http://www.universetoday.com/94800/spit ... es-in-one/
Spitzer Spots Two Galaxies in One
The Sombrero galaxy has a split personalty, according to recent observations by NASA’s Spitzer Space Telescope. Infrared imaging has revealed a hazy elliptical halo of stars enveloping a dual-structured inner disk; before this, the Sombrero galaxy was thought to be only disk-shaped.
“The Sombrero is more complex than previously thought,” said Dimitri Gadotti of the European Southern Observatory in Chile and lead author of the report. “The only way to understand all we know about this galaxy is to think of it as two galaxies, one inside the other.”
Although it might seem that the Sombrero is the result of a collision between two separate galaxies, that’s actually not thought to be the case. Such an event would have destroyed the disk structure that’s seen today; instead, it’s thought that the Sombrero accumulated a lot of extra gas billions of years ago when the Universe was populated with large clouds of gas and dust. The extra gas fell into orbit around the galaxy, eventually spinning into a flattened disk and forming new stars.
This is one of the first galaxies to be seen with such a dual structure — even though M104 has been known about since the mid-1700s.
The way I understand this, the Sombrero could be understood as an elliptical galaxy encircled by a spiral "belt". The Sombrero Galaxy acquired this "belt" by having gas gently falling into orbit around it, eventually becoming compressed enough to form stars.
Indeed, the Sombrero Galaxy is every so slightly reminiscent of gas giant planet Saturn and its regular, complex system of equatorial rings
It is interesting to compare the Sombrero Galaxy with galaxies where a fully formed spiral is colliding with an elliptical galaxy. The two prime (moderately nearby) examples are Cen A, NGC 5128
, and Perseus A, NGC 1275
. In both cases, a spiral is clearly colliding with the elliptical, and it could be that Cen A and Perseus A will never acquire the "settled" look of M104. Interestingly, the warped disk of Cen A is revealed in this infrared ESO image.
Finally, let's look at this HST image of NGC 1316. Judging by appearances, NGC 1316 could be what a galaxy like Perseus A may look like in the future, when the shattered spiral is almost completely absorbed into the giant elliptical. Only tattered strands of dust remains of the hapless spiral galaxy.