arXiv subjects

P. Kroupa

Publications and source records attributed to P. Kroupa.

At least 19 recordsLinked to original sources

arXiv2025

The p-Laplacian as a Framework for Generalizing Newtonian Gravity and MoND

The Radial Acceleration Relation (RAR) follows from Milgromian gravitation (MoND). Velocity dispersion data of many dwarf spheroidal galaxies (dSphs) and galaxy clusters have been reported to be in tension with it. We consider the Generalized Poisson Equation (GPE), expressed in terms of the p-Laplacian, which has been applied in electrodynamics. We investigate whether it can address these tensions. From the GPE we derive a generalized RAR characterized by the $p$-parameter from the p-Laplacian and a velocity dispersion formula for a Plummer model. We apply these models to Milky Way and Andromeda dSphs and HIFLUGS galaxy clusters and derive a $p$-parameter for each dSph and galaxy cluster. We explore a relation of $p$ to the mass density of the bound system, and alternatively a relation of $p$ to the external field predicted from Newtonian gravity. This ansatz allows the deviations of dSphs and galaxy clusters from the RAR without introducing dark matter. Data points deviate from the Milgromian case, $p=3$, with up to $5\sigma$-confidence. Also, we find the model predicts velocity dispersions, each of which lies in the 1$\sigma$-range of their corresponding data point allowing the velocity dispersion to be predicted for dSphs from their baryonic density. The functional relation between the mass density of the bound system and $p$ suggests $p$ to increase with decreasing density. We find for the critical cosmological density $p(\rho_{\text{crit}}) = 12.27 \pm 0.39$. This implies significantly different behaviour of gravitation on cosmological scales. Alternatively, the functional relation between $p$ and the external Newtonian gravitational field suggests $p$ to decrease with increasing field strength.

astro-ph.GA↗

arXiv2024

The mass distribution of stellar mergers: A new scenario for several FS CMa stars

(Abreviated) FS CMa stars belong to a diverse group of stars exhibiting the B[e] phenomenon, which is manifested mainly by the presence of forbidden emission lines and a~strong infrared (IR) excess in their spectra. Recently, a strong magnetic field has been discovered in the FS CMa star IRAS 17449+2320. Its strength and an unusually high space velocity point to a post-merger nature. Stellar mergers may provide an explanation for the complex and sometimes chaotic behaviour of some of the FS CMa stars. We did a statistical study of numerical simulations using Aarseth's NBODY6 code. We show the importance of stellar mergers of low- to intermediate-mass stars (from $\approx$ 1.4 to $\approx$ 8 $\text{M}_{\odot}$) and for B-type stars in particular. We analysed two sets of N-body simulations with different initial orbital period distributions. In the simulations, more massive binaries are treated differently than less massive binaries and the mass limit usually used is 5 $\text{M}_{\odot}$. We also used the value of 2 $\text{M}_{\odot}$ to test the influence of this ambiguous limit on the results. Looking at mass, distance from their birth cluster, and velocity distributions, we investigated the statistical significance of individual spectral types in terms of merger dynamics and how merger events affect the stellar evolution. We found that around 50 \% of stars in the simulated open clusters involved in the formation of mergers are B-type stars. As a result, more than 50 \% of the merger products end up as a B-type star as well. Between 12.54 \% and 23.24 \% of all B-type stars are mergers. These results are a natural consequence of the initial mass function, initial distribution of the binary star parameters, and large range of masses for B-type stars. We present a comparison of the W component and the space velocity of the simulated mergers with a sample of observed FS CMa stars.

astro-ph.GA↗

arXiv2023

The El Gordo galaxy cluster challenges {\Lambda}CDM for any plausible collision velocity

El Gordo (ACT-CL J0102-4915) is an extraordinarily large and bright galaxy cluster collision. In a previous study, we found that El Gordo is in $6.2\sigma$ tension with the $\Lambda$CDM standard model when assuming the nominal mass and infall velocity values from the hydrodynamical simulations of Zhang et al. ($M_{200} = 3.2 \times 10^{15} M_{\odot}$ and $V_{\textrm{infall}} = 2500~\textrm{km~s}^{-1}$, respectively). The recent weak lensing study of Kim et al. showed that the mass of El Gordo is actually $2.13^{+0.25}_{-0.23} \times 10^{15} M_{\odot}$. Here we explore the level of tension between El Gordo and $\Lambda$CDM for the new mass estimate, assuming several $V_{\textrm{infall}}$ values. We find that in order to reduce the tension below the $5\sigma$ level, the El Gordo subclusters should have $V_{\textrm{infall}} < 2300~\textrm{km~s}^{-1}$ ($V_{\textrm{infall}} < 1800~\textrm{km~s}^{-1}$ when considering the combined tension with the Bullet Cluster). To the best of our knowledge, the El Gordo hydrodynamical simulations conducted so far require $V_{\textrm{infall}} \geq 2500~\textrm{km~s}^{-1}$ to simultaneously reproduce its morphology and its high X-ray luminosity and temperature. We therefore conclude that El Gordo still poses a significant challenge to $\Lambda$CDM cosmology. Whether the properties of El Gordo can be reconciled with a lower $V_{\textrm{infall}}$ should be tested with new hydrodynamical simulations that explore different configurations of the interaction.

astro-ph.CO↗

arXiv2023

On the degree of stochastic asymmetry in the tidal tails of star clusters

Context: Tidal tails of star clusters are commonly understood to be populated symmetrically. Recently, the analysis of Gaia data revealed large asymmetries between the leading and trailing tidal tail arms of the four open star clusters Hyades, Praesepe, Coma Berenices and NGC 752. Aims: As the evaporation of stars from star clusters into the tidal tails is a stochastic process, the degree of stochastic asymmetry is quantified in this work. Methods: For each star cluster 1000 configurations of test particles are integrated in the combined potential of a Plummer sphere and the Galactic tidal field over the life time of the particular star cluster. For each of the four star clusters the distribution function of the stochastic asymmetry is determined and compared with the observed asymmetry. Results: The probabilities for a stochastic origin of the observed asymmetry of the four star clusters are: Praesepe ~1.7 sigma, Coma Berenices ~2.4 sigma, Hyades ~6.7 sigma, NGC 752 ~1.6 sigma. Conclusions: In the case of Praesepe, Coma Berenices and NGC 752 the observed asymmetry can be interpreted as a stochastic evaporation event. However, for the formation of the asymmetric tidal tails of the Hyades additional dynamical processes beyond a pure statistical evaporation effect are required.

astro-ph.GA↗

arXiv2022

The distribution and morphologies of Fornax Cluster dwarf galaxies suggest they lack dark matter

Due to their low surface brightness, dwarf galaxies are particularly susceptible to tidal forces. The expected degree of disturbance depends on the assumed gravity law and whether they have a dominant dark halo. This makes dwarf galaxies useful for testing different gravity models. In this project, we use the Fornax Deep Survey (FDS) dwarf galaxy catalogue to compare the properties of dwarf galaxies in the Fornax Cluster with those predicted by the Lambda cold dark matter ($\Lambda$CDM) standard model of cosmology and Milgromian dynamics (MOND). We construct a test particle simulation of the Fornax system. We then use the MCMC method to fit this to the FDS distribution of tidal susceptibility $\eta$ (half-mass radius divided by theoretical tidal radius), the fraction of dwarfs that visually appear disturbed as a function of $\eta$, and the distribution of projected separation from the cluster centre. This allows us to constrain the $\eta$ value at which dwarfs should get destroyed by tides. Accounting for an $r'$-band surface brightness limit of 27.8 magnitudes per square arcsec, the required stability threshold is $\eta_{\textrm{destr}} = 0.25^{+0.07}_{-0.03}$ in $\Lambda$CDM and $ 1.88^{+0.85}_{-0.53}$ in MOND. The $\Lambda$CDM value is in tension with previous $\textit{N}$-body dwarf galaxy simulations, which indicate that $\eta_{\textrm{destr}} \approx 1$. Our MOND $\textit{N}$-body simulations indicate that $\eta_{\textrm{destr}} = 1.70 \pm 0.30$, which agrees well with our MCMC analysis of the FDS. We therefore conclude that the observed deformations of dwarf galaxies in the Fornax Cluster and the lack of low surface brightness dwarfs towards its centre are incompatible with $\Lambda$CDM expectations but well consistent with MOND.

astro-ph.GA↗

arXiv2022

First detection of a magnetic field in low-luminosity B[e] stars: New scenarios for the nature and evolutionary stages of FS CMa stars

We report the first detection of the magnetic field in a star of FS CMa type, a subgroup of objects characterized by the B[e] phenomenon. The split of magnetically sensitive lines in IRAS 17449+2320 determines the magnetic field modulus of 6.2+/-0.2 kG. Spectral lines and their variability reveal the presence of a B-type spectrum and a hot continuum source in the visible. The hot source confirms GALEX UV photometry. Because there is a lack of spectral lines for the hot source in the visible, the spectral fitting gives only the lower temperature limit of the hot source, which is 50 000 K, and the upper limit for the B-type star of 11 100 K. The V/R ratio of the H alpha line shows quasiperiodic behavior on timescale of 800 days. We detected a strong red-shifted absorption in the wings of Balmer and OI lines in some of the spectra. The absorption lines of helium and other metals show no, or very small, variations, indicating unusually stable photospheric regions for FS CMa stars. We detected two events of material infall, which were revealed to be discrete absorption components of resonance lines. The discovery of the strong magnetic field together with the Gaia measurements of the proper motion show that the most probable nature of this star is that of a post-merger object created after the leaving the binary of the birth cluster. Another possible scenario is a magnetic Ap star around Terminal-Age Main Sequence (TAMS). On the other hand, the strong magnetic field defies the hypothesis that IRAS 17449+2320 is an extreme classical Be star. Thus, IRAS 17449+2320 provides a pretext for exploring a new explanation of the nature of FS CMa stars or, at least, a group of stars with very similar spectral properties.

astro-ph.SR↗

arXiv2021

A possible solution to the Milky Way's binary-deficient retrograde stellar population. Evidence that $ω$ Centauri has formed in an extreme starburst

Context. The fraction of field binaries on retrograde orbits about the Milky Way is significantly lower compared to its prograde counterpart. Chemical and dynamical evidence suggests that the retrograde stellar population originates from $ω$ Centauri, which is either the most massive globular cluster (GC) of the Milky Way or the putative core of a former dwarf galaxy. Aims. Star formation conditions required to produce the retrograde binary population are constrained assuming that the retrograde stellar population originates from $ω$ Centauri's progenitor. Methods. We match the observed low binary fraction with dynamical population synthesis models, including a universal initial binary population and dynamical processing in star clusters, making use of the publicly available binary population synthesis tool BiPoS1. Results. It is found that either the GC progenitor of $ω$ Centauri must have formed with a stellar density of $\approx 10^8 \; M_{sun} \; pc^{-3}$ or the $ω$ Centauri dwarf galaxy's progenitor star cluster population must have formed in an extreme starburst with a star formation rate exceeding $1000 \; M_{sun} \; yr^{-1}$ and probably a top-heavy embedded-cluster mass function with suppressed low-mass cluster formation. The separation and mass ratio distribution for retrograde field binaries are predicted for comparison with future observations. Conclusions. A viable solution for the deficiency of binaries on retrograde orbits is presented, and star formation conditions for $ω$ Centauri as well as orbital parameter distributions for the Milky Way's retrograde binary population are predicted. The dwarf galaxy origin for $ω$ Centauri is tentatively preferred within the present context.

astro-ph.GA↗

arXiv2021

BiPoS1 -- a computer programme for the dynamical processing of the initial binary star population

The first version of the Binary Population Synthesizer (BiPoS1) is made publicly available. It allows to efficiently calculate binary distribution functions after the dynamical processing of a realistic population of binary stars during the first few Myr in the hosting embedded star cluster. Instead of time-consuming N-body simulations, BiPoS1 uses a stellar dynamical operator which determines the fraction of surviving binaries depending on the binding energy of the binaries. The stellar dynamical operator depends on the initial star cluster density as well as the time until the residual gas of the star cluster is expelled. BiPoS1 has also a galactic-field mode, in order to synthesize the stellar population of a whole galaxy. At the time of gas expulsion, the dynamical processing of the binary population is assumed to efficiently end due to the subsequent expansion of the star cluster. While BiPoS1 $has been used previously unpublished, here we demonstrate its use in the modelling of the binary populations in the Orion Nebula Cluster, in OB associations and as an input for simulations of globular clusters.

astro-ph.GA↗

arXiv2020

A massive blow for $Λ$CDM $-$ the high redshift, mass, and collision velocity of the interacting galaxy cluster El Gordo contradicts concordance cosmology

El Gordo (ACT-CL J0102-4915) is an extremely massive galaxy cluster ($M_{200} \approx 3 \times 10^{15}$ $M_{\odot}$) at redshift $z = 0.87 $ composed of two subclusters with mass ratio 3.6 merging at speed $V_{infall} \approx 2500$ km/s. Such a fast collision between individually rare massive clusters is unexpected in Lambda cold dark matter ($Λ$CDM) cosmology at such high $z$. However, this is required for non-cosmological hydrodynamical simulations of the merger to match its observed properties (Zhang et al. 2015). Here, we determine the probability of finding a similar object in a $Λ$CDM context using the Jubilee simulation box with side length $6 \, h^{-1}$ Gpc. We search for galaxy cluster pairs that have turned around from the cosmic expansion with properties similar to El Gordo in terms of total mass, mass ratio, redshift, and collision velocity relative to virial velocity. We fit the distribution of pair total mass quite accurately, with the fits used in two methods to infer the probability of observing El Gordo in the surveyed region. The more conservative (and detailed) method involves considering the expected distribution of pairwise mass and redshift for analogue pairs with similar dimensionless parameters to El Gordo in the past lightcone of a $z = 0$ observer. Detecting one pair with its mass and redshift rules out $Λ$CDM cosmology at $6.16σ$. We also use the results of Kraljic & Sarkar (2015) to show that the Bullet Cluster is in $2.78σ$ tension once the sky coverage of its discovery survey is accounted for. Using a $χ^2$ approach, the combined tension can be estimated as $6.43 σ$. Both collisions arise naturally in a Milgromian dynamics (MOND) cosmology with light sterile neutrinos.

astro-ph.CO↗

arXiv2018

The impact of the metallicity and star formation rate on the time-dependent galaxy-wide stellar initial mass function

The stellar initial mass function (IMF) is commonly assumed to be an invariant probability density distribution function of initial stellar masses being represented by the canonical IMF. As a consequence the galaxy-wide IMF (gwIMF), defined as the sum of the IMFs of all star forming regions, should also be invariant. Recent observational and theoretical results challenge the hypothesis that the gwIMF is invariant. In order to study the possible reasons for this variation we use the IMF determined in resolved star clusters and apply the IGIMF-theory to calculate a grid of gwIMF models for metallicities, -3<[Fe/H]<1, and galaxy-wide star formation rates, $10^{-5}$ 1\,M_\odot$/yr, which is a common condition in the early Universe, we find that the gwIMF is top-heavy (more massive stars), when compared to the canonical IMF. For a SFR $< 1\,\mathrm{M_{\odot}/yr}$ the gwIMF becomes top-light regardless of the metallicity. For metallicities $\mathrm{[Fe/H]} > 0$ the gwIMF can become bottom-heavy regardless of the SFR. The IGIMF models predict that massive elliptical galaxies should have formed with a gwIMF that is top-heavy within the first few hundred Myr of the galaxy's life and that it evolves into a bottom-heavy gwIMF in the metal-enriched galactic center. We study the SFR$-$H$α$ relation, its dependency on metallicity and the SFR, the correction factors to the Kennicutt SFR$_{\rm K}-$H$α$ relation, and provide new fitting functions Late-type dwarf galaxies show significantly higher SFRs with respect to Kennicutt SFRs, while star forming massive galaxies have significantly lower SFRs than hitherto thought. This has implications for the gas-consumption time scales and for the main sequence of galaxies. The Leo P and ultra-faint dwarf galaxies are discussed explicitly. [abridged]

astro-ph.GA↗

arXiv2017

The optimally-sampled galaxy-wide stellar initial mass function - Observational tests and the publicly available GalIMF code

Here we present a full description of the integrated galaxy-wide initial mass function (IGIMF) theory in terms of the optimal sampling and compare it with available observations. Optimal sampling is the method we use to discretize the IMF into stellar masses deterministically. Evidence has been indicating that nature may be closer to deterministic sampling as observations suggest a smaller scatter of various relevant observables than random sampling would give, which may result from a high level of self-regulation during the star formation process. The variation of the IGIMFs under various assumptions are documented. The results of the IGIMF theory are consistent with the empirical relation between the total mass of a star cluster and the mass of its most massive star, and the empirical relation between a galaxy's star formation rate (SFR) and the mass of its most massive cluster. Particularly, we note a natural agreement with the empirical relation between the IMF's power-law index and a galaxy's SFR. The IGIMF also results in a relation between the galaxy's SFR and the mass of its most massive star such that, if there were no binaries, galaxies with SFR $<10^{-4}$ M$_\odot$/yr should host no Type II supernova events. In addition, a specific list of initial stellar masses can be useful in numerical simulations of stellar systems. For the first time, we show optimally-sampled galaxy-wide IMFs (OSGIMF) which mimics the IGIMF with an additional serrated feature. Finally, A Python module, GalIMF, is provided allowing the calculation of the IGIMF and OSGIMF in dependence on the galaxy-wide SFR and metallicity.

astro-ph.GA↗

arXiv2017

Generation of inclined protoplanetary discs and misaligned planets through mass accretion I: Coplanar secondary discs

We study the three-dimensional evolution of a viscous protoplanetary disc which accretes gas material from a second protoplanetary disc during a close encounter in an embedded star cluster. The aim is to investigate the capability of the mass accretion scenario to generate strongly inclined gaseous discs which could later form misaligned planets. We use smoothed particle hydrodynamics to study mass transfer and disc inclination for passing stars and circumstellar discs with different masses. We explore different orbital configurations to find the parameter space which allows significant disc inclination generation. \citet{Thi2011} suggested that significant disc inclination and disc or planetary system shrinkage can generally be produced by the accretion of external gas material with a different angular momentum. We found that this condition can be fullfilled for a large range of gas mass and angular momentum. For all encounters, mass accretion from the secondary disc increases with decreasing mass of the secondary proto-star. Thus, higher disc inclinations can be attained for lower secondary stellar masses. Variations of the secondary disc's orientation relative to the orbital plane can alter the disc evolution significantly. The results taken together show that mass accretion can change the three-dimensional disc orientation significantly resulting in strongly inclined discs. In combination with the gravitational interaction between the two star-disc systems, this scenario is relevant for explaining the formation of highly inclined discs which could later form misaligned planets.

astro-ph.EP↗

arXiv2017

The origin of discrete multiple stellar populations in globular clusters

Recent observations have revealed that at least several old globular clusters (GCs) in the Galaxy have discrete distributions of stars along the Mg-Al anti-correlation. In order to discuss this recent observation, we construct a new one-zone GC formation model in which the maximum stellar mass (m_max) in the initial mass function (IMF) of stars in a forming GC depends on the star formation rate (SFR), as deduced from independent observations. We investigate the star formation histories of forming GCs. The principal results are as follows. About 30 Myr after the formation of the first generation (1G) of stars within a particular GC, new stars can be formed from ejecta from asymptotic giant branch (AGB) stars of 1G. However, the formation of this second generation (2G) of stars can last only for [10-20] Myr, because the most massive SNe of 2G expel all of the remaining gas. The third generation (3G) of stars are then formed from AGB ejecta ~ 30$ Myr after the truncation of 2G star formation. This cycle of star formation followed by its truncation by SNe can continue until all AGB ejecta is removed from the GC by some physical process. Thus, it is inevitable that GCs have discrete multiple stellar populations in the [Mg/Fe]-[Al/Fe] diagram. Our model predicts that low-mass GCs are unlikely to have discrete multiple stellar populations, and young massive clusters may not have massive OB stars owing to low m_max ([20-30] M_sun) during the secondary star formation.

astro-ph.GA↗

arXiv2017

Using binary statistics in Taurus-Auriga to distinguish between brown dwarf formation processes

Whether BDs form as stars through gravitational collapse ("star-like") or BDs and some very low-mass stars constitute a separate population which form alongside stars comparable to the population of planets, e.g. through circumstellar disk ("peripheral") fragmentation, is one of the key questions of the star-formation problem. For young stars in Taurus-Auriga the binary fraction is large with little dependence on primary mass above ~0.2Msun, while for BDs it is <10%. We investigate a case in which BDs in Taurus formed dominantly through peripheral fragmentation. The decline of the binary frequency in the transition region between star-like and peripheral formation is modelled. A dynamical population synthesis model is employed in which stellar binary formation is universal. Peripheral objects form separately in circumstellar disks with a distinctive initial mass function (IMF), own orbital parameter distributions for binaries and a low binary fraction. A small amount of dynamical processing of the stellar component is accounted for as appropriate for the low-density Taurus-Auriga embedded clusters. The binary fraction declines strongly between the mass-limits for star-like and peripheral formation. The location of characteristic features and the steepness depend on these mass-limits. Such a trend might be unique to low density regions hosting dynamically unprocessed binary populations. The existence of a strong decline in the binary fraction -- primary mass diagram will become verifiable in future surveys on BD and VLMS binarity in the Taurus-Auriga star forming region. It is a test of the (non-)continuity of star formation along the mass-scale, the separateness of the stellar and BD populations and the dominant formation channel for BDs and BD binaries in regions of low stellar density hosting dynamically unprocessed populations.

astro-ph.GA↗

arXiv2017

DGSAT: Dwarf Galaxy Survey with Amateur Telescopes II. A catalogue of isolated nearby edge-on disk galaxies and the discovery of new low surface brightness systems

The connection between the bulge mass or bulge luminosity in disk galaxies and the number, spatial and phase space distribution of associated dwarf galaxies is a discriminator between cosmological simulations related to galaxy formation in cold dark matter and generalized gravity models. Here, a nearby sample of isolated Milky Way class edge-on galaxies is introduced, to facilitate observational campaigns to detect the associated families of dwarf galaxies at low surface brightness. Three galaxy pairs with at least one of the targets being edge-on are also introduced. About 60% of the catalogued isolated galaxies contain bulges of different size, while the remaining objects appear to be bulge-less. Deep images of NGC 3669 (small bulge, with NGC 3625 at the edge of the image) and NGC 7814 (prominent bulge), obtained with a 0.4-m aperture, are also presented, resulting in the discovery of two new dwarf galaxy candidates, NGC3669-DGSAT-3 and NGC7814-DGSAT-7. Eleven additional low surface brightness galaxies are identified, previously notified with low quality measurement flags in the Sloan Digital Sky Survey (SDSS). Integrated magnitudes, surface brightnesses, effective radii, Sersic indices, axis ratios, and projected distances to their putative major hosts are displayed. At least one of the galaxies, NGC3625-DGSAT-4, belongs with a surface brightness of approximately 26 mag per arcsec^2 and effective radius >1.5 kpc to the class of ultra-diffuse galaxies (UDGs). NGC3669-DGSAT-3, the galaxy with lowest surface brightness in our sample, may also be an UDG.

astro-ph.GA↗

arXiv2016

The bound fraction of young star clusters

The residual gas within newly formed star clusters is expelled through stellar feedback on timescales ~ 1 Myr. The subsequent expansion of the cluster results in an unbinding of a fraction of stars before the remaining cluster members can re-virialize and form a surviving cluster. We investigate the bound fraction after gas expulsion as a function of initial cluster mass in stars and gauge the influence of primordial mass segregation, stellar evolution and the tidal field at the solar distance. We also assess the impact of the star-formation efficiency and gas expulsion velocity. We perform N-body simulations using Sverre Aarseth's NBODY7 code, starting with compact clusters in their embedded phase and approximate the gas expulsion by means of an exponentially depleting external gravitational field. We follow the process of re-virialization through detailed monitoring of different Lagrange radii over several Myr, examining initial half-mass radii of 0.1 pc, 0.3 pc and 0.5 pc and initial masses usually ranging from $5\times10^3 M_\odot$ to $5\times10^4 M_\odot$. The adopted star-formation efficiency of 0.33 in the cluster volume results in a distinct sensitivity to the gas expulsion velocity over a wide mass range, while a variation of the star-formation efficiency can make the cluster robust to the rapidly decreasing external potential. We confirm that primordial mass segregation leads to a smaller bound fraction, its influence possibly decreasing with mass. Stellar evolution has a higher impact on lower mass clusters, but heating through dynamical friction could expand the cluster to a similar extent. The examined clusters expand well within their tidal radii and would survive gas expulsion even in a strong tidal field.

astro-ph.GA↗

arXiv2016

Understanding the internal dynamics of elliptical galaxies without non-baryonic dark matter

Assuming virial equilibrium and Newtonian dynamics, low-mass early-type galaxies have larger velocity dispersions than expected from the amount of baryons they contain. The conventional interpretation of this finding is that their dynamics is dominated by non-baryonic matter. However, there is also strong evidence that many low-mass early-type galaxies formed as tidal dwarf galaxies, which would contain almost no dark matter. Using an extensive catalogue of early-type galaxies, we therefore discuss how the internal dynamics of early-type galaxies in general can be understood by replacing the assumption of non-baryonic dark matter with two alternative assumptions. The first assumption is that Milgromian dynamics (i.e., MOND) is valid, which changes the effective gravitational force in the weak-field limit. The second assumption is that binary stars affect the observed line-of-sight velocity dispersions. Some moderate discrepancies between observed and predicted velocity dispersions remain also when these effects are implemented. Nevertheless, the observed velocity dispersions in early-type galaxies can then easily be explained without invoking the presence of non-baryonic dark matter in them, but with already documented variations of the galaxy-wide stellar initial mass function and non-equilibrium dynamics in some of the low-mass early-type galaxies.

astro-ph.GA↗

arXiv2016

Distribution of star formation rates during the rapid assembly of NGC 1399 as deduced from its globular cluster system

Ultra-compact dwarf galaxies (UCDs) share many properties with globular clusters (GCs) and are found in similar environments. A large sample of UCDs and GCs around NGC 1399, the central giant elliptical of the Fornax galaxy cluster, is used to infer their formation history and also that of NGC 1399. We assumed that all GCs and UCDs in our sample are star clusters (SCs) and used them as tracers of past star formation activities. After correcting our GC/UCD sample for mass loss, we interpreted their overall mass function to be a superposition of SC populations that formed coevally during different times. The SC masses of each population were distributed according to the embedded cluster mass function (ECMF), a pure power law with the slope $-β$ and a stellar upper mass limit, $M_{\mathrm{max}}$, which depended on the star formation rate (SFR). We decomposed the observed GC/UCD mass function into individual SC populations and converted $M_{\mathrm{max}}$ of each SC population to an SFR. The overall distribution of SFRs reveals how the GC/UCD sample formed. Considering the age of the GCs/UCDs and the present stellar mass of NGC 1399, we found that the formation of the GCs/UCDs can be well explained for $β<2.3$. This agrees very well with the observation in young SCs where $β\approx2.0$ is usually found. Even if taking into account that some of the most massive objects might not be genuine SCs and applying different corrections for the mass loss, the outcome is not influenced much. We found peak SFRs between approximately 300 and 3000 $M_{\odot}\mathrm{yr}^{-1}$, which matches the SFRs observed in massive high-$z$ sub-mm galaxies and an SFR estimate inferred from NGC 1399 based on "downsizing", i.e. more massive galaxies must have formed over shorter times. Our results suggest that NGC 1399 and its GC/UCD system formed in an early, short, and intense starburst.

astro-ph.GA↗