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Register a new userThe origin of globular cluster systems from cosmological simulations
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We investigate the structural, kinematical, and chemical properties of globular cluster systems (GCSs) in galaxies of different Hubble types in a self-consistent manner based on high-resolution cosmological N-body simulations combined with semi-analytic models of galaxy and globular cluster (GC) formation. We focus on correlations between the physical properties of GCSs and those of their host galaxies for about 10^5 simulated galaxies located at the centres of dark matter halos (i.e. we do not consider satellite galaxies in sub-halos). Our principal results, which can be tested against observations, are as follows. The majority (about 90%) of GCs currently in halos are formed in low-mass galaxies at redshifts greater than 3 with mean formation redshifts of z = 5.7 (12.7 Gyrs ago) and 4.3 (12.3 Gyrs ago) for metal-poor GCs (MPC) and metal-rich GCs (MRCs), respectively. About 52 % of galaxies with GCs show clear bimodality in their metallicity distribution functions, though less luminous galaxies with M_B fainter than -17 are much less likely to show bimodality owing to little or no MRCs. The number fraction of MRCs does not depend on Hubble type but is generally smaller for less luminous galaxies. The specific frequencies (S_ N) of GCSs are typically higher in ellipticals (S_ N ~ 4.0) than in spirals (S_ N ~ 1.8), and higher again (S_N ~ 5.0) for galaxies located at the centers of clusters of galaxies. The total number of GCs per unit halo mass does not depend strongly on M_B or Hubble type of the host galaxy. The mean metallicities of MPCs and MRCs depend on M_B such that they are higher in more luminous galaxies, though the dependence is significantly weakerfor MPCs.
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We report evidence for dynamically significant rotation in the globular cluster systems of two luminous Virgo dwarf ellipticals, VCC1261 and VCC1528. Including previous results for VCC1087, the globular cluster systems of all three Virgo dwarf ellipticals studied in detail to date exhibit v_rot/sigma > 1. Taking the rotation seen in the globular clusters as maximal disk rotation, we find all three dEs lie on the r-band Tully-Fisher relation. We argue that these data support the hypothesis that luminous dEs are the remnants of transformed disk galaxies. We also obtained deep, longslit data for the stars in VCC1261 and VCC1528. Both these galaxies show rapid rotation in their inner regions, with spatial scales of ~0.5 kpc. These rotation velocities are similar to those seen in the GC systems. Since our longslit data for Virgo dEs extend out to 1-2 effective radii (typical of deep observations), whereas the globular clusters extend out to 4--7 effective radii, we conclude that non-detections of rotation in many luminous dEs may simply be due to a lack of radial coverage in the stellar data, and that globular clusters represent singularly sensitive probes of the dynamics of dEs. Based on these data, we suggest that gas disks are significant sites of globular cluster formation in the early universe.
The evolution of globular cluster systems in some galaxies can be cause of merging of globulars in the very central regions. This high stellar density favours the growth of a central nucleus via swallowing of surrounding stars. The infall of stars into a nuclear black hole is here shown to be, under certain conditions, not only source of electromagnetic radiation but also a significant source of gravitational waves.
Using a state-of-the-art cosmological simulation of merging proto-galaxies at high redshift from the FIRE project, with explicit treatments of star formation and stellar feedback in the interstellar medium, we investigate the formation of star clusters and examine one of the formation hypothesis of present-day metal-poor globular clusters. We find that frequent mergers in high-redshift proto-galaxies could provide a fertile environment to produce long-lasting bound star clusters. The violent merger event disturbs the gravitational potential and pushes a large gas mass of ~> 1e5-6 Msun collectively to high density, at which point it rapidly turns into stars before stellar feedback can stop star formation. The high dynamic range of the reported simulation is critical in realizing such dense star-forming clouds with a small dynamical timescale, t_ff <~ 3 Myr, shorter than most stellar feedback timescales. Our simulation then allows us to trace how clusters could become virialized and tightly-bound to survive for up to ~420 Myr till the end of the simulation. Because the clusters tightly-bound core was formed in one short burst, and the nearby older stars originally grouped with the cluster tend to be preferentially removed, at the end of the simulation the cluster has a small age spread.
Globular clusters in the Milky Way are thought to have either an {it in situ} origin, or to have been deposited in the Galaxy by past accretion events, like the spectacular Sagittarius dwarf galaxy merger. We aim to probe the origin of the recently discovered globular cluster FSR 1758, often associated with some past merger event, and which happens to be projected toward the Galactic bulge, by a detailed study of its Galactic orbit, and to assign it to the most suitable Galactic component. We employ three different analytical time-independent potential models to calculate the orbit of the cluster by using the Gauss Radau spacings integration method. In addition, a time-dependent bar potential model is added to account for the influence of the Galactic bar. We run a large suite of simulations to account for the uncertainties in the initial conditions, in a Montecarlo fashion. We confirm previous indications that the globular cluster FSR 1758 possesses a retrograde orbits with high eccentricity. The comparative analysis of the orbital parameters of star clusters in the Milky Way, in tandem with recent metallicity estimates, allows us to conclude that FSR1758 is indeed a Galactic bulge intruder. The cluster can therefore be considered an old metal poor halo globular cluster formed {it in situ} and which is passing right now in the bulge region. Its properties, however, can be roughly accounted for also assuming that the cluster is part of some stream of extra-Galactic origin. We conclude that assessing the origin, either Galactic or extra-galactic, of globular clusters is surely a tantalising task. In any case, by using an {it Occams razor} argument, we tend to prefer an {it in situ} origin for FSR 1758.
We examine the properties of a previously discovered population of globular clusters in the heart of the rich galaxy cluster Abell 1185 that might be intergalactic in nature. Deep images obtained with the Advanced Camera for Surveys (ACS) aboard Hubble Space Telescope (HST) confirm the presence of ~ 1300 globular clusters brighter than I_{F814W} = 27.3 mag in a field devoid of any large galaxies. The luminosities and colors of these objects are found to be similar to those of metal-poor globular clusters observed in many galaxies to date. Although a significant fraction of the detected globular clusters undoubtedly reside in the outer halos of galaxies adjacent to this field, detailed modeling of their distribution suggests that the majority of these objects are likely to be intergalactic, in the sense that they are not gravitationally bound to any individual galaxy. We conclude that the true nature and origin of the globular cluster population in the core of A1185 -- galactic residents or intergalactic wanderers -- remains uncertain, and suggest how future observation could resolve this ambiguity.
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