AGN in the RASS
BL Lac
Source Catalogue
Hamburger Sternwarte - Research

ROSAC - A ROSAT based Search for AGN-Clusters

We use the AGN detected by the ROSAT All-Sky Survey to search for large scale structures in the universe at z<0.5. The surface density in the extragalactic sky is ~0.5 AGN/deg2, which has not been reached until now by any other survey for this redshift range. AGN candidates are picked up from the RASS using their identifications (Hamburg/RASS collaboration) on the Schmidt Plates of the Hamburg Quasar Survey.

Follow-up spectroscopy was made for these RASS AGN candidates to prove their nature and to determine their redshift. In general, about 90% of the AGN candidates turn out to be Seyfert galaxies, quasars or BL Lac objects. To minimize selection effects, several regions with low galactic hydrogen absorption were chosen, in which complete optical spectroscopy was attempted. The regions selected are given in the following table with their current status of observations.

Region alpha delta area AGNAGN Complete-
(2000)(2000)[deg2] candidatesness
Pisces 23h 30m - 02h 00m 00 - 10d 675 160 70 70%
23h 30m - 01h 30m 10 - 20d
Aries-Taurus 01h 30m - 04h 00m 10 - 20d 375 47 7 87%
Ursa Major 08h 00m - 11h 00m 45 - 58d 585 173 38 82%
Coma Berenices 11h 00m - 14h 00m 25 - 35d 450 147 78 65%

To obtain the required redshifts an ambitious spectroscopic observing program was performed 1997 -- 2000. In total we determined 366 new redshifts in 79 nights allocated.

A Minimal-Spanning-Tree analysis was made in the region with the highest completenes of follow-up spectorscopy, the Ursa Major region. Four large scale structures ("AGN groups") were found at significance levels 2.1 -3.2 σ, containing 6 to 26 members. Two structures at z = 0.1 with 19 to 26 members extend beyond the region boundaries, and might be parts of even larger structures.

The other two structures are reasonable candidates for AGN groups. Group 1000+5100 (0.183 < z < 0.246) has dimensions 120 x 86 x 141 h-1Mpc and group 0950+5230 (0.285 < z < 0.305) extends over 53 x 58 x 104 h-1Mpc.

ROSAC is a collaborative effort of astronomers from several institutes:

Dieter Engels, Frank Tesch Hamburger Sternwarte
Jingyao Hu, Jianyan Wei, Xuebing Wu Beijing Astronomical Observatory
Volker Müller AIP Potsdam
Cedric Ledoux, David Valls-Gabaud Observatoire de Strasbourg
Wolfgang Voges MPE Garching
Andrew Ugryumov Special Astronomical Observatory

Documentation available:
  • Project description (9/97)
  • Status Report No. 1 (3/98)
  • Engels D., Tesch F., Ledoux C., et al., Large-scale structures in the distribution of X-ray selected AGN,
    MPE-Report (in press) (9/98)


    For more Information please contact: D. Engels.

  • Last changed  Wednesday, 31-March-2004 | D.Engels, A.Mertz