I will discuss the current state of our understanding of GPS and CSS sources. This will include the current outstanding issues and possible paths forward.

Energetic feedback driven by the large-scale (100’s of kpc) lobes of classical radio galaxies is known to play an important role in shaping galaxy evolution. However, the prevalence of young and compact jets - and their impact on the interstellar medium - remains an open question. Progress requires the development of strategies for systematically finding and characterizing jets in their youth. Multi-epoch radio surveys with cadences of years to decades offer a promising means of identifying even faint (mJy-level) jets that are compact and potentially young on the basis of variability. Recently, a comparison of images from the Very Large Array Sky Survey (VLASS) and the FIRST survey has revealed a population of distant (0.2 < z < 3.2) quasars that have brightened dramatically in the past 1-2 decades. These quasars appear to have transitioned from "radio-quiet" non-detections in FIRST to "radio-loud" detections in VLASS. Extensive multi-band follow-up observations with the VLA have revealed that these quasars harbor compact (sub-kpc) radio sources that are consistent with young and growing jets. Here, I will present the results of an on-going multiwavelength study of young quasar jets identified in VLASS and discuss implications for our understanding of radio jet life cycles and their potential impact on galaxy evolution.

We have carried out a Chandra X-ray and multifrequency radio Very Long Baseline Array study of TXS 0128+554, which is associated with the Fermi γ-ray source 4FGL J0131.2+5547. The AGN is unresolved in a target 19.3 ks Chandra image, and its spectrum is well fit by a simple absorbed power-law model, with no distinguishable spectral features. Its relatively soft X-ray spectrum compared to other compact symmetric objects may be indicative of a thermal emission component, for which we were able to obtain an upper temperature limit of kT = 0.08 keV. The compact radio morphology and measured advance speed of 0.32 c ± 0.07 c indicate a kinematic age of only 82 yr ± 17 yr, placing TXS 0128+554 among the youngest members of the CSO class. The lack of compact, inverted spectrum hotspots and an emission gap between the bright inner jet and outer radio lobe structure indicate that the jets have undergone episodic activity, and were relaunched a decade ago. The predicted γ-ray emission from the lobes, based on an inverse Compton-emitting cocoon model, is three orders of magnitude below the observed Fermi-LAT flux. A comparison to other Fermi-detected and non-Fermi-detected CSOs with redshift z < 0.1 indicates that the γ-ray emission likely originates in the inner jet/core region, and that nearby, recently launched AGN jets are primary candidates for detection by the Fermi-LAT instrument.

I will cover some of the recent results obtained with LOFAR on the life cycle and evolution of radio AGN, their hosts, and their large-scale environments, and how this emerging picture fits in our understanding of GPS and CSS sources.

Cross-correlating the INTEGRAL/IBIS and Swift/BAT AGN population with radio catalogs (NVSS, FIRST, SUMSS), we found that 25\% of extended radio sources are Giant Radio Galaxes (GRG), i.e. the largest individual objects in the Universe. This fraction is four time more abundant than what found in previous studies. In 2014, we observed a pilot sample of these soft-gamma ray selected GRG at low radio frequencies with the GMRT, with the aim of studying the morphological and spectral properties of these objects. Thanks to these data, we discovered the second known X-shaped GRG, and a previously unidentified radio galaxy. Another object, observed both at kpc and pc scales, showed an extreme jet re-orientation of about 90 degrees. Given these intriguing premises, we embarked on a radio and X-ray follow-up to probe the lifecycle of these soft gamma-ray selected GRG. We found that more than half of these GRG have a nucleus with typical GPS radio spectrum, while the remaining ones show a restarting radio morphology from low-frequency radio surveys or literature data. Among these two could host up to three distinct radio phases. The next step will be to take advantage of LOFAR and EVN data we have at hand to study in more details the properties and duty cycle of these hard X-ray selected GRG.

Peaked-spectrum sources have been predominately identified and studied at gigahertz-frequencies. However, a renaissance is occurring in low-frequency (<300 MHz) astronomy with the maturation of telescopes such as the Murchison Widefield Array (MWA) and LOw-Frequency ARray (LOFAR). The wide-field surveys being conducted by these two instruments allow us to isolate large sample of sources with low-frequency spectral peaks, and statistically compare these populations to those peaked-spectrum samples isolated at higher frequencies. In this talk I will detail the selection of peaked-spectrum sources at low-frequencies, some of the unique challenges low-frequency samples represent, and how these samples compare to canonical peaked-spectrum sources.

We present a sample of eight low-luminosity GPS and CSS sources, selected from deep radio continuum observations, six of which we detect within VLBI. We determine their linear sizes, model their radio spectra, derive statistical model ages and spectral ages, and find general consistency with the hypothesis that they are young and evolving. We resolve the morphology of one CSS source and find what appear to be two hotspots spanning 1.7 kpc. We find that our sources follow the turnover–linear size relation, and that both homogeneous SSA and an inhomogeneous FFA model can account for the spectra with observable turnovers. We will present the properties of the six detected sources, including their luminosities, spectral shape, turnover frequencies, linear sizes, and ages, and how they differ to the higher luminosity population.

Active galactic nuclei (AGN) show episodic activity, which can be evident in galaxies that exhibit restarted radio jets. These restarted jets can interact with their environment, leaving signatures on the radio spectral energy distribution. Tracing these signatures is a powerful way to explore the life of radio galaxies. This requires resolved spectral index measurements over a broad frequency range including low frequencies. We present such a study for the radio galaxy 3C 293, which has long been thought to be a restarted galaxy on the basis of its radio morphology. Using the ILT we probe spatial scales as fine as ~0.2 arcsec at 144 MHz and to constrain the spectrum we combine these data with MERLIN and VLA archival data at frequencies up to 8.4 GHz that have comparable resolution. In the inner lobes (~2 kpc) we detect the presence of a spectral turnover that peaks at ~225 MHz and is most likely caused by free-free absorption from the rich surrounding medium. We confirm that these inner lobes are part of a jet dominated young CSS radio source (spectral age <0.1 Myr) strongly interacting with the rich ISM of the host galaxy. The diffuse emission surrounding these lobes on scales of up to ~4.5 kpc shows steeper spectral indices (a difference of ~0.2-0.5) and spectral age of <0.18 Myr. The outer lobes (extending up to ~100 kpc) have a spectral index of 0.6-0.8 from 144-4850MHz with a remarkably uniform spatial distribution and only mild spectral curvature (of ~0.2). We propose that intermittent fuelling and jet flow disruptions are powering the mechanisms that keep the spectral index in the outer lobes from steepening and maintain their spatial uniformity. Overall, it appears that 3C 293 has gone through multiple (2 to 3) epochs of activity. This study adds 3C 293 to the new sub-group of restarted galaxies with short interruption time periods. This is the first time a spatially resolved study has been performed that simultaneously studies a young CSS source as well as the older outer lobes at such low frequencies. This illustrates the potential of the International LOFAR telescope to study young CSS sources in larger samples of restarted galaxies. The paper from this study has been submitted to Astronomy & Astrophysics.

Over the last few years, increasingly ambitious deep-wide radio surveys conducted with powerful facilities like LOFAR have started to map the universe at the lowest radio frequencies visible from Earth. In doing so, these surveys open a largely unexplored window of the electromagnetic spectrum for astronomical studies. Particularly, LOFAR wide-area surveys are much deeper than the traditional mid-frequency radio surveys (e.g., NVSS and FIRST), and routinely detect “radio-quiet” quasars. In this talk, I will first present our work on the optimized detection of quasars from using LOFAR and optical/IR surveys using machine-learning algorithms. I will then discuss the cosmic evolution of radio-selected quasars between redshifts 1.4 < z < 5.0 and the possible origins (black hole accretion or star-formation activity in the host galaxy) of the radio emission in quasars. Finally, I will discuss the implications of our results for our understanding of the evolution of radio sources and AGN feedback.

The evolution of extragalactic sources has been an important issue in the study of active galactic nuclei for many years. Numerous observations led to creation of a standard evolutionary model for radio sources, according to which the younger and smaller Gigahertz-Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) sources expand and become large-scale FRI and FRII objects.

However, the excess of compact sources in comparison to fully developed radio-galaxies indicates that a part of GPS and CSS sources never evolve into extended structures.

Among several explanations for this early cessation of radio activity, there are indications that some sources may be transient objects on timescales of 10⁴ - 10⁵ years, representing the new population of active galaxies that needs to be explored.

We would like to present a comprehensive multi-epoch and multi-wavelength study of twelve radio transient sources, discovered recently by Caltech-NRAO Stripe 82 Survey (CNSS). They were undetected in the FIRST survey (<0.5 mJy at 1.4 GHz; epoch 1995) but discovered in the CNSS observations (2012-2015) to have brightened significantly and become radio-loud sources. These objects represent a new group of AGNs that have not been thoroughly researched so far and may exhibit behaviour typical of many sources of this class.

The evolutionary stage of a powerful radio source originated by an AGN is related to its linear size. Following the evolution models, intrinsically compact objects would evolve into the population of classical radio galaxies. However, the fraction of young radio sources in flux density-limited samples is much larger than what is expected from the number counts of large radio sources, suggesting the existence of short-lived objects.

We will present results of multi-epoch VLA and VLBA observations of a sub-sample of extreme GPS sources that should be at the very beginning of their radio evolution. We will also discuss preliminary results on VLBA and JVLA observations of a sample of short-lived radio source candidates and discuss their properties in the context of the radio source evolution.

According to radiative models, radio galaxies are predicted to produce gamma rays from the earliest stages of their evolution onwards.The study of the high-energy emission from young radio sources is crucial for providing information on the most energetic processes associated with these sources, the actual region responsible for this emission, as well as the structure of the newly born radio jets.

Despite systematic searches for young radio sources at gamma-ray energies, only a handful of detections have been reported so far. Taking advantage of more than 11 years of Fermi-LAT data, we investigate the gamma-ray emission of 162 young radio sources (103 galaxies and 59 quasars), the largest sample of young radio sources used so far for a gamma-ray study. We analyse the Fermi-LAT data of each individual source separately to search for a significant detection. In addition, we perform the first stacking analysis of this class of sources in order to investigate the gamma-ray emission of the young radio sources that are undetected at high energies.

We find that 11 young radio source are individually detected at high confidence (>5 sigma), including the discovery of significant gamma-ray emission from the compact radio galaxy PKS 1007+142. Although the stacking analysis of individually-undetected young radio sources does not result in a significant detection, it provides stringent upper limits to constrain the gamma-ray emission from these objects.

In this talk we present the results of our study and we discuss their implications for the predictions of gamma-ray emission from this class of sources.

Most of the pulsar radio spectra can be described by a simple power-law function. However, spectra of some pulsars show a different behavior, among them are the so-called gigahertz-peaked spectra (GPS) characterized by a positive spectral index in the frequency range below one gigahertz (Kijak et al. 2017). Recently the model of the free-free thermal absorption was used in order to explain the turnover around one gigahertz in pulsars spectra by Lewandowski et al. (2015). Such absorption may be caused by pulsars environments such as pulsar wind nebulae, dense filaments in supernovae remnants or cooler H II regions.

So far we know of about thirty pulsars showing GPS characteristics. In my talk I will show an overview of this sample, with a special focus on three objects: J1745-2900 – a radio-magnetar near the Galactic Center black hole, B1800-21 – Vela-like GPS pulsar with a variable spectrum, and J1740+1000 – a pulsar for which spectral analysis appear to be a real challenge.

Bibliography:

J. Kijak, R. Basu, W. Lewandowski, K. Rozko, M. Dembska, 2017, ApJ, 840, id. 108

W. Lewandowski, K. Rozko, J. Kijak, G. Melikidze, 2015, ApJ, 808, id:18

I will present the current state of the studies focusing on the X-ray emissions from Compact Symmetric Objects (CSOs), sources with radio linear sizes not exceeding 1 kpc and observed from the direction perpendicular to the jet axis which facilitates dynamic age measurements of the radio structures with multi epoch radio observations. CSOs are believed to be young and their broadband properties are important for our understanding of the initial conditions for the evolutionary paths of radio sources. In particular, observations in the X-ray band are instrumental in measuring the properties of the environment surrounding the expanding jets, and in pinpointing the high energy processes that operate in AGN at the time of renewed radio activity.

The centres of galaxy groups and clusters are an especially favourable location for radio galaxies. Cluster-dominant ellipticals host the most massive black holes, cooling from the intra-cluster medium provides abundant fuel, and in some systems we can trace multiple cycles of jet activity via radio and X-ray observations. We will describe one such cluster at z=0.263, whose dominant galaxy hosts the compact steep spectrum radio source 1321+045. We estimate the age of the 5kpc-scale double-lobed source to be around 2 Myr, but new VLBI data suggest that these lobes may no longer be powered by the AGN, which has launched a new 20pc jet on a different axis, probably within the last few hundred years. A deep Chandra observation allows us to map the surrounding cluster, showing it to be strongly cooling and disturbed by a recent minor merger, with properties very similar to the population of FR-I hosting cool-core clusters in the local universe. Estimating the energy required to form the radio lobes, we find that this can only balance cooling in the intra-cluster medium for a short time, implying that another, larger outburst must be expected in future. We will discuss the source in the context of other group and cluster radio galaxies, and the small number of peaked sources known to live in these environments.

We aim to leverage the transformational science enabled by the Event Horizon Telescope (EHT) to study the physics of, and near, the black holes in a sample of galaxies covering a large parameter space in SMBH mass, accretion rate, and jet power. To this end, we work on a sample of nearby galaxies whose directly measured black hole masses and distances imply that 40 micro-arcsec EHT observations will resolve the central engine at < 100 Schwarzschild radius resolution. I will present the ongoing study of radio spectra of AGNs and will discuss the impact of this finding on future EHT observations.

We studied a distant (z=2.152) GPS quasar 0858-279 with a strongly variable convex radio spectrum and extended odd-looking parsec-scale structure provided by early VLBI surveys. Its spectrum was observed by us at RATAN-600 while VLBA delivered detailed information on its structure from 1.4 to 22 GHz within a dedicated experiment. High VLBA resolution at 15 and 22 GHz as well as the reconstructed spectral index maps allowed us to identify an opaque weak core and a jet with a component which dominates in the total continuum peaked spectrum. The brightest jet feature turned out to be located 20 parsecs away from the core. We estimated the magnetic field in this feature and found it to be unusually strong, about 6 G. This suggested a strong interaction between the jet and the interstellar medium that could have caused the formation of a shock. We also studied the polarization properties of the quasar obtaining the rotation measure maps. Those maps enabled us to trace the direction of the magnetic field in the brightest feature. High rotation measure values (more than 6000 rad/m²) and the magnetic field direction being perpendicular to the jet were yet another evidence in support of the shock hypothesis. We estimated the magnetic field value in the core as 0.1 G using two approaches, the core shift effect and the conservation of the magnetic flux through the jet flow.

Less than 200 radio-emitting AGNs are known above redshift 4. Around 40 of them had been observed at milliarcsecond resolutions with very long baseline interferometry (VLBI) technique. Currently, another dozen sources are being observed with the European VLBI Network in our ongoing project. Among the already known sources, there are unresolved, compact, relativistically beamed objects, blazars with jets pointing at small angles to the observer's line of sight. But we also found misaligned sources with larger jet inclination angles. There are sources showing radio structures more reminiscent to medium-sized symmetric objects or young radio galaxies. Interestingly, we even found arcsec-scale separated double sources among those objects which were classified as blazar candidates based upon their X-ray characteristics. The prominent radio emission detected from the hot spots of these sources might be related to the denser interstellar medium around these young radio galaxies at early cosmological epochs.

We present the analysis of the archival Chandra observations of the central parts of the Centaurus A radio source. Our analysis focuses on four distinct regions characterized by the soft X-ray emission excess: two regions located at kiloparsec distances from the nucleus to the west and east, the extended bow-shock structure placed further to the south, and a fragment of a thin arc to the north. Our spectral analysis reveals the presence of a dense and cold gas in the east and west regions, as well as a power-law emission component in the north, east, and the south regions, all three coinciding with the edges of the inner radio lobes in the system. We discuss the emerging best-fit parameters of the cold and dense thermal gas, in particular the number density of ~0.3cm-3 and the tempereture of ~0.2 keV, speculating in particular that it could be related to a massive gaseous outflow from the central regions of the galaxy.

The clearest indications of restarting activity among radio-loud AGN come from the discoveries of 'double-double' radio galaxies (DDRG), displaying multiple pairs of radio lobes from different generations of AGN activity. They are a comparatively rare population, and offer direct insights on radio AGN life cycles. Polarization measurements offer complementary information of the observed radio emission, and are useful to describe the line of sight environments toward the radio lobes and their thermal particle populations. I present high resolution VLA and low resolution LOFAR polarimetry of a sample of DDRGs selected in the LOFAR Two Metre Sky Survey. Our early results indicate that only a small fraction of DDRGs have detectable polarization, coming from the hotspots of the inner restarting jets. On the other hand, a small number of sources have polarization in the outer remnant lobes only, which is likely due to these sources living in dense, highly concentrated Faraday rotating environments. We discuss the implications of our results in the context of the episodic activity of radio-loud AGN.

In this study, we report our ongoing search for extremely inverted spectrum compact radio galaxies, for which the defining feature in the radio spectrum is not the spectral peak, but instead the slope of the spectrum (alpha) in the high-opacity (i.e., lower frequency) part of the radio spectrum. Specifically, our focus is on the spectral regime with spectral index, alpha_thick > +2.5. The motivation for our study is, firstly, extragalactic sources with such extreme spectral index are extremely rare, probably because of the unavailability of the right combination of sensitivity and resolution over a range of low frequencies. The second reason was more physically motivated since alpha = +2.5 is the maximum slope theoretically possible for a standard non-thermal radio source, believed to radiate incoherent synchrotron radiation from a relativistic particle population accelerated with single power-law energy distribution. Therefore such sources could be the test-bed for some already proposed alternative scenarios for synchrotron self-absorption (SSA), involving non-power-law energy distributions.

A detailed comparison of the two low-frequency surveys TGSS-ADR1 (150 MHz) and WENSS/WISH (325 MHz) undertaken with the GMRT and Westerbork telescope respectively, led us to shortlist ~35 candidates with alpha (150-325 MHz) > +2.5. We carried out quasi-simultaneous observations of all the candidate sources with the upgraded GMRT, at frequencies in the range 150 - 500 MHz (for some sources, up to 900 MHz), to rule out variability, flux scale errors and other possible issues that would arise because the two surveys were conducted using two different telescope arrays at different resolutions and almost a decade apart from each other. Quasi-simultaneous observations done within a week help us rule out any variability between the range of frequencies on which the spectral index is based. We report 6 sources clearly above the spectral index limit of alpha (150-325 MHz) >2.5 and 10 sources have a spectral index above +2.0. As mentioned above these sources violate the SSA limit and it is shown that their spectra can be very well reproduced in terms of free-free absorption (FFA) occurring in an inhomogeneous external screen of thermal plasma, taking reasonable values for the input parameters. MHD simulations of jets in AGNs (from literature) point to the change in the spectral slope with the evolution of the radio jet. Thus, these sources could very well be young radio galaxies that will eventually evolve into large structures. Milli arc-second resolution VLBI images of these sources also point to similar conclusions.

Here we discuss the sample of confirmed young radio galaxies with measured kinematic ages and nuclear X-ray fluxes. The analyzed targets are sufficiently young so that the currently observed luminosities of compact lobes and accretion disks correspond to exactly same episode of the AGN activity. For the targets, we analyze the available optical data, estimating the bolometric luminosities of the accretion disks L_bol, and the black hole masses; we also derive the minimum jet kinetic kinetic luminosities, P_j. With such, we investigate the distribution of our sample in the three-dimensional space of the accretion parameter λ_Edd ≡ L_bol / L_Edd, the nuclear X-ray luminosity L_X considered here as a limit for the emission of the accretion disk coronae, and the jet power P_j. We find that (i) the accretion parameter λ_Edd in our sample is distributed within a narrow range from λ_Edd ∼ 0.01 up to ∼ 0.2; (ii) the normalized jet power formally correlates with the accretion rate; (iii) the jet production efficiency η_jet ≡ P_j / Ṁ_acc c² spans a range from η_jet ≲ 10^-3 up to ∼ 0.2 at maximum, which is below the level expected for magnetically arrested disks around maximally spinning black holes; and (iv) there is an interesting diversification in η_jet on the hardness--intensity diagram L_X / L_bol - λ_Edd, with the jets being produced the most efficiently during the high/hard states, and suppressed during the soft states.

I will present VLBA data for 90 members of the sample of WISE-NVSS selected obscured and ultra-luminous quasars (Lonsdale et al. 2015). Many of these sources have peaked or curved radio spectra, as discussed in the companion paper by Dr. Pallavi Patil. The VLBA observations probe the most compact radio source components, while deeper, multi-frequency VLA, VLBA and e-MERLIN observations of a subset of the sample are yielding spectral index maps that further reveal the nature and evolution of these enigmatic radio sources.

Radio galaxies (RGs) are active galactic nuclei (AGN) able to launch relativistic jets, the most energetic phenomena in the Universe which can have a large impact on galaxy evolution. RGs are typically associated with red giant elliptical galaxies hosting the most massive black holes (> 10⁸ solar masses). In the local Universe (z < 0.3) the optical classifications of AGN reflect a clearer separation of the local population of RGs (LERGs/HERGs) in terms of nuclear and host properties than that based on the radio morphology (FRI/FRII). However, a different pitcure is emerging from recent large-area high-sensitivity radio-optical surveys, which have allowed us to explore the very low end of the RG luminosity function, where low-power FRI (LERGs) were expected to dominate. This has instead unearthed a large population of RGs which differ from classical FRI/IIs, by showing 'compact' radio structures and more heterogeneous host properties: the FR0s, which lack of large scale (>10 kpc) radio emission. Considering their multi-band properties and large-scale environments, I will speculate about the possible origins of FR0s and the possible cosmological scenarios they imply.

We present the result of 34 FR0 radio galaxies observations with the RATAN-600 radio telescope in 2020-2021 during 2-6 epochs. We obtained FR0 radio spectra at 2.25-22.3 GHz quasi-simultaneously and revealed moderate radio luminosities with the typical values ~5*10³⁹ erg/s at 4.7 GHz. The most FR0 have flat radio spectra, and we also found many peaked spectra. Due this fact and a compact nature of the FR0 radio galaxies we suggest their probable relationship with CSS/GPS radio sources. For study this issue we analyzed FR0s broadband radio spectra using RATAN-600 measurements and available literature data, including LOFAR and VLASS. There are 17 objects out of 34 which can be candidates to the CSS/GPS radio sources. FR0s have broader radio spectra than GPS: FWHM > 2 for the most objects. Spectral indices at the frequency range below and above the peak don’t reach canonical values which used for classify the genuine GPS. We classified 3 FR0s as low power GPS sources according the canonical criteria. Long time observations are needed for the further study the relationship between FR0s and GPS sources. The FR0 galaxies variability properties are key issue. Some FR0s are quite variable at the time scale of one year according to the RATAN-600 measurements. This fact can mean the flare phenomena in a compact radio core and leads the issue about relation between FR0s and blazars.

Most of large VLBI surveys observed only the sources with a flat continuum radio spectrum. For this reason, the population of compact steep-spectrum (CSS) radio sources remains poorly studied at parsec scales. We observed with the Very Long Baseline Array (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. With a detection limit of 30 mJy, we detected with the VLBA 34% of the sample. In particular, 82 sources have a steep VLBA spectrum, which makes them good candidates to CSS sources; most of these candidates are reported for the first time. The CSS source candidates are 54% of the sources detected by the VLBA at 2.3 GHz in our sample and 17\% of the whole sample. We investigated the relation between the total continuum spectra of the sources and their parsec-scale structure and found that the compactness and the brightness temperature of the sources in our sample show a positive correlation with single-dish and VLBA spectral indices, in agreement with common AGN models. Our results show that future VLBI surveys aimed at searching for new sources with parsec-scale structure should include not only flat-spectrum sources but also steep-spectrum ones in order to reach an acceptable level of completeness.

Gigahertz-Peaked spectrum (GPS) sources are compact active galactic nuclei, presumably young precursors of bright radio sources, which are of interest as the early stages of their evolution. The study of GPS radio properties provides information about origin of synchrotron radiation in the radio-loud AGNs. We present the results of a multifrequency GPS study based on quasi-simultaneous measurements with the RATAN-600 radio telescope in 2006-2021. We observed several GPS samples and obtained multifrequency catalogs of their measurements at six frequencies (1.1, 2.3, 4.8, 7.7/8.2, 11.2, and 22 GHz). The GPS broad-band radio spectra were constructed and analyzed using available literature data. We confirm a relatively small number of genuine GPS sources (2-3%) with convex spectra when they are monitored densely and for long periods of time. A large proportion of the current GPS samples are contaminated by blazars that temporarily exhibit a peaked spectrum. We constructed an aver age radio spectrum of GPS quasars and GPS galaxies and found statistical differences in them.

We observed two samples of quasars at z≥3 (S_1.4 > 100 mJy) and z≥4 (S_1.4 > 20 mJy) in 2017-2021. The peaked radio spectrum (PS) is a common feature for the bright high-redshift quasars, 45% of objects have observed maximum in their radio spectra in the GHz range. We defined the spectrum of the newly discovered most distant blazar PSO J047.4478+27.2992 (z=6.1) as a MPS candidate by RATAN measuring in 2020. Comparing with other distant blazars at z>5 (J1026+25, J0906+69, and J1648+46) revealed the PS for three out of four of them. We compared the spectra of GPS quasars of several complete samples at different redshifts (Mingaliev et al., 2012, 2013, and Sotnikova et al., 2019). The high redshift GPS (z>3) have on average the same spectral indices of optically thick and thin spectral parts as the low (z<1) and medium redshift (1 < z < 3) GPS within the measurement uncertainties.

New GPS and MPS candidates were suggested according to the RATAN multifrequency observations. The MPS sources with an observed spectral peak ν_peak ≤ 1 GHz can be highly compact objects and useful for finding high-redshift radio-loud AGN. Further study of their variability is needed to classify them precisely.

The Tenth Cambridge (10C) survey is the deepest high-frequency (>10 GHz) radio survey to date - it is complete to 0.5 mJy across 10 extragalactic fields. Follow-up VLA observations of a complete subsample of 10C sources have shown that compact radio galaxies, or radio galaxies with very faint jets, are the dominant population in the 10C survey. As such, this survey provides a unique view of compact radio galaxies. I will present the results of an extensive study of 10C sources, using a range of radio and multi-wavelength data. The 10C sample of compact, high-frequency selected radio galaxies is a mixture of high-excitation and low-excitation radio galaxies and displays a range of radio spectral shapes, demonstrating that they are a mixed population of objects. I will discuss these results and the implications for the nature of compact radio galaxies.

Old global VLBI observations at 330 MHz on a sample of CSS sources from the 3C and the PW (Peacock & Wall) catalogues provide information on the oldest electrons in these objects. A significant fraction of the total flux density is missing in many objects, implying the presence of ~ arc second scale emission not sampled by our observations. The ages of the oldest electron population is consistent with the hypothesis of radio sources in an early stage of their evolution. Consequently, the growth (expansion) speed is of the order of 0.1c.

In recent years, the old paradigm according to which only high-mass black holes can launch powerful relativistic jets in active galactic nuclei (AGN) has begun to crumble. The discovery of γ-rays coming from narrow-line Seyfert 1 galaxies (NLS1s), usually considered young and growing AGN harboring a central black hole with mass typically lower than 1e8 solar masses, indicated that also these low-mass AGN can produce powerful relativistic jets. The search for parent population of gamma-ray emitting NLS1s revealed their connection with compact steep-spectrum sources (CSS) and gigahertz-peaked sources (GPS). In this talk, I will outline the fundamental properties of NLS1s, and I will explore their link with CSS and GPS. I will also present the recent discovery of a new class of jetted NLS1s where the radio emission of their kinematically young relativistic jets is completely absorbed by ionized gas, making them undetectable at radio frequencies below 10 GHz even with the most sensitive instrumentation. Finally, I will discuss the implications of this groundbreaking discovery on our knowledge of other young jetted AGN, and how it can impact our understanding of AGN life cycle.

Interplanetary Scintillation (IPS) causes sources of size < 1" to vary in brightness on timescales of ~1s. The Murchison Widefield Array (MWA), with its very wide field of view and excellent instantaneous imaging capability, is ideal for performing a rapid sky survey of sources showing IPS. This survey identifies all compact sources down to a well-defined flux density limit. The strongest scintillators (size < 0.3") are composed of flat-spectrum, peaked-spectrum, and steeper sources in roughly equal measure. All peaked sources classified by Callingham (2017) and observed by us show scintillation, though not quite all have scintillation indices consistent with a completely compact source.

IPS with the MWA therefore has the potential to probe the size of peaked-spectrum sources, as well as identifying other compact sources that cannot be identified by their spectra alone. In this talk I will give an overview of our IPS work, the compact sources we have identified, and our efforts to find infra-red counterparts and thereby estimate redshifts. I will demonstrate how we can estimate source sizes and how these size estimates correlate with other parameters such as peaking frequency and flux density. Finally, I will discuss the prospects for more sophisticated measurement of source structure from IPS observables.

High-redshift radio galaxies (HzRGs) are vital laboratories for studying massive galaxy formation and evolution in the early Universe (review by Miley & De Breuck, 2008, A&ARv, 15, 67). We have developed a new selection technique for finding these rare, powerful systems, making use of spectral curvature in the Murchison Widefield Array (MWA) 70-230 MHz GLEAM survey (Hurley-Walker et al., 2017, MNRAS, 464, 1146). Distant radio galaxies are expected to be young, compact radio sources due to significant inverse-Compton losses at high redshift (e.g. Blundell & Rawlings, 1999, Nature, 399, 330; Saxena et al. 2017, MNRAS, 469, 4083), i.e. we are searching for the redshifted analogues of gigahertz peaked-spectrum and compact steep‐spectrum sources.

In this talk, I will give an overview of our pilot study in the 60 deg² GAMA-09 field. From just four targets, we discovered the second-most distant radio galaxy currently known (z = 5.55; Drouart et al. 2020, PASA, 37, e026), with the possibility that an additional source is also at z > 5 (Drouart et al. 2021, PASA, submitted). This latter source has a projected linear radio size <= 10 kpc; moreover, an interplanetary scintillation constraint from the MWA implies that half of the low-frequency flux density is emitted from a region < 3 kpc in size. We are currently analysing Hubble Space Telescope (HST) spectroscopic data for the z > 5 candidate. Additionally, Low-Frequency Array (LOFAR) 34-66 MHz data have been obtained to constrain and model the low-frequency spectral turnover for both the z > 5 candidate and our z = 5.55 discovery.

I will conclude by briefly outlining ongoing efforts to expand our selection technique over the full 1200 deg² sky area covered by the ESO VIKING near-infrared survey. Our goal is to build a sample of active galactic nuclei at z > 6.5 during the Epoch of Reionization that is not affected by orientation-dependent obscuration, thereby allowing us to study the co-evolution of the supermassive black hole and host galaxy during the first billion years of the Universe.

Spectral variability of radio sources encodes information about the conditions of intervening media, source structure and emission processes. With new low-frequency radio interferometers observing over wide fractional bandwidths, studies of spectral variability for a large population of extragalactic radio sources are now possible. Using two epochs of observations from the GaLactic and Extragalactic All-sky MWA (GLEAM) survey that were taken a year apart, we search for spectral variability across 100-230 MHz for roughly 21,000 sources. In this talk, I present a population of peaked spectrum sources (PSS) whose variability brings into question their original PSS classification. Furthermore, I show PSS do not follow the same distribution of variability as the population of typical radio galaxies with power-law spectra. This talk will discuss the viability of several potential explanations of the observed spectral variability, such as interstellar scintillation and jet evolution.

I will review the latest optical, infrared and mm studies of compact radio sources, which provide important clues to how the radio sources are triggered and the impact they have on their host galaxies.

There is now strong evidence that powerful CSS/GPS objects are triggered in galaxy mergers; however, their star formation rates and cool ISM masses tend to be higher than those of their more extended counterparts. I will explore the idea this is a consequence of such sources being triggered in unusually dense environments, leading to their radio fluxes being boosted through jet-cloud interactions, so that they are preferentially selected in flux-limited samples.

It is also clear that powerful CSS/GPS sources drive multi-phase outflows that have a major impact on their host galaxies on kpc-scales. These outflows are manifest in the disturbed kinematics in the warm ionized (detected via optical emission lines), neutral (HI 21cm absorption lines) and molecular (mm CO emission lines) gas. Recently, the properties of the outflows have been precisely measured across the various phases, demonstrating that the molecular gas can dominate in terms of mass outflow rates and kinetic power. The feedback effect associated with these outflows is not confined to the most powerful CSS/GPS objects, but is also observed in compact radio sources that are orders of magnitude less powerful at radio wavelengths. In terms of the evolution of the host galaxies, currently the debate about these jet-accelerated outflows centres on whether their effects are only confined to the central few kpc or are more widespread.

Recent studies of young radio galaxies have revealed the presence of dense and dusty gas near the active nucleus traced by both 21cm HI absorption and soft X-ray absorption (e.g. Ostorero et al. 2010, Siemiginowska et al. 2016, Glowacki et al. 2017, Moss et al. 2017), offering new insight into the physical nature of the circumnuclear medium in these distant galaxies. These absorbers are often found in CSS/GPS radio sources, suggesting an influence of age and evolutionary state in terms of the physical circumstances of dense circumnuclear hydrogen. To explore this, it is instructive to analyse the absorption trends seen for the entire radio galaxy population as well as for CSS/GPS sources explicitly. In order to further investigate the correlation between HI absorption and X-ray absorption alongside host galaxy properties, we are carrying out the SEAFOG project (Studies of eROSITA And FLASH Obscured Galaxies). SEAFOG will give us a clear view for the first time on the nature of the relationship between HI and X-ray absorption with a sample of hundreds of galaxies. I will give a status update on SEAFOG, in particular with respect to ongoing analysis of the eROSITA performance verification field (eFEDS) alongside both FLASH Pilot Survey data and ASKAP data observed as part of the SWAG-X Observatory Project.

Young radio active galactic nuclei (young AGN) are compact (≲ 20 kpc), radioloud AGN. Their radio spectra has an convex characteristic, with a peak frequency from 100 MHz to tens of GHz. It is generally believed that they are at early stage of AGN evolution, and will evolve into large-scale radio sources, known as FR II/FR I. According to the dynamical age measured by the radio structure and the radiation age determined by the radio spectrum, it is about 10² − 10⁵ yr, which strongly supports the youth scenario. Thus, the research of young AGN is conducive to study many core issues of AGN: uncovering the radio/accretion activity at the initial stage of AGN, understand the evolution of AGN, investigate the relationship between the accretion and jet, jet formation and launching mechanism, and radiative processes dominating the X-ray/𝛾-ray output throughout the evolution of AGN. However, at present, the research of young AGN mainly focuses on the radio band, and other bands have not been adequately studied. In this talk, I will introduce our studies on the optical properties by building the largest optical sample based on SDSS DR12 and X-ray emission by constructing the largest X-ray sample and combining multi-band data for young AGN.

We present the first science results from ASKAP-FLASH, a 34,000 square degree survey for HI absorption towards radio sources at cosmological distances. HI 21-cm absorption provides a unique perspective on the neutral gas in galaxies; it is sensitive to the coldest (T~100K) atomic gas, thereby directly tracing the reservoir of fuel available for star formation and SMBH growth in galaxies. FLASH is now underway and expected to discover more than a thousand intervening and associated 21-cm absorbers out to redshifts z~1. The kinematic information from this survey will reveal accretion and jet-cloud interactions in young radio galaxies, providing crucial constraints on models of the gaseous environments in which these objects are triggered.

We have recently completed observations of the first 1000 square degrees of the survey, covering well-studied fields in SDSS-BOSS, GAMA, VIKING and DES/SPT regions. We will present several of our absorbers that are associated with the host galaxies of GPS and CSS radio sources and, as a demonstrative case study, we discuss in detail our detection towards PKS 1740-517. This is a young and luminous radio galaxy at z = 0.44 that is undergoing an interaction with a gas-rich minor companion. Such minor interactions are important in fueling luminous radio AGN that generate jet-driven neutral gas outflows in their host galaxy. Our ASKAP and ALMA observations revealed a reservoir of atomic and molecular gas in the central kpc of the radio galaxy, and a distinct cloud of cold atomic gas that is associated with accretion from the companion.

Radio jets are a spectacular manifestation of the nuclear activity in galaxies. The energy they release can strongly impact the ISM/IGM. Interestingly, this interaction can be traced also by the molecular gas. The high spatial resolution of ALMA is, therefore, ideal to trace the details of this process. The way the radio plasma couples with the surrounding medium, the dependence on the jet power and whether this coupling changes as the jet expands, are all key parameters for quantifying their effect on the conditions of the ISM and, ultimately, on the evolution of their host galaxy. However, building a complete picture requires tracing the details of this interaction in a variety of radio sources to cover this large parameter space. Since Cycle 1 we are in the process of doing this, building up a small but rich sample. The latest target observed in Cycle 6 is the young (CSS) radio galaxy PKS 0023-26. In this talk I will report the results of C(2-1) and 176 GHz continuum observations and what they are telling us about the way the radio plasma affects the kpc-scale gas. A comparison with the other objects in the sample is giving some hints on how such effects may change from pc to kpc scales.

We present a systematic analysis of the mid-infrared (MIR) properties of the most compact radio galaxies, based on low-resolution data provided by the Wide-field Infrared Survey Explorer (WISE). We restrict our analysis to sources with available X-ray data that constitute the earliest phase of radio galaxy evolution, i.e. those classified as Gigahertz Peaked Spectrum (GPS) and/or Compact Symmetric Objects (CSOs). In our sample of 29 objects, we discuss the host galaxy morphology based on available optical images, as well as investigate the distribution of their MIR colors with respect to differing active galactic nuclei (AGN) populations. We conclude that the triggering of radio jets in AGN does not differentiate between elliptical hosts with substantially different fractions of young stars; instead there is a relationship between the jet duty cycle and the ongoing star formation. Finally, we comment on the star formation rates of the two gamma-ray detected sources in our sample, 1146+596 & 1718-649.

We report the detection of outflowing molecular gas in the center of the nearby (z=0.014) massive radio galaxy NGC6328. The radio core of the galaxy, PKS B1718-649, is identified as a Gigahertz Peak Spectrum source with a compact (2 pc) double radio lobe morphology tracing a young jet. We used ALMA CO(2-1) and CO(3-2) observations at 100 pc resolution to study the kinematics of the emission up to 6 kpc from the galaxy center. Most of the molecular gas is settled in a highly warped disk that possibly originated from a recent merger event. In the inner 400 pc of the disk, and along with the orientation of the radio jet we identified excited gas with velocity dispersion of more than 150km/s. These results suggest the presence of a previously undetected molecular outflow of 1-3 M_sun/yr, possibly originated from the interaction of the Jet with the dense interstellar medium. Our work adds PKS B1718-649 to the increasing number of Peaked Radio Sources with a detected molecular outflow. The proximity of the source and the well-studied young jet provides us a unique opportunity to better understand the physical mechanisms of Peaked Sources and their role in the context of AGN feedback.

Imaging the UV continuum from hot massive stars is the best way to study recently triggered and ongoing star formation. Following our pilot study that detected spatially extended UV radiation in 3/3 CSS sources, we obtained high spatial resolution HST/UVIS images for a larger sample of 9 sources. Our goal is to morphologically separate and characterize the generic star formation (due to gas infall) from the star formation aligned with the jet axis which is attributed to shock-triggering by the radio source. We will determine the luminosity and spatial extent of the star formation regions as well as the relation to radio emission and estimate SFRs and their time scales for triggering of jet-induced star formation in the target CSS sources.

Active Galactic Nucleus (AGN) feedback at z ~ 1-3 is believed to take place in the presence of thick columns of gas and dust, leading to heavily obscured systems that are challenging to detect at optical and X-ray wavelengths but are transparent at radio and Mid-IR wavelengths. Mid-IR color diagnostics using WISE observations can identify the most luminous and heavily obscured AGNs, which are believed to represent a transient phase of rapid massive black hole growth. By combining both Mid-IR and radio diagnostics, we have identified a sample of 155 ultra-luminous and obscured quasars (0.4 < z < 3) selected to have extremely red Mid-IR colors in WISE and compact, bright (>7 mJy) radio emission in the NVSS Survey and FIRST. In this talk, I present results from radio and submillimeter follow-ups of our sample. High-resolution VLA imaging has revealed compact source morphologies on angular scales < 0.2” (1.7 kpc at z ~ 2) for the majority of our sources. Then, I present broadband radio spectra of the entire sample, constructed from our 10 GHz VLA observations and archival radio data. About half of our sample exhibits peaked or curved spectral shapes consistent with those typically seen in young radio AGN (e.g., Gigahertz Peaked Spectrum and Compact Steep Spectrum sources). The application of a simple adiabatic lobe expansion model is consistent with the radio jets that are relatively young (< 0.01-10 Myr) and propagating into a dense ambient medium. The presence of a dense ISM is further supported by the direct detection of large molecular gas reservoirs in a pilot follow-up study with the ALMA. Overall, our sample is consistent with a population of recently triggered, young radio jets caught in a unique evolutionary stage in which they reside in a dense ISM. Finally, I discuss the implications of our study for understanding the impact of young jets on the ISM and star formation rates in powerful young AGN.

Compact Symmetric Objects (CSOs) are young jetted-AGN with overall projected size < 1 kpc. The classification was introduced in order to provide a useful distinction between the ∼ 90% of powerful compact extragalactic radio sources that are dominated by asymmetric emission due to relativistically boosted emission from jets aligned close to the line of sight, and the ∼ 10% of symmetric objects having jet axes aligned closer to the plane of the sky than to the line of sight, in which the observed radiation is not boosted in the direction of the observer. The original classification criteria for CSOs were three: (i) overall projected diameter smaller than ~ 1 kpc, (ii) an identified center of activity, and (iii) symmetric jet structure about the center. However there has been much confusion in the literature and erosion of the value of the CSO classification due to misclassified CSOs, because many jets contain compact bright features outside of the core, resulting in a GPS total spectrum and a 'compact double' appearance, and some relativistically boosted objects with jet axes aligned close to the line of sight appear symmetric because the approaching jet is projected on both sides of the core. We propose adding two new criteria to the CSO classification criteria based on (iv) the radio variability and (v) the apparent velocities of bright features moving along the jets. We are engaged in compiling a comprehensive catalog of CSOs drawn from the literature based on these five criteria.

Relativistic jets from AGNs are an important driver of feedback in galaxies. Interaction of such jets with an inhomogeneous ISM significantly affect the evolution of the galaxy. A class of galaxies where such interactions are thought to be occurring are the Gigahertz Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) galaxies. Such galaxies are characterised by a distinct turnover in the radio spectrum, which is often well explained by free-free absorption from a turbulent ISM surrounding the jet. Peaked spectrum radio galaxies are thus an important probe of young jets, possibly embedded in a gaseous environment, which represent the early phase of radio jets before they affect larger scales. I shall review the results of recent 3D relativistic hydrodynamic simulations probing the interaction of such AGN jets with a turbulent ISM. These simulations explore the effect of evolving jets on the host galaxy, which is under-resolved in large scale cosmological simulations. Our simulations probe the differences in interaction arising from differing ISM morphology (spherical and disk), differing densities and jet power. We find that the jet couples strongly with the ISM launching fast lateral outflows of shocked ionised gas. The increased turbulence and densities in the immediate vicinity of the jet result in sharp turnover in the simulated radio spectrum, corresponding to the GPS phase of the galaxy's evolution, eventually becoming a CSS type. I shall also discuss latest advancement of numerical techniques in predicting synchrotron emission from simulations of relativistic jets and its potential to differentiate between different models of jet dynamics.

In the past decade, the remarkable improvement in the performance of millimeter-VLBI arrays has led to important progresses in the investigation of jet formation in active galaxies, recently culminated in the imaging of M87 on event horizon scales. While high-resolution studies have been performed so far in a few bright radio galaxies besides M87, the boost in sensitivity due to the use of wide bandwidth receivers and to the inclusion of large telescopes is now opening a new window in the exploration of AGN cores, enabling us to study fainter objects for the first time. This is fundamental for testing theoretical models describing the launching, acceleration and collimation of the relativistic plasma in different luminosity classes. In this talk, I will present a first millimeter-VLBI study of the jet collimation region in the radio galaxy NGC315, which could be imaged with a resolution down to ∼100 Schwarzschild radii. I will then consider a sample of nearby objects to investigate a possible relation between the accretion properties and those of the jet collimation region.

The interstellar medium (ISM) around AGN is clumpy and possibly highly inhomogeneous. Furthermore, jets produce strong shocks in this medium and can trigger, e.g., ionization, which can result in observable line emission. With the aim of studying these processes, we have programmed a version of our 3D RHD code that includes (atomic) Hydrogen ionization. It is our plan to present the first test simulations of jets propagating through an inhomogenous ISM composed of both atomic and ionised Hydrogen, representing the first hundreds of parsecs of jet evolution. These simulations represent an initial step into a deeper search of the effects of jets on the evolution host galaxy.

Co-authors: José López-Miralles, Álvaro Labiano and Victoria Reynaldi

Among the very heterogeneous population of compact radio galaxies, the recently discovered "FR0" represents a puzzling class of AGNs. While the radio size is reminiscent of young AGNs, the large abundance of FR0s with respect to extended FRIs excludes an evolutionary scenario. It has been suggested that their lack of extended radio emission derives either from an intrinsic jet weakness, or from an hostile environment limiting the growth of the radio galaxy. While it seems unlikely that the galaxy scale ISM is halting the expansion of the radio source, the effect of the intracluster medium (ICM) for sources residing in galaxy clusters has never been probed. To investigate these aspects, we performed a detailed study of the galaxy cluster Abell 795, which hosts a central FR0 radio galaxy. From an archival Chandra observation and radio survey data we found a dynamically disturbed environment with evidence for ICM sloshing. The associated turbulence could possibly power the extended radio source revealed at 150 MHz and explain the observed ~18 kpc offset between the central galaxy and the X-ray peak. We argue that the environment alone cannot explain the compactness of the FR0 radio galaxy, as similar conditions are also found around extended FRI sources, thus the jet propagation is likely hampered by an intrinsic weakness. An unexpected discovery was the identification of a pair of X-ray cavities in the proximity of the FR0: these could have been created in a past outburst of the FR0, and dragged away from the AGN by the large scale turbulence. The presence of X-ray cavities associated with a FR0 in a galaxy cluster could be the long sought signature of feedback and could open a new window on the study of recurrency, jet power and evolution of this new class of compact radio galaxies.

The interplay between the nuclear activity and the interstellar medium of galaxies plays an important role in their evolution: the gas accreting onto the dormant supermassive black hole turns it into an AGN and the ensuing activity is believed to starve the host galaxy of the fuel needed to form stars. The contribution of radio-loud AGN to this feedback effect is yet to be well understood, more so that of low luminosity radio AGN. These make up a significant fraction of the radio-loud AGN population but are generally believed to be too weak to cause any significant impact. I will present a detailed study of the conditions of cold gas in one such low-luminosity compact radio AGN B2 0258+35. Our recent NOEMA CO(1-0) emission study tracing gas in the nuclear region of this radio galaxy has shown the presence of a very turbulent circumnuclear disc interacting with the radio source as well as a fast outflow at the location of radio jets. Our findings, in combination with numerical simulations, indicate that low-power AGN activity, under favourable circumstances can inject significant disturbance in the surrounding cold gas, and also perhaps result in the formation of large scale radio structures. This thereby highlights the importance of low-luminosity radio AGN in the context of feedback.

Here we present in detail analysis of the archival and previous unpublished 40 ksec-long Chandra exposure of NGC 3894 (z=0.01075), observed by the ACIS-S detector. It is hosted by an elliptical galaxy in a non-interacting pair, again with the LINER-type nucleus, and possesses an extremely compact, mas-scale double-lobed structure with the projected extent of a few parsecs, indicative of the newly born jets (Taylor et al. 1998) where we clearly see neutral Fe Kα line at ~6.4 keV due to compression and heating of the hot diffuse component by the expanding jets/lobes; the excess X-ray emission is consistent with a halo of a hot gas characteristic for an early-type host galaxy.