A New Formation Scenario for the Milky Way Cluster NGC 2419

Abstract

We present a new formation scenario for NGC 2419, which is one of the most luminous, one of the most distant, and as well one of the most extended globular clusters of the Milky Way. We propose that NGC 2419 is the remnant of a merged star cluster complex, which was possibly formed during an interaction between a gas-rich galaxy and the Milky Way. To test this hypothesis, we performed numerical simulations of 27 different models of star cluster complexes (CCs) moving on a highly eccentric orbit in the Galactic halo. We vary the CC mass, the CC size, and the initial distribution of star clusters in the CC to analyze the influence of these parameters on the resulting objects.In all cases, the vast majority of star clusters merged into a stable object. The derived parameters mass, absolute V-band magnitude, effective radius, velocity dispersion and the surface brightness profile are, for a number of models, in good agreement with those observed for NGC 2419. Despite the large range of CC sizes, the effective radii of the merger objects are constrained to a relatively small interval. A turnover in the r eff vs. M encl space leads to degenerate states, i.e. relatively compact CCs can produce an object with the same structural parameters as a more massive and larger CC. In consequence, a range of initial conditions can form a merger object comparable to NGC 2419 preventing us to pinpoint the exact parameters of the original CC, which formed NGC 2419.We conclude that NGC 2419 can be well explained by the merged cluster complex scenario. Some of the recently discovered stellar streams in the Galactic halo might be related to the parent galaxy, which produced the cluster complex in our scenario. Measurements of the proper motion of NGC 2419 are necessary to prove an association with one of the stellar streams.

A parametric study on the formation of extended star clusters and ultra-compact dwarf galaxies

“…We used the same seeds for the random number generator to generate the same distribution of star clusters scaled to the corresponding Plummer radius of the CC model. In a previous paper on the extended Milky Way cluster NGC 2419 (Brüns & Kroupa 2011), we have demonstrated that the exact initial distribution of star clusters in an extended CC leads to variations in the structural parameters mass and size of the order 10 to 20%. The initial velocity distribution of the CC models is chosen such that each CC is in virial equilibrium.…”

Strömgrenuvbyphotometry of the peculiar globular cluster NGC 2419

“…The photometric catalogue is available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ E-mail: mfrank@lsw.uni-heidelberg.de Present address: European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748, Garching bei München, Germany 1 Throughout this paper we adopt a distance modulus of 19.71±0.08 mag for NGC 2419, measured by Di based on RR Lyrae; the distance of the Sun to the Galactic centre is assumed to be 8.3 kpc (Gillessen et al 2009). mer Milky Way satellite (e.g. van den Bergh & Mackey 2004), although to date no firm link with Galactic halo substructure, such as the Virgo, Gemini, Orphan, or Sagittarius streams has been established (Casetti-Dinescu et al 2009;Drake et al 2013;Brüns & Kroupa 2011;Casey et al 2014;Belokurov et al 2014). Given its unique location in the size-luminosity plane, it has also been suggested that NGC 2419 is a local example at the lowsurface-brightness end of the ultra-compact dwarf galaxy population (Brodie et al 2011).…”

Mass segregation in the outer halo globular cluster Palomar 14

“…As noted in the introduction, Beccari et al (2011) Ripepi et al 2007;Cohen et al 2010;Brüns & Kroupa 2011;di Criscienzo et al 2011). For ωCen, Sollima et al (2007) found no difference in the luminosity function of stars in fields at 12 and 20 arcmin from the cluster center.…”