NGC 7320C, NGC 7319, NGC 7318A, NGC 7318B, and NGC 7317 all have similar velocities, and so are believed to be physically associated. NGC 7320 has a much lower velocity, and so is generally thought to be a foreground galaxy, not part of the group.
Stephan's Quintet is a compact group of
five galaxies that are strongly gravitationally interacting.
This group
is shown below, in an optical image from the
Digitized Sky Survey.
NGC 7320C, NGC 7319, NGC 7318A, NGC 7318B,
and NGC 7317 all have similar velocities, and so
are believed to be physically associated. NGC 7320
has a much lower velocity, and so is generally thought
to be a foreground galaxy, not part of the group.
To the north of NGC 7318B is a concentration of atomic hydrogen gas with on-going star formation (identified as Star Formation Region A in this figure).
Using the
12 meter telescope
on Kitt Peak in Arizona,
with Curt Struck (Iowa State),
I have searched for the 2.6mm line of carbon monoxide
at this position.
Note that
two CO lines are detected at this position,
at 6000 km/s and 6700 km/s. Using
the standard Galactic CO to H2
conversion factor, these lines
correspond to molecular gas
masses of 109 solar masses
(from
Smith and
Struck 2001,
Astronomical Journal,
Volume 121, p. 710).
To see another example of extra-disk molecular gas, see my web page on NGC 2782.
To return to my home page, click here.
To see Jane Charlton's Hubble Space Telescope image of Stephan's Quintet, click here.