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Properties of the Cluster Population of NGC 1566 and Their Implications

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Properties of the Cluster Population of NGC 1566 and Their Implications MNRAS Advance Access published May 13, 2016 Properties of the cluster population of NGC 1566 and their implications K. Hollyhead1, A. Adamo2, N. Bastian1, M. Gieles3, J. E. Ryon4 1 Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK 2 Department of Astronomy, Oscar Klein Centre, Stockholm University, AlbaNova, Stockholm SE-106 91, Sweden 3 Department of Physics, University of Surrey, Guildford, GU2 7XH, UK 4 Department of Astronomy, University of Wisconsin-Madison, 475 North Charter Street, Madison, WI 53706, USA Accepted. Received; in original form ABSTRACT We present results of a photometric study into the cluster population of NGC 1566, a nearby grand design spiral galaxy, sampled out to a Galactocentric radius of ≈ 5:5 kpc. Downloaded from The shape of the mass-limited age distribution shows negligible variation with radial distance from the centre of the galaxy, and demonstrates three separate sections, with a steep beginning, flat middle and steep end. The luminosity function can be approximated by a power law at lower luminosities with evidence of a truncation at higher luminosity. The power law section of the luminosity function of the galaxy is http://mnras.oxfordjournals.org/ best fitted by an index ≈ −2, in agreement with other studies, and is found to agree with a model luminosity function, which uses an underlying Schechter mass function. The recovered power law slope of the mass distribution shows a slight steepening as a function of galactocentric distance, but this is within error estimates. It also displays a possible truncation at the high mass end. Additionally, the cluster formation efficiency (Γ) and the specific U-band luminosity of clusters (TL(U)) are calculated for NGC 1566 and are consistent with values for similar galaxies. A difference in NGC 1566, however, is that the fairly high star formation rate is in contrast with a low ΣSF R and Γ, indicating that Γ can only be said to depend strongly on ΣSF R, not the star at 07988000 on June 16, 2016 formation rate. Key words: galaxies: individual: ngc1566, galaxies: star clusters: general 1 CLUSTER POPULATIONS agreement with hierarchical star formation models, which indicate that clusters are not well defined and unique ob- The study of stellar clusters is vital to the understanding jects at young ages (Bastian 2011). of star formation and galaxy evolution, as current theory proposes that the majority of all stars form in groups or The problem with defining and determining a cluster clustered environments (e.g. Hopkins 2013; Kruijssen 2012). from an unbound grouping also lies in the limited spatial A grouping is defined as any sized collection of stars, inde- resolution of the telescope and lack of information on stellar pendent of whether or not the collection is gravitationally dynamics (e.g. Bastian et al. 2012). Without knowledge of bound. More specifically, a cluster refers to a bound group- the stellar dynamics within clusters/groups, it is impossible ing while associations are unbound groupings (Blaauw 1964; to unambiguously determine whether the collection is bound Gieles & Portegies Zwart 2011). It is difficult to define a or not. One simplistic way is to use the size of the grouping. cluster at young (< 10 Myr) ages, as without detailed kine- As shown in the Small Magellanic Cloud, groupings with an matical information of all group members (and potentially effective radius above 6 pc rapidly decline in number as a −1 the gas in the region as well) it is not possible to determine function of age (t ), i.e., 90% of groups get disrupted every if the grouping is bound. At young ages there exists a con- decade in age, but for groups with sizes below 6 pc, there tinuous distribution of structures, whereas at ages greater is a flat distribution suggesting little disruption (Portegies than 10 Myr, a bimodal distribution arises with bound (com- Zwart et al. 2010). By limiting cluster population studies pact) and unbound (expanding associations) groups (Porte- to systems containing dense, centrally concentrated groups gies Zwart et al. 2010). with ages > 10 Myr, issues with identifying bound structures can be resolved. Hence, an object can only definitely be defined and clas- sified as a cluster once it is dynamically evolved. This is in Much work has been done on cluster populations in sev- eral nearby galaxies, most notably on M83 and the Small and slow disruption and disruption from internal mechanisms or Large Magellanic Clouds. By studying the distributions of outside influence (such as nearby clouds) with certain as- different cluster properties across the galaxy you can infer sumptions within reasonable limits (Elmegreen & Hunter how the clusters formed and determine how they evolve over 2010). Additionally, if tidal shocks are sufficiently strong, time. disruption may be mass independent (with an age distribu- tion as t−1) (Kruijssen et al. 2011), however there will still be a strong environmental influence. This indicates that fully 1.1 Cluster disruption constraining disruption mechanisms is a difficult process. Clusters that survive any initial mass loss during formation Age distributions have been widely studied for different will not survive indefinitely, as they will continue to dissolve galaxies and can provide insight into the process of disrup- through a combination of internal and external processes. tion in a galaxy. The general shape of the distribution is a This can be seen in the decrease in the number of clusters power law section with a steepening at high ages. The shape with increasing age (e.g. Elmegreen & Hunter 2010). of the distribution is determined by the star formation his- Gas loss at very young ages is expected to be entirely tory of the galaxy and the amount of disruption present. If ζ environmentally independent as this should be an internal approximated as a single power-law, log(dN/dt) / t , where process, and occurs on very short timescales (less than a studies to date have found ζ to vary between 0 and -1 (e.g. crossing time). Early cluster evolution due to gas expulsion Fall et al. 2005, Gieles et al. 2007, Chandar et al. 2010, should additionally be mass independent, as violent relax- Silva-Villa & Larsen 2011, Bastian et al. 2012, Ryon et al. ation is responsible for dynamical restructuring after rapid 2014). gas loss (e.g. Bastian & Goodwin 2006). One the other hand, Studies of M83 show environmental dependence in the 0 the gas rich birth environment of clusters has been suggested age distribution, ranging from nearly flat (dN=dt ∼ t ) to Downloaded from −0:7 to actually cause many of the young clusters to disrupt be- relatively steep (dN=dt ∼ t ) (Silva-Villa et al. 2014) at fore they can migrate to areas that are more gas poor (e.g., different galactocentric distances. This was also found by Elmegreen & Hunter 2010; Kruijssen et al. 2011) in which Chandar et al. (2014) (radially dependent index of the age case the local environment would play a strong role in the distribution) who needed to invoke radially dependent dif- ferences in the cluster formation history in order to bring determination of the survival fraction of young clusters. The- http://mnras.oxfordjournals.org/ oretically, disruption should depend on the cluster's initial the age distributions in the inner and outer regions of the mass and its environment, with clusters in weaker tidal fields galaxy into agreement. or higher masses surviving longer (e.g. Baumgardt & Makino Bastian et al. (2012) found that both the MID and 2003; Gieles et al. 2006c). However, empirical studies have MDD scenarios could provide good fits to the observed resulted in two separate theories for the disruption process. age distributions of clusters in M83, however, disruption The process of disruption over the lifetime of the clus- needed to be strongly dependent on environment. Addition- ters is more questionable. There are two main empirical ally, these authors found that the mass function for the clus- scenarios to explain the process: MID (Mass Independent ters was truncated, and the truncation value depended on Disruption) and MDD (Mass Dependent Disruption). The environment. The truncation in the mass function was nec- age and mass distributions of clusters, as studied in this essary to include in disruption analysis in order to explain at 07988000 on June 16, 2016 project, strongly depend on how disruption is modelled dur- the age distribution in the MDD framework. ing the cluster lifetime. MDD has been evidenced and dis- played via empirical studies (Boutloukos & Lamers 2003; 1.2 Cluster population properties Lamers et al. 2005) and N-body simulations (Baumgardt 2001; Gieles et al. 2004) where disruption is found to de- The luminosity function (LF) of a cluster population pro- pend on the initial mass of the cluster and the environ- vides insight into the mass function of the clusters. It is ment within the galaxy. The timescale for disruption has found to behave as dN=dL / L−α, although some devi- been found to depend on the cluster mass as Mγ , where γ ations from a pure single valued power-law have been re- varies slightly between different galaxies, with a mean value ported (Gieles et al. 2006a), including a steepening at the of γ = 0:62 (Boutloukos & Lamers 2003). Simply, this indi- bright end. An example is the bend observed in the LF in cates that higher mass clusters live longer. In this scenario, the Antennae galaxies by Whitmore et al. (1999), which can disruption is also dependent on environment due to tidal be fit with a double power law. When the power law section effects and the ambient density within the galaxy (Lamers of the distribution is fit, α is usually found to be ≈ 2, with et al.
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