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Scientific Report 2007-2009
Astronomy & Astrophysics
A10. Search and analysis of galaxy clusters in the optical and X-ray
bands
Galaxy clusters are the largest and most massive gravitationally
bound systems and represent a powerful tool
to investigate dark matter, the evolution in cosmic time
of the large scale structure of the Universe and galaxy
formation and evolution. Originally selected as local enhancements
of the galaxy number density on the celestial
sphere, they were successfully modeled as hydrostatic
equilibrium structures, once X-ray observations made
the study of their intergalactic hot gas possible. Since
the 80-ies, galaxy morphology and colour segregation in
low redshift clusters were discovered, and quantified indicating
the interaction of galaxies with the environment.
For this reason, finding end studying high redshift cluster
is the main way to understand the origin of the properties
observed at low redshift, and the nature of the physical
processes which determine the evolution of galaxies and
their interaction with the environment. Studies of X-ray
detected massive clusters up to redshift z 1.4 have shown
little evolution of their properties, despite the large look
back time ( 65 % of the age of the Universe). However,
only very massive structures have been detected so far,
due to the strong dependence of X-ray luminosity on the
gas mass. Surveys based on the Sunyaev-Zeldovich (SZ)
effect will open invaluable perspectives for the future,
but do not reach yet the sensitivity to detect any of the
known clusters at z¿1. Searching for Ly-alpha emitters
near radio galaxies is limited to z¿2 for ground-based
observations and other methods used at low redshift become
unpractical for finding distant clusters in the range
1¡z¡2 where the first hints of colour segregation are expected
to appear. The use of broad band images in several
wavelength intervals, typically from the ultraviolet
to the near infrared, makes it possible to derive a spectral
energy distribution (SED), essentially equivalent to
a low resolution spectrum, for all the galaxies in the
observed field. Fitting the observed SEDs, with either
empirical templates or with models derived from population
syntheses, provides the determination of the so
called photometric redshift. We developed the (2+1)D
algorithm [1] which estimates a three-dimensional galaxy
number density from the angular position and a radial
distance determined from the photometric redshift obtained
from multi-band photometry down to the deepest
observational limits.
The application of this method in the GOODS field
[2] allowed us to identify a galaxy cluster at redshift 1.6,
to estimate its mass and to measure its the X-ray luminosity,
from the deepest X-ray observation existing
nowadays, obtained with the 2 Ms exposure of the Chandra
X-ray observatory in the Chandra Deep Field South.
This is the most distant galaxy cluster ever detected on
the sole basis of an over-density in the galaxy distribution.
While at low redshift the fraction of elliptical (red)
galaxies is larger in regions of higher density, this effect
Figure 1: The highest redshift, z=1.61, galaxy cluster detected
on the sole basis of galaxy overdensity [2]. Optical
image in the z 850 band from the ACS camera on board of
the Hubble Space Telescope. Yellow lines: cluster iso-density
contours; black lines 0.4-3 keV X-ray contours from Chandra
X-ray Observatory data.
tends to vanish at redshift greater than 1, due to a high
fraction of star forming galaxies, which is present even in
the over-dense regions. Our study of the segregation of
galactic types for different environmental densities, as a
function of cosmic time, has extended to redshifts grater
than 2 [3] the evidence of this trend.
Thanks to the X-ray observations from Chandra
and XMM-Newton satellites, at lower redshift
(0.1 < z < 0.5) is now possible to study also the
properties of relatively small (10 14 M ⊙ ) and cool (kT
4 keV) clusters, which are more likely to display the
effects of non-gravitational energy (star formation, active
galactic nuclei) into the intra-cluster medium. One
of these objects, Zw 1305.4+2941, has been observed
with a medium-deep exposure of XMM-Newton and its
properties were compared with those of other objects
in the same range of parameters [4]. The study adds
evidence in favour of a deviation of the main scaling
relations, between X-ray luminosity, gas temperature
and density and galaxy velocity dispersion, obtained for
more massive galaxy clusters.
References
1. Trevese D., et al. Astron.&Astrophys 463, 853 (2007)
2. Castellano M. et al. Astrophys. J. 671, 1497 (2007)
3. Salimbeni S. et al. Astron.&Astrophys 501, 865 (2009)
4. Gastaldello F. et al. Astrophys. J. 673, 176 (2008)
Authors
D. Trevese, M. Castellano, K. Boutsia, A. Fontana 5 , E.
Giallongo 5 , S. Salimbeni 5
http://astrowww.phys.uniroma1.it/scae.html
Sapienza Università di Roma 157 Dipartimento di Fisica