A false-color,
multi-wavelength image of the interacting galaxies M51A and M51B (The
Whirlpool system). Blue corresponds to
ultraviolet light from hot young stars, green to light from evolved
stars, and red from warm dust heated by all stars. A new study of
interacting galaxies analyzes their colors across this very broad
spectral range. Credit: NASA/L.Lanz. Low Resolution Image (jpg)
Collisions between galaxies are common.
Indeed, most galaxies have probably been involved in one or more
encounters during their lifetimes. One example is our own Milky Way,
which is bound by gravity to the
Andromeda galaxy, our neighbor, and towards which we are approaching at a
speed of about 50 kilometers per second, perhaps to meet in another
billion years or so. Galaxy-galaxy interactions are thought to
stimulate vigorous star formation because the encounters somehow induce
the interstellar gas to condense into stars. These stimulated
starbursts in turn light up the galaxies, especially at infrared
wavelengths, making some systems hundreds or even thousands of times
brighter than the Milky Way while they are active. Many of the massive
stars that are produced become supernovae whose explosive deaths enrich
the environment with carbon, oxygen, and all the other elements that are
essential for life. Interacting galaxies are important not only in
shedding light on how galaxies evolve, form stars, and seed the
interstellar medium, but because they can be very bright and seen across
cosmological distances.
The details of galaxy collisions are only approximately understood, in
part because most observed interactions involve galaxies of unequal
sizes, morphologies, and stages of the interaction. Since an
interaction takes billions of years to run its course, it is not
possible to watch an entire sequence of events. Scientists trying to
figure out the evolution of a collision can only observe many different
systems at different stages, and then try to correct for all the other
factors (like mass or shape) that might influence the analysis. New
space-based telescopes offer some help because they can collectively
observe at all wavelengths from the ultraviolet to far infrared
wavelengths. These wavelengths capture most of the global activity
present in galaxies from star formation: The UV detects the hottest and
youngest new stars, the far infrared senses dust warmed by otherwise
obscured stellar radiation, while the intermediate wavelengths sample a
range of other contributing phenomena.
CfA astronomers Lauranne Lanz, Andreas Zezas, Howard Smith, Matt Ashby,
Giovanni Fazio, Lars Hernquist, and Patrik Jonsson have used new
observations of thirty-one interacting galaxies in fourteen systems to
publish the first systematic analysis of the energy distribution of
interacting galaxies across this key, broad spectral range. The
galaxies are from a sample that includes all stages of interaction, from
early stages when disruption has only just began to near final stages
when the effects of the collision are prominent; the published set
contain every interacting galaxy in the sample for which the full
dataset was available.
The team measured - and then modeled - these objects at twenty-five
different wavelength bands in an effort to test how star
formation and related galaxy properties are influenced in an
interaction. They report that the radiative output of the dust, and its
temperature, increase as the interaction progresses, and provide
evidence that the star formation rate does increase as the interaction
progresses. But somewhat surprisingly, they find after taking into
account the different galaxy masses that the rate enhancements are not
as dramatic as had been expected. This perhaps reflects the limited
size of the current sample and/or the fact that because induced bursts
of star formation are confined to short time intervals, they just happen
to be under-represented in the current sample. The astronomers
conclude by outlining future analyses that incorporate the results of
simulations of interacting galaxies in order to fill in some of the
missing details.
