The origin of the Milky Way globular clusters Renaud, Florent; Agertz, Oscar; Gieles, Mark
Monthly Notices of the Royal Astronomical Society,
03/2017, Letnik:
465, Številka:
3
Journal Article
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We present a cosmological zoom-in simulation of a Milky Way-like galaxy used to explore the formation and evolution of star clusters. We investigate in particular the origin of the bimodality ...observed in the colour and metallicity of globular clusters, and the environmental evolution through cosmic times in the form of tidal tensors. Our results self-consistently confirm previous findings that the blue, metal-poor clusters form in satellite galaxies that are accreted on to the Milky Way, while the red, metal-rich clusters form mostly in situ, or, to a lower extent, in massive, self-enriched galaxies merging with the Milky Way. By monitoring the tidal fields these populations experience, we find that clusters formed in situ (generally centrally concentrated) feel significantly stronger tides than the accreted ones, both in the present day, and when averaged over their entire life. Furthermore, we note that the tidal field experienced by Milky Way clusters is significantly weaker in the past than at present day, confirming that it is unlikely that a power-law cluster initial mass function like that of young massive clusters, is transformed into the observed peaked distribution in the Milky Way with relaxation-driven evaporation in a tidal field.
We present a hydrodynamical simulation of an Antennae-like galaxy merger at parsec resolution, including a multicomponent model for stellar feedback and reaching numerical convergence in the global ...star formation rate for the first time. We analyse the properties of the dense stellar objects formed during the different stages of the interaction. Each galactic encounter triggers a starburst activity, but the varying physical conditions change the triggering mechanism of each starburst. During the first two pericentre passages, the starburst is spatially extended and forms many star clusters. However, the starburst associated with the third, final passage is more centrally concentrated: stars form almost exclusively in the galactic nucleus and no new star cluster is formed. The maximum mass of stars clusters in this merger is more than 30 times higher than those in a simulation of an isolated Milky Way-like galaxy. Antennae-like mergers are therefore a formation channel of young massive clusters possibly leading to globular clusters. Monitoring the evolution of a few clusters reveals the diversity of formation scenarios including the gathering and merger of gas clumps, the monolithic formation and the hierarchical formation in sub-structures inside a single cloud. Two stellar objects formed in the simulation yield the same properties as ultracompact dwarf galaxies. They share the same formation scenario than the most massive clusters, but have a larger radius either since birth, or get it after a violent interaction with the galactic centre. The diversity of environments across space and time in a galaxy merger can account for the diversity of the stellar objects formed, both in terms of mass and size.
How runaway stars boost galactic outflows Andersson, Eric P; Agertz, Oscar; Renaud, Florent
Monthly Notices of the Royal Astronomical Society,
05/2020, Letnik:
494, Številka:
3
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ABSTRACT
Roughly 10 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 that 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 $10^{-5}\, \mathrm{\,cm^{-3}}$ to $10^{-3}\, \mathrm{\,cm^{-3}}$. 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 multiphase 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.
Using hydrodynamical simulations of entire galactic discs similar to the Milky Way (MW), reaching 4.6 pc resolution, we study the origins of observed physical properties of giant molecular clouds ...(GMCs). We find that efficient stellar feedback is a necessary ingredient in order to develop a realistic interstellar medium, leading to molecular cloud masses, sizes, velocity dispersions, and virial parameters in excellent agreement withMWobservations. GMCscaling relations observed in the MW, such as the mass-size (M-R), velocity dispersion-size (σ-R), and the σ-RΣ relations, are reproduced in a feedback-driven ISM when observed in projection, with M∝R2.3 and σ∝R0.56.When analysed in 3D, GMC scaling relations steepen significantly, indicating potential limitations of our understanding of molecular cloud 3D structure from observations. Furthermore, we demonstrate how a GMC population's underlying distribution of virial parameters can strongly influence the scatter in derived scaling relations. Finally, we show that GMCs with nearly identical global properties exist in different evolutionary stages, where a majority of clouds being either gravitationally bound or expanding, but with a significant fraction being compressed by external ISM pressure, at all times.
Interacting galaxies favour the formation of star clusters but are also suspected to affect their evolution through an intense and rapidly varying tidal field. Treating this complex behaviour remains ...out-of-reach of (semi-)analytical studies. By computing the tidal field from galactic models and including it into star-by-star N-body simulations of star clusters, we monitor the structure and mass evolution of a population of clusters in a galaxy major merger, taking the Antennae galaxies (NGC 4038/39) as a prototype. On the long time-scale (∼109 yr), the merger only indirectly affects the evolution of clusters by modifying their orbits in or around the galaxies: the mass-loss of clusters in the merger remnant is faster, while clusters ejected in the tidal debris survive much longer, compared to in an isolated galaxy. The tidal perturbations of the galactic collisions themselves are too short lived and not strong enough to significantly influence the structure and dissolution of realistically dense/massive star clusters.
ABSTRACT
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 disc 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 disc, thus establishing chemically bimodal thin and thick discs, 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.
We study the impact of stellar feedback in shaping the density and velocity structure of neutral hydrogen (H i) in disc galaxies. For our analysis, we carry out ∼4.6 pc resolution N-body+adaptive ...mesh refinement hydrodynamic simulations of isolated galaxies, set up to mimic a Milky Way and a Large and Small Magellanic Cloud. We quantify the density and velocity structure of the interstellar medium using power spectra and compare the simulated galaxies to observed H i in local spiral galaxies from THINGS (The H i Nearby Galaxy Survey). Our models with stellar feedback give an excellent match to the observed THINGS H i density power spectra. We find that kinetic energy power spectra in feedback-regulated galaxies, regardless of galaxy mass and size, show scalings in excellent agreement with supersonic turbulence (E(k) ∝ k
−2) on scales below the thickness of the H i layer. We show that feedback influences the gas density field, and drives gas turbulence, up to large (kpc) scales. This is in stark contrast to density fields generated by large-scale gravity-only driven turbulence. We conclude that the neutral gas content of galaxies carries signatures of stellar feedback on all scales.
ABSTRACT
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 are 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 (SFR) is enhanced with an associated decrease in the depletion time. Before redshift 5, mergers were 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 SFR. 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 SFR 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.
Evolution of star clusters in arbitrary tidal fields Renaud, Florent; Gieles, Mark; Boily, Christian M.
Monthly Notices of the Royal Astronomical Society,
December 2011, Letnik:
418, Številka:
2
Journal Article
Recenzirano
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We present a novel and flexible tensor approach to computing the effect of a time-dependent tidal field acting on a stellar system. The tidal forces are recovered from the tensor by polynomial ...interpolation in time. The method has been implemented in a direct-summation stellar dynamics integrator (NBODY6) and test-proved through a set of reference calculations: heating, dissolution time and structural evolution of model star clusters are all recovered accurately. The tensor method is applicable to arbitrary configurations, including the important situation where the background potential is a strong function of time. This opens up new perspectives in stellar population studies reaching to the formation epoch of the host galaxy or galaxy cluster, as well as for starburst events taking place during the merger of large galaxies. A pilot application to a star cluster in the merging galaxies NGC 4038/39 (the Antennae) is presented.
ABSTRACT
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 fuelled 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 disc, 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 disc, 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, similar to what is observed in the Milky Way, due to the inclination of the outer disc that eventually aligns with the inner one via gravitational torques. We show that this tilting disc 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 disc and could be tested observationally.
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