In the early 1960s, astronomers were puzzled by quasars — sources of intense radio emission that seemed to be stars, but had unintelligible optical spectra. In 1963, Maarten Schmidt solved the puzzle by recognizing the Balmer lines of hydrogen, strongly redshifted, in the spectrum of the quasar 3C 273. Schmidt reached the "most direct and least objectionable" conclusion, that 3C 273 was no star, but the enormously bright nucleus of a distant galaxy.

3C 273: A Star-like Object with Large Red-Shift

High-redshift quasars are currently the only probes of the growth of supermassive black holes and potential tracers of structure evolution at early cosmic time. Here we present our candidate selection criteria from the Panoramic Survey Telescope & Rapid Response System 1 and follow-up strategy to discover quasars in the redshift range 5.7 lsim z lsim 6.2. With this strategy we discovered eight new 5.7 ≤ z ≤ 6.0 quasars, increasing the number of known quasars at z > 5.7 by more than 10%. We additionally recovered 18 previously known quasars. The eight quasars presented here span a large range of luminosities (–27.3 ≤ M 1450 ≤ –25.4; 19.6 ≤ z P1 ≤ 21.2) and are remarkably heterogeneous in their spectral features: half of them show bright emission lines whereas the other half show a weak or no Lyα emission line (25% with rest-frame equivalent width of the Lyα +N V line lower than 15 A). We find a larger fraction of weak-line emission quasars than in lower redshift studies. This may imply that the weak-line quasar population at the highest redshifts could be more abundant than previously thought. However, larger samples of quasars are needed to increase the statistical significance of this finding.

/pdf/discovery-of-eight-z-6-quasars-from-pan-starrs1-307sud7lpd.pdf

DISCOVERY OF EIGHT z ∼ 6 QUASARS FROM Pan-STARRS1

The relationship of an AGN to its host galaxy is a crucial question in the study of galaxy evolution We perform stellar population synthesis in the central regions of galaxies of different activity levels A large number of stellar features are measured both in the optical and near-infrared We find the nuclear stellar population to be related to the level of activity These differences are no more conspicuous further away in the bulge of the galaxy

http://arxiv.org/pdf/astro-ph/0111331.pdf

Host galaxies of AGN

SDSS repeat spectroscopic observations have resulted in multiepoch spectroscopy for ~2500 quasars observed more than 50 days apart. From this sample, calibrating against stars observed simultaneously, we identify 315 quasars that have varied significantly between observations (with respect to assumed nonvariable stars observed concurrently). These variable quasars range in redshift from 0.5 to 4.72. This is the first large quasar sample studied spectroscopically for variability and represents a potentially useful sample for future high-redshift reverberation mapping studies. This also marks the first time the precise wavelength dependence of quasar variability has been determined, allowing both the continuum and emission-line variability to be studied. We create an ensemble difference spectrum (bright phase minus faint phase) covering rest-frame wavelengths from 1000 to 6000 A. This average difference spectrum is bluer than the average single-epoch quasar spectrum; a power-law fit to the difference spectrum yields a spectral index αλ = -2.00, compared to an index of αλ = -1.35 for the single-epoch spectrum. This confirms that quasar continua are bluer when brighter. The difference spectrum also exhibits very weak or absent emission-line features; the strongest emission lines vary only 30% as much as the continuum. This small emission-line variability with respect to the continuum is consistent with the intrinsic Baldwin effect. Due to the lack of variability of the lines, measured photometric color is not always bluer in brighter phases but depends on redshift and the filters used. Lastly, the difference spectrum is bluer than the ensemble quasar spectrum only for λrest < 2500 A, indicating that the variability cannot result from a simple scaling of the average quasar spectrum.

Spectral variability of quasars in the Sloan Digital Sky Survey

High-redshift quasars are currently the only probes of the growth of supermassive black holes and potential tracers of structure evolution at early cosmic time. Here we present our candidate selection criteria from the Panoramic Survey Telescope & Rapid Response System 1 and follow-up strategy to discover quasars in the redshift range 5.7 5.7 by more than 10%. We additionally recovered 18 previously known quasars. The eight quasars presented here span a large range of luminosities (-27.3 < M_{1450} < -25.4; 19.6 < z_ps1 < 21.2) and are remarkably heterogeneous in their spectral features: half of them show bright emission lines whereas the other half show a weak or no Ly$\alpha$ emission line (25% with rest-frame equivalent width of the Ly$\alpha$ + Nv line lower than 15{\AA}). We find a larger fraction of weak-line emission quasars than in lower redshift studies. This may imply that the weak-line quasar population at the highest redshifts could be more abundant than previously thought. However, larger samples of quasars are needed to increase the statistical significance of this finding.

/pdf/discovery-of-eight-z-6-quasars-from-pan-starrs1-24wrk939ps.pdf

Discovery of eight z ~ 6 quasars from Pan-STARRS1

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 ) ≈ 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 radio-luminous 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.

/pdf/unusual-quasars-from-the-sloan-digital-sky-survey-selected-3h0jem5kgk.pdf

Unusual quasars from the Sloan Digital Sky Survey selected by means of Kohonen self-organising maps

Context. The galaxy population in the contemporary Universe is characterised by a clear bimodality, blue galaxies with significant ongoing star formation and red galaxies with only a little. The migration between the blue and the red cloud of galaxies is an issue of active research. Post starburst (PSB) galaxies are thought to be observed in the short-lived transition phase. Aims. We aim to create a large sample of local PSB galaxies from the Sloan Digital Sky Survey (SDSS) to study their characteristic properties, particularly morphological features indicative of gravitational distortions and indications for active galactic nuclei (AGNs). Another aim is to present a tool set for an efficient search in a large database of SDSS spectra based on Kohonen self-organising maps (SOMs). Methods. We computed a huge Kohonen SOM for ~ 10 6 spectra from SDSS data release 7. The SOM is made fully available, in combination with an interactive user interface, for the astronomical community. We selected a large sample of PSB galaxies taking advantage of the clustering behaviour of the SOM. The morphologies of both PSB galaxies and randomly selected galaxies from a comparison sample in SDSS Stripe 82 (S82) were inspected on deep co-added SDSS images to search for indications of gravitational distortions. We used the Portsmouth galaxy property computations to study the evolutionary stage of the PSB galaxies and archival multi-wavelength data to search for hidden AGNs. Results. We compiled a catalogue of 2665 PSB galaxies with redshifts z EW (H δ ) > 3 A and z EW (H δ ) > 5 A, significantly higher than in the 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 E+A sample that may indicate hidden AGNs, though other sources are also possible.

/pdf/a-large-sample-of-kohonen-selected-e-a-post-starburst-5cjhjfj1e7.pdf

A large sample of Kohonen selected E+A (post-starburst) galaxies from the Sloan Digital Sky Survey

/pdf/aspect-a-spectra-clustering-tool-for-exploration-of-large-k1u9cbsp1t.pdf

ASPECT: A spectra clustering tool for exploration of large spectral surveys

We exploit the spectral archive of the Sloan Digital Sky Survey (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 10^5 spectra classified as quasars by the SDSS pipeline. Particular attention was paid to minimise possible contamination by rare peculiar stellar spectral types. We present a catalogue of 1005 quasars with unusual spectra. 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. The spectra are grouped into six types. 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. Both the unusual broad absorption line (BAL) quasars and the strong iron emitters have significantly lower radio luminosities than normal quasars. We also confirm that strong BALs avoid the most radio-luminous quasars. Finally, we create a sample of quasars similar to the two "mysterious" objects discovered by Hall et al. (2002) and briefly discuss the quasar properties and possible explanations of their highly peculiar spectra. (Abstract modified to match the arXiv format)

/pdf/unusual-quasars-from-the-sloan-digital-sky-survey-selected-8md5k0j4ig.pdf

P. Schalldach

Papers

Unusual quasars from the Sloan Digital Sky Survey selected by means of Kohonen self-organising maps

A large sample of Kohonen selected E+A (post-starburst) galaxies from the Sloan Digital Sky Survey

ASPECT: A spectra clustering tool for exploration of large spectral surveys

Unusual quasars from the Sloan Digital Sky Survey selected by means of Kohonen self-organising maps