Tracing Ghost Cavities with Low Frequency Radio Observations

We present X-ray and multi-frequency radio observations of the central radio sources in several X-ray cavity systems. We show that targeted radio observations are key to determining if the lobes are being actively fed by the central AGN. Low frequency observations provide a unique way to study both the lifecycle of the central radio source as well as its energy input into the ICM over several outburst episodes.Comment: 6 pages, 4 figures, To appear in the Proceedings of "Heating vs. Cooling in Galaxies and Clusters of Galaxies", eds. H. Boehringer, P. Schuecker, G. W. Pratt & A. Finoguenov (ESO Astrophysics Symposia, Springer-Verlag), Garching (Germany), August 200

Dark matter annihilation and non-thermal Sunyaev-Zel'dovich effect: II. dwarf spheroidal galaxy

We calculate the CMB temperature distortion due to the energetic electrons and positrons produced by dark matter annihilation (Sunyaev-Zel'dovich effect), in dwarf spheroidal galaxies (dSphs). In the calculation we have included two important effects which were previously ignored. First we show that the electron-positron pairs with energy less than GeV, which were neglected in previous calculation, could contribute a significant fraction of the total signal. Secondly we also consider the full effects of diffusion loss, which could significantly reduce the density of electron-positron pairs at the center of cuspy halos. For neutralinos, we confirm that detecting such kind of SZ effect is beyond the capability of the current or even the next generation experiments. In the case of light dark matter (LDM) the signal is much larger, but even in this case it is only marginally detectable with the next generation of experiment such as ALMA. We conclude that similar to the case of galaxy clusters, in the dwarf galaxies the SZ_2DM} effect is not a strong probe of DM annihilations.Comment: 22 pages, 9 figures, version accepted by JCA

LOFAR low-band antenna observations of the 3C295 and Bootes fields: source counts and ultra-steep spectrum sources

We present LOFAR Low Band observations of the Bootes and 3C295 fields. Our images made at 34, 46, and 62 MHz reach noise levels of 12, 8, and 5 mJy beam−1, making them the deepest images ever obtained in this frequency range. In total, we detect between 300 and 400 sources in each of these images, covering an area of 17 to 52 deg 2. From the observations we derive Euclidean-normalized differential source counts. The 62 MHz source counts agree with previous GMRT 153 MHz and VLA 74 MHz differential source counts, scaling with a spectral index of −0.7. We find that a spectral index scaling of −0.5 is required to match up the LOFAR 34 MHz source counts. This result is also in agreement with source counts from the 38 MHz 8C survey, indicating that the average spectral index of radio sources flattens towards lower frequencies. We also find evidence for spectral flattening using the individual flux measurements of sources between 34 and 1400 MHz and by calculating the spectral index averaged over the source population. To select ultra-steep spectrum (α<−1.1 ) radio sources, that could be associated with massive high redshift radio galaxies, we compute spectral indices between 62 MHz, 153 MHz and 1.4 GHz for sources in the Booötes field. We cross-correlate these radio sources with optical and infrared catalogues and fit the spectral energy distribution to obtain photometric redshifts. We find that most of these ultra-steep spectrum sources are located in the 0.7≲z≲2.5 range

A radio ridge connecting two galaxy clusters in a filament of the cosmic web

Galaxy clusters are the most massive gravitationally bound structures in the Universe. They grow by accreting smaller structures in a merging process that produces shocks and turbulence in the intracluster gas. We observed a ridge of radio emission connecting the merging galaxy clusters Abell 0399 and Abell 0401 with the Low-Frequency Array (LOFAR) telescope network at 140 megahertz. This emission requires a population of relativistic electrons and a magnetic field located in a filament between the two galaxy clusters. We performed simulations to show that a volume-filling distribution of weak shocks may reaccelerate a preexisting population of relativistic particles, producing emission at radio wavelengths that illuminates the magnetic ridge

Comptonization of the cosmic microwave background by relativistic plasma

We investigate the spectral distortion of the cosmic microwave background (CMB) caused by relativistic plasma. Within the Thomson regime, an exact analytic expression for the photon scattering kernel of a momentum power-law electron distribution is given. The ultra-relativistic Sunyaev-Zeldovich (SZ) decrement measures the electron number and not the energy content. On the other hand, the relativistic SZ increment at higher frequencies depends strongly on the spectral shape of the electrons. We calculate the expected Comptonization due to the energy release of radio galaxies, which we estimate to be 3 × 1066 erg Gpc-3. We investigate Comptonization from (a) the part of the released energy which is thermalized and (b) the relativistic, remnant radio plasma, which may form a second, relativistic phase in the inter galactic medium, nearly unobservable for present day instruments (presence of so called 'radio ghosts'). We find a thermal Comptonization parameter due to (a) of y = 10-6 and (b) an optical depth of relativistic electrons in old radio plasma of τrel &lt; 10-7. If a substantial fraction of the volume of clusters of galaxies is filled with such old radio plasma the SZ effect based determination of the Hubble constant is biased to lower values. Finally, it is shown that a supra-thermal population of electrons in the Coma cluster would produce a signature in the Wien-tail of the CMB, which is marginally detectable with a multifrequency measurement by the Planck satellite. Such a population is expected to exist, since its bremsstrahlung would explain Coma's recently reported high energy X-ray excess

Modification of cluster radio halo appearance by the thermal Sunyaev-Zeldovich effect

SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Radio plasma as a cosmological probe

Plasma containing relativistic particles appears in various forms in the inter galactic medium (IGM): As radio plasma released by active radio galaxies, as fossil radio plasma from former radio galaxies - so called radio ghosts -, as cluster radio relics in some clusters of galaxies, and as cluster radio halos. The impact of the different forms of radio plasma on the IGM and their use as diagnostic tools are briefly discussed. (orig.)SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Relativistic particles in clusters of galaxies

SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

The fate of intracluster radio plasma

SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Does circular polarisation reveal the rotation of quasar engines?

SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman