Galaxies NGC 3718, NGC 3729, Hickson 56

Object Description

NGC 3718 + NGC 3729
NGC 3718 + NGC 3729: Galaxien NGC (links), NGC 3718 (rechts) und Hickson 56 (unterhalb von NGC 3718); 500 mm Cassegrain 3625 mm f/7.2; SBIG STL11K; 280+120+120+120 min LRGB; Bernese Highlands; © 2017 Radek Chromik

The data on the two galaxies are quite extensive and not particularly consistent in places.

For NGC 3718 the NGC says «round». The MCG (Morphological Catalog of Galaxies) has a 3x2.5 arc-minute, bright (class 3) inner part and an 8x2.5 arc-minute, faint (class 5) outer part. The brightness classification is: 1 = brightest, 6 = weakest. Presumably only the inner part of the NGC is described. On the blue POSS, this galaxy even has a size of 11x5 arc minutes. The angle of inclination of the axis of rotation of the galaxy to the line of sight is almost 68 degrees, whereby 90 degrees would mean an edge-on position. The radial speed of this galaxy is just under 1100 km / s. The position angle, i.e. the angle between north and elongation axis, is 15 degrees.

NGC 3729 also has a radial speed of 1100 km/s. Its angle of inclination is 51.3 degrees. The surface brightness is approx. 13 mag; the difference in brightness between the inner (class 1) and outer (class 4) part according to MCG is greater than with 3718. The position angle here is 15 degrees.

«Catalogue of Principal Galaxies (PGC)», Paturel et al. 1989 [144]
NameRA [hms]Dec [dms]mTypeDim [']Btot [mag]HRV [km/s]PA [°]
NGC 371811 32 35.6+53 03 59SB M8.2 x 3.511.699415
NGC 372911 33 49.8+53 07 36SB2.9 x 1.912.0102315

Hickson 56 / Arp 322

Hickson 56
Hickson 56: Crop from PanSTARRS DR1 color [147]

Paul Hickson wrote in his «Atlas of Compact Groups of Galaxies» [166]: «Group 56 consists of five galaxies, three of them are apparently in contact and interacting. Two of these three (b and d) are radio sources. Infrared emission is also detected from the interacting system». The mean redshift of the galaxy group is given as 0.0270, which suggests a distance of about 425 million light years.

«Catalogue of Principal Galaxies (PGC)», Paturel et al. 1989 [144]
NameRA [hms]Dec [dms]mTypeDim [']Btot [mag]HRV [km/s]PA [°]
HICK 56E11 32 32.7+52 56 21L M x 16.57884
HICK 56D11 32 35.3+52 56 50L M x 16.88346
HICK 56C11 32 36.7+52 56 51L M x 15.78110
UGC 6527, HICK 56B11 32 38.8+52 56 53C M1.1 x .314.88115
HICK 56A11 32 46.6+52 56 27S M1.1 x .216.08245

Finder Chart

This delightful pair of galaxies stands south of the Big Dipper's box in the constellation Ursa Maior. You align the telescope with the help of the Telrand and the map below and you should then find the two galaxies without any major problems. The inlet with the section from the DSS helps to identify the faint Hickson 56 group of galaxies.

Chart NGC 3718
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

Visual Observation

200 mm aperture: The galaxies NGC 3718 and NGC 3729 form a pretty pair that can be seen together at a 50 arc-minute field of view. Both galaxies are clearly elongated at 1: 4, whereby the long axes are almost exactly parallel to each other. In addition, the direction of elongation is hardly tilted against the north-south direction. At the southern end of each galaxy there are stars - with NGC 3718 there are two, with NGC 3729, which is a good bit smaller (almost a factor of 1/2), there is one. In photos or in the POSS, these stars are in front of the galaxies, ie. Depending on the telescope aperture, a bit more size and structure can be expected in both galaxies. — 1996, Frank H. Leiter

NGC 3718, NGC 3729, Hickson 56
NGC 3718, NGC 3729, Hickson 56: Pencil drawing; 14" PWO-Dobson f/4.6 + TV-Radian 8mm, 200x, 0.3°, D: 6.7, air: moderate; Langis 1500m; © 2. 4. 2005, 23:30 Eduard von Bergen

350 mm aperture: NGC 3718 is the larger of the two NGC objects and shows a bright core and fine outer regions. NGC 3729 has an almost star-shaped core and an elongated elliptical overall shape. Behind the abbreviation Hickson 56 there are actually five galaxies, which one is glad when they are perceived as a group. On closer inspection, the individual galaxies can be seen. — 14" PWO-Dobson, F:4.6 / TV-Radian 8mm, 200x, 0.3°, 2005, Eduard von Bergen

762 mm aperture: The galaxies NGC 3718 and NGC 3729, as well as the galaxy group Hikson 56, could not be more different. The largest of these galaxies is NGC 3718 and shows an elliptical shape and a dark indentation below the core. Galaxy NGC 3729 has an elongated core oriented toward a nearby star. The five visible galaxies of the Hickson 56 group are lined up as 1 2 1, with the «right» one only clearly visible with averted vision. The fifth galaxy lies as a knife edge at right angles to the line of four. — 30" f/3.3 Slipstream Dobsonian, Hasliberg Reuti, 3. 3. 2022, Eduard von Bergen

References

144Catalogue of Principal Galaxies (PGC); Paturel G., Fouque P., Bottinelli L., Gouguenheim L.; Astron. Astrophys. Suppl. Ser. 80, 299 (1989); cdsarc.unistra.fr/viz-bin/cat/VII/119 (2021-02-18)
147Aladin Lite; aladin.u-strasbg.fr/AladinLite (2020-12-23)
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
166Atlas of Compact Groups of Galaxies (Special Issue); Hickson, P.; Astrophysical Letters and Communications, Vol. 29, p. 1; Bibcode:1993ApL&C..29....1H