M87 und der Jet home


M87 and its jet

Visual observation of a monster


March 2009

M87 is the central galaxy of the Virgo galaxy cluster. Its magnitude is 8.6 and it belongs to the class of the elliptic cD giant galaxies, which over time cannibalize their entire neighborhood. M87 is at the position of the strong radio source Virgo A. In the centre of M87 a super massive black hole is suspected with more than 6 billion times the mass of our sun, which is surrounded by a disk of accreted material. A jet originates from this nucleus, which emits broadband polarized synchrotron radiation. The jet is collimated by strong magnetic fields of the accretion disk or the black hole itself. The image to the left (Hubble Space Telescope) shows the jet consisting of single knots.


The image to the left of M87 together with its jet was taken by Carsten Strübig using the 1.2 m Monet telescope (see here). 

Here is another image of the jet by Gernot Stenz (using a webcam!), which comes quite close to the visual impression with a large Dob.

Here finally an image by Gerald Willems, showing both the jet and the smaller galaxies in the immediate neighborhood (note that the image is mirror-reverse!)

Inspired by observing reports in particular of Stathis Kafalis  and of Dave Healy I tackled the challenge of a visual observation in March 2009. That night, the seeing was exceptionally good (at least as compared with the usual nights in the Black Forest) and I observed with my 22" Dob on an equatorial platform.

Quite unexpectedly, I was successful already at the first attempt. At lower magnification, M87 was as always a not particularly impressive elliptical blob. M87 as a cD giant galaxy is hardly structured and hence a rather boring object for visual observation, as there is not much to see.

After upping the magnification to 400x, the jet could be discerned with surprising ease as a small appendix to the nucleus of the galaxy. Its length was about 20 arc seconds, in agreement with the references in the literature. I had tried to get a feeling for the size of 20 arc seconds by observing Saturn ahead of M87. The jet is visible in the negative image to the left as the distinct dark structure in the core of M87. As expected, I could not discern structures, such as the knots shown above, in the jet. Using the small galaxies in M87's immediate vicinity, the observed position of the jet was found to be in agreement with the photographs. These small galaxies might be helpful to direct your way to the jet under less optimal conditions where the observation is borderline.

I guess that good seeing is the most important condition for a successful observation of the jet, as the structure is very small and of low contrast, such that it blends into the background of M87's central part under suboptimal seeing. On the other hand, you will not need exceptionally dark sky, precisely because of the bright background of M87. This was confirmed in later observations under less than optimal seeing, which were not successful.  In summary, an observation of the jet is really exciting, that's astrophysics at its best, live at the eyepiece!

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