11institutetext: INAF - Istituto di Radioastronomia, via Gobetti 101 40129
Bologna
11email<EMAIL_ADDRESS>22institutetext: Dipartimento di Fisica e
Astronomia - University of Bologna, via Gobetti 93/2 33institutetext: INAF -
Osservatorio Astronomico di Torino 44institutetext: Instituto de Astrofísica
de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain.

# Jets in FR0 radio galaxies

G. Giovannini 1122 R. D. Baldi 11 A. Capetti 33 M. Giroletti 11 R. Lico 4411

(Received …….; accepted ……)

###### Abstract

Context. The local radio-loud AGN population is dominated by compact sources
named FR0s. These sources show features, for example the host type, the mass
of the supermassive black hole (SMBH), and the multi-band nuclear
characteristics, that are similar to those of FRI radio galaxies. However, in
the radio band, while FR0 and FRI share the same nuclear properties, the
kiloparsec-scale diffuse component dominant in FRI is missing in FR0s.
Previous very-long-baseline interferometry (VLBI) observations of a small
sample of FR0s show a complex structure, mostly symmetric (two-sided jets)
with respect to the central core.

Aims. With this project we would like to study the parsec-scale structure in
FR0s in comparison with that of FRI sources. Jets in FRI are relativistic on
the parsec scale and decrease their velocity becoming subrelativistic on the
kiloparsec scale. We would like to test whether this result also applies to
the jets in FR0s or, alternatively, whether they are subrelativistic on the
parsec scale. This might be the reason why they are unable to grow, because of
instabilities, related to a low jet bulk velocity.

Methods. To this end we observed 18 FR0 galaxies with the VLBA at 1.5 and 5
GHz and/or with the EVN at 1.7 GHz and produced detailed images at milliarcsec
resolution of their nuclear emission to study the jet and core structure.

Results. All sources have been detected but one. Four sources are unresolved,
even in these high-resolution images; jets have been detected in all other
sources. We derived the distribution of the jet-to-counter-jet ratio of FR0s
and found that it is significantly different from that of FRIs, suggesting
different jet bulk speed velocities.

Conclusions. Combining the present data with published data of FR0 with VLBI
observations, we derive that the radio structure of FR0 galaxies shows strong
evidence that parsec-scale jets in FR0 sources are mildly relativistic with a
bulk velocity on the order of 0.5c or less. A jet structure with a thin inner
relativistic spine surrounded by a low-velocity sheath could be in agreement
with the SMBH and jet launch region properties.

###### Key Words.:

radio continuum: galaxies; galaxies: active; galaxies: jets

##  1 Introduction

It is now clear that most of the radio sources associated with low-redshift
galaxies (e.g., Heckman & Best 2014) are compact, with corresponding linear
sizes smaller than $\sim$10 kpc (Baldi & Capetti, 2009, 2010). These compact
sources are located in red massive ($\sim$1011 solar masses) early-type
galaxies, with high supermassive black hole (SMBH) masses ($\sim$
10${}^{8}M_{\odot}$), and spectroscopically classified as low-excitation radio
galaxies (LERGs). They also show the same correlation between radio core power
and [O III] line luminosity of FRIs, but are more core-dominated (by a factor
of $\sim$ 30) than FRIs and show a clear deficit of extended radio emission.
For these reasons these compact sources were named FR0 (Ghisellini, 2011;
Sadler et al., 2014; Baldi et al., 2015), being similar to FRI from the
optical and nuclear radio emission point of view, but without an extended
emission dominant at low frequencies.

The available information from observations of FR0s is still limited, even in
the radio band, but recently major results have been obtained (see, e.g., JVLA
observations presented in Baldi et al. 2015, 2019a). Most of the sources
appear unresolved down to an angular resolution of $\sim$ 0.3 arcsec, which
corresponds to 100–300 pc. A few sources show radio emission extended over a
few arcseconds, on a scale of 2–14 kpc.

From an X-ray study of a sample of 19 FR0 radio galaxies Torresi et al. (2018)
find that the X-ray photons are likely produced by the jet, as attested by the
observed correlation between X-ray (2–10 keV) and radio (5 GHz) luminosities,
similar to FRI radio galaxies. Overall, they find that the X-ray properties of
FR0s are indistinguishable from those of FRIs. A comparison between FR0s and
low-luminosity BL Lacs rules out important beaming effects in the X-ray
emission of the compact radio galaxies.

A complete sample of FR0 radio galaxies have been selected from radio and
optical surveys, the FR0CAT catalog (FR0CAT, Baldi et al. 2018) to constitute
a workbench sample for the following studies of these compact radio-loud AGN.
Thanks to LOFAR and GMRT observations of FR0 galaxies from the FR0CAT, Capetti
et al. (2019, 2020) conclude that the fraction of FR0s showing evidence for
the presence of jets at low radio frequencies, by including both spectral and
morphological information, is at least $\sim$40%. This study confirms that
FR0s and FRIs can be interpreted as two extremes of a continuous population of
jetted sources (Capetti et al., 2017; Baldi et al., 2018), with the FR0s
representing the low end in size and radio power.

Studying the properties and the origin of the radio emission in the most
luminous, early-type galaxies (ETGs) in the nearby Universe with LOFAR
observations, Capetti et al. (2022) found that about two-thirds of the
detected ETGs are unresolved, with sizes on the order of 4 kpc or lower,
confirming the prevalence of compact radio sources in local universe. Because
these observations are at low radio frequencies, they exclude the presence of
extended steep spectrum lobes and the authors conclude that these sources are
FR0 candidates.

Various evolutionary scenarios were investigated (e.g., Baldi et al. 2019a,
b). First, the possibility that all FR0s are young sources eventually evolving
into extended sources was ruled out by the distribution of radio sizes.
Similarly, a time-dependent scenario where accretion or jet launching
properties prevent the formation of large-scale radio structures appears to be
implausible, owing to the large abundance of subkiloparsec objects. Finally, a
scenario in which FR0s are produced by mildly relativistic jets is consistent
with the data, but requires observations of a larger sample to be properly
tested.

Garofalo et al. (2010) discussed a model where powerful jets are due to fast
spin retrograde accreting SMBHs, while a prograde configuration is identified
with radio quiet galaxies. The natural time evolution of fast spin retrograde
SMBHs is toward low-spin retrograde SMBHs followed by a prograde configuration
(Garofalo et al., 2016). Garofalo & Singh (2019) identified a region of the
jet power versus spin space corresponding to relatively weak jets (see their
Fig. 1), possibly identified with FR 0 jets properties. Prograde low-spin BHs
can shape compact and/or weak FR 0 jets.

Massaro et al. (2020) compared the properties of the large-scale environment,
an orientation-independent property, of BL Lacs with those of samples of radio
galaxies. This study revealed that BL Lacs are found in regions characterized
by a significantly lower galaxy density with respect to FRIs, challenging
predictions of the unification scenario (e.g., Urry & Padovani 1995). They
noted that instead large-scale environments of BL Lacs are statistically
consistent with those of the FR 0s. This implies that a substantial fraction
of BL Lacs must be associated with FR0 sources oriented at a small angle from
the line of sight and that FR0s are able to produce relativistic jets.

To test these suggestions VLBI observations of FR0s are necessary to check the
presence of small scale jets and to study their properties and evolution.
Cheng & An (2018) cross-matched the FR0CAT catalogue with the mJy imaging VLBA
exploration at 20 cm (mJIVE-20) catalog and the Astrogeo database (see details
and references in Cheng & An 2018). They found 14 FR0CAT sources with VLBI
data, with fluxes at 1.4 GHz in the $\sim$50-400 mJy range. In the VLBI images
all of them show a radio structure. In four sources the high brightness
temperature suggests Doppler boosting; in most of the sources the core
emission is likely blended with that of the inner jet. They discuss that the
VLBI properties of these FR0s are consistent with low-power compact symmetric
object (CSO) properties, but observations of more FR0s are necessary to
investigate this point. The authors noted that these 14 sources are more
compact and brighter than the FR0 general population introducing possible
selection effects.

In a follow up project, Chen et al. (2021) selected, from the 14 sources
studied by Cheng & An (2018), the 8 FR0 that are resolved sources on parsec
scale and without multi-epoch VLBI observations. They obtained EVN and VLBA
data to investigate the presence of proper motions, to identify the core
position, and to classify the radio source structure. In all eight sources
they confirmed the presence of compact structures. In more detail: six sources
show a two-sided morphology and two are one-sided jet sources. The low proper
motion detected and the presence in most sources of a two-sided structure
suggest a relatively large viewing angle and/or a low bulk velocity of FR0
jets.

Baldi et al. (2021) presented e-MERLIN observations of five FR0s and
preliminary results from 1.7 GHz EVN observations of ten FR0s, which are
discussed in more details here. The main result is that the large fraction of
two-sided structures further support the presence of mildly relativistic jets
in FR0s.

In this scenario we present here the results of EVN (at 1.7 GHz) and/or VLBA
observations (at 1.5 and 5 GHz) of 18 FR0 sources. In Sect. 2 we describe
these observations and discuss the data reduction. The results are presented
in Sect. 3. In Sect. 4 we discuss FR0 parsec scale properties, and compare our
results with the literature data. Our conclusions are presented in Sect. 5.

The intrinsic parameters quoted in this paper are computed assuming a
$\Lambda$CDM cosmology with $H_{0}$ = 70 km s-1Mpc-1, $\Omega_{m}$ = 0.3, and
$\Omega_{\Lambda}$ = 0.7.

##  2 Observations and data reduction

Among the 25 FR0s studied with JVLA observations in the A array (Baldi et al.,
2015, 2019a), we selected sources with a core flux density at 1.4 GHz greater
than 20 mJy and z $<$ 0.05. The 11 sources selected were observed with EVN at
1.7 GHz. Observations were carried in May 2019 in phase reference mode with a
data rate of 1024 Mbit/sec (project EG100). Nearby phase calibrators selected
from the VLBA calibrator list, and target sources were observed with a $\sim$
5 min cycle. The total observing time was $\sim$ 2hr/source with at least 60
min on the target source and the remaining time on calibrator source and
motion. Data were correlated at the JIVE correlator in Dwingeloo. In Fig. 1 we
show the typical uv-coverage obtained using the following array: Jodrell
(Lovell), Westerbork single antenna, Effelsberg, Medicina, Onsala 25m, Urumqi,
Torun, Hartebeesthoek, Svetloe, Zelenchukaskaya, Badary, and Irbene. Data
calibration was carried out in the standard mode using the EVN pipeline. Data
reduction was done in AIPS, using the task IMAGR to obtain images. Self-
calibration in phase was applied (when possible because of the low correlated
flux density) to improve the quality of the images. The final parameters are
reported in the next section.

Figure 1: uv-coverage for source J1703+24 observed with EVN.  Figure 2: uv-
coverage for source J1621+25 observed with VLBA at 1.5 GHz (left) and at 5 GHz
(right).

Moreover, in the time range September – December 2021 we observed 11 FR0s with
the VLBA at 5 and 1.5 GHz (project BG272). Selected sources are characterized
by a complex and steep radio spectrum in the high-frequency range, with a
flatter or self-absorbed spectrum in the low-frequency range. Sources were
observed in phase-reference mode at 5 and 1.5 GHz for 2 hr each including
calibration and dwell time. After the correlation at the NRAO Socorro
correlator, data were imported in AIPS and calibrated using the standard
technique. Images were produced using the AIPS task IMAGR; bad data were
flagged mostly thanks to the task TVFLG; self-calibration in phase only was
applied when possible to improve the image quality. Dual frequency
observations are useful to properly identify the radio core and turbulence
regions using spectral index information and comparing different source
morphology; we have high-resolution images of the source core and lower
resolution, high-sensitivity images of the diffuse steep spectrum regions. In
Fig. 2 the typical uv-coverage of a source at the two frequencies is shown.

##  3 Results

In Table LABEL:tab1 we present the parameters of the images obtained from
these observations and in Table LABEL:tab2 we present estimated source
parameters from the images. The core structure was identified from spectral
index information, when available. In other sources it was identified with the
brightest compact component. More details are reported in the next subsection
(comments on the individual sources). Core parameters were measured using the
task JMFITS in AIPS. This task fits up to four Gaussian components to a
portion of an image. The starting model is a Gaussian with a width equal to
the half power beam width of the image, centered on the peak of the component.
In most sources a convergence is obtained after 40–50 iterations. In diffuse
sources we integrated the flux density within the 2$\sigma$ area to avoid
negative regions. We checked that the total flux density is not sensitive to a
larger integration area. The source size is obtained from the deconvolved
Gaussian fit in slightly extended regions, while it is directly measured on
the image for well-resolved sources. The size uncertainty is 0.1 times the
HPBW for unresolved or slightly extended sources (Gaussian fit) and $\sim$ 0.3
times the HPBW in resolved sources. The source PA uncertainty is a few
degrees. We note that we produced images with a redundant cell size,
approximately ten times smaller than the HPBW, for a better quality of the
images. The flux density uncertainty was obtained combining the image noise
level reported in Table LABEL:tab1 with the gain calibration uncertainty as

$\Delta$ S = ((0.03$\times$S)2 \+ noise2)0.5,
where S is the flux density; the calibration uncertainty has been evaluated at
$\sim$ 3%. We note that, given the low flux density of most of our sources,
the density flux uncertainty is noise dominated. In a few sources the flux
density uncertainty can be higher because of a peculiar source structure or
other problems commented on in single source notes (Sect. 3.1).

Table 1: Observational parameters

Name | EVN 1.7 | EVN 1.7 | VLBA 1.5 | VLBA 1.5 | VLBA 5 | VLBA 5
---|---|---|---|---|---|---
| HPBW | noise | HPBW | noise | HPBW | noise
| mas |  $\mu$Jy/beam | mas |  $\mu$Jy/beam | mas |  $\mu$Jy/beam
J0020-00 | – | – | 8 x 4 | 59 | 2.5 x 2.5 | 17
J0115+00 | – | – | 10 x 4 | 46 | 3 x 2 | 18
J0907+32 | 15 x 7 | 50 | 10 x 10 | 77 | 4 x 3 | 17
|  |  | 15 x 15 | 54 |  |
J0930+34 | 16 x 9(-8) | 35 | – | – | – | –
J0943+36 | 17 x 7(-25) | 198 | – | – | – | –
J1025+10 | 21 x 11(-5) | 100 | – | – | – | –
J1040+09 | 20 x 20 | 120 | 14 x 13 | 15 | 5 x 5 | 20
J1136+51 | – | – | 13 x 8(-20) | 50 | 4 x 2.5 | 30
J1213+50 | 25 x 25 | 270 | – | – | – | –
J1230+47 | 24 x 15 | 140 | – | – | – | –
J1530+27 | 21 x 12(-10) | 400 | – | – | – | –
J1559+25 | 15 x 10(-25) | 70 | – | – | – | –
J1621+25 | – | – | 12 x 12 | 50 | 2.5 x 2.5 | 30
J1628+25 | 20 x 12(-40) | 100 | 4 x 3.5 | 35 | 0.9 x 0.9 | 17
J1658+25 | – | – | 6 x 6 | 37 | 2.5 x 2.5 | 31
J1703+24 | 15 x 10(-14) | 50 | 5 x 3 | 29 | 3 x 2 | 25
J2336+00 | – | – | 10 x 10 | 30 | 5 x 3 | 40
J2346+00 | – | – | 5 x 5 | 28 | 2 x 2 | 15

Col. 1: source name; Col. 2: EVN HPBW at 1.7 GHz, the orientation angle is in
parentheses, if not reported it is at zero degree or it is a circular beam;
Col. 3: noise level (1 $\sigma$) of EVN images at 1.7 GHz; Col. 4: VLBA HPBW
at 1.5 GHz, the orientation angle is in parentheses, if not reported it is at
zero degree or it is a circular beam; Col. 5: noise level (1 $\sigma$) of VLBA
images at 1.5 GHz; Col. 6: VLBA HPBW at 5 GHz, the orientation angle is in
parentheses, if not reported it is at zero degree or it is a circular beam;
Col. 7: noise level (1 $\sigma$) of VLBA images at 5 GHz.

Images of resolved sources are in Figures 3 to 15. No image is presented for
unresolved sources since they appear as a beam image and no information (e.g.,
field sources or noise structures) is present. Single sources are discussed in
Sect. 3.1 where more details are presented.

Table 2: Source data

Name | RA | z | EVN 1.7 | VLBA 1.5 | VLBA 5 | LAS | PA | Note
---|---|---|---|---|---|---|---|---
| DEC |  | core(mJy) | core(mJy) | core(mJy) | mas | deg |
|  |  | total(mJy) | total(mJy) | total(mJy) | pc |  |
J0020-00 | 00 20 34.700 | 0.0720 | – | 0.94(0.07) | 0.35(0.02) | 10 | 160 | one-sided
| -00 28 14.40 |  | – | 1.48(0.08) | 0.82(0.03) | 14 | 140 |
J0115+00 | 01 15 15.702 | 0.0450 | – | 7.26(0.05) | 5.77(0.02) | 40 | 95 | two-sided
| 00 12 48.59 |  | – | 38.42(1.10) | 35.34(1.00) | 35 |  |
J0907+32 | 09 07 34.827 | 0.0490 | – | 0.45(0.05) |  $<$0.04 | 65 | 80 | double+core
| 32 57 22.80 |  | 13.28(0.70) | 12.47(0.07) | ND | 62 |  | Spiral galaxy
J0930+34 | 09 30 03.560 | 0.0420 | – | – | – | 15 | 90 | double
| 34 13 25.34 |  | 7.18(0.36) | – | – | 12 |  | structure
J0943+36 | 09 43 19.159 | 0.0223 | 140(7.0) | – | – | P | – | unresolved
| 36 14 52.07 |  | – | – | – | – |  | variable
J1025+10 | 10 25 44.213 | 0.0457 | 106(5.0) | – | – | P | 110 | see text
| 10 22 30.48 |  | – | – | – | – |  |
J1040+09 | 10 40 28.373 | 0.0195 | – | 2.45(0.20) | 3.08(0.25) | 90 | 180 | two-sided
| 09 10 57.25 |  | 14.57(0.74) | 19.25(0.58) | 5.68(0.28) | 36 |  | complex
J1136+51 | 11 36 37.148 | 0.0498 | – | 1.43(0.05) | 0.30(0.03) | P | – | unresolved
| 51 00 08.44 |  | – | – | – | – |  | steep spectrum
J1213+50 | 12 13 29.280 | 0.0308 | – | – | – | 15 | 145 | see text
| 50 44 29.38 |  | 54.7(2.8) | – | – | 9 |  |
J1230+47 | 12 30 11.852 | 0.0391 | 11.85(0.83) | – | – | 100 | 0 | two-sided
| 47 00 22.63 |  | 68.8(2.1) | – | – | 77 |  | symmetric
J1530+27 | 15 30 16.153 | 0.0325 | 48.5(2.4) | – | – | 15 | 65 | one-sided?
| 27 05 51.01 |  | 62.6(3.1) | – | – | 10 |  |
J1559+25 | 15 59 51.615 | 0.0447 | 6.98(0.71) | – | – | P | – | see text
| 25 56 26.33 |  | – | – | – | – |  |
J1621+25 | 16 21 46.069 | 0.0480 | – | 0.50(0.06) | 0.28(0.03) | 180 | 90 | two-sided
| 25 49 14.45 |  | – | 5.95(0.19) | 0.30(0.03) | 169 | 130 | z-shaped
J1628+25 | 16 28 46.129 | 0.0401 | – | 8.27(0.31) | 7.94(0.28) | 12 | 115-90 | 2-sided
| 25 29 40.97 |  | 27.4(1.4) | 27.62(0.83) | 26.50(0.80) | 10 |  |
J1658+25 | 16 58 30.054 | 0.0327 | – | 6.51(0.10) | 10.26(0.16) | 73 | 60 | 2-sided
| 25 23 24.97 |  | – | 13.83(0.17) | 12.94(0.16) | 48 |  | distorted
J1703+24 | 17 03 58.503 | 0.0310 | 4.00(0.20) | ?? | ?? | 40 | 70? | core?
| 24 10 39.52 |  | 11.00(0.55) | 9.34(0.30) | 3.46(0.15) | 25 |  | complex
J2336+00 |  | 0.0763 | – | ND | ND | – | – | not detected
|  |  | – | – | – | – |  |
J2346+00 | 23 46 09.103 | 0.0927 | – | 4.61(0.25) | 2.00(0.21) | 30 | 105 | 2-sided
| 00 59 08.83 |  | – | 23.91(0.72) | 9.84(0.30) | 52 |  | z-shaped

Col. 1: source name; Col. 2: core position obtained with a gaussian fit; Col.
3: source redshift; Cols. 4,5,6: core (top) and total (bottom) flux density in
mJy in the different available images; Col. 7: largest measured size of the
source in mas (top) and parsec (bottom); Col. 8: source orientation in the
image in degrees; Col. 9: comments.

Table 3 provides the source parameters of our sample The jet-to-counter-jet
ratio, reported in Col. 5 is the ratio of the surface brightness of the two
opposite jets in the region near the VLBI core, $R_{\rm JC}=F_{\rm jet}/F_{\rm
cjet}$. When the counter-jet was not detected we report an upper limit
obtained using the one-sigma noise level as counter-jet surface brightness. In
other sources the ratio uncertainty is in parentheses; it does not affect
observed beaming effects.

Table 3: Source properties

Name |  Log Ptot ($\nu$) |  Log Pcore ($\nu$) |  SVLBI/Score-JVLA | jet-to-cjet ratio
---|---|---|---|---
| W/Hz | W/Hz |  |
J0020-00 | 22.75 (1.5) | 21.65 (5.0) | 1.59 |  $>$ 2.7
J0115+00 | 23.26 (1.5) | 22.44 (5.0) | 1.05 | 1 (0.1)
J0907+32 | 22.88 (1.7) | 21.41 (1.5) | 3.34 | 1 (0.2)
J0930+34 | 22.47 (1.7) | – | 0.57 |  $>$ 10
J0943+36 | – | 23.20 (1.7)? | 0.50 | –
J1025+10 | 23.72 (1.7) | – | 0.92 | –
J1040+09 | 22.22 (1.5) | 21.42 (5.0) | 0.77 | 1 (0.2)
J1136+51 | 21.92 (1.5) | 21.25 (5.0) | 1.23 |
J1213+50 | 23.08 (1.7) |  | 0.74 | 1 (0.1
J1230+47 | 23.39 (1.7) | 22.63 (1.7) | 1.35 | 1.4 (0.1)
J1530+27 | 23.18 (1.7) | 23.07 (1.7) | 1.39 |  $>$ 8
J1559+25 |  | 22.57 (1.7) | 0.52 |
J1621-25 | 22.51 (1.5) | 21.18 (5.0) | 2.70 | 1 (0.1)
J1628+25 | 23.02 (1.5) | 22.47 (5.0) | 1.14 | 1 (0.1)
J1658+25 | 22.53 (1.5) | 22.40 (5.0) | 1.01 | 1 (0.1)
J1703+24 | 22.39 (1.7) | ??? | 0.44 | 1 (0.1)
J2236+00 | — | — | … |
J2346+00 | 23.71 (1.5) | 22.64 (5.0) | 2.63 | 1 (0.1)

Col. 1: source name; Col. 2: total radio power in W/Hz, in parentheses the
observed radio frequency in GHz; Col. 3: core radio power in W/Hz, in
parentheses the observed radio frequency in GHz; Col. 4: the ratio of the
correlated VLBI flux density to the core flux density in JVLA observations;
Col. 5: the jet-to-counter-jet surface brightness ratio in the region near to
the VLBI core, the ratio uncertainty is in parentheses.

###  3.1 Comments on the individual sources

J0020-00 \- This source shows a two-sided symmetric structure in the JVLA
image at 7.5 GHz published in Baldi et al. (2015). eMERLIN show a core with
scattered components along the JVLA radio axis Baldi et al. (2021). In the
VLBA images it shows (Fig. 3) a one-sided structure at 1.5 GHz at PA = 140∘
(the same as the arcsec structure) and an inner core in the VLBA image at 5
GHz with a symmetric short extension along PA 160∘. The jet-to-counter-jet
ratio at 1.5 GHz is $>$2.7. The JVLA core flux density at 7.5 GHz is
significantly lower than the correlated flux density at 1.5 GHz (the ratio is
1.59), suggesting variability and/or complex nuclear spectral index.

J0115+00 \- This source, unresolved in JVLA images (Baldi et al., 2015), shows
a symmetric two-sided structure in VLBA images at 1.5 (Fig. 4) and 5 GHz. The
ratio of the VLBA correlated flux at 1.5 GHz to the JVLA flux density at 7.5
GHz is 1.05, suggesting that in the VLBA image all the nuclear structure is
visible. The jet-to-counter-jet ratio is $\sim$ 1\. The source spectrum in
both the JVLA and VLBA images is flat.

J0907+32 \- This peculiar source is identified with a spiral galaxy and should
not be included among the FR0 sources. However, because of its peculiar
structure we report here data and images. We do not use it in statistical
considerations. A similar source (B2 0722+30) was found by Liuzzo et al.
(2013), who studied a sample of compact FRI sources. In JVLA images (Baldi et
al., 2019a) J0907+32 shows a nuclear emission and a symmetric structure
identified with the disc emission easily confused with two-sided jets.
Interestingly the nuclear emission is slightly resolved in the direction
perpendicular to the disk. EVN images at 1.7 GHz and VLBA at 1.5 GHz shows two
symmetric small-scale lobes in agreement with the nuclear arcsecond extension
perpendicular to the disk. In the VLBA image a possible central emission is
present (Fig. 5). The VLBA data at 5 GHz completely resolve this structure and
no significant structure is visible. It is a puzzling steep spectrum source,
suggesting AGN activity in the core of this spiral galaxy but with no jet-like
structure. The presence of a similar structure in B2 0722+30, as noted before,
could suggest the possibility of FR0-like structures in a few cores of spiral
galaxies. More data are necessary to discuss this point.

J0930+34 \- This source, unresolved but with a steep spectrum in JVLA
observations (Baldi et al., 2019a), is resolved in EVN observations in a
double structure (Fig. 6). The core identification is not clear; we note that
the correlated flux density is significantly lower than the JVLA flux density
at 7.5 GHz. The ratio is 0.57, due to the source steep spectrum and a possibly
missing structure in VLBI image. In any case, if the stronger lobe (5.42 mJy)
is the core and the secondary lobe (1.76 mJy) a one-sided jet, the jet-to-
counter-jet ratio does not require a fast jet (v $>$ 0.4c).

Figure 3: Contour image of J0020-00 obtained with VLBA at 1.5 GHz  Figure 4:
Contour image of J0115+00 obtained with VLBA at 1.5 GHz  Figure 5: Contour
image obtained with VLBA at 1.5 GHz of J0907+32  Figure 6: Contour image of
J0930+34 obtained with EVN at 1.7 GHz

J0943+36 \- This source is unresolved in our EVN observations and in VLBA
observations at 8 GHz (Cheng & An, 2018). The source flux density is 140 mJy
and in the image no dynamic range problem is present. The unresolved source is
the only visible structure, also in large field images. The radio spectrum is
inverted and it shows evidence of variability. This property and the
relatively high radio power suggest a relativistic source oriented at a small
angle from the line of sight. The ratio of the correlated EVN flux density to
the JVLA flux density at 7.5 MHz (Baldi et al., 2019a) is 0.50. This can be
due to an inverted spectrum and source variability.

J1025+10 \- Unresolved in EVN observations at 1.7 GHz, it is resolved with a
two-sided structure (13 pc in size) in the EVN 5 GHz image by Cheng et al.
(2021). In their image the inner core is extended at PA = 90∘ and the two-
sided jets are at PA 130∘, suggesting a Z-shaped structure in the inner
($\sim$ 1 pc) region. The source size in the 5 GHz image is in agreement with
the unresolved structure at 1.7 GHz. In our image no dynamic range problem is
present. The unresolved source is the only visible structure, also in large
field images. The core flux density ratio of the EVN correlated flux to the
JVLA flux density at 7.5 GHz (Baldi et al., 2019a) is 0.92. The radio spectrum
from the total flux density estimated with JVLA is peaked at 5 GHz in
agreement with the source’s small size.

J1040+09 \- This is a very complex source. The VLBA image at 5 GHz shows an
unresolved core, a northern curved jet, and some structure in the south (Fig.
7, top panel). At 1.5 GHz the core is surrounded by a diffuse emission; the
northern jet is extended $\sim$ 20 mas, transversely resolved, while the
southern jet is extended $\sim$ 60 mas and it is heavily transversely resolved
(Fig. 7, bottom panel). The classification of the structure as a jet is
uncertain; it could be a parsec-scale lobe, given its size of 15.80 x 8.7 pc
(PA 180∘). Lower resolution images confirm this structure, with a slightly
larger size. The comparison between the correlated VLBI flux density and the
flux density of the unresolved source imaged by JVLA at 7.5 GHz (Baldi et al.,
2019a) is 0.77.

Because of the low correlated flux, no self-calibration was possible. We
carefully edited the data and compared the 1.5 and 5 GHz images with the same
uv-range and angular resolution between the images shown here. In these images
the radio structures are in agreement; therefore, we are confident that the
observed complex structure is real.

J1136+51 \- This is an unresolved source with a steep spectrum,
$\alpha_{1.5}^{5}$ = 1.3, which could be an old source. No other radio source
is visible, even in large field images. The ratio of the VLBI correlated flux
to the JVLA flux density (Baldi et al., 2019a) is 1.22.

J1213+50 \- This is a slightly extended source with a nearly symmetric
structure in the EVN image at 1.7 GHz (Fig. 8). The source extension is at PA
145∘, but at PA 0∘ in the JVLA 5 GHz image (Baldi et al., 2019a) and 90∘ in
the VLBA 8 GHz image by Cheng & An (2018). No jet boosting is evident in all
images. The core flux density ratio is 0.74.

J1230+47 \- The extended symmetric structure in the EVN images at 1.7 GHz is
more than 70 pc in size (Fig. 9). The central core is resolved in the images
by Cheng & An (2018) in a two-sided structure 7-8 pc in size at the same PA of
the more extended structure, suggesting a possible recurrent activity. The
source is unresolved in JVLA images by Baldi et al. (2019a).

J1530+27 \- A core dominates in the EVN images at 1.7 GHz with a short one-
sided emission (Fig. 10) possibly identified with a jet. Because of the low
brightness of this structure, Doppler boosting is not evident. The JVLA flux
densities show an unresolved self-absorbed structure with the highest flux
density at 5 GHz (Baldi et al., 2019a).

Figure 7: Contour image of J1040+09 obtained with VLBA at 5 GHz (top) and at
1.5 GHz (bottom)  Figure 8: Contour image of J1213+50 obtained with EVN at 1.7
GHz  Figure 9: Contour image of J1230+47 obtained with EVN at 1.7 GHz  Figure
10: Contour image of J1530+27 obtained with EVN at 1.7 GHz

J1559+25 \- This source is one-sided in JVLA images (Baldi et al., 2019a) with
a jet size of 3 kpc; it is unresolved in our EVN data, but one-sided in the
8GHz VLBA image by Cheng & An (2018). The parsec- and kiloparsec-scale
structure are at the same PA. The jet is transversely resolved in the VLBA
image. The jet-to-counter-jet ratio in the VLBA image is higher than 6. The
minimum velocity is 0.34c and the maximum orientation angle is 70∘.

Our image is not affected by dynamic range problems. No other radio structure
is visible, even in large field images.

J1621+25 \- In the 5 GHz and 1.5 GHz high-resolution VLBA images this source
shows a short (28 pc in size) symmetric nuclear structure at PA = 90∘. At
lower resolution (HPBW = 12 x 12 mas), a symmetric complex double-jet
structure $\sim$ 170 pc in size is clearly evident at PA 130∘. The jet is
transversely resolved and its extension is noise limited (see Fig. 11). The
correlated flux density is 2.7 times the unresolved core estimated at 7.5 GHz
in JVLA data by Baldi et al. (2019a), suggesting an extended steep spectrum
structure.

J1628+25 \- This source is unresolved in JVLA high-resolution images (Baldi et
al., 2019a) and in the EVN image. At higher resolution (VLBA images) it is
resolved in a symmetric structure. The inner 10 pc are in PA 115∘ (1.5 and 5
GHz image), but the more external regions visible only at 1.5 GHz rotate to a
PA $\sim$ 90∘ (Fig. 12).

J1658+25 \- Two-sided jets are present in the VLBA 1.5 GHz image (Fig. 13)
slightly bent with respect to the inner structure. In the 5 GHz VLBA image
(not shown here) only a slightly extended core emission is visible. The
correlated VLBA flux is in agreement with the JVLA flux density by Baldi et
al. (2019a).

J1703+24 \- This is a complex source. In the VLBA images we see a complex two-
sided structure oriented in E-W (Fig. 14, top panel). In the EVN image a
symmetric two-sided structure is confirmed, but at a different PA: 70∘ (Fig.
14, bottom panel). We note that the JVLA PA is $\sim$ 110∘ (Baldi et al.,
2019a). The source is detected also in LOFAR observations (Capetti et al.,
2020) and it is slightly extended (5-6 kpc), in agreement with the JVLA image.

To confirm this complex structure we self-calibrated in phase only our data at
1.5 and 1.7 GHz. Self-calibrated images have a better noise level; the source
structure is confirmed. We also compared images at different frequencies at
different angular resolution using the same uv-range (cutting long or short
baselines). The present results are in agreement with all the tests we
performed.

Figure 11: Contour image of J1621+25 obtained with VLBA at 1.5 GHz (top) and 5
GHz (bottom)

Figure 12: Contour image of J1628+25 obtained with VLBA at 1.5 GHz (top) and
1.5 GHz (bottom)

J2336+00 \- We did not detect this source with VLBA at 1.5 and 5 GHz. This
could be due to a bad choice of the phase calibrator (J2340-0053), too faint
and resolved. However, this source has been slightly resolved by eMERLIN
(Baldi et al., 2021), which shows a core of 0.64 mJy beam-1 with a weaker
second component. Moreover, the target source is resolved in JVLA observations
(Baldi et al., 2015) with a faint nuclear emission (1.96 mJy, at 7.5 GHz) with
a steep spectrum: 1.00 between 1.4 and 7.5 GHz suggesting a faint resolved
core.

J2346+00 \- We observed this source with the VLBA at 1.5 and 5 GHz. The images
show a two-sided symmetric emission $\sim$ 50 pc in size. The inner nuclear
emission ($\sim$ 10pc) is at PA $\sim$ 90∘; the two-sided extended jet
emission is at PA 105∘, suggesting a Z-shaped structure (Fig. 15). It is
unresolved in JVLA images by Baldi et al. (2015).

Figure 13: Contour image of J1658+25 obtained with VLBA at 1.5 GHz.

Figure 14: Contour image of J1703+24 at 1.5 GHz with VLBA (top) and EVN at 1.7
GHz (bottom). Figure 15: Contour image of J2346+00 at 5 GHz (VLBA).

###  3.2 Other sources

VLBI observations of FR0 sources not included here have been mostly presented
in Cheng & An (2018) and Cheng et al. (2021). We already quoted their results
for the sources in common with our sample; here we briefly report their
results for sources not present in our sample to increase our knowledge of FR0
parsec-scale jets.

J0906+41 \- This is a core-dominated source with a two-sided symmetric
structure. The ratio of the NVSS flux density at 1.4 GHz to the total
correlated flux density at 8.4 GHz is $\sim$ 1.7–1.8 suggesting an inverted
radio spectrum. The jet-to-counter-jet ratio is low, 3.9, and no relativistic
boosting is necessary.

J0909+19 \- This is a two-sided source; the southern jet shows a large bend,
from PA 180∘ to PA 110∘ (see Cheng & An 2018; Cheng et al. 2021). The 8.4 GHz
correlated flux (104.5 mJy in Cheng & An 2018 and 156 mJy in Cheng et al.
2021) is significantly higher than the NVSS flux density (69.1 mJy),
suggesting an inverted radio spectrum and/or a source variability.

J0910+18 \- This is a point-like source in the VLBA 8 GHz image (Cheng & An,
2018); it shows a diffuse low-brightness halo structure in a LOFAR image
(Capetti et al., 2020). No connection is evident between the central structure
and the halo low-brightness structure, suggesting that the diffuse low-
brightness emission is the remnant of a previous activity. This structure
shows a relatively steep radio spectrum ($\alpha$ = 0.96): 480.7 mJy at 150
MHz (Capetti et al., 2020) and 50 mJy at 1.4 GHz (NVSS).

J0933+10 \- This is a one-sided source. The ratio of the correlated VLBI flux
to NVSS is only $\sim$ 0.6, suggesting a missing structure in VLBI images. The
jet-to-counter-jet ratio is $>$ 8.

J1037+43 \- This is a one-sided core dominated structure (Cheng et al., 2021).
The correlated flux is significantly lower than the NVSS source flux density
(the ratio is $\sim$ 0.2), suggesting the presence of a missing structure in
VLBI images and/or source variability. The jet-to-counter-jet ratio is $>$
20.8; assuming that it is due to Doppler relativistic beaming, it requires a
velocity larger than 0.54 c and an angle smaller than 57∘.

J1205+20 \- In images by Cheng & An (2018) and Cheng et al. (2021), this
source shows a one-sided jet and a diffuse lobe-like structure on the other
side. The correlated flux is 41.2 mJy at 8.4 MHz and the NVSS flux density is
89.9 mJy. The jet-to-counter-jet ratio near the core is $\sim$ 33 (v $>$ 0.6c
and $\theta$ $<$ 50∘). The spectral index of the diffuse NE structure is 0.75
or steeper. It could be the remnant of a previous activity and/or a peculiar
structure related to the counter-jet interaction with the ISM.

J1246+11 \- This is a one-sided jet structure in Cheng & An (2018). The source
flux density in the NVSS is 61.2 mJy, 38 mJy in Cheng & An (2018) at 8 GHz and
only 7–8 mJy in Cheng et al. (2021) at 5 and 8 GHz. More observations are
necessary to understand this source.

J1604+17 \- Only a core-like structure is visible in the image reported by
Cheng & An (2018), even if in the table it is reported as a jet-like
structure. The correlated flux is in agreement with the NVSS flux density
measure for this source. A tail-like structure interacting with a nearby
galaxy is present in the LOFAR image in Capetti et al. (2020).

J1606+18 \- A one-sided source is reported in Cheng & An (2018). The
correlated flux density is 360 mJy; the NVSS flux density is 396 mJy. The jet-
to-counter-jet ratio is $>$ 8.

NGC5322 \- This source at z= 0.00594 was studied by Feretti et al. (1984) and
more recently by Dullo et al. (2018). The source shows a two-sided symmetric
extended jet structure.

NGC1052 \- This source at z = 0.00496 is a well-known active radio galaxy
because of its low redshift. Baczko et al. (2022) confirms the presence of a
remarkably straight low-velocity double-sided jet.

##  4 Discussion

The low radio power and the lack of a kiloparsec-scale radio emission in FR0
sources could suggest an origin of the radio emission different from the
classic non-thermal emission from a SMBH and jets, for example star formation
or a disc–corona wind emission, similarly to what has been suggested for radio
quiet AGN (e.g., Panessa et al. 2019). The widespread detection of compact
parsec-scale jets in FR0s indicates that the origin of the radio emission is
synchrotron emission from outflows produced by a SMBH at the center of the
parent galaxy.

###  4.1 The jet-to-counter-jet ratio

The jet-to-counter-jet ratio, $R_{\rm JC}=F_{\rm jet}/F_{\rm cjet}$, can be
ascribed to the relativistic boosting of two otherwise symmetric jets (see,
e.g., Giovannini 2004). Therefore, this parameter can be linked to the jet
bulk speed, and it is very useful to study the jet velocity distribution. To
this end, Giovannini et al. (2005) selected a sample of radio galaxies from
the B2 Catalogue of Radio Sources and the Third Cambridge Revised Catalog
(3CR), forming the Bologna Complete Sample (BCS), with no selection constraint
on the nuclear properties. The jet-to-counter-jet ratio distribution of the
BCS sources was discussed by Liuzzo et al. (2009a), who found that $\sim$ 30%
of the sources show evidence of a two-sided structure. This result is in
agreement with a jet velocity of $\beta\sim$ 0.9, and a random orientation of
radio galaxies with respect to our line of sight.

Figure 16: Distributions of the logarithm of the ratio RJC between the flux
densities of the jet and counter-jet for the FR0s (top panel) and the source
in the BCS sample (bottom panel). The dashed portions of the histograms
correspond to lower limits on RJC.

In new sources presented here and in the literature data (see Sects. 3.1 and
3.2), we find that 56% of FR0s show a two-sided structure (52% if we exclude
NGC5322 and NGC1052). The FR0 sample with VLBI observations is not a complete
sample, but it is representative of the parsec-scale structures of this class
of sources if we take into consideration selection effects. Only sources with
an unresolved nuclear emission in JVLA images larger than 20 mJy at 1.4 GHz
(see Sect. 2) were selected for EVN observations. Moreover, sources observed
by Cheng & An (2018) have a higher nuclear flux density with respect to the
general FR0 population. These selection biases in our sample favor sources
whose nuclear emission is boosted by Doppler beaming, that is, those seen at a
smaller angle with respect to the line of sight. Therefore, we would expect as
selection effect a lower percentage of two-sided structures with respect to a
sample of randomly distributed sources as the BCS. Conversely, the percentage
of two-sided sources (56%) found here is significantly higher than the 30%
found in the sample by Liuzzo et al. (2009a).

We conclude that the high percentage of FR0 with a two-sided structure is
strong evidence that parsec-scale jets in FR0 sources are mildly relativistic
or not relativistic (v $\sim$ 0.5c or lower). The lower bulk speed of the jets
in FR0s with respect to the BCS sources is also supported by the distribution
of the jet-to-counter-jet ratio for the two samples presented in Fig. 16. The
fraction of FR0s with $R_{\rm JC}>2$ is 23%, while this fraction is between
60% and 66% (depending on the actual values of the lower limits) in the FRIs.
By considering the upper limits, the mean values of the two distributions
(Kaplan Meier estimator) are 2.7$\pm$1.0 and 1.2$\pm$0.2 for BCG galaxies and
FR 0s, respectively. The probability that the two populations are drawn from
different distributions of $R_{\rm JC}$ is $>$98%.111The tests were performed
using a censored statistical analysis with the task twosampt within the ASURV
package (Lavalley et al., 1992) implemented within the PyRAF software (Science
Software Branch at STScI, 2012). Furthermore, because the FR0s sample is
biased toward sources seen at a smaller angle with respect to the line of
sight, this difference is likely to be underestimated. Unfortunately, the
incompleteness of the FR 0s sample prevents us from deriving a robust estimate
of their jet speed.

###  4.2 Jet morphology

The jet morphology in many sources presented in the previous section is
complex. Many FR0 jets are transversely resolved, with blobs and substructures
suggesting a strong interaction with the surrounding ISM. One of the best case
is J1040+09 (Fig. 7) where at 5 GHz we see a northern jet with an S-shaped
structure, and at 1.5 GHz we have a well-resolved two-sided structure. The
southern one is more extended and is lobe-like extended to the east.

These features are instead not seen in the inner region of FRIs. This
difference can be interpreted as being due to different jet speeds in the two
classes: the slower and less energetic jets of FR0s are more affected by the
presence of a nonhomogeneous external medium; the relativistic jets of FRIs
are instead able to penetrate it without being deflected. Only at a distance
from the jet basis of several kiloparsec, when the jet velocity decreases, do
FRIs show distorted and complex morphologies.

Three FR0s, namely J1621+25, J1628+25, and J2346+00, show a Z-shaped
structure. The jets are symmetric very near to the core, and move to a
different PA after a few milliarcseconds at about the same distance where they
form symmetric extended structures at a different PA. A similar structure has
been rarely found in FRI sources and on a larger scale. The best cases are 3C
293 (van Breugel et al., 1984) and 4C 26.42 (Liuzzo et al., 2009b). These
structures have been interpreted as the interaction between a low-velocity jet
and a surrounding rotating gas disk structure (3C293) or as a low-velocity jet
interacting with rotating accretion gas at the center of the cooling cluster
A1795 (Liuzzo et al., 2009b) for 4C 26.42. This suggests that in FR0 sources
the Z-shaped structure could also be due to a gas rotating disk-like structure
coupled with a low-velocity jet.

We conclude that the FR0 morphology found in available VLBI images also show
strong evidence that parsec-scale jets in FR0 sources are not relativistic or
at most are mildly relativistic.

###  4.3 Missing correlated flux

We compared the correlated VLBI flux with the nuclear emission detected in
JVLA images at 7.5 GHz, when available, and with other VLA images in other
cases. In 40% of FR0 sources we find an agreement between the two results
suggesting that VLBI images show all or most of the source nuclear structure.
In a lower number (20%) we found a higher flux density in VLBI images,
possibly due to source variability and/or an inverted or peculiar radio
spectrum. In other sources (40%) a significant fraction of flux density is
missing in VLBI images with respect to the arcsec-scale images. This loss can
be due to variability, but mainly to the lack of sensitivity in VLBI images to
diffuse emission; the VLBI images resolve and do not see low-brightness
extended structures because of super-resolution and spectral index effects.
This is also evident when comparing high- and low-frequency VLBI images (see,
e.g., the EVN images at 1.7 GHz of J1230+47 with the 8.4 GHz of the same
source by Cheng & An 2018). A deep study of the sources where a significant
fraction of their flux density is missing in VLBI images will allow us to
understand why jets stop and die. To understand jet properties we need to
understand how the jets evolve. The FR0 jets could be able to create a diffuse
low-brightness lobe-like emission or an extended low-brightness jetted
structure, both not visible in VLBI images. The interaction between the host
galaxy and the nonthermal jet emission would be different in the two cases.
Moreover, in the one-sided sources the missing flux could be due to the
invisible counter-jet emission. Intermediate-scale observations (e.g., with
EVN and eMERLIN) and the combination of long and short baselines (as
demonstrated in Baldi et al. 2021 with eMERLIN and JVLA combined data sets)
represent a powerful tool to study the missing flux, for example in a lobe-
like structure created by the jet, in a longer jet becoming increasingly
fainter moving far from the core, or a contribution of the counter-jet side in
asymmetric sources.

###  4.4 Origin of the radio emission

Theoretical models indicate that an accreting SMBH is the origin of symmetric
relativistic jets (see, e.g., Blandford & Königl 1979; Gomez et al. 1995 and
references therein) in contrast with the low-velocity jets found here.
However, observational evidence of low-velocity jets cannot exclude the
presence of an inner relativistic spine surrounded by a low-velocity sheath.
The main difference between FRI and FR0 jets could be the relative importance
of the relativistic spine in the jet structure.

The present data are in agreement with the Garofalo & Singh model where low-
power relativistic jets are expected in sources with a low prograde spinning
black hole surrounded by advection dominated accretion flows. In these sources
the relativistic inner spine of the jet launched by the black hole ergosphere
associated with the Blandford-Znajek process (Blandford & Znajek, 1977) is
less prominent than that produced by the Blandford-Payne portion of the
accretion disk (Blandford & Payne, 1982).

In this framework we expect a jet with a subtle relativistic spine unable to
extend, in most cases, to the kiloparsec scale due to instabilities. This jet
is surrounded by a low-velocity sheath, which is the dominant structure
visible in our images. This jet structure accounts for the nature of the very
few compact FR0s, also in VLBI images (e.g., J0943+36) where the structure,
the high flux density, and the high variability suggest that we are seeing a
relativistic jet well aligned with the line of sight.

FR0 jets, because of the large diffuse sheath, are influenced by the ISM, but
also strongly interact with it and modify its properties and physical
conditions. Mapping HI on parsec scales (Murthy et al., 2022) show that a low-
power jet is also able to remove cold gas from the center of a galaxy, and
therefore to influence the accretion into the SMBH and the properties of the
host galaxy. The large number of FR0 galaxies implies that the feedback
between the energy produced by the AGN and the galaxy evolution is also
present in sources where the total radio power is low because the low-velocity
jet is not able to create and sustain extended radio lobes.

##  5 Conclusions

FR0 galaxies are the dominant local population in the radio sky. They have
multiwavelength nuclear properties similar to FRI radio galaxies, but show a
clear deficit of extended radio structures and are largely core dominated with
respect to FRI sources. Most FR0s are unresolved in radio images at arcsecond
resolution. For this reason VLBI observations are necessary to investigate the
nature and properties of their radio emission.

From the new observations presented here and literature data we conclude the
following:

  1. 1.

All FR0 sources observed with VLBI, with the exception of J2236+00, have been
detected. This result, together with the observed jet structures visible in
most VLBI images, show that the FR0 radio emission is related to the nuclear
activity of an accreting SMBH from where two symmetric synchrotron jets are
launched.

  2. 2.

The complex jet structures and the large number of two-sided structures are
strong evidence that the FR0 jets present in VLBI images are not relativistic,
contrary to what is observed in FRI sources.

  3. 3.

The properties of a few FR0s also suggest the presence of a relativistic faint
inner spine in these sources, as requested by the proposed identification as
misoriented BL Lacs. These results are in agreement with the suggestion that
FR0 jets are associated with a low (and prograde) spin SMBH. In these sources
jets are structured; a relativistic faint inner spine launched by the black
hole ergosphere (BZ jet) is surrounded by a dominant nonrelativistic shear
launched by the accretion disk (BP jet). Because of the strong interaction
between the shear and the ISM, the jet cannot survive to reach the kiloparsec
scale.

  4. 4.

In several FR0 galaxies the correlated parsec-scale flux density is a small
fraction of the nuclear flux density measured at subarcsec resolution,
indicating that a significant amount of radio emission is produced at
intermediate scales. EVN and MERLIN observations are necessary to understand
and probe this component, most likely associated with low surface brightness
jets or diffuse lobes.

  5. 5.

Despite the low power and small size of radio emission in FR0s, our results
represent clear evidence for the presence of collimated jets. This implies
that AGN feedback is also at work in these sources. Recent observations have
shown that a low-power jet is able to remove cold gas from the center of a
galaxy, and therefore to influence the accretion onto the SMBH.

###### Acknowledgements.

We would like to thank the Referee for useful comments. RL acknowledges
financial support from the State Agency for Research of the Spanish MCIU
through the “Center of Excellence Severo Ochoa” award for the Instituto de
Astrofísica de Andalucía (SEV-2017-0709), from the Spanish Ministerio de
Economía y Competitividad, and Ministerio de Ciencia e Innovación (grants
AYA2016-80889-P, PID2019-108995GB-C21), the Consejería de Economía,
Conocimiento, Empresas y Universidad of the Junta de Andalucía (grant
P18-FR-1769), the Consejo Superior de Investigaciones Científicas (grant
2019AEP112). The National Radio Astronomy Observatory is a facility of the
National Science Foundation operated under cooperative agreement by Associated
Universities, Inc. The European VLBI Network is a joint facility of
independent European, African, Asian, and North American radio astronomy
institutes.

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