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Register a new user20 cm VLA Radio-Continuum Study of M31 - Images and Point Source Catalogues DR2: Extraction of a supernova remnant sample
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We present Data Release 2 of the Point Source Catalogue created from a series of previously constructed radio-continuum images of M31 at lambda=20 cm (nu=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 lambda=92 cm, the spectral index of 98 sources has been derived. The majority (73%) of these sources exhibit a spectral index of alpha <-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 object in our images within a 5 arcsec search area. Auxiliary optical, radio and X-ray catalogs were cross referenced highlighting a small population of SNR and SNR candidates common to multi-frequency domains.
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We present a series of new high-sensitivity and high-resolution radio-continuum images of M31 at lambda=20 cm ( u=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 mu Jy and feature high angular resolution (<10). A complete sample of discrete radio sources have been catalogued and analysed 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 lambda=20 cm. We compare here, detected sources to those listed in Gelfand et al. (2004) at lambda=92 cm and find 118 sources in common to both surveys. The majority (61%) of these sources exhibit a spectral index of alpha <-0.6 indicating that their emission is predominantly non-thermal in nature. That is more typical for background objects.
We present and discuss a new catalogue of 52 compact HII regions in the Small Magellanic Cloud (SMC) and a newly created deep 1420 MHz (lambda=20 cm) radio-continuum image of the N19 region located in the southwestern part of the SMC. The new images were created by merging 1420 MHz radio-continuum archival data from the Australian Telescope Compact Array. The majority of these detected radio compact HII regions have rather flat spectral indices which indicates, as expected, that the dominant emission mechanism is of thermal nature.
We present a multi-wavelength study of the radio source G296.7-0.9. This source has a bilateral radio morphology, a radio spectral index of -0.5 +/- 0.1, sparse patches of linear polarisation, and thermal X-rays with a bright arc near the radio boundary. Considering these characteristics, we conclude that G296.7-0.9 is a supernova remnant (SNR). The age and morphology of the SNR in the context of its environment suggest that the source is co-located with an HII region, and that portions of the shock front have broken out into a lower density medium. We see no evidence for a neutron star or pulsar wind nebula associated with SNR G296.7-0.9.
Using HST photometry, we age-date 59 supernova remnants (SNRs) in the spiral galaxy M31 and use these ages to estimate zero-age main sequence masses (MZAMS) for their progenitors. To accomplish this, we create color-magnitude diagrams (CMDs) and use CMD fitting to measure the recent star formation history (SFH) of the regions surrounding cataloged SNR sites. We identify any young coeval population that likely produced the progenitor star and assign an age and uncertainty to that population. Application of stellar evolution models allows us to infer the MZAMS from this age. Because our technique is not contingent on precise location of the progenitor star, it can be applied to the location of any known SNR. We identify significant young SF around 53 of the 59 SNRs and assign progenitor masses to these, representing a factor of 2 increase over currently measured progenitor masses. We consider the remaining 6 SNRs as either probable Type Ia candidates or the result of core-collapse progenitors that have escaped their birth sites. The distribution of recovered progenitor masses is bottom heavy, showing a paucity of the most massive stars. If we assume a single power law distribution, dN/dM proportional to M^alpha, we find a distribution that is steeper than a Salpeter IMF (alpha=-2.35). In particular, we find values of alpha outside the range -2.7 to -4.4 inconsistent with our measured distribution at 95% confidence. If instead we assume a distribution that follows a Salpeter IMF up to some maximum mass, we find that values of M_max greater than 26 Msun are inconsistent with the measured distribution at 95% confidence. In either scenario, the data suggest that some fraction of massive stars may not explode. The result is preliminary and requires more SNRs and further analysis. In addition, we use our distribution to estimate a minimum mass for core collapse between 7.0 and 7.8 Msun.
We present new deep multi-frequency radio-polarimetric images of a sample of high redshift radio galaxies (HzRGs), having redshift between 1.7 and 4.1. The radio data at 4.7 and 8.2 GHz were taken with the Very Large Array in the A configuration and provide a highest angular resolution of 0.2. Maps of total intensity, radio spectral index, radio polarization and internal magnetic field are presented for each source. The morphology of most objects is that of standard FRII double radio sources, but several contain multiple hot-spots in one or both lobes. Compared to similar samples of HzRGs previously imaged, there is a higher fraction (29%) of compact steep spectrum sources (i.e. sources with a projected linear size less than 20 kpc). Radio cores are identified in about half of the sample and tend to have relatively steep spectra (alpha < -1). Polarization is detected in all but 4 sources, with typical polarization at 8.2 GHz of around 10-20%. The Faraday rotation can be measured in most of the radio galaxies: the observed rotation measure (RM) of 8 radio sources exceeds 100 rad m^{-2} in at least one of the lobes, with large gradients between the two lobes. We find no dependence of Faraday rotation with other properties of the radio sources. If the origin of the Faraday rotation is local to the sources, as we believe, then the intrinsic RM is more than a 1000 rad m^{-2}. Because low redshift radio galaxies residing at the center of clusters usually show extreme RMs, we suggest that the high-z large RM sources also lie in very dense environments. Finally, we find that the fraction of powerful radio galaxies with extreme Faraday rotation increases with redshift, as would be expected if their average environment tends to become denser with decreasing cosmic epoch.
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