| 1. |
- Agertz, Oscar, et al.
(författare)
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Characterizing gravitational instability in turbulent multicomponent galactic discs
- 2015
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 449:2, s. 2156-2166
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Tidskriftsartikel (refereegranskat)abstract
- Gravitational instabilities play an important role in galaxy evolution and in shaping the interstellar medium (ISM). The ISM is observed to be highly turbulent, meaning that observables like the gas surface density and velocity dispersion depend on the size of the region over which they are measured. In this work, we investigate, using simulations of Milky Way-like disc galaxies with a resolution of ∼ 9 pc, the nature of turbulence in the ISM and how this affects the gravitational stability of galaxies. By accounting for the measured average turbulent scalings of the density and velocity fields in the stability analysis, we can more robustly characterize the average level of stability of the galaxies as a function of scale, and in a straightforward manner identify scales prone to fragmentation. Furthermore, we find that the stability of a disc with feedback-driven turbulence can be well described by a ‘Toomre-like' Q stability criterion on all scales, whereas the classical Q can formally lose its meaning on small scales if violent disc instabilities occur in models lacking pressure support from stellar feedback.
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| 2. |
- Agertz, Oscar, et al.
(författare)
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EDGE : The mass-metallicity relation as a critical test of galaxy formation physics
- 2020
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 491:2, s. 1656-1672
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Tidskriftsartikel (refereegranskat)abstract
- 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 (Mhalo = 109M⊙), simulated to redshift z = 0 at amass and spatial resolution of ∼ 20 M⊙ and ∼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 (∼104K) even before cosmic reionization. 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.
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| 3. |
- Agertz, Oscar, et al.
(författare)
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THE IMPACT of STELLAR FEEDBACK on the STRUCTURE, SIZE, and MORPHOLOGY of GALAXIES in MILKY-WAY-SIZED DARK MATTER HALOS
- 2016
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Ingår i: Astrophysical Journal. - 0004-637X. ; 824:2
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Tidskriftsartikel (refereegranskat)abstract
- We use cosmological zoom-in simulations of galaxy formation in a Milky-Way-sized halo started from identical initial conditions to investigate the evolution of galaxy sizes, baryon fractions, morphologies, and angular momenta in runs with different parameters of the star formation-feedback cycle. Our fiducial model with a high local star formation efficiency, which results in efficient feedback, produces a realistic late-type galaxy that matches the evolution of basic properties of late-type galaxies: stellar mass, disk size, morphology dominated by a kinematically cold disk, stellar and gas surface density profiles, and specific angular momentum. We argue that feedback's role in this success is twofold: (1) removal of low angular momentum gas, and (2) maintaining a low disk-to-halo mass fraction, which suppresses disk instabilities that lead to angular momentum redistribution and a central concentration of baryons. However, our model with a low local star formation efficiency, but large energy input per supernova, chosen to produce a galaxy with a similar star formation history as our fiducial model, leads to a highly irregular galaxy with no kinematically cold component, overly extended stellar distribution, and low angular momentum. This indicates that only when feedback is allowed to become vigorous via locally efficient star formation in dense cold gas do resulting galaxy sizes, gas/stellar surface density profiles, and stellar disk angular momenta agree with observed z = 0 galaxies.
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| 4. |
- Agertz, Oscar, et al.
(författare)
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Vintergatan - i. The origins of chemically, kinematically, and structurally distinct discs in a simulated milky way-mass galaxy
- 2021
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 503:4, s. 5826-5845
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Tidskriftsartikel (refereegranskat)abstract
- 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 MilkyWay-mass disc galaxy. We find that in connection to the last major merger at z ∼ 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 ∼ 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 disc to the outer disc. Secular disc heating is found to give rise to increasing velocity dispersions and scale heights with stellar age, which together with disc flaring explains several trends observed in the MilkyWay, including shallower radial [Fe/H] profiles above the mid-plane.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 scale height 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.
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| 5. |
- Agertz, Oscar, et al.
(författare)
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Virtual Universes
- 2011
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Ingår i: Tofsen. ; :4, s. 38-39
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 6. |
- Andersson, Eric P., et al.
(författare)
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INFERNO : Galactic winds in dwarf galaxies with star-by-star simulations including runaway stars
- 2023
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 521:2, s. 2196-2214
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Tidskriftsartikel (refereegranskat)abstract
- 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 multiphase, spanning almost 8 orders of magnitude in temperature, and with a clear dichotomy between mass ejected in cold, slow-moving (T ≲ 5 × 104 K, v < 100 km s-1) gas and energy ejected in hot, fast-moving (T > 106 K, v > 100 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.
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| 7. |
- Andersson, Eric P., et al.
(författare)
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Pre-supernova feedback sets the star cluster mass function to a power law and reduces the cluster formation efficiency
- 2024
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Ingår i: Astronomy and Astrophysics. - 0004-6361. ; 681
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Tidskriftsartikel (refereegranskat)abstract
- Context. 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 (CFE) 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. Aims. We study how feedback mechanisms acting on different timescales and with different energy budgets affect the star cluster mass function and CFE. Methods. 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 (SNe). Results. Each feedback mechanism affects the CFE and cluster mass function. Increasing the feedback budget by combining the different types of feedback decreases the CFE 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 CFE, albeit with a steeper slope than that usually observed.
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| 8. |
- Andersson, Eric P., et al.
(författare)
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Runaway stars masquerading as star formation in galactic outskirts
- 2021
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Ingår i: Monthly Notices of the Royal Astronomical Society: Letters. - : Oxford University Press (OUP). - 1745-3925 .- 1745-3933. ; 502:1, s. 29-34
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Bland-Hawthorn, Joss, et al.
(författare)
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The Rapid Onset of Stellar Bars in the Baryon-dominated Centers of Disk Galaxies
- 2023
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Ingår i: Astrophysical Journal. - 0004-637X. ; 947:2
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Tidskriftsartikel (refereegranskat)abstract
- Recent observations of high-redshift galactic disks (z ≈ 1-3) show a strong negative trend in the dark-matter (DM) fraction f DM with increasing baryon surface density. For this to be true, the inner baryons must dominate over DM in early massive galaxies, as observed in the Milky Way today. If disks are dominant at early times, we show that stellar bars form promptly within these disks, leading to a high bar fraction. New James Webb Space Telescope observations provide the best evidence for mature stellar bars in this redshift range. The disk mass fraction f disk within R s = 2.2 R disk is the dominant factor determining how rapidly a bar forms. Using 3D hydro simulations of halo-bulge-disk galaxies, we confirm the “Fujii relation” for the exponential dependence of the bar formation time τ bar as a function of f disk. For f disk > 0.3, the bar formation time declines exponentially fast with increasing f disk. Instead of Fujii's arbitrary threshold for when a bar appears, for the first time, we exploit the exponential growth timescale associated with the positive feedback cycle as the bar emerges from the underlying disk. A modified, mass-dependent trend is observed for halos relevant to systems at cosmic noon ( 10.5 < log M halo < 12 ), where the bar onset is slower for higher-mass halos at a fixed f disk. If baryons dominate over DM within R ≈ R s, we predict that a high fraction of bars will be found in high-redshift disks long before z = 1.
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| 10. |
- Ejdetjärn, Timmy
(författare)
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Exploring the nature of ISM turbulencein disc galaxies
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Galaxy formation is a continuous process that started only a few hundred million yearsafter the Big Bang. The first galaxies were very volatile, with bursts of star formationand disorganised gas motions. However, even as these galaxies evolved to have orderlyrotating gas discs, the gas within the disc, referred to as the interstellar medium (ISM),still remained highly turbulent. In fact, the ISM is supersonically turbulent, meaning thatthe disorganised gas motion exceeds the speed of sound in the medium. This supersonicturbulence has been connected to several crucial properties related to galaxy evolution; forexample, increasing (and decreasing in some regions) the ISM gas density, star formation,and gas mixing.Many observation have shown that all of the gas phases in the ISM experience su-personic levels of turbulence, with line widths (an observational method to quantify theamount of turbulence) as high as σg ≲ 100 km s−1 in high-redshift (younger) disc galaxies,while local quiescent discs have σg ≲ 40 km s−1 . However, the ISM contains a variety ofgas phases that cover a wide range of temperatures and densities, which exhibit differentlevels of turbulence. For example, the warm ionised gas phase represents the upper limitsquoted above, while colder denser gas only reaches σg ≲ 40 km s−1 and σg ≲ 15 km s−1 inhigh-redshift and local galaxies, respectively.The physical processes driving this turbulence are not fully understood, but a combi-nation of stellar feedback (e.g. supernova) and gravitational instability (e.g. during cloudcollapse) have been suggested to provide a majority of the turbulent energy. In particular,stellar feedback is crucial in the formation of warm ionised gas and may therefore have asignificant contribution on the turbulence within ionised gas. Furthermore, heterogeneousdata of widely different galaxies (in terms of e.g. mass and size) at different resolutions(which causes artificial line broadening) complicates understanding the underlying cause.A commonly used tracer of ionised gas is the Hα emission line and has been usedextensively in high-redshift surveys. However, the contribution of the Hα signal comesfrom two primary sources: the radiatively ionised regions around massive newborn starsembedded in molecular gas (called H II regions) and diffuse ionised gas (DIG) filling theentire galactic disc. Observations have found that these two sources contribute, on average,roughly the same amount to the Hα signal (although with a large spread), but the levelsof turbulence is starkly different; with the DIG being roughly 2-3 times more turbulethan the gas in H II regions.Numerical simulations have come a long way and are now able to simulate entire discgalaxies at parsec-scale resolution (in regions of interest). Furthermore, galaxy simulationshave been able to reproduce the level of turbulence observed in local and high-redshiftgalaxies. Direct comparisons between numerical and observational studies are crucial tounderstand the relevant physics driving observed correlations. However, numerical andobservational work have different data available and the reduction/analysis varies betweenauthors, and so diligence is required to perform qualitative comparisons.In this work, I perform numerical simulations to investigate ISM turbulence in differentgas phases. My simulations model a Milky Way-like galaxy at two different redshifts(using gas fraction as a proxy for redshift) and with/without stellar feedback physics, toevaluate its impact. I perform mock observations to explore the relation between the starformation rate and turbulence, and investigate what is driving this relation. Additionally, Ianalyse the Hα emission line and compare the contribution in intensity and line broadening(turbulence) from H II regions and DIG.
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