HUBBLE SEES GALAXY ON EDGE
This is a unique NASA Hubble Space Telescope view of the disk galaxy NGC
5866 tilted nearly edge-on to our line-of-sight.
Hubble's sharp vision reveals a crisp dust lane dividing the galaxy into
two halves. The image highlights the galaxy's structure: a subtle,
reddish bulge surrounding a bright nucleus, a blue disk of stars running
parallel to the dust lane, and a transparent outer halo.
Some faint, wispy trails of dust can be seen meandering away from the
disk of the galaxy out into the bulge and inner halo of the galaxy. The
outer halo is dotted with numerous gravitationally bound clusters of
nearly a million stars each -- known as globular clusters. Background
galaxies that are millions to billions of light-years farther away than
NGC 5866 are also seen through the halo.
NGC 5866 is a disk galaxy of type "S0" (pronounced s-zero). Viewed face
on, it would look like a smooth, flat disk with little spiral structure.
It remains in the spiral category because of the flatness of the main
disk of stars as opposed to the more spherically rotund (or ellipsoidal)
class of galaxies called "ellipticals." Such S0 galaxies, with disks
like spirals and large bulges like ellipticals, are called 'lenticular'
The dust lane is slightly warped compared to the disk of starlight. This
warp indicates that NGC 5866 may have undergone a gravitational tidal
disturbance in the distant past, by a close encounter with another
galaxy. This is feasible because it is the largest member of a small
cluster known as the NGC 5866 group of galaxies. The starlight disk in
NGC 5866 extends well outside the dust absorption. This means that dust
and gas still in the galaxy and potentially available to form stars does
not stretch nearly as far out in the disk as it did when most of these
stars in the disk were formed.
The Hubble image shows that NGC 5866 shares another property with the
more gas-rich spiral galaxies. Numerous filaments that reach out
perpendicular to the disk punctuate the edges of the dust lane. These
are short-lived on an astronomical scale, since clouds of dust and gas
will lose energy to collisions among themselves and collapse to a thin,
For spiral galaxies, the incidence of these fingers of dust correlates
well with tracers of how many stars have been formed recently, as
massive stars' energy input moves gas and dust around to create these
structures. The thinness of dust lanes in S0s has been discussed in
ground-based galaxy atlases, but it took the resolution of Hubble to
show that they can have their own smaller fingers and chimneys of dust.
NGC 5866 lies in the Northern constellation Draco, at a distance of 44
million light-years (13.5 Megaparsecs). It has a diameter of roughly
60,000 light-years (18,400 parsecs) only two-thirds the diameter of the
Milky Way, although its mass is similar to our galaxy. This Hubble image
of NGC 5866 is a combination of blue, green and red observations taken
with the Advanced Camera for Surveys in November 2005.
Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
Acknowledgment: W. Keel (University of Alabama, Tuscaloosa)