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Scientific Report 2007-2009
Astronomy & Astrophysics
A12. Ground-based observations of the Secondary Anisotropy of the
Cosmic Microwave Background
The photons of the Cosmic Microwave Background
(CMB), on their way towards us from the last scattering
surface, interact with cosmic structures and their frequency,
energy or direction of propagation are affected.
These effects are included in the so called Secondary
Anisotropies that arise from two major families of interactions.
The first includes the interactions between
photons and gravitational potential wells (i.e. gravitational
lensing, the Rees-Sciama effect and the integrated
Sachs-Wolfe effect). The second family incorporates the
effects of scattering between CMB photons and free electrons
such as inverse Compton interaction, the Sunyaev-
Zel’dovich (SZ) effect, and velocity-induced scatterings,
the Ostriker-Vishniac (OV) effect.
Observations of the SZ effect, developing instruments
for this purpose, and the study of its implications in cosmology
are the main goals of this research activity. The
expected distortion in the CMB spectra is evident in the
millimeter band and this allows its observation even from
the ground. The Experimental Cosmology Group G31
has developed a 2.6 m in diameter on-axis aplanatic telescope
mainly devoted to millimeter wavelength observations.
The project, named MITO (Millimeter and Infrared
Testagrigia Observatory) enjoys the logistical support
of the IFSI/INAF laboratory on the Alps (Breuil-
Cervinia 3480 m a.s.l.). The advantage of an observational
cold and dry site is a stable and high atmospheric
transmission in the mm-band. The cross-elevation modulation
in the sky, for reducing the sky-noise, is ensured
by a wobbling 41-cm in diameter subreflector. Several
instruments have been installed at the telescope focal
plane and new ones are almost ready (MAD, Multi Array
of Detectors, a 3x3 pixels for 4 bands: 143, 214,
272 and 353 GHz) [1], or planned (WCAM, 7x7 arrays
of TES in the W-band). An atmospheric spectrometer,
CASPER2, has been designed and realised in order to
continuously monitor the atmospheric opacity in the 2
mm ÷ 850 micron band. CASPER2 is a small (62-cm in
diameter) telescope with a Martin-Puplett spectrometer
and 2 detectors cooled down to 300 mK.
The SZ effect has a continuous increasing number of
applications in cosmology. Among the many, we have
oriented our research on the possibility of constraining
the scaling of the CMB temperature along the redshift,
T CMB (z), deriving it from multifrequency observations
of SZ effect towards cluster of galaxies [2,3]. Incoming
all sky surveys collecting a large number of clusters will
constrain better the temperature standard scaling law.
So far, T CMB (z) has been only determined from measurements
of microwave transitions in interstellar clouds
due to atoms and molecules excited by CMB photons:
an approach with substantial systematic uncertainties.
The clusters of galaxies are the main scatterers producing
SZ distortion but the effect is generated by all
the gas present along the line of sight. For this reason
the SZ effect is also a useful challenging probe for detecting
clusters having no detectable X-ray emission and for
revealing the so-called missing baryons in the local universe.
In fact half of the expected baryons are not yet
counted mainly due to the difficulty of their detection
due to their low gas density and temperature. Gasdynamical
simulations suggest that these missing baryons
could be accounted for in a diffuse gas phase with temperatures
10 5 < T < 10 7 K and moderate overdensities
(δ ≤ 10÷100), known as the warm/hot intergalactic
medium (WHIM). The superclusters are suitable sky regions
for this purpose as derived by gasdinamical simulations
of the Universe: long filaments are present connecting
cluster members.
We have performed observations of SZ effect towards
Corona Borealis supercluster in collaboration with
IAC in Tenerife and we studied in the MareNostrum
Universe, a gasdynamical simulation provided us by a
collaboration with UAM in Madrid, the expected SZ
signal due to different gas components [4]. Incoming
experiments, ground based or space missions in which
the authors are involved, will allow to fully explore
this topic reaching higher angular resolution and larger
spectral range.
References
1. M. De Petris, et al., New Astronomy Rev. 51, 368 (2007).
2. L. Lamagna, et al. New Astronomy Rev., 51, 381 (2007).
3. G. Luzzi, et al., Astrophysical Journal 705, 1122 (2009).
4. I. Flores-Cacho, et al., MNRAS 400, 1868 (2009).
Authors
M. De Petris, E.S. Battistelli, B. Comis, A. Conte, P. de
Bernardis, S. De Gregori, L. Lamagna, V. Lattanzi, G.
Luzzi, S. Masi
http://oberon.roma1.infn.it/
Figure 1: MITO telescope on the top of Testa Grigia
mountain (left) and the atmospheric spectrometer CASPER2
(right).
Sapienza Università di Roma 159 Dipartimento di Fisica