|
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 |
AGN | AGN | Complete- |
| (2000) | (2000) | [deg2] |
| candidates | ness |
|---|
| 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:
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)
Publications:
- Tesch F., Engels D., Carrera F.J., Hu J., Wei J., Ledoux C., Ugryumov A.,
Valls-Gabaud D., Voges W., 2001
Delving into large-scale
structures of the X-ray Universe
- F. Tesch:
"The spatial
distribution of soft X-ray selected AGNs at low redshifts" (PDF)
2000, PhD thesis, Hamburg University
- . Tesch, D. Engels :
Detection of the first
X-ray selected large AGN group
- Jiang X.J., Engels D., Wei J.Y., Tesch F., Hu J.Y.,
New cataclysmic variables from the
RASS
- D.Engels, F. Tesch, C. Ledoux, J. Wei, A. Ugryumov, D. Valls-Gabaud, J. Hu, W. Voges :
Large-scale structures in the
distribution of X-ray selected AGN
For more Information please contact:
D. Engels.
|
