download report - Sapienza
download report - Sapienza
download report - Sapienza
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
Scientific Report 2007-2009<br />
Astronomy & Astrophysics<br />
A13. Balloon-borne and satellite measurements<br />
of the Cosmic Microwave Background<br />
and its interaction with Clusters of Galaxies<br />
The measurement of cosmic microwave background<br />
(CMB) with experiments like BOOMERanG, WMAP,<br />
and currently Planck, has provided extraordinary images<br />
of the early universe, allowing a precise estimation<br />
of the cosmological parameters. The future of this research<br />
consists of the study of its detailed fine-scale and<br />
polarization properties.<br />
Space missions allow the measurement of the spectral<br />
properties of the CMB. While the COBE satellite has<br />
measured very precisely the specific brightness of the<br />
CMB, the spectral distribution of CMB anisotropy is<br />
largely unexplored. Normally being the derivative of a<br />
Planck spectrum, its distribution is slightly modified by<br />
the interaction of CMB photons with matter along the<br />
path from recombination to here.<br />
A well known effect is the inverse Compton scattering<br />
CMB photon undergo crossing the hot ionized plasma of<br />
clusters of galaxies (the Sunyaev-Zeldovich effect). Low<br />
frequency photons are boosted to higher frequencies, so<br />
that in the direction of cluster there is a deficit of brightness<br />
at frequencies lower than 217 GHz, and an excess at<br />
frequencies higher than 217 GHz. This is a very characteristic<br />
spectral feature, with an amplitude of the order<br />
of 10-100 ppm of the brightness of the CMB, allowing a<br />
clean separation from competing foregrounds. This effect<br />
can be used in a number of ways, ranging from the<br />
discovery of early clusters (this effect does not depend<br />
on the distance of the cluster) to the use of clusters as<br />
standard rulers for the determination of cosmological parameters<br />
(H o , Ω Λ ), to the study of hidden baryons, or<br />
the study of the nature of non-baryonic dark matter in<br />
interacting clusters [1].<br />
Other spectral features in the same frequency range<br />
are due to the interaction of CMB photons with early<br />
molecules, and the emission of lines (like the very strong<br />
[CII] line at 158 µm rest wavelength) from early galaxies.<br />
A first important step in the measurement of these<br />
weak spectral features is the mission OLIMPO (fig.1),<br />
coordinated by our group and funded by the Italian<br />
Space Agency. This is a 2.6 m telescope featuring<br />
bolometer arrays at 150, 220, 340 and 450 GHz [2].<br />
OLIMPO will produce maps of about 100 selected<br />
clusters in both hemispheres, significantly improving<br />
over the current Planck survey [3], due the longer<br />
integration time on clusters (by a factor >100), the<br />
larger number of detectors (by a factor ∼ 3) and of the<br />
finer angular resolution (by a factor ∼ 2). The first<br />
flight of OLIMPO is planned for 2011, in a circum-polar<br />
long-duration flight from Svalbard. In 2008 we have<br />
carried out a detailed phase-A study of a small satellite<br />
mission using a telescope similar to OLIMPO and a<br />
differential Fourier Transform Spectrometer with four<br />
with photon-noise limited bolometer arrays, to cover<br />
continuously the bands 100-450 GHz and 720-760 GHz,<br />
Figure 1: The OLIMPO payload, with the shields removed<br />
to display the 2.6 m Cassegrain telescope. The primary mirror<br />
can tilt, so that the focal plane arrays scan the sky in<br />
cross-elevation to make deep maps of the sky around selected<br />
targets (mainly clusters of galaxies).<br />
with spectral resolution tunable between 1 and 30 GHz.<br />
The instrument, called SAGACE (Spectroscopic Active<br />
Galaxies And Clusters Explorer), flies on a Molniya<br />
orbit, and can be built and operated within the tight<br />
budget of a small mission. This pathfinder mission can<br />
provide spectroscopic surveys of the Sunyaev-Zeldovich<br />
effects of thousands of galaxy clusters, of the spectral<br />
energy distribution of active galactic nuclei, and of the<br />
[CII] line of a thousand galaxies in the redshift desert.<br />
This would qualify the Italian community in view of the<br />
future large space-observatory Millimetron.<br />
References<br />
1. S. Colafrancesco et al., A.& A., 467, L1 (2007)<br />
2. S. Masi, et al., Mem. S.A.It. 79, 887 (2008)<br />
3. J.M. Lamarre et al., A.&A., Planck pre-launch status: the<br />
HFI instrument, from specification to actual performance,<br />
submitted (2009)<br />
Authors<br />
S. Masi, E. Battistelli, M. Calvo, A. Cruciani, P. de<br />
Bernardis, M. De Petris, C. Giordano, L. Lamagna, L. Nati,<br />
F. Nati, F. Piacentini, A. Schillaci<br />
<strong>Sapienza</strong> Università di Roma 160 Dipartimento di Fisica