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Object of the Week March 23, 2014 - Centaurus A (NGC 5128)
NGC 5128, Centaurus A, Arp 153
Peculiar Galaxy (S0p)
RA: 13h 25.5'
Dec: −43° 01'
Size: 16’ x 13'
Mag: 6.7v
NGC 5128, or Centaurus A, is a superb galaxy that is magnificent in every way. It shines brightly in X-rays, it is one of the strongest radio sources in the entire sky, and it is one of the brightest galaxies in the visual spectrum. As if its brightness is not enough, the visual appearance of NGC 5128 is totally out of the ordinary: A bright, roundish patch of nebulosity perfectly bisected by a thick - almost black - dark lane. Even a casual look at its puzzling face is more than enough to pique the interest of even casual observers, let alone hard-core deep sky enthusiasts.
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Photo from the STScI Digitized Sky Survey
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Photo by the author, taken from Mosfiloti, Cyprus on March 22, 2014
The “pathologic specimen” appearance (as Harlow Shapley mentioned in his classic book “Galaxies” way back in 1947) has puzzled professional astronomers for decades. For many years Centaurus A was like a ping-pong ball, thrown from the field of galactic gaseous nebulae to the realm of external galaxies and vice versa. By the middle of the 20th century though NGC 5128’s nature as a galaxy was firmly established. And what a galaxy! With an estimated diameter of 80,000 light years, a radio energy emission of about 1000 times that of the Milky Way, and a population of more than a trillion suns, Centaurus A is a superlative galaxy in all the means of the word! The unique appearance of NGC 5128 earned it a place as the 153th entry in Halton Arp's "Atlas of Peculiar Galaxies". The latest research seems to indicate that the bisected appearance of Centaurus A is the “cannibalization” of a spiral galaxy (hence the dark dust lane) by a super – massive black hole in the center of a huge elliptical galaxy. A search through many scientific internet sites indicates that now there is more or less a consensus about the nature of this cosmic predator – prey interaction that is presented to our own eyes from the safe distance of about 16 million light years away…
My own “love affair” with NGC 5128 dates from more than ten years ago. During my early days as a dedicated visual deep-sky observer, way back in the spring of 2003, I happened to be at the seaside on the south shores of Cyprus, equipped only with my old trusty 10x50 binoculars and my Sky Atlas 2000. At the time I did not know about NGC 5128, though I had “encountered” it years ago as a cover of “Astronomy” magazine. A few nights earlier I had seen for the first time the grand Omega Centauri globular cluster with my 4” refractor, and I was trying to see if I could also locate it with binoculars. I more or less estimated the approximate position of Ω Centauri and raised my binoculars to my not-yet-dark-adapted eyes. To my surprise, instead of the roundish starry ball, I immediately noticed a bright haziness that seemed to be roundish, ill-defined – yet bright. Wow, what was that! I knew it was not Ω as I found that immediately afterwards due south of the bright patch. I scanned again, and there it was – nice, distinct, and almost as bright as M81 was in my binos! ? Soon, thanks to my star atlas, I had the answer as to what the bright patch was: I had “discovered” Centaurus A!
Ever since that spring night of 2003, Centaurus A has become of one of my favorite spring targets. I have seen it through a plethora of instruments, from 8x22 binoculars all the way to my 18” dob. I have even tried with my naked eyes, but as the galaxy culminates at just a bit more than twelve degrees above my southern horizon (I observe from 34 degrees 45 seconds north), I have yet to achieve that.
Whatever instrument is being used, NGC 5128 has many observing challenges to offer. For many observers, the first challenge is getting to somewhere where they can actually hope to see it, as the galaxy’s declination is -43 degrees. Observers in southern Europe and the southern one third of the United States have a distinct advantage here – needless to say, observers from the southern hemisphere are really privileged! Obviously, my own observations (and challenges) reflect my southern European location, but I would be very pleased to read observations from people living further south!
As I mentioned above, I have tried – unsuccessfully – to see this galaxy with the naked eyes. At magnitude 6.7, and taking into account its brightness profile, it should be possible to see it with the naked eye under transparent conditions, though I doubt that this is possible from the northern hemisphere.
With binoculars, the issue is not seeing it – it should be readily visible under good conditions. The main challenge should be the detection of the dark lane. I have not been able to confirm its presence, though with my 15x70 binoculars on a tripod I could detect a shadow play over the bright patch. I could not hold this shadow steady though, nor could pinpoint its position over the galaxy, so I can’t say I have seen the dark lane.
With small and medium sized scopes, the galaxy appears as two bright patches separated by a dark lane, the southern patch appearing brighter (the presence of two superimposed stars helps on that). The main challenge is to trace the extend of each bright hemisphere, and also to see whether the dark lane is straight, with the two edges parallel, or whether it is wider on one end compared to the other.
With larger telescopes, there is a wealth of detail to be seen, provided the atmospheric conditions are favorable. For us observing from the southern parts of the northern hemisphere, with the galaxy barely reaching ten degrees (or less) above the horizon, the amount of detail to be seen is strongly dependent on the sky transparency. At medium to high powers, the galaxy appears two bright semicircular patches separated by a distinctly dark grey band. The southern patch appears slightly larger and brighter than the northern patch. If conditions are good, it is evident that the dark band is not straight, but rather it is narrower on the western end. Observers from more southerly latitudes have mentioned that the edges of the dark lane are not straight but rather they are scalloped. I have noticed myself some irregularities on the edges of the dark lane, but I was unable to keep them steady. In addition, there seems to be some brighter material towards the western end of the dark lane, right where a 14th magnitude star is located. My notes from my April 12th 2012 observation indicate that I had noticed some very indistinct brightness within the narrower western end of the dark lane, but I was not certain if it was real or if it was due the presence of the faint star.
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Weather conditions did not allow me to have a decent observation of Centaurus A in 2013, but a few weeks ago I was able to observe it with my 4” refractor under quite good sky conditions. Using powers of 67x and 109x I was able to detect that the dark band appears narrower at its western edge, and that the southern half of the galaxy appeared distinctly brighter. It certainly reminded me of a mutant “Pac-man” with one of its halves bigger than the other!
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So, whatever instrument you have, if Centaurus A clears your horizon by a few degrees, during your next clear, transparent night don’t hesitate and…
“GIVE IT A GO AND LET US KNOW”
GOOD LUCK AND GREAT VIEWING!
