Key Events in the Observational History of Centaurus A



1826: What we now know as NGC 5128, or its radio name Centaurus A (Cen A) was discovered on August 4, 1826 by James Dunlop at the Parramatta observatory in Australia. It is number 482 in his "A Catalogue of Nebulae and Clusters of Stars in the Southern Hemisphere observed in New South Wales", in: Philosophical Transactions of the Royal Society, Vol. 118, p. 113-151 (1828).

1847: Sir John Herschel reports observations of it in his "Outlines of Astronomy," published in 1849, Herschel described it as "two semi-ovals of elliptically formed nebula appearing to be cut asunder and separated by a broad obscure band parallel to the larger axis of the nebula, in the midst of which a faint streak of light parallel to the sides of the cut appears."

1848-1948: Despite Herschel's observation, astronomers paid little attention to Centaurus A for about 100 years because they considered it another one of those nebulous, fuzzy objects originally thought to be in our own galaxy. Heber D. Curtis (probably in "A Study of Occulting Matter in Spiral Nebulae", Publ. Lick. Obs. 13, 45-54 (1918)) describes the unusual wide dust band crossing the nebula.
Even Edwin Hubble called it a nebulous object. Later astronomers with more powerful telescopes identified many of these objects as galaxies. Centaurus A also was largely ignored because of the lack of large optical telescopes in the southern hemisphere, where this galaxy can be seen.

1948/49: Astronomers had just developed another technique to study celestial objects besides optical light: special instruments to collect radio waves. Using a sea interferometer at Dover Heights, Australia which uses direct radio waves and radio waves reflected off the nearby sea, astronomer John Bolton announced the discovery of discrete sources of radio emission (Bolton 1948).
In a paper published a year later, astronomers John Bolton, Gordon Stanley, and Bruce Slee were the first to identify Centaurus A as a powerful radio galaxy. (Bolton et al. 1949)
Radio waves from Centaurus A were among the first to be linked to an extragalactic object.
For further information on the early Australian optical and radio observations of Centaurus A see Robertson et al. (2009).

1954: Studying Centaurus A with telescopes at Palomar Observatory in California, Walter Baade and Rudolph Minkowski confirmed that it is a galaxy. The pair also proposed that the peculiar structure of Centaurus A - with a bar of dark dust bisecting the galaxy - is the result of a merger between two galaxies, a giant elliptical and a small spiral.
( Baade and Minkowski 1954)

1968: The probably first attempt to measure X-rays (0.25 - 10 keV) from Centaurus A was made in April this year by Byram et al. 1970 using an Arobee rocket. Only a marginal signal had been detected.

1969-1971: Stuart Bowyer, using also a sounding rocket, detected X-rays emanating from Centaurus A (Bowyer et al. 1970). In late 1970, the UHURU satellite confirmed that the X-rays were indeed associated with the galaxy. In 1971, Bill Kunkel and Hale Bradt used a telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile to observe infrared light originating from a compact source in the galaxy's core. X-ray observations in the early 1970s with the Astronomical Netherlands Satellite (ANS) and the Orbiting Solar Observatory (OSO-7) showed changes in the intensity of X-ray emissions. Significant changes over a very short time indicated that the emission source was confined to a small region of space. The results suggested that a black hole could be the culprit.
(Kunkel and Bradt 1971)

1972-74: Using atmospheric Cerenkov technique, Josh Grindlay et al. detected gamma-ray emission from NGC 5128 at energies above 100 GeV
( Grindlay et al. 1975 )

1974-76: Using information from high-flying research balloons, R.D. Hall detected gamma-ray emissions (33 keV - 12.25 MeV) which were probably coming from the nucleus.
( Hall et al. 1976 )

1975: Looking at the galaxy in visible light with a CTIO telescope, Victor Blanco discovered a faint jet out in the galaxy. He also noted blue stellar objects in this region.
( Blanco et al. 1975)

Late 1970s: John Graham, using a CTIO telescope, found a series of faint shells of gas in the outer regions of the galaxy. These shells could have been produced by collisions of gas from a galaxy merger.
( Graham 1979)

1978: J.H. Beall and colleagues publish a paper that for the first time reports variability in both radio and X-ray frequencies at the same time in 1975 and 1976. The radio observations were carried out from North America (Stanford University and Kitt Peak) and the X-ray observations are from the OSO-8 satellite.
(J.H. Beall et al. 1978)

1979: Using the Einstein Observatory (2-10 keV), Ethan Schreier discovered an X-ray jet emanating from the nucleus of Centaurus A. Working with Eric Feigelson and Jack Burns, Schreier used the Very Large Array in New Mexico to find the jet's radio counterpart.
(Schreier et al. 1979; Schreier et al. 1981)

1981: With the Einstein Observatory, Feigelson et al. also observed extended X-ray emissions in the galaxy, including ridges of emissions along the north and south edges of the dust lane.
( Feigelson et al. 1981)

1986: Supernova 1986G in NGC 5128 is detected on May 3.5 (Evans 1986) in the middle of the south-eastern part of the obscuring dust lane. This bright object was subsequently used as a probe to investigate the properties of NGC 5128 and the dust lane.

Closeup of the SE part of the dust lane in Cen A. The supernova 1986G is the conspicious blue object in the dust lane. The blue color results from the fact, that the green and blue plate for this composite color picture were taken when the supernova was close to maximum, and the red plate was taken a year later when the supernova had faded considerably.
(Enlarged section of an Anglo Australian Telescope picture (AAT7).)
Same region as above, but ten years later. The supernova is no longer visible.
(Enlarged section of a NOAO picture).

1996: Schreier used the Hubble Space Telescope's Wide Field and Planetary Camera 2 (WFPC2) to study the dust lane's polarization properties and the distribution of young stars along its northern edge. The young, blue stars provided further proof that Centaurus A, an elliptical galaxy, merged with a spiral galaxy. Elliptical galaxies such as Centaurus A would not have had enough dust and gas to form clusters of new stars.

1997: Kenneth Kellermann, Anton Zensus, and Marshall Cohen, using the Very Long Baseline Array (VLBA) in New Mexico, observed that the core of Centaurus A is only 10 light-days across, making it the smallest known extragalactic radio source. They also noted that the energy produced by this source varies in intensity every day. This core has a mass of about 100 million suns.

1997-98: Schreier used WFPC2 of the Hubble Space Telescope to further examine the nucleus and the bright, young stars. Since WFPC2 cannot peer through the dusty core, Schreier and Alessandro Marconi turned to the Hubble telescope's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) to explore the core. They found a very compact nucleus and also discovered a central, warped disk. The disk could have been warped by a collision with another galaxy.

1999: The new Chandra X-ray Observatory takes images of Cen A with unprecedented resolution. More than 200 point sources can be identified in those images.
(Kraft et al. 2001)

2000: Steinle, Dennerl, and Englhauser report the detection of a bright X-ray transient close (2'.5) to the nucleus of Cen A in ROSAT images taken in 1995. This may shed new light on the reported (X-ray) variability of Cen A, which so far was attributed to the nucleus, if measured with instruments with low spatial resolution.
(Steinle et al. 2000)

2005: TeV observations with the new HESS imaging atmospheric-Cherencov telescope did not detect Cen A > 190 GeV (190 x 109 eV) resulting in a 3 sigma upper flux limit of 5.68 10-11 photons cm-2 s-1.
(Aharonian et al. 2005)

2006: Like almost all new telescopes and satellites in the past, the new Spitzer Space Telscope, sensitive in the infrared, was aimed at Centaurus A as one of the first objects to demonstrate the new capabilities. Peering into the "gut" of the galaxy, Spitzer has captured in unprecedented detail this massive galaxy's last big meal: a spiral galaxy twisted into a parallelogram-shaped structure of dust. The geometric shape can be explained using a model that describes a flat spiral galaxy falling into an elliptical galaxy and becoming twisted and warped in the process.
( Quillen et al. (2006))

2007: Analyzing data obtained at the Pierre Auger Observatory (PAO) in Argentina between January 2004 and August 2007, the Pierre Auger Collaboration announced the detection of 21 Cosmic Ray events with energies above 57 EeV (57 x 1018 eV). Their distribution on the sky is such, that ony two events overlap with their 3.1° error radius. This two events are within 3° from Cen A!  And this is the only case in which high energy cosmic rays are detected close to the location of an AGN.
This detection triggered a surge in theoretical papers explaining why Cen A has to be a source of high energy Cosmic Rays.
( Abraham J., and the Pierre Auger Collaboration (2007))

2008: A new method to determine the mass of the central black hole was applied to Centaurus A for the first time. Using AO-assisted integral-field observations of stellar kinematics in the vicinity of the massive black hole, the derived mass value is in excellent agreement with previous determinations from the gas kinematics.
The new value for the mass of the black hole is (5.5 ± 3.0) * 107 Msun (3 sigma errors).
( Cappellari et al. (2008))

2009: Finally, after many years of repeated attempts to detect Centaurus A at very high energies (VHE, E>100 GeV), the H.E.S.S. collaboration announced the discovery of faint very high energy gamma-ray emission. Cen A has been observed for more than 120 hours over a time span of 4 years and a signal with a statistical significance of 5.0 sigma is detected from the region including the radio core and the inner kpc jets.
The discovery of VHE gamma-ray emission from Centaurus A reveals particle acceleration in the source to >TeV energies and, together with M 87, establishes radio galaxies as a class of VHE emitters.   ( Aharonian, et al. (HESS Collaboration, 2009); image (Fig. 2))

The very successful international conference The Many Faces of Centaurus A (Sydney, Australia, June 28 - July 03, 2009) brought together a broad range of astronomers and high-energy physicists that traditionally form separate research communities. It was the first time that a conference solely devoted to Cen A was organized.
Most of the review talks have been published as articles in a special issue on Cen A of the
Publications of the Astronomical Society of Australia, Volume 27 Number 4 2010, The Many Faces of Centaurus A

The stellar light emerging from the kiloparsec-scale, ring-like structure of Cen A found with Spitzer observations in 2006 is revealed in unprecedented detail by using arcsecond-scale resolution near infrared images to create a "dust-free" view of the central region of the galaxy. The ring-like structure contains several hundreds of discreet, point-like or slightly elongated sources. Diffraction limited (FWHM resolution of ~ 0.1" , or 1.6 pc) near infrared data taken with the NACO instrument on the ESO VLT show that the structure decomposes into thousands of separate, mostly point-like sources, most of them red supergiants or relatively low-mass star clusters.
( Kainulainen, et al. (2009) and ESO Photo Release 44/09)

2010/2011: The data obtained by the Fermi Gamma-ray Space Telescope (NASA; launched in June 2008) lead to the detection of Cen A and its lobes at GeV energies with high statistical significance (Yang R. et al. 2012).
The first results of the analysis of several years of routine measurements after the completion of the Pierre Auger Cosmic Ray Observatory in Argentina in 2008 hint to a possible connection of the origin of Ultra High Energy Cosmic Rays and Centaurus A (The Pierre Auger Collaboration; Abreu et al. 2010).

Both results have been expected since long, but the confirmation shows that Cen A is one of the few objects in the sky that are detected in all energy ranges over the whole electromagnetic spectrum up to very high energies and may even be considered to be a source of Ultra High Energy Cosmic Rays.

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Last update: 2012-04-11     |     Copyright © Helmut Steinle, MPE |     Impressum

 

 Centaurus A