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Register a new userSearching for nuclear stellar discs in simulations of star cluster mergers
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Added by
Enrico Maria Corsini
Publication date
2013
fields
Physics
and research's language is
English
Authors
E. Portaluri
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The nuclei of galaxies often host small stellar discs with scale-lengths of a few tens of parsecs and luminosities up to 10^7 Lsun. To investigate the formation and properties of nuclear stellar discs (NSDs), we look for their presence in a set of N-body simulations studying the dissipationless merging of multiple star clusters in galactic nuclei. A few tens of star clusters with sizes and masses comparable to those of globular clusters observed in the Milky Way are accreted onto a pre-existing nuclear stellar component: either a massive super star cluster or a rapidly rotating, compact disc with a scale-length of a few parsecs, mimicing the variety of observed nuclear structures. Images and kinematic maps of the simulation time-steps are then built and analysed as if they were real and at the distance of the Virgo cluster. We use the Scorza-Bender method to search for the presence of disc structures via photometric decomposition. In one case the merger remnant has all the observed photometric and kinematic properties of NSDs observed in real galaxies. This shows that current observations are consistent with most of the NSD mass being assembled from the migration and accretion of star clusters into the galactic centre. In the other simulation instead, we detect an elongated structure from the unsharp masked image, that does not develop the photometric or kinematic signature of a NSD. Thus, in the context of searches for a disc structure, the Scorza-Bender method is a robust and necessary tool.
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We present observations and dynamical models of the stellar nuclear clusters (NCs) at the centres of NGC 4244 and M33. We then compare these to an extensive set of simulations testing the importance of purely stellar dynamical mergers on the formation and growth of NCs. Mergers of star clusters are able to produce a wide variety of observed properties, including densities, structural scaling relations, shapes (including the presence of young discs) and even rapid rotation. Nonetheless, difficulties remain, most notably that the second order kinematic moment V_rms = (V^2 + sigma^2)^(1/2) of the models is too centrally peaked to match observations. This can be remedied by the merger of star clusters onto a pre-existing nuclear disc, but the line-of-sight velocity V is still more slowly rising than in NGC 4244. Our results therefore suggest that purely stellar dynamical mergers cannot form NCs, and that gas dissipation is a necessary ingredient for at least ~50% of a NCs mass. The negative vertical anisotropy found in NGC 4244 however requires at least 10% of the mass to be accreted as stars, since gas dissipation and in situ star formation leads to positive vertical anisotropy.
Nuclear stellar discs (NSDs) can help to constrain the assembly history of their host galaxies, as long as we can assume them to be fragile structures that are disrupted during merger events. In this work we investigate the fragility of NSDs by means of N-body simulations reproducing the last phases of a galaxy encounter, when the nuclear regions of the two galaxies merge. For this, we exposed a NSD set in the gravitational potential of the bulge and supermassive black hole of a primary galaxy to the impact of the supermassive black hole from a secondary galaxy. We explored merger events of different mass ratios, from major mergers with a 1:1 mass ratio to intermediate and minor interactions with 1:5 and 1:10 ratios, while considering various impact geometries. We analyse the end results of such mergers from different viewing angles and looked for possible photometric and kinematic signatures of the presence of a disc in the remnant surface density and velocity maps, while adopting detection limits from real observations. Our simulations show that indeed NSDs are fragile against major mergers, which leave little trace of NSDs both in images and velocity maps, while signatures of a disc can be found in the majority of the intermediate to minor-merger remnants and in particular when looking at their kinematics. These results show that NSDs could allow to distinguish between these two modes of galaxy assembly, which may indeed pertain to different kinds of galaxies or galactic environments.
We study the effect of dissipational gas physics on the vertical heating and thickening of disc galaxies during minor mergers. We produce a suite of minor merger simulations for Milky Way-like galaxies. This suite consists of collisionless simulations as well as hydrodynamical runs including a gaseous component in the galactic disc. We find that in dissipationless simulations minor mergers cause the scale height of the disc to increase by up to a factor of ~2. When the presence of gas in the disc is taken into account this thickening is reduced by 25% (50%) for an initial disc gas fraction of 20% (40%), leading to a final scale height z0 between 0.6 and 0.7 kpc, regardless of the initial scale height. We argue that the presence of gas reduces disc heating via two mechanisms: absorption of kinetic impact energy by the gas and/or formation of a new thin stellar disc that can cause heated stars to recontract towards the disc plane. We show that in our simulations most of the gas is consumed during the merger and thus the regrowth of a new thin disc has a negligible impact on the z0 of the post merger galaxy. Final disc scale heights found in our simulations are in good agreement with studies of the vertical structure of spiral galaxies where the majority of the systems are found to have scale heights of 0.4 kpc < z0 < 0.8 kpc. We also found no tension between recent measurements of the scale height of the Milky Way thin disc and results coming from our hydrodynamical simulations. We conclude that the existence of a thin disc in the Milky Way and in external galaxies is not in obvious conflict with the predictions of the CDM model.
Recent simulation work has successfully captured the formation of the star clusters that have been observed in merging galaxies. These studies, however, tend to focus on studying extreme starbursts, such as the Antennae galaxies. We aim to establish whether there is something special occurring in these extreme systems or whether the mechanism for cluster formation is present in all mergers to a greater or lesser degree. We undertake a general study of merger-induced star formation in a sample of 5 pc resolution adaptive mesh refinement simulations of low redshift equal-mass mergers with randomly-chosen orbital parameters. We find that there is an enhanced mass fraction of very dense gas that appears as the gas density probability density function evolves during the merger. This finding has implications for the interpretation of some observations; a larger mass fraction of dense gas could account for the enhanced HCN/CO ratios seen in ULIRGs and predicts that alpha_CO is lower in mergers, as for a given mass of H_2, CO emission will increase in a denser environment. We also find that as the star formation rate increases, there is a correlated peak in the velocity dispersion of the gas, which we attribute to increasing turbulence driven by the interaction itself. Star formation tends to be clumpy: in some cases there is extended clumpy star formation, but even when star formation is concentrated within the inner kpc (i.e. what may be considered a nuclear starburst) it still often has a clumpy, rather than a smooth, distribution. We find no strong evidence for a clear bimodality in the Kennicutt-Schmidt relation for the average mergers simulated here. Instead, they are typically somewhat offset above the predicted quiescent relation during their starbursts.
The past few years have seen dramatic improvements in the scope and realism of star cluster simulations. Accurate treatments of stellar evolution, coupled with robust descriptions of all phases of binary evolution, have been incorporated self-consistently into several dynamical codes, allowing for the first time detailed study of the interplay between stellar dynamics and stellar physics. The coupling between evolution, dynamics, and the observational appearance of the cluster is particularly strong in young systems and those containing large numbers of primordial binary systems, and important inroads have been made in these areas, particularly in N-body simulations. I discuss some technical aspects of the current generation of N-body integrators, and describe some recent results obtained using these codes.
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