A Tidally Disturbed Galaxy, a Galaxy with a Double Nuclei, and Other Distant Members are a Group of Enigmatic Galaxies in the Southern Skies to be Explored by Amateur Observers
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Figure 1.Pointing toward the zone where the galaxies are situated, in constellation
Vela
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In the eastern part of constellation Vela (the Sail) a group of faint galaxies (at least for an 8-inch telescope) can be found. March is a good month to observe this constellation because it is at a very good altitude as soon as the sky gets dark, so you can observe objects there for several hours during the last nights of the summer in the Southern Hemisphere. To find the area where this galaxies lie is easy because you can use one of the bright stars of Vela, the 2.7 visual magnitude m (mu) velorum, a class G (G5) yellow-white giant. From there we can move to the stars HD 92139 and HD 91504 (visual magnitudes 3.8 and 5 respectively) to finally find the faint pair of stars of magnitudes 5.7 and 5.9 that are visible from any site that offers dark skies. Once there, the field of interest (centered at R.A. 10h 29m 25s Dec. -44° 20`32") is roughly 52 arc minutes northwest of the mentioned pair. The galaxies are distributed roughly South-North (see Figure 3 below). Some researchers state that two groups of galaxies lie there, NGC 3256 and 3263, but are difficult to
distinguish from each other spatially. However, many researchers consider at least 15 galaxies to be members of a single large
group spread over roughly a few degrees that is called the "NGC 3256 group of galaxies". According to a group of researchers led by Jayanne English, "The most spectacular HI feature in the NGC 3256 Group is
a galaxy-sized intergalactic HI cloud (English 1994), which we
will refer to as “the Vela Cloud.” The structure, as projected on
the sky, is not clearly associated with an individual galaxy but
appears to be part of the group" (see the paper "The Vela Cloud: A Giant HI Anomaly in the NGC 3256 Group" (2010).
At low magnification (42x) the field does not show any bright and conspicuous galaxy. Focusing on the southern edge of the field of view (see Figure 3) a very faint hazy patch, somewhat elongated, can be glimpsed. Four stars in a row or a chain seems to lie there, among the galaxy. One of them seems to be the core of the galaxy (see Figure 2). Averted vision is necessary for a better detection. That is the barred galaxy NGC 3261, discovered in 1836 by John Herschel. An historical identification by Dreyer is "faint, small, round, among stars". SN 2008fw, a supernova of type Ia, was discovered in this galaxy in 2008.
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| Figure 2. NGC 3261 Credit: The Carnegie-Irvine Galaxy Survey (CGS) |
Moving farther north, another 18 arc minutes, we find a tidally disturbed galaxy, NGC 3263 (m_v: 11.1mag , SB: 11.8mag per square arcmin), included in category 7b "Jets from Spirals" in the Catalogue of Southern Peculiar Galaxies and Associations by Arp & Madore. At this magnification the galaxy is extremely difficult to detect visually. A very faint hazy structure seems to be there when viewing with averted vision. A close companion, NGC 3262, was not visible.
Reaching the northern edge of the field of view we find, at least on nice deep astrophotos, a couple of galaxies (i.e. NGC 3256 and 3256C), see Figure 3. The last one was not detected at this magnification. Although it is a faint galaxy, NGC 3256 (also AM 1025-433) is the "easiest" member of the group in the field appearing round and rather smooth in brightness when using averted vision. A. J. Mulia et al. in their paper "Star Cluster Formation and Destruction in the Merging Galaxy NGC 3256" (2016) state that NGC 3256 is a merging pair of galaxies ≈ 36 Mpc away and the most luminous LIRG1 in the local universe. The main body of NGC 3256 contains a dense population of clusters, many of which are embedded in the galaxy’s dusty interstellar medium. This galaxy shows two long tidal tails in optical photographs
(e.g., Sandage & Bedke 1994).
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Figure 3. DSS image of the galaxies in the field
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At 78x, NGC 3261 is visible faint but with direct vision. It is situated among three stars (see Figure 2 above). Using averted vision, NGC 3261 looks rather round in shape. A small structure, slightly brighter, seems to lie in its interior.
NGC 3263 is a faint member detected using averted vision that makes possible to see it rather elongated with its central zone somewhat brighter.
NGC 3256C, a SBcd type galaxy, was not visible at this magnification. Its magnitude is 12.6, SB: 13.2 (mag per square arcmin).
The galaxy NGC 3256 is visible with direct vision at this magnification. Averted vision of course helps for a better detection. It looks round with a brighter core.
An observation at 106x shows NGC 3261 clearly distinguished from the surrounding stars. It is round and wider than NGC 3256. Toward its center the brightness increases and for moments a star-like structure shows up when observing carefully.
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| Figure 4. NGC 3263 Credit: The Carnegie-Irvine Galaxy Survey (CGS) |
NGC 3256C is not visible while the nearby companion, NGC 3256, is easy to see at 106x, round and with a bright core.
A new analysis of the galaxies using even higher magnification was carried out. At 166x NGC 3261, which looks faint and round, shows a small "granulated" structure when using averted vision. Two stars, very faint and elusive, are visible there for moments.
NGC 3263 is still a faint and elusive galaxy, better viewed using averted vision. At 166x its elongated shape was not clearly discerned.
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| Figure 5. NGC 3256 Credit: The Carnegie-Irvine Galaxy Survey (CGS) |
Is NGC 3262 visible through an 8-inch scope?
You know that a faint galaxy, a challenging object for a given type of telescope, can be hard to see or glimpsed. The degree of difficulty will depend on how dark, steady, and clean is the sky when you are observing. According to an optimum detection method, an observer under a 6.2 limiting magnitude sky should detect this small galaxy, a companion of NGC 3263 (see Figure 4), using around 100x. Even if it was not a clear image at all, a very small hazy patch, round, seems to be barely seen for moments using averted vision. I was very careful identifying the nearby stars surrounding that target to realized if the gosthly image I glimpsed is the small lenticular companion of NGC 3263. It was also barely visible at higher magnification (266x). Observers with 8-inch telescopes are encouraged to observe this galaxy and get their own conclusion about its visibility. Even though NGC 3256B has the same magnitude than NGC 3262, it was not detected through an 8-inch telescope that night. Maybe the reason is that NGC 3256B is a little bigger in angular dimension (1.90 x 0.5 arcmin). NGC 3256C, with its magnitude of 12.6 and surface brightness 13.2 (mag per square arcmin) should be visible in an 8-inch scope, but for some reason it was not visible during the entire observing session.
NGC 3256 is clearly visible showing its bright core that for a moment looked offset (maybe suggestion of the more expanded structure of the galaxy visible upper left on Figure 5?). NGC 3256 is included in Category 15 "Galaxies with Tails Loops of Material or Debris" in the Arp-Madore Catalogue of Southern Peculiar Galaxies & Associations. This galaxy also has a peculiar feature, a double (northern and southern) nuclei. Presence of an AGN2 at the optically obscured nucleus
has been suspected for a long time. AGNs are often found among LIRGs.
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| Figure 6 |
NGC 3263 looks, with averted vision, elongated. Brighter areas in the galaxy briefly jump to the view.
The central zone of the merging galaxy NGC 3256 looks rather smooth in brightness. Fainter structure of the galaxy is visible surrounding its core.
In the silence of the vast Universe, these galaxies await being observed by amateur astronomers from sites far away from big urban centers.
In the silence of the vast Universe, these galaxies await being observed by amateur astronomers from sites far away from big urban centers.
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1_ LIRG (Luminous Infrarred Galaxies)are galaxies with luminosities above 1011 L☉. These galaxies emit more energy in the infrared portion of the spectrum, not visible to the naked eye. LIRGs are more abundant than starburst galaxies. Infrared galaxies appear to be single, gas-rich spirals whose infrared luminosity is created largely by the formation of stars within them. However, some galaxies' luminosity comes from an active galactic nucleus or AGN.
2_ AGN (Active Galactic Nuclei) are galaxies where the nucleus (or central core) produces more radiation than the entire rest of the galaxy. Current theory suggests that there is a supermassive black hole (millions of times the mass of the sun) at the center of AGN.