NGC 6438

A Distorted Galaxy Pair, or Maybe a Triplet System, can be Found in the Modest Constellation Octans, Home of the South Pole Star.

The faint constellation Octans seen from Pampa El Leoncito (San Juan, Argentina). 

Photo by the author.

Octans, a constellation with 3 primary stars charted by Abbe Nicholas Louis de Lacaille in the 1750s encompasses the south pole star, Sigma Octantis, a faint 5.5 magnitude star you can glimpse with the naked eye from a dark sky site. Another object contained in this constellation is NGC 6438, discovered in 1835 by John Herschel. First, let`s try to answer the question formulated on the tittle above. Is this galactic pair the closest one to the south celestial pole? Certainly no, another interacting system (i.e. NGC 2573A & B) is situated even closer to the pole star and not too far from the southernmost NGC object, the galaxy NGC 2573 (polarissima australis). However, that pair, with a magnitude around 14, is a target out of reach for owners of an 8-inch telescope, even in 6.5 limiting magnitude skies. Thus, we could say that NGC 6438 is the southernmost interacting system to observe with a common amateur telescope (e.g. 6, 8 or 10-inch). More than that, we can consider NGC 6438 the southernmost interacting system with its components very close to each other in projection on the sky (the components of NGC 2573 are not so close, as you can see in Figure 2). Of course, you need to observe somewhere south of the Equator. Another requirement is that you will need at least a latitude that makes possible to see this object at a reasonable altitude above the horizon. Being a circumpolar object for most of the Southern Hemisphere, another advantage is that NGC 6438 is a target you can study at any time throughout the year, specially for those living south of latitude -20° where this galaxies never go too low in the sky.

Figure 1. The NGC 6438 system and its surrounding field. North is up.
Photo by STSCI DSS.

I observed this system around local midnight from a dark sky site in Uspallata Valley (central west Argentina), not far from the Aconcagua Provincial Park in Los Andes mountains). The seeing conditions that night were really good, I should say excellent, to try to see as many details as possible using an 8-inch telescope. The system under study, NGC 6438 / 6438A* (also ESO 10-1 / ESO 10-2), lies about 3° 50` from the south pole star Sigma Octantis. 


Morphology of the system

NGC 6438, described in Dreyer`s catalog as "pretty bright, very gradually brighter in the middle" was also described by Shapley & Paraskevopoulos (1939) as a strange pair, probably a physical double, one spheroidal and the other belonging to the Magellanic type. A second description is given well afterward by Sérsic (1966) in which he notes the weak extension toward the north-preceding of the SO (NGC 6438) galaxy and the remarkable appearance of the irregular object (NGC 6438A) showing a nucleus and a disk of slightly smaller dimensions than the main body of the SO galaxy” (read more in the paper NGC 6438: A Triple System? C. J. Donzelli1, and M. Espíndola 1996). These researchers state that their analysis suggests that NGC 6438 is an interacting triplet, one S0 galaxy and two disk galaxies undergoing a merger, rather than a double system (E+S). In its now old paper "Southern Peculiar Galaxies II - NGC 6438" Sèrsic (1966) states that there are no radio sources coinciding with these galaxies. The Revised Data for NGC 6438 says that one of the galaxies is S0 type while the other one is a type ring B. This system appears also in the "Catalogue of Southern Peculiar Galaxies and Associations" (H. Arp, B. Madore, and W. Roberton) as AM 1806-852, in the category 2b: E+S interaction. In this sub-category an E-like galaxy is interacting with another galaxy which is classifiable as a disk or spiral galaxy. The sub-category contains some very disrupted pairs shown in the middle of the section, and spirals with long, open arms at the end.

Figure 2. The interacting system NGC 6438 (upper panel) and the fainter 

components of the system NGC 2573A & B (lower panel).
Photo by STSCI DSS.

At low magnification (42x), the brightest star in the 1° field of view is the 6.5 magnitude star HD 159517, situated just on the border of the field when the galactic pair is centered on it (see Figure 1). A very subtle nebulosity is visible through this kind of telescope at low power. The fact that the system lies between some faint stars makes me think that we are observing both the system and the faint nearby stars so higher magnification is necessary to try to discern the galaxies from the surrounding field. From the beginning, I recommend covering your head with a black blanket to avoid any unwanted surrounding light, however weak it may be. Also, try to use your averted vision as help as this object is definitively faint for a telescope of this size. Observing carefully, some of those stars could be identified. Applying averted vision a small hazy patch could be glimpsed, without a doubt the brighter part of the system that, according to Svend Laustsen, Claus Madsen, and Richard M. West in their book "Exploring the Southern Sky: A Pictorial Atlas from the European Southern Observatory (ESO), lies at 110 million light-years from us.

At 78x the view improves a little, the group of stars and the galaxy itself can be identified, but it is not an easy view. The shape of the very faint and smooth nebulosity looks, for moments, irregular (maybe a suggestion of the fainter companion NGC 6438A, a type Ring B galaxy with a visual magnitude of 11.6 and a surface brightness 12.5 (mag per square arcmin) according to the Wolfgang Steinicke's Revised NGC and IC Catalog web page).  

106x is a good magnification. Observing carefully and always with averted vision as help NGC 6438 appears, for moments, round and easier to see (or less hard to detect) than its elongated companion, NGC 6438A, that seems to be visible because a very faint, elongated East-West hazy structure reaches the star GSC-9527-1716, a faint 12.8 magnitude star (see upper panel on Figure 2). The whole systems look rather smooth in brightness, situated close to the chain of stars labeled with blue circles in Figure 2. All those stars could be glimpsed as very faint points of lights that night. The stars of the tight pair at one of the ends have magnitudes of 12.9 and 14. The star at the opposite end has a visual magnitude of 14.7 and it could be barely glimpsed through the 8-inch telescope, indicative of a dark, clean, and steady night for deep-sky observations.

196x was, maybe, the magnification that made it possible to see the galaxy pair more clearly, being always a challenging picture for an 8-inch. Taking advantage of the excellent seeing and very good transparency that night NGC 6438 was seen, at this power, round and with a slightly brighter core. An elongated hazy structure can be glimpsed roughly East-West as was aforementioned. Remember, an appropriate dark adaptation and the use of averted vision is a must to see this obscure target. A little worse view was obtained at even higher magnification (266x), with NGC 6438 barely glimpsed in the field of view, and with NGC 6438A not clearly identified.

A close encounter seems to take place in the Universe, not far from the southernmost point where you can aim a telescope from Earth, the south celestial pole.

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* NGC 6438A is not an NGC object but it is sometimes called in that way. This is the galaxy PGC 61793.