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Centaurus A

Centaurus A, also known as NGC 5128, is a giant elliptical galaxy and one of the brightest and most studied radio galaxies in the sky, located approximately 13 million light-years from Earth in the southern constellation of Centaurus.^[1] It is characterized by a prominent lane of dark dust and gas that bisects its otherwise smooth stellar disk, a feature resulting from a past collision and merger with a smaller spiral galaxy that warped its structure.^[2] At its core lies a supermassive black hole with a mass of about 55 million solar masses, which powers an active galactic nucleus (AGN) that ejects relativistic jets of plasma and produces enormous radio lobes extending up to 1.5 million light-years from the galaxy.^[3] Discovered optically by Scottish astronomer James Dunlop in 1826, it was later identified as a powerful radio source in 1949, making it the fifth-brightest galaxy visible to the naked eye with an apparent magnitude of 6.7.^[4] This active galaxy serves as a nearby laboratory for understanding galactic mergers, supermassive black hole accretion, and high-energy astrophysical processes, given its proximity—the closest such AGN to our Milky Way.^[2] Observations across multiple wavelengths reveal young blue star clusters forming along the edges of the dust lane, triggered by the merger, as well as regions of intense star formation amid reddish patches of ionized gas.^[2] The galaxy spans about 60,000 light-years in diameter and exhibits a complex halo of dark matter and globular clusters, with two notable supernovae recorded in 1986 and 2016.^[4] Its radio emissions, mapped by telescopes like the Very Large Array, highlight vast lobes that dominate the sky in radio views, while high-resolution observations from the Event Horizon Telescope reveal inner jets approximately 15 light-days across near the nucleus.^[5] Centaurus A continues to be a prime target for telescopes such as the Hubble Space Telescope, Chandra X-ray Observatory, and the James Webb Space Telescope, with recent JWST observations enabling detailed studies of its dusty core, ionized gas outflows, jet dynamics, and feedback mechanisms that influence galaxy evolution.^[6]^[7] These observations confirm its role as a prototype for radio-loud AGN, with the black hole's outflows regulating star formation and dispersing heavy elements into the intergalactic medium.^[8]

Discovery and Observational History

Early Optical Observations

Centaurus A, cataloged as NGC 5128, was first observed optically on 29 April 1826 by Scottish astronomer James Dunlop during a systematic survey of southern celestial objects using a 9-inch reflecting telescope at the Parramatta Observatory near Sydney, Australia.^[9] Dunlop described it as a "pretty large, irregular round nebula, 4 or 5 arcminutes in diameter, with a small bright nucleus," initially classifying it as a nebula without resolving its structure due to the limitations of his instrument.^[9] This observation marked the earliest recorded detection of the object, entered into Dunlop's 1828 catalog as entry number 482.^[9] The galaxy was independently rediscovered in 1834 by British astronomer John Herschel during his extensive telescopic survey of the southern skies from the Cape of Good Hope, South Africa, using an 18.7-inch reflecting telescope.^[10] On June 1, 1834, Herschel noted its striking peculiarity, describing it as "a most wonderful object: two large patches of light separated by a broad dark lane, like a double nebula cut asunder by a black band," highlighting the prominent dark dust lane that bisects the galaxy's apparent disk.^[11] He cataloged it as h 3501 and emphasized its unusual morphology in his 1847 publication, interpreting the dark band as an obscuring feature within what appeared to be a single irregular nebula.^[12] Throughout the 19th century, subsequent observers reinforced descriptions of the object's distinctive dust lane, often portraying it as a planetary nebula or an anomalous gaseous structure due to its apparent bifurcation and irregular outline.^[13] For instance, early catalogers like John Dreyer in the New General Catalogue (1888) retained Herschel's notation of its divided appearance, listing it as a "remarkable object" with a "broad, dark lane" crossing the nebula, which fueled speculation about its nature as a local interstellar phenomenon rather than a distant system.^[13] These visual accounts, based on eyepiece sketches and qualitative notes, underscored the galaxy's warped and obscured structure without resolving its true scale or composition.^[14] In the early 20th century, photographic advancements enabled more detailed imaging, with plates from observatories like Mount Wilson capturing its irregular, warped form and prominent absorption lane.^[15] These early photographs revealed the object's elliptical envelope surrounding the twisted dust features, though its extragalactic nature remained debated until the mid-20th century due to the prominent dust lane.

Identification as Radio Source

Centaurus A was first detected as a discrete radio source in 1949 by astronomers John G. Bolton, Gordon J. Stanley, and Bruce A. Slee at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) radio astronomy field station in Dover Heights, Australia. Using a simple interferometer operating at around 100 MHz, they identified three prominent extragalactic sources, with the strongest in the constellation Centaurus designated as Centaurus A (or Radio Source No. 1 in Centaurus) due to its position and intensity. This marked one of the earliest confirmations of discrete, non-thermal radio emissions from beyond the Milky Way. In the early 1950s, refined positional measurements using enhanced interferometric techniques confirmed the coincidence between the radio source Centaurus A and the optically peculiar galaxy NGC 5128. Observations by B. Y. Mills with the newly constructed Mills cross antenna—a large east-west and north-south array at Fleurs Observatory near Sydney—provided positions accurate to within 2 arcminutes, aligning the radio centroid precisely with the optical center of NGC 5128 and solidifying the identification. These efforts built on the initial tentative link from 1949 but required such precision to rule out unrelated foreground or background objects. Subsequent radio mapping in the 1950s, leveraging instruments like the Mills cross and swept-lobe interferometers, unveiled the source's vast extended structure, including symmetric giant lobes stretching approximately 10 degrees across the sky—corresponding to over 2 million light-years in physical extent at the then-estimated distance of about 3.5 megaparsecs. This scale, combined with the source's proximity, positioned Centaurus A as the nearest known radio galaxy, highlighting its role as a prototype for understanding the energetic processes driving extragalactic radio emissions.^[16] Advancements in the 1960s, particularly the aperture synthesis interferometry technique developed by Martin Ryle and his team at Cambridge University, achieved angular resolutions down to arcseconds, allowing the separation of Centaurus A's compact central core from the surrounding lobes. These observations, conducted with multi-element arrays like the one-mile telescope, also resolved narrow, linear features interpreted as relativistic jets emerging from the core at radio wavelengths, providing early insights into the collimated outflows characteristic of active galactic nuclei.

Physical Characteristics

Morphology and Structure

Centaurus A is classified as a peculiar lenticular galaxy (S0 pec) or a peculiar elliptical galaxy (E pec), characterized by a prominent equatorial dust lane that bisects the galaxy and obscures its central regions. This unusual morphology distinguishes it from typical ellipticals or lenticulars, with the dust lane appearing as a dark band superimposed on the otherwise smooth stellar envelope. The lane's presence suggests an atypical evolutionary path for an early-type galaxy, where interstellar material has been incorporated into an otherwise gas-poor system. Deep imaging reveals evidence for a past merger between a larger elliptical galaxy and a smaller spiral or gas-rich companion, inferred from the warped inner disk and a system of faint shells and filaments in the outer halo. These shell structures, visible in optical and near-infrared observations, are tidal remnants typically produced during galaxy interactions, with the warp in the disk indicating dynamical disturbances from the merger event approximately 2 billion years ago. Such features support a formation scenario involving the accretion of material that disrupted the original symmetry of the host elliptical. The prominent dust lane briefly referenced here contributes to obscuring the active nucleus but is primarily a morphological hallmark of this merger history.^[17] The overall stellar body spans approximately 91,000 light-years (28 kpc) in diameter along the major axis, comprising a smooth elliptical envelope that encases a warped inner disk. This extended structure highlights the galaxy's composite nature, with the outer envelope representing the dominant older stellar population and the inner components showing signs of recent dynamical mixing. Kinematic studies, including integral-field spectroscopy, detect counter-rotating stellar populations within the central regions, where the stars rotate in the opposite sense to the ionized gas disk, further corroborating the merger origin by indicating the preservation of angular momentum from the infalling companion. These counter-rotations are observed on scales of several kiloparsecs, providing direct evidence of the incomplete relaxation following the interaction.^[17]

Distance and Scale

Centaurus A, also known as NGC 5128, lies at a distance estimated between 3 and 5 megaparsecs (Mpc), corresponding to approximately 10 to 16 million light-years, based on multiple independent measurement techniques.^[18] Cepheid variable stars provide a distance of 3.4 Mpc, derived from Hubble Space Telescope observations of twelve such variables in the galaxy's halo. The tip of the red giant branch (TRGB) method yields values around 3.8 Mpc from optical and near-infrared photometry of resolved stars.^[18] Surface brightness fluctuations (SBF) in the galaxy's resolved stellar population indicate 3.5 Mpc, while the planetary nebula luminosity function (PNLF) suggests 3.8 Mpc from spectroscopy of emission-line nebulae.^[18] These methods converge on a weighted mean distance of 3.8 ± 0.1 Mpc, though earlier planetary nebulae estimates contributed to a lower value of 3.4 Mpc in some analyses.^[13] Uncertainties arise from the galaxy's complex dust obscuration and line-of-sight depth, leading to a spread of up to 20% in distance moduli across techniques. At the adopted distance of 3.8 Mpc, the galaxy's optical angular diameter of 25.7 arcminutes along its major axis translates to a physical extent of approximately 28 kiloparsecs (kpc).^[19] This scale underscores its classification as a giant elliptical galaxy, with the minor axis measuring about 20 arcminutes or 22 kpc, encompassing the prominent dust lane and stellar halo.^[18] The physical dimensions highlight the galaxy's large-scale structure, where fields at projected distances of 20–40 kpc reveal extended stellar populations.^[18] The total bolometric luminosity of Centaurus A is approximately 10^{10} solar luminosities (L_\sun), consistent with its status as a luminous giant elliptical.^[20] This value derives from its integrated apparent V-band magnitude of 6.20, corrected for foreground extinction and distance modulus, yielding an absolute magnitude M_V ≈ -21.7 and reinforcing its high stellar mass.^[21] Variations in distance estimates propagate to luminosity uncertainties of about 0.4–0.5 magnitudes, but the 10^{10} L_\sun figure establishes its dominance within the Centaurus A group.^[18]

Active Galactic Nucleus

Supermassive Black Hole

The central supermassive black hole in Centaurus A has an estimated mass of (5.5 \pm 3.0) \times 10^7 solar masses, determined through a combination of stellar dynamics and gas kinematics in the nuclear region.^[8] This measurement was obtained using adaptive optics-assisted integral-field spectroscopy with the SINFONI instrument on the ESO Very Large Telescope (VLT), which resolved the rotation of molecular hydrogen gas and stellar orbits within the central few parsecs.^[8] Historical estimates evolved from dynamical modeling in the 1990s and early 2000s, which provided lower limits such as greater than $1.1 \times 10^8 solar masses based on infrared spectroscopy of ionized gas motions, to more precise values refined by subsequent VLT observations and Atacama Large Millimeter/submillimeter Array (ALMA) mapping of the circumnuclear molecular gas disk.^[8] These advancements highlight the challenges posed by the dusty nuclear environment, which initially obscured direct dynamical probes. Centaurus A's active galactic nucleus qualifies as a low-luminosity active galactic nucleus (LLAGN), characterized by subdued emission powered by sub-Eddington accretion onto the black hole at a rate of approximately 0.001 times the Eddington limit, inferred from its bolometric luminosity of about $8 \times 10^{42} erg s^{-1}.^[22] This low accretion efficiency aligns with radiatively inefficient accretion flows typical of LLAGNs, where the inferred mass accretion rate is around $10^{-4} to $10^{-3} solar masses per year.^[22] The nucleus displays a weak broad-line region, with no prominent broad emission lines detected in optical or near-infrared spectra, consistent with heavy obscuration by the circumnuclear dust lane or intrinsically low activity.^[23] Observations with the James Webb Space Telescope (JWST) Mid-Infrared Instrument (MIRI) in 2025 have uncovered direct evidence of black hole feedback through fast ionized gas outflows in the central region.^[24] These outflows, traced by broad emission lines such as [Fe II] and [Ne VI], exhibit velocity dispersions up to \pm 1400 km s^{-1} and are confined within approximately 100 pc, with kinematics showing blueshifted and redshifted components perpendicular to the inner radio jet axis.^[24] The outflow energetics, including an ionized gas mass-outflow rate of several solar masses per year, suggest they are driven by the AGN's radiative and mechanical power, potentially regulating star formation in the nuclear interstellar medium.^[24]

Relativistic Jets and Lobes

Centaurus A exhibits twin relativistic jets that emanate from its active nucleus, extending over thousands of light-years and revealing complex structures at various scales. Observations with the Event Horizon Telescope have resolved the inner jet at parsec scales, uncovering a bright, edge-brightened morphology with prominent knots indicative of shocks or instabilities in the plasma flow. These knots display superluminal apparent motion, with speeds up to approximately 0.77c in the X-ray regime, as measured in Chandra observations spanning over two decades, confirming the relativistic nature of the outflow and suggesting bulk Lorentz factors greater than 2.^[25]^[26] The jets terminate in expansive giant radio lobes that span approximately 1.8 million light-years across the sky, corresponding to an angular extent of about 8 degrees due to the galaxy's proximity. These lobes are filled with diffuse synchrotron-emitting plasma, primarily from aged relativistic electrons spiraling in magnetic fields of order microgauss, as mapped by high-resolution radio interferometry. The northern and southern lobes exhibit asymmetric morphologies, with the southern lobe showing a more elongated structure, reflecting the historical evolution of the jet injection.^[27]^[28] Evidence for dynamical interactions is apparent in the jets' curvature and the lobes' overall bending, attributed to ram pressure from the intracluster medium as the system moves through the Centaurus cluster environment. This bending manifests as a clockwise twist in the inner structures and broader distortions in the outer lobes, consistent with simulations of jet propagation in a stratified gaseous halo. At the jet terminations, X-ray hotspots detected by Chandra highlight regions of particle reacceleration where the jet impacts the lobe edges, producing non-thermal emission up to keV energies.^[29]^[30]^[31] The total energy content in the lobes is estimated at around $10^{58} ergs, dominated by relativistic particles and magnetic fields, with the particle energy comprising about 75% of the total based on synchrotron and inverse-Compton modeling. This vast reservoir, accumulated over approximately $10^7 years, underscores the jets' role in galactic feedback, where the outflow influences star formation and gas dynamics on kiloparsec scales by heating and displacing the interstellar medium.^[32]^[33]

Stellar Population and Transients

Stellar Content

Centaurus A features a predominantly old stellar population in its elliptical envelope, consisting mainly of red giants and other evolved stars with ages exceeding 10 billion years and relatively low metallicities peaking around [M/H] ≈ -0.6 dex.^[34] This ancient component dominates the galaxy's halo, where resolved photometry reveals a mean stellar age of approximately 11 Gyr, with the majority of stars formed in a burst around 12 Gyr ago and spanning a broad metallicity range from Z = 0.0001 to Z = 0.04.^[34] The low-metallicity nature of these stars reflects the early formation history of the elliptical host, prior to the disruptive merger event that shaped the galaxy's current morphology. In contrast, the warped inner disk hosts a younger population of blue stars, indicative of merger-induced star formation triggered by the collision that created the prominent dust lane.^[35] Ultraviolet surveys, such as those from GALEX, detect these hot, massive blue stars concentrated along the disk, where extended UV emission traces recent star-forming regions amid the warped structure rich in gas and dust. This younger component contributes a minor fraction to the overall stellar content, adding roughly 20-30% of the halo stars in some outer regions, but is more prominent in the central disk where shockwaves from the merger enhanced star birth.^[34] The total stellar mass of Centaurus A is estimated at approximately 2 × 10¹¹ solar masses, underscoring its status as a massive elliptical galaxy.^[35] Accompanying this is a rich system of roughly 2000-3200 globular clusters, exhibiting multiple subpopulations distinguished by color and metallicity, with blue (metal-poor) and red (metal-rich) groups reflecting the galaxy's composite formation history from the merger.^[36] Kinematic studies of the stellar dynamics rely on tracers such as asymptotic giant branch (AGB) stars and planetary nebulae (PNe), which reveal rotation patterns in the disk and envelope.^[20] These evolved populations, including upper AGB stars brighter than the red giant branch tip and PNe with measured velocities, indicate a velocity gradient along the minor axis, supporting models of a rotating, warped disk embedded in the older elliptical structure.^[20] Transients such as novae occasionally erupt in this stellar disk, providing additional probes of the younger population.

Novae and Supernovae

Centaurus A has hosted several notable supernovae, providing insights into its stellar evolution and serving as tools for distance measurements. SN 1986G, a Type Ia supernova discovered on May 3, 1986, by amateur astronomer Robert Evans, exhibited a light curve and spectral evolution closely resembling prototypical Type Ia events like SN 1981B, with an apparent peak B-band magnitude of 12.45 ± 0.05 after correction for foreground extinction of A_B = 4.4 mag.^[20] Its proximity in Centaurus A (at a distance of 3.8 ± 0.1 Mpc) made it one of the closest Type Ia supernovae observed, enabling detailed studies that contributed to calibrating the Cepheid period-luminosity relation for extragalactic distances.^[37] Spectral analysis of SN 1986G revealed abundance stratification typical of Type Ia explosions, with oxygen and iron-group elements dominating the outer layers and higher-velocity silicon and calcium features in the inner regions, reflecting metal enrichment from the host galaxy's chemical history shaped by its merger event.^[37] Another significant supernova is SN 2016adj, a stripped-envelope Type Ic event discovered on February 8, 2016 within the prominent dust lane of Centaurus A.^[38] Its extinction-corrected light curve peaked at an absolute B-band magnitude of M_B ≈ -18.0 ± 0.1, consistent with carbon-rich stripped-envelope supernovae, and spectroscopic follow-up confirmed a carbon-dominated ejecta composition with minimal hydrogen and helium, indicating a progenitor from a massive star that lost its envelope through binary interaction.^[38] The spectra of SN 2016adj displayed broad absorption lines from intermediate-mass elements, highlighting the metal-rich environment of the dust lane, which traces the chemical evolution from the infalling spiral galaxy in Centaurus A's merger.^[38] These progenitors likely arise from the younger stellar populations in the dust lane, distinct from the older elliptical halo.^[38] Classical novae in Centaurus A are more frequently detected than supernovae, with recent examples including AT 2025nok, a classical nova discovered by the ATLAS survey on June 14, 2025, at an apparent magnitude of 18.706 and located in the dust lane.^[39] The expected nova rate in Centaurus A is approximately 28 per year, significantly higher than in typical elliptical galaxies (where normalized rates are 2–3 times lower), owing to enhanced binary populations fostered by the galaxy's recent merger history that introduced gas-rich, star-forming components.^[40] Light curve analyses of these novae typically show rapid rises and declines over weeks to months, while spectra reveal metal-rich ejecta enriched in neon, oxygen, and iron, signatures of the progenitors' accretion from companions in the chemically evolved interstellar medium of the dust lane.^[40]

Interstellar Medium

Dust Lane

The prominent dust lane in Centaurus A manifests as a dark, equatorial band visible in optical images, spanning approximately 10 kpc across the galaxy's disk and creating a striking silhouette against the underlying stellar halo. This feature arises from dense concentrations of interstellar dust that absorb and scatter light, with a typical visual extinction of A_V ≈ 10 mag, rendering the central regions nearly opaque at optical wavelengths. The lane's thickness is estimated at around 100 pc, though it varies, being thinner (∼35 pc) in the inner regions and up to 200 pc overall, as inferred from near-infrared and submillimeter observations that penetrate the obscuration.^[41]^[42] The dust composition is dominated by silicates and carbonaceous grains, including polycyclic aromatic hydrocarbons (PAHs), akin to those in the Milky Way's interstellar medium, as revealed by prominent silicate absorption at 9.7 μm and PAH emission features in mid-infrared spectra. Evidence suggests this dust formed relatively recently, likely during the galaxy's merger with a gas-rich spiral companion approximately 100 million years ago, which supplied the raw material for the lane's structure and triggered ongoing star formation within it.^[42]^[35] The lane exhibits a warped geometry, viewed nearly edge-on with a position angle of about 122°, where the disk twists dynamically due to the differential precession induced by the merger, extending to a warp angle of up to 90° in the outer parts. This twisting is evident in infrared and submillimeter maps, which trace the dust's thermal emission and reveal tilted ring-like structures. The obscuration plays a critical role in hiding the active galactic nucleus, with extinction values reaching up to 30 mag near the center, complicating direct optical views but allowing multi-wavelength studies to probe the hidden core.^[41]^[43]

Gas and Molecular Components

The neutral atomic gas in Centaurus A is primarily traced through HI 21-cm emission, which reveals an extended disk spanning approximately 15 kpc in diameter and closely aligned with the warped dust lane that serves as a major gas reservoir.^[44] This disk exhibits regular rotation in its inner regions (r < 6 kpc) but displays kinematics rotating oppositely to the nuclear stellar component, a signature of the galaxy's merger history.^[45] The total HI mass is estimated at around 5 × 10^8 solar masses, contributing significantly to the galaxy's interstellar medium.^[44] Ionized gas, observed in HII regions within the inner disk, shows evidence of dynamical disturbances driven by active galactic nucleus (AGN) feedback.^[7] These regions feature outflows with line-of-sight velocities reaching up to 500 km/s, as mapped in emission lines such as [Ne III] and [Ar II], indicating shock excitation and jet interactions on scales of ~35 pc.^[7] Molecular gas, detected via CO emission, is predominantly concentrated in the circumnuclear disk and core regions, with a total mass of approximately 3 × 10^8 solar masses.^[46] This gas reservoir fuels limited star formation, consistent with the low specific star formation rate typical of gas-rich early-type galaxies like Centaurus A.^[46] Anomalous velocity features in the gas distribution, attributed to unsettled remnants from the progenitor merger, are evident in both HI and CO kinematics, with outer HI showing non-regular orbits and molecular structures revealing warped and irregular flows.^[44] High-resolution ALMA mapping of CO(1-0) and higher transitions has resolved these features on parsec scales, highlighting the ongoing dynamical relaxation in the interstellar medium.^[47]

Local Environment

Nearby Galaxies

Centaurus A serves as the dominant member of a local group featuring a rich population of approximately 42 confirmed satellite galaxies and over 30 candidates, primarily low-mass dwarfs that influence the overall dynamics through gravitational interactions.^[48] These satellites are generally located within relative physical distances of several hundred kpc, with recession velocities differing by under 200 km/s from Centaurus A's heliocentric velocity of 547 km/s, consistent with a bound system exhibiting a velocity dispersion of 136 km/s.^[49]^[50] Key companions include the lenticular NGC 5102 at a similar distance of 3.5–4.0 Mpc and velocity of 464 km/s, as well as HI-rich irregular dwarfs such as those identified in blind surveys (e.g., KK 197 and similar low-surface-brightness objects). These companions highlight the diverse morphology of the satellite population, from ellipticals to gas-bearing irregulars.^[50]^[51]^[52] A 2018 study revealed that 14 of the 16 Centaurus A satellites with kinematic data lie in a thin, rotating plane, suggesting coherent accretion or dynamical alignment similar to structures observed in the Local Group.^[53] Interactions among these nearby galaxies are evident from disturbed morphologies and HI distributions in several satellites, indicative of ongoing minor mergers involving gas-rich dwarfs. Such events likely supply neutral hydrogen to the central galaxy and its environs, enhancing group-wide gas dynamics without the dramatic features of major mergers. The satellite dwarfs, with typical HI masses around 10^7–10^8 M_⊙, show irregular profiles that support this accretion scenario, fostering subtle evolutionary processes within the local environment.^[51]^[50]

Centaurus A/M83 Group Membership

The Centaurus A/M83 Group is recognized as the third-largest galaxy group within the Local Volume, comprising approximately 44 known member galaxies with measured radial velocities and an estimated total mass of (6.0 ± 1.4) × 10^{12} M_\odot within its zero-velocity surface radius of 1.40 ± 0.11 Mpc.^[54] This mass estimate is derived from virial theorem applications to the subgroup dynamics, highlighting the group's significant gravitational influence in the nearby universe.^[54] The group stands out for its relatively high mass-to-light ratio of approximately 125 M_\odot / L_\odot, indicative of a dominance by dark matter.^[54] Centaurus A serves as the dominant central galaxy of the primary subgroup, an elliptical galaxy exerting substantial control over the group's core dynamics, while M83, a barred spiral galaxy, anchors the secondary subgroup approximately 1 Mpc away.^[54] This spatial separation underscores the complex's bimodal structure, with the Centaurus A subgroup exhibiting a higher velocity dispersion of 136 km/s compared to 61 km/s in the M83 subgroup, reflecting differing evolutionary stages.^[54] The two subgroups are gravitationally bound, forming a cohesive assembly that bridges isolated field galaxies and denser clusters.^[54] As a constituent of the Virgo Supercluster, the Centaurus A/M83 Group occupies a position at the edge of the Local Sheet, a flattened structure encompassing the Local Group and nearby associations with minimal peculiar velocities perpendicular to its plane.^[54] This location positions the group as a key influencer on Local Group dynamics, contributing to the broader gravitational field that shapes the motion of our galactic neighborhood within the supercluster.^[54] Observations reveal infall patterns around the group, where surrounding field galaxies exhibit small peculiar velocities (<25 km/s relative to the Hubble flow), suggesting gentle accretion onto the complex.^[54] Peculiar velocity analyses indicate structured infall, with the Centaurus A subgroup displaying a systemic radial velocity of 547 km/s relative to the Local Group centroid, consistent with Hubble expansion modulated by local gravitational effects.^[54] Recent dynamical modeling reinforces this, estimating a virial mass of (7.3 ± 2.0) × 10^{12} M_\odot for the overall complex and noting transitional behaviors between virialization and cosmic expansion.^[55]

Multi-Wavelength Observations

Radio Observations

High-resolution radio mapping of Centaurus A has been conducted using the Very Large Array (VLA) and the Australia Telescope Compact Array (ATCA), revealing detailed structures in the relativistic jets, including discrete knots with apparent proper motions up to approximately 0.6c on scales of hundreds of parsecs. These observations, spanning multi-year baselines, demonstrate knot proper motions consistent with bulk relativistic speeds projected close to the line of sight, providing insights into the jet's dynamics and particle acceleration processes.^[56] The radio emission from the jets and giant lobes primarily arises from synchrotron radiation by relativistic electrons in magnetic fields. Spectral index measurements across the jet show variations around α ≈ -0.7, indicative of energy losses through synchrotron radiation and inverse Compton scattering, while the giant lobes exhibit steeper spectra due to the aging of relativistic electron populations over larger timescales and volumes. These spectral properties highlight the evolution of particle populations as they propagate outward from the active nucleus.^[57]^[29] The compact core of Centaurus A has a flux density of approximately 1 Jy at 5 GHz, with variability observed on timescales of months attributed to fluctuations in the accretion flow onto the central supermassive black hole. Such changes in core brightness reflect episodic variations in the inner jet launching and non-thermal emission processes.^[58]^[59]

Optical, Infrared, X-ray, and Gamma-ray Observations

In optical wavelengths, Hubble Space Telescope observations reveal intricate details of the prominent dust lane bisecting Centaurus A, appearing as dark, warped bands of interstellar material that obscure the underlying stellar distribution and suggest a past merger event with a spiral galaxy.^[2] These images also resolve numerous globular clusters scattered across the galaxy's halo, with studies identifying over 1,300 candidates, many associated with low-mass X-ray binaries, highlighting the galaxy's evolved stellar population.^[60] Optical spectroscopy further discloses absorption lines dominated by old stars, such as Ca II H and K lines and G-band features, indicating a predominantly elliptical stellar component with ages exceeding 10 billion years, though traces of younger populations appear in emission-line regions along the dust lane.^[61] Infrared observations complement these findings by penetrating the dust-obscured regions. Mid-infrared data from the Spitzer Space Telescope highlight heated dust emission along the dust lane, with prominent polycyclic aromatic hydrocarbon (PAH) features at 11.3 μm and 17 μm tracing photoionized gas and star-forming complexes, where the dust temperatures reach up to 100 K driven by nearby massive stars.^[62] Recent James Webb Space Telescope (JWST) observations using the Mid-Infrared Instrument (MIRI) Medium Resolution Spectrometer (MRS) in 2025 have detected fast outflows of ionized gas in the central region, with velocities exceeding 500 km/s and extending over 100-200 pc, likely powered by the active galactic nucleus and interacting with the surrounding interstellar medium.^[24] Chandra X-ray Observatory observations reveal a bright central point source attributed to the accretion disk around the supermassive black hole, with a luminosity of approximately 10^{41} erg/s in the 2-10 keV band, varying on timescales of months due to obscuration by the dusty torus.^[63] Extended X-ray emission traces shocks from the relativistic jet interacting with the interstellar medium, producing knotty structures with luminosities up to 10^{39} erg/s, while diffuse hot halo gas at temperatures around 10^7 K envelops the galaxy out to 50 kpc, with a total thermal energy content of ~10^{56} erg indicating ongoing heating from the central engine.^[64]^[65] Gamma-ray detections by the Fermi Large Area Telescope show GeV emission from the core, with a spectrum peaking around 1 GeV and extending to ~10 GeV, characterized by a power-law index of ~2.7 and a flux of ~10^{-8} photons cm^{-2} s^{-1}, primarily attributed to inverse Compton scattering of seed photons by relativistic electrons in the jet base.^[66] This emission aligns spatially with the inner jet regions observed at radio wavelengths, providing evidence for non-thermal processes linking the low-energy jet to high-energy particle acceleration.

Visibility and Significance

Observational Visibility

Centaurus A, with an apparent magnitude of 6.84 in the V-band, is visible to the naked eye under dark southern skies, though its low surface brightness requires excellent conditions for unaided detection.^[67]^[68] It is best observed from latitudes south of 30° N, where the constellation Centaurus rises higher in the sky.^[69] The galaxy is located at equatorial coordinates RA 13h 25m 27.6s, Dec −43° 01′ 09″ (J2000).^[67] In the southern hemisphere, it is circumpolar for observers south of approximately 47° S and reaches its highest point in the sky during May.^[70] From the northern hemisphere, visibility is limited to spring months when the object is low on the southern horizon, becoming unobservable mid-year due to its proximity to the Sun.^[71] Amateur astronomers can spot the prominent dust lane with binoculars under dark skies, while small telescopes readily resolve the bright core and surrounding structure.^[72] It serves as a key target for radio telescopes due to its proximity and strong emissions.^[73]

Role in Astronomical Research

Centaurus A, located at a distance of approximately 3.8 Mpc, is the nearest radio galaxy to Earth, providing an unparalleled opportunity for high-resolution observations of active galactic nucleus (AGN) processes.^[74] Its proximity enables detailed studies of AGN feedback mechanisms, where the relativistic jets interact with the interstellar medium, potentially regulating star formation and gas dynamics in the host galaxy NGC 5128.^[75] For instance, observations reveal inefficient jet-induced star formation in the northern filaments, highlighting how feedback can suppress rather than enhance starburst activity.^[76] Similarly, high-resolution imaging resolves particle acceleration along the jet, offering insights into jet propagation and energy dissipation over kiloparsec scales. Recent JWST/MIRI observations in 2025 have revealed fast ionized gas outflows in the central region (~100–200 pc), providing new insights into AGN feedback mechanisms.^[77] As a key calibrator in the cosmic distance ladder, Centaurus A has been instrumental in refining measurements within the Local Volume. Cepheid variable stars observed in NGC 5128 contribute to a precise distance estimate of 3.8 Mpc, serving as a benchmark for anchoring nearby supernova distances. Type Ia supernovae, such as SN 1986G in this galaxy, provide standardized candles calibrated against these Cepheids, contributing to tests of the Hubble constant and cosmological models like ΛCDM in low-redshift regimes. Recent analyses of the Centaurus A/M83 group dynamics further utilize these distances to probe peculiar velocities and mass distributions, aiding evaluations of local Hubble flow discrepancies. Centaurus A exemplifies merger-driven radio sources, informing broader models of galaxy evolution and supermassive black hole growth. The prominent dust lane across NGC 5128 evidences a recent merger between an elliptical and a gas-rich spiral galaxy, likely triggering the central AGN activity and jet formation. As a prototype Fanaroff-Riley Class I radio galaxy, it illustrates how such mergers fuel black hole accretion and extended radio lobes, influencing host galaxy morphology over cosmic time. In multi-messenger astronomy, Centaurus A contributes significantly through potential associations with high-energy particles. IceCube observations have explored neutrino fluxes from its direction, with events like IC35 prompting models of neutrino production in the jets or lobes, though definitive associations remain elusive.^[78] Its active nucleus also positions it as a candidate for future gravitational wave detections from supermassive black hole binaries formed during the merger, enhancing prospects for joint electromagnetic-gravitational wave studies.^[78]

References

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Centaurus A | ESO - Eso.org
Jan 28, 2009 · Centaurus A is an active galaxy with lobes and jets from its central black hole, located 13 million light years away in the Centaurus ...
[2]
Active Galaxy Centaurus A - NASA Science
Jan 3, 2012 · At a distance of just over 11 million light-years, Centaurus A contains the closest active galactic nucleus to Earth.
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Radio Telescopes Capture Best-Ever Snapshot of a Black Hole's Jets
May 20, 2011 · Centaurus A is a giant elliptical active galaxy 12 million light years away. Radio and X-ray images reveal features associated with jets ...
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Caldwell 77 - NASA Science
Centaurus A was discovered by astronomer James Dunlop in 1826. It is the fifth brightest galaxy in the sky, making it an ideal target for amateur astronomers.
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Radio galaxy Centaurus A - Eso.org
Centaurus A, also known as NGC 5128, is a radio-loud active galaxy, a merger of elliptical and spiral galaxies, with a small radio source and a supermassive ...
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Peering into a Galaxy's Dusty Core to Study an Active Supermassive ...
Mar 17, 2021 · Centaurus A sports a warped central disk of gas and dust, which is evidence of a past collision and merger with another galaxy. It also has an ...
[7]
[1002.0965] The Supermassive Black Hole at the Heart of Centaurus A
Feb 4, 2010 · Using data from SINFONI at the ESO Very Large Telescope, the best-fit black hole mass is M_BH=4.5 +1.7/-1.0 x 10^7 Msolar (from H_2 kinematics) ...
[8]
Early Australian Optical and Radio Observations of Centaurus A - ADS
NGC 5128 was first observed in August 1826 by James Dunlop during a survey of southern objects at the Parramatta Observatory, west of the settlement at Sydney ...
[9]
Early Australian Optical and Radio Observations of Centaurus A
Jan 2, 2013 · NGC 5128 was first observed in August 1826 by James Dunlop during a survey of southern objects at the Parramatta Observatory, west of the ...<|control11|><|separator|>
[10]
Centaurus A (NGC 5128) | Deep⋆Sky Corner
Apr 29, 2024 · The galaxy NGC 5128 was discovered by Scottish astronomer James Dunlop on 29 April 1826 using the self-built 9-inch reflector at Parramatta ...
[11]
John Herschel's drawing of NGC 5128 made in June 1834,...
NGC 5128 was first observed in August 1826 by James Dunlop during a survey ... In early June 1834 Herschel made four observations of Dunlop"s nebula ...
[12]
Centaurus A - NGC 5128 - F.P. Israel
They described it as "... an unresolved E0 nebula with an unusually strong and wide central dark band, a combination highly anomalous for a spherical nebula" ...
[13]
The nebula with the dark band - Max-Planck-Gesellschaft
Jul 20, 2021 · Astronomer John Herschel discovered it while observing the southern sky from the Cape of Good Hope between 1834 and 1838. In his directory, ...
[14]
The Instrinsic Shape of Elliptical Galaxies - NASA ADS
“Classical” Photometric Methods In his original paper on “extra-galactic nebulae,” Hubble (1926) performed an intuitively simple inversion of the apparent ...
[15]
The classification of galaxies - Book chapter - IOPscience
After Edwin Hubble discovered that galaxies outside of the Milky Way existed ... Left panel: NGC 5128 (or Centaurus A) is a type EO peculiar elliptical galaxy.
[16]
https://www.publish.csiro.au/ph/PH590471
[17]
A Recent Major Merger Tale for the Closest Giant Elliptical Galaxy ...
Aug 31, 2020 · We have used hydrodynamical simulations to model the formation of the closest giant elliptical galaxy, Centaurus A. We find that a single major ...Missing: counter- rotating studies
[18]
https://arxiv.org/abs/0911.3180
[19]
NGC 5128 (Centaurus A) - AstroPixels
Apr 14, 2021 · Centaurus A, Caldwell 77; Object Type: giant elliptical galaxy; Object Data: Apparent Magnitude = 6.8, Angular Size = 25.7 × 20.0 arc-minutes ...
[20]
the galaxy - Centaurus A - NGC 5128 - F.P. Israel
Consequently, NGC 5128 is sometimes also classified as a polar ring galaxy although it fails to share several of the characteristics common to these (see ...Missing: SB0 pec
[21]
Basic parameters - Centaurus A (NGC 5128)
Centaurus A (Cen A) is the entire radio galaxy, the large-scale structure, about 9° at maximum extent, between approximately -39° and -48°, including the giant ...
[22]
https://ntrs.nasa.gov/api/citations/20110008613/downloads/20110008613.pdf
[23]
[PDF] Twelve and a Half Years of Observations of Centaurus A with
Cen A was found to. "correspond within limits of experimental error" to the "extra-galactic nebula" NGC 5128 ... Mills, B.Y. 1952, Nature, 170, 1063. Mills, B.Y. ...Missing: identification | Show results with:identification
[24]
Searching for the hidden broad-line region in Centaurus A
... AGN is whether all objects contain a broad-line region (BLR). ... Additional data reduction consisted of interpolating over known bad pixels, sky subtraction, ...Missing: weak | Show results with:weak
[25]
[2506.15286] MICONIC: JWST/MIRI MRS reveals a fast ionized gas ...
We present a kinematical study of the ionized and molecular gas in the central region (~7-14"~100-200pc) of the nearby radio galaxy Cen A.
[26]
Event Horizon Telescope observations of the jet launching and collimation in Centaurus A - Nature Astronomy
- **Mass Estimate**: Supermassive black hole in Centaurus A is (5.5 ± 3) × 10⁷ M⊙.
[27]
Superluminal proper motion in the X-ray jet of Centaurus A - arXiv
Aug 26, 2024 · Most of the knots had an apparent proper motion below the detection limit. However, one knot at a transverse distance of 520~\mathrm{pc} had an ...
[28]
THE RADIO CONTINUUM STRUCTURE OF CENTAURUS A AT 1.4 ...
Almost the entirety of the giant southern lobe is resolved into a largely chaotic and mottled structure which appears considerably different (morphologically) ...
[29]
Giant lobes of Centaurus A as seen in radio and γ-ray images ...
The γ-ray data of Fermi-LAT on the giant lobes of Centaurus A are analysed together with the high frequency radio data obtained with the Planck satellite.
[30]
giant lobes of Centaurus A observed at 118 MHz with the Murchison ...
The structure of Cen A is complicated by the bending of the lobes, which is probably due to both the interaction with the ambient medium (Morganti et al. 1999), ...
[31]
Hardcastle et al., Centaurus A Jet - IOP Science
In this paper we present new, high dynamic range radio data and new, high spatial resolution X-ray imaging spectroscopy, which together shed new light on the ...<|control11|><|separator|>
[32]
Precession in the jet of NGC 5128 (Centaurus A). - ADS
New optical and radio data are presented on the outer parts of Centaurus A (NGC 5128) and their relationship with recently published optical, ...Missing: bending intracluster medium
[33]
Deep observation of the giant radio lobes of Centaurus A with the ...
... 1058 erg and the energy in the magnetic fields is roughly 25% of this. Dividing the total energy by the epoch time 107 yr would imply a mean kinetic power ...
[34]
Interaction between the intergalactic medium and central radio ...
We confirm the presence of cavities associated with the radio lobes and estimate their enthalpy to be ~2.4x10^58 erg. The mechanical power output of the jets is ...
[35]
How old are the stars in the halo of NGC 5128 (Centaurus A)?
However, the well known age/metallicity degeneracy that strongly affects the old RGB stars prevented any more precise statements about the age distribution.
[36]
A recent major merger tale for the closest giant elliptical galaxy ...
ABSTRACT. We have used hydrodynamical simulations to model the formation of the closest giant elliptical galaxy Centaurus A. We find that a single major me.
[37]
The colour–magnitude relation of globular clusters in Centaurus and ...
We measure the relation between mean colour and luminosity for the blue and red subpopulation of the globular cluster samples.<|control11|><|separator|>
[38]
Abundance stratification in Type Ia supernovae – V. SN 1986G ...
SN 1986G was located in NGC 5128, also known as Centaurus A, which is at a distance 3.42 ± 0.18 Mpc (Ferrarese et al. 2007). This made it one of the closest ...
[39]
The carbon-rich type Ic supernova 2016adj in the iconic dust lane of ...
We present a comprehensive data set of supernova (SN) 2016adj located within the central dust lane of Centaurus A. SN 2016adj is significantly reddened.
[40]
2025nok | Transient Name Server
Jun 20, 2025 · AT 2025nok ; Reporting Group. ATLAS ; Discovering Data Source. ATLAS ; Discovery Date. 2025-06-14 21:56:57.120 ; TNS AT. Y ; Public. Y.Missing: Centaurus | Show results with:Centaurus
[41]
[PDF] Ground-based variability surveys towards Centaurus A
Considering the expected Nova rate for Centaurus A of ∼28 per year (Ciardullo et al. 1990), which is several factors higher than the halo microlensing rate ...
[42]
The dusty disk - Centaurus A - NGC 5128 - F.P. Israel
(1992) shows that in spite of appearances, the distribution of dust in NGC 5128 is that of a warped thin disk of about 200 pc thickness (Sect. 6.1) along ...
[43]
Observations and a model for the infrared continuum of Centaurus A
It shows evidence of a merger, star ... (1996), implying that this structure may be an ionization cone rather than the suggested warped accretion disc.<|control11|><|separator|>
[44]
Disk in Centaurus A - IOP Science
It is significantly smaller than the 100 or more parsec stellar disks presumed to exist at the centers of many galaxies. Even at this small scale, the disk is ...Missing: size | Show results with:size
[45]
Centaurus A: morphology and kinematics of the atomic hydrogen
The prominent warped dust and gas disk in the central region, and the outer partial ring structure, suggest that Cen A experienced a recent merging event where ...
[46]
mass of the black hole in Centaurus A from SINFONI AO-assisted ...
The median value is D= 3.8 Mpc, with extreme ranges of 3.4 and 4.4 Mpc, respectively. Here, we adopt a value2D= 3.5 Mpc to be consistent with all the ...
[47]
MICONIC: JWST/MIRI MRS reveals a fast ionized gas outflow in the ...
We present a kinematical study of the ionized and molecular gas in the central region (∼7–14′′∼100–200 pc) of the nearby radio galaxy Centaurus A (Cen A).
[48]
Observations of CO isotopic emission and the far-infrared continuum ...
Jul 1, 1990 · The total molecular mass of the disk and bulge is of the order of 3 x 10 to the 8th power solar mass. The molecular gas in the nucleus is warm ...Missing: masses | Show results with:masses
[49]
Disentangling the Circumnuclear Environs of Centaurus A. III. An ...
We present the distribution and kinematics of the molecular gas in the circumnuclear disk (CND; 400 pc × 200 pc) of Centaurus A with resolutions of ∼5 pc (0 ...
[50]
New distances to galaxies in the Centaurus A group - ADS
We present Hubble Space Telescope/WFPC2 images of seventeen dwarf galaxies in the Centaurus A group. Their distances derived from the magnitudes of the tip ...Missing: 5090 270-017 4296
[51]
New Galaxies Discovered in the First Blind H I Survey ... - IOP Science
New Galaxies Discovered in the First Blind H I Survey of the Centaurus A Group. G. D. Banks, M. J. Disney, P. M. Knezek, H. Jerjen, D. G. Barnes, R. Bhatal ...
[52]
MUSE view of the low-luminosity S0 galaxy NGC 5102 | Monthly ...
NGC 5102 is a low-luminosity S0 galaxy in the Centaurus group (Karachentsev et al. 2002) at a distance of 4.0 ± 0.2 Mpc, as determined via the surface ...
[53]
The Hubble flow around the CenA / M83 galaxy complex - arXiv
Mar 3, 2006 · The study uses HST/ACS images of 24 galaxies near CenA and M83, finding two groups with small peculiar velocities relative to the Hubble flow.
[54]
Hubble Constant and Mass Determination of Centaurus A & M83 ...
Oct 10, 2025 · In this study, we analyze the dynamics of the Centaurus~A (CenA) and M83 galaxies, along with their associated dwarf companions identified via ...
[55]
Radio and X-ray observations of the jet in Centaurus A - arXiv
Apr 24, 2003 · Abstract: We present new, high dynamic range VLA images of the inner jet of the closest radio galaxy, Centaurus A. Over a ten-year baseline ...Missing: ATCA superluminal
[56]
(PDF) Centaurus A - NGC 5128 - ResearchGate
Aug 7, 2025 · ... Mills (1952) conﬁrmed the identiﬁcation. of Centaurus A with NGC 5128. Discussing the nature of optical counterparts. to the newly ...
[57]
the radio source and associated features
Centaurus A (PKS 1322-427) is the largest extragalactic radio source in the sky. It extends predominantly in a north-south direction.
[58]
[PDF] The millimeter-wave continuum spectrum of Centaurus A and ... - arXiv
Apr 29, 2008 · 4 we illustrate this variability of core flux density. Over the period monitored, the flux density never drops much below 7 Jy, but at times ...
[59]
Globular Clusters and X-Ray Point Sources in Centaurus A (NGC ...
We detect 353 X-ray point sources, mostly low-mass X-ray binaries (LMXBs), in four Chandra observations of Centaurus A (NGC 5128), the nearest giant early-type ...
[60]
Centaurus A - IOP Science
Some are likely to have measurable emission and absorption lines, and this seems to be the most promising direction to follow to obtain a sharper definition of ...
[61]
Spitzer Space Telescope Infrared Spectrograph mapping of the ...
The S(3) and S(5) emission also lies in the vicinity of the dust shell that is most prominent in the 11.3-μm dust (PAH) emission feature. A two-temperature ...
[62]
Chandra Observations of the X-Ray Point Source Population in ...
The core of M87 has a luminosity of 4.4 × 10 40 erg s −1 in the 0.5-10 keV band (Marshall et al. 2002), while the HST-1 feature reached an order of higher ...
[63]
Chandra Observations of the X-ray Jet in Centaurus A - arXiv
Nov 16, 2001 · We find that the X-ray emission from the jet is composed of a low surface brightness diffuse component extending continuously from within at ...Missing: halo gas 10^ 7 K<|separator|>
[64]
X-raying galaxies: A Chandra legacy - PNAS
Mar 8, 2010 · The gas typically has a mean characteristic temperature of ∼106.3 K, although there is evidence for a high-temperature component (up to ≳107 K) ...
[65]
The γ-ray spectrum of the core of Centaurus A as observed with ...
Nov 29, 2018 · This work on the Cen A core does not require a high-precision model for the lobes, as the angular size of the Cen A lobes is sufficiently larger ...
[66]
https://www.aanda.org/articles/aa/full_html/2018/11/aa32640-18/aa32640-18.html
[67]
NGC 5128 The Hamburger Galaxy
It is bright enough to be visible in binoculars, and can be observed with the naked eye under very dark skies. ... visible to the naked eye. In an 8 ...
[68]
Centaurus
Visible between latitudes 30 and -90 degrees; Best seen in May (at 9:00 PM) ... This constellation represents Chiron who is frequently mentioned in Greek mythology ...
[69]
Centaurus Constellation: Stars, Myth, Facts, Location
NGC 5090 and NGC 5091 form a pair of galaxies that are in the process of merging and colliding. They are 150 million light years distant from Earth.
[70]
Centaurus Constellation | Star Map & Facts - Go-Astronomy.com
Centaurus is located in the second quadrant of the southern hemisphere (SQ2) and can be observed at latitudes between +25? and -90?. It is surrounded by several ...
[71]
The strange galaxy Centaurus A in the constellation of ... - ESO
May 16, 2012 · This galaxy is quite bright and can be seen well in small amateur telescopes or even binoculars. The dark band is also clearly visible in ...Missing: observational naked tips
[72]
[1006.1486] Sub-parsec scale imaging of Centaurus A - arXiv
Jun 8, 2010 · At a distance of about 3.8 Mpc, the radio galaxy Centaurus A is the closest active galaxy. Therefore it is a key target for studying the ...Missing: MeerKAT surveys structures 2020s<|control11|><|separator|>
[73]
ALMA Observations of the Physical and Chemical Conditions ... - arXiv
Dec 1, 2017 · Abstract:Centaurus A, with its gas-rich elliptical host galaxy, NGC 5128, is the nearest radio galaxy at a distance of 3.8 Mpc. Its ...Missing: anomalous merger remnants
[74]
The jet/wind outflow in Centaurus A: a local laboratory for AGN ...
Nov 6, 2017 · Thus, our observational overview of Centaurus A reveals a number of features proposed to be associated with AGN feedback mechanisms, often cited ...
[75]
[1710.09851] Inefficient jet-induced star formation in Centaurus A
Oct 26, 2017 · ... Centaurus A: High resolution ALMA observations of the northern filaments. ... Abstract:NGC 5128 is one of the best targets to study AGN-feedback ...
[76]
Centaurus A, GW 170817, and KM3-230213A - arXiv
Jul 22, 2025 · We review recent advances in multimessenger astrophysics, with particular emphasis on Centaurus A and on two pivotal events: the gravitational- ...Missing: multi- messenger