Extragalactic Astrophysics &
Observational Cosmology Group
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Christian Knobel
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Postal Address:
ETH Hönggerberg Campus
Physics Department, HIT 23.5
CH-8093 Zürich
Switzerland
Phone: +41 (0)44 633-6330
Fax: +41 (0)44 633-1238
email: christian.knobel@phys.ethz.ch
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Curriculum Vitae
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Education:
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Employment:
| 2006 | PhD student at the Institute of Astronomy, ETH Zurich, Switzerland |
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Research Interests
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Galaxy groups in zCOSMOS
My research interest encompasses the analysis of high redshift large scale structure with the zCOSMOS survey.
My supervisors is Prof. Dr. Simon Lilly.
Groups and clusters of galaxies are the most massive virialized structures in
the Universe. They provide information about how the local
environment affects the evolution of galaxies, about the galactic
content of dark matter (DM) halos, and
their number density yields strong constraints on certain cosmological parameters.
Thus knowing the groups opens the door for several important
scientific investigations. However, there are many different
observational techniques to identify groups in the local and distant
Universe. Each of these methods has its own advantages and problems,
and the choice of a particular method might depend on the desired
application.
In my recent paper (Knobel et al. 2009), we present a galaxy group catalogue spanning the
redshift range 0.1 < z < 1 in the 1.7 sqrdeg
COSMOS field, based on the first 10,000 zCOSMOS spectra. To
identify the groups, we used the most fundamental signature of
groups – overdensities in redshift space – without recourse to
additional color information. We applied two different group-finding
algorithms, the Friends-of-Friends (FOF) and the
Voronoi-Delaunay-Method (VDM), whose performances are extensively
tested and compared.
We find that the statistics of the classic FOF algorithm is slightly
superior to that of the VDM algorithm, and that the
performance improves substantially if groups are found by progressively
optimizing the groupfinding parameters for successively smaller
groups. However, the highest fidelity catalogue, in terms of
completeness and purity, is obtained by combining the independently
created FOF and VDM catalogues resulting in the “1-way-matched”
(1WM) sub-catalogue. The basic FOF catalogue along with its 1WM
sub-catalogue constitute the final group catalogue released along
with the paper. The completeness and purity of this
catalogue, both in terms of the groups and of individual members,
compares favorably with recent results in the literature.
Figure 1
shows the groups #8 and #795-#799
forming a “super-group” at z = 0.22 which is just about to
merge. Black points denote field galaxies, and the other symbols
(squares, triangles, etc.) are group members, whereas each
group has its own symbol and color. The blue contours exhibit X-ray
emission as observed with XMM-Newton.
Figure 2
exhibits the fraction of galaxies in groups for
groups with more than 2 (upper panel) and more than 5
members (lower panel) respectively. The red lines correspond to
the real 10k groups, while the black and the green lines
correspond to the groups in the 24 mock catalogues. The 10k
group catalogue is broadly consistent with that expected from the
semi-analytic evolution model underlying the mock catalogues.
Figure 3
shows the fraction of galaxies in groups as a
fraction of corrected
richness Ncorr in volume limited samples. The blue line corresponds
to a 2dfGRS sample (z ~ 0.1) and two black lines to the 10k
samples at z ~ 0.3 and z ~ 0.7 respectively. The decline of
the fraction with redshift can straightforwardly be interpreted in
terms of growth of structure as expected in a hierarchical structure
formation scenario.
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| Selected Papers |
An optical group catalogue to z = 1 from the zCOSMOS 10k sample
C. Knobel et al.
The Astrophysical Journal, Volume 697, Issue 2, pp. 1842-1860 (2009)
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| Links |
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About us