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Register a new userNonlinear Color-Metallicity Relations of Globular Clusters. IX. Different Radial Number Density Profiles between Blue and Red Clusters
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The optical colors of globular clusters (GCs) in most large early-type galaxies are bimodal. Blue and red GCs show a sharp difference in the radial profile of their surface number density in the sense that red GCs are more centrally concentrated than blue GCs. An instant interpretation is that there exist two distinct GC subsystems having different radial distributions. This view, however, was challenged by a scenario in which, due to the nonlinear nature of the GC metallicity-to-color transformation for old ($gtrsim$10 Gyr) GCs, a broad unimodal metallicity spread can exhibit a bimodal color distribution. Here we show, by simulating the radial trends in the GC color distributions of the four nearby giant elliptical galaxies (M87, M49, M60, and NGC 1399), that the difference in the radial profile between blue and red GCs stems naturally from the metallicity-to-color nonlinearity plus the well-known radial metallicity gradient of GC systems. The model suggests no or little radial variation in GC age even out to $sim$20${R}_{rm eff}$. Our results provide a simpler solution to the distinct radial profiles of blue and red GCs that does not necessarily invoke the presence of two GC subsystems and further fortify the nonlinearity scenario for the GC color bimodality phenomenon.
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Recent spectroscopy on the globular cluster (GC) system of M31 with unprecedented precision witnessed a clear bimodality in absorption-line index distributions of old GCs. Such division of extragalactic GCs, so far asserted mainly by photometric color bimodality, has been viewed as the presence of merely two distinct metallicity subgroups within individual galaxies and forms a critical backbone of various galaxy formation theories. Given that spectroscopy is a more detailed probe into stellar population than photometry, the discovery of index bimodality may point to the very existence of dual GC populations. However, here we show that the observed spectroscopic dichotomy of M31 GCs emerges due to the nonlinear nature of metallicity-to-index conversion and thus one does not necessarily have to invoke two separate GC subsystems. We take this as a close analogy to the recent view that metallicity-color nonlinearity is primarily responsible for observed GC color bimodality. We also demonstrate that the metallicity-sensitive magnesium line displays non-negligible metallicity-index nonlinearity and Balmer lines show rather strong nonlinearity. This gives rise to bimodal index distributions, which are routinely interpreted as bimodal metallicity distributions, not considering metallicity-index nonlinearity. Our findings give a new insight into the constitution of M31s GC system, which could change much of the current thought on the formation of GC systems and their host galaxies.
One of the conundrums in extragalactic astronomy is the discrepancy in observed metallicity distribution functions (MDFs) between the two prime stellar components of early-type galaxies-globular clusters (GCs) and halo field stars. This is generally taken as evidence of highly decoupled evolutionary histories between GC systems and their parent galaxies. Here we show, however, that new developments in linking the observed GC colors to their intrinsic metallicities suggest nonlinear color-to-metallicity
Several recent studies have reported a mean size difference of about 20% between the metal-rich and metal-poor subpopulations of globular clusters (GCs) in a variety of galaxies. In this paper we investigate the possibility that the size difference might be a projection effect, resulting from a correlation between cluster size and galactocentric distance, combined with different radial distributions of the GC subpopulations. We find that projection effects may indeed account for a size difference similar to the observed one, provided that there is a steep relation between GC size and galactocentric distance in the central parts of the GC system and that the density of GCs flattens off near the center in a manner similar to a King profile. For more centrally peaked distributions, such as a de Vaucouleurs law, or for shallower size-radius relations, projection effects are unable to produce the observed differences in the size distributions.
We used a proper combination of high-resolution HST observations and wide-field ground based data to derive the radial star density profile of 26 Galactic globular clusters from resolved star counts (which can be all freely downloaded on-line). With respect to surface brightness (SB) profiles (which can be biased by the presence of sparse, bright stars), star counts are considered to be the most robust and reliable tool to derive cluster structural parameters. For each system a detailed comparison with both King and Wilson models has been performed and the most relevant best-fit parameters have been obtained. This is the largest homogeneous catalog collected so far of star count profiles and structural parameters derived therefrom. The analysis of the data of our catalog has shown that: (1) the presence of the central cusps previously detected in the SB profiles of NGC 1851, M13 and M62 is not confirmed; (2) the majority of clusters in our sample are fitted equally well by the King and the Wilson models; (3) we confirm the known relationship between cluster size (as measured by the effective radius) and galactocentric distances; (4) the ratio between the core and the effective radii shows a bimodal distribution, with a peak at ~ 0.3 for about 80% of the clusters, and a secondary peak at ~ 0.6 for the remaining 20%. Interestingly, the main peak turns out to be in agreement with what expected from simulations of cluster dynamical evolution and the ratio between these two radii well correlates with an empirical dynamical age indicator recently defined from the observed shape of blue straggler star radial distribution, thus suggesting that no exotic mechanisms of energy generation are needed in the cores of the analyzed clusters.
Blue hook (BHk) stars are a rare class of horizontal branch stars that so far have been found in only very few Galactic globular clusters (GCs). The dominant mechanism for producing these objects is currently still unclear. In order to test if the presence of BHk populations in a given GC is linked to specific physical or structural cluster properties, we have constructed a parent sample of GCs for which existing data is sufficient to establish the presence or absence of BHk populations with confidence. We then compare the properties of those clusters in our parent sample that do contain a BHk population to those that do not. We find that there is only one compelling difference between BHk and non-BHk clusters: all known BHk clusters are unusually massive. However, we also find that the BHk clusters are consistent with being uniformly distributed within the cumulative mass distribution of the parent sample. Thus, while it is attractive to suggest there is is a lower mass cut-off for clusters capable of forming BHk stars, the data do not require this. Instead, the apparent preference for massive clusters could still be a purely statistical effect: intrinsically rare objects can only be found by searching a sufficiently large number of stars.
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