Early-type galaxies in low-density environments: NGC 6876 explored through its globular cluster system

Abstract

We present the results of a photometric study of the early-type galaxy NGC 6876 and the surrounding globular cluster system (GCS). The host galaxy is a massive elliptical, the brightest of this type in the Pavo Group. According to its intrinsic brightness (M v ∼ -22.7), it is expected to belong to a galaxy cluster instead of a poor group. Observational material consists of g ′ , r ′ , i ′ images obtained with the Gemini/GMOS camera. The selected globular cluster (GC) candidates present a clear bimodal colour distribution at different galactocentric radii, with mean colours and dispersions for the metal-poor ("blue") and metal-rich ("red") typical of old GCs. The red subpopulation dominates close to the galaxy centre, in addition to the radial projected distribution showing that they are more concentrated towards the galaxy centre. The azimuthal projected distribution shows an overdensity in the red subpopulation in the direction of a trail observed in X-ray that could be evidence of interactions with its spiral neighbour NGC 6872. The turn-over of the luminosity function gives an estimated distance modulus (m − M) ≈ 33.5 and the total population amounts to 9400 GCs, i.e. a quite populous system. The halo mass obtained using the number ratio (i.e. the number of GCs with respect to the baryonic and dark mass) gives a total of ∼ 10 13 , meaning it is a very massive galaxy, given the environment.

Nonlinear Color-Metallicity Relations of Globular Clusters. IX. Different Radial Number Density Profiles between Blue and Red Clusters

“…For most early-type galaxies, the fraction of red (blue) GCs is highest (lowest) at the galactic center and decreases (increases) with radius. Accordingly, the surface number density of red GCs drops faster than that of blue GCs as the radius increases (Forbes et al 2004;Forte et al 2005;Bassino et al 2006Bassino et al , 2008Tamura et al 2006;Harris 2009b;Faifer et al 2011;Strader et al 2011;Blom et al 2012;Pota et al 2013;Usher et al 2013;Cho et al 2016;Harris et al 2016Harris et al , 2017Escudero et al 2018;Ennis et al 2019;Ko et al 2019;De Bórtoli et al 2020). In Figure 1, we show the surface number density profiles of blue and red GCs in the four largest elliptical galaxies (M87, M49, M60, and NGC 1399) in the Virgo and Fornax galaxy clusters.…”

“…Globular clusters (GCs) play a vital role in understanding the formation process of their host galaxies because GC formation accompanies star-forming episodes of their hosts, and GCs are easier to observe than field stars of their hosts. Among others, a well-established observational phenomenon for GCs, the bimodal optical color distributions in early-type galaxies have shed light on galaxy and GC formation (e.g., Ostrov et al 1993;Zepf & Ashman 1993;Gebhardt & Kissler-Patig 1999;Kundu & Whitmore 2001;Larsen et al 2001;Forte et al 2005Forte et al , 2012Harris et al 2006Harris et al , 2017Peng et al 2006;Lee et al 2008a;Blakeslee et al 2010;Faifer et al 2011;Forbes et al 2011;Foster et al 2011;Liu et al 2011;Blom et al 2012;Chies-Santos et al 2012;Cho et al 2012;Kim et al 2013a;Usher et al 2013;Cantiello et al 2014Cantiello et al , 2018Kartha et al 2014Kartha et al , 2016Richtler et al 2015;Cho et al 2016;Caso et al 2017;Escudero et al 2018Escudero et al , 2020Hudson & Robison 2018;Ennis et al 2019;Ko et al 2019;De Bórtoli et al 2020, to name a few; see also West et al 2004;Brodie & Strader 2006;Beasley 2020 for reviews and references therein). The bimodal distribution of colors suggests the existence of two distinct GC subpopulations.…”

“…In relation to the GC color bimodality, a well-known phenomenon observed in most GC systems is the systematic variation in the GC color histogram morphology along the galactocentric radius (Geisler et al 1996;Kundu et al 1999;Lee & Kim 2000;Dirsch et al 2003Dirsch et al , 2005Forbes et al 2004Forbes et al , 2011Bassino et al 2006;Tamura et al 2006;Strader et al 2011;Blom et al 2012;Kim et al 2013a;Escudero et al 2015Escudero et al , 2018Harris et al 2016;Ennis et al 2019;Ko et al 2019;De Bórtoli et al 2020). For most early-type galaxies, the fraction of red (blue) GCs is highest (lowest) at the galactic center and decreases (increases) with radius.…”

The Ancient Globular Clusters of NGC 1291

“…Lenticular and early-type galaxies have bimodal GC populations: one blue and metal-poor, and the other red and metal-rich, whose colors correlate with the host galaxy luminosity and color (Peng et al 2006). This has been studied in galaxies such as the lenticular galaxy NGC 1172 (Ennis et al 2020) and the early-type NGC 6876 (Ennis et al 2019). These GC populations have distinct spatial distributions, with redder, metal-rich GC's more centrally concentrated than their metal-poor counterparts.…”

“…These GC populations have distinct spatial distributions, with redder, metal-rich GC's more centrally concentrated than their metal-poor counterparts. The distinct subpopulations, along with with high specific frequencies (the number of GCs normalized to a galaxy with M V = −15) are believed to result from earlier merger events (Ennis et al 2020(Ennis et al , 2019.…”