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  • Thomas Grohmann

Red clouds in a distant galaxy

Aktualisiert: 3. Okt. 2022

Encouraging reflection on our stay at Tivoli Southern Sky Guest Farm in Namibia in May 2022

My image of the galaxy NGC 5128, Centaurus A, showing some "red clouding" (see red marker 1)*, a blue jet (blue marker 2)*, a blue star forming arc (marker 3)* and of course the outer galactic shell structure, became the "Astrophoto of the Week, AdW" on 27.9.2022. Peter Riepe, a physicist, founder and head of the astrophotography section of the Vereinigung der Sternfreunde (VdS), has analysed the photo in great depth. I thank him very much for that. In particular, I had asked whether he had an explanation for the obvious red HII regions and whether they were foreground material of our galaxy or could be attributed to NGC 5128. His explanations below reveal the answer.

*The submitted image as a "pretty picture" can be seen above or below at the AdW. To clarify the structures and nebulae visible in the image, I have strengthened the background and added markings:

Click image for full screen view

From Peter Riepe's conclusion for eager readers: "Our image author has succeeded in providing photographic evidence of Graham's HII regions. To the best of my (P.R.) knowledge, Thomas must be one of the few amateurs who have consciously perceived these nebulae. Supplementary image 6 shows in direct comparison that the nebulae are identical in both Graham's and our image author's work."

And here are Peter Riepe's explanations to the Astrophoto of the Week including links to additional material:

Tuesday, 27 September 2022 - Astrophoto of the Week

Click image for full screen view

In the southern constellation of Centaurus is the galaxy NGC 5128, which is a "must" for many astrophotographers, especially on their first excursions to the southern hemisphere. In today's AdW, Thomas Grohmann shows us his image of NGC 5128. Thomas is new here in the circle of AdW astrophotographers, so a warm welcome! The series of images was taken at the Tivoli Astrofarm in Namibia (therefore a greeting to the Schreiber family!). The date of recording was 01.05.2022. The telescope used was a 12-inch reflector with 305 mm aperture and 1200 mm focal length (an ASA Newtonian astrograph). With the camera, a ZWO ASI2600MC Pro (colour), a total of 7 h 24 min was exposed at gain 0 and -10°C. Further technical details: Astro-Pphysics GTO1200 mount, guiding, optics with 3-inch Wynne corrector, image control with N.I.N.A., image processing with PixInsight, Photoshop and Topaz DeNoise. North is in the upper left corner of the image as seen from the galaxy centre and the field size is 73.4' x 48.9'.

The reader knows that the AdW is not only a presentation of astrophotos with the communication of exposure data and information on image processing. Because if a great image was created with some effort, then the astrophotographer should also take a closer look at what can be said about the image apart from the object name and constellation, which objects are included in the image and what special features are present. Therefore: There is much worth knowing about NGC 5128, including astronomical facts that have an astrophotographic impact. Our image author has already noticed this himself in the preparation of his submission. So dear people: Please patiently endure the rest of my text. …

NGC 5128 is the closest giant elliptical galaxy to us. Its distance is 11.9 million light years (Tully et al., 2013). This means it is about as far away from the Milky Way as M 81 in the Big Dipper. Short-exposure images show a diffuse, elliptical-round main galaxy with a massive, bent dust belt surrounding it in an east-west direction. Here in the AdW, this galaxy area shows up in the centre, surrounded by a huge halo. The halo measures 28.5' x 23' in the image with a handle-shaped continuation in the north and south areas (left and right).

In 1949, NGC 5128 was identified as the optical counterpart of the radio galaxy Centaurus A. The entire radio structure consists of two huge lobes in the north-south direction and a long extension to the east. The dimensions of this radio structure can be seen in the declination information in Supplementary Image 1 from 1960 (with source reference). Pioneers of such radio studies were Shain C.A.: The radio emission fron Centaurus A and Fornax A Australian J. Phys. 11, p. 517-529 (12/1958) and Piddington J.H., Trent G.H.: Cosmic Radio Sources Observed at 600 Mc/s; Australian J. Phys. 9, p. 74 (3/1956).

Furthermore, NGC 5128 is known as the Seyfert-2 galaxy (with active nucleus). Supplementary image 2 shows the core area in an X-ray image from the Chandra X-Ray Observatory. Here a violent dynamic is visible, in particular a jet directed to the northeast, and on the opposite side a bubble-shaped shockfront. This jet is most likely directly associated with a supermassive black hole. The closer jet environment with an approx. 18' extended radio plume is shown in Supplementary Figure 3 in a representation by ESO.

Now back to the AdW. If such violent processes take place in the centre, then this must also have an effect on the outer halo area of Centaurus A. Supplementary image 4 shows the AdW original in an edited version: In a clear shell structure of the halo sits centrally the brightest region of NGC 5128. Such a shell structure was already uncovered some 40 years ago by Malin D.F., Quinn P.J., Graham J.A.: Shell structure in NGC 5128; Astrophys. J. 272, L5-L7 (9/1983). Incidentally, Malin was also able to detect a shell structure in other giant elliptical galaxies. He used the "unsharp masking ", which is still known today and which he developed. In addition, there are still other treatments today to bring out the shell structure. But these treatments are, to my (P.R.) conviction, heavily exaggerated.

In Centaurus A, numerous star-forming regions have been discovered. Malin and his colleagues, for example, already pointed out the blue colouration of the fringes of the dust belt. And Appenzeller I. & Moellenhoff C. made spectra of the dust belt with clear confirmation of such young, blue stars (Astron. & Astrophys. 81, 54-58 (1/1980). Massive young blue stars were then directly detected optically with the Hubble Space Telescope. In addition, however, other blue jets and arcs have been found in the halo of Centaurus A, e.g. Peng E.W.. Ford H.C., Freeman K.C., White R.L.: A Young Blue Tidal Stream in NGC 5128; Astronom. J. 124, pp. 3144-3156 (12/2002). So such "hot material" is transported to the outside.

Neutral hydrogen HI has already been found at NGC 5128. First HI observations by van Gorkom J.H. et al. (Astronom. J. 99, p. 1781, 6/1990) show that HI is mainly distributed around the dust belt. Further HI measurements by Schiminovich (1994) clearly prove that HI also occurs in the area of the shells. Are there then also interactions of the above-mentioned jets with this neutral hydrogen? The answer is a resounding "yes". When hot jets push into an HI cloud, spontaneous star formation can take place, accompanied by HII regions that are ionised in this way. Such HII regions were then also found. The research work on this comes from Blanco V.M., Graham J.A., Lasker B.M. and Osmer P.S.: Optical condensations and filaments in the northeast radio lobe of NGC 5128; Astrophys. J. 198, L63-L64 (1975). In the northeast region, where the halo merges into the lobe and where neutral hydrogen is present, extensive diffuse and filamentary fragments were found by J.A. Graham.

This image shows the HII regions described in a version in which I have removed the stars for better visibility of the nebulae.

And now we get to the highlight of this AdW: Let's look at supplementary image 5. Colour-wise, it is exaggerated in saturation, but to prove Graham's HII regions, it could not be any other way. The yellow squiggles clearly contain the red glowing nebulae. Before sending in the AdW, Thomas Grohmann and astrophotographers of his home observatory had considered the objects, also made researches, but came to no result. Now the matter is clear: Our image author has succeeded in providing photographic evidence of Graham's HII regions. According to my (P.R.) knowledge, Thomas must be one of the few amateurs who have consciously perceived these nebulae. Supplementary image 6 shows in direct comparison that the nebulae are identical in both Graham's and our image author's photographs.

Postscript: NGC 5128, with all the details discussed here, is the result of a merger. A smaller disk galaxy with a spiral structure and a lot of gas and dust has collided with the progenitor of the larger elliptical galaxy. The dust band is the remnant of the smaller galaxy, as evidenced by the fact that dust and young stars in and around the dust band rotate at a very different speed than the elliptical galaxy. All observed phenomena (jets, X-ray source, secondary star formation) are consequences of this merger.

Notes: At f3.9, this image was exposed for a total of 7.4 hours. That is quite powerful, but we know that such faint nebulae must be exposed much longer to get a strong enough signal above the noise of the background. Well, Thomas was not at all aware of such a "catch" when he took the picture. His aim was to document the shell structure and extension of the halo. But maybe in the near future there will be the opportunity to add another "lump of coal" ... that is: to expose a little more. Because: In the last second before setting the AdW, I find a much deeper image of the red nebulae with 130 h exposure under this link.

Incidentally, the colouring is quite moderate in the original. But that is, as one likes to put it, a matter of personal taste. In any case, we would like to thank Thomas for this successful astrophoto of Centaurus A and are pleased that after Fabian Neyer's recent AdW with the external HII regions at M 51, Centaurus A can now also be mentioned as a second example. Thomas, our hearty congratulations on the astrophoto of the week!

Peter Riepe

Image author: Thomas Grohmann

Coordinates of NGC 5128 (J2000.0):

RA = 13 h 25 min 27.6 s, Dec = -43° 01' 09''


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