arXiv subjects

Pavel Kroupa

Publications and source records attributed to Pavel Kroupa.

At least 73 records · Page 4Linked to original sources

arXiv2024

Mass segregation and velocity dispersion as evidence for a dark star cluster

A dark star cluster (DSC) is a system in which the cluster potential is dominated by stellar remnants, such as black holes and neutron stars having larger masses than the long-lived low-mass stars. Due to mass segregation, these remnants are located in the central region of the cluster and form a dark core. We expect that at a few kpc from the Galactic centre, the efficient evaporation of the lower-mass stars caused by the strong tidal force exposes the dark core, because the dynamical properties of the DSC are dominated by the remnants. Due to the invisibility of the remnants, finding a DSC by observation is challenging. In this project, we use $N$-body simulations to obtain models of DSCs and try to discern observables that signify a DSC. We consider four observables: the mass spectrum, the observational mass density profile, the observational velocity dispersion profile and the mass segregation. The models show that a DSC typically exhibits two distinct characteristics: for a given mass in stars and a given half-light radius the expected velocity dispersion is underestimated when only visible stars are considered, and there is a lack of measurable mass segregation among the stars. These properties can be helpful for finding DSCs in observational data, such as the Gaia catalogue.

astro-ph.GA↗

arXiv2024

The impact of a top-heavy IMF on the formation and evolution of dark star clusters

The Spitzer instability leads to the formation of a black hole sub-system (BHSub) at the center of a star cluster providing energy to luminous stars (LSs) and increasing their rate of evaporation. When the self-depletion time of the BHSub exceeds the evaporation time of the LSs, a dark star cluster (DSC) will appear. Using the NBODY7 code, we performed a comprehensive set of direct \Nbody simulations over a wide range of initial conditions to study the pure effect of the top-heaviness of the IMF on the formation of the DSC phase. In the Galactic tidal field, top-heavy IMFs lead to the fast evaporation of LSs and the formation of DSCs. Therefore, DSCs can be present even in the outer region of the Milky Way (MW). To successfully transition to the DSC phase, the MW Globular Clusters (GCs) must possess an initial BH mass fraction of $\widetilde{\mathit{M}}_\mathrm{BH}(0)>0.05$. For star clusters with $\widetilde{\mathit{M}}_\mathrm{BH}(0)>0.08$, the DSC phase will be created for any given initial density of the cluster and Galactocentric distance. The duration of the cluster's lifetime spent in the DSC phase shows a negative (positive) correlation with the initial density, and Galactocentric distance of the star cluster if $\widetilde{\mathit{M}}_\mathrm{BH}(0)\leq 0.12$ ($\widetilde{\mathit{M}}_\mathrm{BH}(0)\geq 0.15$). Considering the canonical IMF, it is unlikely for any MW GCs to enter the DSC phase. We discuss the BH retention fraction in view of the observed properties of the GCs of the MW.

astro-ph.GA↗

arXiv2024

Effects of physical conditions on the stellar initial mass function: The low-metallicity star-forming region Sh 2-209

Recent work suggested that the variation of the initial mass function (IMF) of stars depends on the physical conditions, notably, the metallicity and gas density. We investigated the properties of two clusters, namely the main cluster (MC) and the subcluster (SC), in the low-metallicity HII region Sh 2-209 (S209) based on recently derived IMFs. We tested three previously published correlations using previous observations: the top-heaviness of the IMF in dependence on metallicity, the half-mass radius, and the most massive star in dependence on the stellar mass of the embedded clusters. For this region, two different galactocentric distances, namely 10.5 kpc and 18 kpc, were considered, where an age-distance-degeneracy was found for the previously determined IMF to be consistent with other formulated metallicity and density dependent IMFs. The determined half-mass radius (0.080 +/- 0.005) pc and the embedded cluster density (0.2 +/- 0.1) 10^6 Msun pc^-3 for the MC with an age of 0.5 Myr in S209 assuming a galactocentric distance of 18 kpc support the assumption that a low-metallicity environment results in a denser cluster, which leads to a top-heavy IMF. Thus, all three tests are consistent with the previously published correlations. The results for S209 are placed in the context with the IMF determination within the metal-poor cluster in the star-forming region NGC 346 in the Small Magellanic Cloud.

astro-ph.GA↗

arXiv2024

The spin, expansion and contraction of open star clusters

Empirical constraints on the internal dynamics of open clusters are important for understanding their evolution and evaporation. High precision astrometry from Gaia DR3 are thus useful to observe aspects of the cluster dynamics. This work aims to identify dynamically peculiar clusters such as spinning and expanding clusters. We also quantify the spin frequency and expansion rate and compare them with $N$-body models to identify the origins of the peculiarities. We used the latest Gaia DR3 and archival spectroscopic surveys to analyse the radial velocities and proper motions of the cluster members in 1379 open clusters. A systematic analysis of synthetic clusters is performed to demonstrate the observability of the cluster spin along with effects of observational uncertainties. $N$-body simulations were used to understand the evolution of cluster spin and expansion for initially non-rotating clusters. We identified spin signatures in 10 clusters (and 16 candidates). Additionally, we detected expansion in 18 clusters and contraction in 3 clusters. The expansion rate is compatible with previous theoretical estimates based on expulsion of residual gas. The orientation of the spin axis is independent of the orbital angular momentum. The spin frequencies are much larger than what is expected from simulated initially non-rotating clusters. This indicates that >1% of the clusters are born rotating and/or they have undergone strong interactions. Higher precision observations are required to increase the sample of such dynamically peculiar clusters and to characterise them.

astro-ph.GA↗

arXiv2024

The Relevance of Dynamical Friction for the MW/LMC/SMC Triple System

Simulations of structure formation in the standard cold dark matter cosmological model quantify the dark matter halos of galaxies. Taking into account dynamical friction between the dark matter halos, we investigate the past orbital dynamical evolution of the Magellanic Clouds in the presence of the Galaxy. Our calculations are based on a three-body model of rigid Navarro-Frenk-White profiles for the dark matter halos, but were verified in a previous publication by comparison to high-resolution N-body simulations of live self-consistent systems. Under the requirement that the LMC and SMC had an encounter within 20 kpc between 1 and 4 Gyr ago, in order to allow the development of the Magellanic Stream, and using the latest astrometric data, the dynamical evolution of the MW/LMC/SMC system is calculated backwards in time. With the employment of the genetic algorithm and a Markov-Chain Monte-Carlo method, the present state of this system is unlikely with a probability of <10^{-9} (6 sigma complement), because solutions found do not fit into the error bars for the observed plane-of-sky velocity components of the Magellanic Clouds. This implies that orbital solutions that assume dark matter halos according to cosmological structure formation theory to exist around the Magellanic Clouds and the Milky Way are not possible with a confidence of more than 6 sigma

astro-ph.GA↗

arXiv2024

A co-rotating gas and satellite structure around the interacting galaxy pair NGC 4490/85

The interacting binary system NGC 4490/85 = Arp 269 is intermediate in mass between the Milky Way/Large Magellanic Cloud and the Large/Small Magellanic Cloud binary systems. It is a system of 14 known galaxies. We estimate the total Newtonian gravitating mass of the NGC\,4490/85 group to be M_T = (1.37 +- 0.43) times 10^{12} M_Sun using radial velocities and projected separations of its 13~candidate members. The system of dwarf satellites in the group demonstrates signs of coherent rotation in the same direction as that of the extended HI-shell surrounding the central interacting galaxy pair. The origin of this phase-space correlated population of star-forming late-type satellite galaxies raises questions in view of the planes-of-satellites observed around more massive galaxy pairs that are, however, made up of old early-type dwarf galaxies. We also report the detection of a candidate stellar Plume near the binary. This elongated structure of low surface brightness is a likely optical counterpart to the HI-tail north of NGC 4490/85, recently discovered by the FAST radio telescope.

astro-ph.GA↗

arXiv2024

The formation of compact massive relic galaxies in MOND

Compact massive relic galaxies are a class of galaxies that exhibit characteristics suggesting they have remained largely unchanged since their initial formation, making them 'relics' of the early universe. These galaxies represent a distinct class characterized by strongly peaked high-velocity dispersion profiles with large rotational velocities. This study investigates the formation of such galaxies within the framework of Milgromian Dynamics (MOND), offering a unique perspective on their origin without invoking the presence of cold or warm dark matter. Our analysis focuses on the collapse dynamics of isolated non-rotating post-Big-Bang gas clouds, revealing kinematic and density profiles comparable to observed compact massive relic galaxies like NGC 1277, Mrk 1216, and PGC 032873. The findings underscore the natural emergence of compact massive relic galaxies within a MOND-based Universe, providing valuable insights into the interplay between gravitational dynamics and galaxy formation.

astro-ph.GA↗

arXiv2023

A simultaneous solution to the Hubble tension and observed bulk flow within 250 ${h^{-1}}$ Mpc

The $Λ$ cold dark matter ($Λ$CDM) standard cosmological model is in severe tension with several cosmological observations. Foremost is the Hubble tension, which exceeds $5σ$ confidence. Galaxy number counts show the Keenan-Barger-Cowie (KBC) supervoid, a significant underdensity out to 300~Mpc that cannot be reconciled with $Λ$CDM cosmology. Haslbauer et al. previously showed that a high local Hubble constant arises naturally due to gravitationally driven outflows from the observed KBC supervoid. The main prediction of this model is that peculiar velocities are typically much larger than expected in the $Λ$CDM framework. This agrees with the recent discovery by Watkins et al. that galaxies in the CosmicFlows-4 catalogue have significantly faster bulk flows than expected in the $Λ$CDM model on scales of $100-250 \, h^{-1}$~Mpc. The rising bulk flow curve is unexpected in standard cosmology, causing $4.8σ$ tension at $200 \, h^{-1}$~Mpc. In this work, we determine what the semi-analytic void model of Haslbauer et al. predicts for the bulk flows on these scales. We find qualitative agreement with the observations, especially if our vantage point is chosen to match the observed bulk flow on a scale of $50 \, h^{-1}$~Mpc. This represents a highly non-trivial success of a previously published model that was not constrained by bulk flow measurements, but which was shown to solve the Hubble tension and explain the KBC void consistently with the peculiar velocity of the Local Group. Our results suggest that several cosmological tensions can be simultaneously resolved if structure grows more efficiently than in the $Λ$CDM paradigm on scales of tens to hundreds of Mpc.

astro-ph.CO↗

arXiv2023

A modern view of galaxies and their stellar populations

A critical discourse is provided on the current status of the astrophysics of galaxies in view of open fundamental questions on the law of gravitation and the physics-driven variation of the galaxy-wide stellar initial mass function (GWIMF). The Einstein/Newtonian plus cold or warm dark-matter-based models face many significant but unresolved tensions (e.g. planes of satellites, the highly organised and symmetrical structure of the Local Group, the local Gpc-scale void). The accumulating nature of these indicates quite compellingly the need for a different theoretical framework. An example is the prediction, made in 1997, of the existence of satellite galaxies with near-exact properties to the Hercules dwarf spheroidal satellite galaxy discovered in 2007 if it is a tidal dwarf galaxy void of dark matter. Such results indicate that once the evidence is accepted that dark matter particles have no role in galaxies with gravitation being effectively Milgromian, then the resulting theoretical understanding of galaxies becomes much simpler and highly predictive with remarkable successes. Along with these scientific advancements, the recent observational data solidify the existence of one of the most important relations in star-formation theory, namely the mmax-Mecl relation. The data rule out stochastic star formation and confirm that the IMF becomes increasingly top-heavy with increasing star-formation rate density. Galaxies and their embedded-star-cluster building blocks are therefore self-regulated dynamical systems which are computationally accessible. Finding a theory for the formation of galaxies involves the development of a new cosmological model, which may differ substantially from the dark-matter based one. Very significant new opportunities have thus emerged for inquisitive and daring researchers which may appear risky now but almost certainly lead to major breakthroughs.

astro-ph.GA↗

arXiv2023

The many tensions with dark-matter based models and implications on the nature of the Universe

(Abridged) Fundamental tensions between observations and dark-matter based cosmological models have emerged. This updated review has two purposes: to explore new tensions that have arisen in recent years, compounding the unresolved tensions from previous studies, and to use the shortcomings of the current theory to guide the development of a successful model. Tensions arise in view of the profusion of thin disk galaxies, the pronounced symmetrical structure of the Local Group of Galaxies, the common occurrence of planes of satellite systems, the El Gordo and Bullet galaxy clusters, significant matter inhomogeneities on scales much larger than 100 Mpc, and the observed rapid formation of galaxies and super-massive black holes at redshifts larger than 7. Given the nature of the tensions, the real Universe needs to be described by a model in which gravitation is effectively stronger than Einsteinian/Newtonian gravitation at accelerations below Milgrom's acceleration scale. The promising nuHDM model, anchored on Milgromian dynamics but keeping the standard expansion history with dark energy, solves many of the above tensions. However galaxy formation appears to occur too late in this model, model galaxy clusters reach too large masses, and the mass function of model galaxy clusters is too flat and thus top-heavy in comparison to the observed mass function. Classes of models that reassess inflation, dark energy and the role of the CMB should be explored.

astro-ph.CO↗

arXiv2023

The convective kissing instability in low-mass M-dwarf models: convective overshooting, semi-convection, luminosity functions, surface abundances and star cluster age dating

Low-mass models of M-dwarfs that undergo the convective kissing instability fluctuate in luminosity and temperature resulting in a gap in the main sequence that is observed in the $Gaia$ data. During this instability, the models have repeated periods of full convection where the material is mixed throughout the model. Stellar evolution models are performed using MESA with varying amounts of convective overshooting and semi-convection. We find that the amplitude and intensity of the instability is reduced with increasing amounts of overshooting but sustained when semi-convection is present. This is reflected in the loops in the evolutionary tracks in the Hertzsprung-Russell diagram. The surface abundances of $^1$H, $^3$He, $^4$He, $^{12}$C, $^{14}$N and $^{16}$O increase or decrease over time due to the convective boundary, however the relative abundance changes are very small and not likely observable. The mass and magnitude values from the models are assigned to a synthetic population of stars from the mass-magnitude relation to create colour-magnitude diagrams, which reproduce the M-dwarf gap as a large indent into the blueward edge of the main sequence (MS). This is featured in the luminosity function as a small peak and dip. The width of the MS decreases over time along with the difference in width between the MS at masses higher and lower than the instability. The parallel offset and relative angle between the upper and lower parts of the MS also change with time along with the mass-magnitude relation. Potential age-dating methods for single stars and stellar populations are described.

astro-ph.SR↗

arXiv2023

The integrated galaxy-wide stellar initial mass function over the radial acceleration range of early-type galaxies

The observed radial accelerations of 462 Early-type galaxies (ETGs) at their half-mass radii are discussed. They are compared to the baryonic masses of the same galaxies, which are derived from theoretical expectations for their stellar populations and cover a range from $\approx 10^4 \, {\rm M}_{\odot}$ to $\approx 10^{11} \, {\rm M}_{\odot}$. Both quantities are plotted against each other, and it is tested whether they lie (within errors) along theoretical radial acceleration relations (RARs). We choose the Newtonian RAR and two Milgromian, or MONDian RARs. At low radial accelerations (corresponding to low masses), the Newtonian RAR fails without non-baryonic dark matter, but the two MONDian ones may work, provided moderate out-of-equilibrium dynamics in some of the low-mass ETGs. However all three RARs fail at high accelerations (corresponding to high masses) if all ETGs have formed their stellar populations with the canonical stellar initial mass function (IMF). A much better agreement with the observations can however be accomplished, if the theory of the integrated galaxy-wide stellar initial mass functions (IGIMFs) is used instead. This is because the IGIMF-theory predicts the formation of an overabundance of stellar remnants during the lifetime of the massive ETGs. Thus their baryonic masses today are higher than they would be if the ETGs had formed with a canonical IMF. Also the masses of the stellar-mass black holes should be rather high, which would mean that most of them probably formed when the massive ETGs were not as metal-enriched as they are today. The IGIMF-approach confirms downsizing.

astro-ph.GA↗

arXiv2023

The cosmological star formation history from the Local Cosmological Volume of galaxies and constraints on the matter homogeneity

The Lilly-Madau plot is commonly interpreted as the history of the cosmic star formation of the Universe by showing the co-moving star formation rate density (SFRD) over cosmic time. Therefore, the Lilly-Madau plot is sensitive not only to the star formation history (SFH) but also to the number density of galaxies. Assessing the Catalogue of Neighbouring Galaxies, we reconstruct the SFHs of galaxies located in the Local Cosmological Volume (LV) based on delayed-$τ$ and power-law SFH models. Galaxies with stellar masses of $M_{*} \gtrsim 10^{10}\,\rm{M_{\odot}}$ typically evolve according to the delayed-$τ$ model by having first increasing followed by exponentially declining SFRs, while the majority of less massive star-forming galaxies have an almost constant or increasing SFH. Deducing the cosmic SFRD evolution of the LV reveals that the SFHs of local galaxies are inconsistent with the Lilly-Madau plot. The SFRDs of the LV are significantly lower at redshifts of $z \lesssim 3$ underestimating the Lilly-Madau peak at $z = 1.86$ by a factor of $2.16\pm0.32$ (delayed-$τ$ model) and $5.90\pm0.88$ (power-law model). Assuming the delayed-$τ$ model for galaxies with $M_{*} \geq 10^{10}\,\rm{M_{\odot}}$ and a power-law model for less massive galaxies, the SFRD is $2.22\pm0.33$ lower than measured at $z = 1.86$. This inconsistency between the evolution of the local and global SFRD has cosmological implications. Since the Lilly-Madau plot also constrains the cosmological matter field, the near-constancy of SFHs of LV galaxies could imply that the peak of the Lilly-Madau plot at $z = 1.86$ is the imprint of an $\approx~5$ co-moving Gpc-scale inhomogeneity.

astro-ph.GA↗

arXiv2023

Do old globular clusters in low mass galaxies disprove modified gravity?

The controversy "dark matter vs. modified gravity" constitutes a major topic of discussion. It was proposed that dynamical friction could be used to discriminate between the two alternatives. Analytic calculations indicate that, with modified gravity, globular clusters (GCs) of low-mass galaxies experience much stronger dynamical friction than in the equivalent system with Newtonian gravity and dark matter. As a result, in modified gravity the old GCs of low mass galaxies should have already settled in the centers of the galaxies. This is not observed. Here we report on our efforts to verify the analytic results by self-consistent simulations with the MOND-type (modified Newtonian dynamics) gravity. The core stalling mechanism, that was not considered in the analytic calculations, prevents GCs to settle in centers of ultra-diffuse galaxies. For isolated dwarf galaxies, which are gas-rich objects, supernova explosions prevent the GCs from settling.

astro-ph.GA↗

arXiv2023

Hydrodynamical structure formation in Milgromian cosmology

We present the first hydrodynamical cosmological simulations in the $ν$HDM framework based on Milgromian dynamics (MOND) with light (11~eV) sterile neutrinos. $ν$HDM can explain the expansion history, CMB anisotropies, and galaxy cluster dynamics similarly to standard cosmology while preserving MOND's successes on galaxy scales, making this the most conservative Milgromian framework. We generate initial conditions including sterile neutrinos using \textsc{camb} and \textsc{music} and modify the publicly available code \textsc{phantom of ramses} to run $ν$HDM models. The simulations start at redshift $z_e=199$, when the gravitational fields are stronger than $a_{_0}$ provided this does not vary. We analyse the growth of structure and investigate the impact of resolution and box size, which is at most 600 comoving Mpc. Large density contrasts arise at late times, which may explain the KBC void and Hubble tension. We quantify the mass function of formed structures at different redshifts. We show that the sterile neutrino mass fraction in these structures is similar to the cosmic fraction at high masses (consistent with MOND dynamical analyses) but approaches zero at lower masses, as expected for galaxies. We also identify structures with a low peculiar velocity comparable to the Local Group, but these are rare. The onset of group/cluster scale structure formation at $z_e\approx4$ appears to be in tension with observations of high redshift galaxies, which we discuss in comparison to prior analytical work in a MONDian framework. The formation of a cosmic web of filaments and voids demonstrates that this is not unique to standard Einstein/Newton-based cosmology.

astro-ph.CO↗

arXiv2023

The Possible Emergence of an Attractive Inverse-Square Law from the Wave-Nature of Particles

A model of a particle in finite space is developed and the properties that the particle may possess under this model are studied. The possibility that particles attract each other due to their own wave nature is discussed. The assumption that the particles are spatially confined oscillations (SCO) in the medium is used. The relation between the SCO and the refractive index of the medium in the idealized universe is derived. Due to the plane wave constituents of SCOs, the presence of a refractive index field with a nonzero gradient causes the SCO to accelerate. The SCO locally changes the refractive index such that another SCO is accelerated towards it, and vice versa. It is concluded that the particles can attract each other due to their wave nature and an inverse-square-type acceleration emerges. The constant parameter in the inverse-square-type acceleration is used to compare with the gravitational constant $G_N$, and the possibility of non inverse-square-type behavior is preliminary discussed.

physics.gen-ph↗

arXiv2022

Simulations of star forming main sequence galaxies in Milgromian gravity

We conduct hydrodynamical MOND simulations of isolated disc galaxies over the stellar mass range $M_{\star}/M_\odot = 10^7 - 10^{11}$ using the adaptive mesh refinement code \textsc{phantom of ramses} (\textsc{por}), an adaptation of the \textsc{ramses} code with a Milgromian gravity solver. The scale lengths and gas fractions are based on observed galaxies, and the simulations are run for 5~Gyr. The main aim is to see whether existing sub-grid physics prescriptions for star formation and stellar feedback reproduce the observed main sequence and reasonably match the Kennicutt-Schmidt relation that captures how the local and global star formation rates relate to other properties. Star formation in the models starts soon after initialisation and continues as the models evolve. The initialized galaxies indeed evolve to a state which is on the observed main sequence, and reasonably matches the Kennicutt-Schmidt relation. The available formulation of sub-grid physics is therefore adequate and leads to galaxies that largely behave like observed galaxies, grow in radius, and have flat rotation curves $-$ provided we use Milgromian gravitation. Furthermore, the strength of the bars tends to be inversely correlated with the stellar mass of the galaxy, whereas the bar length strongly correlates with the stellar mass. Irrespective of the mass, the bar pattern speed stays constant with time, indicating that dynamical friction does not affect the bar dynamics. The models demonstrate Renzo's rule and form structures at large radii, much as in real galaxies. In this framework, baryonic physics is thus sufficiently understood to not pose major uncertainties in our modelling of global galaxy properties.

astro-ph.GA↗

arXiv2022

The most massive stars in very young star clusters with a limited mass: Evidence favours significant self-regulation in the star formation processes

The stellar initial mass function (IMF) is commonly interpreted to be a scale-invariant probability density distribution function (PDF) such that many small clusters yield the same IMF as one massive cluster of the same combined number of stars. Observations of the galaxy-wide IMF challenge this as dwarf galaxies do not form as many massive stars as expected. This indicates a highly self-regulated star formation process in which stellar masses are not stochastically sampled from the IMF and are instead related to the environment of star formation. Here, the nature of star formation is studied using the relation between the most massive star born in a star cluster and its parental stellar cluster mass (the $m_{\rm max}$--$M_{\rm ecl}$ relation). This relation has been argued to be a statistical effect if stars are sampled randomly from the IMF. By comparing the tightness of the observed $m_{\rm max}$--$M_{\rm ecl}$ distribution with synthetic star clusters with stochastically sampled stellar masses, we find that the expected dispersion of the mock observations is much larger than the observed dispersion. Assuming that $m_{\rm max}$ and $M_{\rm ecl}$ uncertainties from the literature are correct, our test rejects the hypothesis that the IMF is a PDF at a more than $4.5σ$ confidence level. Alternatively, we provide a deterministic stellar mass sampling tool which reproduces the observed $m_{\rm max}$--$M_{\rm ecl}$ distribution and compares well with the luminosities of star-forming molecular clumps. In addition, we find that there is a significant flattening of the $m_{\rm max}$--$M_{\rm ecl}$ relation near $m_{\rm max}=13~M_\odot$. This may suggest strong feedback of stars more massive than about $13~M_\odot$ and/or the ejections of the most massive stars from young clusters in the mass range 63 to $400~M_\odot$ to be likely important physical processes in forming clusters.

astro-ph.GA↗