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Open Access
Issue
A&A
Volume 695, March 2025
Article Number A118
Number of page(s) 7
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/202453138
Published online 12 March 2025

© The Authors 2025

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article is published in open access under the Subscribe to Open model.

Open Access funding provided by Max Planck Society.

1. Introduction

Extra-galactic jets in active galactic nuclei (AGN), observed for the first time a century ago in the optical (Curtis 1918), have become the subject of renewed interest from the scientific community in recent years (see Blandford et al. 2019, for a review). In the millimeter (mm) band, the development of sensitive interferometric arrays, providing angular resolutions down to tens of micro-arcseconds, has enabled the exploration of the immediate surroundings of the supermassive black hole, where jets are formed (Boccardi et al. 2017, and references therein). The most exemplary case is the imaging of M 87 on event horizon scales, performed with the Event Horizon Telescope (EHT) at 230 GHz (Event Horizon Telescope Collaboration 2019).

M 87 is the only extra-galactic source where such an exceptional goal can be achieved using current arrays. However, several other objects have been investigated on slightly larger spatial scales, where the jet is undergoing acceleration up to relativistic velocities and collimation down to narrow opening angles. Studies of this region, which extends up to 103 − 107 Schwarzschild radii (RS) from the black hole (e.g., Kovalev et al. 2020; Boccardi et al. 2021), are important for constraining theoretical models of jet launching (Blandford & Znajek 1977; Blandford & Payne 1982) and serve as a crucial input for numerical simulations (e.g., Chatterjee et al. 2019; Lalakos et al. 2022). As is the case for M 87, the ideal targets for such studies are nearby radio galaxies hosting very massive black holes, with jets oriented at relatively large angles with respect to the observer. Indeed, these properties enable the jet base to be imaged with a high spatial resolution (in units of RS). Most of the radio galaxies examined in detail so far exhibit jets with relatively low power and host low-luminosity nuclei; that is, they are optically classified as low-excitation galaxies (LEGs, see Heckman & Best 2014). Some notable examples, besides M 87, include: Cen A (Janssen et al. 2021), NGC 1052 (Baczko et al. 2022), and NGC 4261 (Yan et al. 2023). The classification as LEG signals the presence of a radiatively inefficient, hot accretion flow at the center of these sources.

Significant progress in our understanding of jet formation could be made by extending such studies to objects characterized by different accretion modes and jet powers. A first attempt in this direction was made by Boccardi et al. (2021), who examined the properties of the jet acceleration and collimation region in a small sample comprising not only LEGs, but also high-excitation galaxies (HEGs). The latter are thought to be powered by cold, thin disks and generally produce high-power jets developing Fanaroff-Riley (FR) II radio morphologies (Fanaroff & Riley 1974). Some interesting trends emerged in this study: jets in HEGs were suggested to collimate for larger distances and to present of a wider sheath surrounding the spine with respect to LEGs. However, the numbers are still too low to draw solid conclusions. In particular, jets in HEGs are underrepresented due to their higher redshifts and lower flux densities; with the exception of Cygnus A (Boccardi et al. 2016a,b), they have not been imaged on scales smaller than ∼104 − 105RS in the mm band.

The nearby Universe hosts numerous, potentially excellent new targets (e.g., Ramakrishnan et al. 2023). These include radio galaxies often well-studied on kiloparsec (kpc) scales, but poorly explored on parsec (pc) and subparsec (subpc) scales due to the jet faintness; in the past, this has precluded imaging via very long baseline interferometry (VLBI) at wavelengths shorter than a few centimeters (cm). Today, the most sensitive VLBI arrays are capable of detecting some of these faint targets also in the mm band, thanks to the use of wide bandwidth receiving systems and to the inclusion of elements with large collecting area, such as the phased Very Large Array (VLA). In this article, we investigate the high-frequency VLBI properties of a sample of poorly explored radio galaxies spanning different luminosity classes. The goal of this work is to identify the most interesting targets, so that follow-up studies can be performed through dedicated observations. In the longer term, this and future experiments will contribute to the compilation of larger samples for performing first population studies of jet formation in AGN. This is especially relevant in view of the planned construction of next generation VLBI facilities, such as next generation Event Horizon Telescope (ngEHT, Johnson et al. 2023) and next generation Very Large Array (ngVLA) (Murphy et al. 2018), which will push the resolution and sensitivity limits much further.

2. Sample description

The sample comprises sixteen objects (see Table 1 for details). With one exception (IC 310), these belong to the Bologna sample (Giovannini et al. 2001, 2005; Liuzzo et al. 2009), which implies that they have been imaged with VLBI at least once at 6 cm. This ensured the existence of a compact core and enabled us to make detectability predictions for this experiment. One basic criterion was indeed for the sources to be bright enough for self-calibration in the fringe-fitting. Objects with sufficiently high declination have been preferred to facilitate good mutual visibility on intercontinental baselines. The sample originally included the giant radio galaxy 3C 236; however, this was not observed due to a misidentification in the source catalog used in the observation scheduling. In turn, IC 310 was not included originally, but was observed by taking advantage of the pointing gaps scheduled for the largest telescopes.

Table 1.

Sources in the sample and their main properties.

The targets are found at z < 0.1 and host large black holes (log MBH ≳ 8.5), which results in a spatial resolution of < 103 − 104RS (Fig. 2)1. Due to the low-redshift cut, the sample is dominated by LEGs. However, three HEGs are also present (3C 33, 3C 382, and 3C 452) and this is still a significant number when considering that Cygnus A has so far been the only HEG imaged on similar scales. The total radio power spans a broad interval, 21.75 < log(Pt) < 26.64 at 408 MHz, and the sources also present a variety of large-scale radio morphologies, with nine FRI s, five FR IIs, and two compact (C); the latter class includes objects not developing extended structures, with clear morphology on kpc scales. The sample includes five γ-ray sources (NGC 315, 4C 39.12, 3C 264, IC 310, and NGC 4278, Ajello et al. 2020; Cao et al. 2024), with the latter three emitting up to TeV-energies2.

3. Dataset and analysis

The VLBI observations were performed with High Sensitivity Array (HSA) at 1 cm and 7 mm (experiment code BB393). In addition to the VLBA (Very Long Baseline Array), the phased-VLA and the Effelsberg radio telescope participated in the experiment. The observations were organized in four blocks (A, B, C, and D), each of 12 hours. The on-source time for each target was typically in the range of 30–45 min at 1 cm and 45–60 min at 7 mm. In addition to the calibrators, compact and bright sources in the targets vicinity were observed for the VLA-phasing, as the targets themselves were in most cases inadequate for this purpose (too faint and/or resolved on VLA scales). Data were recorded in dual polarization mode with a bandwidth of 256 MHz per polarization (two sub-bands per polarization, each with 64 channels), resulting in an aggregate bit rate of 2 Gbit/s.

The data calibration was performed with the Astronomical Image Processing System (AIPS, Greisen 1990) through the standard procedure, including an opacity correction at both frequencies. The amplitude calibration was challenged by problems in the system temperature values recorded at the VLA, which were nonsensical during parts of the experiment. These were replaced by estimates made considering the sources elevation. In the fringe fitting at 1 cm, a solution interval of 1 minute was adopted for all sources except 3C 338, for which 2 minutes allowed us to recover significantly more solutions. At 7 mm, we used a solution interval of 1 minute for all sources. The imaging was carried out using the software package DIFMAP (DIFference MAPping, Shepherd et al. 1994). The main properties of the clean maps are reported in Table 2, and the entire set of images is presented in Appendix A.

Through the MODELFIT subroutine we have then modeled the main emission features in each source through circular Gaussian components, which has allowed us to derive their integrated flux density, S, radial distance, r, position angle, pa, and size, d, the latter assumed to correspond to the full width at half maximum (FWHM) of the Gaussian (Appendix B, Tables B.1–B.15). For each of these quantities, we have calculated the associated uncertainties following the approach of Lee et al. (2008), which is based on the determination of the signal-to-noise ratio (S/N) of each emission feature. The S/N also determines the resolution limit for each component, which we have considered when estimating the brightness temperature of the VLBI cores at the two frequencies (Table B.16). A detailed description of this analysis can be found in Appendix B. We notice that this error analysis only considers the statistical image errors. Systematic errors, for instance, affecting the amplitude calibration, are harder to quantify. High-frequency imaging of such faint objects could be affected by large uncertainties in the amplitudes, up to ∼20 − 30%, and in the image fidelity. However, the comparison between results obtained in the two bands indicates a good agreement. This applies both to the observed structures and to the flux density distribution, with the targets generally showing flat spectrum cores and steep spectrum jets, as expected.

4. Results

All sources were detected and imaged at both bands. In Fig. 1, we present the maps obtained for three representative sources: the FR II-HEG 3C 452 (left panel), showing a twin jet both at 7 mm and 1 cm, the FRI-LEG 3C 31 (central panel), revealing jet limb-brightening on scales < 103RS, and the compact-LEG NGC 4278 (right panel), the closest and weakest of the targets, which we find to be characterized by a complex core structure and poorly collimated two-sided emission (Fig. A.9). Notes on each source are reported, together with the full set of images, in Appendix A. Below we discuss the main properties of the observed subpc scale structures, highlighting the most relevant findings. Results obtained from this experiment for the brightest target, NGC 315, were already published by Boccardi et al. (2021), Ricci et al. (2022). In the following, we include this source in the discussion of the sample properties, but we refer to these papers for the images and detailed analysis of the source.

thumbnail Fig. 1.

VLBI images for three representative sources in the sample. Left: 7 mm HSA image of 3C 452 (FRII-HEG). The beam size and position angle are 0.29 × 0.16 mas, −10°. Center: 7 mm HSA image of 3C 31 (FRI-LEG). The beam size and position angle are 0.48 × 0.18 mas, −16°. Right: 1 cm HSA image of NGC 4278 (compact LEG). The beam size and position angle are 0.61 × 0.23 mas, −8°. Contours start at −3σ and increase by a factor of two until allowed by the peak intensity. These maps were produced with uniform weighting in the case of 3C 452 and NGC 4278, and with natural weighting in the case of 3C 31, which better highlights the faint limb-brightened structure of this jet. The full set of images is shown in Appendix A.

4.1. Flux density and spatial resolution

The total flux densities span an interval 17 mJy  <  Stot < 447 mJy at 1 cm and 19 mJy <  Stot < 268 mJy at 7 mm (Table 2). Assuming the BH masses in Table 1, the maximum angular resolution achieved at 7 mm, ∼0.1 mas, translates into a spatial resolution in the range 85–2034 RS.

Table 2.

Observations and characteristics of the VLBI clean maps for each source.

In Fig. 2, we display the ranges of flux densities and resolutions in the sample, as well as those of better known, previously studied objects. This shows that, while this is clearly a fainter population, the number of sources imaged on scales ≲103RS is more than doubled by the present study. Given the very large mass estimated by North et al. (2019), the highest spatial resolution is achieved for 3C 31, followed by NGC 4278 and NGC 315, which were all classified as LEGs. The lowest resolution is instead obtained, as expected, for the powerful HEGs 3C 33 and 3C 452, which are found at higher redshift. These are, nevertheless, still good candidates for probing the jet acceleration and collimation region and, due to the high level of symmetry between jet and counter-jet (Sect. 4.3), they are excellent targets for testing the AGN unified schemes.

thumbnail Fig. 2.

Spatial resolution in RS for an angular resolution of 0.1 mas versus flux density at 7 mm. In blue, we plot the sources in our sample, in red, the well-known radio galaxies previously studied on these scales (Boccardi et al. 2021). For Cyg A, NGC 1052, and M 87, we assume the average 7 mm flux densities from Boccardi et al. (2016b), Baczko et al. (2019), and Walker et al. (2018), respectively. For NGC 6251, we refer to Cheng et al. (2020). For Cen A, no 7 mm VLBI flux density is reported in the literature and we estimated it from the 1 cm one (Müller et al. 2011), assuming α = −0.5.

We note that the spatial resolution values in Fig. 2 scale linearly with the BH mass, and mass estimates can often vary significantly, by a factor of two or more (e.g., Barth et al. 2016; GRAVITY Collaboration 2018), depending on the observational method. The black hole masses in Table 1 were selected based on a careful literature search. In the presence of multiple studies for a given source, we have preferred estimates from methods which are considered more accurate, such as reverberation mapping in the case of 3C 382 (Fausnaugh et al. 2017), or resolved kinematics of cold gas in the case of 3C 31 (North et al. 2019). Nevertheless, by considering a “worst-case scenario” where the masses are smaller by one order of magnitude, the spatial resolution range for this sample would increase by the same factor. A resolution of ≲104RS would still be sufficient, in most cases, to probe the jet acceleration and collimation region.

4.2. Core brightness temperature

We estimate the core brightness temperature T B c [ K ] $ T_{\mathrm{B}}^{\mathrm{c}} [\rm K] $ of each source at the two frequencies by taking into account the parameters of the core MODELFIT components. The observed brightness temperature T B obs $ T_{\mathrm{B}}^{\mathrm{obs}} $, which differs from the intrinsic one by the Doppler factor δ ( T B obs = δ T B intr $ T_{\mathrm{B}}^{\mathrm{obs}}=\delta T_{\mathrm{B}}^{\mathrm{intr}} $), is expressed as a function of the component flux density, S [Jansky], size, d [mas], redshift, z, and frequency, νobs [GHz], as:

T B obs = 1.22 × 10 12 · S ( 1 + z ) d 2 ν obs 2 . $$ \begin{aligned} T_{\rm B}^\mathrm{obs}=\frac{1.22\times 10^{12}\cdot S(1+z)}{d^2\nu _{\rm obs}^2} .\end{aligned} $$(1)

The obtained values are reported in Table B.16 and displayed in the histogram in Fig. 3, where we overlay the distributions observed at 1 cm and 7 mm. The 7 mm histogram does not include the value obtained for 3C 33, due to the fact the core size was found to be smaller than the resolution limit. In this single case we estimate a lower limit for the brightness temperature, T B c ≥ 1.5 × 10 10 $ T_{\mathrm{B}}^{\mathrm{c}}\geq1.5\times10^{10} $ K.

thumbnail Fig. 3.

Distributions of the observed core brightness temperatures at 1 cm and 7 mm. The values on the x-axis are displayed in logarithmic scale. In linear scale, the six bins span the ranges 0.150–0.339, 0.339–0.763, 0.763–1.720, 1.720–3.878, 3.878–8.741, and 8.741–19.703 in units of 1010 K.

With respect to results typically observed in blazars, which have core temperatures that often exceed the inverse Compton limit (TB ∼ 5 × 1011 K, Kellermann & Pauliny-Toth 1969), our sources are characterized by lower values, with a few exceptions well below this limit – and even below the equipartition value of TB ∼ 5 × 1010 K. This is approximately the temperature expected to signal equipartition conditions between the particles and the magnetic energy densities (Readhead 1994) and the one typically estimated in blazars in the quiet state after correcting for Doppler boosting (Homan et al. 2021). There are several, possibly concurrent effects that could explain our low values. First, due to the fact that we are probing the jet base at short distance from the BH, equipartition conditions might not have been reached yet; namely, the jet is still strongly magnetized on these scales. This may be supported by the fact that, in most cases (in 13/16 sources), we measured lower TB values at 7 mm with respect to 1 cm, i.e., at 7 mm we are probing regions even closer to the central engine and, therefore, they more strongly magnetized than at 1 cm. Second, due to the larger viewing angle of these jets, the Doppler factors can be smaller than 1 also in the approaching side for sufficiently high intrinsic speeds (see Sect. 4.4), which will result in T B obs < T B intr $ T_{\mathrm{B}}^{\mathrm{obs}} < T_{\mathrm{B}}^{\mathrm{intr}} $. 3) Finally, it is possible that some of these cores have become fully optically thin at the observed frequencies. This could indeed be the case for the lowest power sources in particular, where less extreme physical conditions at the jet base could induce milder opacity effects.

4.3. Jets morphology and orientation

Except for 3C 388 and 3C 33, which appear point-like at 7 mm, extended jet emission was detected in all cases, with half the objects (8/16) showing two-sided structures. For these objects, we report the jet-to-counter-jet intensity ratio RJ/CJ, which we used to estimate some jet parameters, in Table 3.

Table 3.

Two-sided sources and estimated jet parameters.

We performed this analysis at 1 cm, since a larger number of counter-jets is detected in this band. We defined a source as “two-sided” if at least two Gaussian features could be fitted in the region upstream of the core. The approaching jet flux density was then calculated as the sum of the integrated flux density of all components in the brightest side, including the core, and the receding jet flux density as the sum of the features in the opposite side. Objects such as 3C 66B and 3C 264, which present a single upstream feature in the proximity of the core, are of an uncertain classification. Indeed, such features could still be part of the approaching jet, marking optically thick regions between the black hole and the VLBI core. Core shift and spectral studies will be necessary to clarify their nature. The upstream emission in NGC 315 was also modeled with a single component in this specific epoch; thus, we did not classify the source as “two-sided” based on these images. However, for this target, we performed detailed alignment and spectral analysis considering a broad multi-wavelength dataset, which clearly confirmed the presence of a counter-jet (Boccardi et al. 2021; Ricci et al. 2022).

By considering that RJ/CJ depends on the viewing angle, θ, the intrinsic speed, β and the spectral index, α, as

R J / CJ = ( 1 + β cos θ ) ( 1 − β cos θ ) 2 − α , $$ \begin{aligned} R_{\rm J/CJ}=\frac{(1+\beta \cos \theta )}{(1-\beta \cos \theta )}^{2-\alpha }, \end{aligned} $$(2)

we can use this observable for constraining β and θ (we assume α = −0.7, with Sν ∝ να) under the hypothesis of intrinsic symmetric and constant speed. In particular, we can derive a lower limit for the speed, βmin, by setting θ = 0, along with an upper limit for the angle, θmax, by setting β = 1. We can also derive exact values for θ by making an assumption on the Lorentz factor, Γ. Relatively low jet speeds are inferred in kinematic studies of radio galaxies, with Γ typically in the range ∼1 − 2 (e.g., Lister et al. 2019). By assuming Γ ∼ 1.4, corresponding to β = 0.7 c, as an average value for this sample, we obtained reasonable estimates for the jet viewing angle of the two-sided sources, which is in the range 25° −81° (Table 3). However, the true θ value might deviate significantly from these estimates in few cases. The low RJ/CJ observed in the lowest power object NGC 4278 is also compatible with a much lower jet speed (βmin ∼ 0.1 c) and viewing angle. For this source, a smaller jet viewing angle would be consistent with its recent detection at TeV energies by the Large High Altitude Air Shower Observatory (Cao et al. 2024), as well as with previous cm-VLBI results (Giroletti et al. 2005b). Moreover, it would explain the rather complex morphology of the diffuse two-sided emission we have imaged in this source (Fig. 1 and Appendix A.9), which could be ascribed to projection effects.

As mentioned, Eq. (2) is strictly valid only in the hypothesis of intrinsically symmetric emission from two flows propagating at constant speed. Since we may be imaging the acceleration region of these jets, RJ/CJ is likely to increase with distance from the BH, due to the increasing Lorentz factor. This effect was indeed observed in NGC 315 based on these data (Ricci et al. 2022). Asymmetries between the approaching and receding sides, either intrinsic (e.g., Baczko et al. 2019) or induced by external absorbers (e.g., Haga et al. 2015), are also possible. A detailed analysis of the variation of RJ/CJ with distance can therefore provide a unique insight on these effect, particularly if combined with spectral information. We are currently following this approach for the sources in Table 3, in a work which will be subject of a future publication.

4.4. Jets internal structure

As discussed in Sect. 4.3, a moderate Lorentz factor seems adequate to explain the properties of the observed radio emission in our galaxies. However, the true maximum Γ in most of these jets can be expected to be much higher. Indeed, the measurement of low speeds in misaligned sources is likely due to the presence of transverse velocity gradients in the jet, with the fast central spine becoming faint for larger angles due to Doppler de-boosting. The presence of this gradient may manifest in the limb brightening of the jet emission, previously observed in several LEGs (Boccardi et al. 2017, and references therein). Based on our images, sources showing hints of this feature, most often in the innermost 7 mm jet, are 3C 31 (Fig. 1), 3C 66B, 4C 39.12, 3C 264, and 3C 465 (see Appendices A.1, A.3, A.6, A.8, and A.15, respectively), which all belong to the LEG class. Furthermore, NGC 315, which is also a LEG, did not clearly show any limb brightening in our images, but this feature was observed in more recent data presented by Park et al. (2024). However, before concluding that limb brightening only characterizes jets in LEGs, we should notice that these are also the best resolved in the transverse direction due to their proximity with respect to HEGs. Indeed, the only HEG jet that is well resolved transversely is the one in Cygnus A, and this is also limb brightened (Boccardi et al. 2016b). Future, higher resolution VLBI studies of jets in HEGs will help clarify this aspect.

5. Prospects for future observations

Throughout this work, we have identified new interesting targets for jet formation studies, several of which are bright enough to be examined in detail by performing dedicated experiments using current VLBI arrays. Even 3 mm-imaging with the upgraded global mm-VLBI array (GMVA, e.g., Kim et al. 2023), including the phased-ALMA (Atacama Large Millimeter Array), is expected to be feasible for mm-flux densities of few tens of mJy. Other than NGC 315, which has already been subject of several studies (see also Park et al. 2021, 2024), the targets that offer the best combination of a superb spatial resolution (< 500 RS) with a relatively high flux density (> 50 mJy) at 7 mm are 3C 31, 3C 66B, and 3C 465, which all belong to the LEG class. While fainter, the LEG NGC 4278 is also extremely well resolved, providing a unique view into the base of a low-power, poorly collimated jet on milli-pc scales. Among the HEGs in the sample, 3C 33 and 3C 452 show highly symmetric two-sided jets, and 3C 452 is also rather bright (53 mJy at 7 mm). This makes it a highly interesting target to investigate the nuclear regions of a powerful source. In the longer term, we expect most of these objects to become prime targets of next-generation arrays. In the mm band, the ngVLA (Murphy et al. 2018) will have a similar capability to the GMVA in terms of angular resolution, but its sensitivity will be improved by several orders of magnitude. It will be sensitive to thermal emission as well, which will allow us to probe the full outflow stratification properties in these objects for the first time. Several of our targets are also of interest for future observations with ngEHT (Johnson et al. 2023; Ramakrishnan et al. 2023), which will be aimed at probing the black hole shadow in other extragalactic objects besides M 87.

Data availability

The FITS files of the images in Appendix A are available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/695/A118

The appendices are available at https://doi.org/10.5281/zenodo.14627546. They cite the references: Aleksić et al. (2014a), Boccardi et al. (2019), Capetti et al. (2000), Giovannini et al. (1998), Giroletti et al. (2005a), Hardcastle et al. (2005), Sudou et al. (2003).

1

A ΛCDM cosmology with H0= 71 km s−1 Mpc −1, ΩM = 0.27, ΩΛ = 0.73 (Komatsu et al. 2009) is assumed.

Acknowledgments

The authors thank the anonymous referee for providing valuable feedback. The authors thank DaeWon Kim for reading the article. BB acknowledges the financial support of a Otto Hahn research group from the Max Planck Society. This research has been partially funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – project number 443220636 (DFG research unit FOR 5195: “Relativistic Jets in Active Galaxies”). The VLBA is a facility of the National Science Foundation under cooperative agreement by Associated Universities, Inc.

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All Tables

Table 1.

Sources in the sample and their main properties.

In the text
Table 2.

Observations and characteristics of the VLBI clean maps for each source.

In the text
Table 3.

Two-sided sources and estimated jet parameters.

In the text

All Figures

thumbnail Fig. 1.

VLBI images for three representative sources in the sample. Left: 7 mm HSA image of 3C 452 (FRII-HEG). The beam size and position angle are 0.29 × 0.16 mas, −10°. Center: 7 mm HSA image of 3C 31 (FRI-LEG). The beam size and position angle are 0.48 × 0.18 mas, −16°. Right: 1 cm HSA image of NGC 4278 (compact LEG). The beam size and position angle are 0.61 × 0.23 mas, −8°. Contours start at −3σ and increase by a factor of two until allowed by the peak intensity. These maps were produced with uniform weighting in the case of 3C 452 and NGC 4278, and with natural weighting in the case of 3C 31, which better highlights the faint limb-brightened structure of this jet. The full set of images is shown in Appendix A.
In the text
thumbnail Fig. 2.

Spatial resolution in RS for an angular resolution of 0.1 mas versus flux density at 7 mm. In blue, we plot the sources in our sample, in red, the well-known radio galaxies previously studied on these scales (Boccardi et al. 2021). For Cyg A, NGC 1052, and M 87, we assume the average 7 mm flux densities from Boccardi et al. (2016b), Baczko et al. (2019), and Walker et al. (2018), respectively. For NGC 6251, we refer to Cheng et al. (2020). For Cen A, no 7 mm VLBI flux density is reported in the literature and we estimated it from the 1 cm one (Müller et al. 2011), assuming α = −0.5.
In the text
thumbnail Fig. 3.

Distributions of the observed core brightness temperatures at 1 cm and 7 mm. The values on the x-axis are displayed in logarithmic scale. In linear scale, the six bins span the ranges 0.150–0.339, 0.339–0.763, 0.763–1.720, 1.720–3.878, 3.878–8.741, and 8.741–19.703 in units of 1010 K.
In the text

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