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Register a new userBright globular clusters in NGC 5128: the missing link between young massive clusters and evolved massive objects
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M. Rejkuba
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In order to investigate whether the brightest globular clusters (GCs) in the giant elliptical galaxies are similar to the less luminous GCs like those found in Local Group galaxies, we study the velocity dispersion and structural parameter correlations of a sample of bright GCs in the nearest gE galaxy NGC 5128. UVES echelle spectrograph on the ESO VLT, and EMMI on the ESO NTT were used to obtain high resolution spectra of bright GCs in NGC 5128. The velocity dispersions were obtained for all the targets. The structural parameters were either taken from the existing literature, or derived from our VLT FORS1 images using the ISHAPE software. The velocity dispersion and structural parameter measurements were used to obtain masses and M/L_V ratios of 22 clusters. The masses of the clusters in our sample range from M_vir=10^5-10^7 M_sun and the average M/L_V is 3+/-1. The three GCs harbouring X-ray point sources are the second, third and sixth most massive in our sample. The most massive cluster, HCH99-18, is also the brightest and the largest in size. It has the mass (M_vir=1.4x10^7 M_sun) an order of magnitude larger than the most massive clusters in the Local Group, and a high M/L_V ratio (4.7+/-1.2). We discuss briefly possible formation scenarios for this object. The correlations of structural parameters, velocity dispersion, masses and M/L_V for the bright GCs in NGC 5128 extend the properties established for the most massive Local Group clusters towards those characteristic of dE galaxy nuclei and Ultra Compact Dwarfs (UCDs). The detection of the mass-radius and the mass-M/L_V relations for the GCs with masses greater than ~2x10^6 M_sun provides the missing link between ``normal old globular clusters, young massive clusters, and evolved objects like UCDs. (Abridged)
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VLT images in BVI are used to identify the ionizing source centered on Sersic 13, the largest HII region of the giant nearby galaxy NGC 5128 with log L_Halpha=39.6 erg/s. This ionizing source turns out to be a close pair of bright and blue star cluster candidates. Spectroscopy obtained with the Magellan I telescope confirms that these are massive young clusters physically associated with the giant HII region Sersic 13. The spectra of both clusters show prominent Wolf-Rayet type emission features, and prominent lines of HI and HeI, indicative of a very young age (t ~ few x 10^6 yr). Their luminosities make each of them at least as luminous as the massive young cluster R136 in 30 Doradus in the LMC, and their individual masses are estimated to be 1-7.5x10^5 M_sun. In addition, the projected separation of the cluster pair is 42 pc. The measured velocity difference between the clusters is small, Delta V=49+/-21 km/s, and within 2 sigma of the expected orbital velocity V_orb=5-12 km/s if they are bound. Dynamical models predict that binary clusters with these properties would merge in a short timescale of a few orbital periods (P=20-50x10^6 yr). The discovery of this binary cluster suggests that mergers of young massive clusters could lead to the formation of the most massive globular clusters such as omega Cen in our Galaxy and G1 in M31. Alternatively, if they are not gravitationally bound, these objects would individually evolve into two normal globular clusters.
In the last decade we have come to realize that the traditional classification of stellar clusters into open and globular clusters cannot be easily extended beyond the realm of the Milky Way, and that even for our Galaxy it is not fully valid. The main failure of the traditional classification is the existence of Massive Young Clusters (MYCs), which are massive like Globular Clusters (GCs) but also young like open clusters. We describe here the mass and age distributions of clusters in general with an emphasis on MYCs. We also discuss the issue of what constitutes a cluster and try to establish a general classification scheme.
As of August 2019, among the more than 4000 confirmed exoplanets, only one has been detected in a globular cluster (GC) M4. The scarce of exoplanet detections motivates us to employ direct $N$-body simulations to investigate the dynamical stability of planets in young massive clusters (YMCs), which are potentially the progenitors of GCs. In an $N=128{rm k}$ cluster of virial radius 1.7 pc (comparable to Westerlund-1), our simulations show that most wide-orbit planets ($ageq 20$~au) will be ejected within a timescale of 10 Myr. Interestingly, more than $70%$ of planets with $a<5$~au survive in the 100 Myr simulations. Ignoring planet-planet scattering and tidal damping, the survivability at $t$ Myr as a function of initial semi-major axis $a_0$ in au in such a YMC can be described as $f_{rm surv}(a_0, t)=-0.33 log_{10}(a_0) left(1 - e^{-0.0482t} right) + 1$. Upon ejection, about $28.8%$ of free-floating planets (FFPs) have sufficient speeds to escape from the host cluster at a crossing timescale. The other FFPs will remain bound to the cluster potential, but the subsequent dynamical evolution of the stellar system can result in the delayed ejection of FFPs from the host cluster. Although a full investigation of planet population in GCs requires extending the simulations to multi-Gyr, our results suggest that wide-orbit planets and free-floating planets are unlikely to be found in GCs.
The purpose of this research is to study the connection of global properties of eight young stellar clusters projected in the Vista Variables in the Via Lactea (VVV) ESO Large Public Survey disk area and their young stellar object population. The analysis in based on the combination of spectroscopic parallax-based reddening and distance determinations with main sequence and pre-main sequence ishochrone fitting to determine the basic parameters (reddening, age, distance) of the sample clusters. The lower mass limit estimations show that all clusters are low or intermediate mass (between 110 and 1800 Mo), the slope Gamma of the obtained present-day mass functions of the clusters is close to the Kroupa initial mass function. On the other hand, the young stellar objects in the surrounding clusters fields are classified by low resolution spectra, spectral energy distribution fit with theoretical predictions, and variability, taking advantage of multi-epoch VVV observations. All spectroscopically confirmed young stellar objects (except one) are found to be massive (more than 8 Mo). Using VVV and GLIMPSE color-color cuts we have selected a large number of new young stellar object candidates, which are checked for variability and 57% are found to show at least low-amplitude variations. In few cases it was possible to distinguish between YSO and AGB classification on the basis of the light curves.
We present integrated-light spectra of 8 Young Massive Clusters (YMCs) in the metal-rich spiral galaxy NGC 5236 (M 83). The observations were taken with the X-Shooter spectrograph on the ESO Very Large Telescope. Through the use of theoretical isochrones and synthetic integrated-light (IL) spectra we derive metallicities and study the radial metallicity gradient observed through these young populations. For the inner regions of the galaxy we observe a relatively shallow metallicity gradient of $-$0.37 $pm$0.29 dex R$_{25}^{-1}$, agreeing with chemical evolution models with an absence of infall material and a relatively low mass loss due to winds in the inner parts of the disk. We estimate a central metallicity of [$Z$] = $+$0.17 $pm$ 0.12 dex, finding excellent agreement with that obtained via other methods (e.g. blue supergiants and J-band). We infer a metallicity of 12+log(O/H) = 8.75 $pm$ 0.08 dex at R/R$_{25}$ = 0.4, which fits the stellar mass-metallicity relation (MZR) compilation of blue supergiants and IL studies.
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