Nonlinear Color-Metallicity Relations of Globular Clusters. Iii. On the Discrepancy in Metallicity Between Globular Cluster Systems and Their Parent Elliptical Galaxies

Abstract

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 conversions, which translate observed color distributions into strongly-peaked, unimodal MDFs with broad metal-poor tails. Remarkably, the inferred GC MDFs are similar to the MDFs of resolved field stars in nearby elliptical galaxies and those produced by chemical evolution models of galaxies. The GC MDF shape, characterized by a sharp peak with a metal-poor tail, indicates a virtually continuous chemical enrichment with a relatively short timescale. The characteristic shape emerges across three orders of magnitude in the host galaxy mass, suggesting a universal process of chemical enrichment among various GC systems. Given that GCs are bluer than field stars within the same galaxy, it is plausible that the chemical enrichment processes of GCs ceased somewhat earlier than that of field stellar population, and -2if so, GCs preferentially trace the major, vigorous mode of star formation events in galactic formation. We further suggest a possible systematic age difference among GC systems, in that the GC systems in more luminous galaxies are older. This is consistent with the downsizing paradigm whereby stars of brighter galaxies, on average, formed earlier than those of dimmer galaxies; this additionally supports the similar nature shared by GCs and field stars. Although the sample used in this study (the HST ACS/WFC, WFPC2, and WFC3 photometry for the GC systems in the Virgo galaxy cluster) confines our discussion to R R e for giant ellipticals and 10 R e for normal ellipticals, our findings suggest that GC systems and their parent galaxies have shared a more common origin than previously thought, and hence greatly simplify theories of galaxy formation.

VEGAS-SSS. A VST early-type galaxy survey: analysis of small stellar systems

“…Because of its proximity, the galaxy and its satellites were targeted by many photometric and spectroscopic studies (Hanes & Harris 1986;Capaccioli et al 1987;Kundu & Whitmore 1998;Puzia et al 2000Puzia et al , 2002Norris et al 2006;Arnold et al 2011;Usher et al 2012). Moreover, we recall that the GC system of the galaxy is the first system beyond the Local Group with a confirmed bimodal metallicity distribution, as shown by Brodie et al (2012) from calcium triplet analysis and by Cantiello et al (2014) using optical to near-IR photometry (see also Blakeslee et al 2010;Yoon et al 2011). The paper is organized as follows: the next section briefly describes the observations and data reduction procedures.…”

Isolated ellipticals and their globular cluster systems

“…Our finding of a dominant blue peak despite the likely absence of a rich accretion history favors in situ formation for GCs, where then the color bimodality would instead be explained by a nonlinear color-metallicity relation in GCs (e.g., Yoon et al 2006Yoon et al , 2011.…”

Globular Cluster Systems in Brightest Cluster Galaxies. Iii. Beyond Bimodality

“…Many indices from near-UV through to near-IR have now been tested and compared. Several discussions have claimed that a unimodal MDF is capable of being converted into a bimodal CDF if the transformation is sufficiently nonlinear (e.g., Yoon et al 2006Yoon et al , 2011aYoon et al , 2011bCantiello & Blakeslee 2007;Chies-Santos et al 2012a;Kim et al 2013;Chung et al 2016). As a numerical exercise, this claim is certainly true, although the continuing issue with these discussions is that they rely very heavily on single-stellarpopulation (SSP) theoretical modeling to develop translation curves from [Fe/H] to a given color index.…”