We present an independent catalog (FRIIRGcat) of 45,241 Fanaroff-Riley Type II (FR-II) radio galaxies compiled from the Very Large Array Faint Images of the Radio Sky at Twenty-centimeters (FIRST) survey and employed the deep learning method. Among them, optical and/or infrared counterparts are identified for 41,425 FR-IIs. This catalog spans luminosities 2.63 x 10(22) <= L-rad <= 6.76 x 10(29) W Hz(-1) and redshifts up to z = 5.01. The spectroscopic classification indicates that there are 1431 low-excitation radio galaxies and 260 high-excitation radio galaxies. Among the spectroscopically identified sources, black hole masses are estimated for 4837 FR-IIs, which are in 10(7.5) less than or similar to M-BH less than or similar to 10(9.5)M(circle dot). Interestingly, this catalog reveals a couple of giant radio galaxies (GRGs), which are already in the existing GRG catalog, confirming the efficiency of this FR-II catalog. Furthermore, 284 new GRGs are unveiled in this new FR-II sample; they have the largest projected sizes ranging from 701 to 1209 kpc and are located at redshifts 0.31 < z < 2.42. Finally, we explore the distribution of the jet position angle and it shows that the faint Images of the FIRST images are significantly affected by the systematic effect (the observing beams). The method presented in this work is expected to be applicable to the radio sky surveys that are currently being conducted because they have finely refined telescope arrays. On the other hand, we are expecting that further new methods will be dedicated to solving this problem.

The Square Kilometre Array Observatory (SKAO) will explore the radio sky to new depths in order to conduct transformational science. SKAO data products made available to astronomers will be correspondingly large and complex, requiring the application of advanced analysis techniques to extract key science findings. To this end, SKAO is conducting a series of Science Data Challenges, each designed to familiarise the scientific community with SKAO data and to drive the development of new analysis techniques. We present the results from Science Data Challenge 2 (SDC2), which invited participants to find and characterise 233245 neutral hydrogen (Hi) sources in a simulated data product representing a 2000~h SKA MID spectral line observation from redshifts 0.25 to 0.5. Through the generous support of eight international supercomputing facilities, participants were able to undertake the Challenge using dedicated computational resources. Alongside the main challenge, `reproducibility awards' were made in recognition of those pipelines which demonstrated Open Science best practice. The Challenge saw over 100 participants develop a range of new and existing techniques, with results that highlight the strengths of multidisciplinary and collaborative effort. The winning strategy -- which combined predictions from two independent machine learning techniques to yield a 20 percent improvement in overall performance -- underscores one of the main Challenge outcomes: that of method complementarity. It is likely that the combination of methods in a so-called ensemble approach will be key to exploiting very large astronomical datasets.

Disk winds and jets are ubiquitous in active galactic nuclei (AGN), and how these two components interact remains an open question. We study the radio properties of the radio-intermediate quasar III Zw 2. We detect two jet knots, J1 and J2, on parsec scales that move at a mildly apparent superluminal speed of 1.35 c . Two γ -ray flares were detected in III Zw 2 in 2009–2010, corresponding to the primary radio flare in late 2009 and the secondary radio flare in early 2010. The primary 2009 flare was found to be associated with the ejection of J2. The secondary 2010 flare occurred at a distance of ∼0.3 pc from the central engine, probably resulting from the collision of the jet with the accretion disk wind. The variability characteristics of III Zw 2 (periodic radio flares, unstable periodicity, multiple quasiperiodic signals and the possible harmonic relations between them) can be explained by the global instabilities of the accretion disk. These instabilities originating from the outer part of the warped disk propagate inward and can lead to modulation of the accretion rate and consequent jet ejection. At the same time, the wobbling of the outer disk may also lead to oscillations of the boundary between the disk wind and the jet tunnel, resulting in changes in the jet–wind collision site. Object III Zw 2 is one of the few cases observed with jet–wind interactions, and the study in this paper is of general interest for gaining insight into the dynamic processes in the nuclear regions of AGN.

Jets may have contributed to promoting the growth of seed black holes in the early Universe, and thus observations of radio-loud high-redshift quasars are crucial to understanding the growth and evolution of the early supermassive black holes. Here we report the radio properties of an X-ray bright z = 5.5 quasar, SRGE J170245.3 + 130104 (J1702 + 1301). Our high-resolution radio images reveal the radio counterpart at the optical position of J1702 + 1301, while another radio component is also detected at similar to 23.5 arcsec to the south-west. Our analysis suggests that this south-west component is associated with a foreground galaxy at z asymptotic to 0.677, which is mixed with J1702 + 1301 in low-frequency low-resolution radio images. After removing the contamination from this foreground source, we recalculated the radio loudness of J1702 + 1301 to be R > 1100, consistent with those of blazars. J1702 + 1301 exhibits a flat radio spectrum ( alpha = -0.17 +/- 0.05, S proportional to nu(alpha)) between 0.15 and 5 GHz; abo v e 5 GHz, it shows a rising spectrum shape, and the spectral index alpha(8 . 2)(4 . 7) appears to be correlated with the variation of the flux density: in burst states, alpha(8 . 2)(4 . 7) becomes larger. J1702 + 1301 displays distinct radio variability on time-scales from weeks to years in the source's rest frame. These radio properties, including high radio loudness, rising spectrum, and rapid variability, tend to support it as a blazar.

Observational studies of GHz peaked spectrum (GPS) sources contribute to the understanding of the radiative properties and interstellar environment of host galaxies. We present the results from the multi-frequency high-resolution VLBI observations of a sample of nine GPS sources at 8, 15, and 43 GHz. All sources show core-jet structure. Four sources show relativistic jets with Doppler boosting factors ranging from 2.0 to 5.0 and a jet viewing angle between 10° and 30°. The core brightness temperatures of the other five sources are below the equipartition brightness temperature limit with their jet viewing angles in the range of 13.6° to 71.9°, which are systematically larger than those of relativistic jets in this sample. The sources show diverse variability properties, with variability levels ranging from 0.11 to 0.56. The measured turnover frequency in the radio spectrum ranges from 6.2 and 31.8 GHz (in the source’s rest frame). We estimate the equipartition magnetic field strength to be between 9 and 48 mG. These results strongly support the notion that these GPS sources are young radio sources in the very early stage of their evolution.

Modern large radio continuum surveys have high sensitivity and resolution, and can resolve previously undetected extended and diffuse emissions, which brings great challenges for the detection and morphological classification of extended sources. We present HeTu-v2, a deep learning-based source detector that uses the combined networks of Mask Region-based Convolutional Neural Networks (Mask R-CNN) and a Transformer block to achieve high-quality radio sources segmentation and classification. The sources are classified into 5 categories: Compact or point-like sources (CS), Fanaroff-Riley Type I (FRI), Fanaroff-Riley Type II (FRII), Head-Tail (HT), and Core-Jet (CJ) sources. HeTu-v2 has been trained and validated with the data from the Faint Images of the Radio Sky at Twenty-one centimeters (FIRST). We found that HeTu-v2 has a high accuracy with a mean average precision ($AP_{\rm @50:5:95}$) of 77.8%, which is 15.6 points and 11.3 points higher than that of HeTu-v1 and the original Mask R-CNN respectively. We produced a FIRST morphological catalog (FIRST-HeTu) using HeTu-v2, which contains 835,435 sources and achieves 98.6% of completeness and up to 98.5% of accuracy compared to the latest 2014 data release of the FIRST survey. HeTu-v2 could also be employed for other astronomical tasks like building sky models, associating radio components, and classifying radio galaxies.

Interplanetary scintillation (IPS) provides an approach for identifying the presence of subarcsec structures in radio sources, and very long baseline interferometry (VLBI) technique can help verify whether the IPS sources have fine structures on milli-arcsec (mas) scales. We searched the available VLBI archive for the 244 IPS sources detected by the Murchison Widefield Array at 162 MHz and found 63 cross-matches. We analysed the VLBI data of the 63 sources and characterised the compactness index in terms of the ratio R of the VLBImeasured flux density at 4.3 GHz to the flux density estimated using the Very Large Array Sky Survey (VLASS) at 3 GHz and NRAOVLA Sky Survey (NVSS) at 1.4 GHz (SVLBI/SSA). Eleven sources are identified as blazars according to their flat spectra and strong variability. They show high compactness indices with R > 0.4, compact core-jet structure, and a broad distribution of normalised scintillation index (NSI). Other sources show diverse morphologies (compact core, core and one-sided jet, core and two-sided jets), but there is a correlation between R and NSI with a correlation coefficient r = 0.47. A similar R–NSI correlation is found in sources showing single steep power-law or convex spectra. After excluding blazars (which are already known to be compact sources) from theVLBI-detected IPS sources, a strong correlation is found between the compactness and scintillation index of the remaining samples, indicating that stronger scintillating sources are more compact. This pilot study shows that IPS offers a convenient method to identify compact radio sources without the need to invoke high-resolution imaging observations, which often require significant observational time.

Observational searches for intermediate-mass black holes (IMBHs; 102–106 M ⊙) include relatively isolated dwarf galaxies. For those that host active galactic nuclei (AGNs), the IMBH nature may be discerned through the accretion–jet activity. We present radio observations of four AGN-hosting dwarf galaxies, which potentially harbor IMBHs. Very large array (VLA) observations indicate steep spectra (indices of −0.63 to −1.05) between 1.4 and 9 GHz. However, a comparison with the 9 GHz in-band spectral index shows a steepening for GH047 and GH158 (implying older/relic emission) and flattening for GH106 and GH163 (implying recent activity). Overlapping emission regions in the VLA 1.4 GHz and our very long baseline array (VLBA) 1.5 GHz observations, and possibly symmetric pc-scale extensions, are consistent with recent activity in the latter two. Using the compact VLBA radio luminosity, X-ray luminosity (probing the accretion activity), and the black hole masses, all AGNs are found to lie on the empirical fundamental plane relation. The four AGNs are radio-quiet with relatively higher Eddington ratios (0.04–0.32) and resemble X-ray binaries during spectral state transitions that entail an outflow ejection. Furthermore, the radio to X-ray luminosity ratio logRX of −3.9 to −5.6 in these four sources support the scenarios that include corona mass ejection from the accretion disk and wind activity. The growth to kpc-scales likely proceeds along a similar trajectory to young AGNs and peaked spectrum sources. These complex clues can thus aid in the detection and monitoring of IMBHs in the nearby universe.

As the largest radio telescope in the world, the Square Kilometre Array (SKA) will lead the next generation of radio astronomy. The feats of engineering required to construct the telescope array will be matched only by the techniques developed to exploit the rich scientific value of the data. To drive forward the development of efficient and accurate analysis methods, we are designing a series of data challenges that will provide the scientific community with high-quality datasets for testing and evaluating new techniques. In this paper we present a description and results from the first such Science Data Challenge (SDC1). Based on SKA MID continuum simulated observations and covering three frequencies (560 MHz, 1400MHz and 9200 MHz) at three depths (8 h, 100 h and 1000 h), SDC1 asked participants to apply source detection, characterization and classification methods to simulated data. The challenge opened in November 2018, with nine teams submitting results by the deadline of April 2019. In this work we analyse the results for 8 of those teams, showcasing the variety of approaches that can be successfully used to find, characterise and classify sources in a deep, crowded field. The results also demonstrate the importance of building domain knowledge and expertise on this kind of analysis to obtain the best performance. As high-resolution observations begin revealing the true complexity of the sky, one of the outstanding challenges emerging from this analysis is the ability to deal with highly resolved and complex sources as effectively as the unresolved source population.

ABSTRACT Mrk 231 is the closest radio-quiet quasar known and one of the most luminous infrared galaxies in the local Universe. It is characterized by the co-existence of a radio jet and powerful multiphase multiscale outflows, making it an ideal laboratory to study active galactic nucleus (AGN) feedback. We analyse the multi-epoch very long baseline interferometry data of Mrk 231 and estimate the jet head advance speed to be ≲0.013 c, suggesting a sub-relativistic jet flow. The jet position angle changes from −113○ in the inner parsec to −172○ at a projected distance of 25 pc. The jet structure change might result from either a jet bending following the rotation of the circum-nuclear disc or the projection of a helical jet on the plane of the sky. In the large opening angle (∼60○) cone, the curved jet interacts with the interstellar medium and creates wide-aperture-angle shocks that subsequently dissipate a large portion of the jet power through radiation and contribute to powering the large-scale outflows. The low power and bent structure of the Mrk 231 jet, as well as extensive radiation dissipation, are consistent with the obstruction of the short-length jet by the host galaxy’s environment.

Compact symmetric objects (CSOs) are radio-emitting active galactic nuclei (AGNs) typically with a double-lobed radio structure confined to within 1 kpc. CSOs represent the earliest evolutionary phase of jetted AGNs. Some of them may eventually evolve into large-scale extended double sources, while others stall within the host galaxy and die out, depending on the longevity of nuclear activity, the jet power, and parameters of the surrounding galactic environment. Studying CSOs is a useful tool for understanding the evolution of the galaxies and the interactions between the jets and the medium of the host galaxy. Based on milliarcsec-resolution imaging observations using very long baseline interferometry (VLBI), it is not always straightforward to distinguish between a compact double-lobed or a core-jet structure. The quasar J1110+4817 was considered a CSO candidate in the literature earlier, but because of the lack of clear evidence, it could not be securely classified as a CSO. Here we present a comprehensive analysis of archival multi-frequency VLBI observations combined with accurate Gaia optical astrometric information. Lower-frequency VLBI images reveal an extended radio feature nearly perpendicular to the main structural axis of the source, apparently emanating from the brighter northern feature, that is rare among the known CSOs. While the presence of a binary AGN system cannot be fully excluded, the most plausible explanation is that J1110+4817 is a CSO.

Space very long baseline interferometry (VLBI) has unique applications in high-resolution imaging of the fine structure of astronomical objects and high-precision astrometry due to the key long space–Earth or space–space baselines beyond the Earth’s diameter. China has been actively involved in the development of space VLBI in recent years. This review briefly summarizes China’s research progress in space VLBI and the future development plan.

The Giant Meter-wave Radio Telescope observations of the HI 21 cm-line emission from 13 nearby dwarf star-forming galaxies are presented. These galaxies are selected from the catalogues of Wolf-Rayet galaxies having very young (<= 10 Myr) star formation. The ranges of star formation rates and stellar masses of the sample galaxies are 0.03-1.7M(circle dot) yr(-1) and 0.04-22.3 x 10(8) M-circle dot, respectively. The HI line emission is detected from 12 galaxies with peak column density >1 x 10(21) cm(-2). The 3 sigma HI column density sensitivities per channel width of 7 km s-1 for low (60 arcsec x 60 arcsec) resolution images are in the range 0.8-1.9 x 10(19) cm(-2). The HI channel images, moment images, global profiles, and mass surface density profiles are presented here. The average value of the peak HI mass surface density is estimated to be similar to 2.5M(circle dot) pc(-2), which is significantly less compared to that in massive spiral galaxies. The scaling relations of (M-stars + M-HI + M-He)versus M-dyn, gas fraction versus M-B, M(HI)versus M-stars, HI-to-stellar mass ratio versus M-stars, and M(HI)versus D(HI)for the sample galaxies are estimated. These scaling relations can be used to constraint the key parameters in the galaxy evolution models. These galaxies are residing in group environment with galaxy density up to eight galaxy Mpc(-3). An HI mass deficiency (with DEFHI > 0.3) is noticed in majority of galaxies for their optical diameters as compared to galaxies in field environments. Clear signatures of tidal interactions in these galaxies could be inferred using the HI images. Isolated HI clouds without known optical counterparts are seen in the vicinity of several galaxies. HI emission envelope is found to be having an offset from the optical envelope in several galaxies. Consistent with the previous studies on galaxy evolution in group environments, tidal interactions seem to play an important role in triggering recent star formation.

The search and study of close pairs of supermassive black holes (SMBHs) is important in the study of galaxy mergers that can possibly trigger active galactic nucleus (AGN) activity, and in the context of their evolution into the gravitational wave emitting regime. The quasar SDSS J1048+0055 was identified as an SMBH binary (SMBHB) candidate based on the observed double-peaked [O iii] λλ4959, 5007 emission lines and two distinct radio components separated by ∼20 pc (Zhou et al. ). To ascertain the binary nature of this source, we analyzed multifrequency, multiepoch very long baseline interferometry (VLBI) data to investigate its parsec-scale radio properties. The source shows double components with the western feature being brighter than the eastern one. This brighter component has a brightness temperature of ∼1010 K, a spectral index of α = −0.09 ± 0.09 (flat), and is indicative of mildly relativistic beaming. In contrast, the faint component has a lower brightness temperature of ∼108−9 K and a steep spectrum. These clues are consistent with a core–jet structure; moreover, the apparent separation speed between the two components is much higher than the expected orbital motion in a binary SMBH. Thus the present study excludes the association of the two VLBI components with the cores of an SMBHB; though, the SMBHB possibility (e.g., a pair of radio-loud and radio-quiet AGNs) is not fully ruled out. In the single AGN scenario, the double-peaked optical emission lines can originate from the jet interacting with the narrow-line region as indicated by a change in the jet direction at ∼140 pc.

We present here spatially-resolved optical spectroscopic observations of four nearby dwarf Wolf-Rayet (WR) galaxies. The ages of the most recent starburst events in these galaxies are found between 3 and 10 Myr. The gas-phase metallicities [12+log(O/H)] for the spatially-resolved star-forming regions are derived using several indicators. The star-forming regions within the galaxies are found chemically homogeneous within the uncertainties in the estimates. Nitrogen-enrichment as expected in the WR regions is not detected. This implies that metal-enrichment due to supernovae explosions in the most recent star-forming episode is not being detected here. It is suggested that the newly synthesized metals still reside in hot gas-phase. The metals from the previous episodes, cooled by now and well mixed across the whole extent of galaxies, are making galaxies chemically homogeneous with normal N/O ratio. These galaxies are residing in dense environments with galaxy density in the range of $8-80$ Mpc$^{-3}$.

Space Very Long Baseline Interferometry (VLBI) has unique applications in high-resolution imaging of fine structure of astronomical objects and high-precision astrometry, owing to the key long space-Earth or space-space baselines beyond the Earth's diameter. China has been actively involved in the development of space VLBI in recent years. This paper briefly summarizes China's research progress in space VLBI and the development plan in future.

A space-based very long baseline interferometry (VLBI) programme, named as the Cosmic Microscope, is proposed to involve dual VLBI telescopes in the space working together with giant ground-based telescopes (e.g., Square Kilometre Array, FAST, Arecibo) to image the low radio frequency Universe with the purpose of unraveling the compact structure of cosmic constituents including supermassive black holes and binaries, pulsars, astronomical masers and the underlying source, and exoplanets amongst others. The operational frequency bands are 30, 74, 330 and 1670 MHz, supporting broad science areas. The mission plans to launch two 30-m-diameter radio telescopes into 2,000 km x 90,000 km elliptical orbits. The two telescopes can work in flexibly diverse modes: (i) space-ground VLBI. The maximum space-ground baseline length is about 100,000 km; it provides a high-dynamic-range imaging capacity with unprecedented high resolutions at low frequencies (0.4 mas at 1.67 GHz and 20 mas at 30 MHz) enabling studies of exoplanets and supermassive black hole binaries (which emit nanoHz gravitational waves); (ii) space-space single-baseline VLBI. This unique baseline enables the detection of flaring hydroxyl masers, and more precise position measurement of pulsars and radio transients at milli-arcsecond level; (iii) single dish mode, where each telescope can be used to monitor transient bursts and rapidly trigger follow-up VLBI observations. The large space telescope will also contribute in measuring and constraining the total angular power spectrum from the Epoch of Reionization. In short, the Cosmic Microscope offers astronomers the opportunity to conduct novel, frontier science.

The optical spectroscopic and radio interferometric HI 21 cm-line observations of the blue compact dwarf galaxy Mrk 22 are presented. The Wolf-Rayet (WR) emission line features corresponding to high ionization lines of HeII $\lambda$4686 and CIV $\lambda$5808 from young massive stars are detected. The ages of two prominent star forming regions in the galaxy are estimated as $\sim$10 Myr and $\sim$ 4 Myr. The galaxy has non-thermal radio deficiency, which also indicates a young star-burst and lack of supernovae events from the current star formation activities, consistent with the detection of WR emission lines features. A significant N/O enrichment is seen in the fainter star forming region. The gas-phase metallicities [12 + log(O/H)] for the bright and faint regions are estimated as 7.98$\pm$0.07 and 7.46$\pm$0.09 respectively. The galaxy has a large diffuse HI envelop. The HI images reveal disturbed gas kinematics and HI clouds outside the optical extent of the galaxy, indicating recent tidal interaction or merger in the system. The results strongly indicate that Mrk 22 is undergoing a chemical and morphological evolution due to ongoing star formation, most likely triggered by a merger.

The H alpha and optical broad-band images of 25 nearby Wolf-Rayet (WR) galaxies are presented. The WR galaxies are known to have a recent (<= 10 Myr) and massive star formation episode. The photometric H alpha fluxes are estimated and corrected for extinction and line contamination in the filter pass-bands. The star formation rates (SFRs) are estimated using H alpha images and from archival data in the far-ultraviolet (FUV), far-infrared (FIR) and 1.4-GHz radio continuum wavebands. A comparison of SFRs estimated from different wavebands is made after including similar data available in the literature for other WR galaxies. The H alpha-based SFRs are found to be tightly correlated with SFRs estimated from the FUV data. The correlations also exist with SFR estimates based on the radio and FIR data. The WR galaxies also follow the radio-FIR correlation known for normal star-forming galaxies, although it is seen here that the majority of dwarf WR galaxies have a radio deficiency. An analysis using the ratio of non-thermal to thermal radio continuum and the ratio of the FUV to H alpha SFRs indicates that WR galaxies have lower non-thermal radio emission compared to normal galaxies, most likely due to a lack of supernovae in the very young star formation episode in the WR galaxies. The morphologies of 16 galaxies in our sample are highly suggestive of an ongoing tidal interaction or a past merger in these galaxies. This survey strengthens the conclusions obtained from previous similar studies indicating the importance of tidal interactions in triggering star-formation in WR galaxies.

The Devasthal Fast Optical Telescope (DFOT) is a 1.3 meter aperture optical telescope, recently installed at Devasthal, Nainital. We present here the first results using an H α filter with this telescope on a Wolf–Rayet dwarf galaxy Mrk 996. The instrumental response and the H α sensitivity obtained with the telescope are (3.3 ± 0.3) × 10 − 15 erg s −1 cm −2 /counts s −1 and 7.5 × 10 − 17 erg s −1 cm −2 arcsec −2 respectively. The H α flux and the equivalent width for Mrk 996 are estimated as (132 ± 37) × 10 −14 erg s −1 cm −2 and ∼96 Å respectively. The star formation rate is estimated as 0.4 ± 0.1 M ⊙ yr −1 . Mrk 996 deviates from the radio-FIR correlation known for normal star forming galaxies with a deficiency in its radio continuum. The ionized gas as traced by H α emission is found in a disk shape which is misaligned with respect to the old stellar disk. This misalignment is indicative of a recent tidal interaction in the galaxy. We believe that galaxy–galaxy tidal interaction is the main cause of the WR phase in Mrk 996.