We report results on multi-epoch Very Large Array (VLA) and pc-scale Very Long Baseline Array (VLBA) observations of candidate compact symmetric objects (CSOs) from the faint sample of high frequency peakers. New VLBA observations could resolve the radio structure in about 42 per cent of the observed sources, showing double components that may be either mini-lobes or core-jet structures. Almost all the sources monitored by the VLA show some variability on time scale of a decade, and only 1 source does not show any significant variation. In 17 sources the flux density changes randomly as it is expected in blazars, and in 4 sources the spectrum becomes flat in the last observing epoch, confirming that samples selected in the GHz regime are highly contaminated by beamed objects. In 16 objects, the pc-scale and variability properties are consistent with a young radio source in adiabatic expansion, with a steady decrease of the flux density in the optically-thin part of the spectrum, and a flux density increase in the optically-thick part. For these sources we estimate dynamical ages between a few tens to a few hundreds years. The corresponding expansion velocity is generally between 0.1c and 0.7c, similar to values found in CSOs with different approaches. The fast evolution that we observe in some CSO candidates suggests that not all the objects would become classical Fanaroff-Riley radio sources.
The jet in active galactic nuclei (AGN) is a key ingredient in understanding the co-evolution of galaxies and their central supermassive black holes (SMBHs). Unfortunately, the mechanism of jet launching and collimation is still elusive. The observational evidence of decreasing radio loudness with increasing Eddington ratio implies that the jet should be coupled with the accretion process. To further explore the relationship between the jet and accretion, it is necessary to extend our knowledge of the jet to an extreme end of the Eddington ratio distribution of AGN. Using Very Long Baseline Ar…
We observed 20 Palomar-Green (PG) quasars at low redshift ($z<0.5$) with total flux density > 1 mJy, including 4 radio-loud quasars (RLQs) and 16 radio-quiet quasars (RQQs), using the Very Long Baseline Array (VLBA) at 5 GHz. Ten RQQs are clearly detected in the VLBA images, and a compact radio core is identified in eight of them, indicating the prevalence of active galactic nucleus (AGN)-related radio emission in this flux-density-limited RQQ sample. The RQQs and RLQs in our sample have a division at $\sim$30~mJy. The radio emission from RQQs appears to be the result of a combination of star …
WISE J224607.56$-$052634.9 (W2246-0526) is a hyperluminous ($L_{\rm bol}\approx 1.7\times 10^{14}~L_\odot$), dust-obscured and radio-quiet quasar at redshift $z=4.6$. It plays a key role in probing the transition stage between dusty starbursts and unobscured quasars in the co-evolution of galaxies and supermassive black holes (SMBHs). To search for the evidence of the jet activity launched by the SMBH in W2246-0526, we performed very long baseline interferometry (VLBI) observations of its radio counterpart with the European VLBI Network (EVN) plus the enhanced Multi Element Remotely Linked Int…
We report the high-resolution radio observations of eight Fanaroff-Riley type 0 radio galaxies (FR 0s), selected from the published FR 0 sample. These observations were carried out with the Very Long Baseline Array (VLBA) and European VLBI Network (EVN) at frequencies of 5 and 8 GHz with a highest resolution of 0.6 mas. All eight sources show compact structures on projected physical sizes of 0.3-10 parsec. Six sources show a two-sided structure and two sources show a one-sided jet structure. J1025+1022 shows an X-shaped jet structure, which could result from a reorientation of the jet axis due…
We observed with the VLBA at 2.3 and 8.6 GHz a complete flux-density limited sample of 482 radio sources with declination >+75 degrees brighter than 200 mJy at 1.4 GHz drawn from the NVSS catalog. 34% of the sources show parsec-scale emission above the flux density detection limit of 30 mJy; their accurate positions and parsec-scale structure parameters are determined. Among all the sources detected at least at the shortest VLBA baselines, the majority, or 72%, has a steep single-dish spectrum. The fraction of the sources with a detectable parsec-scale structure is above 95% among the flat-spe…
Observational information on high-energy astrophysical neutrinos is being continuously collected by the IceCube observatory. However, the sources of neutrinos are still unknown. In this study, we use radio very-long-baseline interferometry (VLBI) data for a complete VLBI-flux-density limited sample of active galactic nuclei (AGN). We address the problem of the origin of astrophysical neutrinos with energies above 200 TeV in a statistical manner. It is found that AGN positionally associated with IceCube events have typically stronger parsec-scale cores than the rest of the sample. The post-tria…
Massive outflows of neutral atomic hydrogen (HI) have been observed in absorption in a number of radio galaxies and are considered a signature of AGN feedback. These outflows on kpc-scales have not been investigated in great detail as they require high-angular resolution observations to be spatially resolved. In some radio AGN, they are likely the result of the radio jets interacting with the interstellar medium. We have used a global VLBI array to map the HI outflow in a small sample of young and restarted radio galaxies which we previously observed with the VLA and the WSRT at lower resoluti…
We present global VLBI observations of the 21-cm transition of atomic hydrogen seen in absorption against the radio source J0855+5751. The foreground absorber (SDSS~J085519.05+575140.7) is a dwarf galaxy at $z$ = 0.026. As the background source is heavily resolved by VLBI, the data allow us to map the properties of the foreground HI gas with a spatial resolution of 2pc. The absorbing gas corresponds to a single coherent structure with an extent $>$35pc, but we also detect significant and coherent variations, including a change in the HI optical depth by a factor of five across a distance of $\…
VLBI Detections of Compact Nuclei in Spiral-hosted Double-lobed Radio-loud Active Galactic Nuclei (DRAGNs): Evidence for Weak Parsec-Scale Jet Activity
Ulsan National Institute of Science and Technology (UNIST)（蔚山科学技术院）
Double-Peaked Emission Lines due to A Radio Outflow in KISSR1219
给出了1.5GHz和5GHz相位参考VLBA和1.5GHz Karl G. Jansky甚大阵列（VLA）观测的塞弗特2星系KISSR 1219，它在其光谱中具有双峰发射线。VLA和VLBA的数据显示，在大致相同的位置角，一个单侧的核心喷流结构，提供了活动星系核外流的证据。双秒差距尺度的射电核心的缺失使得KISSR 1219的双星黑洞图像受到质疑。秒差距尺度的核心和喷流组件（$>10^6$ K）的高亮度温度与这种解释是一致的。喷流速度为$\gtrsim0.55c$到$\gtrsim0.25c$，从秒差距到千秒差距尺度，喷流倾角为$\gtrsim50^\circ$的多普勒增强可以解释这个塞弗特2星系中喷流的单侧性。在[O { \sc iii}]中的蓝移宽发射线分量也指示在发射线气体中以$\sim350$ km s $^{-1}$的速度流出，而[O { \sc i}]双峰线表明存在冲击加热的气体。使用MAPPINGS III代码进行的详细线比研究进一步表明，激波+前兆模型可以很好地解释线电离数据。总的来说，我们的数据表明，在KISSR 1219的无线电外流是推动发射线云，无论是在喷流的前面，并在横向方向，引起双峰发射线光谱。
Double-peaked emission line AGN (DPAGN) have been regarded as binary black hole candidates. We present here results from parsec-scale radio observations with the Very Long Baseline Array (VLBA) of five DPAGN belonging to the KISSR sample of emission-line galaxies. This work concludes our pilot study of nine type 2 Seyfert and LINER DPAGN from the KISSR sample. In the nine sources, dual compact cores are only detected in the "offset AGN", KISSR 102. The overall incidence of jets however, in the eight sources detected with the VLBA, is $\ge$60%. We find a difference in the "missing flux density"…
The past decade has experienced an explosive increase of optically-discovered tidal disruption events (TDEs) with the advent of modern time-domain surveys. However, we still lack a comprehensive observational view of their infrared (IR) echoes in spite of individual detections. To this end, we have conducted a statistical study of IR variability of the 23 optical TDEs discovered between 2009 and 2018 utilizing the full public dataset of Wide-field Infrared Survey Explorer. The detection of variability is performed on the difference images, yielding out 11 objects with significant (>$3σ$) varia…
Resolving the sub-parsec circumnuclear density profiles of quiescent galaxies: Evidence for Bondi accretion flows in tidal disruption event hosts
International Centre for Radio Astronomy Research – Curtin University（国际射电天文中心 - 科廷大学）；School of Natural Sciences, Institute for Advanced Study（自然科学与高级研究院）
Using the JVLA, we explored the Galactic center (GC) with a resolution of 0.05" at 33.0 and 44.6 GHz. We detected 64 hyper-compact radio sources (HCRs) in the central parsec. The dense group of HCRs can be divided into three spectral types: 38 steep-spectrum ($α\le-0.5$) sources; 10 flat-spectrum ($-0.5<α\le0.2$) sources; and 17 inverted-spectrum sources having $α>0.2$, assuming $S\proptoν^α$. The steep-spectrum HCRs are likely represent a population of massive stellar remnants associated with nonthermal compact radio sources powered by neutron stars and stellar black holes. The surface-densit…
Context. Many questions on the nature of astrophysical counterparts of high-energy neutrinos remain unanswered. There is increasing evidence of a connection between blazar jets and neutrino events, with the flare of the gamma-ray blazar TXS0506+056 in spatial and temporal proximity of IC170922A representing one of the most outstanding associations of high-energy neutrinos with astrophysical sources reported so far. Aims. With the purpose of characterising potential blazar counterparts to high-energy neutrinos, we analysed the parsec-scale regions of gamma-ray blazars in spatial coincidence wit…
