The Ancient Globular Clusters of NGC 1291

Abstract

We present a new catalog of 81 ancient globular clusters (GCs) in the early-type spiral (SB0/a) galaxy NGC 1291. Candidates have been selected from B, V, and I band images taken with the Hubble Space Telescope, which also reveal 17 younger (τ ≲ few × 100 Myr) clusters. The luminosity function shows a peaked shape similar to that found for GC systems in other spiral and elliptical galaxies. The ancient clusters have a bimodal color distribution, with approximately 65% (35%) of the population having blue (red) colors. The red, presumably metal-rich, GCs are more centrally concentrated, as expected for a bulge population; while the blue, presumably metal-poor, GCs are more broadly distributed, consistent with expectations of a halo population. The specific frequency of GCs in NGC 1291 is higher than found previously in most spiral galaxies. However, if we consider just the blue subpopulation, we find S N,blue = 0.50 ± 0.06, quite similar to that found for other spirals. This result supports the hypothesis of a universal population of halo GCs in spirals. The fraction of red GCs in NGC 1291, when compared with those found in other galaxies, suggests that these correlate with host galaxy type rather than with host galaxy luminosity.

The X-Ray Binary-star Cluster Connection in Late-type Galaxies

“…The majority of studies regarding XRBs within star clusters focus on the GCs of elliptical galaxies (e.g., Angelini et al 2001;Jordán et al 2004;Smits et al 2006;Peacock & Zepf 2016;Lehmer et al 2020;Ferrell et al 2021;Riccio et al 2022), a few more recent papers report on XRB associations with GCs in late-type galaxies (e.g., Pfahl et al 2003;Peacock et al 2009;Generozov et al 2018;Hailey et al 2018;Hixenbaugh et al 2022), and fewer still include XRBs in younger clusters (e.g., Rangelov et al 2012;Johns Mulia et al 2019;Avdan et al 2022;Hunt et al 2023). In this paper, we take the first step toward (i) assessing the fraction of star clusters-both old and young-that host XRBs in late-type galaxies, and (ii) comparing the fraction of field versus cluster XRBs in late-type versus early-type galaxies.…”

“…In contrast to elliptical galaxies, we find that XRB-hosting GCs in the star-forming galaxies under consideration are not redder than the general GC population-rather, their colors are consistent with those of the full population (see Figure 2(c)). For early-type galaxies, it is believed that metallicity plays a large role in this correlation, possibly due to enhanced tidal capture rates and core concentrations (smaller radii) in highermetallicity GCs, which in turn facilitates the formation of LMXBs (Jordán 2004;Schulman et al 2012;Ivanova 2013;Vulic et al 2018;Hixenbaugh et al 2022). It is interesting to note that while GCs typically display bimodal colors corresponding to metal-rich and metal-poor populations (e.g., Peacock & Zepf 2016;Hixenbaugh et al 2022), the GCs in our sample lack a clear distinction between red and blue GCs (Figures 2(b) and (c)).…”

“…For early-type galaxies, it is believed that metallicity plays a large role in this correlation, possibly due to enhanced tidal capture rates and core concentrations (smaller radii) in highermetallicity GCs, which in turn facilitates the formation of LMXBs (Jordán 2004;Schulman et al 2012;Ivanova 2013;Vulic et al 2018;Hixenbaugh et al 2022). It is interesting to note that while GCs typically display bimodal colors corresponding to metal-rich and metal-poor populations (e.g., Peacock & Zepf 2016;Hixenbaugh et al 2022), the GCs in our sample lack a clear distinction between red and blue GCs (Figures 2(b) and (c)). This could be due to the fact that (i) the metal-poor mode becomes very broad when different late-type galaxies (each with different star-forming histories) are added together, and (ii) the metal-rich mode may be triggered by mergers, which are rarer in spirals (Muratov & Gnedin 2010;Pfeffer et al 2023).…”