $z \approx 0$, and compare them with star formation rate (SFR) and gas surface density observations of the Milky Way's Central Molecular Zone (CMZ). Despite not being tuned to reproduce the detailed structure of the CMZ, we find that four of these galaxies are consistent with CMZ observations at some point during this 22 Myr period. The galaxies presented here are not homogeneous in their central structures, roughly dividing into two morphological classes; (a) several of the galaxies have very asymmetric gas and SFR distributions, with intense (compact) starbursts occurring over a period of roughly 10 Myr, and structures on highly eccentric orbits through the CMZ, whereas (b) others have smoother gas and SFR distributions, with only slowly varying SFRs over the period analyzed. In class (a) centers, the orbital motion of gas and star-forming complexes across small apertures ($R \lesssim 150$pc, analogously $|l, Comment: 9 pages, 3 figures, accepted to ApJ Letters. One appended supplemental appendix and figure, which do not appear in the ApJ version