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Register a new userYoung Star Clusters in The Antennae: A Clue to their Nature from Evolutionary Synthesis
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Added by
Uta Fritze-v. Alvensleben
Publication date
1998
fields
Physics
and research's language is
English
Authors
Uta Fritze-v. Alvensleben
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We analyse the population of bright star clusters in the interacting galaxy pair NGC 4038/39 detected with HST WFPC1 by Whitmore & Schweizer (1995) using our spectrophotometric evolutionary synthesis models for various initial metallicities. We derive individual ages for all clusters with V-I colour and follow the time evolution of their luminosity function (LF). The age distribution clearly reveals the presence of a number of old Globular Clusters (GCs) from the parent galaxies. Their effective radii do not significantly differ from those of the young star cluster population. We confirm Meurers (1995) conjecture that the shape of the LF changes in the course of evolution if age spread effects are accounted for. We find that over a Hubble time the observed exponential LF of the young star clusters will evolve into a typical Gaussian GCLF with parameters M$_{V_0} = -7.1$ mag and $sigma (M_{V_0}) = 1.3$ mag. We discuss the influence of metallicity, the effects of an inhomogeneous internal dust distribution, as well as the possible influence of internal -- through stellar mass loss -- and external dynamical effects on the secular evolution of the LF. Referring YSC luminosities to a uniform age and combining with model M/L, we recover the intrinsic mass distribution of the YSC system. It is Gaussian in shape to good approximation thus representing a quasi-equilibrium distribution that $-$ according to Vesperinis (1997) dynamical modelling for the Milky Way GC system $-$ will not be altered in shape over a Hubble time of dynamical evolution, allthough a substantial number of clusters will be destroyed. Comparing young star cluster populations in an age sequence of interacting/merged and merger remnant galaxies will directly reveal the role of external dynamical effects.
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Stars mostly form in groups consisting of a few dozen to several ten thousand members. For 30 years, theoretical models provide a basic concept of how such star clusters form and develop: they originate from the gas and dust of collapsing molecular clouds. The conversion from gas to stars being incomplete, the left over gas is expelled, leading to cluster expansion and stars becoming unbound. Observationally, a direct confirmation of this process has proved elusive, which is attributed to the diversity of the properties of forming clusters. Here we take into account that the true cluster masses and sizes are masked, initially by the surface density of the background and later by the still present unbound stars. Based on the recent observational finding that in a given star-forming region the star formation efficiency depends on the local density of the gas, we use an analytical approach combined with mbox{N-body simulations, to reveal} evolutionary tracks for young massive clusters covering the first 10 Myr. Just like the Hertzsprung-Russell diagram is a measure for the evolution of stars, these tracks provide equivalent information for clusters. Like stars, massive clusters form and develop faster than their lower-mass counterparts, explaining why so few massive cluster progenitors are found.
The study of young star cluster (YSC) systems, preferentially in starburst and merging galaxies, has seen great interest in the recent past, as it provides important input to models of star formation. However, even some basic properties (like the luminosity function [LF]) of YSC systems are still under debate. Here we study the photometric properties of the YSC system in the nearest major merger system, the Antennae galaxies. We find evidence for the existence of a statistically significant turnover in the LF.
We report on a multi-wavelength study of the relationship between young star clusters in the Antennae galaxies (NGC 4038/9) and their interstellar environment, with the goal of understanding the formation and feedback effects of star clusters in merging galaxies. This is possible for the first time because various new observations (from X-rays to radio wavelengths) have become available in the past several years. Quantitative comparisons are made between the positions of the star clusters (broken into three age groups) and the properties of the interstellar medium by calculating the two-point correlation functions. We find that young star clusters are distributed in a clustered fashion. The youngest star clusters are associated with molecular cloud complexes with characteristic radii of about 1 kpc. In addition, there is a weak tendency for them to be found in regions with higher HI velocity dispersions. No dominant triggering mechanism is identified for the majority of the clusters in the Antennae. Feedback from young bright cluster complexes show large H_alpha bubbles and H_alpha velocity gradients in shells around the complexes. We estimate the current star formation rate to be 20 solar mass/yr, and the gas consumption timescale to be 700 Myr. The latter is comparable to the merging time scale and indicates that star formation has been enchanced by the merger event. Finally, we find that the Schmidt law, with index N=-1.4, is also a good description of the cluster formation triggered by merging in the Antennae. There is some evidence that feedback effects may modify the Schmidt law at scales below 1 kpc.
The young star clusters we observe today are the building blocks of a new generation of stars and planets in our Galaxy and beyond. Despite their fundamental role we still lack knowledge about the conditions under which star clusters form and the impact of these often harsh environments on the evolution of their stellar and substellar members. We demonstrate the vital role numerical simulations play to uncover both key issues. Using dynamical models of different star cluster environments we show the variety of effects stellar interactions potentially have. Moreover, our significantly improved measure of mass segregation reveals that it can occur rapidly even for star clusters without substructure. This finding is a critical step to resolve the controversial debate on mass segregation in young star clusters and provides strong constraints on their initial conditions.
The luminosity functions (LFs) of star cluster systems (i.e. the number of clusters per luminosity interval) are vital diagnostics to probe the conditions of star cluster formation. Early studies have revealed a clear dichotomy between old globular clusters and young clusters, with the former characterised by Gaussian-shaped LFs, and the latter following a power law. Recently, this view was challenged by studies of galaxy merger remnants and post-starburst galaxies. In this paper we re-evaluate the young ($lta$ few hundreds of Myrs, with the majority $lta$ few tens of Myrs) star cluster system in the ongoing spiral-spiral major merger system NGC 4038/39, the Antennae galaxies. The Antennae galaxies represent a very active and complex star-forming environment, which hampers cluster selection and photometry as well as the determination of observational completeness fractions. A main issue of concern is the large number of bright young stars contained in most earlier studies, which we carefully exclude from our cluster sample by accurately determining the source sizes. The resulting LFs are fitted both with Gaussian and with power-law distributions, taking into account both the observational completeness fractions and photometric errors, and compared using a likelihood ratio test. The likelihood ratio results are rigidly evaluated using Monte Carlo simulations. We perform a number of additional tests, e.g. with subsets of the total sample, all confirming our main result: that a Gaussian distribution fits the observed LFs of clusters in this preferentially very young cluster system significantly better than a power-law distribution, at a (statistical) error probability of less than 0.5 per cent.
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