A universal initial mass function (IMF) is not intuitive, but so far no convincing evidence for a variable IMF exists. The detection of systematic variations of the IMF with star-forming conditions ...would be the Rosetta Stone for star formation. In this contribution an average or Galactic-field IMF is defined, stressing that there is evidence for a change in the power-law index at only two masses: near 0.5 M⊙ and near 0.08 M⊙. Using this supposed universal IMF, the uncertainty inherent in any observational estimate of the IMF is investigated by studying the scatter introduced by Poisson noise and the dynamical evolution of star clusters. It is found that this apparent scatter reproduces quite well the observed scatter in power-law index determinations, thus defining the fundamental limit within which any true variation becomes undetectable. The absence of evidence for a variable IMF means that any true variation of the IMF in well-studied populations must be smaller than this scatter. Determinations of the power-law indices α are subject to systematic errors arising mostly from unresolved binaries. The systematic bias is quantified here, with the result that the single-star IMFs for young star clusters are systematically steeper by Δα≈0.5 between 0.1 and 1 M⊙ than the Galactic-field IMF, which is populated by, on average, about 5-Gyr-old stars. The MFs in globular clusters appear to be, on average, systematically flatter than the Galactic-field IMF (Piotto & Zoccali; Paresce & De Marchi), and the recent detection of ancient white-dwarf candidates in the Galactic halo and the absence of associated low-mass stars (Ibata et al.; Méndez & Minniti) suggest a radically different IMF for this ancient population. Star formation in higher metallicity environments thus appears to produce relatively more low-mass stars. While still tentative, this is an interesting trend, being consistent with a systematic variation of the IMF as expected from theoretical arguments.
It has been known for a long time that the satellite galaxies of the Milky Way (MW) show a significant amount of phase-space correlation, and they are distributed in a highly inclined disc of ...satellites (DoS). We have extended the previous studies on the DoS by analysing for the first time the orientations of streams of stars and gas, and the distributions of globular clusters within the halo of the MW. It is shown that the spatial distribution of MW globular clusters classified as young halo clusters (YH GC) is very similar to that of the DoS, while seven of the 14 analysed streams align with the DoS. The probability to find the observed clustering of streams is only 0.3 per cent when assuming isotropy. The MW thus is surrounded by a vast polar structure (VPOS) of subsystems (satellite galaxies, globular clusters and streams), spreading from Galactocentric distances as small as 10 kpc out to 250 kpc. These findings demonstrate that a near-isotropic infall of cosmological substructure components on to the MW is essentially ruled out because a large number of infalling objects would have had to be highly correlated, to a degree not natural for dark matter substructures. The majority of satellites, streams and YH GCs had to be formed as a correlated population. This is possible in tidal tails consisting of material expelled from interacting galaxies. We discuss the tidal scenario for the formation of the VPOS, including successes and possible challenges. The potential consequences of the MW satellites being tidal dwarf galaxies are severe. If all the satellite galaxies and YH GCs have been formed in an encounter between the young MW and another gas-rich galaxy about 10-11 Gyr ago, then the MW does not have any luminous dark matter substructures and the missing satellites problem becomes a catastrophic failure of the standard cosmological model.
We have carried out a large set of N-body simulations studying the effect of residual-gas expulsion on the survival rate, and final properties of star clusters. We have varied the star formation ...efficiency (SFE), gas expulsion time-scale and strength of the external tidal field, obtaining a three-dimensional grid of models which can be used to predict the evolution of individual star clusters or whole star cluster systems by interpolating between our runs. The complete data of these simulations are made available on the internet. Our simulations show that cluster sizes, bound mass fraction and velocity profile are strongly influenced by the details of the gas expulsion. Although star clusters can survive SFEs as low as 10 per cent if the tidal field is weak and the gas is removed only slowly, our simulations indicate that most star clusters are destroyed or suffer dramatic loss of stars during the gas removal phase. Surviving clusters have typically expanded by a factor of 3 or 4 due to gas removal, implying that star clusters formed more concentrated than as we see them today. Maximum expansion factors seen in our runs are around 10. If gas is removed on time-scales smaller than the initial crossing time, star clusters acquire strongly radially anisotropic velocity dispersions outside their half-mass radii. Observed velocity profiles of star clusters can therefore be used as a constraint on the physics of cluster formation.
ABSTRACT
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.
Abstract
According to the standard model of cosmology, galaxies are embedded in dark matter haloes that are made of particles beyond the standard model of particle physics, thus extending the mass ...and the size of the visible baryonic matter by typically two orders of magnitude. The observed gas distribution throughout the nearby M81 group of galaxies shows evidence for past significant galaxy–galaxy interactions but without a merger between the present-day members having occurred. This group is here studied for possible dynamical solutions within the dark matter standard model. In order to cover a comprehensive set of initial conditions, the inner three core members M81, M82 and NGC 3077 are treated as a three-body model based on Navarro–Frenk–White profiles. The possible orbits of these galaxies are examined statistically taking into account dynamical friction. Long living, non-merging initial constellations that allow multiple galaxy–galaxy encounters comprise unbound galaxies only, which are arriving from a far distance and happen to simultaneously encounter each other within the recent 500 Myr. Our results are derived by the employment of two separate and independent statistical methods, namely a Markov chain Monte Carlo method and the genetic algorithm using the sap
system environment. The conclusions reached are confirmed by high-resolution simulations of live self-consistent systems (N-body calculations). Given the observed positions of the three galaxies, the solutions found comprise predictions for their proper motions.
The stellar initial mass function (IMF) is commonly assumed to be an invariant probability density distribution function of initial stellar masses. These initial stellar masses are generally ...represented by the canonical IMF, which is defined as the result of one star formation event in an embedded cluster. As a consequence, the galaxy-wide IMF (gwIMF) should also be invariant and of the same form as the canonical IMF; gwIMF is defined as the sum of the IMFs of all star-forming regions in which embedded clusters form and spawn the galactic field population of the galaxy. Recent observational and theoretical results challenge the hypothesis that the gwIMF is invariant. In order to study the possible reasons for this variation, it is useful to relate the observed IMF to the gwIMF. Starting with the IMF determined in resolved star clusters, we apply the IGIMF-theory to calculate a comprehensive grid of gwIMF models for metallicities, Fe/H ∈ (−3, 1), and galaxy-wide star formation rates (SFRs), SFR ∈ (10−5, 105) M⊙ yr−1. For a galaxy with metallicity Fe/H < 0 and SFR > 1 M⊙ yr−1, which is a common condition in the early Universe, we find that the gwIMF is both bottom light (relatively fewer low-mass stars) and top heavy (more massive stars), when compared to the canonical IMF. For a SFR < 1 M⊙ yr−1 the gwIMF becomes top light regardless of the metallicity. For metallicities 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 life of the galaxy and that it evolves into a bottom heavy gwIMF in the metal-enriched galactic centre. Using the gwIMF grids, we study the SFR−Hα relation and its dependency on metallicity and the SFR. We also study the correction factors to the Kennicutt SFRK − Hα 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 gas-consumption timescales and for the main sequence of galaxies. We explicitly discuss Leo P and ultra-faint dwarf galaxies.
Context. The Milky Way (MW) and Andromeda (M 31) galaxies possess rotating planes of satellites. The formation of these planes has not been explained satisfactorily so far. It has been suggested that ...the MW and M 31 satellites are ancient tidal dwarf galaxies; this might explain their configuration. This suggestion gained support by an analytic backward-calculation of the relative MW–M 31 orbit in the MOND modified dynamics paradigm. The result implied that the galaxies experienced a close flyby 7–11 Gyr ago. Aims. Here we explore the Local Group history in MOND in more detail using a simplified first-ever self-consistent simulation. We describe the features induced by the encounter in the simulation and identify possible real counterparts of these features. Methods. The initial conditions were set to eventually roughly reproduce the observed MW and M 31 masses, effective radii, separation, relative velocity, and disk inclinations. We used the publicly available adaptive-mesh-refinement code Phantom of RAMSES. Results. Matter was transferred from the MW to M 31 along a tidal tail in the simulation. The encounter induced the formation of several structures resembling the peculiarities of the Local Group. Most notably are that 1) a rotating planar structure formed around M 31 from the transferred material. It had a size similar to the observed satellite plane and was oriented edge-on to the simulated MW, just as the real plane. 2) The same structure also resembled the tidal features observed around M 31 by its size and morphology. 3) A warp in the MW developed with an amplitude and orientation similar to that observed. 4) A cloud of particles formed around the simulated MW, with the extent of the actual MW satellite system. The encounter did not end by merging in a Hubble time. The simulated stellar disks also thickened as a result of the encounter. Conclusions. The simulation demonstrated that MOND might explain many peculiarities of the Local Group; this needs to be verified with additional simulations. The simulation moreover showed that tidal features observed in galaxies, usually interpreted as merger remnants, could have been formed by matter exchange during non-merging galactic flybys in some cases.
Observational studies are showing that the galaxy-wide stellar initial mass function (IMF) is top-heavy in galaxies with high star formation rates (SFRs). Calculating the integrated galactic stellar ...initial mass function (IGIMF) as a function of the SFR of a galaxy, it follows that galaxies which have or which formed with SFRs >... would have a top-heavy IGIMF in excellent consistency with the observations. Consequently and in agreement with observations, elliptical galaxies would have higher mass-to-light ratios as a result of the overabundance of stellar remnants compared to a stellar population that formed with an invariant canonical stellar IMF. For the Milky Way, the IGIMF yields very good agreement with the disc- and the bulge IMF determinations. Our conclusions are that purely stochastic descriptions of star formation on the scales of a parsec and above are falsified. Instead, star formation follows the laws, stated here as axioms, which define the IGIMF theory. We also find evidence that the power-law index β of the embedded cluster mass function decreases with increasing SFR. We propose further tests of the IGIMF theory through counting massive stars in dwarf galaxies. (ProQuest: ... denotes formulae/symbols omitted.)
Abstract
The enormous velocities of the so-called hypervelocity stars (HVSs) derive, likely, from close interactions with massive black holes, binary stars encounters or supernova explosions. In this ...paper, we investigate the origin of HVSs as consequence of the close interaction between the Milky Way central massive black hole and a passing-by young stellar cluster. We found that both single and binary HVSs may be generated in a burst-like event, as the cluster passes near the orbital pericentre. High-velocity stars will move close to the initial cluster orbital plane and in the direction of the cluster orbital motion at the pericentre. The binary fraction of these HVS jets depends on the primordial binary fraction in the young cluster. The level of initial mass segregation determines the value of the average mass of the ejected stars. Some binary stars will merge, continuing their travel across and out of the Galaxy as blue stragglers.
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