An optical/NIR survey of globular clusters in early-type galaxies

Abstract

Context. The interpretation that bimodal colour distributions of globular clusters (GCs) reflect bimodal metallicity distributions has been challenged. Non-linearities in the colour to metallicity conversions caused for example by the horizontal branch (HB) stars may be responsible for transforming a unimodal metallicity distribution into a bimodal (optical) colour distribution. Aims. We study optical/near-infrared (NIR) colour distributions of the GC systems in 14 E/S0 galaxies. Methods. We test whether the bimodal feature, generally present in optical colour distributions, remains in the optical/NIR ones. The latter colour combination is a better metallicity proxy than the former. We use KMM and GMM tests to quantify the probability that different colour distributions are better described by a bimodal, as opposed to a unimodal distribution. Results. We find that double-peaked colour distributions are more commonly seen in optical than in optical/NIR colours. For some of the galaxies where the optical (g − z) distribution is clearly bimodal, a bimodal distribution is not preferred over a unimodal one at a statistically significant level for the (g − K) and (z − K) distributions. The two most cluster-rich galaxies in our sample, NGC 4486 and NGC 4649, show some interesting differences. The (g − K) distribution of NGC 4649 is better described by a bimodal distribution, while this is true for the (g − K) distribution of NGC 4486 GCs only if restricted to a brighter sub-sample with small K-band errors (<0.05 mag). Formally, the K-band photometric errors cannot be responsible for blurring bimodal metallicity distributions to unimodal (g − K) colour distributions. However, simulations including the extra scatter in the colour-colour diagrams (not fully accounted for in the photometric errors) show that such scatter may contribute to the disappearance of bimodality in (g − K) for the full NGC 4486 sample. For the less cluster-rich galaxies results are inconclusive due to poorer statistics. Conclusions. A bimodal optical colour distribution is not necessarily an indication of an underlying bimodal metallicity distribution. Horizontal branch morphology may play an important role in shaping some of the optical GC colour distributions. However, we find tentative evidence that the (g − K) colour distributions remain bimodal in the two cluster-rich galaxies in our sample (NGC 4486 and NGC 4649) when restricted to clusters with small K-band photometric errors. This bimodality becomes less pronounced when including objects with larger errors, or for the (z − K) colour distributions. Deeper observations of large numbers of GCs will be required to reach more secure conclusions.

Optical-near-IR analysis of globular clusters in the IKN dwarf spheroidal: a complex star formation history

“…This problem can be partially resolved with a combination of optical and NIR colour indices, as demonstrated by earlier work (e.g. Puzia et al 2002;Hempel et al 2003Hempel et al , 2007Cantiello & Blakeslee 2007;Chies-Santos et al 2011a,b;Chies-Santos et al 2012;Georgiev et al 2012, and references therein). Additional complications in the optical domain come from the hot evolved stars (HB, blue stragglers) which contribute significantly to the integrated light, whereas in the NIR and for ages ≥2 Gyr, the red giant stars completely dominate the total energy emission.…”

Globular clusters of NGC 3115 in the near-infrared

“…If the nonlinear projection is at work, then the [Fe/H] distributions inferred from linear inversion of different color indices for the same GC sample will show some degree of inconsistency (discordant [Fe/H] peaks and/or fractions of GCs in each [Fe/H] component). The analysis of opticalto-NIR GC colors in various ETGs, as well as u to z photometry in some Virgo cluster members, indicates that the nonlinear projection effect is present at some level in these galaxies (Blakeslee et al 2012;Chies-Santos et al 2012;Yoon et al 2011Yoon et al , 2013.…”

“…Recently, the interpretation of one of the most intriguing properties of GC systems in early-type galaxies, the nearly universal presence of two distinct peaks in the optical color distribution, has inspired a vigorous and prolific debate (Yoon et al 2006(Yoon et al , 2011Kundu & Zepf 2007;Chies-Santos et al 2012;Blakeslee et al 2012;Usher et al 2012;Vanderbeke et al 2014).…”

Isolated ellipticals and their globular cluster systems

“…The globular cluster systems (GCSs) of old, massive, elliptical galaxies present an almost ubiquitous optical color bimodality (e.g., Kundu & Whitmore 2001;Larsen et al 2001), which is thought to correspond to a metallicity bimodality (e.g., Strader et al 2007;Usher et al 2012;Brodie et al 2012), with caveats coming from possible nonlinearities of the color-metallicity transformations that are introduced by horizontal-branch stars (Richtler 2006(Richtler , 2013Yoon et al 2006;Chies-Santos et al 2012;Blakeslee et al 2012).…”