Where are the stars ?

The Two degree Field Galaxy Redshift Survey (2dFGRS) is used in conjunction with the Two Micron All Sky Survey (2MASS) Extended Source Catalogue (XSC) to study the near-infrared light and stellar mass content of the local Universe. Mock galaxy catalogues, constructed from cosmological N-body simulations and semi-analytical galaxy formation models, are used to gauge the accuracy with which quantities can be recovered. The mean luminosity densities of the Universe are found to be ρ̄J = (3.57 ± 0.11) × 10 8hL⊙Mpc −3 and ρ̄KS = (7.04 ± 0.23) × 10 8hL⊙Mpc −3 (statistical uncertainty only, and not accounting for the 2MASS low surface brightness incompleteness). Using the 2dFGRS Percolation-Inferred Galaxy Group (2PIGG) catalogue, the group mass-to-light ratio in the KS band is found to increase by a factor of ∼ 3 when going from groups with total bJ-band luminosities of 3 × 10 10 h −2 L⊙ to the richest clusters. These clusters have typical dynamical mass-to-light ratios of ΥK ≈ 80hΥ⊙. Galaxy luminosities are used to estimate stellar masses. Taking into account the bias introduced by uncertainties in estimating galaxy stellar masses, a value of Ωstarsh = (0.99±0.03)×10 −3 is measured, assuming that a Kennicutt stellar initial mass function (IMF) is applicable to all galaxies. Changing this to a Salpeter stellar IMF gives Ωstarsh ≈ 2.1× 10 . The 2PIGGs are then used to study the distribution of the stellar content of the local Universe among groups of different size. The three main conclusions are: (1) a slowly rising stellar mass-to-KS band light ratio is found with the clusters having the largest value of ∼ 0.6Υ⊙, (2) in contrast, the fraction of mass in stars decreases with increasing group size, reaching ∼ 5× 10h for the rich clusters, and (3) in answer to the question posed in the title, most stellar mass is contained in Local Group-sized objects (M ∼ 2 × 10 h M⊙) with only ∼ 2 per cent in clusters with M ∼ > 5× 10 h M⊙.