arXiv subjects

Oscar Agertz

Publications and source records attributed to Oscar Agertz.

At least 91 records · Page 5Linked to original sources

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

EDGE: What shapes the relationship between HI and stellar observables in faint dwarf galaxies?

We show how the interplay between feedback and mass-growth histories introduces scatter in the relationship between stellar and neutral gas properties of field faint dwarf galaxies ($M_{\star} \lessapprox 10^{6} M_{\odot}$). Across a suite of cosmological, high-resolution zoomed simulations, we find that dwarf galaxies of stellar masses $10^5 \leq M_{\star} \leq 10^{6} M_{\odot}$ are bimodal in their cold gas content, being either HI-rich or HI-deficient. This bimodality is generated through the coupling between (i) the modulation of HI contents by the background of ultraviolet radiation (UVB) at late times and (ii) the significant scatter in the stellar-mass-halo-mass relationship induced by reionization. Furthermore, our HI-rich dwarfs exhibit disturbed and time-variable neutral gas distributions primarily due to stellar feedback. Over the last four billion years, we observe order-of-magnitude changes around the median $M_{HI}$, factor-of-a-few variations in HI spatial extents, and spatial offsets between HI and stellar components regularly exceeding the galaxies' optical sizes. Time variability introduces further scatter in the $M_{\star}-M_{HI}$ relation and affects a galaxy's detectability in HI at any given time. These effects will need to be accounted for when interpreting observations of the population of faint, HI-bearing dwarfs by the combination of optical and radio wide, deep surveys.

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↗

arXiv2021

Runaway stars masquerading as star formation in galactic outskirts

In the outskirts of nearby spiral galaxies, star formation is observed in extremely low gas surface densities. Star formation in these regions, where the interstellar medium is dominated by diffuse atomic hydrogen, is difficult to explain with classic star formation theories. In this letter, we introduce runaway stars as an explanation for this observation. Runaway stars, produced by collisional dynamics in young stellar clusters, can travel kiloparsecs during their main-sequence lifetime. Using galactic-scale hydrodynamic simulations including a treatment of individual stars, we demonstrate that this mechanism enables the ejection of young massive stars into environments where the gas is not dense enough to trigger star formation. This results in the appearance of star formation in regions where it ought to be impossible. We conclude that runaway stars are a contributing, if not dominant, factor to the observations of star formation in the outskirts of spiral galaxies.

astro-ph.GA↗

arXiv2021

VINTERGATAN II: the history of the Milky Way told by its mergers

Using the VINTERGATAN cosmological zoom simulation, we explore the contributions of the in situ and accreted material, and the effect of galaxy interactions and mergers in the assembly of a Milky Way-like galaxy. We find that the initial growth phase of galaxy evolution, dominated by repeated major mergers, provides the necessary physical conditions for the assembly of a thick, kinematically hot disk populated by high-[$α$/Fe] stars, formed both in situ and in accreted satellite galaxies. We find that the diversity of evolutionary tracks followed by the simulated galaxy and its progenitors leads to very little overlap of the in situ and accreted populations for any given chemical composition. At a given age, the spread in [$α$/Fe] abundance ratio results from the diversity of physical conditions in VINTERGATAN and its satellites, with an enhancement in [$α$/Fe] found in stars formed during starburst episodes. Later, the cessation of the merger activity promotes the in situ formation of stars in the low-[$α$/Fe] regime, in a radially extended, thin and overall kinematically colder disk, thus establishing chemically bimodal thin and thick disks, in line with observations. We draw links between notable features in the [Fe/H] - [$α$/Fe] plane with their physical causes, and propose a comprehensive formation scenario explaining self-consistently, in the cosmological context, the main observed properties of the Milky Way.

astro-ph.GA↗

arXiv2021

VINTERGATAN III: how to reset the metallicity of the Milky Way

Using the cosmological zoom simulation VINTERGATAN, we present a new scenario for the onset of star formation at the metal-poor end of the low-[$α$/Fe] sequence in a Milky Way-like galaxy. In this scenario, the galaxy is fueled by two distinct gas flows. One is enriched by outflows from massive galaxies, but not the other. While the former feeds the inner galactic region, the latter fuels an outer gas disk, inclined with respect to the main galactic plane, and with a significantly poorer chemical content. The first passage of the last major merger galaxy triggers tidal compression in the outer disk, which increases the gas density and eventually leads to star formation, at a metallicity 0.75 dex lower than the inner galaxy. This forms the first stars of the low-[$α$/Fe] sequence. These in situ stars have halo-like kinematics, similarly to what is observed in the Milky Way, due to the inclination of the outer disk which eventually aligns with the inner one via gravitational torques. We show that this tilting disk scenario is likely to be common in Milky-Way like galaxies. This process implies that the low-[$α$/Fe] sequence is populated in situ, simultaneously from two formation channels, in the inner and the outer galaxy, with distinct metallicities. This contrasts with purely sequential scenarios for the assembly of the Milky Way disk and could be tested observationally.

astro-ph.GA↗

arXiv2021

EDGE: Two routes to dark matter core formation in ultra-faint dwarfs

In the standard Lambda cold dark matter paradigm, pure dark matter simulations predict dwarf galaxies should inhabit dark matter haloes with a centrally diverging density `cusp'. This is in conflict with observations that typically favour a constant density `core'. We investigate this `cusp-core problem' in `ultra-faint' dwarf galaxies simulated as part of the `Engineering Dwarfs at Galaxy formation's Edge' (EDGE) project. We find, similarly to previous work, that gravitational potential fluctuations within the central region of the simulated dwarfs kinematically heat the dark matter particles, lowering the dwarfs' central dark matter density. However, these fluctuations are not exclusively caused by gas inflow/outflow, but also by impulsive heating from minor mergers. We use the genetic modification approach on one of our dwarf's initial conditions to show how a delayed assembly history leads to more late minor mergers and, correspondingly, more dark matter heating. This provides a mechanism by which even ultra-faint dwarfs ($M_* < 10^5\,\text{M}_{\odot}$), in which star formation was fully quenched at high redshift, can have their central dark matter density lowered over time. In contrast, we find that late major mergers can regenerate a central dark matter cusp, if the merging galaxy had sufficiently little star formation. The combination of these effects leads us to predict significant stochasticity in the central dark matter density slopes of the smallest dwarfs, driven by their unique star formation and mass assembly histories.

astro-ph.GA↗

arXiv2020

EDGE: A new approach to suppressing numerical diffusion in adaptive mesh simulations of galaxy formation

We introduce a new method to mitigate numerical diffusion in adaptive mesh refinement (AMR) simulations of cosmological galaxy formation, and study its impact on a simulated dwarf galaxy as part of the 'EDGE' project. The target galaxy has a maximum circular velocity of 21 km/s but evolves in a region which is moving at up to 90 km/s relative to the hydrodynamic grid. In the absence of any mitigation, diffusion softens the filaments feeding our galaxy. As a result, gas is unphysically held in the circumgalactic medium around the galaxy for 320 Myr, delaying the onset of star formation until cooling and collapse eventually triggers an initial starburst at z=9. Using genetic modification, we produce 'velocity-zeroed' initial conditions in which the grid-relative streaming is strongly suppressed; by design, the change does not significantly modify the large scale structure or dark matter accretion history. The resulting simulation recovers a more physical, gradual onset of star formation starting at z=17. While the final stellar masses are nearly consistent ($4.8 \times 10^6\,M_\odot$ and $4.4\times 10^6\,M_\odot$ for unmodified and velocity-zeroed respectively), the dynamical and morphological structure of the z=0 dwarf galaxies are markedly different due to the contrasting histories. Our approach to diffusion suppression is suitable for any AMR zoom cosmological galaxy formation simulations, and is especially recommended for those of small galaxies at high redshift.

astro-ph.GA↗

arXiv2020

From lenticulars to blue compact dwarfs: the stellar mass fraction is regulated by disc gravitational instability

The stellar-to-halo mass relation (SHMR) is not only one of the main sources of information we have on the connection between galaxies and their dark matter haloes, but also an important indicator of the performance of galaxy formation models. Here we use one of the largest sample of galaxies with both high-quality rotation curves and near-infrared surface photometry, and perform a detailed comparative analysis of the SHMR. Our analysis shows that there are significant statistical differences between popular forms of the SHMR, and illustrates the predictive power of a new physically motivated scaling relation, which connects the stellar mass fraction ($M_{\star}/M_{\mathrm{h}}$) to the stellar specific angular momentum ($j_{\star}$) and the stellar radial velocity dispersion ($σ_{\star}$) via disc gravitational instability. Making use of such a relation, we demonstrate (i) how challenging it is to reproduce the efficiency of galaxy formation even for state-of-the-art cosmological hydrodynamical simulations, and (ii) that the evolution of the stellar mass fraction is regulated by disc gravitational instability: when $M_{\star}/M_{\mathrm{h}}$ varies, $j_{\star}$ and $σ_{\star}$ also vary as predicted by our scaling relation, thus erasing the memory of such evolution. This implies that the process of disc gravitational instability is intriguingly uniform across disc galaxies of all morphological types: from lenticulars to blue compact dwarfs. In particular, the cosmic variance of Toomre's $Q$ is 0.2 dex, a universal value for both stars and atomic gas.

astro-ph.GA↗

arXiv2020

Rapid Filamentary Accretion as the Origin of Extended Thin Discs

Galactic outflows driven by stellar feedback are crucial for explaining the inefficiency of star formation in galaxies. Although strong feedback can promote the formation of galactic discs by limiting star formation at early times and removing low angular momentum gas, it is not understood how the same feedback can result in diverse objects such as elliptical galaxies or razor thin spiral galaxies. We investigate this problem using cosmological zoom-in simulations of two galaxies forming within $10^{12}~\mathrm{M_\odot}$ halos with almost identical mass accretion histories and halo spin parameters. However, the two resulting galaxies end up with very different bulge-to-disc ratios at $z = 0$. At $z>1.5$, the two galaxies feature a surface density of star formation $Σ_{\rm SFR}\simeq 10~\mathrm{M_\odot}~{\rm yr}^{-1}~{\rm kpc}^{-2}$, leading to strong outflows. After the last starburst episode, both galaxies feature a dramatic gaseous disc growth from 1~kpc to 5~kpc during 1~Gyr, a decisive event we dub "the Grand Twirl". After this event, the evolutionary tracks diverge strongly, with one galaxy ending up as a bulge-dominated galaxy, whereas the other ends up as a disc-dominated galaxy. The origins of this dichotomy are the angular momentum of the accreted gas, and whether it adds constructively to the initial disc angular momentum. The build-up of this extended disc leads to a rapid lowering of $Σ_{\rm SFR}$ by over two orders of magnitude with $Σ_{\rm SFR} \lesssim 0.1~\mathrm{M_\odot}~{\rm yr}^{-1}~{\rm kpc}^{-2}$, in remarkable agreement with what is derived from Milky Way stellar populations. As a consequence, supernovae explosions are spread out and cannot launch galactic outflows anymore, allowing for the persistence of a thin, gently star forming, extended disc.

astro-ph.GA↗

arXiv2020

EDGE: From quiescent to gas-rich to star-forming low-mass dwarf galaxies

We study how star formation is regulated in low-mass field dwarf galaxies ($10^5 \leq M_{\star} \leq 10^6 \, \text{M}_{\odot}$), using cosmological high-resolution ($3 \, \text{pc}$) hydrodynamical simulations. Cosmic reionization quenches star formation in all our simulated dwarfs, but three galaxies with final dynamical masses of $3 \times 10^{9} \,\text{M}_{\odot}$ are subsequently able to replenish their interstellar medium by slowly accreting gas. Two of these galaxies re-ignite and sustain star formation until the present day at an average rate of $10^{-5} \, \text{M}_{\odot} \, \text{yr}^{-1}$, highly reminiscent of observed low-mass star-forming dwarf irregulars such as Leo T. The resumption of star formation is delayed by several billion years due to residual feedback from stellar winds and Type Ia supernovae; even at $z=0$, the third galaxy remains in a temporary equilibrium with a large gas content but without any ongoing star formation. Using the "genetic modification'' approach, we create an alternative mass growth history for this gas-rich quiescent dwarf and show how a small $(0.2\,\mathrm{dex})$ increase in dynamical mass can overcome residual stellar feedback, re-igniting star formation. The interaction between feedback and mass build-up produces a diversity in the stellar ages and gas content of low-mass dwarfs, which will be probed by combining next-generation HI and imaging surveys.

astro-ph.GA↗

arXiv2020

VINTERGATAN I: The origins of chemically, kinematically and structurally distinct discs in a simulated Milky Way-mass galaxy

Spectroscopic surveys of the Milky Way's stars have revealed spatial, chemical and kinematical structures that encode its history. In this work, we study their origins using a cosmological zoom simulation, VINTERGATAN, of a Milky Way-mass disc galaxy. We find that in connection to the last major merger at $z\sim 1.5$, cosmological accretion leads to the rapid formation of an outer, metal-poor, low-[$α$/Fe] gas disc around the inner, metal-rich galaxy containing the old high-[$α$/Fe] stars. This event leads to a bimodality in [$α$/Fe] over a range of [Fe/H]. A detailed analysis of how the galaxy evolves since $z\sim 1$ is presented. We demonstrate the way in which inside-out growth shapes the radial surface density and metallicity profile and how radial migration preferentially relocates stars from the inner to the outer disc. Secular disc heating is found to give rise to increasing velocity dispersions and scaleheights with stellar age, which together with disc flaring explains several trends observed in the Milky Way, including shallower radial [Fe/H]-profiles above the midplane. We show how the galaxy formation scenario imprints non-trivial mappings between structural associations (i.e. thick and thin discs), velocity dispersions, $α$-enhancements, and ages of stars, e.g. the most metal-poor stars in the low-[$α$/Fe] sequence are found to have a scaleheight comparable to old high-[$α$/Fe] stars. Finally, we illustrate how at low spatial resolution, comparable to the thickness of the galaxy, the proposed pathway to distinct sequences in [$α$/Fe]-[Fe/H] cannot be captured.

astro-ph.GA↗

arXiv2020

How runaway stars boost galactic outflows

Roughly ten per cent of OB stars are kicked out of their natal clusters before ending their life as supernovae. These so-called runaway stars can travel hundreds of parsecs into the low-density interstellar medium, where momentum and energy from stellar feedback is efficiently deposited. In this work we explore how this mechanism affects large scale properties of the galaxy, such as outflows. To do so we use a new model which treats OB stars and their associated feedback processes on a star-by-star basis. With this model we compare two hydrodynamical simulations of Milky Way-like galaxies, one where we include runaways, and one where we ignore them. Including runaway stars leads to twice as many supernovae explosions in regions with gas densities ranging from 1e-5 cm^-3 to 1e-3 cm^-3. This results in more efficient heating of the inter-arm regions, and drives strong galactic winds with mass loading factors boosted by up to one order of magnitude. These outflows produce a more massive and extended multi-phase circumgalactic medium, as well as a population of dense clouds in the halo. Conversely, since less energy and momentum is released in the dense star forming regions, the cold phase of the interstellar medium is less disturbed by feedback effects.

astro-ph.GA↗

arXiv2019

EDGE: the mass-metallicity relation as a critical test of galaxy formation physics

We introduce the "Engineering Dwarfs at Galaxy formation's Edge" (EDGE) project to study the cosmological formation and evolution of the smallest galaxies in the Universe. In this first paper, we explore the effects of resolution and sub-grid physics on a single low mass halo ($M_{\rm halo}=10^{9}~M_\odot$), simulated to redshift $z=0$ at a mass and spatial resolution of $\sim 20~M_\odot$ and $\sim 3$ pc. We consider different star formation prescriptions, supernova feedback strengths and on-the-fly radiative transfer (RT). We show that RT changes the mode of galactic self-regulation at this halo mass, suppressing star formation by causing the interstellar and circumgalactic gas to remain predominantly warm ($\sim 10^4$ K) even before cosmic reionisation. By contrast, without RT, star formation regulation occurs only through starbursts and their associated vigorous galactic outflows. In spite of this difference, the entire simulation suite (with the exception of models without any feedback) matches observed dwarf galaxy sizes, velocity dispersions, $V$-band magnitudes and dynamical mass-to-light-ratios. This is because such structural scaling relations are predominantly set by the host dark matter halo, with the remaining model-to-model variation being smaller than the observational scatter. We find that only the stellar mass-metallicity relation differentiates the galaxy formation models. Explosive feedback ejects more metals from the dwarf, leading to a lower metallicity at a fixed stellar mass. We conclude that the stellar mass-metallicity relation of the very smallest galaxies provides a unique constraint on galaxy formation physics.

astro-ph.GA↗

arXiv2019

EDGE: The origin of scatter in ultra-faint dwarf stellar masses and surface brightnesses

We demonstrate how the least luminous galaxies in the Universe, ultra-faint dwarf galaxies, are sensitive to their dynamical mass at the time of cosmic reionization. We select a low-mass ($\sim \text{1.5} \times 10^{9} \, \text{M}_{\odot}$) dark matter halo from a cosmological volume, and perform zoom hydrodynamical simulations with multiple alternative histories using "genetically modified" initial conditions. Earlier forming ultra-faints have higher stellar mass today, due to a longer period of star formation before their quenching by reionization. Our histories all converge to the same final dynamical mass, demonstrating the existence of extended scatter ($\geq$ 1 dex) in stellar masses at fixed halo mass due to the diversity of possible histories. One of our variants builds less than 2 % of its final dynamical mass before reionization, rapidly quenching in-situ star formation. The bulk of its final stellar mass is later grown by dry mergers, depositing stars in the galaxy's outskirts and hence expanding its effective radius. This mechanism constitutes a new formation scenario for highly diffuse ($\text{r}_{1 /2} \sim 820 \, \text{pc}$, $\sim 32 \, \text{mag arcsec}^2$), metal-poor ($\big[ \mathrm{Fe}\, / \mathrm{H} \big]= -2.9$), ultra-faint ($\mathcal{M}_V= -5.7$) dwarf galaxies within the reach of next-generation low surface brightness surveys.

astro-ph.GA↗

arXiv2019

Constraining churning and blurring in the Milky Way using large spectroscopic surveys -- an exploratory study

We have investigated the possibilities to quantify how much stars move in the Milky Way stellar disk due to diffuse processes (i.e. so called blurring) and due to influences from spiral arms and the bar (i.e. so called churning). To this end we assume that it is possible to infer the formation radius of a star if we know their elemental abundances and age as well as the metallicity profile of the interstellar medium at the time of the formation of the star. Using this information, coupled with orbital information derived from Gaia DR2 data and radial velocities from large spectroscopic surveys, we show that it is possible to isolate stellar samples such that we can start to quantify how important the role of churning is. We use data from APOGEE DR14, parallaxes from Gaia and stellar ages based on C and N elemental abundances in the stars. In our sample, we find that about half of the stars have experienced some sort of radial migration (based solely on their orbital properties), 10 % have likely have suffered only from churning, whilst a modest 5-7 % of stars have never experienced either churning or blurring making them ideal tracers of the original properties of the cool stellar disk. Our investigation shows that it is possible to put up a framework where we can begin to quantify churning and blurring an important. Important aspects for future work would include to investigate how selection effects should be accounted for.

astro-ph.GA↗

arXiv2019

On the Observed Diversity of Star Formation Efficiencies in Giant Molecular Clouds

Observations find a median star formation efficiency per free-fall time in Milky Way Giant Molecular Clouds (GMCs) on the order of $ε_{\rm ff}\sim 1\%$ with dispersions of $\sim0.5\,{\rm dex}$. The origin of this scatter in $ε_{\rm ff}$ is still debated and difficult to reproduce with analytical models. We track the formation, evolution and destruction of GMCs in a hydrodynamical simulation of a Milky Way-like galaxy and by deriving cloud properties in an observationally motivated way, measure the distribution of star formation efficiencies which are in excellent agreement with observations. We find no significant link between $ε_{\rm ff}$ and any measured global property of GMCs (e.g. gas mass, velocity dispersion). Instead, a wide range of efficiencies exist in the entire parameter space. From the cloud evolutionary tracks, we find that each cloud follow a \emph{unique} evolutionary path which gives rise to wide diversity in all properties. We argue that it is this diversity in cloud properties, above all else, that results in the dispersion of $ε_{\rm ff}$.

astro-ph.GA↗