Abstract of the PhD thesis of V. Beckmann
Abstract
The evolution and nature of AGN is still one of the enigmatic questions in
astrophysics. While large and complete Quasar samples are available, special
classes of AGN, like BL Lac objects and Seyfert II galaxies, are still rare
objects. In this work I present two new AGN samples. The first one is the
HRX-BL Lac survey, resulting in a sample of X-ray selected BL Lac objects. This
sample results from 223 BL Lac candidates based on a correlation of X-ray
sources with radio sources. The identification of this sample is 98% complete.
77 objects have been identified as BL Lac objects and form the HRX-BL
Lac complete sample, the largest homogeneous sample of BL Lac objects
existing today. For this sample, redshifts are now known for 62
objects (81 %). In total I present 101 BL Lac objects in the enlarged
HRX-BL Lac survey, for which redshift information is available for 84 objects.
During the HRX-BL Lac survey I found several objects of special interest. 1ES 1517+656 turned out to be the brightest known BL Lac object in the universe. 1ES 0927+500 could be the first BL Lac object with a line detected in the X-ray region. RX J1211+2242 is probably the the counterpart of the up to now unidentified gamma-ray source 3EG J1212+2304. Additionally I present seven candidates for ultra high frequency peaked BL Lac objects.
RX J1054+3855 and RX J1153+3517 are rare high redshift X-ray bright QSO or accreting binary systems with huge magnetic fields.
For the BL Lac objects I suggest an unified scenario in which giant elliptical
galaxies, formed by merging events of spiral galaxies at z > 2, start as powerful,
radio dominated BL Lacs. As the jet gets less powerful, the
BL Lacs start to get more X-ray dominated, showing less total luminosities
(for z < 1). This effect is seen in the different evolutionary behavior
detected in high and low frequency cut off BL Lac objects (HBL and LBL,
respectively). The model of negative evolution is supported by assumptions
about the energetic effects which contribute to the BL Lac phenomenon.
I also suggest an extension of the BL Lac definition to objects with a calcium
break up to 40%, but do not support for the HBL the idea of allowing
emission lines in the spectra of BL Lac galaxies.
A way to find high redshift BL Lac objects might be the identification of
faint X-ray sources (e.g. from the ROSAT All-Sky Survey) with neither optical
nor radio counterpart in prominent databases (e.g. POSS plates for the
optical, and NVSS/FIRST radio catalogues).
The Seyfert II survey on the southern hemisphere derived a sample of 29
galaxies with 22 in a complete sample. The selection procedure developed in
this work is able to select Seyfert II candidates with a success rate of
~40%. The Seyfert II galaxies outnumber the
Seyfert I by a factor of 3...4 when comparing the total flux of the objects, but are less numerous than the type I objects when studying the core luminosity function. This luminosity function of the Seyfert II cores is the first one presented up to now. Hence it is possible to estimate the number of luminous Type II AGN, and the conclusion is drawn that absorbed AGN with MV < -28 mag might not exist within the universe.
In 25 % of the Seyfert II galaxies I find evidence for merging events.
In collaboration with Roberto Della Ceca I also showed that it is possible to
find Type II AGN by selecting "hard" X-ray sources. I present a prototype
of a Type II AGN found within this project.
This work might be the basis to explore the universe for rare objects like BL
Lacs and Seyfert II galaxies at higher redshifts. This could give an answer to
the question: Whether there are BL Lac objects at redshifts $z \gg 1$ and Type
II Quasars or not.
In summary the AGN phenomenon appears to be linked closely to merging and
interacting events. For the BL Lac phenomenon the merging area seems to form the progenitor, while the Seyfert II phenomenon could be triggered by merging events. The role of star burst activity in terms of activity of
the central engine remains illusive.

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