11institutetext: 1 Joint ALMA Observatory - ESO, Av. Alonso de Córdova, 3104,
Santiago, Chile
2 Bosscha Observatory and Astronomy Research Division, FMIPA, Institut
Teknologi Bandung, Indonesia
3 CAUP, Porto University, Portugal
4 Instituto de Astrofísica de Andalucía-CSIC, Granada, Spain
5 National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA,
22903, USA

# Anatomy of a blazar in the (sub-)millimeter: ALMA Observations of PKS
0521-365

S. Leon 11 P. C. Cortes 1155 M. Guerard 11 E. Villard 11 T. Hidayat 22 B.
Ocaña Flaquer 33 B. Vila-Vilaro 11

(Received September 15, 1996; accepted March 16, 1997)

###### Abstract

Aims. We aim at analyzing the (sub-)millimeter emission in a nearby blazar,
PKS 0521-365, to study the synchrotron and thermal emission in the different
components detected at low frequency.

Methods. We analyze the archive public data of the ALMA Cycle 0 where PKS
0521-365 is used as a calibrator. A total of 13 projects with 23 dataset is
analyzed in band 3, 6 and 7 and combined. The whole set of data is combined
and wavelet filtered to obtain a deep image towards PKS 0521-365 reaching a
dynamic range of 47000. The individual emission flux is measured at different
date over a period of 11 months in various components. Finally we analyze the
Spectral Energy Distribution (SED) in each different component, including the
radio jet and counter jet.

Results. The point sources detected in the field follow a similar distribution
to previous studies. The blazar flux shows large variation especially in band
3. Different components are observed: core, radio jet and newly detected
counter jet, Hot Spot (HS) and a disky structure roughly perpendicular to the
jet. The HS emission is formed by a point source surrounded by an extended
emission. The viewing angle of the jet is about $30^{\circ}$ with a Doppler
factor of $\delta=1.6$. The HS is at a distance of 19 kpc from the center. The
SED analysis shows a strong variation of the core spectral index, especially
in band 3. The two components in the radio jet have roughly a flat spectral
index in band 6 and 7.

Conclusions. Using these ALMA data the different weak and extended components
in PKS 0521-365 are detected. The analysis of both jets constrains the
geometrical distance of the HS to the center. The SED presents a different
shape in time and frequency for each component. Finally a new structure is
detected roughly perpendicular to the radio jet and a thermal emission origin
is currently favoured. Further observations at higher spatial resolution are
needed to confirm that hypothesis.

###### Key Words.:

galaxies: jets – galaxies: ISM – galaxies: active – galaxies: BL Lacertae
objects: PKS 0521-365

##  1 Introduction

Inside the large range of different type of active galactic nuclei (AGN)
blazars are the most powerful sources. Showing intense emission across the
electromagnetic spectrum, it is thought that this strong continuum emission is
due to an ejection of relativistic plasma nearly on the line of sight. A radio
jet is commonly observed in blazars at lower frequencies (Antonucci &
Ulvestad, 1985) and their strong flux is accounted for the relativistic
beaming. A blazar is then a particular case of a head-on radio galaxy from an
underlying population with a spread geometry distribution of the jet
(Antonucci & Ulvestad, 1985; Urry & Padovani 1995) originating from an AGN and
oriented nearly head-on towards the observer according to the unified scheme
proposed two decades ago (e.g. Urry & Padovani, 1995). The jet can be detected
up to large energy ranges (Gamma, X-ray) because of the strong beaming effect.
Blazars can be divided into two types, the low power AGNs, mainly without
Broad-Line Regions (BLRs) and a large synchrotron break frequency (Capetti et
al. 2010, Tavecchio et al. 1998), and the Flat-Spectrum Radio Quasars (FSRQs)
with a lower break frequency (Maraschi & Tavecchio 2003). The first kind forms
the family of BL Lac radio galaxies which was the first of its kind detected
(e.g. Blandford & Rees 1978). The SED of the blazar shows normally two bumps,
the synchrotron bump at low energy peaking in the infrared (IR)-X-ray and the
Inverse Compton (IC) bump normally in $\gamma$-ray. Each bump is characterized
by a peak frequency $\nu_{1}$ and $\nu_{2}$ at the peak of the emission in
each regime. Fossati et al. (1998) found an anti-correlation between the peak
frequency $\nu_{1}$ and the bolometric and synchrotron luminosity. They
suggested a link between the source power and the frequency peak. The sequence
may match a spectroscopic one, from powerful FSRQ sources at the low-peak,
high-luminosity end and the BL Lacs with high-peak and low-luminosity. The BL
Lacs do not show normally signs of BLRs. It was suggested that the Inverse
Compton (IC) emission in FSRQ was produced through the Synchrotron Self
Compton (SSC) mechanism or from external seed photons from the BLRs. These
photons would be up-scattered by the synchrotron electrons (e.g. Chen et al.
2012). The long-standing Unified Scheme for radio-loud AGN proposes that the
Fanaroff-Riley I and II radio galaxies (Fanaroff & Riley, 1974) are the
underlying progenitors of BL Lacs and FSRQs galaxies (Bicknell 1995, Fossati
et al. 1998).

In a population synthesis study, Lister & Marscher (1997) used realistic
luminosity functions to show that the majority of compact extragalactic radio
sources have jets with low Lorentz factors. The AGN in general and the jet in
particular are known to drive the evolution of their host galaxy (Best &
Heckman 2012, Khalatyan et al. 2008, Sturm et al. 2011) through a feedback
mechanism of depositing energy in the interstellar medium (ISM). At large
scale the radio jet can slow down or stop the cooling of the massive X-ray
halo (Fabian 2012) that surrounds these galaxies. At the galactic scale the
jet interacts with the local ISM (molecular gas, X-ray halo, etc), and it has
been suggested that for host galaxies with a dense ISM phase the radio jet
could be completely confined to the central part with no outwards propagation
(Carvalho 1998) forming Compact Steep Spectrum galaxies. It was argued
recently (Rashed et al., 2013) that the backflow along the radio jet in the
elliptical host galaxy SDSS J080800.99+483807.7 might have triggered star
formation. This star formation triggering mechanism in radio galaxies has been
used to explain the correlation between the radio and optical morphologies of
distant 3C radio galaxies (McCarthy et al., 1987). Some of the most dramatic
jet-induced star formation, with star formation rates (SFR) as high as 1000
$M_{\odot}$yr-1, is associated with very luminous radio galaxies at redshifts
up to $z\sim 4$ (Dey et al. 1997; Bicknell et al. 2000).

One study has found a clear interaction of the radio jet and the ISM in the
Narrow Line Regions (NLRs) of Seyfert galaxies (Falcke et al., 1998). Such
interactions play a major role in determining the morphology of the NLR, as
the radio jets sweep up and compress ambient gas, producing ordered structures
with enhanced surface brightness in line emission. The nearby radio galaxy M
87 which can be classified as a BL Lac was recently found to have a giant
radio-to-X-ray synchrotron (Stawarz et al., 2006). It was argued that the
presence of a gaseous concentration in the central part would provoke the
interaction of the jet with such a feature resulting in the formation of a
stationary shock structure in the innermost parts of the M 87 jet.

The radio source PKS 0521-365 (J0522-364) is a nearby AGN (Danziger et al.
1979) and it is one of the most remarkable extragalactic objects of the
southern sky, because it exhibits a variety of nuclear and extra-nuclear
phenomena. In this work we assume a cosmology with $\Omega_{m}=0.27$,
$\Omega_{\Lambda}=0.73$, $H_{0}=71$ km s-1 Mpc-1 (Spergel et al. 2003). At
$z=0.0565$, PKS 0521-365 is at a comoving radial distance of 236 Mpc, and have
a linear scale size of 1.15 kpc/arcsec. Classified as a BL Lac object, it
shows strong narrow and broad emission lines typical of type 1 AGN both in the
optical and the UV (Ulrich 1981; Danziger et al. 1983; Scarpa et al. 1995).
The source has a prominent radio, optical, and X-ray jet (Danziger et al.
1979; Keel 1986; Falomo 1994; Macchetto et al. 1991; Scarpa et al. 1999;
Birkinshaw et al. 2002), which resembles that of the nearby radio galaxy M 87
(Sparks et al. 1994).

Here we present archival ALMA observations of the radio source PKS 0521-365.
We retrieved public Cycle-0 observation where PKS 0521-365 was used as
calibrator in band 3, 6, and 7. Section 2 presents the observations and data
reduction, section 3 the millimeter and sub-millimeter distribution of the
emission, section 4 the variability in the emission, section 5 computes the
spectral index, section 6 the discussion, and section 7 the summary and
conclusions.

##  2 Observations and data reduction

Publicly available data from ALMA Cycle 0 observations were used to analyze
the sub-millimeter properties of the blazar PKS 0521-365. As shown on Table 1
the observations span a period of about 1 year from 2012/01/10 to 2012/11/19.
The observed frequencies go from 87.3 GHz up to 372.5 GHz, which covers bands
3, 6 and 7. PKS 0521-365 was generally used as a bandpass calibrator for the
science projects sharing the same spectral setup, and observed between 2 to 4
minutes per execution. If two ExecBlocks (hereafter EBs) were observed
consecutively, we merged them in a single dataset and the observing time was
chosen as the starting time of the first EB. Each EB has four spectral windows
of, generally, 2 GHz bandwidth distributed over 4 basebands, two in the USB
window and two in the LSB window. For the flux determination of each dataset
the four spectral windows were combined and the LSB/USB spectral windows were
used to determine the spectral index $\alpha$, hereafter defined as
$S_{\nu}\propto\nu^{\alpha}$ with $S_{\nu}$ the flux density at the frequency
$\nu$, in one dataset as indicated further. Before combining the different
dataset in each band, a self-calibration was performed on each of them to
improve the phase/amplitude gain solutions thanks to the strong continuum
emission. The effective frequency quoted for the combined bands was computed
using the deconvolution CLEAN task in CASA with the multi-frequency synthesis
(mfs) mode, see Rau & Cornwell (2011) for details.

The two most prominent feature of PKS 0521-365 are the strong core and the so-
called ”Hot Spot” (HS) located at 8.6″at the South-East of the core. The main
properties of each dataset and the measured flux of the core and the Hot Spot
are summarized in Table 2. For the core the flux in the central area was
measured directly since the jet contribution is less than 1% of the total
flux. The HS flux was measured including the pixels above 3-$\sigma$ of the
RMS in the HS area.

Table 1: Journal of the observations Id | Project UID | Date | Frequency | Integration
---|---|---|---|---
|  |  | (GHz) | (seconds)
J0522-364_99_1 | 2011.0.00099.S | 2012/04/06 22:19 | 98.7/110.3 | 123
J0522-364_99_2 | 2011.0.00099.S | 2012/07/04 11:53 | 98.7/110.3 | 110
J0522-364_99_3 | 2011.0.00099.S | 2012/07/03 11:25 | 100.6/112.5 | 171
J0522-364_131_1 | 2011.0.00131.S | 2012/01/24 23:20 | 101.4/114.5 | 296
J0522-364_467_1 | 2011.0.00467.S | 2012/03/27 21:23 | 96.5/108.3 | 175
J0522-364_108_3 | 2011.0.00108.S | 2012/07/29 10:35 | 87.3/99.0 | 233
J0522-364_268_1 | 2011.0.00268.S | 2012/01/10 21:55 | 252.8/241.3 | 200
J0522-364_268_2 | 2011.0.00268.S | 2012/01/13 02:26 | 252.8/241.3 | 200
J0522-364_210_1_2 | 2011.0.00210.S | 2012/08/09 09:58 | 219.7/231.2 | 463
J0522-364_210_3 | 2011.0.00210.S | 2012/08/26 09:14 | 219.7/231.2 | 236
J0522-364_108_1 | 2011.0.00108.S | 2011/11/05 06:51 | 342.8/354.3 | 115
J0522-364_108_2 | 2011.0.00108.S | 2011/11/06 02:05 | 342.8/354.3 | 115
J0522-364_397_1 | 2011.0.00397.S | 2011/11/16 05:24 | 339.0/351.0 | 141
J0522-364_124_1_2 | 2011.0.00124.S | 2012/07/18 09:41 | 318.1/330.1 | 464
J0522-364_768_1 | 2011.0.00768.S | 2012/07/31 09:54 | 339.8/351.7 | 175
J0522-364_768_2_3 | 2011.0.00768.S | 2012/08/14 09:15 | 339.8/351.7 | 351
J0522-364_83_1 | 2011.0.00083.S | 2012/06/16 11:25 | 342.8/354.7 | 326
J0522-364_83_2 | 2011.0.00083.S | 2012/06/17 11:43 | 342.8/354.7 | 326
J0522-364_83_3 | 2011.0.00083.S | 2012/08/27 06:59 | 342.8/354.7 | 326
J0522-364_208_1 | 2011.0.00208.S | 2012/10/22 03:56 | 342.4/354.4 | 200
J0522-364_223_1 | 2011.0.00223.S | 2012/10/21 06:21 | 330.8/345.6 | 342
J0522-364_223_2 | 2011.0.00223.S | 2012/10/22 05:08 | 330.8/345.6 | 342
J0522-364_340_1_2 | 2011.0.00340.S | 2012/11/19 07:36 | 358.0/372.5 | 130
Table 2: Dataset properties (natural weighting) Id | MJD | $\nu_{eff}$ | Beam | RMS | Flux (Core) | Flux (HS)
---|---|---|---|---|---|---
|  | (GHz) | (arcsec) | (mJy) | (mJy) | (mJy)
J0522-364_99_1 | 56023.93 | 104.55 | $1.34\times 1.72$ | 3.9 | 7174 | 83
J0522-364_99_2 | 56112.50 | 104.33 | $1.69\times 2.41$ | 3.0 | 6111 | 178
J0522-364_99_3 | 56111.48 | 106.62 | $1.43\times 2.46$ | 0.6 | 4858 | 163
J0522-364_131_1 | 55950.97 | 107.99 | $3.14\times 4.46$ | 1.4 | 5317 | 142
J0522-364_467_1 | 56013.89 | 102.40 | $1.47\times 1.69$ | 3.0 | 9623 | 133
J0522-364_108_3 | 56137.44 | 93.16 | $1.52\times 2.64$ | 1.3 | 5474 | 177
J0522-364_268_1 | 55936.91 | 247.08 | $1.32\times 2.68$ | 2.1 | 4208 | 33
J0522-364_268_2 | 55939.10 | 247.08 | $1.19\times 1.78$ | 1.0 | 4524 | 44
J0522-364_210_1_2 | 56148.42 | 225.43 | $0.67\times 0.91$ | 0.3 | 4732 | 53
J0522-364_210_3 | 56165.38 | 225.44 | $0.68\times 0.97$ | 0.7 | 4735 | 52
J0522-364_108_1 | 55870.29 | 348.58 | $1.18\times 1.35$ | 0.7 | 5345 | 17
J0522-364_108_2 | 55871.09 | 348.58 | $1.17\times 2.95$ | 0.6 | 5077 | 14
J0522-364_397_1 | 55881.22 | 344.99 | $1.23\times 1.39$ | 0.9 | 5254 | 19
J0522-364_124_1_2 | 56126.40 | 324.15 | $0.43\times 0.80$ | 0.3 | 3310 | 25
J0522-364_768_1 | 56139.41 | 345.77 | $0.41\times 0.75$ | 1.8 | 3973 | 22
J0522-364_768_2_3 | 56153.39 | 345.77 | $0.45\times 0.66$ | 0.2 | 4087 | 23
J0522-364_83_1 | 56094.48 | 348.82 | $0.58\times 0.91$ | 0.6 | 4870 | 21
J0522-364_83_2 | 56095.49 | 348.82 | $0.47\times 0.82$ | 0.8 | 4683 | 26
J0522-364_83_3 | 56166.29 | 348.82 | $0.44\times 0.95$ | 4.1 | 3733 | 31
J0522-364_208_1 | 56222.16 | 348.42 | $0.52\times 0.79$ | 0.5 | 4245 | 26
J0522-364_223_1 | 56221.26 | 338.19 | $0.51\times 0.60$ | 0.3 | 4518 | 24
J0522-364_223_2 | 56222.21 | 338.19 | $0.52\times 0.63$ | 0.4 | 4588 | 19
J0522-364_340_1_2 | 56250.32 | 365.28 | $0.50\times 0.61$ | 0.9 | 7666 | 10

The self-calibrated datasets were combined in each band and imaged. A final
superset of all self-calibrated dataset, called ”band 3+6+7”, was created and
imaged to produce the most sensitive continuum image at an effective frequency
of 221.02 GHz. The total integration time is of 1.55 hours with an equivalent
of 66 pad positions and 2145 baselines. The self-calibration of these data
allowed us to achieve a RMS noise of 158 $\mu$Jy for the band 3+6+7 with a
dynamic range (SNR) of 27100 without wavelet filtering. The final spatial
resolution with a natural weighting is of $0.53\arcsec\times 0.71\arcsec$. It
is worth noting that the band 7 combined image has the deepest sensitivity (91
$\mu$Jy) which is similar to the all-in-one combined data. The properties of
the combined dataset in band 3, 6, 7 and 3+6+7 are summarized in Table 3
together with the Hot Spot flux which is not varying with time. Each band
reaches an effective bandwidth of 27 GHz (band 3) to 285 GHz (band 3+6+7). The
effects of the lowest spatial frequencies observed at the different bands was
checked by computing the flux of the HS in band 3 using the spatial
frequencies of band 7. The HS flux is in that case lower by 5% compared to the
measured flux with the full uv-coverage. Given the calibration uncertainty we
can assume safely that the differential sensitivity to spatial scales between
bands is negligible in the case of the observations of PKS 0521-365.

Table 3: Sets of combined data (natural weighting) together with the Hot Spot flux. The properties indicated are with the wavelet filtering. See text for more details. Set | $\nu_{eff}$ | Beam | RMS | Dynamic Range | Flux (HS)
---|---|---|---|---|---
| (GHz) | (arcsec) | ($\mu$Jy) |  | (mJy)
band 3 | 100.43 |  $1.59\times 2.49$ (PA=78∘) | 306 | 15300 | 153
band 6 | 237.06 |  $0.69\times 0.93$ (PA=93∘) | 147 | 28500 | 39
band 7 | 336.44 |  $0.48\times 0.64$ (PA=90∘) | 91 | 40000 | 22
band 3+6+7 | 221.02 |  $0.53\times 0.71$ (PA=90∘) | 90 | 47000 | 43

###  2.1 Wavelet filtering

The wavelet transform is a powerful signal processing technique which provides
a decomposition of the signal into elementary local contribution labeled by a
scale parameter (Grossman & Morlet 1985). They are the scalar products with a
family of shifted and dilated functions of constant shape called wavelets. The
data are unfolded in a space-scale representation which is invariant with
respect to dilation of the signal. Such an analysis is particularly suited to
study signals which exhibit space-scale discontinuities and/or hierarchical
features.
The wavelet analysis of the PKS 0521-365 datasets is performed on the combined
dataset in each band after the deconvolution process using the CLEAN algorithm
with natural weighting. As described in Leon et al. (2000) we use the ”A
trous” algorithm (see Bijaoui 1991). It allows to get a discrete wavelet
decomposition within a reasonable CPU time. The kernel function $B_{s}(x,y)$
for the convolution is a $B_{3}$ spline function.

The wavelet transform (WT) $W(i,x,y)$ of the image $I(x,y)$ is then obtained
from the following steps:

$\displaystyle c_{o}(x,y)$ $\displaystyle=$ $\displaystyle I(x,y),$ (1)
$\displaystyle c_{i}(x,y)$ $\displaystyle=$ $\displaystyle
c_{i-1}\mbox{$\ast$}B_{s}(\frac{x}{2^{i}},\frac{y}{2^{i}}),$ (2)
$\displaystyle W(i,x,y)$ $\displaystyle=$ $\displaystyle
c_{i}(x,y)-c_{i-1}(x,y).$ (3)

The last plane, called Last Smoothed Plane (LSP), is the residuals of the last
convolution and not a wavelet plane, but for convenience we will speak of
wavelet plane for all these planes. Each plane $W(i,x,y)$ represents the
details of the image at the scale $i$. A wavelet plane $i$ has a spatial
resolution of $0.86\times 2^{i}$ pixels which translates into 0.21″ in band 3
and 0.08″ in band 6, 7 and combined for the wavelet plane 0.

Then each component of the raw map $W(i,x,y)$ was filtered, above a given
threshold $\beta$, using the RMS noise $\sigma_{i}$ on each wavelet scale $i$
to get the filtered wavelet planes $W_{f}(i,x,y)$ of the radio continuum
distribution in the different bands:

$\displaystyle W_{f}(i,x,y)$ $\displaystyle=$ $\displaystyle
W(i,x,y)\mbox{\small\hskip 2.84544pt If $|W(i,x,y)|>\beta\sigma_{i}$}$
$\displaystyle=$ $\displaystyle 0\mbox{\small\hskip 39.83368pt Otherwise}$

In this study the wavelet coefficients are filtered at the 4.5-$\sigma$ level.
After WT filtering for the continuum distribution in the different bands the
actual dynamic range increases in the different filtered set of data. Thus the
combined band 3+6+7, after WT filtering, has a dynamic range of 47000 and the
combined band 7 reaches a dynamic range of 40000. By combining the wavelet
scales one can achieve a different spatial resolution with a specific RMS
noise. Since the contribution of an image $I$ is spread over the different
spatial scales, the reconstruction through a limited set of wavelet scales is
a lower limit to the total emission contribution. That effect is visible in
Table 4 where the core flux is computed for the different wavelet
reconstructions. A final equivalent RMS noise of 90 $\mu$Jy can be achieved
for the band 3+6+7. It is worth noting that the ”A trous” wavelet
decomposition is not strictly orthogonal and the spatial scale contribution
can be spread over different wavelet scales. But the algorithm has a quasi-
isotropic kernel which makes the physical analysis of each scale easier to
perform. In our notation of the WT for the continuum images the scale 0 has
the highest spatial resolution and the scale 7 is the LSP. The maximum scale
was chosen to be 7 since largest wavelet scales are not giving fundamental
informations.

Table 4: Properties of different wavelet reconstruction of the band 3+6+7 combined and WT filtered at 4.5-$\sigma$. In our notation the scale 7 is the LSP (see text). Wavelet Planes | RMS | Core Flux
---|---|---
| ($\mu$Jy) | (%)
6 - 7 | 3.7 | 60
5 - 7 | 17.3 | 68
4 - 7 | 38.9 | 77
3 - 7 | 64.8 | 86
2 - 7 | 99.3 | 95
1 - 7 | 107.9 | 100

##  3 (Sub-)millimeter distribution

In order to analyze the sub-millimeter emission towards PKS 0521-365 the point
source distribution was searched in that field. The point sources were
searched in the band 3+6+7 WT-filtered field with an effective RMS noise of 90
$\mu$Jy. The list of the point sources was obtained by using the SExtractor
software (Bertin & Arnouts, 1996) at a 4.5-$\sigma$ level with a minimum of 5
pixels per sources to avoid spurious detections. A total of 16 sources are
detected in a $1\arcmin\times 1\arcmin$ box and their properties are listed in
Table 5. Few sources are associated with the continuum emission in PKS
0521-365 and most of them should be related with background sources as shown
on Figure 1. A comparison of the maximum peaks detected in the psf image was
done with the point sources and no correlation was found. The source #10 and
#12 are respectively associated with the core and the HS emission. It is
important to note that the spatial distribution of these sources are not
clustered on the radio continuum emission of PKS 0521-365 but rather uniform
through the band 3+6+7 field. Because of the RMS noise of 90 $\mu$Jy in the
WT-filtered band 3+6+7 field we can estimate the number of sources with a flux
greater than 0.5 mJy to be $S(>0.5\mbox{mJy})\sim 5760\deg^{-2}$. An
estimation of the cumulative count at 221 GHz is given on Figure 2 between a
flux of 0.1 mJy and 3 mJy. This estimation is not addressing entirely the
completeness of the detection and the possible spurious detections and bias.
But we note that this estimation is similar, but lower, to the one derived by
Hatsukade et al. (2013) using ALMA fields at 238 GHz. Figure 3 shows the band
6 and 7 emission by selecting the wavelet planes 2 to 4 which biases the
slightly extended emission. The analysis of the point sources is beyond the
scope of this article.

Table 5: Properties of the point sources detected in the band 3+6+7 ($\nu_{eff}=221.02$ GHz) at a 4-$\sigma$ level in a box of $1\arcmin\times 1\arcmin$. The $X,Y$ spatial positions are relative to the nucleus of PKS 0521-365. The spatial resolution of the band 3+6+7 dataset is $0.53\arcsec\times 0.71\arcsec$.  Id | $\alpha_{2000}$ | $\delta_{2000}$ | X | Y | Flux | Flux Error | FWHM
---|---|---|---|---|---|---|---
| (degree) | (degree) | (″) | (″) | (mJy) | (mJy) | (″)
1 | 80.7422245 | -36.4503386 | -1.7 | 29.7 | 1.05 | 0.05 | 0.59
2 | 80.7440029 | -36.4505140 | -6.9 | 29.1 | 1.06 | 0.05 | 0.51
3 | 80.7331005 | -36.4518921 | 24.7 | 24.1 | 0.58 | 0.03 | 0.35
4 | 80.7479270 | -36.4527443 | -18.2 | 21.1 | 0.80 | 0.04 | 0.44
5 | 80.7500829 | -36.4533700 | -24.5 | 18.8 | 1.50 | 0.05 | 0.55
6 | 80.7338043 | -36.4552554 | 22.7 | 12.0 | 1.39 | 0.06 | 0.66
7 | 80.7413304 | -36.4558385 | 0.9 | 9.9 | 0.85 | 0.04 | 0.49
8 | 80.7476982 | -36.4563809 | -17.5 | 8.0 | 1.50 | 0.06 | 0.62
9 | 80.7434403 | -36.4564975 | -5.2 | 7.6 | 0.57 | 0.03 | 0.35
10 | 80.7415967 | -36.4585668 | 0.1 | 0.1 | 4302.94 | 0.07 | 0.66
11 | 80.7460065 | -36.4575836 | -12.7 | 3.6 | 0.84 | 0.04 | 0.44
12 | 80.7440576 | -36.4598546 | -7.0 | -4.5 | 45.47 | 0.11 | 0.98
13 | 80.7472396 | -36.4606248 | -16.2 | -7.3 | 1.12 | 0.05 | 0.51
14 | 80.7397929 | -36.4608532 | 5.3 | -8.1 | 0.72 | 0.04 | 0.35
15 | 80.7373352 | -36.4610799 | 12.5 | -8.9 | 1.64 | 0.05 | 0.55
16 | 80.7487821 | -36.4662021 | -20.7 | -27.4 | 1.97 | 0.07 | 0.87
Figure 1: Point sources detected in the combined field of band 3, 6 and 7
($\nu_{eff}=221.02$ GHz) after WT filtering at 4.5$\sigma$. The symbols are
proportional to the flux. The field is centered on PKS 0521-365. The colored
circles represent the primary beamsize of each band. Figure 2: Cumulative
number count at 221 GHz in a field of $1\arcmin\times 1\arcmin$ between a flux
of 0.1 mJy and 3 mJy. The effective RMS noise in the WT-filtered image is 100
$\mu$Jy. Figure 3: Contour levels at [4, 4.5, 5, 5.5, 6]-$\sigma$ of the B6+B7
WT for the scales 2 to 4. The image is the band 7 emission.

In Figures 4, 5, 6 the WT-filtered emission is shown towards PKS 0521-365 in
band 3 (100 GHz), band 6 (237 GHz) and band 7 (336 GHz). Figure 7 shows the
full WT-filtered combined dataset in band 3+6+7. The radio continuum emission
shares common features in the three bands: a strong core emission, a radio jet
oriented in the N-W direction with a $PA\sim 126^{\circ}$, a Hot Spot in the
symmetric direction of the jet and an elongated jet-like feature close to the
nucleus and nearly perpendicular to the main jet with a $PA\sim 66^{\circ}$.
That latter feature is more visible in band 6 and 7 than in band 3 mainly
because of the better spatial resolution. The main jet has a length of 6.8″,
i.e. 7.7 kpc in projection at the distance of PKS 0521-365. The jet-like
feature has a quite symmetric feature with a length of about 2.6″(2.9 kpc) in
each direction. In band 6 the counter-jet is visible towards the S-E with a
length of 3.9″(4.4 kpc). It is probably visible as well in band 3 but the
poorer spatial resolution is not good enough. Because of the dimming at high
frequency the counter-jet is not detected so clearly in band 7. We discuss
below the comparison between the jet and counter-jet. In band 6 and 7 the Hot
Spot is spatially resolved with some extended emission as it was checked by
removing a Gaussian component fitted on the Hot Spot and is indicated by the
FWHM of the source #36 detected by SExtractor (see Table 5). Inside the jet
two main spots, dubbed A and B, are visible and were already detected in
different wavelengths (Tingay & Edwards 2002, and references therein).
Together with the HS we give their coordinates in Table 6 and their positions
in Figure 7. We discuss hereafter the diffuse emission outside of the nucleus,
jet and HS which should trace the interaction of the jet with the local
interstellar medium.

Figure 4: Continuum emission towards PKS 0521-365 in band 3 ($\nu_{eff}=100.43$ GHz) after filtering at 4.5-$\sigma$ using a WT. The contours are at [3, 5, 7, 9, 11, 15, 20, 30, 40, 50, 100, 200, 1000, 5000]-$\sigma$ for a RMS noise of 306 $\mu$Jy. Figure 5: Continuum emission towards PKS 0521-365 in band 6 ($\nu_{eff}=237.06$ GHz) after filtering at 4.5-$\sigma$ using a WT. The contours are at [3, 4, 5, 7, 9, 11, 15, 20, 30, 40, 50, 100, 200, 1000, 5000]-$\sigma$ for a RMS noise of 147 $\mu$Jy. Figure 6: Continuum emission towards PKS 0521-365 in band 7 ($\nu_{eff}=336.44$ GHz) after filtering at 4.5-$\sigma$ using a WT. The contours are at [3, 4, 5, 7, 9, 11, 15, 20, 30, 40, 50, 100, 200, 1000, 5000]-$\sigma$ for a RMS noise of 91 $\mu$Jy. Figure 7: Continuum emission towards PKS 0521-365 combining the band 3,6 and 7 data ($\nu_{eff}=221.02$ GHz) after filtering at 4.5-$\sigma$ using a WT. The contours are at [3, 4, 5, 7, 9, 11, 15, 20, 30, 40, 50, 100, 200, 1000, 5000]-$\sigma$ for a RMS noise of 90 $\mu$Jy. The red crosses point on 3 regions indicated in the text. Table 6: Coordinates (J2000) of some regions in PKS 0521-365. See Figure 7. Region | RA | Dec
---|---|---
| (J2000) | (J2000)
Hot Spot (HS) | 05:22:58.58 | -36:27:35.6
A | 05:22:57.85 | -36:27:29.9
B | 05:22:57.62 | -36:27:27.9

As indicated before, the WT-filtered decomposition allows to analyze the
emission present at specific scales. On Figure 8 the emission in band 3+6+7 at
different scales is restored using the WT-filtered decomposition. The most
emphasized features using that technique are the counter-jet emission and the
extended emission of the HS. The discrete emission outside the central part is
emphasized too in the different reconstructions and its relevance was
partially addressed before for the low brightness emission in band 6 and 7
using the wavelet planes 2 to 4 (see Figure 3) which select the spatial range
0.4″-1.4″(see Leon et al., 2000), i.e. 450-1600 pc in the galaxy PKS 0521-365.

Figure 8: Image restoration of the combined band 3+6+7 emission using
different range of the wavelet filtered planes: planes 6-to-7 (low spatial
frequency) on the top left figure to planes 1-to-7 (high spatial frequency) on
the bottom right figure. Note that the full reconstruction is achieved with
the planes 0-to-7.

Because of the small viewing angle of the radio jet in blazar and in
particular in PKS 0521-365 the relativistic beaming enhances the approaching
blobs of plasma and dims the receding plasma. The counter-jet is then more
visible at low frequencies as it is observed here in PKS 0521-365 (see Table
7). Because of the low spatial resolution in band 3 we searched for the
counter-jet emission by using uniform weights during the imaging step
achieving detection in only 3 datasets. In band 6, 4 datasets reveal the
counter-jet emission and because of its low brightness, the uncertainty on its
flux is quite large. To compute the jet/counter-jet emission ratio we use the
band 6 estimation with a better spatial resolution, i.e the band 3+6+7 dataset
which have an effective frequency in band 6 and a low RMS noise. The jet and
counter-jet flux was estimated by subtracting to the image a Gaussian source
fitting the nucleus and selecting a polygonal area on the jet and the counter-
jet. The emission ratio between the jet and the counter-jet was found to be
$\Re=10.2$.

Assuming an equal intrinsic emission between the jet and the counter-jet, the
ratio $J$ between both componet can be estimated as following (e.g. in
Ghisellini et al. 1993):

$J=\left(\frac{1+\beta\cos\theta}{1-\beta\cos\theta}\right)^{p}$ (4)

where $\beta$ is the bulk velocity expressed in units of the speed of light,
$\theta$ is the viewing angle and $p$ can be expressed in the simplest cases
as $p=2+\alpha$ (a moving isotropic source) or $p=3+\alpha$ (a continuous jet)
with $\alpha$ the spectral index of the jet. On Figure 9 the ratio $J$ is
shown for different bulk velocities taking an intermediate exponent $p=2.5$
which is used for the analysis. To match the PKS 0521-365 ratio it appears
that the viewing angle must be in the range
$\theta\subset[16^{\circ},38^{\circ}]$ with a moderate bulk velocity
$\beta\subset[0.45,0.55]$. These values give us a Lorentz factor
$\Gamma\subset[1.1,1.2]$ and a Doppler factor $\delta\subset[1.3,1.7]$.

Figure 9: Jet/counter-jet ratio $J$ for an exponent $p=2.5$ in function of the
viewing angle $\theta$. The curves for different bulk velocity $\beta$ are
plotted. The horizontal line (dashed red line) indicates the flux ratio in PKS
0521-365.

These results are similar to the values obtained by Pian et al. (1996) which
have estimated the Lorentz factor of the plasma responsible for the radio jet
to be $\Gamma=1.2$ and a viewing angle of $\theta\sim 30^{\circ}$ giving a
Doppler factor of $\delta=1.6$. It is also consistent with the space parameter
exploration of Tingay et al. (2002) (See Figure 5 in their work).

Table 7: Flux of the counter-jet in PKS 0521-365. band | $\nu_{eff}$ | Flux | RMS | Comments
---|---|---|---|---
| (GHz) | (mJy) | (mJy) |
3 | 106.62 | 15.0 | 1.0 | J0522-364_99_3 uniform weighting
3 | 104.33 | 20.0 | 3.9 | J0522-364_99_3 uniform weighting
3 | 102.40 | 25.5 | 4.7 | J0522-364_467_1 uniform weighting
6 | 221.02 | 0.8 | 0.16 | band 3,6,7 combined and WT filtered
6 | 237.06 | 1.8 | 0.26 | band 6 combined and WT filtered
6 | 225.43 | 1.2 | 0.3 | J0522-364_210_1_2
6 | 225.43 | 3.1 | 0.7 | J0522-364_210_3
7 | - | - | - | Not detected

The HS is located at 8.6″ from the core position, or 9.9 kpc in projection.
Assuming a viewing angle of $30^{\circ}$ for the jet we estimate the distance
from the center to be 19.8 kpc. The HS exhibits an extended structure,
especially in bands 3 and 7 (see Figure 4 and 5). Removing a fitted Gaussian
on the HS, the extended structure is extended mainly towards the North
direction (see Figure 10), with a projected position angle similar to the jet.

Figure 10: Emission of the full data set in band 3, 6 and 7. The contours are at [3,4,5,6,7,8,9,10]-$\sigma$ for a RMS noise of 90 $\mu$Jy. A Gaussian was fitted and removed from the core emission. The counter-jet is visible at the South-East between the HS and the core. Table 8: Flux of regions A and B. Id | $\nu_{eff}$ | Flux (A) | Flux (B)
---|---|---|---
| (GHz) | (mJy) | (mJy)
B367-combined | 221.02 | 5.0 | 2.3
B6-combined | 237.06 | 3.3 | 1.6
J0522-364_268_2 | 247.08 | - | 1.9
J0522-364_210_1_2 | 225.43 | 3.6 | 1.6
J0522-364_210_3 | 225.44 | 4.4 | -
B7-combined | 336.44 | 5.3 | 2.1
J0522-364_108_2 | 348.58 | - | 1.6
J0522-364_397_1 | 344.99 | - | 1.7
J0522-364_124_1_2 | 324.15 | 4.0 | -
J0522-364_768_2_3 | 345.77 | 5.0 | 1.6
J0522-364_83_1 | 348.82 | 3.3 | -
J0522-364_208_1 | 348.42 | 3.9 | -
J0522-364_223_1 | 338.19 | 6.1 | -
J0522-364_223_2 | 338.19 | 6.7 | -
J0522-364_340_1_2 | 365.28 | 7.0 |

##  4 (Sub-)millimeter variability

The flux evolution of PKS 0521-365 in three bands of ALMA, 3, 6 and 7, were
measured during nearly 1 year using calibration data of several unrelated
scientific projects. The time sampling for each band was irregular and did not
allow to study completely the very fast variation (intra-days) or to measure a
precise time variation of the individual blobs in the jet given the low signal
to noise of the individual measurements, e.g. using a Fourier decomposition in
the time domain. Nevertheless, it was possible to disclose important flux
variation. On Figure 11 the evolution of the total flux in the three bands is
shown. No correction for the frequency was applied but given the spectral
index of the core at the different bands (see Table 10) the maximum correction
would be in band 7 of the order of 4%.

The variability index VI (Ciaramella et al., 2004) was introduced to
parametrize the flux variation of a source:

$VI=\frac{S_{max}-S_{min}}{S_{max}+S_{min}}$ (5)

For each band, the VI of PKS 0521-365 is then estimated on the sampling period
of 1 year to be respectively $VI_{B3}=0.33$, $VI_{B6}=0.06$ and $VI_{B7}=0.40$
(see Table 10). These values of the VI must be considered as lower limit
during the year 2012 since the sampling is sparser than the variability in PKS
0521-365. The sparse and irregular sampling makes difficult to draw clear
conclusions on the variability between the bands. Nevertheless, it seems that
the emission in band 3, the lowest frequencies, is more variable than the
other two bands.

Figure 11: Flux evolution of the core of PKS 0521-365. For the data from the
Source Catalogue we removed the Hot Spot flux using the spectral fit.

##  5 Spectral index

Figure 12: Spectral index using the 4 simultaneous basebands observed with
ALMA. Here is the example of the set J0522-364_99_2. The spectral index fit is
indicated by the dashed line.

On Table 9 the spectral index is given per dataset. The spectral index was
computed measuring the core flux at the 4 basebands of a given dataset (see
Figure 12). The mean value of the spectral index was computed for each band
(see Table 10) and shows a clear correlation with the frequency with a
steepening towards the high frequency, $-0.43$ in band 7 versus $-0.08$ in
band 3. The time evolution of the spectral index for each band (see Figure 13)
shows a very quick change for the spectral index in band 7, which in less than
5 days it can change by more than 0.5 dex. That behavior was already shown at
lower frequencies in other blazars (e.g. Venturi et al., 2001).

The dynamic range achieved for the imaging of the different dataset allows to
measure the flux of the other components. Since the regions A and B are
embedded in the jet emission we take a fixed circular aperture of 1″around the
peak emission to estimate the flux. The SED of these components is shown on
Figure 14 (HS) and 15 (counter-jet, A and B). The very low value, $-2.9$, of
the spectral index of the counter-jet is not only reflecting the intrinsic
flux distribution but as well the relativistic beaming of the jet/counter-jet
as discussed before. The HS discloses a steep distribution with a spectral
index of $-1.6$ similar to Hot Spots at high frequency in other radio galaxies
(Carilli et al. 1999, Okayasu et al., 1992).

The regions A and B in the radio jet have a different behavior with a flat or
even inverted spectrum. We note that because of the spatial resolution
necessary to discriminate the regions A and B the spectral index was measured
from the band 6 and 7. Figure 15 makes clear the large dispersion of the
measurements. The time variation cannot be discarded even if it is not
expected. A most likely reason for these variations is the difficulty to
extract the flux from a region with a single dataset because of the low
dynamic range in the radio jet and the ill-defined location. Despite these
issues we can confidently affirm that regions A and B have a relatively flat
spectrum in band 6 and 7.

Figure 13: Evolution of the spectral index $\alpha$ in the core of PKS 0521-365 vs. MJD. The data from the calibrator monitoring were added to the project data. In that case we remove the Hot Spot flux estimated from the SED fit. Figure 14: SED of the Hot Spot in band 3, 6 and 7 and the fit of the spectral index $\alpha=-1.60$. Figure 15: SED of the measured continuum data in the regions A, B and the counter-jet. A spectral index is fitted on the three regions and indicated in the plot. Table 9: Spectral index of the core of PKS 0521-365 measured on the basebands. Id | $\nu_{eff}$ | $\alpha$
---|---|---
| (GHz) |
J0522-364_99_1 | 104.55 | -0.03
J0522-364_99_2 | 104.33 | -0.20
J0522-364_99_3 | 106.62 | 0.0
J0522-364_131_1 | 107.99 | 0.00
J0522-364_467_1 | 102.40 | -0.06
J0522-364_108_3 | 93.16 | -0.18
J0522-364_268_1 | 247.08 | -0.15
J0522-364_268_2 | 247.08 | -0.05
J0522-364_210_1_2 | 225.43 | -0.28
J0522-364_210_3 | 225.44 | -0.26
J0522-364_108_1 | 348.58 | -0.05
J0522-364_108_2 | 348.58 | -0.51
J0522-364_397_1 | 344.99 | -0.42
J0522-364_124_1_2 | 324.15 | -0.31
J0522-364_83_1 | 348.82 | -0.51
J0522-364_83_2 | 348.82 | -0.22
J0522-364_83_3 | 348.82 | -0.42
J0522-364_208_1 | 348.42 | -1.57
J0522-364_223_1 | 338.19 | -0.37
J0522-364_223_2 | 338.19 | -0.35
J0522-364_340_1_2 | 365.28 | 0.03
Table 10: Mean values of core spectral index in PKS 0521-365 and the VI per band without correcting for the spectral dependence. band | # | $\alpha$ | VI
---|---|---|---
3 | 6 | -0.08 | 0.40
6 | 4 | -0.19 | 0.23
7 | 13 | -0.43 | 0.48

##  6 Discussion

###  6.1 Beamed radio jet

The detection of the jet and counter-jet emission allows to use their ratio,
$\mathfrak{R}$, as an alternative beaming indicator assuming no isotropic
component in the jet emission (Ghisellini et al. 1993). This beaming indicator
is independent of other methods such as the proper motion of blobs and the
core to extended radio flux usually used with VLBI observations. Also, the
resolution required to detect kinematic motions within the jet by ALMA is
difficult to achieve frequently enough to monitor the movement of blobs within
the jet. But taking advantadge of the ALMA sensitivity, the detection of the
counter-jet by the stacking approach followed here is a promising avenue at
these frequencies. Our derived value for $\mathfrak{R}$ is similar to those
values derived from previous VLBI observations. Tingay et al. (1996) found
lower limits for $\mathfrak{R}$ in the range of our findings (5.0 and 9.3 at
4.8 GHz, and greater than 2.9 at 8.4 GHz). Further observations by Tingay &
Edwards (2002) obtained $\mathfrak{R}>20$ from the VLBI observations at 5 and
8.4 GHz increasing the previously obtained lower limit given the higher
sensitivity of the new VLBI data. Although our result differs in a factor of
2, it is still within the expected range when compared to other highly beamed
blazars (Ghisellini et al. 1993). As previously described, from
$\mathfrak{R}>20$ we derived bounded ranges for $\delta$, $\Gamma$, $\beta$,
and $\theta$ (see Section 3) which mean values are in good agreement with
previous studies, where $\langle\delta\rangle=1.5$,
$\langle\Gamma\rangle=1.15$, $\langle\theta\rangle=32.5^{\circ}$, and
$\langle\beta\rangle=0.5$. In a highly beamed blazar, the ratio $\mathfrak{R}$
get enhanced due to the beaming effect in the jet. Thus, a small value of
$\mathfrak{R}$ suggests that we are seeing the true bulk motions in the jet
and not emission due to beaming effects. Tingay & Edwards (2002) observations
constrain the parameter space to a well defined region which centered the jet
angle close to $30^{\circ}$ and $\beta\sim 0.5$. These exploration of the
$\beta-\theta$ plane also considered the calculations done by Ghisellini et
al. (1993) for the Doppler parameter ($\delta>1$) and Dondi & Ghisellini
(1995) ($\delta>1.3$), which we can now further constrain with our ALMA data.
It is no coincidence that our derived parameters lie just inside the box
allowed by Tingay & Edwards (2002) in the $\beta-\theta$ plane and is also
consistent with the values used for $\delta$. Therefore, we found little
support for highly beamed emission in PKS 0521-365 and thus, though not
detected here, it is unlikely that there are super-luminal motions along the
jet.

###  6.2 Double jet, dust or free-free emission

One of the striking results present in the millimeter continuum emisson
towards PKS 0521-365 is an elongated structure nearly perpendicular to the
well-defined radio jet (see Fig. 7). In the combined dataset with an effective
frequency of 221 GHz that structure has a size of 4.8″, i.e. 5.5 kpc at the
distance of PKS 0521-365, and a position angle of $72^{\circ}$. The isophote
fitting of the R-band image (Falomo, 1994) indicates that this feature is
aligned with the major axis of the host galaxy (PA$\approx 75^{\circ}$). The
thermal (dust) emission or free-free emission from central star forming region
or synchrotron emission are then possible explanations for that feature.
Previous observations have shown the presence of kpc-scale disk of molecular
gas and dust in the center of radio galaxies (Martel et al. 1999, García-
Burillo et al. 2007, Ocaña Flaquer et al. 2010). It has been observed as well
X-shaped radio sources with normal pairs of active lobes and lower surface
brightness wings of emission (Leahy & Parma 1992, Dennett-Thorpe et al. 2002,
Cheung 2007). The 15 GHz VLA map shown in Falomo et al. (2009) has a similar
angular resolution ($\sim 0.5\arcsec$) to the ALMA data and a RMS noise of 200
$\mu$Jy which is sufficient to detect the perpendicular feature if it is a
synchrotron emission. No hint of an elongated feature along the major axis is
visible on the 15 GHz map where all the other features are clearly detected
(HS, jet). Boisson et al. (1989) found a nebular line emission with a low
surface brightness extending up to 21 kpc on the opposite side of the radio
jet. It does not match spatially with the secondary emission found in band 6
and 7 with ALMA. The spectral index of the emission in the elongated feature
between the band 3 and 7 would allow to discriminate between synchrotron and
thermal emission since in most of the case the SED slope are negative and
positive respectively. The spatial resolution of the current band 3 data does
not allow to resolve that feature. The lack of evidence of that feature at 15
GHz favors the dust emission but it will need to be confirmed by observations
in band 3 at a higher spatial resolution. In that case the presence of dust in
the center of PKS 0521-365 should be associated with molecular gas that we
should be able to detect in CO(1-0) emission or other tracers (Ocaña Flaquer
et al. 2010).

##  7 Conclusions

The blazar PKS 0521-365 was studied using the ALMA public archive from the
projects using that source as a calibrator over more than 11 months. The data
in band 3, 6 and 7 were analyzed separately to follow the time variability and
combined for each band in a superset at an effective frequency of 221 GHz. To
enhance the imaging we applied a wavelet filtering using the ”A trous”
algorithm to reach a final dynamic range of 47000 in the superset of data.
From that analysis we can draw several conclusions about the (sub-)millimeter
emission of PKS 0521-365:

  * •

The detection of the submillimeter point sources in the field of PKS 0521-365
are consistent with sub-millimeter galaxy count obtained with similar data
using ALMA observations. They show the strong potential of the numerous
calibrator fields used in ALMA observations to build a large sample of high-
redshift sources over the whole sky spanned by the ALMA scientific projects. A
careful analysis of the sources, beyond the scope of this study, would be
needed to discard spurious detections.

  * •

The (sub-)millimeter emission of the extended structures in PKS 0521-365 is
detected in band 3, 6 and 7. The Hot Spot was clearly detected at 8″ from the
core. The strongly beamed radio jet was detected together with its internal
regions (A and B). We detected the presence of a weak counter-jet, symmetric
to the radio jet. A roughly perpendicular disky structure is detected in band
3 and 6. That structure may be the relic of a previous jet but a thermal
origin cannot be discarded without more observations at higher spatial
resolution and frequency.

  * •

The analysis of the time variability shows large variations during the nearly
1 year of measuremens. The irregular sampling makes difficult to perform a
frequency analysis of the variation. The spectral index variation at the three
bands shows large variation reaching 0.4-0.6 dex difference.

  * •

The spectral index analysis indicate a steepening of the SED in the core
towards the highest frequencies (band 7) whereas the regions A and B have a
quite flat spectrum. The counter-jet displays a very steep SED because of the
relativistic beaming in the blazar. Analyzing the data of the jet and counter-
jet we were able to put constraint on the Lorentz factor and the viewing angle
confirming previous studies at lower frequencies. According to that analysis
the Hot Spot would be at a distance of nearly 20 kpc from the core.

  * •

The Hot Spot shows an extended structure apart from its strong central
emission. Despite the lower spatial resolution in band 3 it seems to present
more extended structures towards the low frequencies.

  * •

Our derived parameters for $\langle\delta\rangle=1.5$,
$\langle\Gamma\rangle=1.15$, $\langle\theta\rangle=32.5^{\circ}$, and
$\langle\beta\rangle=0.5$, are within the range of the constraints imposed by
VLBI observation in the $\beta-\theta$ plane indicating that the emission from
PKS 0521-365 is definitely not highly beamed and thus, support minimal super-
luminal motions along the jet.

###### Acknowledgements.

We are grateful to the anonymous referee for the very useful comments. M. G.
thanks ESO for his studentship. This paper makes use of the following ALMA
data: ADS/JAO.ALMA#Id (Id are the project UID indicated in Table 1). ALMA is a
partnership of ESO (representing its member states), NSF (USA), and NINS
(Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation
with the Republic of Chile. The Joint ALMA Observatory is operated by ESO,
AUI/NRAO, and NAOJ.

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