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تسجيل مستخدم جديدSpatial segregation of massive clusters in dwarf galaxies
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The relative average minimum projected separations of star clusters in the Legacy ExtraGalactic UV Survey (LEGUS) and in tidal dwarfs around the interacting galaxy NGC 5291 are determined as a function of cluster mass to look for cluster-cluster mass segregation. Class 2 and 3 LEGUS clusters, which have a more irregular internal structure than the compact and symmetric class 1 clusters, are found to be mass segregated in low mass galaxies, which means that the more massive clusters are systematically bunched together compared to the lower mass clusters. This mass segregation is not present in high-mass galaxies nor for class 1 clusters. We consider possible causes for this segregation including differences in cluster formation and scattering in the shallow gravitational potentials of low mass galaxies.
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Several dynamical scenarios have been proposed that can lead to prompt mass segregation on the crossing time scale of a young cluster. They generally rely on cool and/or clumpy initial conditions, and are most relevant to small systems. As a counterp
oint, we present a novel dynamical mechanism that can operate in relatively large, homogeneous, cool or cold systems. This mechanism may be important in understanding the assembly of large mass-segregated clusters from smaller clumps.
(Abridged) Using luminosities and structural parameters of globular clusters (GCs) in the nuclear regions (nGCs) of low-mass dwarf galaxies from HST/ACS imaging we derive the present-day escape velocities (v_esc) of stellar ejecta to reach the cluste
r tidal radius and compare them with those of Galactic GCs with extended (hot) horizontal branches (EHBs-GCs). For EHB-GCs, we find a correlation between the present-day v_esc and their metallicity as well as (V-I)_0 colour. The similar v_esc, (V-I)_0 distribution of nGCs and EHB-GCs implies that nGCs could also have complex stellar populations. The v_esc-[Fe/H] relation could reflect the known relation of increasing stellar wind velocity with metallicity, which in turn could explain why more metal-poor clusters typically show more peculiarities in their stellar population than more metal-rich clusters of the same mass do. Thus the cluster v_esc can be used as parameter to describe the degree of self-enrichment. The nGCs populate the same Mv vs. rh region as EHB-GCs, although they do not reach the sizes of the largest EHB-GCs like wCen and NGC 2419. We argue that during accretion the rh of an nGC could increase due to significant mass loss in the cluster vicinity and the resulting drop in the external potential in the core once the dwarf galaxy dissolves. Our results support the scenario in which Galactic EHB-GCs have originated in the centres of pre-Galactic building blocks or dwarf galaxies that were later accreted by the Milky Way.
A promising mechanism to form intermediate-mass black holes (IMBHs) is the runaway merger in dense star clusters, where main-sequence stars collide and form a very massive star (VMS), which then collapses to a black hole. In this paper we study the e
ffects of primordial mass segregation and the importance of the stellar initial mass function (IMF) on the runaway growth of VMSs using a dynamical Monte Carlo code for N-body systems with N as high as 10^6 stars. Our code now includes an explicit treatment of all stellar collisions. We place special emphasis on the possibility of top-heavy IMFs, as observed in some very young massive clusters. We find that both primordial mass segregation and the shape of the IMF affect the rate of core collapse of star clusters and thus the time of the runaway. When we include primordial mass segregation we generally see a decrease in core collapse time (tcc). Moreover, primordial mass segregation increases the average mass in the core, thus reducing the central relaxation time, which also decreases tcc. The final mass of the VMS formed is always close to sim 10^-3 of the total cluster mass, in agreement with the previous studies and is reminiscent of the observed correlation between the central black hole mass and the bulge mass of the galaxies. As the degree of primordial mass segregation is increased, the mass of the VMS increases at most by a factor of 3. Flatter IMFs generally increase the average mass in the whole cluster, which increases tcc. For the range of IMFs investigated in this paper, this increase in tcc is to some degree balanced by stellar collisions, which accelerate core collapse. Thus there is no significant change in tcc for the somewhat flatter global IMFs observed in very young massive clusters.
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The population of massive black holes (MBHs) in dwarf galaxies is elusive, but fundamentally important to understand the coevolution of black holes with their hosts and the formation of the first collapsed objects in the Universe. While some progress
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