Low-excitation Radio Galaxies Research Articles

The LOFAR Two-metre Sky Survey: The nature of the faint source population and SFR–radio luminosity relation using prospector

ABSTRACT Spectral energy distribution (SED) fitting has been extensively used to determine the nature of the faint radio source population. Recent efforts have combined fits from multiple SED-fitting codes to account for the host galaxy and any active nucleus that may be present. We show that it is possible to produce similar-quality classifications using a single energy-balance SED fitting code, prospector, to model up to 26 bands of UV–far-infrared aperture-matched photometry for ∼31 000 sources in the ELAIS-N1 field from the LOFAR Two-Metre Sky Survey (LoTSS) deep fields first data release. One of a new generation of SED-fitting codes, prospector accounts for potential contributions from radiative active galactic nuclei (AGN) when estimating galaxy properties, including star formation rates (SFRs) derived using non-parametric star formation histories. Combining this information with radio luminosities, we classify 92 per cent of the radio sources as a star-forming galaxy, high-/low-excitation radio galaxy, or radio-quiet AGN and study the population demographics as a function of 150 MHz flux density, luminosity, SFR, stellar mass, redshift, and apparent r-band magnitude. Finally, we use prospector SED fits to investigate the SFR–150 MHz luminosity relation for a sample of ∼133 000 3.6 μm-selected z < 1 sources, finding that the stellar mass dependence is significantly weaker than previously reported, and may disappear altogether at $\log _{10} (\mathrm{SFR}/M_\odot \, \mathrm{yr}^{-1}) \gt 0.5$. This approach makes it significantly easier to classify radio sources from LoTSS and elsewhere, and may have important implications for future studies of star-forming galaxies at radio wavelengths.

A Machine Learning Made Catalog of FR-II Radio Galaxies from the FIRST Survey

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 × 1022 ≤ L rad ≤ 6.76 × 1029 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 107.5 ≲ M BH ≲ 109.5 M ⊙. 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 AGN fuelling/feedback cycle in nearby radio galaxies

We present Australia Compact Telescope Array (ATCA) 21 cm observations of the nearby low-excitation radio galaxy (LERG) NGC 3100. This is the brightest galaxy of a loose group and it hosts a young (∼2 Myr) radio source. The ATCA observations reveal for the first time the presence of neutral hydrogen (H I) gas in absorption in the centre of this radio galaxy, and in emission in two low-mass galaxies of the group and in a diffuse dark cloud in the proximity of NGC 3100. The sensitivity to low-column density gas (NHI ∼ 1019 cm−2) allows us to reveal asymmetries in the periphery of most of the H I-detected galaxies, suggesting that tidal interactions may be ongoing. The diffuse cloud does not show a stellar counterpart down to 27 mag arcsec−2 and could be the remnant of these interactions. The analysis of the H I absorption complex in NGC 3100 indicates that the atomic phase of the hydrogen is distributed in the same way as its molecular phase (observed at arcsecond resolution through several carbon monoxide emission lines). We suggest that the interactions occurring within the group are causing turbulent cold gas clouds in the intra-group medium to be slowly accreted towards the centre of NGC 3100. This caused the recent formation of the cold circum-nuclear disc, which is likely sustaining the young nuclear activity.

Cosmic evolution of radio-AGN feedback: confronting models with data

ABSTRACT Radio-mode feedback is a key ingredient in galaxy formation and evolution models, required to reproduce the observed properties of massive galaxies in the local Universe. We study the cosmic evolution of radio-active galactic nuclei (AGN) feedback out to z ∼ 2.5 using a sample of 9485 radio-excess AGN. We combine the evolving radio luminosity functions with a radio luminosity scaling relationship to estimate AGN jet kinetic powers and derive the cosmic evolution of the kinetic luminosity density, Ωkin (i.e. the volume-averaged heating output). Compared to all radio-AGN, low-excitation radio galaxies dominate the feedback activity out to z ∼ 2.5, with both these populations showing a constant heating output of $\Omega _{\rm {kin}} \approx (4\!-\!5) \times 10^{32}\, \rm {W\, Mpc^{-3}}$ across 0.5 &amp;lt; z &amp;lt; 2.5. We compare our observations to predictions from semi-analytical and hydrodynamical simulations, which broadly match the observed evolution in Ωkin, although their absolute normalization varies. Comparison to the Semi-Analytic Galaxy Evolution (sage) model suggests that radio-AGN may provide sufficient heating to offset radiative cooling losses, providing evidence for a self-regulated AGN feedback cycle. We integrate the kinetic luminosity density across cosmic time to obtain the kinetic energy density output from AGN jets throughout cosmic history to be $\sim 10^{50}\, \rm {J\, Mpc^{-3}}$. Compared to AGN winds, the kinetic energy density from AGN jets dominates the energy budget at z ≲ 2; this suggests that AGN jets play an important role in AGN feedback across most of cosmic history.

AGNs in post-mergers from the ultraviolet near infrared optical northern survey

ABSTRACT The kinematic disturbances associated with major galaxy mergers are known to produce gas inflows, which in turn may trigger accretion onto the supermassive black holes (SMBH) of the participant galaxies. While this effect has been studied in galaxy pairs, the frequency of active galactic nuclei (AGNs) in fully coalesced post-merger systems is poorly constrained due to the limited size or impurity of extant post-merger samples. Previously, we combined convolutional neural network (CNN) predictions with visual classifications to identify a highly pure sample of 699 post-mergers in deep r-band imaging. In the work presented here, we quantify the frequency of AGNs in this sample using three metrics: optical emission lines, mid-infrared (mid-IR) colour, and radio detection of low-excitation radio galaxies (LERGs). We also compare the frequency of AGNs in post-mergers to that in a sample of spectroscopically identified galaxy pairs. We find that AGNs identified by narrow-line optical emission and mid-IR colour have an increased incidence rate in post-mergers, with excesses of ~4 over mass- and redshift-matched controls. The optical and mid-IR AGN excesses in post-mergers exceed the values found for galaxy pairs, indicating that AGN activity in mergers peaks after coalescence. Conversely, we recover no significant excess of LERGs in post-mergers or pairs. Finally, we find that the [O iii] luminosity (a proxy for SMBH accretion rate) in post-mergers that host an optical AGN is ~0.3 dex higher on average than in non-interacting galaxies with an optical AGN, suggesting that mergers generate higher accretion rates than secular triggering mechanisms.

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 environments of the radio galaxy population in simba

ABSTRACT We investigate the environmental properties of the z = 0 radio galaxy population using the simba cosmological hydrodynamic simulation. We identify centrals and satellites from a population of high and low excitation radio galaxies (HERGs and LERGs) in simba, and study their global properties. We find that $\sim 20{{\ \rm per\ cent}}$ of radio galaxies are satellites, and that there are insignificant differences in the global properties of LERGs based on their central/satellite classification. HERG satellites display lower values of star formation, 1.4 GHz radio luminosity, and Eddington fractions than HERG centrals. We further investigate the environments of radio galaxies and show that HERGs typically live in less dense environments, similar to star-forming galaxies. The environments of high-mass LERGs are similar to non-radio galaxies, but low-mass LERGs live in underdense environments similar to HERGs. LERGs with overmassive black holes reside in the most dense environments, while HERGs with overmassive black holes reside in underdense environments. The richness of a LERG’s environment decreases with increasing Eddington fraction, and the environments of all radio galaxies do not depend on radio luminosity for $\, P_{\rm 1.4~GHz}\lt 10^{24} \rm {~W~Hz}^{-1}$. Complementing these results, we find that LERGs cluster on the same scale as the total galaxy population, while multiple HERGs are not found within the same dark matter halo. Finally, we show that high density environments support the growth of HERGs rather than LERGs at z = 2. Simba predicts that with more sensitive surveys, we will find populations of radio galaxies in environments much similar to the total galaxy population.

MIGHTEE: the nature of the radio-loud AGN population

ABSTRACT We study the nature of the faint radio source population detected in the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) Early Science data in the COSMOS field, focusing on the properties of the radio-loud active galactic nuclei (AGNs). Using the extensive multiwavelength data available in the field, we are able to classify 88 per cent of the 5223 radio sources in the field with host galaxy identifications as AGNs (35 per cent) or star-forming galaxies (54 per cent). We select a sample of radio-loud AGNs with redshifts out to z ∼ 6 and radio luminosities 1020 &amp;lt; L1.4 GHz/W Hz−1 &amp;lt; 1027 and classify them as high-excitation and low-excitation radio galaxies (HERGs and LERGs). The classification catalogue is released with this work. We find no significant difference in the host galaxy properties of the HERGs and LERGs in our sample. In contrast to previous work, we find that the HERGs and LERGs have very similar Eddington-scaled accretion rates; in particular we identify a population of very slowly accreting AGNs that are formally classified as HERGs at these low radio luminosities, where separating into HERGs and LERGs possibly becomes redundant. We investigate how black hole mass affects jet power, and find that a black hole mass ≳ 107.8 M⊙ is required to power a jet with mechanical power greater than the radiative luminosity of the AGN (Lmech/Lbol &amp;gt; 1). We discuss that both a high black hole mass and black hole spin may be necessary to launch and sustain a dominant radio jet.

SDSS-IV MaNGA: Identification and multiwavelength properties of Type-1 AGN in the DR15 sample

ABSTRACT We present a method to identify type-1 active galactic nuclei (AGNs) in the central 3 arcsec integrated spectra of galaxies in the MaNGA DR15 sample. It is based on flux ratios estimates in spectral bands flanking the expected h α broad component h αBC. The high signal-to-noise ratio obtained (mean S/N = 84) permits the identification of h αBC without prior subtraction of the host galaxy (HG) stellar component. A final sample of 47 type-1 AGN is reported out of 4700 galaxies at z &amp;lt; 0.15. The results were compared with those from other methods based on the SDSS DR7 and MaNGA data. Detection of type-1 AGN in those works compared to our method goes from 26 per cent to 81 per cent. Spectral indexes were used to classify the type-1 AGN spectra according to different levels of AGN-HG contribution, finding 9 AGN-dominated, 14 intermediate, and 24 HG-dominated objects. Complementary data in NIR-MIR allowed us to identify type I AGN-dominated objects as blue and HG-dominated as red in the WISE colours. From NVSS and FIRST radio continuum data, we identify 5 HERGs (high-excitation radio galaxies) and 4 LERGs (low-excitation radio galaxies), three showing evidence of radio-jets in the FIRST maps. Additional X-ray data from ROSAT allowed us to build [O iii] and h αBC versus X-ray, NIR-MIR, and radio continuum diagrams, showing that L(h αBC) and L([O iii]) provide good correlations. The range in h αBC luminosity is wide 38 &amp;lt; logL(h αBC) &amp;lt; 44, with log FWHM(h αBC) ∼ 3–4, covering a range of Eddington ratios of −5.15 &amp;lt; log Lbol/LEdd &amp;lt; 0.70. Finally, we also identify and report ten possible changing-look AGN candidates.

Do radio active galactic nuclei reflect X-ray binary spectral states?

Context.Over recent years there has been mounting evidence that accreting supermassive black holes in active galactic nuclei (AGNs) and stellar mass black holes have similar observational signatures: thermal emission from the accretion disk, X-ray coronas, and relativistic jets. Further, there have been investigations into whether or not AGNs have spectral states similar to those of X-ray binaries (XRBs) and what parallels can be drawn between the two using a hardness-intensity diagram (HID).Aims.To address whether AGN jets might be related to accretion states as in XRBs, we explore whether populations of radio AGNs classified according to their (a) radio jet morphology, Fanaroff-Riley classes I and II (FR I and II), (b) excitation class, high- and low-excitation radio galaxies (HERG and LERG), and (c) radio jet linear extent, compact to giant, occupy different and distinct regions of the AGN HID (total luminosity vs. hardness).Methods.We do this by cross-correlating 15 catalogs of radio galaxies with the desired characteristics from the literature withXMM-NewtonandSwiftX-ray and ultraviolet (UV) source catalogs. We calculate the luminosity and hardness from the X-ray and UV photometry, place the sources on the AGN HID, and search for separation of populations and analogies with the XRB spectral state HID.Results.We find that (a) FR Is and IIs, (b) HERGs and LERGs, and (c) FR I-LERGs and FR II-HERGs occupy distinct areas of the HID at a statistically significant level (p-value &lt; 0.05), and we find no clear evidence for population distinction between the different radio jet linear extents. The separation between FR I-LERG and FR II-HERG populations is the strongest in this work.Conclusions.Our results indicate that radio-loud AGNs occupy distinct areas of the HID depending on the morphology and excitation class, showing strong similarities to XRBs.

Cosmic evolution of low-excitation radio galaxies in the LOFAR two-metre sky survey deep fields

ABSTRACT Feedback from low-excitation radio galaxies (LERGs) plays a key role in the lifecycle of massive galaxies in the local Universe; their evolution, and the impact of these active galactic nuclei on early galaxy evolution, however, remain poorly understood. We use a sample of 10 481 LERGs from the first data release of the LOFAR two-metre Sky Survey Deep Fields, covering ∼25 deg2, to present the first measurement of the evolution of the radio luminosity function (LF) of LERGs out to z ∼ 2.5; this shows relatively mild evolution. We split the LERGs into those hosted by quiescent and star-forming galaxies, finding a new dominant population of LERGs hosted by star-forming galaxies at high redshifts. The incidence of LERGs in quiescent galaxies shows a steep dependence on stellar mass out to z ∼ 1.5, consistent with local Universe measurements of accretion occurring from cooling of hot gas haloes. The quiescent-LERGs dominate the LFs at z &amp;lt; 1, showing a strong decline in space density with redshift, tracing that of the available host galaxies, while there is an increase in the characteristic luminosity. The star-forming LERG LF increases with redshift, such that this population dominates the space densities at most radio-luminosities by z ∼ 1. The incidence of LERGs in star-forming galaxies shows a much weaker stellar-mass dependence, and increases with redshift, suggesting a different fuelling mechanism compared to their quiescent counterparts, potentially associated with the cold gas supply present in the star-forming galaxies.

The discovery of a radio galaxy of at least 5 Mpc

Context. Giant radio galaxies (GRGs, or colloquially ‘giants’) are the Universe’s largest structures generated by individual galaxies. They comprise synchrotron-radiating active galactic nucleus ejecta and attain cosmological (megaparsec-scale) lengths. However, the main mechanisms that drive their exceptional growth remain poorly understood. Aims. To deduce the main mechanisms that drive a phenomenon, it is usually instructive to study extreme examples. If there exist host galaxy characteristics that are an important cause for GRG growth, then the hosts of the largest GRGs are likely to possess them. Similarly, if there exist particular large-scale environments that are highly conducive to GRG growth, then the largest GRGs are likely to reside in them. For these reasons, we aim to perform a case study of the largest GRG available. Methods. We reprocessed the LOFAR Two-Metre Sky Survey DR2 by subtracting compact sources and performing multi-scale CLEAN de-convolutions at 60″ and 90″ resolution. The resulting images constitute the most sensitive survey yet for radio galaxy lobes, whose diffuse nature and steep synchrotron spectra have allowed them to evade previous detection attempts at higher resolution and shorter wavelengths. We visually searched these images for GRGs. Results. We have discovered Alcyoneus, a low-excitation radio galaxy with a projected proper length lp = 4.99 ± 0.04 Mpc. Both its jets and lobes are detected at very high significance, and the SDSS-based identification of the host, at spectroscopic redshift zspec = 0.24674 ± 6 × 10−5, is unambiguous. The total luminosity density at ν = 144 MHz is Lν = 8 ± 1 × 1025 W Hz−1, which is below average, though near median (percentile 45 ± 3%) for GRGs. The host is an elliptical galaxy with a stellar mass M⋆ = 2.4 ± 0.4 × 1011 M⊙ and a super-massive black hole mass M• = 4 ± 2 × 108 M⊙, both of which tend towards the lower end of their respective GRG distributions (percentiles 25 ± 9% and 23 ± 11%). The host resides in a filament of the Cosmic Web. Through a new Bayesian model for radio galaxy lobes in three dimensions, we estimate the pressures in the megaparsec-cubed-scale northern and southern lobes to be Pmin, 1 = 4.8 ± 0.3 × 10−16 Pa and Pmin, 2 = 4.9 ± 0.6 × 10−16 Pa, respectively. The corresponding magnetic field strengths are Bmin, 1 = 46 ± 1 pT and Bmin, 2 = 46 ± 3 pT. Conclusions. We have discovered what is in projection the largest known structure made by a single galaxy – a GRG with a projected proper length lp = 4.99 ± 0.04 Mpc. The true proper length is at least lmin = 5.04 ± 0.05 Mpc. Beyond geometry, Alcyoneus and its host are suspiciously ordinary: the total low-frequency luminosity density, stellar mass, and super-massive black hole mass are all lower than, though similar to, those of the medial GRG. Thus, very massive galaxies or central black holes are not necessary to grow large giants, and, if the observed state is representative of the source over its lifetime, neither is high radio power. A low-density environment remains a possible explanation. The source resides in a filament of the Cosmic Web, with which it might have significant thermodynamic interaction. The pressures in the lobes are the lowest hitherto found, and Alcyoneus therefore represents the most promising radio galaxy yet to probe the warm–hot inter-galactic medium.

The AGN fuelling/feedback cycle in nearby radio galaxies – IV. Molecular gas conditions and jet–ISM interaction in NGC 3100

ABSTRACT This is the fourth paper of a series investigating the AGN fuelling/feedback processes in a sample of 11 nearby low-excitation radio galaxies (LERGs). In this paper, we present follow-up Atacama Large Millimeter/submillimeter Array (ALMA) observations of one source, NGC 3100, targeting the 12CO(1-0), 12CO(3-2), HCO+(4-3), SiO(3-2), and HNCO(6-5) molecular transitions. 12CO(1-0) and 12CO(3-2) lines are nicely detected and complement our previous 12CO(2-1) data. By comparing the relative strength of these three CO transitions, we find extreme gas excitation conditions (i.e. Tex ≳ 50 K) in regions that are spatially correlated with the radio lobes, supporting the case for a jet–ISM interaction. An accurate study of the CO kinematics demonstrates that although the bulk of the gas is regularly rotating, two distinct non-rotational kinematic components can be identified in the inner gas regions: one can be associated to inflow/outflow streaming motions induced by a two-armed spiral perturbation; the second one is consistent with a jet-induced outflow with vmax ≈ 200 km s−1 and $\dot{M}\lesssim 0.12$ M⊙ yr−1. These values indicate that the jet-CO coupling ongoing in NGC 3100 is only mildly affecting the gas kinematics, as opposed to what expected from existing simulations and other observational studies of (sub-)kpc scale jet–cold gas interactions. HCO+(4-3) emission is tentatively detected in a small area adjacent to the base of the northern radio lobe, possibly tracing a region of jet-induced gas compression. The SiO(3-2) and HNCO(6-5) shock tracers are undetected: this – along with the tentative HCO+(4-3) detection – may be consistent with a deficiency of very dense (i.e. ncrit &amp;gt; 106 cm−3) cold gas in the central regions of NGC 3100.

Characterising the Extended Morphologies of BL Lacertae Objects at 144 MHz with LOFAR

Abstract We present a morphological and spectral study of a sample of 99 BL Lac objects using the LOFAR Two-Metre Sky Survey Second Data Release (LDR2). Extended emission has been identified at gigahertz frequencies around BL Lac objects, but with LDR2 it is now possible to systematically study their morphologies at 144 MHz, where more diffuse emission is expected. LDR2 reveals the presence of extended radio structures around 66/99 of the BL Lac nuclei, with angular extents ranging up to 115″, corresponding to spatial extents of 410 kpc. The extended emission is likely to be both unbeamed diffuse emission and beamed emission associated with relativistic bulk motion in jets. The spatial extents and luminosities of the extended emission are consistent with the unification scheme for active galactic nuclei, where BL Lac objects correspond to low-excitation radio galaxies with the jet axis aligned along the line of sight. While extended emission is detected around the majority of BL Lac objects, the median 144–1400 MHz spectral index and core dominance at 144 MHz indicate that the core component contributes ∼42% on average to the total low-frequency flux density. A stronger correlation was found between the 144 MHz core flux density and the γ-ray photon flux (r = 0.69) than between the 144 MHz extended flux density and the γ-ray photon flux (r = 0.42). This suggests that the radio-to-γ-ray connection weakens at low radio frequencies because the population of particles that give rise to the γ-ray flux are distinct from the electrons producing the diffuse synchrotron emission associated with spatially extended features.

Ionized Gas Outflows in Low-excitation Radio Galaxies Are Radiation Driven

Abstract Low-excitation radio galaxies (LERGs) are weakly accreting active galactic nuclei (AGNs) believed to be fueled by radiatively inefficient accretion processes. Despite this, recent works have shown evidence for ionized and neutral hydrogen gas outflows in these galaxies. To investigate the potential drivers of such outflows, we select a sample of 802 LERGs using the Best &amp; Heckman (2012) catalog of radio galaxies. By modeling the [O iii] λ5007 profile in Sloan Digital Sky Survey spectra of a sample of 802 LERGs, we determine that the ionized outflows are present in ∼1.5% of the population. Using 1.4 GHz imaging from the Faint Images of the Radio Sky at Twenty Centimeters survey we analyze the radio morphology of LERGs with outflows and find these to be consistent with the parent LERG population. However, we note that unlike the majority of the LERG population, those LERGs showing outflows have Eddington-scaled accretion rates close to 1%. This is indicative that ionized outflows in LERGs are driven by the radiation pressure from the accretion disk of the AGN rather than the radio jets. We report specific star formation rates in the range of 10−12 &lt; sSFR &lt; 10−9 yr−1. Moreover, we observe higher mass outflow rates, 7–150 M ☉ yr−1, for these LERGs than luminous quasars for a given bolometric luminosity, which could possibly be due to the radio source in LERGs boosting the mass loading. This scenario could indicate that these outflows could potentially drive feedback in LERGs.

Multi-transonic spherical-type flows around massive black holes: galactic potential induced standing isothermal shocks

Accretion on length-scales of interstellar or even intergalactic is particularly relevant, in the context of spherical or quasi-spherical hot mode accretion, supposedly powering low excitation radio galaxies in the maintenance-mode feedback paradigm. In the present study, we aim to analyze such a spherical-type flow around the host active nucleus in the backdrop of a five-component galactic system (SMBH, stellar, dark matter, diffuse hot gas, Λ), with the principal intent to address the issue of galactic potential induced shock formation in the flow, that may contain (dissipative) isothermal standing shocks. The present paper is an extension of Raychaudhuri et al. (2018) [83], who conducted a preliminary investigation of such a problem. The galactic potential, not only renders the flow to be multi-transonic in nature, the flow topology resembles `Xα' and `α X' type trajectories of advective flows in the vicinity of the BHs/compact objects. Owing to the influence of the galactic potential, the entire range of galactic mass-to-light ratio (ΥB) allows shock formation in central to the outer radial regions of our wind-type flows, with the strength of those galactic induced shocks found to be comparable to that of the shocks one would expect in the advective flows in the vicinity of a BH. We also observe that the shock parameters remain sensitive to ΥB. We discussed the possible implication of these shocks in the context of radio source dynamics as well as their potential association with flaring in radio jets. Our study also reveals that galactic potential could substantially augment the mass inflow rate.

The relativistic jet dichotomy and the end of the blazar sequence

ABSTRACT Our understanding of the unification of jetted AGN has evolved greatly as jet samples have increased in size. Here, based on the largest-ever sample of over 2000 well-sampled jet spectral energy distributions, we examine the synchrotron peak frequency – peak luminosity plane, and find little evidence for the anticorrelation known as the blazar sequence. Instead, we find strong evidence for a dichotomy in jets, between those associated with efficient or ‘quasar-mode’ accretion (strong/type II jets) and those associated with inefficient accretion (weak/type I jets). Type II jets include those hosted by high-excitation radio galaxies, flat-spectrum radio quasars (FSRQ), and most low-frequency-peaked BL Lac objects. Type I jets include those hosted by low-excitation radio galaxies and blazars with synchrotron peak frequency above 1015 Hz (nearly all BL Lac objects). We have derived estimates of the total jet power for over 1000 of our sources from low-frequency radio observations, and find that the jet dichotomy does not correspond to a division in jet power. Rather, type II jets are produced at all observed jet powers, down to the lowest levels in our sample, while type I jets range from very low to moderately high jet powers, with a clear upper bound at L 300MHz ∼1043 erg s−1. The range of jet power in each class matches exactly what is expected for efficient (i.e. a few to 100 % Eddington) or inefficient ( &amp;lt;0.5% Eddington) accretion on to black holes ranging in mass from $10^7{\, {\rm to}\,}10^{9.5}\, {\rm M}_\odot$.

Jet–Accretion System in the Nearby mJy Radio Galaxies

Abstract It is generally thought that FRII radio galaxies host thin optically thick disks, while FRIs are powered by advection-dominated accretion flows. Sources with an efficient engine are optically classified as high-excitation radio galaxies (HERGs) and those with an inefficient motor as low-excitation radio galaxies (LERGs). Recently, the study of radio galaxies down to mJy fluxes has cast serious doubts on the LERG-FRI and HERG-FRII correspondence, revealing that many LERGs show FRII radio morphologies. The FR catalogs recently compiled by Capetti et al. and Baldi et al. have allowed us to explore this issue in the local (z ≤ 0.15) mJy universe. Our statistical study shows that the majority of nearby mJy objects are in a late stage of their life. FRII-LERGs appear more similar to the old FRI-LERGs than to the young FRII-HERGs. FRII-LERGs may be aged HERGs that, having exhausted their cold fuel, have changed their accretion regime or are a separate LERG class particularly efficient in launching jets. Exploiting the empirical relations that convert L [O III] and L 1.4 GHz into accretion power and jet kinetic power, respectively, we observed that LERGs with similar masses and accretion rates seem to expel jets of different powers. We speculate that intrinsic differences related to the black hole properties (spin and magnetic field at its horizon) can determine the observed spread in jet luminosity. In this view, FRII-LERGs should have the fastest spinning black holes and/or the most intense magnetic fluxes. On the contrary, compact LERGs (i.e., FR0s) should host extremely slow black holes and/or weak magnetic fields.

The radio galaxy population in the simba simulations

ABSTRACT We examine the 1.4 GHz radio luminosities of galaxies arising from star formation and active galactic nuclei (AGNs) within the state-of-the-art cosmological hydrodynamic simulation Simba. Simba grows black holes via gravitational torque limited accretion from cold gas and Bondi accretion from hot gas, and employs AGN feedback including jets at low Eddington ratios. We define a population of radio loud AGNs (RLAGNs) based on the presence of ongoing jet feedback. Within RLAGN, we define high and low excitation radio galaxies (HERGs and LERGs) based on their dominant mode of black hole accretion: torque limited accretion representing feeding from a cold disc, or Bondi representing advection-dominated accretion from a hot medium. Simba predicts good agreement with the observed radio luminosity function (RLF) and its evolution, overall as well as separately for HERGs and LERGs. Quiescent galaxies with AGN-dominated radio flux dominate the RLF at $\gtrsim 10^{22-23}$ W Hz−1, while star formation dominates at lower radio powers. Overall, RLAGNs have higher black hole accretion rates and lower star formation rates than non-RLAGN at a given stellar mass or velocity dispersion, but have similar black hole masses. Simba predicts an LERG number density of 8.53 Mpc−3, ∼10× higher than for HERGs, broadly as observed. While LERGs dominate among most massive galaxies with the largest black holes and HERGs dominate at high specific star formation rates, they otherwise largely populate similar-sized dark matter haloes and have similar host galaxy properties. Simba thus predicts that deeper radio surveys will reveal an increasing overlap between the host galaxy demographics of HERGs and LERGs.

The Emergence of the X-Ray Luminosity/Cluster Richness Relation for Radio Galaxies

Abstract The idea that mergers are more likely in dense groups or clusters coupled with the assumption that such events lead to cold gas flows onto black holes suggests a direct relationship between the radiative efficiency of an active galactic nucleus and environmental richness. Observations, however, increasingly challenge this and other basic expectations. Mounting evidence, for example, shows an inverse trend between near-Eddington accreting objects and environmental richness. Broken down by radio galaxy subgroup, recent work has explored connections between low excitation radio galaxies with Fanaroff–Riley II jet morphology (FR II LERGs) and other radio galaxies. We make contact with that work by adding a discussion of the recently discovered FR0 radio galaxies and show how to fit them in a picture in which FRII LERGs are not initial or final phases in the lifetime of a radio galaxy, but de facto transition states. We describe how to understand the observed X-ray luminosity/cluster richness relation as a fundamental correlation to the nature of the jet–disk connection.