We present the fourth data release (DR4) of the SkyMapper Southern Survey (SMSS), the last major step in our hemispheric survey with six optical filters: u, v, g, r, i, z. SMSS DR4 covers 26 000 deg$^{2}$ from over 400 000 images acquired by the 1.3 m SkyMapper telescope between 2014-03 and 2021-09. The 6-band sky coverage extends from the South Celestial Pole to $\delta=+16^{\circ}$, with some images reaching $\delta\sim +28^{\circ}$. In contrast to previous DRs, we include all good-quality images from the facility taken during that time span, not only those explicitly taken for the public Survey. From the image dataset, we produce a catalogue of over 15 billion detections made from $\sim$700 million unique astrophysical objects. The typical 10$\sigma$ depths for each field range between 18.5 and 20.5 mag, depending on the filter, but certain sky regions include longer exposures that reach as deep as 22 mag in some filters. As with previous SMSS catalogues, we have cross-matched with a host of other imaging and spectroscopic datasets to facilitate additional science outcomes. SMSS DR4 is now available to the worldwide astronomical community.

We report the discovery of the ultra-luminous quasi-stellar object SMSS J215728.21−360215.1 with magnitude z = 16.9 and W4 = 7.42 at redshift 4.75. Given absolute magnitudes of M145, AB = −29.3, M300, AB = −30.12, and logLbol/Lbol, ⊙ = 14.84, it is the quasi-stellar object with the highest unlensed UV-optical luminosity currently known in the Universe. It was found by combining proper-motion data from Gaia DR2 with photometry from SkyMapper DR1 and the Wide-field Infrared Survey Explorer. In the GAIA database, it is an isolated single source and thus unlikely to be strongly gravitationally lensed. It is also unlikely to be a beamed source as it is not discovered in the radio domain by either NRAO-VLA Sky Survey or Sydney University Molonglo Southern Survey. It is classed as a weak-emission-line quasi-stellar object and possesses broad absorption line features. A lightcurve from ATLAS spanning the time from 2015 October to 2017 December shows little sign of variability.

We use a WISE-2MASS-Pan-STARRS1 galaxy catalog to search for a supervoid in the direction of the Cosmic Microwave Background Cold Spot. We obtain photometric redshifts using our multicolor data set to create a tomographic map of the galaxy distribution. The radial density profile centred on the Cold Spot shows a large low density region, extending over 10's of degrees. Motivated by previous Cosmic Microwave Background results, we test for underdensities within two angular radii, 5°, and 15°. Our data, combined with an earlier measurement by Granett et al. 2010, are consistent with a large Rvoid=(192 ± 15)h−1 Mpc (2σ) supervoid with δ ≃ −0.13 ± 0.03 centered at z=0.22 ± 0.01. Such a supervoid, constituting a ∼3.5 σ fluctuation in the ΛCDM model, is a plausible cause for the Cold Spot.

We present a brief update on the ACS Virgo and Fornax Cluster Surveys — Hubble Space Telescope programs to obtain ACS imaging for 143 early-type galaxies in the two galaxy clusters nearest to the Milky Way. We summarize a selection of science highlights from the surveys as including new results on the central structure of early-type galaxies, the apparent continuity of photometric and structural parameters between dwarf and giant galaxies, and the properties of globular clusters, diffuse star clusters and ultra-compact dwarf galaxies.

We examine the photometric and structural properties of early-type galaxies derived from the ACS Virgo and Fornax Cluster Surveys, supplementing these data with previously published or newly reanalysed data for additional early-type galaxies in Virgo, Fornax and the Local Group. As we have noted elsewhere, Sérsic models are found to provide accurate representations of the observed brightness profiles on scales greater than a few percent of the effective radius, Re. On smaller scales, the brightness profiles of bright (MB ≳ −20) galaxies show central deficits with respect to the inward extrapolation of the Sérsic models; fainter galaxies usually show central excesses. Fainter than MB ≈ −20, we find a continuity in the photometric and structural scaling relations: i.e., between absolute magnitude, central surface brightness, Sérsic index, effective radius, and the surface brightness measured at, and averaged within, the effective radius. There is no evidence for a “dwarf/giant dichotomy” — a conclusion consistent with a number of recent studies but contrary to some earlier claims.

cD galaxies are the most luminous galaxies in the universe. They are characterized by a surface brightness profile that falls off more slowly with radius than most elliptical galaxies. In most respects D galaxies are a continuous extrapolation from other ellipticals: their M/L and their colors are comparable to other ellipticals, their inner parts are fitted by an r1/4 law, and they follow the same relation between L and σ. On the other hand, their luminosity is too bright to be consistent with the luminosity function of other ellipticals and they are always found at the center of a cluster of other galaxies. Being at the center of a cluster of galaxies often endows D galaxies with a very faint, very extended halo of luminosity and multiple nuclei, but these are more properly associated with the cluster than the D galaxy itself. The connection between the formation of cD galaxies and the formation of clusters remains a mystery. It is still unresolved whether cDs are a byproduct of cluster evolution, whether they formed in parallel with clusters, or whether primeval D are galaxies the seed around which clusters accreted.