Galaxy Centaurus A

VLT Kueyen + FORS2, ESO



Larger image.




Clusters of luminous, young blue stars
can be seen to have formed in and around
the dark, gas- and dust-rich band that
bisects the "Hamburger Galaxy" (more).

In 1949, NGC 5128 was found to be a powerful radio galaxy by early radio astronomers (which was first suggested by John Bolton, Gorden Stanley, and Bruce Slee in Nature, I64, 101, 1949; according to Baade and Minkowski, 1954, pp. 223-225, footnote on 217). It then shared the additional designation of Centaurus A, which had been determined to be the brightest radio source in the direction of (13:25:28-43:1:11 J2000; and 13:25:27.6-43:1:8.8, ICRS 2000) of Constellation Centaurus, the Centaur, north of globular cluster Omega Centauri (NGC 5139) and west of Mu Centauri -- map. Found to be the closest "radio-loud" galaxy to the Solar System (José Luis Sérsic, 1960), its radio emissions comes from two giant lobes that extend over a million light-years (ly), roughly at right angles to the galaxy's dark dust band.

ESA/ISO, ISOCAM Team,
Mirabel et al, 1998


Larger infrared image with radio contours.



Some of the dust inside Centaurus A maps
out what appears to be a barred spiral
galaxy, which has recently merged with its
giant elliptical host and is feeding gas
into the host's central hole to produce
bi-polar jets that are bright in radio
wavelengths (more from APOD and ESA).

Now sometimes called the "Hamburger Galaxy" for its dark band, Centaurus A has been recently estimated to be located around 13.7 ± 1.9 million light-years away from the Solar System (ESO press release), in line with estimates of 10 to 15 million ly derived with various methods made since 1998. Located outside the Local Group of galaxies that includes the Milky Way, it is a member of a nearby group of galaxies that is sometimes named after Centaurus A itself (as the Centaurus A or NGC 5128 Group), but the group of galaxies is also commonly referred to as the M83 Group -- after prominent member M83, a spiral galaxy that is also called the Southern Pinwheel (or NGC 5236). Centaurus A is not visible to Human eyes without the aid of a telescope.

Jocelyn Keene et al, SSC,
JPL, Caltech, NASA


Larger infrared image.



The warped and twisted disk of
a spiral galaxy devoured by
Centaurus A has dust shaped
like a parallelogram around
1,000 ly wide (more from APOD
and the Spitzer Space Telescope).

In visible light, Centaurus A resembles an elliptical galaxy. Its prominent dust band, however, is not a feature normally associated with ellipticals which are generally quite dust poor and populated with old stars. Recent observations suggest that the galaxy is actually a giant elliptical that is in the process of devouring a smaller dusty, barred spiral galaxy, similar in size to prominent nearby spiral M33 (Mirabel et al, 1998).

X-ray (CXC/NASA/Karovska et al, 2002);
radio 21-cm (NRAO/VLA/Schiminovich et al),
and continuum (NRAO/VLA/Condon et al); and
optical (Digitized Sky Survey/UK Schmidt/STScI)


Larger optical (orange and yellow),
radio (pink and green), and x-ray (blue),
composite image created in 2002.



X-ray, radio, and optical images suggest
that the galaxy is in tremendous turmoil
with energetic arcs and jets after merging
with the barred spiral some 100 million
years ago, (more from APOD and CXC).

Detection of the spiral's dust in a bi-symmetric structure provides strong evidence of its position deep inside the giant elliptical host while the vast amounts of radio, visual, and x-ray emissions are a result of the energy released by this continuing galactic merger (or "consumption" of a satellite galaxy). Astronomers (who uncovered the infrared image of the embedded spiral using the ISO satellite) believe that the giant elliptical's gravity has helped the consumed, barred spiral galaxy to maintain its shape thus far. In turn, gas and dust funneled along the spiral's bar fuels the central black hole which powers the elliptical's enormous radio lobes.

X-ray (CXC/NASA/CfA/Hardcastle et al, 2007);
radio (VLA/NSF/U.Hertfordshire/Hardcastle et al); and
optical (ESO/VLT/ISAAC/Rejkuba et al)




Larger composite image.




A 2008 composite image provides
more detail that outlines how the
arcs may be created by tilting
bi-polar jets erupting from the
rotating center of Centaurus A
(more from APOD and CXC).

Centaurus A's broad band of dust and cold gas is bisected at an angle by opposing jets of high-energy particles blasting away from the supermassive black hole at its core. In recent years, two large arcs of X-ray emitting hot gas were discovered in the outskirts of the galaxy on a plane perpendicular to the jets. The arcs of multi-million degree gas appear to be part of a projected ring with a diameter around 25,000 light-years, whose size and location suggest that it may have been created by a titanic explosion around 10 million years ago. Such an explosion would have produced the high-energy jets, as well as a galaxy-sized shockwave that is moving outward at speeds of a million miles (more than a million kilometers) per hour. The age of 10 million years for the outburst is consistent with optical and infrared observations which indicate that the rate of star formation in the galaxy increased dramatically at about that time.

Submillimeter (Weiss et al, 2008;
MPIfR / APEX);
X-ray (Kraft et al, NASA / CXC / CfA);
optical (ESO / WFI)





Larger composite image.





A 2009 composite image clarifies
the arcs (or radio lobes) and jets
emanating from the active galaxy's
central black hole (more).

The galaxy's unusually energetic activity may have begun around 100 million years ago with its consumption of the small spiral galaxy now found inside it. Such a merger could have triggered both the burst of star formation and the violent activity in its galactic nucleus. The tremendous energy released when a galaxy becomes "active" can have a profound influence on the subsequent evolution of the galaxy and its neighbors. The mass of the central black hole can increase, the gas reservoir for the next generation of stars can be expelled, and local intergalactic space may be enriched with heavier elements.

Eric Peng, Holland Ford,
Ken Freeman, Rick White,
CTIO/NOAO/NSF, (STScI)/NASA



Larger image and blue-arc close-up.



Concentric shells and a blue arc of
young blue stars around the galaxy
are indicative of recent collisions with
smaller gas-rich galaxies (more from
APOD, NOAO, and Peng et al, 2002).

Additional evidence that Centaurus A's activity is the result of mergers with smaller, gas-rich galaxies are the bright young blue stars seen along the edges of the dust band (or "lane"), that elliptical galaxies normally do not have enough gas to form. The galaxy is also surrounded by faint shells of gas and dust with a blue arc of young hot stars and star clusters that suggest a collision with a gas-rich, dwarf irregular galaxy (such as POX 186) around 200 to 400 million years ago. Although faint shells around galaxies are not unusual, they are considered by astronomers as strong evidence of previous galaxy mergers, which leave ripples like the surface of a pond. An unexpected attribute of these shells is the abundance of gas, which can become separated from smaller galaxies during a collision. The tidal forces of the larger, host elliptical galaxy caused a burst of star formation within the infalling galaxy, and these young stars were then spread along the remnant of the incoming orbit (Peng et al, 2002).

VLT ISAAC, ESO




More images.





Thousands of Mira-type red giant,
pulsating variable stars have been
detected in the halo of Centaurus A
near its bi-polar jets, including
an area coinciding with a stellar
shell associated with the ongoing
galactic merger (more from ESO).

In recent years, intermediate-age and young stellar populations in the halo of Centaurus A had been detected by a team of astronomers currently working with the European Southern Observatory. The youngest stars appear to be aligned with the powerful jet produced by the massive black hole at the galaxy's center. In 2003, the team announced the discovery of thousands of very luminous and red, long-term pulsating variable stars that are similar to Mira (or Omicron Ceti), thus confirming the presence of a population of intermediate-age stars in Centaurus A's halo. The Mira-type variables were found in a special survey of two halo regions coinciding with a stellar shell containing many blue and luminous stars produced by Centaurus A's on-going galactic merger, lying at distances of 57,000 and 30,000 ly from the galaxy's center (ESO press release; and Rejkuba et al, 2003).

Schreier et al, 1998, STScI/NASA



Larger image.




The Centaurus A is observed to
have an active galactic nucleus,
with a supermassive black hole
that is spewing bi-polar jets of
high-energy particles (more
from APOD and STScI).

The core of Centaurus A contains one of the smallest known extragalactic radio sources, at only 10 light-days across. One of a pair of bi-polar jets of high energy particles from this center has been observed with extensive detail in radio and X-ray images. The core probably contains a supermassive black hole with a mass of 100 million to as much as a billion Solar-masses (ESO press release; and Marconi et al, 2001). As a result of a galactic collision around 100 million years ago, gas and dust from a smaller barred, spiral galaxy is steadily being pulled to the black hole, which is shooting off some of that substance at its poles. Astronomers believe that supermassive, central black holes generate the radio, X-ray, and gamma-ray energy radiated by active galaxies such as Centaurus A, as well as quasars like SDSS J1030+0524.

Hardcastle et al, 2007,
CfA, CXC, NASA






Larger annotated x-ray image.






One entire jet erupting towards
the upper left with an opposing
counterjet can be observed amid
many bright x-ray sources,
including numerous black holes
actively feeding on gas and
dust (more from APOD and CXC).

The galaxy's bi-polar jets blast out as linear sprays of bright x-ray knots that emanate from a bright central source (Kraft et al, 2002), with one jet apparently extending out around 15,000 light-years, or 4,500 parsecs (Hardcastle et al, 2007). Centaurus A's central region is also seen to be teeming with smaller x-ray sources, which are likely to be neutron stars as well as stellar-mass black holes that are accreting material from stellar companions (Kraft et al, 2001). Also observed is a diffuse x-ray glow which is apparently radiated by gas that has been heated to temperatures of millions of degrees Celsius.

Kraft et al, 2001, CXC/ NASA


Larger x-ray image.


Blasting from a black hole with as much as
a billion Solar-masses, one jet travels
30,000 light-years in this image, among
neutron stars and black holes that are x-ray
bright from accreting material from companion
stars (more from APOD and CXC).

The bi-polar jets are apparently confined to relatively narrow angles. Most of the high-energy x-rays are produced farther away from the core, apparently where a jet begins to collide with the galaxy's denser gas. In contrast, radio emissions have been observed all along one observable jet, as well as along its obscured twin ("counterjet") that emanates from the opposite pole. Clusters of young blue stars have been found along the jets, probably because patches of gas and dust have been shocked to collapse and trigger stellar formation (Graham and Fassett, 2002).

Hardcastle et al, 2003,
CXC/NASA, VLA/NRAO



Larger radio (red) and
x-ray (blue), composite image.



The inner 4,000 light-years of a jet
can be seen to erupt from the
galaxy's core at the lower right to
produce more x-rays when it collides
with denser gas at the upper left
(more from APOD and CXC).