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Observations and Properties of Candidate High Frequency GPS Mon. Not. R. Astron. Soc. 000, 1–21 () Printed 30 October 2018 (MN LATEX style file v2.2) Observations and properties of candidate high frequency GPS radio sources in the AT20G survey Paul J. Hancock1,2, Elaine M. Sadler1, Elizabeth K. Mahony1,2, Roberto Ricci3 1Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, NSW 2006, Australia 2Australia Telescope National Facility, CSIRO, PO Box 76, Epping, NSW 1710, Australia 3INAF-Istituto di Radioastronomia, Bologna, Via P. Gobetti, 101, 40129 Bologna, Italy ABSTRACT We used the Australia Telescope Compact Array (ATCA) to obtain 40GHz and 95GHz ob- servations of a number of sources that were selected from the Australia Telescope Compact Array 20GHz (AT20G) survey . The aim of the observations was to improve the spectral cov- erage for sources with spectral peaks near 20GHz or inverted (rising) radio spectra between 8.6GHz and 20GHz. 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 evi- dence 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. 1 INTRODUCTION vious epochs of activity. The cores of these sources may still be considered to blue genuine GPS galaxies. Gigahertz Peaked Spectrum (GPS) radio sources are important to • A typical total flux variability of ∼10% or less in a year. our understanding of the evolution of radio galaxies as they are • Little or no variability in spectral shape over time. thought to be the progenitors of large radio galaxies. The youth scenario described in O’Dea (1998) relates the spectral turnover • An optical spectrum indicative of AGN activity with broad or frequency to the age of the radio source, with the youngest sources narrow emission lines or stellar absorption lines, but not the flat, having the highest intrinsic turnover frequencies. featureless continuum shown by many blazars. As noted by Torniainen et al. (2007) and others, it is important to distinguish between the different causes for a peaked radio spec- trum. Nearly all QSO-type objects that have been observed to have a peaked spectrum have done so either due to non-simultaneous arXiv:1008.1401v2 [astro-ph.GA] 10 Aug 2010 observations of variable flat spectrum sources, or to beamed com- ponents that are observed in a flaring state. Such sources show a peaked spectrum only for a short period of time and are not related to the evolution of radio galaxies, so are a major source of contam- ination to any sample of GPS radio sources. Monitoring of sources to exclude variability as a source of the spectral peak is essential, 2 TARGET SELECTION as is optical spectroscopy to identify QSO like sources. To investigate the properties of the youngest radio galaxies, Sources were selected from the Australia Telescope 20 GHz radio and optical observations were carried out on 21 candidate (AT20G, Murphy et al. 2010) survey’s list of confirmed sources high frequency peaking GPS sources. The motivation for the radio available as of August 2007, and were restricted to 18h < RA < observations is to identify variability and the high frequency radio 01h30m to obtain the best visibility during the allotted ATCA ob- 20 properties, including the turnover frequency of each source. The serving time. Sources that showed a rising (α4.8 > +0.2) spec- goal of this paper is to investigate the candidate high-frequency trum that either remained inverted, or appeared to be peaking above GPS sources, and to classify them as being either genuine GPS 10 GHz were identified as potential high frequency GPS sources. In galaxies, possible GPS galaxies and non GPS galaxies. total there were 148 sources that met this criteria, however not all Genuine high-frequency GPS galaxies are characterised by: were able to be observed due to timing constraints as discussed in section 4.1. Previous observations by Sadler et al. (2008) have • A convex radio spectrum with a peak above 5 GHz. shown that fewer than 2% of all sources with rising a spectrum 20 • A compact radio morphology, with a low level of extended (α8.6 > 0) continued to rise at 95 GHz, and thus we expect to see flux. Restarted radio sources can show jets and hot-spots from pre- a turnover in nearly all the sources selected. c RAS 2 Hancock et al. (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) Name AT20G position S95 S40 S20 S20 Vrms ∆ t S8 S5 S1.4 S0.8 AT20G J2000 mJy mJy mJy mJy % Years mJy mJy mJy mJy Epoch 2 Epoch 2 Epoch 2 Epoch 1 Epoch 1 Epoch 1 NVSS SUMSS J002616-351249 00:26:16.40 −35:12:49.3 434 ± 38 1150 ± 25 1156 ± 79 1123 ± 43c <6 2.92 357 ± 18 136 ± 7 24.6 ± 0.9 12.2 ± 1.5 J004417-375259 00:44:17.06 −37:52:59.2 105 ± 9 136 ± 2 94 ± 6 114 ± 5c 7.7 2.92 86 ± 4 66 ± 3 18.4 ± 0.7 45.6 ± 1.8 J004905-552110 00:49:05.62 −55:21:10.5 128 ± 11 204 ± 4 142 ± 9 89 ± 5b 22 1.92 76 ± 4 59 ± 3 - 11.8 ± 1.0 J005427-341949 00:54:27.92 −34:19:49.2 69 ± 6 90 ± 1 82 ± 5 110 ± 4c 13 2.92 90 ± 5 80 ± 4 51.3 ± 2.2 32.9 ± 1.9 J010333-643907 01:03:33.63 −64:39:07.4 279 ± 24 437 ± 9 311 ± 21 323 ± 16b <6 1.92 235 ± 12 187 ± 9 - 144.7 ± 4.5 J011102-474911 01:11:02.93 −47:49:11.3 <87 82 ± 1 72 ± 4 83 ± 4c <6 2.92 64 ± 3 30 ± 2 - 10.5 ± 0.9 J012714-481332 01:27:14.87 −48:13:32.1 187 ± 16 276 ± 6 174 ± 12 237 ± 12c 14 2.92 181 ± 9 140 ± 7 - 80.3 ± 2.6 J012820-564939 01:28:20.38 −56:49:39.7 120 ± 10 222 ± 4 131 ± 9 189 ± 10b 17 1.92 141 ± 7 130 ± 7 - 82.6 ± 2.6 J180859-832526 18:08:59.53 −83:25:26.8 <128 93 ± 2 80 ± 5 95 ± 5a 6.1 0.92 71 ± 4 68 ± 4 - 106.8 ± 3.3 J181225-712006 18:12:25.07 −71:20:06.3 <97 25 ± 1 31 ± 2 43 ± 2a 15 0.92 39 ± 2 26 ± 2 - <6 J181857-550815 18:18:57.99 −55:08:15.0 <99 71 ± 1 55 ± 3 75 ± 4a 14 0.92 54 ± 4 42 ± 5 - 124.4 ± 4.0 J203540-694407 20:35:40.43 −69:44:07.9 <104 106 ± 2 157 ± 10 160 ± 8b <6 1.92 236 ± 12 188 ± 9 - 16.4 ± 1.0 J203958-720655 20:39:58.06 −72:06:55.0 151 ± 13 266 ± 5 332 ± 22 417 ± 21b 9.7 1.92 244 ± 12 186 ± 9 - 99.1 ± 3.1 J205503-635207 20:55:03.83 −63:52:07.0 <91 28 ± 1 43 ± 2 44 ± 2a <6 0.92 29 ± 2 12 ± 1 - <6 J212222-560014 21:22:22.81 −56:00:14.6 <106 64 ± 1 56 ± 3 58 ± 3b <6 1.92 34 ± 2 28 ± 2 - <6 J212402-602808 21:24:02.96 −60:28:08.9 <98 60 ± 1 89 ± 6 135 ± 7a 20 0.92 (125 ± 6)e (81 ± 4)e - 33.0 ± 1.3 J213622-633551 21:36:22.08 −63:35:51.2 224 ± 19 354 ± 7 372 ± 25 411 ± 21c <6 2.92 (281 ± 14)d (198 ± 10)d - 154.5 ± 4.7 J214447-694654 21:44:47.42 −69:46:54.9 104 ± 9 143 ± 3 124 ± 8 119 ± 6b <6 1.92 94 ± 5 81 ± 4 - 23.5 ± 1.7 J230737-354828 23:07:37.27 −35:48:28.7 98 ± 8 110 ± 2 88 ± 6 190 ± 9c 36 2.92 186 ± 9 112 ± 7 58.7 ± 2.2 93.2 ± 3.1 J233159-381147 23:31:59.43 −38:11:47.3 132 ± 11 316 ± 6 443 ± 30 523 ± 25c <6 2.92 747 ± 37 592 ± 30 543.2 ± 16.3 429.7 ± 13.0 J234743-494627 23:47:43.68 −49:46:27.8 275 ± 24 411 ± 9 403 ± 27 284 ± 12c 16 2.92 251 ± 13 211 ± 11 - 107.7 ± 3.3 Table 1. Candidate high-frequency GPS sources. Data in columns 1, 2, 6, 9, and 10 are taken from the AT20G catalog. Columns 11 and 12 are from the NVSS and SUMSS surveys. Columns 3, 4, and 5 are new observations from this paper. The 20 GHz variability index in column 7 is defined by Vrms in eq 2. Column 8 shows the time in years between the Epoch 1 and Epoch 2 20 GHz observations. The year of observation for Epoch 1 is denoted by: a - 2004.75, b - 2005.75, c - 2006.75, d - 2006.83, e - 2007.83. Sources with an upper limit in column 12 are present within the SUMSS survey maps, but are below the 6 mJy detection limit of the catalog. 3 OPTICAL IDENTIFICATION shown to provide robust phases at 95 GHz (Sadler et al.
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