Observing Relativistic Jets in Active Galactic Nuclei with ngVLAKazuhiro Hada 1,21

Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2–12 Hoshigaoka,Mizusawa, Oshu, Iwate 023–0861, Japan2
Department of Astronomical Science, The Graduate University for Advanced Studies (SOKENDAI),
2–21–1 Osawa, Mitaka-shi, Tokyo 181–8588
kazuhiro.hada@nao.ac.jp

Abstract
Active galactic nuclei (AGN) are the most energetic objects in the Universe that are ultimately powered by theaccretion onto the central supermassive black holes (SMBH). In particular, powerful relativistic jets are the manifestation of AGN activities, while the formation and emission mechanisms of relativistic jets remain a central questionin high-energy astrophysics since the first discovery of such phenomena. Here we overview some potential sciencecases on AGN jets addressed by ngVLA. Its superb observing capability (µJy sensitivity at sub-milliarcsecond angular resolution with the long-baseline-array mode) operated at centimeter wavelengths is best suited to study thephysics of relativistic jets.
Key words: galaxies:active — radio continuum:galaxies — technique: interferometric

1. Introduction
It is widely believed that active galactic nuclei (AGN) are
powered by the accretion of material onto the supermassive
black holes (SMBH). Approximately 10% of known AGN are
radio-loud and exhibit powerful relativistic jets. AGN jets are
one of the most spectacular phenomena in the Universe and observable on a wide range of spacial scales from sub-pc to Mpc.
Elucidating the formation, propagation and internal physicalproperties (radiative and kinetic energy densities, magnetic
field strength and configuration) of AGN jets is a half-centurylong quest in high-energy astrophysics. Besides, as they cantransport a significant fraction of gravitational energy back into
their host galaxies and intergalactic space, detailed high resolution studies are essential in the above contexts and in understanding their role in the cosmological evolution of galaxies.
Recently, the Event Horizon Telescope (EHT) operated atmillimeter wavelengths has obtained the first SMBH shadow
image in the nucleus of M87, a nearby radio galaxy with apoweful radio jet (EHT Collaboration et al. 2019). This has
opened a new window for high frequency observational studiesof AGN. However, the millimeter-VLBI image of M87 does
not show any extended emission around the photon ring, despite the dominance of prominent jet at larger scales. This
could be due to the limited imaging sensitivity of the EHT array and/or due to the low-level (∼mJy) jet emission at suchhigh frequencies.
In this context, radio observations at centimeter wavelengths
are advantageous to observe jet formation scales after its
launching from SMBH, thanks to the steep spectral natureof nonthermal synchrotron emission of the jet. Indeed, various important jet physics and questions such as the collimation and acceleration are associated with a wide range of distances above subpc/pc scales, which cannot be probed by mmwavelength observations. The ngVLA and its long-baselinearray (LBA) capability that achieves unprecedented sensitivity

and mas/submas-scale resolution at ∼1–100 GHz can uniquelyprobe a vast range of physical scales associated with the AGN
jets, from their formation/propagation region to termination.
Here we introduce some potential science cases on AGN jetsthat can be best addressed with ngVLA, in particular with useof the LBA mode.

Fig. 1. Radio images of M87 jet at various spatialscales.
(Top) VLA 15 GHz image (NRAO). (Middle)
EAVN 43GHz image (EAVN Collaboration).
(Bottom)
EHT 230 GHz image (EHT Collaboration et al. 2019).

2[Vol. ,2. Imaging the acceleration and collimation zone
One of the remarkable features of AGN jets is that they arevery well collimated (<5 deg) even at large scales (up to ∼kpcMpc). The key question is how and where the flow starts tobe collimated after launching. A widely favored scenario of
powerful AGN jet formation suggests that a jet is produced via
strong magnetic fields amplified by a spinning BH (Blandford& Znajek 1977) or/and the inner accretion disk (Blandford &Payne 1982). The flow is initially slow and the subsequent
acceleration is realized by gradually conversion of the magnetic energy into kinetic one, while the ambient pressure fromthe external medium collimates the flow. As a consequence,
the theory suggests that an acceleration and collimation zone
(ACZ) is formed over distances of ∼10–105 Schwarzschildradii (Rs ) from BH (e.g., Komissarov et al. 2009). To test thisobservationally, in recent years high-resolution VLBI analysesof jet morphology have been performed for a growing numberof objects. The best example is M87, where the jet collimation
profile was robustly determined by various authors (e.g., Asada& Nakamura 2012; Hada et al. 2013) over in total 7 orders ofmagnitude in gravitational radii of SMBH (see Fig.4 for a compilation plot by Nakamura et al. 2018). It was found that thejet shows a parabolic collimation profile over ∼100–106 Rs ,which is consistent with the magnetic models. Remarkably, thejet transitions into a conical shape at the Bondi radius, suggesting that the surrounding gases bound by the gravitational potential of SMBH play a key role in confining the jet. However,the number of sources for which the jet collimation profile
was determined is still very limited (of the order of ten) andmostly biased towards Fanaroff-Riley-I (FR-I) type jets whose
jet power is relatively low. To test how such features are common to AGN jets, it is important to investigate the jet geometry
and structural transitions for a variety of objects (including FRII objects) by imaging the ACZ of jet flows over a wide rangeof distances. For FR-II sources (i.e., highly debeamed fromus) this would require imaging rms noise levels of ≤10 µJy
even for nearby targets (e.g., 3C 452; Giovannini et al. 2001).
Ultra-high-sensitivity ngVLA along with its mas-submas angular resolution at 1–115 GHz, will be a unique way to achieve
this requirement.
3. Imaging multi-layered jet
An increasing number of AGN jets show edge-brightened
structures when resolved transversely. Broadband spectral
modeling studies on powerful AGN jets suggest that a relativistic jet is not a simple uniform flow but composed of
two distinct layers, i.e., an outer “sheath” flow and a central“spine” flow (Ghisellini et al. 2005). Similar arguments arealso yielded from GRMHD simulations (e.g., McKinney &Blandford 2009), suggesting that the central spine can be produced by extracting the rotational energy of the spinning BH
while the outer sheath may be connected to the innermost partof the accretion disk. Hence, probing the transverse structureof AGN jets is essential to constrain their formation mechanisms. Since jet transverse structures can be typically resolved
on mas-submas scales and the central stream is much dimmer than the bright limbs, high-sensitivity radio imaging at cm

Fig. 2. Jet collimation profile of M87 jet (Nakamura et al. 2018)wavelengths would be best suited to address this. Indeed, recent high-resolution high-sensitivity radio images of the M87
jet at cm wavelengths have begun to discover substructures inthe central dim part of the jet, which could be associated with
the interior spine component (Asada et al. 2016; Hada 2017).
To test how common spine-sheath structures are in AGN jets
(particularly in the relatively dimmer FR-II jets as describedabove), further enhancement of image sensitivity (down to afew µJy levels) at these frequencies is crucial, which is possible with ngVLA.
4. Magnetic fields in the ACZ: helical scenario and estimation
According to the current leading scenario of AGN jet production, differential rotation in the BH ergosphere or inner partof accretion disk creates tightly-twisted magnetic fields (e.g.,Meier et al. 2001). Searching for signatures of such helicalmagnetic fields is a longstanding quest in observational studies
of relativistic jets.
This may be best addressed by high-resolution polarimetricradio observations. If a jet possesses helical magnetic fields,
one should see systematic Faraday rotation-measure (RM) gradients across the jet due to the gradual change of the field component along our line-of-sight (Broderick & McKinney 2010).
The first observational hint of such transverse RM gradientswas reported in the pc-scale jet of 3C273 (Asada et al. 2002),
and to date similar features have been claimed in ∼30 pc-scale
jets (Hovatta et al. 2012; Gabuzda et al. 2017), suggesting thatthe presence of helical fields may be common in AGN jets.
However, the previous arguments are mostly based on patchy
polarization signals covering only a small fraction of the entire
jet area, since the SNR of polarization/RM maps are typically∼10 times lower than those of total intensity images. To definitively test the global magnetic-field configuration in a jet, weneed polarization/RM maps covering the whole jet area. A factor of 10 sensitivity improvement by ngVLA will be able toNo. ]create jet polarization maps at comparable levels as currently
done for total intensity, which may robustly detect RM gradients across the jet in >100 sources. In particular, accurate
RM measurements require multiple bands which covers a wide
frequency range. Therefore, having multi-frequency ngVLA
images simultaneously over 1–100 GHz can greatly facilitate
this study.

Fig. 3. First indication of Faraday rotation measure gradient across thejet in 3C 273 obtained with VLBA at 5 and 8 GHz (Asada et al. 2002)

5. (Sub-)pc-scale imaging of AGN thermal components
Another unique opportunity with ngVLA is that we will
for the first time be able to probe the thermal emission ofAGN at unprecedented resolution. While AGN are an efficientsite of particle acceleration as evident from broadband radioto-γ-ray nonthermal emission, AGN contain various thermal
components such as accretion disks, winds/outflows, tori,
broad/narrow line regions. Understanding the nature of thesecomponents are very important to test the validity of the AGN
unification scenario, and direct imaging these regions would
be the best approach for that. However, these components are
typically confined within the central ∼ pc (or much less) of thenucleus, and this requires mas-scale angular resolution for anAGN at a distance of 100 Mpc. While only VLBI can achieve
this level of resolution, the current VLBI sensitivity (typically
TB > 106−7 K) prevents us from detecting the thermal components via emission, and instead only a few cases were exceptionally probed via absorption (Kameno et al. 2001; Joneset al. 2001; Haga et al. 2015). With its superb sensitivity ofngVLA with the LBA mode, one can reach a sensitivity levelof TB ∼103−4 K at (sub)mas angular resolution. This will forthe first time allow us to resolve and image the thermal accretion disks and also winds/outflows from the disks that confine
the nonthermal jet. Therefore ngVLA will allow us to directly
image a whole system of AGN that is relevant to accretion and3ejection, fueling and feeding, that is key to fully understand thenature of AGN and its evolution.
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