The Acs Fornax Cluster Survey. Viii. The Luminosity Function of Globular Clusters in Virgo and Fornax Early-Type Galaxies and Its Use as a Distance Indicator

Abstract

We use a highly homogeneous set of data from 132 early-type galaxies in the Virgo and Fornax clusters in order to study the properties of the globular cluster luminosity function (GCLF). The globular cluster system of each galaxy was studied using a maximum likelihood approach to model the intrinsic GCLF after accounting for contamination and completeness effects. The results presented here update our Virgo measurements and confirm our previous results showing a tight correlation between the dispersion of the GCLF and the absolute magnitude of the parent galaxy. Regarding the use of the GCLF as a standard candle, we have found that the relative distance modulus between the Virgo and Fornax clusters is systematically lower than the one derived by other distance estimators, and in particular, it is 0.22 mag lower than the value derived from surface brightness fluctuation measurements performed on the same data. From numerical simulations aimed at reproducing the observed dispersion of the value of the turnover magnitude in each galaxy cluster we estimate an intrinsic dispersion on this parameter of 0.21 mag and 0.15 mag for Virgo and Fornax, respectively. All in all, our study shows that the GCLF properties vary systematically with galaxy mass showing no evidence for a dichotomy between giant and dwarf early-type galaxies. These properties may be influenced by the cluster environment as suggested by cosmological simulations.

Globular cluster luminosity function as distance indicator

“…This value is systematically 0.22 mag lower with respect to relative distance modulus obtained from surface brightness fluctuation measurement on the same dataset ). Comparison with the relative distance modulus obtained with the planetary nebulae luminosity function (PNLF) method shows also a systematic offset of 0.1-0.15 mag, and the offset with respect to Cepheids is 0.27 mag, again for both in the sense that the relative GCLF distance Virgo-Fornax is smaller than for PNLF and Cepheids (Villegas et al 2010). This points to a systematic error inherent to the GCLF method that makes this distance indicator less accurate with respect to the SBF, PNLF or Cepheid distance indicators.…”

“…Until today, the universality of the GCLF is debated and debatable (see in particular the work of Villegas et al (2010) discussed in the next section). With deep HST imaging, reaching V ∼ 28 or fainter, the GCLF method, due to the brightness of its TO magnitude, has the potential to step directly into the Hubble flow.…”

“…The lines correspond to simultaneous error-weighted linear fits performed over both samples including an offset of ∆(m − M ) = 0.20 ± 0.04 mag, which is the relative distance modulus between the two clusters. This is Figure 6 from Villegas et al (2010), reproduced by permission of the AAS primarily based on Virgo cluster (and M87) GCLF distances that still require significant correction for recession velocity. Kavelaars et al (2000) and Harris et al (2009) measured GCLF distances to Coma giant ellipticals obtaining H 0 of 69 ± 9 and 73 km/s/Mpc, respectively.…”

“…Based on the most recent, thorough and homogeneous study, that used highest quality deep HST observations of 89 early-type galaxies in the Virgo cluster and 43 galaxies in the Fornax cluster, Villegas et al (2010) derive the relative distance modulus between these two galaxy clusters ∆(m − M ) = 0.20 ± 0.04 mag (see also Figure 8 adopted from that paper). This value is systematically 0.22 mag lower with respect to relative distance modulus obtained from surface brightness fluctuation measurement on the same dataset ).…”

“…The most recent critical comparison of the relative Virgo vs. Fornax cluster distance as measured using GCLF and other The Milky Way (top) and M31 (bottom) globular cluster luminosity functions based on data published by Kron and Mayall (1960) in Tables XV and XX, respectively. In the sample of the Milky Way globular clusters, the absolute magnitudes for the two brightest clusters (47 Tuc and ω Cen) are taken from Gascoigne and Burr (1956), as mentioned in a note added in the proofs by Kron and Mayall (1960) standard candles is presented by Villegas et al (2010) and is discussed briefly here.…”