The Magellanic spiral NGC 4656 at a distance of 7.2 Mpc is interacting with its
companion NGC 4657 to its northeast. ULX1 is on the tenuous tip of the tidal
tail and is coincident with a very faint object in the DSS image.
Pair [with NGC 4656]
Hubble Atlas, p. 40
Feb 18/19, 1963
103aO + GG13
The complex system of NGC 4656/4657 is
listed as two galaxies in the Dreyer NGC Catalog.
It may be a single galaxy in the process of (or
having just completed) a close encounter with
NGC 4631 (Sc on edge; panel 290), which is 31'
distant. NGC 4631 is obviously at the same
distance, shown by the near equality of the redshifts
and the nearly identical levels of resolution
into individual brightest stars. The redshifts are
v_o = 606 km/s for NGC 4631 and v_o = 624 km/s
for NGC 4656/4657. The galaxies resolve
well into brightest stars beginning at about
B = 19.
The largest HII region in NGC 4656/4657
is complex and has an angular diameter (halo) of
11". At an assumed distance of 5 Mpc (Sandage
and Tammann 1974a) the linear diameter of this
HII complex is about 270 psc. At this distance
the projected linear separation of the
NGC 4656/4631 pair is small, at 44 kpc. The 21-cm
content of the pair has been measured by
Weliachew, Sancisi, and Guelin (1978). A model
of the tidal interaction is discussed by Combes
The resolution into individual stars is so
pronounced in part of NGC 4656/4657 as to
make this galaxy a prime candidate for detailed
study of its stellar content.
NGC 4656 and NGC 4657
These form a single nebula, 9' long by about 1' wide, in p.a. 33^deg^. The
northern end corresponds to 4567; is very bright; sharply curved, and shows
several almost stellar condensations. The southern half, southwest of the
apparent nucleus, is quite faint, as though obliterated by absorbing matter.
Description in 4656 in Bull. 219 in error.