We aim to create a large sample of local post-starburst (PSB) galaxies to study their characteristic properties, particularly morphological features indicative of gravitational distortions and indications for active galactic nuclei (AGNs). The selection is based on a huge Kohonen self-organising map (SOM) of about one million SDSS spectra. The SOM is made fully available for the astronomical community, in combination with an interactive user interface. We compiled a catalogue of 2665 PSB galaxies with redshifts z < 0.4. In the colour-mass diagram, the PSB sample is found to be clearly concentrated towards the region between the red and the blue cloud, in agreement with the idea that PSB galaxies represent the transitioning phase between actively and passively evolving galaxies. The relative frequency of morphologically distorted PSB galaxies is at least 57%, significantly higher than in a comparison sample. The search for AGNs based on conventional selection criteria in the radio and MIR results in a low AGN fraction of 2 - 3%. We confirm an MIR excess in the mean SED of the PSB galaxy sample that may indicate hidden AGNs, though other sources are also possible. (Abstract modified to match the arXiv format.)

The Sloan Digital Sky Survey (SDSS) discovered a few unusual quasars with a characteristic break in the continuum around 3000 A that neither shows the typical structure of broad absorption line (BAL) troughs nor is explained by typical intrinsic dust reddening. We used the method of Kohonen self-organising maps for a systematical search for quasars with such properties in the SDSS spectra archive. We constructed a sample of 23 quasars classified as 3000 A break quasars and two comparison samples of quasars with similar properties, to some extent, but also showing typical BAL features. We computed ensemble-averaged broad-band SEDs based on archival data from SDSS, GALEX, 2MASS, UKIDSS, and WISE. The SEDs were corrected for intrinsic dust absorption by the comparison with the average SED of normal quasars. The de-reddened arithmetic median composite SED of the 3000 A break quasars is found to be indistinguishable from that of the unusual BAL quasars. We conclude that 3000 A break quasars are most likely extreme versions of BAL quasars. Assuming that the intrinsic SED of the continuum source is represented by the quasar composite SED, the 3000 A break quasars tend to be intrinsically more luminous than ordinary quasars. (Abstract modified to match the arXiv format.)

We present the novel, semi-automated clustering tool ASPECT for analysing voluminous archives of spectra. The heart of the program is a neural network in form of Kohonen's self-organizing map. The resulting map is designed as an icon map suitable for the inspection by eye. The visual analysis is supported by the option to blend in individual object properties such as redshift, apparent magnitude, or signal-to-noise ratio. In addition, the package provides several tools for the selection of special spectral types, e.g. local difference maps which reflect the deviations of all spectra from one given input spectrum (real or artificial). ASPECT is able to produce a two-dimensional topological map of a huge number of spectra. The software package enables the user to browse and navigate through a huge data pool and helps him to gain an insight into underlying relationships between the spectra and other physical properties and to get the big picture of the entire data set. We demonstrate the capability of ASPECT by clustering the entire data pool of 0.6 million spectra from the Data Release 4 of the Sloan Digital Sky Survey (SDSS). To illustrate the results regarding quality and completeness we track objects from existing catalogues of quasars and carbon stars, respectively, and connect the SDSS spectra with morphological information from the GalaxyZoo project.

Context. Large spectroscopic surveys have discovered very peculiar and hitherto unknown types of active galactic nuclei (AGN). Such rare objects may hold clues to the accretion history of the supermassive black holes at the centres of galaxies.Aims. We aim to create a sizeable sample of unusual quasars from the unprecedented spectroscopic database of the Sloan Digital Sky Survey (SDSS).Methods. We exploit the spectral archive of the SDSS Data Release 7 to select unusual quasar spectra. The selection method is based on a combination of the power of self-organising maps and the visual inspection of a huge number of spectra. Self-organising maps were applied to nearly 105 spectra classified as quasars at redshifts from z = 0.6 to 4.3 by the SDSS pipeline. Particular attention was paid to minimise possible contamination by rare peculiar stellar spectral types. All selected quasar spectra were individually studied to determine the object type and the redshift.Results. We present a catalogue of 1005 quasars with unusual spectra. These spectra are dominated by either broad absorption lines (BALs; 42%), unusual red continua (27%), weak emission lines (18%), or conspicuously strong optical and/or UV iron emission (11%). This large sample provides a useful resource for both studying properties and relations of/between different types of unusual quasars and selecting particularly interesting objects, even though the compilation is not aimed at completeness in a quantifiable sense. The spectra are grouped into six types for which composite spectra are constructed and mean properties are computed. Remarkably, all these types turn out to be on average more luminous than comparison samples of normal quasars after a statistical correction is made for intrinsic reddening (E(B - V) approximate to 0 to 0.4 for SMC-like extinction). Both the unusual BAL quasars and the strong iron emitters have significantly lower radio luminosities than normal quasars. We also confirm that strong BALs avoid the most radioluminous quasars. For 32 particularly interesting objects, individual spectra are presented. Among these objects are quasars with many narrow BAL troughs and one quasar where the continuum is strongly suppressed by overlapping BAL troughs across nearly the whole SDSS spectrum. Finally, we create a sample of quasars similar to the two "mysterious" objects discovered by Hall et al. (2002, ApJS, 141, 267) and briefly discuss the quasar properties and possible explanations of their highly peculiar spectra.