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Object of the Week May 13, 2018 – The Eye of Sauron NGC 4151
Object of the Week May 13, 2018 – The Eye of Sauron NGC 4151
"The Eye of Sauron the Terrible few could endure."
—from The Silmarillion
Sauron was the Dark Lord of Mordor in J. R. R. Tolkien's The Lord of the Rings. The Eye of Sauron, the symbol of a lidless eye, was adopted by the Dark Lord during the Second Age and the Third Age. It was said that few could endure the eye's terrible gaze. Sauron was able to exert his will over Middle-earth, so that the Eye of Sauron became a symbol of power and fear.
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NGC 4151, UGC 7166, MGC +7-25-44, PGC 38739
Constellation: Canes Venatici
Type: SABa
Orientation: face-on
RA: 12h 10m 33s
Dec: +39° 24’ 22”
Magnitude: 11.3B
Size: 6.5’ x 5.0’
Lurking roughly 4.5° WSW of the 4th magnitude star Chara (Beta CVn), near the border of Canes Venatici and Ursa Major, lies the spiral galaxy NGC 4151, one of the nearest and brightest Seyfert galaxies to us. It was discovered by William Herschel on March 17, 1787. His discovery notation was “vB, S, R, vsnbMBN, p of 2” (very bright, small, round, very suddenly much brighter middle bright nucleus, proceeding of 2).
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NGC 4151 is one of the two galaxies studied in the paper that introduced the concept of Seyfert Galaxies. In fact, it remains one of the most researched galaxies in the sky based on the number of reference papers listed for it in NED and SIMBAD. In spite of all this research at least one critical aspect of this galaxy has remained uncertain until recently – its distance from us. Different studies over the years have yielded a wide range of results between 13 million light-years to 95 million light-years. So how far away is NGC 4151? A study conducted at W.M. Keck Observatory, using a new technique called reverberation or echo mapping, and published in 2014 claims to provide a new precise distance of 62 million light-years. Though too complex to explain here the reverberation mapping method relies on measuring light time travel delays rather than a galaxy’s redshift that can be misleading for nearby galaxies (if it’s in a cluster, for example, it can be moving rapidly and mess up the redshift measurements).
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The composite image above shows the central region of the spiral galaxy NGC 4151, dubbed the The Eye of Sauron by astronomers for its similarity to the eye of the malevolent character in "The Lord of the Rings". In the "pupil" of the eye, X-rays (blue) from the Chandra X-ray Observatory are combined with optical data (yellow) showing positively charged hydrogen ("H II") from observations with the 1-meter Jacobus Kapteyn Telescope on La Palma. The red around the pupil shows neutral hydrogen detected by radio observations with the NSF's Very Large Array. This neutral hydrogen is part of a structure near the center of NGC 4151 that has been distorted by gravitational interactions with the rest of the galaxy, and includes material falling towards the center of the galaxy. The yellow blobs around the red ellipse are regions where star formation has recently occurred.
A recent study has shown that the X-ray emission was likely caused by an outburst powered by the supermassive black hole located in the white region in the center of the galaxy. Evidence for this idea comes from the elongation of the X-rays running from the top left to the bottom right and details of the X-ray spectrum. There are also signs of interactions between a central source and the surrounding gas, particularly the yellow arc of H II emission located above and to the left of the black hole.
Two different scenarios to explain the X-ray emission have been proposed. One possibility is that the central black hole was growing much more quickly about 25,000 years ago (in Earth's time frame) and the radiation from the material falling onto the black hole was so bright that it stripped electrons away from the atoms in the gas in its path. X-rays were then emitted when electrons recombined with these ionized atoms.
The second possibility also involved a substantial inflow of material into the black hole relatively recently. In this scenario the energy released by material flowing into the black hole in an accretion disk created a vigorous outflow of gas from the surface of the disk. This outflowing gas directly heated gas in its path to X-ray emitting temperatures. Unless the gas is confined somehow, it would expand away from the region in less than 100,000 years. In both of these scenarios, the relatively short amount of time since the last episode of high activity by the black hole may imply such outbursts occupy at least about 1% of the black hole's lifetime.
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Image courtesy Derek Santiago cn/nj
Now that you have seen the composite Eye of Sauron, what will you see visually in your telescope? With NGC 4151 being nearly face-on, its very bright nucleus will dominate its visual appearance in all telescopes, though it will remain stellar, even in large telescopes. My one recorded observation of the Eye of Sauron in a 17.5” telescope at 150x under good conditions and moderately dark skies reads: “surprisingly bright stellar nucleus; detected halo diffuse and elongated, slight brightening occasionally noticeable toward one edge of halo”. The slight brightening, I noted toward one edge of the halo was one of the ansae of the NGC 4151 that is readily seen in images. There is a lot going on in this galaxy – what do you see? Give it a go and let us know.
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Image courtesy Rick J cn
While in the vicinity of NGC 4151 don’t miss out on some of its nearby neighbors, in particular NGC 4156, NGC 4145, and UGC 7188. The barred spiral NGC 4156 (magnitude 13.2) is small at 1.4’ x 1.1’ however it has an interesting morphology which includes two splayed arms and a projection or possible vestigial arm jutting from the northern part of the disk. NGC 4145 (magnitude 11.3) has a bar imbedded in a large elliptical bulge and exhibits several arms studded with star-forming regions. Last but not least give UGC 7188 (magnitude 15.5) a try if you have the aperture. Images of UGC 7188 show it to be a peculiar galaxy indicating that it has interacted with another galaxy at one time or another.
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Image and annotation courtesy Rick J cn
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NGC 4145
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UGC 7188
Some pretty cool galaxy stuff going on here. Give it a go and let us know.
