Object of the Week May 18, 2014 The Flattest Galaxy NGC 4762
NGC 4762, UGC 8016, MCG +02-33-033, Paper-Kite Galaxy
Edge-on Lenticular Galaxy
RA: 12h 52m 56s Dec: +11° 13′ 49″
Size: 8.7’ x 1.7
Mag: 10.0v
NGC 4754, UGC 8010, MCG +02-33-030
Lenticular Barred Galaxy
RA: 12h 52m 18s Dec: +11° 18′ 50″
Size: 4.6’ x 2.5
Mag: 10.3v
The galaxy pair NGC 4762 and NGC 4754 is easy to find for scopes of almost all sizes under reasonably dark skies, even though they are part of the Virgo galaxy cluster. Start at 3rd magnitude ε Virginis (Vindemiatrix), located on the eastern side of the Virgo cloud. From there, move about 2° west and slightly north. You’ve arrived – just look for an extremely thin, yet reasonably bright, edge-on galaxy – this is NGC 4762. In the same field about 11’ northwest you’ll find NGC 4754, a face-on galaxy. This pair is an example of two similar galaxies at much different orientations. Though these galaxies appear to be interacting they are considered non-interacting pairs due to their significant redshift difference. With a large scope (18” or greater) under dark skies I find this pair to be a stunningly beautiful view.
NGC4762Big.jpg
NGC 4754 is unusual as the outer oval is oriented about 45° counter-clockwise from the inner oval and bar. This would seem to indicate an interaction with another galaxy. NED classifies it as SB0^-(r): while the NGC project has it as SB0.
Our target object this week however is NGC 4762. NED classifies it as SB0^0(r)? edge-on, while the NGC project has it as SB?0. It is an exactly edge-on lenticular galaxy, that is perhaps barred or perhaps not barred. Allan Sandage (1961) noted that this is “the flattest form of any galaxy known”. Both ends of the disk have plumes or winglets extending off of them. To have these extensions the conventional wisdom is the galaxy must have interacted with some other galaxy at some point of time.
William Herschel discovered NGC 4762 with an 18.7-inch f/13 speculum telescope. Of his observation of H II-75 (4762) and H II-74 (4754) on March 15, 1784 he recorded, “Two, nearly parallel, the preceding pretty bright, nearly round [4754]. The following [4762] pretty bright, very much extended, 8 or 10’ in distance.” The NGC description is “Pretty bright, very much extended in position angle 31°, 3 bright stars south, following of two.”
Stephen James O’Meara tagged NGC 4762 with the name Paper-Kite in his book The Secret Deep (2011). The name came from a description of the galaxy by Admiral William Henry Smyth in The Bedford Catalog, From A Cycle of Celestial Objects (1844). “This is a fine object, trending south preceding and north following, nearly in the vertical, but from its superior brightness in the south, or upper end, it rises while gazing from the dumpy egg-shape to that of a paper kite: over it is an arch formed by three telescopic stars the symmetry of which is so peculiar as to add to that appearance.”
O’Meara proceeds to explain what makes NGC 4762 so special. “NGC 4762 belongs to the Virgo Cluster of galaxies and is receding from us at 984 km/sec. If we accept its distance of 65 million light-years, the galaxy spans 170,000 light- years across and shines with a luminosity of nearly 1.5 billion Suns. Most peculiar is that images fail to reveal any dust along the galaxy’s razor-sharp edge. This aspect led astronomers early on to classify it as an armless edge-on lenticular (S01) – one consisting of a very small, very bright nucleus in a narrow, high-surface-brightness, flat central disk (1.70 0.10 ) with faint, twisted (S-shaped), brush-like extensions. These details are surrounded by a quite asymmetric thick disk (lens) of gas.”
“Some astronomers suspected that the galaxy may hide a bar in its perfectly flat inner disk. Indeed, some sources classify NGC 4762 as the prototype SB0 galaxy of the flat-bar variety. But in a 1995 Astronomy Letters and Communications (vol. 31, pp. 165–167), Hervé Wozniak (Geneva Observatory, Switzerland, and Marseille Observatory, France) argues that such a system should have a “thick bar,” while NGC 4762 has only a thin central structure. He also notes that the galaxy’s very luminous bulge is too small for a galaxy classified as an “early type.” And why, Wozniak asks, is the galaxy’s outer lens “bluer” than the central “bar”? (Observations of lenses around stellar bars in early-type galaxies show that their colors are the same.) What’s more, in face-on galaxies, bars and lenses have almost the same major axis length, but NGC 4762’s bar is almost 50 percent the length of the lens!”
“To explain these anomalies, the author suggests that NGC 4762 has cannibalized a small satellite galaxy with 10 percent of the host galaxy’s mass; the companion, then, could have could settled inside the host’s disk – distributing its stars partly in the galaxy’s thin central disk and partly in the thick disk (which formed by thermal heating during the merging). NGC 4762 consuming a small companion would also explain the mysterious warping seen in the disk’s extensions. A merger would also explain the blue color of NGC 4762’s lens, as the interaction may have triggered star formation there. “NGC 4762 is thus not a galaxy with a flat edge-on bar,” Wozniak concludes.”
NGC_4762_SDSS_unlabeled.jpg
Most modern day observations of NGC 4762 with large aperture telescopes note the existence of faint plumes at each end of the galaxy. At the northerly end, the plume extends off to the northeast. At the southerly end, the plume extends off to the southeast. However in researching other amateurs’ observations on the Internet and in print, I found very few that mention these disk extensions.
My sparse notes from a March 2012 observation of NGC 4762 with Barbarella (a large telescope - Jimi’s 48”) read: “Edge-on, very nice galaxy, very bright core, bright straight arms which extend out and then flare or feather at both ends.”
So I challenge you to “Give it a go and let us know.” Share your observations of NGC 4762 and let us know what it takes to see the faint disk extensions of this amazingly flat galaxy.
“GIVE IT A GO AND LET US KNOW”
GOOD LUCK AND GREAT VIEWING!
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