The remnant phase of a radio galaxy begins when the jets launched from an active galactic nucleus are switched off. To study the fraction of radio galaxies in a remnant phase, we take advantage of a $8.31$ deg $^2$ subregion of the GAMA 23 field which comprises of surveys covering the frequency range 0.1–9 GHz. We present a sample of 104 radio galaxies compiled from observations conducted by the Murchison Widefield Array (216 MHz), the Australia Square Kilometer Array Pathfinder (887 MHz), and the Australia Telescope Compact Array (5.5 GHz). We adopt an ‘absent radio core’ criterion to identify 10 radio galaxies showing no evidence for an active nucleus. We classify these as new candidate remnant radio galaxies. Seven of these objects still display compact emitting regions within the lobes at 5.5 GHz; at this frequency the emission is short-lived, implying a recent jet switch off. On the other hand, only three show evidence of aged lobe plasma by the presence of an ultra-steep-spectrum ( $\alpha<-1.2$ ) and a diffuse, low surface brightness radio morphology. The predominant fraction of young remnants is consistent with a rapid fading during the remnant phase. Within our sample of radio galaxies, our observations constrain the remnant fraction to $4\%\lesssim f_{\mathrm{rem}} \lesssim 10\%$ ; the lower limit comes from the limiting case in which all remnant candidates with hotspots are simply active radio galaxies with faint, undetected radio cores. Finally, we model the synchrotron spectrum arising from a hotspot to show they can persist for 5–10 Myr at 5.5 GHz after the jets switch of—radio emission arising from such hotspots can therefore be expected in an appreciable fraction of genuine remnants.

中文翻译：

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在多频调查中发现的残余射电星系

射电星系的残余阶段从活跃星系核发射的喷流关闭时开始。为了研究剩余阶段射电星系的比例，我们利用 $8.31$ 度 $^2$ GAMA 23 领域的子区域，其中包括覆盖 0.1-9 GHz 频率范围的调查。我们展示了 104 个射电星系的样本，这些星系由 Murchison 宽场阵列 (216 MHz)、澳大利亚平方公里阵列探路者 (887 MHz) 和澳大利亚望远镜紧凑阵列 (5.5 GHz) 进行的观测汇编而成。我们采用“不存在无线电核心”标准来识别 10 个没有证据表明存在活动核的无线电星系。我们将它们归类为新的候选残余射电星系。其中 7 个物体在 5.5 GHz 的波瓣内仍显示出紧凑的发射区域；在这个频率下，发射是短暂的，这意味着最近的喷气机关闭。另一方面，只有三个通过超陡光谱的存在显示了老化的叶等离子体的证据（ $\alpha<-1.2$ ) 和漫射的、低表面亮度的射电形态。年轻残余物的主要部分与残余物阶段的快速衰落一致。在我们的射电星系样本中，我们的观察将剩余部分限制为 $4\%\lesssim f_{\mathrm{rem}} \lesssim 10\%$ ; 下限来自极限情况，在这种情况下，所有具有热点的残余候选者只是具有微弱、未被检测到的无线电核心的活跃射电星系。最后，我们对热点产生的同步加速器光谱进行建模，以表明它们可以在射流切换后在 5.5 GHz 持续 5-10 Myr，因此可以预期这些热点产生的无线电发射在相当一部分真正的残余物中。