First results from SAPAC: toward a three-dimensional picture of the Fornax cluster core




Dunn, Laura
Jerjen, Helmut

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University of Chicago Press


A sophisticated surface brightness fluctuation (SBF) analysis package has been developed, designed to measure distances of early-type galaxies by means of SBFs of unresolved stars. This suite of programs, called SAPAC, is made readily available to the astronomical community for extensive testing, with the long-term goal of providing the necessary tools for systematic distance surveys of early-type galaxies using modern optical/near-IR telescopes equipped with wide-field cameras. We discuss the technical and scientific concepts of SAPAC and demonstrate its capabilities by analyzing deep B- and R-band CCD images of 10 dwarf elliptical galaxy candidates in the Fornax Cluster obtained with FORS 1 at the Very Large Telescope. All candidates are confirmed as cluster members. We then turn our attention to the innermost region of the Fornax Cluster. A total of 29 early-type galaxies closer than three cluster core radii (2°) to the central galaxy NGC 1399 have radial velocities and SBF distances. Their Hubble diagram exhibits a pronounced S-shaped infall pattern, suggesting that Fornax is still in the process of formation during the present epoch through a general collapse and possible accretion of distinct groups of galaxies. From fitting a model we estimate the cluster mass within 720 kpc projected distance of NGC 1399 to be 2.3 ± 0.3 × 1014 script M sign ⊙. The associated collapse time is tcoll = 2.9 -0.9+1.6 Gyr. After cleansing our galaxy sample of a few kinematical outliers, the true distance of the Fornax Cluster core is determined at 20.13 ± 0.40 Mpc [(m - M)0 = 31.51 ± 0.04 mag]. Applying a bootstrap resampling technique on the distance distribution with individual distance errors taken into account further reveals a small intrinsic cluster depth of σint = 0.74 -0.74+0.52 Mpc, in best agreement with the cluster's linear extension in the sky: σR.A. = σ decl. ≈ 0.5 Mpc. We conclude that the early-type galaxy population in the Fornax Cluster must be spatially well constrained, with no evidence of elongation along the line of sight, in contrast to the Virgo Cluster. Moreover, we find marginal evidence for substructure, a result that is consistent with the young evolutionary state of the cluster and the overall galaxy infall. Combining the kinematically defined cluster distance with the mean cosmological velocity for the central cluster galaxy sample yields a Hubble constant of H0 = 63 ± 5 km s-1 Mpc-1.



Keywords: Galaxies: clusters: individual (Fornax); Galaxies: distances and redshifts; Galaxies: dwarf; Galaxies: elliptical and lenticular, cD; Methods: data analysis



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