Using the Spitzer Space Telescope in December 2004, the astronomers Beverly Smith and Mark Giroux of East Tennessee State University imaged interacting galaxies Arp 65 in Andromeda and Arp 107 in Leo Minor at two different wavelengths: 3.6 (blue) and 8 (red) microns.
At 3.6 microns, Spitzer sees the stars themselves, so the smooth spiral patterns of the galaxies' old stellar disks show clearly at this wavelength.
"You can see differences between the old and young stellar populations" - Beverly Smith. Younger stars haven't strayed far from the dust-rich molecular clouds in which they formed and appear as clumps at longer wavelengths.
Old stars of the interacting galaxies of Arp 65 in Andromeda trace the spiral patterns in the galaxies' old stellar disks (blue). At longer wavelengths, the galaxies take on a clumpier appearance (red), revealing hot dust around young stars. NASA / JPL / Caltech / M.L. Giroux
Emissions from polycyclic aromatic hydrocarbons (PAHs) dominate the 8-micron image. "We're seeing emission from large molecules to small grains - anywhere from a few hundred up to thousands of atoms. These interstellar molecules produce strong emission at these wavelengths when hot, young stars heat them to temperatures up to about 1,300 degrees Fahrenheit (730 C)." - Beverly Smith
The images carry clues to the star-formation process in these galaxies. "In Arp 65, we're seeing blobs of young stars in the tidal tails. We'd like to understand why that's happening" - Beverly Smith.
Young star clusters form a string of pearls along a ring-like structure in Arp 107, a pair of colliding galaxies in Leo Minor. This composite contains two Spitzer views. An image taken at a wavelength of 8 microns (red) provides a clear view of the young clusters, while a 3.6-micron image (blue) highlights older stars in the small companion to the northeast and the bridge connecting the pair. The field of view is 3 arcminutes; north is up and east is to the left.
For Arp 107, young star clusters form a Cheshire-cat-like grin.
"When you go to 8 microns, it becomes a smile. These are regions of intense star formation, and as we go clockwise around the ring, there's a progression of ages" - Beverly Smith.
The galaxy collision at Arp 107 was more than a glancing blow, but something less than a direct strike. "Something halfway between M51 and the Cartwheel galaxy." This interaction created a distorted spiral density wave. The strongest compression has moved counter clockwise along the ring, so older clusters lie on the left side of the "smile," younger clusters on the right.
Beverly Smith presented the images May 30 at the American Astronomical Society meeting in Minneapolis. The team has submitted a detailed analysis of the images to The Astronomical Journal.
