arXiv subjects

Florent Renaud

Publications and source records attributed to Florent Renaud.

At least 37 records · Page 2Linked to original sources

arXiv2023

Emergence and cosmic evolution of the Kennicutt-Schmidt relation driven by interstellar turbulence

The scaling relations between the gas content and star formation rate of galaxies provide useful insights into processes governing their formation and evolution. We investigate the emergence and the physical drivers of the global Kennicutt-Schmidt (KS) relation at $0.25 \leq z \leq 4$ in the cosmological hydrodynamic simulation NewHorizon capturing the evolution of a few hundred galaxies with a resolution of $\sim$ 40 pc. The details of this relation vary strongly with the stellar mass of galaxies and the redshift. A power-law relation $Σ_{\rm SFR} \propto Σ_{\rm gas}^{a}$ with $a \approx 1.4$, like that found empirically, emerges at $z \approx 2 - 3$ for the most massive half of the galaxy population. However, no such convergence is found in the lower-mass galaxies, for which the relation gets shallower with decreasing redshift. At the galactic scale, the star formation activity correlates with the level of turbulence of the interstellar medium, quantified by the Mach number, rather than with the gas fraction (neutral or molecular), confirming previous works. With decreasing redshift, the number of outliers with short depletion times diminishes, reducing the scatter of the KS relation, while the overall population of galaxies shifts toward low densities. Using pc-scale star formation models calibrated with local Universe physics, our results demonstrate that the cosmological evolution of the environmental and intrinsic conditions conspire to converge towards a significant and detectable imprint in galactic-scale observables, in their scaling relations, and in their reduced scatter.

astro-ph.GA↗

arXiv2023

Pre-supernova feedback sets the star cluster mass function to a power law and reduces the cluster formation efficiency

The star cluster initial mass function is observed to have an inverse power law exponent around 2, yet there is no consensus on what determines this distribution, and why some variation is observed in different galaxies. Furthermore, the cluster formation efficiency covers a range of values, particularly when considering different environments. These clusters are often used to empirically constrain star formation and as fundamental units for stellar feedback models. Detailed galaxy models must therefore accurately capture the basic properties of observed clusters to be considered predictive. We use hydrodynamical simulations of a dwarf galaxy as a laboratory to study star cluster formation. We test different combinations of stellar feedback mechanisms, including stellar winds, ionizing radiation, and supernovae. Each feedback mechanism affects the cluster formation efficiency and cluster mass function. Increasing the feedback budget by combining the different types of feedback decreases the cluster formation efficiency by reducing the number of massive clusters. Ionizing radiation is found to be especially influential. This effect depends on the timing of feedback initiation, as shown by comparing early and late feedback. Early feedback occurs from ionizing radiation and stellar winds with onset immediately after a massive star is formed. Late feedback occurs when energy injection only starts after the main-sequence lifetime of the most massive SN progenitor, a timing that is further influenced by the choice of the most massive SN progenitor. Late feedback alone results in a broad, flat mass function, approaching a log-normal shape in the complete absence of feedback. Early feedback, on the other hand, produces a power-law cluster mass function with lower formation efficiency, albeit with a steeper slope than that usually observed.

astro-ph.GA↗

arXiv2023

Imprint of the galactic acceleration scale on globular cluster systems: Galaxies in the Fornax Cluster

Dark matter is required in galaxies at galactocentric radii that are larger than the $a_0$-radius, which is where the gravitational acceleration generated by baryons of the galaxy equals the constant $a_0=1.2\times 10^{-10}$ms$^{-2}$ known as the galactic acceleration scale. It was found previously for massive early-type galaxies that the radial number-density profiles of their globular cluster (GC) systems follow broken power laws and the breaks occur at the $a_0$-radii. We have newly analyzed the distribution of GCs around galaxies in the Fornax cluster in existing photometric catalogs. We found that 1) the coincidence between $a_0$-radii and the break radii of globular cluster systems is valid for early-type galaxies of all masses and, 2) this also applies to the red and blue sub-populations of GCs separately.

astro-ph.GA↗

arXiv2023

VINTERGATAN-GM: The cosmological imprints of early mergers on Milky-Way-mass galaxies

We present a new suite of cosmological zoom-in hydrodynamical ($\approx 20\, \mathrm{pc}$ spatial resolution) simulations of Milky-Way mass galaxies to study how a varying mass ratio for a Gaia-Sausage-Enceladus (GSE) progenitor impacts the $z=0$ chemodynamics of halo stars. Using the genetic modification approach, we create five cosmological histories for a Milky-Way-mass dark matter halo ($M_{200} \approx 10^{12} \, M_\mathrm{\odot}$), incrementally increasing the stellar mass ratio of a $z\approx2$ merger from 1:25 to 1:2, while fixing the galaxy's final dynamical, stellar mass and large-scale environment. We find markedly different morphologies at $z=0$ following this change in early history, with a growing merger resulting in increasingly compact and bulge-dominated galaxies. Despite this structural diversity, all galaxies show a radially-biased population of inner halo stars like the Milky-Way's GSE which, surprisingly, has a similar magnitude, age, $\rm [Fe/H]$ and $\rm [α/Fe]$ distribution whether the $z\approx2$ merger is more minor or major. This arises because a smaller ex-situ population at $z\approx2$ is compensated by a larger population formed in an earlier merger-driven starburst whose contribution to the GES can grow dynamically over time, with both populations strongly overlapping in the $\rm [Fe/H]-\rm [α/Fe]$ plane. Our study demonstrates that multiple high-redshift histories can lead to similar $z=0$ chemodynamical features in the halo, highlighting the need for additional constraints to distinguish them, and the importance of considering the full spectrum of progenitors when interpreting $z=0$ data to reconstruct our Galaxy's past.

astro-ph.GA↗

arXiv2023

INFERNO: Galactic winds in dwarf galaxies with star-by-star simulations including runaway stars

The formation and evolution of galaxies have proved sensitive to the inclusion of stellar feedback, which is therefore crucial to any successful galaxy model. We present INFERNO, a new model for hydrodynamic simulations of galaxies, which incorporates resolved stellar objects with star-by-star calculations of when and where the injection of enriched material, momentum, and energy takes place. INFERNO treats early stellar kinematics to include phenomena such as walkaway and runaway stars. We employ this innovative model on simulations of a dwarf galaxy and demonstrate that our physically motivated stellar feedback model can drive vigorous galactic winds. This is quantified by mass and metal loading factors in the range of $10-100$, and an energy loading factor close to unity. Outflows are established close to the disc, are highly multi-phase, spanning almost $8$ orders of magnitude in temperature, and with a clear dichotomy between mass ejected in cold, slow-moving ($T\lesssim5\times10^4\,{\rm K}$, $v<100\,{\rm km\,s}^{-1}$) gas and energy ejected in hot, fast-moving ($T>10^6\,{\rm K}$, $v>100\,{\rm km\,s}^{-1}$) gas. In contrast to massive disc galaxies, we find a surprisingly weak impact of the early stellar kinematics, with runaway stars having little to no effect on our results, despite exploding in diffuse gas outside the dense star-forming gas, as well as outside the galactic disc entirely. We demonstrate that this weak impact in dwarf galaxies stems from a combination of strong feedback and a porous interstellar medium, which obscure any unique signatures that runaway stars provide.

astro-ph.GA↗

arXiv2023

The distribution of globular clusters in kinematic spaces does not trace the accretion history of the host galaxy

Reconstructing how all the stellar components of the Galaxy formed and assembled over time, by studying the properties of the stars which make it, is the aim of Galactic archeology. In these last years, thanks to the launch of the ESA Gaia astrometric mission, and the development of many spectroscopic surveys, we are for the first time in the position to delve into the layers of the past of our galaxy. Globular clusters (GCs) play a fundamental role in this research field since they are among the oldest stellar systems in the Milky Way (MW) and so bear witness of its entire past. In the recent years, there have been several attempts to constrain the nature of clusters (accreted or formed in the MW itself) through the analysis of kinematic spaces and to reconstruct from this the properties of the accretions events experienced by the MW through time. This work aims to test a widely-used assumption about the clustering of the accreted populations of GCs in the integrals of motions space. We analyze a set of dissipation-less N-body simulations that reproduce the accretion of one or two satellites with their GC population on a MW-type galaxy. Our results demonstrate that a significant overlap between accreted and "kinematically-heated" in-situ GCs is expected in kinematic spaces, for mergers with mass ratios of 1:10. In contrast with standard assumptions made in the literature so far, we find that accreted GCs do not show dynamical coherence, that is they do not cluster in kinematic spaces. In addition, GCs can also be found in regions dominated by stars which have a different origin (i.e. different progenitor). This casts doubt on the association between GCs and field stars that is generally made in the literature to assign them to a common origin. Our findings severely question the recovered accretion history of the MW based on the phase-space clustering of the GC population.

astro-ph.GA↗

arXiv2022

PHANGS-JWST First Results: Rapid Evolution of Star Formation in the Central Molecular Gas Ring of NGC1365

Large-scale bars can fuel galaxy centers with molecular gas, often leading to the development of dense ring-like structures where intense star formation occurs, forming a very different environment compared to galactic disks. We pair ~0.3" (30pc) resolution new JWST/MIRI imaging with archival ALMA CO(2-1) mapping of the central ~5kpc of the nearby barred spiral galaxy NGC1365, to investigate the physical mechanisms responsible for this extreme star formation. The molecular gas morphology is resolved into two well-known bright bar lanes that surround a smooth dynamically cold gas disk (R_gal ~ 475pc) reminiscent of non-star-forming disks in early type galaxies and likely fed by gas inflow triggered by stellar feedback in the lanes. The lanes host a large number of JWST-identified massive young star clusters. We find some evidence for temporal star formation evolution along the ring. The complex kinematics in the gas lanes reveal strong streaming motions and may be consistent with convergence of gas streamlines expected there. Indeed, the extreme line-widths are found to be the result of inter-`cloud' motion between gas peaks; ScousePy decomposition reveals multiple components with line widths of ~ 19km/s and surface densities of ~ 800M_sun/pc^2, similar to the properties observed throughout the rest of the central molecular gas structure. Tailored hydro-dynamical simulations exhibit many of the observed properties and imply that the observed structures are transient and highly time-variable. From our study of NGC1365, we conclude that it is predominantly the high gas inflow triggered by the bar that is setting the star formation in its CMZ.

astro-ph.GA↗

arXiv2022

PHANGS-JWST First Results: Multi-wavelength view of feedback-driven bubbles (The Phantom Voids) across NGC 628

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.

astro-ph.GA↗

arXiv2022

From starburst to quenching: merger-driven evolution of the star formation regimes in a shell galaxy

Shell galaxies make a class of tidally distorted galaxies, characterised by wide concentric arc(s), extending out to large galactocentric distances with sharp outer edges. Recent observations of young massive star clusters in the prominent outer shell of NGC 474 suggest that such systems host extreme conditions of star formation. In this paper, we present a hydrodynamic simulation of a galaxy merger and its transformation into a shell galaxy. We analyse how the star formation activity evolves with time, location-wise within the system, and what are the physical conditions for star formation. During the interaction, an excess of dense gas appears, triggering a starburst, i.e. an enhanced star formation rate and a reduced depletion time. Star formation coincides with regions of high molecular gas fraction, such as the galactic nucleus, spiral arms, and occasionally the tidal debris during the early stages of the merger. Tidal interactions scatter stars into a stellar spheroid, while the gas cools down and reforms a disc. The morphological transformation after coalescence stabilises the gas and thus quenches star formation, without the need for feedback from an active galactic nucleus. This evolution shows similarities with a compaction scenario for compact quenched spheroids at high-redshift, yet without a long red nugget phase. Shells appear after coalescence, during the quenched phase, implying that they do not host the conditions necessary for in situ star formation. The results suggest that shell-forming mergers might be part of the process of turning blue late-type galaxies into red and dead early-types.

astro-ph.GA↗

arXiv2022

The specific angular momentum of disc galaxies and its connection with galaxy morphology, bar structure and disc gravitational instability

The specific angular momenta ($j\equiv J/M$) of stars ($j_{\star}$), gas ($j_{\mathrm{gas}}$), baryons as a whole ($j_{\mathrm{b}}$) and dark matter haloes ($j_{\mathrm{h}}$) contain clues of vital importance about how galaxies form and evolve. Using one of the largest samples of disc galaxies (S0-BCD) with high-quality rotation curves and near-infrared surface photometry, we perform a detailed comparative analysis of $j$ that stretches across a variety of galaxy properties. Our analysis imposes tight constraints on the "retained" fractions of specific angular momentum ($j_{\star}/j_{\mathrm{h}}$, $j_{\mathrm{HI}}/j_{\mathrm{h}}$ and $j_{\mathrm{b}}/j_{\mathrm{h}}$), as well as on their systematic trends with mass fraction and galaxy morphology, thus on how well specific angular momentum is conserved in the process of disc galaxy formation and evolution. In particular, one of the most innovative results of our analysis is the finding that galaxies with larger baryon fractions have also retained larger fractions of their specific angular momentum. Furthermore, our analysis demonstrates how challenging it is to characterize barred galaxies from a gravitational instability point of view. This is true not only for the popular Efstathiou, Lake & Negroponte bar instability criterion, which fails to separate barred from non-barred galaxies in about 55% of the cases, but also for the mass-weighted Toomre parameter of atomic gas, $\langle Q_{\mathrm{HI}}\rangle$, which succeeds in separating barred from non-barred galaxies, but only in a statistical sense.

astro-ph.GA↗

arXiv2022

The merger-starburst connection across cosmic times

The correspondence between galaxy major mergers and starburst activity is well-established observationally and in simulations of low redshift galaxies. However, the evolution of the properties of interactions and of the galaxies involved suggests that the starburst response of galaxies to merger events could vary across cosmic time. Using the VINTERGATAN cosmological zoom-in simulation of a Milky Way-like galaxy, we show here that starbursts, i.e. episodes of fast star formation, are connected with the onset of tidal compression, itself induced by mergers. However, this compression becomes strong enough to trigger starbursts only after the formation of the galactic disc. As a consequence, starburst episodes are only found during a precise phase of galaxy evolution, after the formation of the disc and until the last major merger. As the depletion time quantifies the instantaneous star formation activity, while the specific star formation rate involves the integrated result of the past activity (via the stellar mass), starburst episodes do not necessarily coincide with elevated specific star formation rate. This suggests that not all starburst galaxies are outliers above the main sequence of galaxy formation.

astro-ph.GA↗

arXiv2022

VINTERGATAN IV: Cosmic phases of star formation in Milky Way-like galaxies

The star formation history of a galaxy is modulated by a plethora of internal processes and environmental conditions. The details of how these evolve and couple together is not fully understood yet. In this work, we study the effects that galaxy mergers and morphological transformations have on setting different modes of star formation at galactic scales and across cosmic time. We monitor the global properties of VINTERGATAN, a 20 pc resolution cosmological zoom-in simulation of a Milky Way-type galaxy. Between redshifts 1 and 5, we find that major mergers trigger multiple starburst episodes, corresponding to a tenfold drop of the gas depletion time down to 100 Myr. Bursty star formation is enabled by the emergence of a galactic disc, when the rotational velocity of gas starts to dominate over its velocity dispersion. Coherent motions of gas then outweigh disordered ones, such that the galaxy responds to merger-induced forcings by redistributing large amounts of gas towards high densities. As a result, the overall star formation rate is enhanced with an associated decrease in the depletion time. Before redshift 5, mergers are expected to be even more frequent. However, a more turbulent interstellar medium, is incapable of reacting in such a collective manner so as to spark rapid star formation. Thus, a constant long depletion time of 1 Gyr is kept, along with a low, but gradually increasing star formation rate. After the last major merger at redshift 1, VINTERGATAN spends the next 8 Gyr evolving secularly. It has a settled and adiabatically growing disc, and a constant star formation rate with gas depletion times of 1-2 Gyr. Our results are compatible with the observed rapid transition between different modes of star formation when galaxies leave the main sequence.

astro-ph.GA↗

arXiv2022

Stellar feedback in M83 as observed with MUSE -- II. Analysis of the HII region population: ionisation budget and pre-SN feedback

We study pre-supernova feedback in a sample of $\sim$ 4700 HII regions in the nearby spiral galaxy M83, identified on their H$α$ emission. We pectroscopically identify Wolf-Rayet (WR) stars populating the star-forming regions. For each HII region, we compute the pressure of ionised gas ($P_{\rm ion}$) and the direct radiation pressure ($P_{\rm dir}$) acting in the region, and investigate how they vary with galactocentric distance, with the physical properties of the region, and with the pressure of the galactic environment ($P_\mathrm{DE}$). For a subset of $\sim$ 500 regions, we also investigate the link between the pressure terms and the properties of the cluster population (age, mass, and LyC flux). We find that $P_{\rm ion}$ dominates over $P_{\rm dir}$ by at least a factor of 10 on average over the disk. Both pressure terms are strongly enhanced and become almost comparable in the central starburst region. In the disk ($R \geq 0.15~R_e$), we observe that $P_{\rm dir}$ stays approximately constant with galactocentric distance. $P_{\rm dir}$ is positively correlated with an increase in radiation field strength (linked to the negative metallicity gradient in the galaxy), while it decreases in low extinction regions. $P_{\rm ion}$ decreases constantly for increasing galactocentric distances. In general, we observe that HII regions near the center are underpressured with respect to their surroundings, whereas regions in the disk are overpressured and hence expanding. We find that regions hosting younger clusters or having more mass in young star clusters have a higher internal pressure, indicating that clustered star formation is likely playing a dominant role in setting the pressure. Finally, we estimate that only 13 % of HII regions hosting young clusters and WR stars have $f_{\rm esc} \geq 0$.[Abridged]

astro-ph.GA↗

arXiv2022

From giant clumps to clouds -- III. The connection between star formation and turbulence in the ISM

Supersonic gas turbulence is a ubiquitous property of the interstellar medium. The level of turbulence, quantified by the gas velocity dispersion ($σ_{\rm g}$), is observed to increase with the star formation rate (SFR) of a galaxy, but it is yet not established whether this trend is driven by stellar feedback or gravitational instabilities. In this work we carry out hydrodynamical simulations of entire disc galaxies, with different gas fractions, to understand the origins of the SFR-$σ_{\rm g}$ relation. We show that disc galaxies reach the same levels of turbulence regardless of the presence of stellar feedback processes, and argue that this is an outcome of the way disc galaxies regulate their gravitational stability. The simulations match the SFR-$σ_{\rm g}$ relation up to SFRs of the order of tens of M$_\odot$ yr$^{-1}$ and $σ_{\rm g}\sim 50$ km s$^{-1}$ in neutral hydrogen and molecular gas, but fail to reach the very large values ($>100$ km s$^{-1}$) reported in the literature for rapidly star-forming galaxies. We demonstrate that such high values of $σ_{\rm g}$ can be explained by 1) insufficient beam smearing corrections in observations, and 2) stellar feedback being coupled to the ionised gas phase traced by recombination lines. Given that the observed SFR-$σ_{\rm g}$ relation is composed of highly heterogeneous data, with $σ_{\rm g}$ at high SFRs almost exclusively being derived from H$α$ observations of high redshift galaxies with complex morphologies, we caution against analytical models that attempt to explain the SFR-$σ_{\rm g}$ relation without accounting for these effects.

astro-ph.GA↗

arXiv2022

EDGE: The sensitivity of ultra-faint dwarfs to a metallicity-dependent initial mass function

Motivated by the observed bottom-light initial mass function (IMF) in faint dwarfs, we study how a metallicity-dependent IMF affects the feedback budget and observables of an ultra-faint dwarf galaxy. We model the evolution of a low-mass ($\approx 8 \, \times \, 10^{8} \, \rm M_{\odot}$) dark matter halo with cosmological, zoomed hydrodynamical simulations capable of resolving individual supernovae explosions, which we complement with an empirically motivated subgrid prescription for systematic IMF variations. In this framework, at the low gas metallicities typical of faint dwarfs, the IMF of newborn stellar populations becomes top-heavy, increasing the efficiency of supernova and photoionization feedback in regulating star formation. This results in a 100-fold reduction of the final stellar mass of the dwarf compared to a canonical IMF, at fixed dynamical mass. The increase in the feedback budget is nonetheless met by increased metal production from more numerous massive stars, leading to nearly constant iron content at $z=0$. A metallicity-dependent IMF therefore provides a mechanism to produce low-mass ($\rm M_{\star}\sim 10^3 \rm M_{\odot}$), yet enriched ($\rm [Fe/H]\approx -2$) field dwarf galaxies, thus opening a self-consistent avenue to populate the plateau in $\rm [Fe/H]$ at the faintest end of the mass--metallicity relation.

astro-ph.GA↗

arXiv2022

Stellar feedback in M83 as observed with MUSE -- I. Overview, an unprecedented view of the stellar and gas kinematics and evidence of outflowing gas

We present a large VLT/MUSE mosaic (3.8 x 3.8 kpc) of the nearby spiral galaxy M83, with a spatial resolution ~20 pc. We obtained the kinematics of the stars and ionised gas, and compared them with molecular gas kinematics from ALMA CO(2-1). We separated the ionised gas into HII regions and diffuse ionised gas (DIG) and determined the fraction of Ha luminosity originating from the DIG (f_DIG). We observe that both stars and gas trace the galactic disk rotation, as well as a fast-rotating nuclear component, likely connected to secular processes driven by the galactic bar. In the gas kinematics, we observe a stream east of the nucleus, redshifted with respect to the disk. The stream is surrounded by an extended ionised gas region with enhanced velocity dispersion and a high ionisation state, which is largely consistent with being ionised by slow shocks. We interpret this feature as either the superposition of the disk and an extraplanar layer of DIG, or as a bar-driven inflow of shocked gas. A double Gaussian component fit to the Ha line also reveals the presence of a nuclear biconic structure whose axis of symmetry is perpendicular to the bar. The two cones appear blue- and redshifted along the line of sight and stand out for having an Ha emission separated by up to 200 km s-1 from that of the disk, and a high velocity dispersion ~80-200 km s-1. At the far end of the cones, we observe that the gas is consistent with being ionised by shocks. These features had never been observed before in M83; we postulate that they are tracing a starburst-driven outflow shocking into the surrounding ISM. Finally, we obtain f_DIG ~ 13% in our field of view. We inspect the emission of the HII regions and DIG in `BPT' diagrams, finding that in HII regions photoionisation accounts for 99.8% of the Ha flux, whereas the DIG has a mixed contribution from photoionisation (94.9%) and shocks (5.1%). [abridged]

astro-ph.GA↗

arXiv2021

From giant clumps to clouds -- II. The emergence of thick disc kinematics from the conditions of star formation in high redshift gas rich galaxies

High redshift disc galaxies are more gas rich, clumpier, and more turbulent than local Universe galaxies. This early era of galaxy formation imprints the distribution and kinematics of the stars that we observe today, but it is not yet well established how. In this work, we use simulations of isolated Milky Way-mass disc galaxies to study how kinematic properties of stars change when varying the gas fraction. This allows us to quantify the roles played by internal processes, e.g. gas turbulence and gravitational scattering off massive gas clumps, in establishing the observed stellar velocity dispersions and orbital eccentricities. We find that models with gas fractions $>20$ per cent feature a turbulent and clumpy interstellar medium (ISM), leading to zero-age stellar velocity dispersions $\sim 20-30~{\rm km\, s}^{-1}$ and high mean orbital eccentricities. Low eccentricities cannot arise from these physical conditions. For gas fractions below $20$ per cent, the ISM becomes less turbulent, with stellar velocity dispersions $<10~{\rm km\, s}^{-1}$, and nearly circular orbits for young stars. The turbulence present in gas-rich high redshift galaxies hence acts as a `barrier' against the formation of thin discs. We compare our findings to the Milky Way's age-velocity dispersion relation and argue that velocity dispersions imprinted already at star formation by the ISM contribute significantly at all times. Finally, we show that observed orbital eccentricities in the Milky Way's thick and thin discs can be explained entirely as imprints by the star-forming ISM, rather than by mergers or secular processes.

astro-ph.GA↗

arXiv2021

From giant clumps to clouds I: the impact of gas fraction evolution on the stability of galactic discs

The morphology of gas-rich disc galaxies at redshift ~1-3 is dominated by a few massive clumps. The process of formation or assembly of these clumps and their relation to molecular clouds in contemporary spiral galaxies are still unknown. Using simulations of isolated disc galaxies, we study how the structure of the interstellar medium and the stability regime of the discs change when varying the gas fraction. In all galaxies, the stellar component is the main driver of instabilities. However, the molecular gas plays a non-negligible role in the inter-clumps medium of gas-rich cases, and thus in the assembly of the massive clumps. At scales smaller than a few 100 pc, the Toomre-like disc instabilities are replaced by another regime, specially in the gas-rich galaxies. We find that galaxies at low gas fraction (10%) stand apart from discs with more gas, which all share similar properties on virtually all the aspects we explore. For gas fractions below approximately 20%, the clump-scale regime of instabilities disappears, only leaving the large-scale disc-driven regime. When translating the change of gas fraction to the cosmic evolution of galaxies, this transition marks the end of the clumpy phase of disc galaxies, and allows for the onset of spiral structures, as commonly found in the local Universe.

astro-ph.GA↗