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