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Based on recent findings of a formation mechanism of substructure in tidal tails by Kuepper, Macleod & Heggie (2008) we investigate a more comprehensive set of N-body models of star clusters on orbits about a Milky-Way-like potential. We find that the predicted epicyclic overdensities arise in any tidal tail no matter which orbit the cluster follows as long as the cluster lives long enough for the overdensities to build up. The distance of the overdensities along the tidal tail from the cluster centre depends for circular orbits only on the mass of the cluster and the strength of the tidal field, and therefore decreases monotonically with time, while for eccentric orbits the orbital motion influences the distance, causing a periodic compression and stretching of the tails and making the distance oscillate with time. We provide an approximation for estimating the distance of the overdensities in this case. We describe an additional type of overdensity which arises in extended tidal tails of clusters on eccentric orbits, when the acceleration of the tidal field on the stellar stream is no longer homogeneous. Moreover, we conclude that a pericentre passage or a disk shock is not the direct origin of an overdensity within a tidal tail. Escape due to such tidal perturbations does not take place immediately after the perturbation but is rather delayed and spread over the orbit of the cluster. All observable overdensities are therefore of the mentioned two types. In particular, we note that substructured tidal tails do not imply the existence of dark-matter sub-structures in the haloes of galaxies.
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We summarize the properties of tidal dwarf candidates in a sample of interacting galaxies and classify objects in tidal tails depending on their morphological appearance. New high-resolution dynamical models are needed to understand how the different structures seen in tidal tails are formed.
Using archival data from ATCA, WSRT, and the VLA, we have analyzed the HI emission of 22 tidal tail regions of the Mullan et al. sample of pairwise interacting galaxies. We have measured the column densities, line-of-sight velocity dispersions, and kinetic energy densities on ~kpc scales. We also constructed a tracer of the line-of-sight velocity gradient over ~10 kpc scales. We compared the distributions of these properties between regions that do and do not contain massive star cluster candidates (M_V < -8.5; ~10^4--10^6 M_(sun) as observed in HST WFPC2 VI data). In agreement with Maybhate et al., we find that a local, ~kpc-scale column density of log N_(HI) = 20.6 cm^(-2) is frequently required for detecting clustered star formation. This HI gas also tends to be turbulent, with line-of-sight velocity dispersions ~10--75 km/s, implying high kinetic energy densities (>46 erg pc^(-2)). Thus, high HI densities and pressures, partly determined by the tail dynamical age and other interaction characteristics, are connected to large-scale cluster formation in tidal tails overall. Lastly, we find that the high mechanical energy densities of the gas are likely not generally due to feedback from star formation. Rather, these properties are more likely to be a cause of star formation than a result.
We present a novel and flexible tensor approach to computing the effect of a time-dependent tidal field acting on a stellar system. The tidal forces are recovered from the tensor by polynomial interpolation in time. The method has been implemented in a direct-summation stellar dynamics integrator (NBODY6) and test-proved through a set of reference calculations: heating, dissolution time and structural evolution of model star clusters are all recovered accurately. The tensor method is applicable to arbitrary configurations, including the important situation where the background potential is a strong function of time. This opens up new perspectives in stellar population studies reaching to the formation epoch of the host galaxy or galaxy cluster, as well as for star-burst events taking place during the merger of large galaxies. A pilot application to a star cluster in the merging galaxies NGC 4038/39 (the Antennae) is presented.
While major mergers and their tidal debris are well studied, they are less common than minor mergers (mass ratios < 0.3). The peculiar spiral NGC 2782 is the result of a merger between two disk galaxies with a mass ratio of ~4:1 occurring ~200 Myr ago. This merger produced a molecular and H I-rich, optically bright eastern tail and an H I-rich, optically faint western tail. Non-detection of CO in the western tail by Braine et al. suggested that star formation had not yet begun to occur in that tidal tail. However, deep H{alpha} narrowband images show evidence of recent star formation in the western tail. Across the entire western tail, we find the global star formation rate per unit area ({Sigma}SFR) to be several orders of magnitude less than expected from the total gas density. Together with extended FUV+NUV emission from Galaxy Evolution Explorer along the tail, this indicates a low global star formation efficiency in the tidal tail producing lower mass star clusters. The H II region that we observed has a local (few-kiloparsec scale) {Sigma}SFR from H{alpha} that is less than that expected from the total gas density, which is consistent with other observations of tidal debris. The star formation efficiency of this H II region inferred from the total gas density is low, but normal when inferred from the molecular gas density. These results suggest the presence of a very small, locally dense region in the western tail of NGC 2782 or of a low-metallicity and/or low-pressure star-forming region.
Context: To investigate how the content of massive OB stars affects the long-term evolution of young open clusters and their tidal streams, and how such an effect influences the constraint of initial conditions by looking at the present-day observations. Aims: OB stars are typically in binaries, have a strong wind mass loss during the first few Myr, and many become black holes. These affect the dynamical evolution of an open star cluster and impact its dissolution in a given Galactic potential. We investigate the correlation between the mass of OB stars and the observational properties of open clusters. Hyades-like star clusters are well represented in the Solar neighborhood and thus allow comparisons with observational data. Methods: We perform a large number of star-by-star numerical $N$-body simulations of Hyades-like star clusters by using the high-performance $N$-body code textsc{petar} combined with textsc{galpy}. We also developed the tool to transfer the simulation data to mock observations of Gaia. Results: We find that OB stars and black holes have a major effect on star cluster evolution. Star clusters with the same initial conditions, but a different initial content of OB stars, follow very different evolutionary paths. Thus, the initial total mass and radius of an observed star cluster cannot be unambiguously determined unless the initial content of OB stars is known. We show that the stellar counts in the corresponding tidal tails, that can be identified in the Gaia data, help to resolve this issues. We thus emphasise the importance of exploring not only star-clusters, but also their corresponding tidal tails. These findings are relevant for studies of the formation of massive stars.
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