Members:

Main Topics:

* * * Clusters of Galaxies * * *

People active in the research field:

Radio sources in clusters and interaction with the ambient medium

Some clusters of galaxies are known to host cluster-wide diffuse radio sources, the so called "radio halos" and "relics". These sources are not identified with any individual galaxy and therefore associated with the intergalactic medium.
They reveal the presence of large scale magnetic fields and of a population of relativistic particles whose origin and evolution is still under discussion.
The importance of these sources is that they represent large scale features, which are related to other cluster properties in the optical and X-ray domain, and are thus directly connected to the cluster history and evolution. Their study is therefore crucial to understand the global evolution of clusters.

Another aspect in the study of galaxy clusters concerns the radio emission from individual galaxies.
The properties of radio galaxies are affected by the environment, therefore a detailed investigation of the cluster radio sources, their extended structure and their relation to the surrounding medium is important to understand the ageing and evolution of the radio emitting plasma. Moreover, Chandra X-ray data has recently revealed the presence of cavities in the X-ray gas at the center of some galaxy clusters with a powerful radio source, confirming earlier results obtained by ROSAT.
The current interpretation is that the gas has been pushed by radio jets and lobes. The study of the radio emitting regions within these X-ray holes gives important information about the interaction between the radio sources and the outer gas.

Connection to the SKA project

The SKA will provide a dramatic improvement in surface brightness sensitivity, coupled with the high angular resolution and large field of view. So, it will be an ideal instrument to study the low brightness radio emission from galaxy clusters, i.e. the diffuse halos and relics and the extended features in radio galaxies, whose detection suffers from the missing short spacings in currently available interferometers.

Major improvements are expected in the study of galaxy clusters, in particular:

- The study of diffuse radio emission (halos and relics). The main problem in the study of radio halos and relics is their low brightness, which makes it difficult their detection and study with the current resources. Very high sensitivity on all angular scales is needed for a proper study of the radio halos and relics. High resolution Chandra X-ray data show structure in the thermal component of the intergalactic medium, wich should be compared with the non-thermal emission, to understand the interaction of these two components.

- The study of cluster radio galaxies, their extended structure, age and interaction with the ambient medium. Moreover, the radio emission filling the X-ray cavities detected in some clusters by Chandra can only be studied with observations at high resolution and high surface brightness sensitivity.

* * * Radio and Optical Surveys * * *

People active in the research field:

The Faint Radio Population

A presentation of this work was given at the Berkeley workshop:
"SKA: Defining the Future", and can be found here (pdf file, 250 kb)

The sub-mJy and mJy faint radio population is composed by different classes of objects (faint AGN, star-forming galaxies, normal elliptical and spiral galaxies), but the relative importance of the different classes is still uncertain. While the fraction of faint AGN is very similar (10-15%) in all samples, there are significant discrepancies in the quoted fractions of early type and star-forming galaxies.
Also, very little is known about the luminosity function and redshift distribution of the different classes composing the sub-mJy population. This is due to both the limited size of the faintest radio surveys and the limited follow-up available. The optical identification workand subsequent spectroscopy needed to investigate the nature of the radio sources are both very demanding in terms of telescope time, since faint radio sources have usually very faint optical counterparts.

Typically, no more than 50-60% of the radio sources in sub-mJy samples have been identified on optical images, while the typical fraction of spectra available is only 20%. Due to the long integration times needed for detection, deeper spectroscopy programmes have been undertaken only for very small sub-mJy radio samples and never went to completion. There is increasing evidence that selection effects could play an important role in this kind of study and that conclusions about the nature and the  evolution of the faint radio population is not only limited by the low identification rate but also biased by the fact that only the brightest counterparts have spectral information.

A full understanding of the sub-mJy population composition and in particular of the relative importance of star-forming galaxies as a function of redshift can have important implications in cosmological studies like, f.i., the study of the global star formation history as a function of cosmic time and the study of the faint tail of the AGN luminosity function and their redshift distribution.
Unless evolutionary effects would play an important role at fainter magnitudes than reached by current studies, we expect that sub-mJy star-forming galaxies would give information up to limited redshifts (z<0.5), while larger redshift values (up to z~1) would be probed by microJy samples. NanoJy samples would instead allow us to trace the evolution of star-forming galaxies up to the epoch of their formation.
In order to verify this hypothesis large radio samples down to nanoJy fluxes, combined with optical follow-ups down to very faint magnitudes are needed.

Connection to the SKA Project

The SKA will provide a dramatic improvement in sensitivity, which coupled with sub-arcsec spatial resolution and large field of view will make it possible to map the radio sky down to the nanoJy regime.
This will allow the detection of statistically relevant samples of normal spiral galaxies up to redshifts of cosmological interest (z~1) and starburst galaxies up to the epoch of their formation. This will have a dramatic impact on the galaxy formation and evolution scenario. High spatial resolution is needed to avoid confusion and, combined with multi-wavelength observations (e.g. deep optical and FIR imaging, good quality spectroscopy and radio spectral index), to distinguish between AGN and star-forming population.
The main advantages of tracing the star formation history and the galaxy evolution using radio selected samples are represented by the fact that radio samples are not affected by extinction effects as for the optical samples and by poor positional accuracies as for the FIR samples. The SKA high brightness sensitivity will allow detailed studies of a largernumber of nearby starburst galaxies, and consequently a better calibration of the star formation rates as derived from radio luminosities.

 

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