Object of the Week April 17, 2016 - NGC 2685 The Most Unusual Galaxy in the Shapley-Ames Catalog
NGC 2685, Arp 336, MCG 10-13-39, PGC 25065, UGC 4666, Helix Galaxy, Pancake Galaxy
Type: Spiral
Class: (R)SB0+ pec
Constellation: Ursa Major
RA: 08h 55m 34.8
Dec: +58’ 44”
Size: 4.1’ x 2.0'
Magnitude: 12.2
In 1996 Halton Arp published his extraordinary Atlas of Peculiar Galaxies, a compilation of the most unusual shaped galaxies photographed with the largest telescopes of that period. What made these galaxies peculiar was they did not fit comfortably into the early morphology classifications of Edwin Hubble and Allan Sandage where “the vast majority of galaxies were beautiful and textbook symmetrical.” This wasn’t the case with the Arp galaxies. Labeling these galaxies peculiar was considered by many to be an understatement. Astronomers and astrophysicists gave the Arp galaxies more illuminating descriptions: galactic malformations, galactic bedlam, bizarre, freaks, and train wrecks. Though further peculiar galaxies were increasingly identified and cataloged over the next thirty years, their total population remained small and statistically insignificant (<2%). It wasn’t until 1995 and 1996 when the Hubble Deep Fields North and South brought to light thousands of previously unknown peculiar galaxies was there the understanding there had to be tens of billions of these galaxies dispersed across the sky. Today we know that peculiar galaxies, caused by galactic interactions, are more the rule than the exception.
Of the 338 peculiar galaxies cataloged by Arp he classified the last six as miscellaneous in the final category of the Atlas, assigning them numbers 333 through 338. In the The Arp Atlas of Peculiar Galaxies, a Chronicle and Observer’s Guide authored by Jeff Kanipe and Dennis Webb these six objects bear the label “unique.” It is not that these six are “more unique” than the other 332 galaxies, but for one reason or another they really stand out. This week’s OOTW is one of these six - Arp 336 – a galaxy first cataloged as NGC 2685.
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DSS Image annotation
©Copyright 1996 through 2008
By Robert E. Erdmann, Jr.
NGC 2685 was discovered by Wilhelm Tempel with an 11” refractor on August 18, 1882. Located in the constellation Ursa Major it is 50,000 light-years across and located some 42 million light-years away. It is a relatively bright galaxy at magnitude of 12.2. At first glance, particularly at lower powers, it appears to be your average lenticular shaped galaxy, but it is not at all. Bearing a classification of (R)SB0+ pec, NGC 2685 is a confirmed rare polar-ring galaxy.
A polar-ring galaxy is a type of galaxy in which an outer ring of gas, stars and dust orbits in a plane almost perpendicular to the normal flat plane of the galaxy. These polar rings are thought to form when two galaxies gravitationally interact with each other. One possibility is that material is tidally striped from a passing galaxy, with the captured debris strung out in a rotating ring. The other possibility is that a smaller galaxy collides perpendicular with the plan of rotation of the larger galaxy, with the smaller galaxy effectively forming the polar ring.
In the case of NGC 2685, several thin filamentary strands, consisting of knots of luminous star-forming regions and hydrogen gas form a helical band perpendicular to the main disk and centered on the galactic nucleus. These structures suggest that NGC 2685 once had a much smaller companion that was captured into a polar orbit and eventually merged with those of the larger system, leaving behind the companion’s interstellar medium. New generations of stars formed from this material to produce the luminous ring seen today. The rotating ring of NGC 2685 is found to be remarkably old and stable.
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A more complex description of this galaxy is given by Sandage in The Hubble Atlas of Galaxies. “NGC 2685 is perhaps the most unusual galaxy in the Shapley-Ames catalogue. There are two axes of symmetry for the projected image; most galaxies have only one. The central, amorphous spindle resembles a normal S0 seen on edge. However, helical filaments surround the spindle. Because of projection effects, it is impossible to tell whether these filaments form complete circles around the spindle or whether they start somewhere on the spindle and spiral outward at right angles to its axis. The filaments are seen in absorption when they pass in front of the bright background, but they are luminous when they arc not silhouetted. Note how the entire northeast end of the spindle is covered with the projected absorption lanes of the helix. A luminous external ring around the entire structure may be either a true ring or a complete shell seen in projection. Many questions are unanswered about this galaxy. Is the central feature a spheroid (with two axes equal as in a plate or pancake), or is it an ellipsoid like a cigar? What is the direction of the angular momentum vector? Is the external ring attached to the central regions, or is it separate?”
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Visual observations of NGC 2685 seem to be as rare as this polar galaxy. Bob Hill from the Amarillo Astronomy Club provides an excellent one. “There is no Arp classification for the last six objects in the atlas, but this one looks as if it would fit into the "interacting double galaxies" class. By the redshift published for this system, 997 kps corrected for the CMB, this is the nearest and brightest polar ring galaxy. That redshift implies that this system is at 2/3 the distance of the Virgo cluster, around 35-40 MLY. I looked at the MAST web site and found Hubble images taken in 1998 and 1999 of 2685, and it is not resolved into stars. This is a case wherein the redshift indicates that the galaxy is much nearer than it actually is. In the 20" at 423x 2685 is a spindle shaped object 2'x.5' aligned NE-SW. It has a bright core region that is .5'x.3' in size. There is a bright field star located 2.5' N of the core. On the side of the galaxy towards that field star there were occasional dark contrast features that would cross the plane of 2685. There is a very faint half oval brightening of the field about 1' in length at right angles to the system on the NW side of the galaxy.”
Now it is your turn to get out and observe a most peculiar and unusual galaxy – NGC 2685.
"Give it a go and let us know!”
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