Simulations of the Polarized Sky for the SKA: How to Constrain Intracluster Magnetic Fields

The advent of the Square Kilometer Array (SKA) will have unprecedented impact on the study of magnetic fields in galaxy clusters. This instrument will be able to perform all-sky surveys in polarization, allowing us to build a rotation-measure (RM) grid based on an enormous number of sources. However, it is not always obvious how to extract correct information about the strength and the structure of magnetic fields from the RM grid. The simulations presented here help us to investigate this topic as they consist of full-Stokes idealized (because we did not add thermal noise) images of a pair of galaxy clusters between 950–1760 GHz, i.e., the SKA1-MID band 2. These images include not just cluster-embedded radio sources but also foreground and background discrete radio sources populating the simulated portion of the universe. To study the magnetic fields of the simulated galaxy clusters, we applied the RM synthesis technique on the simulated images and compared the “true” cluster RM values with those inferred from RM synthesis. The accuracy of our methodology is guarantee by the excellent agreement that we observed when we considered only the signal from the background radio sources. The presence of a Faraday screen, foreground, and cluster sources, introduces degeneracies and/or ambiguities that make the interpretation of the results more difficult.

20 cm VLA radio-continuum study of M31-images and point source catalogues DR2: Extraction of a supernova remnant sample

We present Data Release 2 of the Point Source Catalogue created from a series of previously constructed radio-continuum images of M31 at ?=20 cm (v=1.4 GHz) from archived VLA observations. In total, we identify a collection of 916 unique discrete radio sources across the field of M31. Comparing these detected sources to those listed by Gelfand et al. (2004) at ?=92 cm, the spectral index of 98 sources has been derived. The majority (73%) of these sources exhibit a spectral index of ?<-0.6, indicating that their emission is predominantly non-thermal in nature, which is typical for background objects and Supernova Remnants (SNRs). Additionally, we investigate the presence of radio counterparts for some 156 SNRs and SNR candidates, finding a total of only 13 of these objects in our images within a 500 search area. Auxiliary optical, radio and X-ray catalogues were cross referenced highlighting a small population of SNRs and SNR candidates common to multi frequency domains.

20cm VLA radio-continuum study of M31: Images and point source catalogues

We present a series of new high-sensitivity and high-resolution radio-continuum images of M31 at ?=20 cm (?=1.4 GHz). These new images were produced by merging archived 20 cm radio-continuum observations from the Very Large Array (VLA) telescope. Images presented here are sensitive to rms=60 ?Jy and feature high angular resolution (<10??). A complete sample of discrete radio sources have been catalogued and analyzed across 17 individual VLA projects. We identified a total of 864 unique discrete radio sources across the field of M31. One of the most prominent regions in M31 is the ring feature for which we estimated total integrated flux of 706 mJy at ?=20 cm. We compare here detected sources to those listed in Gelfand et al. (2004) at ?=92 cm and find 118 sources in common to both surveys. The majority (61%) of these sources exhibit a spectral index of ? <-0.6 indicating that their emission is predominantly non-thermal in nature, that is more typical for background objects.

Jets, hotspots and lobes: what X-ray observations tell us about extragalactic radio sources

The brightest and most numerous discrete radio sources in the sky, radio galaxies and quasars, are powered by twin jets of plasma which emerge at relativistic speeds from very small regions at the centre of large elliptical galaxies, powered by mass infall on to supermassive black holes. The jets can carry material out to very large distances (millions of light years) where it forms balloon-like lobes. Until recently it has been impossible to make definite statements about the energy or the nature of the matter supplied by the jets, or the dynamics of the lobes as they expand into the external medium. This has meant that crucial questions about the generation of radio sources and their effect on their environment have gone unanswered. The situation has been revolutionized by the launch at the start of this decade of a new generation of X-ray observatories, Chandra and XMM-Newton . In this article, I explain why observations with these instruments have made such a difference, what we have learned as a result and why the community remains divided on some important features of the interpretation of the data.

A search for candidate radio supernova remnants in the nearby irregular starburst galaxies NGC 4214 and NGC 4395

We present the results of a search for new candidate radio su?pernova remnants (SNRs) in the nearby starburst irregular galaxies NGC 4214 and NGC 4395 using archived radio observations made with the Very Large Array (VLA) at the wavelengths of 3.5 cm, 6 cm and 20 cm for NGC 4214 and 6 cm and 20 cm for NGC 4395. These observations were analyzed as part of our ongoing search for candidate radio SNRs in nearby galaxies: the goal of this search is to prepare a large sample of candidate radio SNRs for the purpose of a robust statistical study of the properties of these sources. Based on our analysis, we have confirmed the nonthermal nature of the discrete radio sources ? and ? in NGC 4214 and classify these sources as candidate radio SNRs based on their positional coincidences with HII regions in that galaxy. We have measured the flux densities of the two candidate radio SNRs at each wavelength and calculated corresponding spectral indices: we have also measured flux densities of two other discrete radio sources in these galaxies - ? in NGC 4214 and #3 in NGC 4395 which we suspect to be additional candidate radio SNRs based on their positional coincidences with other HII regions in these galaxies. However, the radio data presently available for these sources can?not confirm such a classification and additional observations are needed. We have also calculated the radio luminosities Lradio at the wavelength of 20 cm for these two candidate radio SNRs as well as the corresponding values for the minimum total energy Emin required to power these radio sources via synchrotron emission and the corresponding magnetic field strength Bmin. We have compared our mean calculated values for these properties with the mean values for populations of candidate radio SNRs in other starburst galaxies: while the values for Lradio and Bmin are roughly comparable to the values seen in other starburst galaxies, the mean value for Emin is higher than the mean value of any other starburst galaxy. Finally, we include these two candidate radio SNRs in a discussion of the ? ? D relation for extragalactic candidate radio SNRs and find that these sources are located on the shallower end of the master ? ? D relation for all extragalactic SNRs as derived by Urosevic et al.(2005). .

Supernova Remnants in the Magellanic Clouds

A study of the nature and the spatial distribution of over 250 discrete radio sources (HII regions and SNRs) in the Magellanic Clouds (MCs) is underway. A multi-frequency comparison of these sources involves the latest radio, IR, optical and X-ray surveys with angular resolution of <1′. Special emphasis is given to the MC SNRs. Their morphology, birth rate and overall properties are investigated and compared with Galactic SNRs and SNRs from other nearby galaxies. Also, we are investigating the radio source distribution throughout both MCs.