Astronomy Object of the Month: 2025, February
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S-shaped radio jets emanating from a precessing supermassive black hole
Galactic metamorphosis marks a unique stage in a galaxy’s evolution, where its structure and composition undergo a profound transition. This month’s featured astronomical object, the quasar
PKS 2300-18, has been captured in the rare act of transitioning into a radio-loud state forming striking S-shaped radio-emitting lobes. The authors of this study have found that this phenomenon
arises due to the precession of the supermassive black hole at its center, and through a comprehensive multiwavelength investigation, they have unraveled the intricate dynamics of this galaxy
spanning scales from a few light days to several megaparsecs, shedding light on its unusual morphology.

Illustration 1: Radio map of PKS 2300-18 at 1.4 GHz frequency coming from ASKAP interferometer (in orange) superimposed on an optical image. In the enlargement in the right panel insert, the host quasar is visible at the center of the radio source interacting with a companion galaxy and consequentially forming an extended tidal tail. Source: original publication.
Radio galaxies (RGs) launch bipolar relativistic jets that align with the supermassive black hole's (SMBH) spin axis and extend millions of light years, often leading to large-scale radio-emitting lobes. RGs exhibit a wide variety of morphological shapes that include winged RGs resembling an S- or Z-shaped structure and are comparatively rarer. These sources are excellent candidates for studying jet precession, as their S-shaped inversion symmetry strongly indicates underlying precession.
Recently, astronomers conducted a multiwavelength study of PKS 2300-18, a distinctive S-shaped radio quasar with large, inversion-symmetric S-shaped jets embedded in low surface brightness emission. The quasar’s optical morphology appears disturbed, indicative of an ongoing merger with a companion galaxy. Optical studies revealed double-peaked broad Hα and Hβ emission lines, which were analyzed to determine the mass of the SMBH as 108 times the mass of the Sun. The double-peaked emission was attributed to complex gas kinematics in the broad line region (BLR). X-ray spectral modeling using Chandra identified traces of photoionized gas near the SMBH, likely due to quasar-driven emission. Also, in the Chandra map, three X-ray knots coinciding with radio knots were discovered (see Illustration 2). Meanwhile, Very Long Baseline Interferometry (VLBI) radio observations revealed parsec-scale jets moving at superluminal speeds of 2.3 c. Multi-epoch radio core variability was also detected, likely caused by changes in the accretion rate of the AGN or shock propagation within the parsec-scale jets.
To understand the properties of the plasma inside the extensive structure and its evolution over time, an analysis of the radio spectrum at various locations of the radio source was conducted using data from radio telescopes such as the Giant Meterwave Radio Telescope (GMRT) and the Very Large Array (VLA). The results led astronomers to conclude that the S-shaped morphology is the result of continuous jet precession of the SMBH at the center. Using the kinematical jet precession model, the precession period was estimated to be ~12 Myrs (see Illustration 3).
These S-shaped sources undergoing merger act as unique laboratories for studying galaxy evolution, offering a rare glimpse into multiple transitional stages within the lifespan of a galaxy occurring simultaneously. Further in-depth studies will provide deeper insights into the role of nuclear activity in shaping galaxy evolution.
Original publication: Misra A., Jamrozy M., Weżgowiec M., Kozieł-Wierzbowska D., Multiwavelength investigations of PKS 2300–18: S-shaped radio quasar with precessing jets and double-peaked broad emission-line spectrum, Monthly Notices of the Royal Astronomical Society, Volume 536, Issue 3, January 2025, Pages 2025–2045.
The research was conducted at the Department of Stellar and Extragalactic Astronomy of the Jagiellonian University’s Astronomical Observatory (OA UJ). The work was carried out thanks to the financial support of the National Science Center through grant 2021/43/B/ST9/03246.
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Radio galaxies (RGs) launch bipolar relativistic jets that align with the supermassive black hole's (SMBH) spin axis and extend millions of light years, often leading to large-scale radio-emitting lobes. RGs exhibit a wide variety of morphological shapes that include winged RGs resembling an S- or Z-shaped structure and are comparatively rarer. These sources are excellent candidates for studying jet precession, as their S-shaped inversion symmetry strongly indicates underlying precession.
Recently, astronomers conducted a multiwavelength study of PKS 2300-18, a distinctive S-shaped radio quasar with large, inversion-symmetric S-shaped jets embedded in low surface brightness emission. The quasar’s optical morphology appears disturbed, indicative of an ongoing merger with a companion galaxy. Optical studies revealed double-peaked broad Hα and Hβ emission lines, which were analyzed to determine the mass of the SMBH as 108 times the mass of the Sun. The double-peaked emission was attributed to complex gas kinematics in the broad line region (BLR). X-ray spectral modeling using Chandra identified traces of photoionized gas near the SMBH, likely due to quasar-driven emission. Also, in the Chandra map, three X-ray knots coinciding with radio knots were discovered (see Illustration 2). Meanwhile, Very Long Baseline Interferometry (VLBI) radio observations revealed parsec-scale jets moving at superluminal speeds of 2.3 c. Multi-epoch radio core variability was also detected, likely caused by changes in the accretion rate of the AGN or shock propagation within the parsec-scale jets.
To understand the properties of the plasma inside the extensive structure and its evolution over time, an analysis of the radio spectrum at various locations of the radio source was conducted using data from radio telescopes such as the Giant Meterwave Radio Telescope (GMRT) and the Very Large Array (VLA). The results led astronomers to conclude that the S-shaped morphology is the result of continuous jet precession of the SMBH at the center. Using the kinematical jet precession model, the precession period was estimated to be ~12 Myrs (see Illustration 3).
These S-shaped sources undergoing merger act as unique laboratories for studying galaxy evolution, offering a rare glimpse into multiple transitional stages within the lifespan of a galaxy occurring simultaneously. Further in-depth studies will provide deeper insights into the role of nuclear activity in shaping galaxy evolution.

Illustration 2: X-ray image from Chandra telescope of the host PKS 23000-18 (seen in blue). The red contours overlaid on top of the X-ray map are from the VLA telescope at 5 GHz. The radio contours show the location of the core and three knots of the northern jet. The radio knots coincide with points of enhanced X-ray emission. In the insert towards the top-right corner, the first radio knot, K1, is also visible in the optical image. Source: original publication.

Illustration 3: Kinematic jet precession model superimposed on top of the 6 GHz radio map from VLA telescope showing the precessing motion of the jets in the anti-clockwise direction. The color gradient represents flux density in units of Jy/beam. The red and blue lines represent the jet and counterjet, respectively. Source: original publication.
Original publication: Misra A., Jamrozy M., Weżgowiec M., Kozieł-Wierzbowska D., Multiwavelength investigations of PKS 2300–18: S-shaped radio quasar with precessing jets and double-peaked broad emission-line spectrum, Monthly Notices of the Royal Astronomical Society, Volume 536, Issue 3, January 2025, Pages 2025–2045.
The research was conducted at the Department of Stellar and Extragalactic Astronomy of the Jagiellonian University’s Astronomical Observatory (OA UJ). The work was carried out thanks to the financial support of the National Science Center through grant 2021/43/B/ST9/03246.
Arpita Misra Astronomical Observatory Jagiellonian University Amisra [at] oa.uj.edu.pl |