NGC 4038/9, Antennae Galaxies

Object Description

NGC 4038/9
NGC 4038/9: Image of the galaxies in blue light; © François Schweizer [116]

The galaxy pair NGC 4038 and NGC 4039 is also known as the Ringtail Galaxy or Caldwell 60/61. When it was first discovered, astronomers were amazed at its unusual shape. At first it was not clear whether this was a single galaxy or two. However, computer simulations have now shown that these are two colliding galaxies.

In short exposure photographs, the brightest regions of the central areas resemble a reflected question mark or curly tail - an irregular arc about two arc minutes long (see Fig. 2). The curved northern rim of the arc (NGC 4038) is the brightest sub-area and consists of several bright and dark spots from star clusters and dark clouds. The southern extension was numbered NGC 4039 by Dreyer. It is fainter but has roughly the same appearance.

However, long-exposure images reveal more enigmatic structures, giving the pair of galaxies an even more spectacular shape. The two bright central regions merge into two elliptical clumps that appear glued together on their eastern side. From there, two long, curved filaments extend, which reach far out into space (cf. Fig. 1). The more northerly can be traced for about five minutes of arc, the more southerly for more than ten. These two filaments resemble the long feelers of a beetle, hence the second nickname Antennae.

NGC 4038/9
NGC 4038/9: Detail of the two core regions with the 4m AAT; © 1991 David Malin [156]

The two filaments emit strong radio radiation, which was discovered in 1957. The well-known astronomer Fritz Zwicky showed that the tidal forces of two colliding galaxies stimulate new star formation. Indeed, a nebulous region has been spotted at the end of NGC 4039's southern filament, which appears to be a young protogalaxy. Recently formed massive stars excite the surrounding gas from which they were formed to glow.

The remaining, relatively undisturbed regions of the two galaxies (Fig. 2 below) show only subtle color differences, suggesting that the two were galaxies before their collision of the type S0 and not spiral galaxies. S0 galaxies are disc-shaped, relatively gas-poor galaxies whose color is slightly yellower than that of the spiral galaxies. This could explain why star formation occurs mainly in one of the two galaxies - where the gas is.

In the central regions of the two colliding galaxies, young star clusters were found with the help of the (then still uncorrected) Hubble Space Telescope, which should not be older than 10 million years. It is not yet clear whether they are open or globular clusters, but their mean radius of about 60 light-years is comparable to that of typical globular clusters in the Milky Way. Most form dense groups of about a dozen in the large H II region in which they were formed. [4, 112, 116]

According to LEDA [134], the two galaxies are moving away from the center of the local group at a speed of about 1400 km/s, which at a Hubble constant of 75 km/s/Mpc is one distance of about 60 million light years. NGC 4038's spin axis is tilted at about 65° to our viewing direction, and NGC 4039's at about 71°.

«Catalogue of Principal Galaxies (PGC)», Paturel et al. 1989 [144]
NameRA [hms]Dec [dms]mTypeDim [']Btot [mag]HRV [km/s]PA [°]
NGC 403812 01 52.8-18 51 54SB M3.4 x 2.010.9162780
NGC 403912 01 53.8-18 53 06SB M3.3 x 1.711.01655130

How to find NGC 4038/9

West of the diamond of the constellation Corvus, about one degree below halfway from Gienah (γ Corvi , 2.6mag) to ζ Crateris (4.9mag), lies the colliding galaxy pair NGC 4038/9. Since the two galaxies are quite faint, it is best to use an eyepiece with medium magnification.

Chart NGC 4038/9
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

Visual Observation

NGC 4038/9
NGC 4038/9: Pencil drawing; 14" PWO-Dobson f/4.6, TV-Radian 8 mm, 200x, 0.3°, D: 6.3, air: medium-calm; Honegg 1460 m; © 24. 4. 2003, 00:30 Eduard von Bergen

350 mm aperture: Both cores appear very pronounced in the 14-incher. The northern galaxy NGC 4038 appears brighter and larger. The slightly fainter galactic core of NGC 4039 connects almost seamlessly to the south. In photos and with imagination on the sketch, the two galaxies are reminiscent of a fetus. The antennas on both galaxies are not visually accessible with amateur telescopes. [192]

14" PWO-Dobson, F:4.6 / TV-Radian 8mm, 200x, 0.3°
Eduard von Bergen

NGC 4038/39
NGC 4038/39: EAA, roughly represents visual impression; Slipstream 30" f/3.3 Dobson + MallinCam video camera; ca 3s; Hasliberg; © 14. 3. 2012 Eduard von Bergen

762 mm aperture: The visual impression can be roughly reproduced with a MallinCam video camera and an integration time of around three seconds. A tracking 30 inch f/3.3 SlipStream Dobsonian was used. The resulting image comes very close to visual vision with a 13 mm Tele Vue Ethos eyepiece.

— 14. 3. 2012, Eduard von Bergen

References

4«Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System» by Robert Burnham; Dover Publications, Inc.; Voume I: ISBN 0-486-23567-X; Volume II: ISBN 0-486-23568-8; Volume III: ISBN 0-486-23673-0
112Images: NGC 4038/39 by David Malin; S&T 12/94, p.42
116Young Star Clusters Found by HST in the 'Antennae'; S&T 6/95, p.12
134Lyon-Meudon Extragalactic Database (LEDA); leda.univ-lyon1.fr
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)
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
156Photographs from the Anglo-Australian Observatory; aao.gov.au/images.html
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
192Deep-Sky Guide; fernrohr.ch/1d_deep-sky-guide.html (2020-12-25)