Compact Steep Spectrum Sources Research Articles

Exploring the nature of the jetted hybrid AGNs: PKS 2004-447, 3C 286, and PKS 0440-00 through the SED modeling

ABSTRACT In this work, we explore the connection of three jetted $\gamma -$loud AGNs classes: Compact steep-spectrum sources (CSS), Narrow-line Seyfert 1 (NLS1), and flat-spectrum radio quasars, through the modelling of the spectral energy distribution (SED). We selected two sources identified as CSS/NLS1 hybrids, PKS 2004-440 and 3C 286. Additionally, we included the source PKS 0440-00, initially classified as an FSRQ in the first Fermi-LAT catalogue, but recently reclassified as an NLS1. We present the results of their broadband SED modelling using a one-zone leptonic synchrotron-self Compton (SSC) + external Compton (EC) model. By exploring the parameter space and investigating the disc–jet connection in these sources, we analyse their classification in a model-dependent way. Our findings reveal that modeling PKS 2004-447 at relatively large angles, as expected for CSS, results in an SSC-dominated inverse Compton emission. In contrast, at low-observing angles, the inverse Compton emission is dominated by external photon fields. Both scenarios result in a jet with a low-radiative power. For 3C 286, we found that using a one-zone model limits the jet viewing angle to $\sim 7^{\circ }$, mainly due to its impact on the $\gamma$-ray emission. Our model results show a magnetically dominated jet, consistent with $\gamma$-CSS sources. Our results suggest that PKS 0440-00, can be classified as a powerful $\gamma -$NLS1, characterized by high accretion power and a jet dominated by bulk motion, similar to FSRQs.

On the Nature of the Radio Calibrator and Gamma-Ray Emitting NLS1 Galaxy 3C 286 and Its Multiwavelength Variability

The quasar 3C 286, a well-known calibrator source in radio astronomy, was found to exhibit exceptional multiwavelength properties. Its rich and complex optical emission-line spectrum revealed its narrow-line Seyfert 1 (NLS1) nature. Given its strong radio emission, this makes 3C 286 one of the radio-loudest NLS1 galaxies known to date. 3C 286 is also one of very few known compact steep-spectrum (CSS) sources detected in the gamma-ray regime. Observations in the X-ray regime, rarely carried out so far, revealed evidence for variability, raising the question whether it is driven by the accretion disk or jet. 3C 286 is also well known for its damped Lyman alpha system from an intervening absorber at z = 0.692, triggering a search for the corresponding X-ray absorption along the line-of-sight. Here, we present new observations in the radio, X-ray, optical, and UV bands. The nature of the X-ray variability is addressed. Spectral evidence suggests that it is primarily driven by the accretion disk (not the jet), and the X-ray spectrum is well fit by a powerlaw plus soft excess model. The radio flux density and polarization remain constant at the Effelsberg telescope resolution, reconfirming the use of 3C 286 as radio calibrator. The amount of reddening/absorption along the line-of-sight intrinsic to 3C 286 is rigorously assessed. None is found, validating the derivation of a high Eddington ratio (L/LEdd ∼ 1) and of the very high radio-loudness index of 3C 286. Based on the first deep Chandra image of 3C 286, tentative evidence for hard X-ray emission from the SW radio lobe is reported. A large variety of models for the gamma-ray emission of 3C 286 are briefly discussed.

Study of γ-Ray Emission from a Compact Radio Galaxy with the Fermi Large Area Telescope

The radio galaxy PKS 1007+142 is classified as a compact steep-spectrum source (CSS) and belongs to the class of young active galactic nuclei (AGNs). In this paper, we investigate the γ-ray emission from this CSS by conducting a comprehensive analysis of the 15 yr Fermi Large Area Telescope (Fermi-LAT) observation data. The Fermi-LAT latest Source Catalog, 4FGL-DR4, includes an unassociated γ-ray source, 4FGL J1010.0+1416, located at 0.°24 away from the radio position of PKS 1007+142. Using the 15 yr Fermi-LAT observation data, we reestimate the best-fit position of the γ-ray source and find that PKS 1007+142 is in close proximity to the γ-ray source and falls within its 68% error circle. Therefore, we conclude that PKS 1007+142 is the most plausible counterpart to the unassociated LAT source with detection test statistics ∼ 43.4 (∼6.6σ). PKS 1007+142 exhibits a steep power-law spectrum in the 0.1–300 GeV band, with a photon spectral index (Γ γ ) of 2.86 ± 0.17. The average flux in the considered time interval is (2.14 ± 0.34) × 10−12 erg cm−2 s−1. Comparing PKS 1007+142 with other γ-ray emitting AGNs in both the L γ –Γ γ and L γ –L 1.4 GHz planes, it shows a softer γ-ray spectrum and lower luminosity compared to other γ-ray emitting CSSs. Furthermore, the possible origins of γ-ray in PKS 1007+142 are also discussed.

Shocks, clouds, and atomic outflows in active galactic nuclei hosting relativistic jets

Context. A number of observations have revealed atomic and/or molecular lines in active galaxies hosting jets and outflows. Line widths indicate outward motions of hundreds to a few thousands of kilometers per second. They appear to be associated with the presence of radio emission in Gigahert-peaked spectrum (GPS) and/or compact steep spectrum (CSS) sources, with linear sizes of ≤10 kpc. Numerical simulations have shown that the bow shocks triggered by relativistic jets in their host galaxies drive ionization and turbulence in the interstellar medium (ISM). However, the presence of atomic lines requires rapid recombination of ionized gas, which appears challenging to explain from the physical conditions revealed thus far based on numerical simulations of powerful jets. Aims. The aim of this paper is to provide a global framework to explain the presence of lines in terms of jet and shock evolution and to fix the parameter space where the atomic and molecular outflows might occur. Methods. This parameter space is inspired by numerical simulations and basic analytical models of jet evolution as a background. Results. Our results show that a plausible general explanation involves momentum transfer and heating to the interstellar medium gas by jet triggered shocks within the inner kiloparsecs. The presence of post-shock atomic gas is possible in the case of shocks interacting with dense clouds that remain relatively stable after the shock passage. Conclusions. According to our results, current numerical simulations cannot reproduce the physical conditions to explain the presence of atomic and molecular outflows in young radio sources. However, I show that these outflows might occur in low-power jets at all scales and I predict a trend towards powerful jets showing lines at CSS scales, when clouds have cooled to recombination temperatures.

The nature of compact radio-loud AGN: a systematic look at the LOFAR AGN population

ABSTRACT We investigate the nature of low-luminosity radio-loud active galactic nuclei (RLAGN) selected from the LOFAR Two-metre Sky Survey (LoTSS) first data release (DR1). Using optical, mid-infrared, and radio data, we have conservatively selected 55 radiative AGN candidates from DR1 within the redshift range 0.03 < z < 0.1. We show using high-frequency Karl G. Jansky Very Large Array (VLA) observations that 10 out of 55 objects show radio emission on scales >1–3 kpc, 42 are compact at the limiting resolution of 0.35 arcsec (taking an upper limit on the projected physical size, this corresponds to less than 1 kpc), and three are undetected. The extended objects display a wide range of radio morphologies: two-jet (5), one-jet (4), and double-lobed (1). We present the radio spectra of all detected radio sources which range from steep to flat/inverted and span the range seen for other compact radio sources such as compact symmetric objects (CSOs), compact steep spectrum (CSS) sources, and gigahertz peaked-spectrum (GPS) sources. Assuming synchrotron self-absorption (SSA) for flat/inverted radio spectrum sources, we predict small physical sizes for compact objects to range between 2 and 53 pc. Alternatively, using free–free absorption (FFA) models, we have estimated the free electron column depth for all compact objects, assuming a homogeneous absorber. We find that these objects do not occupy a special position on the power/linear size (P − D) diagram but some share a region with radio-quiet quasars (RQQs) and the so-called ‘FR0’ sources in terms of radio luminosity and linear size.

Revisiting the Dragonfly galaxy – I. High-resolution ALMA and VLA observations of the radio hotspots in a hyper-luminous infrared galaxy at z = 1.92

ABSTRACT Radio-loud active galactic nuclei (RLAGNs) are rare among AGN populations. Lacking high-resolution and high-frequency observations, their structure and evolution stages are not well understood at high redshifts. In this work, we report Atacama Large Millimeter/submillimeter Array 237 GHz continuum observation at 0.023 arcsec resolution and Very Large Array (VLA) 44 GHz continuum observation at 0.08 arcsec resolution of the radio continuum emission from a high-redshift radio and hyper-luminous infrared galaxy at z = 1.92. The new observations confirm the South-East (SE) and North-West (NW) hotspots identified by previous low-resolution VLA observations at 4.7 and 8.2 GHz and identify a radio core undetected in all previous observations. The SE hotspot has a higher flux density than the NW one does by a factor of 6, suggesting that there can be a Doppler boosting effect in the SE one. In this scenario, we estimate the advance speed of the jet head, ranging from ∼0.1c to 0.3c, which yields a mildly relativistic case. The projected linear distance between the two hotspots is ∼13 kpc, yielding a linear size (≤20 kpc) of a compact-steep-spectrum (CSS) source. Combined with new high-frequency (νobs ≥ 44 GHz) and archived low-frequency observations (νobs ≤ 8.2 GHz), we find that injection spectra of both NW and SE hotspots can be fitted with a continuous injection (CI) model. Based on the CI model, the synchrotron ages of NW and SE hotspots have an order of 105 yr, consistent with the order of magnitude 103–105 yr observed in CSS sources associated with radio AGNs at an early evolution stage. The CI model also favours the scenario in which the double hotspots have experienced a quiescent phase, suggesting that this RLAGN may have transient or intermittent activities.

Compact Steep Spectrum Radio Sources with Enhanced Star Formation Are Smaller Than 10 kpc

Compact steep spectrum (CSS) radio sources are active galactic nuclei (AGN) that have radio jets propagating only on galactic scales, defined as having projected linear size (LS) of up to 20 kpc. CSS sources are generally hosted by massive early-type galaxies with little ongoing star formation; however, a small fraction are known to have enhanced star formation. Using archival data from the Faint Images of the Radio Sky at Twenty cm survey, the Very Large Array Sky Survey, and the Sloan Digital Sky Survey, we identify a volume-limited sample of 166 CSS sources at z < 0.2 with L 1.4 GHz > 1024 W Hz−1. Comparing the star formation rates and linear sizes of these CSS sources, we find that the ≈14% of CSS sources with specific star formation rates above 0.01 Gyr−1 all have LS < 10 kpc. We discuss the possible mechanisms driving this result, concluding that it is likely the excess star formation in these sources occurred in multiple bursts and ceased prior to the AGN jet being triggered.

Young but fading radio sources: searching for remnants among compact steep-spectrum radio sources

ABSTRACT The incidence of young but fading radio sources provides important information on the life cycle of radio emission in radio-loud active galactic nuclei. Despite its importance for constraining the models of radio source evolution, there are no systematic studies of remnants in complete samples of young radio sources. We report results of the study of 18 compact steep-spectrum (CSS) radio sources, selected from the statistically complete B3-VLA (Very Large Array) CSS sample, characterized by a steep optically thin spectrum (α ≥ 1.0) and no core detection in earlier studies. Our deep multifrequency VLA, pc-scale Very Long Baseline Array, and enhanced Multi Element Remotely Linked Interferometer Network (e-MERLIN) observations allowed us to locate the core component in 10 objects. In three CSS sources, there is no clear evidence of present-time active regions, suggesting that they are likely in a remnant phase. Among sources with core detection, we find three objects that have no clear active regions (hotspots) at the edges of the radio structure, suggesting that the radio emission may have just restarted. Our results support a power-law distribution of the source ages, although the poor statistics prevents us from setting solid constraints on the percentage of remnants and restarted sources in subpopulations of radio sources.

Lunar far-side radio arrays: a preliminary site survey

Abstract The origin and evolution of structure in the Universe could be studied in the Dark Ages. The highly redshifted H i signal between 30 &amp;lt; z &amp;lt; 80 is the only observable signal from this era. Human radio interference and ionospheric effects limit Earth-based radio astronomy to frequencies &amp;gt;30 MHz. To observe the low-frequency window with research from compact steep spectrum sources, pulsars, and solar activity, a 200 km baseline lunar far-side radio interferometer has been much discussed. This paper conducts a preliminary site survey of potential far-side craters, which are few in number on the mountainous lunar far-side. Based on LRO LOLA data, 200 m resolution topographic maps of eight far-side sites were produced, and slope and roughness maps were derived from them. A figure of merit was created to determine the optimum site. Three sites are identified as promising. There is a need to protect these sites for astronomy.

Extragalactic peaked-spectrum radio sources at low frequencies are young radio galaxies

We present a sample of 373 peaked-spectrum (PS) sources with spectral peaks around 150 MHz, selected using a subset of the two LOw Frequency ARray (LOFAR) all-sky surveys, the LOFAR Two Meter Sky Survey and the LOFAR LBA Sky Survey. These LOFAR surveys are the most sensitive low-frequency widefield surveys to date, allowing us to select low-luminosity peaked-spectrum sources. Our sample increases the number of known PS sources in our survey area by a factor 50. The 5 GHz luminosity distribution of our PS sample shows we sample the lowest luminosity PS sources to date by nearly an order of magnitude. Since high-frequency gigahertz-peaked spectrum sources and compact steep-spectrum sources are hypothesised to be the precursors to large radio galaxies, we investigate whether this is also the case for our sample of low-frequency PS sources. Using optical line emission criteria, we find that our PS sources are predominately high-excitation radio galaxies instead of low-excitation radio galaxies, corresponding to a quickly evolving population. We compute the radio source counts of our PS sample, and find they are scaled down by a factor of ≈40 compared to a general sample of radio-loud active galactic nuclei (AGN). This implies that the lifetimes of PS sources are 40 times shorter than large-scale radio galaxies if their luminosity functions are identical. To investigate this, we compute the first radio luminosity function for a homogeneously selected PS sample. We find that for 144 MHz luminosities ≳1025 W Hz−1, the PS luminosity function has the same shape as an unresolved radio-loud AGN population, but shifted down by a factor of ≈-pagination10. We interpret this as strong evidence that these high-luminosity PS sources evolve into large-scale radio-loud AGN. For local low-luminosity PS sources, there is a surplus of PS sources, which we hypothesise to be the addition of frustrated PS sources that do not evolve into large-scale AGN.

The Energetics of the Central Engine in the Powerful Quasar 3C 298

Abstract The compact steep-spectrum radio source 3C 298 (redshift of 1.44) has the largest 178 MHz luminosity in the Third Cambridge Revised Catalogue (3CR); its radio lobes are among the most luminous in the universe. The plasma state of the radio lobes is modeled with the aid of interferometric radio observations (in particular, the new Low Frequency Array observation and archival MERLIN data) and archival single-station data. It is estimated that the long-term time-averaged jet power required to fill these lobes with leptonic plasma is Q ¯ ≈ 1.28 ± 0.51 × 10 47 erg s − 1 , rivaling the largest time-averaged jet powers from any quasar. Supporting this notion of extraordinary jet power is a 0.5–10 keV luminosity of ≈ 5.2 × 1046 erg s−1, comparable to luminous blazars, yet there is no other indication of strong relativistic beaming. We combine two new high signal-to-noise ratio optical spectroscopic observations from the Hobby-Eberly Telescope with archival Hubble Space Telescope, Two Micron All Sky Survey, and Galaxy Evolutionary Explorer data to compute a bolometric luminosity from the accretion flow of L bol ≈ 1.55 ± 0.15 × 1047 erg s−1. The ratio, Q ¯ / L bol ≈ 1 , is the approximate upper limit for quasars. Characteristic of a large Q ¯ / L bol , we find an extreme-ultraviolet (EUV) spectrum that is very steep (the “EUV deficit” of powerful radio quasars relative to radio-quiet quasars), and this weak ionizing continuum is likely a contributing factor to the relatively small equivalent widths of the broad emission lines in this quasar.

GeV γ-Ray Emission of Compact Steep-spectrum Source 4C +39.23B

Abstract Thirteen yr observation data of 4FGL J0824.9+3915 with the Large Area Telescope on board the Fermi Gamma Ray Space Telescope (Fermi/LAT) are analyzed for revisiting whether 4C +39.23B, a compact steep-spectrum (CSS) source close to a flat-spectrum radio quasar (FSRQ) 4C +39.23A in the γ-ray emitting region of 4FGL J0824.9+3915, is a γ-ray emitter. We find that the time-integrated γ-ray emission of 4FGL J0824.9+3915 is overwhelmingly dominated by 4C +39.23A. It shows significant variability at a 6.7σ confidence level and the average γ-ray flux in the 0.1–300 GeV energy band is (1.60 ± 0.15) × 10−8 ph cm−2 s−1 with a power-law photon spectral index of 2.48 ± 0.05. During MJD 57500–58500, 4FGL J0824.9+3915 is in a low state with a steady γ-ray flux. Through the analysis of the Fermi/LAT observation data in this time interval, it is found that the test statistic values of the γ-ray emission from 4C +39.23A and 4C +39.23B are ∼5 and ∼31, respectively, indicating that the γ-ray emission in this time interval is dominated by CSS 4C +39.23B. The derived average flux in this time interval for 4C +39.23B is (9.40 ± 4.10) × 10−9 ph cm −2 s−1 with Γ γ = 2.45 ± 0.17. Attributing the spectral energy distribution (SED) of 4C +39.23B to the radiations from its core and extended region, we show that the SED can be represented with a two-zone leptonic model. Its γ-ray emission is contributed to by the core region. The derived magnetic field strength and Doppler boosting factor of the core are 0.13 G and 6.5. Comparing 4C +39.23B with other γ-emitting CSSs and compact symmetric objects (CSOs) in the Γ γ − L γ plane, it resembles CSSs.

Spectral analysis of spatially resolved 3C295 (sub-arcsecond resolution) with the International LOFAR Telescope

3C295 is a bright, compact steep spectrum source with a well-studied integrated radio spectral energy distribution (SED) from 132 MHz to 15 GHz. However, spatially resolved spectral studies have been limited due to a lack of high resolution images at low radio frequencies. These frequencies are crucial for measuring absorption processes, and anchoring the overall spectral modelling of the radio SED. In this paper, we use International LOw-Frequency ARray (LOFAR) Telescope (ILT) observations of 3C295 to study its spatially resolved spectral properties with sub-arcsecond resolution at 132 MHz. Combining our new 132 MHz observation with archival data at 1.6, 4.8, and 15 GHz, we are able to carry out a resolved radio spectral analysis. The spectral properties of the hotspots provides evidence for low frequency flattening. In contrast, the spectral shape across the lobes is consistent with a Jaffe-Perola spectral ageing model. Using the integrated spectral information for each component, we then fitted low-frequency absorption models to the hotspots, finding that both free-free absorption and synchrotron self-absorption models provide a better fit to the data than a standard power law. Although we can say there is low-frequency absorption present in the hotspots of 3C295, future observations with the Low Band Antenna of the ILT at 55 MHz may allow us to distinguish the type of absorption.

Unravelling the origin of extended radio emission in narrow-line Seyfert 1 galaxies with the JVLA

Narrow-line Seyfert 1 galaxies (NLS1s) are believed to be active galactic nuclei (AGN) in the early stages of their evolution. Several dozen of them have been found to host relativistic jets, whilst the majority of NLS1s have not even been detected in radio, emphasising the heterogeneity of the class in this band. In this paper, our aim is to determine the predominant source of radio emission in a sample of 44 NLS1s, selected based on their extended kiloparsec-scale radio morphologies at 5.2 GHz. We accomplish this by analysing their spatially resolved radio spectral index maps, centred at 5.2 GHz, as the spectral index carries information about the production mechanisms of the emission. In addition, we utilise several diagnostics based on mid-infrared emission to estimate the star formation activity of their host galaxies. These data are complemented by archival data to draw a more complete picture of each source. We find an extraordinary diversity among our sample. Approximately equal fractions (∼10–12 sources) of our sources can be identified as AGN-dominated, composite, and host-dominated. Among the AGN-dominated sources are a few NLS1s with very extended jets, reaching distances of tens of kiloparsecs from the nucleus. One of these, J0814+5609, hosts the most extended jets found in an NLS1 so far. We also identify five NLS1s that could be classified as compact steep-spectrum sources. In addition, one source shows a possible kiloparsec-scale relic that reaches well outside the host galaxy as well as restarted nuclear activity, and one could belong to the sub-class of NLS1s that host relativistic jets that seem to be absorbed at lower radio frequencies (&lt; 10 GHz). We further conclude that, due to the variety seen in NLS1s, simple proxies, such as the star formation diagnostics also employed in this paper and the radio loudness parameter, are not ideal tools for characterising NLS1s. We emphasise the necessity of examining NLS1s as individuals instead of making assumptions based on their classification. When these issues are properly taken into account, NLS1s offer an exceptional environment for studying the interplay between the host galaxy and several AGN-related phenomena, such as jets and outflows.

Extragalactic Radio Quasars: Consequences of the Luminosity/ Redshift Relationship

We have used both analytical and statistical methods to show some plausible consequences of the luminosity/redshift relationships for compact steep spectrum (CSS) quasars and the more extended extragalactic radio (EGR) quasars. From the analytical methods (or theory), we find that luminosity shows an inverse dependence on the redshift; while from the statistical (empirical) the converse is the case for both CSS quasars and their extended counterparts. We know that luminosity selection effects may play some role in the empirical results. However, we find that the magnitudes of departure of the two empirical results from the theoretical result are staggering – the difference for the extended EGR quasars far outweighs that of the CSS quasars. We may state categorically that this discrepancy may simply be a sign of a factor in the intergalactic medium (IGM) that appears to cause the anomaly in the extended EGR quasars. This is because sources with similar powers are expected to produce similar luminosities; and besides, central engines of the more extended EGR quasars have roughly similar powers with the CSS central engines. However, our results show great difference between their radiated powers (luminosities). A likely explanation of this is that the irregularity may be a signature of dark/vacuum energy. It simply indicates that among other factors, dark energy may constitute a factor which influences luminosities of the more extended EGR quasars. The components (jets and lobes) of the more extended EGR quasars lie in the intergalactic medium (IGM). This medium is more of a vacuum than the interstellar medium (ISM) in which the components of the CSS sources are located. Therefore, dark energy is expected to show more effect in the IGM than in the ISM. Besides, we estimate percentage influence caused by luminosity selection effect and that caused by the presumed dark/vacuum energy. The estimates are respectively, and . These results show that intrinsically, dark/vacuum energy influences extended EGR quasars’ luminosities by about times more than the observational bias caused by luminosity selection effects.

The gaseous natal environments of GPS and CSS sources with ASKAP–FLASH

AbstractGHz‐Peaked Spectrum (GPS) and compact steep spectrum (CSS) sources are thought to represent a young and/or confined sub‐population of radio‐loud active galactic nuclei (AGN) that are yet to evacuate their surrounding ambient interstellar gas. By studying the gaseous environments of these objects, we can gain an insight into the inter‐dependent relationship between galaxies and their supermassive black holes (SMBHs). The First Large Absorption Survey in Hi (FLASH) will build a census of the neutral atomic hydrogen (Hi) gas in galaxies at intermediate cosmological redshifts. FLASH is expected to detect at least several hundred Hi absorbers associated with GPS and CSS sources. These absorbers provide an important probe of the abundance and kinematics of line‐of‐sight neutral gas toward radio AGN, in some cases revealing gas associated with infalling clouds and outflows. Observations are now complete for the first phase of the FLASH Pilot Survey and early analysis has already yielded several detections, including the GPS source PKS 2311‐477. Optical imaging of this galaxy reveals an interacting system that could have supplied the neutral gas seen in absorption and triggered the radio‐loud AGN. FLASH will provide a statistically significant sample with which the prevalence of such gas‐rich interactions among compact radio galaxies can be investigated.

The peaked spectrum and compact steep spectrum sources: Issues and opportunities

AbstractThe peaked spectrum (PS) radio sources are sub‐kpc in size and have a peak in the radio spectrum. The compact steep specrum (CSS) sources range from about 1 kpc to an adopted upper limit on projected linear size of 20 kpc. Because of their galactic‐scale range of size, the PS and CSS radio sources interact strongly with the ISM of this host galaxy. This means that the PS and CSS sources can be a significant source of the AGN feedback. The PS and CSS sources are a key phase in radio source evolution. Yet, there is substantial uncertainty regarding the origin of these sources, their lifetimes, and their evolution. There are many interesting open questions regarding the PS and CSS radio sources. Here, in this short introduction to the topic, I focus on the origin stories for these sources and how progress might be made.

Unmasking the history of 3C 293 with LOFAR sub‐arcsecond imaging

AbstractActive galactic nuclei show episodic activity, which can be evident in galaxies that exhibit restarted radio jets. These restarted jets can interact with their environment, leaving signatures in the radio spectral energy distribution. Tracing these signatures requires resolved spectral index measurements over a broad frequency range. We present such a study for the radio galaxy 3C 293. Using the International Low Frequency Array Telescope (ILT), we probe spatial scales as fine as ∼0.2″ at 144 MHz and to constrain the spectrum, we combine these data with Multi‐Element Radio Linked Interferometer Network and Very Large Array archival data at frequencies up to 8.4 GHz. In the inner lobes (∼2 kpc), we detect the presence of a compact steep spectrum (CSS) source with a spectral turnover at ∼225 MHz, caused by free‐free absorption from the rich surrounding medium. We find evidence of interaction of the lobes with the rich interstellar medium of the host galaxy. The outer lobes (extending up to ∼100 kpc) have a remarkably uniform spatial distribution of spectral index and only mild spectral curvature (of ∼0.2). Overall, it appears that 3C 293 has gone through multiple (two to three) epochs of activity. This study adds 3C 293 to the new sub‐group of restarted galaxies with short interruption time periods (≲1 Myr). 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.

Jet‐triggered star formation in young radio galaxies

AbstractEmission in the ultraviolet continuum is a salient signature of the hot, massive, and consequently short‐lived, stellar population that traces recent or ongoing star formation. With the aim of mapping star forming regions and morphologically separating the generic star formation from that associated with the galaxy‐scale jet activity, we obtained high‐resolution ultraviolet (UV) imaging from the Hubble Space Telescope for a sample of nine compact radio sources. Out of these, seven are known Compact Steep Spectrum (CSS) galaxies that host young, kiloparsec‐scale radio sources and hence are the best candidates for studying radio‐mode feedback on galaxy scales, while the other two form a control sample of larger sources. Extended UV emission regions are observed in six of the seven CSS sources showing close spatial alignment with the radio‐jet orientation. If other mechanisms possibly contributing to the observed UV emission are ruled out, this could be evidence in support of jet‐triggered star formation in the CSS phase of radio galaxy evolution and in turn of the “positive feedback” paradigm of host–active galactic nuclei interaction.

1321+045: A compact steep‐spectrum radio source in a cool‐core galaxy cluster

AbstractCluster‐central gigahertz peak and compact steep‐spectrum (CSS) sources offer an opportunity to study the earliest phases of active galactic nucleus (AGN) feedback, but few have yet been examined in detail. We present results from radio and X‐ray observations of 1321+045, a CSS source in a 4.4 keV cluster at . The cluster has a strongly cooling core, and disturbances from a minor cluster merger may have triggered a period of jet activity which formed the 16 kpc radio lobes ago. However, new very long baseline array (VLBA) imaging shows a ∼20 pc jet on a different projected axis, which is probably only a few hundred years old. We consider possible histories for the system, with either one or two periods of jet activity. While this single system is informative, a broader study of the youngest cluster‐central radio sources is desirable.