The most luminous quasars at high-redshift harbour the fastest growing and most massive black holes in the early Universe. They are exceedingly rare and hard to find. Here, we present our search for the most luminous quasars in the redshift range from z = 4.5 to 5 using data from SkyMapper, Gaia, and WISE. We use colours to select likely high-redshift quasars and reduce the stellar contamination of the candidate set with parallax and proper motion data. In ∼12 500 deg2 of Southern sky, we find 92 candidates brighter than Rp = 18.2. Spectroscopic follow-up has revealed 21 quasars at z ≥ 4 (16 of which are within z = [4.5, 5]), as well as several red quasars, Broad-Absorption-Line (BAL) quasars and objects with unusual spectra, which we tentatively label OFeLoBALQSOs at redshifts of z ≈ 1 to 2. This work lifts the number of known bright z ≥ 4.5 quasars in the Southern hemisphere from 10 to 26 and brings the total number of quasars known at Rp < 18.2 and z ≥ 4.5 to 42.

We report the discovery of the ultra-luminous QSO SMSS J215728.21-360215.1 with magnitude z = 16.9 and W4= 7.42 at redshift 4.75. Given absolute magnitudes of M 145,AB = −29.3, M 300,AB = −30.12 and log L bol /L bol, = 14.84, it is the QSO 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 (WISE). 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 NVSS or SUMSS. It is classed as a weak-emission-line QSO and possesses broad absorption line features. A lightcurve from ATLAS spanning the time from October 2015 to December 2017 shows little sign of variability.

We search for ultra-luminous Quasi-Stellar Objects (QSOs) at high redshift using photometry from the SkyMapper Southern Survey Data Release 3 (DR3), in combination with 2MASS, VHS DR6, VIKING DR5, AllWISE, and CatWISE2020, as well as parallaxes and proper motions from Gaia DR2 and eDR3. We report 142 newly discovered Southern QSOs at 3.8 < z < 5.5, of which 126 have M145 < −27 ABmag and are found in a search area of 14 486 deg2. This Southern sample, utilising the Gaia astrometry to offset wider photometric colour criteria, achieves unprecedented completeness for an ultra-luminous QSO search at high redshift. In combination with already known QSOs, we construct a sample that is >80 per cent complete for M145 < −27.33 ABmag at z = 4.7 and for M145 < −27.73 ABmag at z = 5.4. We derive the bright end of the QSO luminosity function at restframe 145 nm for z = 4.7 − 5.4 and measure its slope to be β =−3.60 ± 0.37 and β =−3.38 ± 0.32 for two different estimates of the faint-end QSO density adopted from the literature. We also present the first z ∼ 5 QSO luminosity function at restframe 300 nm.

Abstract Changing Look Active Galactic Nuclei (CLAGN) are rare cases of AGN, where the continuum flux increases/decreases and the broad emission lines appear/disappear within short timescales. These extreme changes challenge our understanding of accretion disk dynamics. We present a sample of four new CLAGN at 0.026 < z < 0.107, which were found by cross-matching the MaNGA survey with AGN from the SDSS spectroscopic database. Our results show that the selection criteria of $>\mathopen |0.5\mathclose |$ magnitude change in SDSS g band is ineffective at lower redshifts. This is in part due to the fact that the g band is probing a different part of the AGN spectral energy distribution at these redshifts. The bigger issue is that the galaxy continuum dominates the spectrum and thus overwhelms any variation of the AGN continuum that might contribute to photometric variations. This suggests the need to use different methods for future low-redshift CLAGN searches.

Changing-look Active Galactic Nuclei (CLAGN) are AGN that change type as their broad emission lines appear or disappear, which is usually accompanied by strong flux changes in their blue featureless continuum. We search for Turn-On CLAGN by selecting type-2 AGN from the spectroscopic 6dF Galaxy Survey, whose colours, observed ∼15 years later by the SkyMapper Southern Survey, are suggestive of type-1 AGN. Starting from 1092 type-2 AGN, we select 20 candidates for follow-up and confirm that 14 of them have changed into type-1 and are thus Turn-On CLAGN; further observations reveal eleven more Turn-On CLAGN. While our search was not tailored to efficiently discover Turn-Off CLAGN, we discover two such cases as well. The result suggests a Turn-On CLAGN rate of over 3 per cent over ∼15 years and imply a total CLAGN rate of more than ∼6 per cent over this period. Finally, we present observations of AGN that are atypical for the CLAGN phenomenology, including J1109146 - a type-1 that did not appear as an AGN at all in 6dFGS; J1406507 - the second reported Changing-look NLS1; and J1340153 - a CLAGN with a change timescale of three months.