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