Elizabeth K. Mahony

Publications (7)0 Total impact

• Article: Australia Telescope Compact Array observations of Fermi unassociated sources

  Leonid Petrov, Elizabeth K. Mahony, Philip G. Edwards, Elaine M. Sadler, Frank K. Schinzel, David McConnell
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  ABSTRACT: We report results of the first phase of observations with the Australia Telescope Compact Array (ATCA) at 5 and 9 GHz of the fields around 411 gamma-ray sources with declinations < +10 deg detected by Fermi but marked as unassociated in the 2FGL catalogue. We have detected 424 sources with flux densities in a range of 2 mJy to 6 Jy that lie within the 99 per cent localisation uncertainty of 283 gamma-ray sources. Of these, 146 objects were detected in both the 5 and 9 GHz bands. We found 84 sources in our sample with a spectral index flatter than -0.5. The majority of detected sources are weaker than 100 mJy and for this reason were not found in previous surveys. Approximately 1/3 of our sample, 128 objects, have the probability of being associated by more than 10 times than the probability of being a background source found in the vicinity of a gamma-ray object by chance. We present the catalogue of positions of these sources, estimates of their flux densities and spectral indices where available.
  01/2013;
• Article: Is the observed high-frequency radio luminosity distribution of QSOs bimodal?

  Elizabeth K. Mahony, Elaine M. Sadler, Scott M. Croom, Ronald D. Ekers, Ilana J. Feain, Tara Murphy
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  ABSTRACT: The distribution of QSO radio luminosities has long been debated in the literature. Some argue that it is a bimodal distribution, implying that there are two separate QSO populations (normally referred to as 'radio-loud' and 'radio-quiet'), while others claim it forms a more continuous distribution characteristic of a single population. We use deep observations at 20 GHz to investigate whether the distribution is bimodal at high radio frequencies. Carrying out this study at high radio frequencies has an advantage over previous studies as the radio emission comes predominantly from the core of the AGN, hence probes the most recent activity. Studies carried out at lower frequencies are dominated by the large scale lobes where the emission is built up over longer timescales (10^7-10^8 yrs), thereby confusing the sample. Our sample comprises 874 X-ray selected QSOs that were observed as part of the 6dF Galaxy Survey. Of these, 40% were detected down to a 3 sigma detection limit of 0.2-0.5 mJy. No evidence of bimodality is seen in either the 20 GHz luminosity distribution or in the distribution of the R_20 parameter: the ratio of the radio to optical luminosities traditionally used to classify objects as being either radio-loud or radio-quiet. Previous results have claimed that at low radio luminosities, star formation processes can dominate the radio emission observed in QSOs. We attempt to investigate these claims by stacking the undetected sources at 20 GHz and discuss the limitations in carrying out this analysis. However, if the radio emission was solely due to star formation processes, we calculate that this corresponds to star formation rates ranging from ~10 solar masses/yr to ~2300 solar masses/yr.
  05/2012;
• Source

  Available from: ArXiv

  Article: Optical Properties of High-Frequency Radio Sources from the Australia Telescope 20 GHz (AT20G) Survey

  Elizabeth K. Mahony, Elaine M. Sadler, Scott M. Croom, Ronald D. Ekers, Keith W. Bannister, Rajan Chhetri, Paul J. Hancock, Helen M. Johnston, Marcella Massardi, Tara Murphy
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  ABSTRACT: Our current understanding of radio-loud AGN comes predominantly from studies at frequencies of 5 GHz and below. With the recent completion of the Australia Telescope 20 GHz (AT20G) survey, we can now gain insight into the high-frequency radio properties of AGN. This paper presents supplementary information on the AT20G sources in the form of optical counterparts and redshifts. Optical counterparts were identified using the SuperCOSMOS database and redshifts were found from either the 6dF Galaxy survey or the literature. We also report 144 new redshifts. For AT20G sources outside the Galactic plane, 78.5% have optical identifications and 30.9% have redshift information. The optical identification rate also increases with increasing flux density. Targets which had optical spectra available were examined to obtain a spectral classification. There appear to be two distinct AT20G populations; the high luminosity quasars that are generally associated with point-source optical counterparts and exhibit strong emission lines in the optical spectrum, and the lower luminosity radio galaxies that are generally associated with passive galaxies in both the optical images and spectroscopic properties. It is suggested that these different populations can be associated with different accretion modes (cold-mode or hot-mode). We find that the cold-mode sources have a steeper spectral index and produce more luminous radio lobes, but generally reside in smaller host galaxies than their hot-mode counterparts. This can be attributed to the fact that they are accreting material more efficiently. Lastly, we compare the AT20G survey with the S-cubed semi-empirical (S3-SEX) models and conclude that the S3-SEX models need refining to correctly model the compact cores of AGN. The AT20G survey provides the ideal sample to do this.
  07/2011;
• Source

  Available from: ArXiv

  Article: Deep 20 GHz observations of X-ray selected QSOs the Compact Array Broadband Backend (CABB)

  Elizabeth K. Mahony, Elaine M. Sadler, Scott M. Croom, Ronald D. Ekers, Ilana J. Feain, Tara Murphy
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  ABSTRACT: The recently commissioned Compact Array Broadband Backend (CABB) on the Australia Telescope Compact Array (ATCA) provides 2 GHz bandwidth in each frequency and polarisation, significantly increasing the sensitivity of the Array. This increased sensitivity allows for larger samples of sources to be targeted whilst also probing fainter radio luminosities. Using CABB, we have observed a large sample of objects at 20 GHz to investigate the high-frequency radio luminosity distribution of X-ray selected QSOs at redshifts less than 1. Observing at high frequencies allows us to focus on the core emission of the AGN, hence recording the most recent activity. Comment: Accepted for publication in Proceedings of Science [PoS(ISKAF2010)072]. Poster presented at 'A New Golden Age for Radio Astronomy', International SKA Forum 2010, 10-14 June 2010, Hof van Saksen, NL
  08/2010;
• Source

  Available from: ArXiv

  Article: Observations and properties of candidate high frequency GPS radio sources in the AT20G survey

  Paul J. Hancock, Elaine M. Sadler, Elizabeth K. Mahony, Roberto Ricci
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  ABSTRACT: We used the Australia Telescope Compact Array (ATCA) to obtain 40 GHz and 95 GHz observations of a number of sources that were selected from the Australia Telescope Compact Array 20 GHz (AT20G) survey . The aim of the observations was to improve the spectral coverage for sources with spectral peaks near 20 GHz or inverted (rising) radio spectra between 8.6 GHz and 20 GHz. We present the radio observations of a sample of 21 such sources along with optical spectra taken from the ANU Siding Spring Observatory 2.3m telescope and the ESO-New Technology Telescope (NTT). We find that as a group the sources show the same level of variability as typical GPS sources, and that of the 21 candidate GPS sources roughly 60% appear to be genuinely young radio galaxies. Three of the 21 sources studied show evidence of being restarted radio galaxies. If these numbers are indicative of the larger population of AT20G radio sources then as many as 400 genuine GPS sources could be contained within the AT20G with up to 25% of them being restarted radio galaxies. Comment: 21 pages, 24 figures, Table 1 truncated at 11 columns
  08/2010;
• Source

  Available from: arxiv.org

  Article: High-frequency radio properties of sources in the Fermi-LAT 1-year Point Source Catalogue

  Elizabeth K. Mahony, Elaine M. Sadler, Tara Murphy, Ronald D. Ekers, Philip G. Edwards, Marcella Massardi
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  ABSTRACT: The high-frequency radio sky, like the gamma-ray sky surveyed by the Fermi satellite, is dominated by flat spectrum radio quasars and BL Lac objects at bright flux levels. To investigate the relationship between radio and gamma-ray emission in extragalactic sources, we have cross-matched the Australia Telescope 20 GHz survey catalog (AT20G) with the Fermi-LAT 1 year Point Source Catalog (1FGL). The 6.0 sr of sky covered by both catalogs ({\delta} < 0\circ, |b| > 1.\circ 5) contains 5890 AT20G radio sources and 604 1FGL gamma-ray sources. The AT20G source positions are accurate to within ~1 arcsec and, after excluding known Galactic sources, 43% of Fermi 1FGL sources have an AT20G source within the 95% Fermi confidence ellipse. Monte Carlo tests imply that at least 95% of these matches are genuine associations. Only five gamma-ray sources (1% of the Fermi catalog) have more than one AT20G counterpart in the Fermi error box. The AT20G matches also generally support the active galactic nucleus (AGN) associations in the First LAT AGN Catalog. We find a trend of increasing gamma-ray flux density with 20 GHz radio flux density. The Fermi detection rate of AT20G sources is close to 100% for the brightest 20 GHz sources, decreasing to 20% at 1 Jy, and to roughly 1% at 100 mJy. Eight of the matched AT20G sources have no association listed in 1FGL and are presented here as potential gamma-ray AGNs for the first time.We also identify an alternative AGN counterpart to one 1FGL source. The percentage of Fermi sources with AT20G detections decreases toward the Galactic plane, suggesting that the 1FGL catalog contains at least 50 Galactic gamma-ray sources in the southern hemisphere that are yet to be identified. Comment: Replaced to match version published in ApJ
  03/2010;
• Source

  Available from: Brian Boyle

  Article: The RASS--6dFGS catalogue: a sample of X-ray selected AGN from the 6dF Galaxy Survey

  Elizabeth K. Mahony, Scott M. Croom, Brian J. Boyle, Alastair C. Edge, Tom Mauch, Elaine M. Sadler
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  ABSTRACT: We present a catalogue of 3405 X-ray sources from the ROSAT All Sky Survey (RASS) Bright Source Catalogue which fall within the area covered by the 6dF Galaxy Survey (6dFGS). The catalogue is count-rate limited at 0.05 cts\s in the X-ray and covers the area of sky with delta < 0 deg and |b|>10 deg. The RASS--6dFGS sample was one of the additional target catalogues of the 6dFGS and as a result we obtained optical spectra for 2224 (65%) RASS sources. Of these, 1715 (77%) have reliable redshifts with a median redshift of z=0.16 (excluding the Galactic sources). For the optically bright sources (b_J < 17.5) in the observed sample, over 90% have reliable redshifts. The catalogue mainly comprises QSOs and active galaxies but also includes 238 Galactic sources. Of the sources with reliable redshifts the majority are Type 1 AGN (69%), while 12% are Type 2 AGN, 6% absorption-line galaxies and 13% are stars. We also identify a small number of optically-faint, very low redshift, compact objects which fall outside the general trend in the b_J-z plane. We detect 918 sources (27%) of the RASS--6dFGS sample in the radio using either the 1.4 GHz NRAO VLA Sky Survey (NVSS) or the 843 MHz Sydney University Molonglo Sky Survey (SUMSS) catalogues and find that the detection rate changes with redshift. At redshifts larger than 1 virtually all of these sources have radio counterparts and with a median flux density of 1.15 Jy, they are much stronger than the median flux density of 28.6 mJy for the full sample. We attribute this to the fact that the X-ray flux of these objects is being boosted by a jet component, possibly Doppler boosted, that is only present in radio-loud AGN. (abridged version) Comment: 17 pages, 15 figures, accepted for publication in MNRAS
  09/2009;