For centuries, biologists and paleontologists have classified
animal species based solely on their appearance and anatomical
differences. Astronomers do the same thing when they look at
photographs and classify galaxies into the elliptical, lenticular, spiral, and

Eight decades ago, the famous astronomer Edwin Hubble developed the
standard visual method of classifying galaxies. In his scheme, galaxies
were divided according to their appearance in blue-light photographs.
While useful, the Hubble method is qualitative because it involves a
researcher’s judgment call, particularly when parts of the galaxy’s image
are obscured by dust.

Modern biological research has undergone a revolution by going beyond
appearances to classify species based on their DNA similarities. Modern
astronomy appears poised for a similar revolution. Today at the 204th
meeting of the American Astronomical Society in Denver, Colorado,
astronomer Michael Pahre (Harvard-Smithsonian Center for Astrophysics)
and colleagues presented new ideas for classifying galaxies based on
infrared observations by NASA’s Spitzer Space Telescope.

"By looking at galaxies in infrared light, Spitzer can see structures hidden
by dust-which is like looking up at the Milky Way at night and seeing
straight through those dark bands," said Dr. Pahre, the lead researcher in
the study.

The fundamental differences among galaxies may not be only their
appearances but also their compositions: What fraction of a galaxy is stars
versus what fraction is clouds of gas and dust that float between the stars?
That dust and gas, called the interstellar medium (ISM), is the material out
of which new stars are born. That difference in galaxy composition leads
to differing outward appearances.

Elliptical and lenticular galaxies have lots of stars, but little dust or gas,
and hence no current star formation activity. Spiral galaxies, on the other
hand, have both stars and interstellar gas and dust, and they are
continuously forming stars. The starlight in the images released today
appears blue, while the interstellar dust emission appears red.

The unique capabilities of Spitzer’s Infrared Array Camera (IRAC)
"provide a direct way of separating the stars from the warm dust, thereby
dissecting a galaxy into its individual components and revealing its true
nature, like a frog in a biology lab" said Giovanni Fazio, a co-author on
the study.

The new infrared Spitzer pictures show two things. One, Spitzer can use
infrared light to peer through the dust that obscures the galaxy, thereby
revealing the entire galaxy. And two, the warm dust itself emits so
strongly that it can be used as a direct tracer of the ISM.

"When you look at blue light photographs and infrared pictures side-by-
side, there is little question that it is easier to classify a galaxy in the
infrared," said Dr. Pahre. "The warm dust emission provides higher
contrast for the spiral arms, for example, than does blue starlight."

One galaxy, NGC 5746, shows these two properties of the dust — both
obscuring and emitting — quite well. The nearly edge-on galaxy is
partially obscured in a blue photograph plate image — but reveals the
underlying starlight in a picture taken at one infrared wavelength, while
revealing a ring of warm dust emitting at another infrared wavelength.

The infrared pictures also revealed another big surprise: some of the
galaxies previously classified to be in the elliptical/lenticular class were
found to have warm dust faintly emitting from spiral arms. It’s akin to
analyzing DNA to find out that two seemingly unrelated species are
actually close cousins.

"Seeing spiral arms in lenticular galaxies was totally unexpected. It could
represent a missing link between lenticulars and spirals that gives us
insight into both their past and current star formation history," said Dr.
Pahre.

Pahre and collaborators also went a step beyond the infrared classification
scheme to propose three quantitative methods, based on the infrared
pictures, which are alternate ways to classify the galaxies. The most
fundamental of these three is the ratio of the light emitted by a galaxy in
starlight versus that emitted via warm dust. All the suggested methods
correlate well with Hubble’s optical classification scheme. With additional
observations, astronomers will test the new methods to determine which
provide the best accuracy and insight into galactic structure and evolution.

"Our ultimate goal is to replace the Hubble classification method with a
new Spitzer classification method. These data put us well on our way
toward realizing that ambitious goal," said Pahre.

This research will be published in a special issue of The Astrophysical
Journal Supplement in a paper co-authored by Michael Pahre, Matthew
Ashby, Giovanni Fazio and Steven Willner (Harvard-Smithsonian Center
for Astrophysics).

The NASA Jet Propulsion Laboratory (JPL) manages the Spitzer Space
Telescope mission for NASA’s Office of Space Science, Washington.
Science operations are conducted at the Spitzer Science Center at the
California Institute of Technology in Pasadena. JPL is a division of
Caltech.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin, evolution
and ultimate fate of the universe.

Note to editors: High-resolution images to accompany this release are
available online at:
http://www.cfa.harvard.edu/press/pr0419image.html