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 rho(J) = (3.57 +/- 0.11) x 10(8) hL circle dot Mpc(-3) and rho(Ks) = (7.04 +/- 0.23) x 10(8) hL circle dot 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 similar to 3 when going from groups with total b(J)-band luminosities of 3 x 10(10) h(-2)L circle dot to the richest clusters. These clusters have typical dynamical mass-to-light ratios of Y-K approximate to 80 hY circle dot. Galaxy luminosities are used to estimate stellar masses. Taking into account the bias introduced by uncertainties in estimating galaxy stellar masses, a value of Omega(stars)h = (1.11 +/- 0.05) x 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 Omega(stars)h approximate to 2.3 x 10(-3). 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 similar to 0.5Y circle dot, (2) in contrast, the fraction of mass in stars decreases with increasing group size, reaching similar to 5 x 10(-3) h 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 similar to 2 x 10(12) h(-1) M circle dot) with only similar to 2 per cent in clusters with M greater than or similar to 5 x 10(14) h(-1) M circle dot.