arXiv subjects

D. Engels

Publications and source records attributed to D. Engels.

79 records · Page 5Linked to original sources

arXiv2000

Detection of the first X-ray selected large AGN group

We have examined the spatial distribution of 856 AGN detected by the ROSAT All-Sky Survey (RASS) using a direct search for structures with the minimal spanning tree. The AGNs were compiled from an area of 7000 deg^2, in which optical identifications of RASS sources were made with the help of the digitized objective prism plates of the Hamburg Quasar Survey (HQS). Redshifts were taken from the literature or from own follow-up observations. The sample probes the spatial distribution at low redshifts, since the redshift distribution peaks at z=0.1. The application of the minimal spanning tree led to a 1.8 sigma discovery of an AGN group with 7 members in a volume V=140*75*75 h^-3Mpc^3 in the Pisces constellation. With a mean redshift z=0.27 this group is only the third discovered group at redshifts z<0.5. The RASS offers excellent possibilities to study large scale structure with AGNs at low redshifts, once these redshifts are determined.

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arXiv2000

The Hamburg/SAO Survey for Emission-Line Galaxies. III. The Third List of 81 Galaxies

We present the third list with results {Tables 2 to 6 are available only in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html. Figures A1 to A9 will be made available only in the electronic version of the journal.} of the Hamburg/SAO Survey for Emission-Line Galaxies (HSS therein, SAO - Special Astrophysical Observatory, Russia). This survey is based on the digitized objective-prism photoplate database of the Hamburg Quasar Survey (HQS). Here, we present new spectroscopic results of candidates which were obtained in 1998 with the 2.1 m KPNO and the 2.2 m Calar Alto telescopes. All candidates are selected in the declination band +35$^{\circ}$ to +40$^{\circ}$. The follow-up spectroscopy with the 2 m class telescopes confirmed 85 emission-line objects out of 113 observed candidates and allowed their quantitative spectral classification. For 80 of them, the redshifts are determined for the first time. For 5 previously known ELGs, line ratios are presented for the first time. We could classify 55 out of the 85 emission-line objects as BCG/H{\sc ii} galaxies or probable BCGs, 4 - as QSOs, 6 - as Seyfert galaxies, 1 - as super-association in a subluminous spiral galaxy, and 11 are low-excitation objects - either starburst nuclear (SBN), or dwarf amorphous nuclear starburst galaxies (DANS). We could not classify 8 ELGs. Further, for 8 more galaxies we did not detect any significant emission lines.

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arXiv1999

HS 1603+3820: a bright z(em)=2.51 quasar with a very rich heavy element absorption spectrum

During the course of Hamburg/CfA Bright Quasar Survey we discovered a bright (B=15.9), high redshift (z=2.51) quasar HS 1603+3820. The quasar has a rich complex of CIV absorbers, containing at least five systems, all within 3000 km/s from one another. Despite large ejection velocity (v(ej)>5000 km/s for all components) the complex is likely to be associated with the quasar. There are at least three more associated heavy element absorbers, two of which have z(abs)>z(em). Together, they make one of the richest known complexes of associated heavy element absorbers, and the richest in objects with B<16. The combination of redshift, brightness, richness of the metal absorption spectrum and richness of the associated absorption is unmatched among known quasars.

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arXiv1998

Large-scale structures in the distribution of X-ray selected AGN

We are searching for large-scale structures in the distribution of AGN discovered by the ROSAT All-Sky Survey. The RASS detected > 60000 X-ray objects, of which approximately 35% are AGN at z < 0.5. The surface density in the extragalactic sky is approximately 0.5 AGN/deg^2, which has not been reached until now by any other survey for this redshift range. We efficiently single out the AGN among all the RASS sources using the Hamburg/RASS database of optical identifications, which contains presently 13867 entries. Follow-up spectroscopy of RASS AGN candidates identified in selected areas of the northern sky is underway to determine the spatial distribution of the AGN in these areas. In every area structures reminiscent of clusters and filaments are found on scales 50 - 100 h^{-1}Mpc. These structures have a similar size as the serendipitously discovered groups of AGN in optical surveys at higher redshifts. Samples of low redshift AGN drawn from the RASS are large enough to be used with samples of higher redshift to study evolutionary effects in the large-scale distribution of AGN.

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arXiv1998

On the evolutionary behaviour of BL Lac objects

We present a new well defined sample of BL Lac objects selected from the ROSAT All-Sky Survey (RASS). The sample consists of 39 objects with 35 forming a flux limited sample down to f_X = 8 x 10^{-13} cgs, redshifts are known for 33 objects (and 31 of the complete sample). X-ray spectral properties were determined for each object individually with the RASS data. The luminosity function of RASS selected BL Lac objects is compatible with results provided by objects selected with the Einstein observatory, but the RASS selected sample contains objects with luminosities at least tenfold higher. Our analysis confirms the negative evolution for X-ray selected BL Lac objects found in a sample by the Einstein observatory, the parameterization provides similar results. A subdivision of the sample into halves according to the X-ray to optical flux ratio yielded unexpected results. The extremely X-ray dominated objects have higher redshifts and X-ray luminosities and only this subgroup shows clear signs of strong negative evolution. The evolutionary behaviour of objects with an intermediate spectral energy distribution between X-ray and radio dominated is compatible with no evolution at all. Consequences for unified schemes of X-ray and radio selected BL Lac objects are discussed.We suggest that the intermediate BL Lac objects are the basic BL Lac population. The distinction between the two subgroups can be explained if extreme X-ray dominated BL Lac objects are observed in a state of enhanced X-ray activity.

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arXiv1998

HS0551+7241: A New Possible Magnetic Cataclysmic Variable in the Hamburg/CfA Bright Quasar Survey

We present the analysis of the spectroscopic observations of a newly discovered cataclysmic variable HS0551+7241. In 1995 the star's brightness dropped by Delta(B)~2.5 mag and HS0551+7241 entered a low state lasting for ~2 yr. The H-alpha, H-beta and HeII(4686) emission line radial velocity curves show evidence of short ~50 min fluctuations superimposed on longer, ~4 hour variations. We found similar modulations in the line fluxes and the line equivalent widths. The continuum magnitude light curve suggest that HS0551+7241 may be an eclipsing system. If the 4 hour period is related to the orbital period then the mass function is ~0.138 M_sun. Relatively strong HeII(4686) emission and short fluctuation may suggest that the binary is an intermediate polar. In that case the ~50 min variations would correspond to the Alfven radius of ~3 x 10^10 cm.

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arXiv1996

SBS 0909+532: A New Double Gravitational Lens or Binary Quasar?

The z=1.377, B=17.0 mag quasar SBS 0909+532 A, B is a double with two images separated by 1.107 +/- 0.006 arcsec. Because the faint image has an emission line at the same wavelength as the MgII 2798 A emission line of the quasar, and lacks the broad MgIb absoption feature expected for a star with the same colors (a K star), we conclude that image B is a quasar with similar redshift to image A. The relative probabilities that the double is the smallest separation (4.8/h kpc for Ω_0=1) correlated quasar pair or a gravitational lens are \sim 1:10^6. If the object is a lens, the mean lens redshift is $ =0.5$ with 90% confidence bounds of 0.18 < z_l < 0.83 for Ω_0=1. If the lens is an elliptical galaxy, we expect it to be brighter than I < 19.5 mag. The broad band flux ratio varies with wavelength, with ΔI=0.31, ΔR=0.58, and ΔB=1.29 magnitudes, which is difficult to reconcile with the lensing hypothesis.

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