A Hubble image of
the central region of the galaxy M60, showing at its outskirts the
small dwarf galaxy M60-UDC1 (inside the red circle). This dwarf is found
to be the galaxy with the highest known density of stars (a more
typical dwarf galaxy is shown inside the dotted red circle. The diagonal
white streak is an artifact). (Credit: ). Credit: NASA HST; J. Strader. Low Resolution Image (jpg)
Our nearest neighbor star, Proxima
Centauri, is about 4.2 light-years away. This distance is typical (on
average) between stars (or gravitationally bound stellar systems) in the
Milky Way and similar galaxies. In terms of density, there are about
0.02 stars per cubic light-year in the general neighborhood of the Sun.
At the other extreme of stellar density, a globular cluster - a
spherical collection of as many as a million stars orbiting a galaxy -
can have an average of over ten stars per cubic light-year, and even
higher densities near its center. Astronomers are trying to understand
what limits the density of stellar neighborhoods, if and how the average
densities differ in different sized galaxies, and the relationship of
stellar densities to the sizes of the supermassive black holes at galaxy
centers.
A dwarf galaxy is small in both size and mass, with 100-1000 times fewer
stars than the Milky Way. Astronomers interested in stellar densities
are naturally curious about the densities in such small systems. CfA
astronomers Pepi Fabbiano and Nelson Caldwell and their colleagues
spotted a strange dwarf galaxy on the outskirts of a much larger galaxy,
Messier 60, about sixty million light-years away (that is, in our
cosmic vicinity). This dwarf is only about 80 light-years in radius
(compared with our Milky Way's radius of tens of thousands of
light-years), but the scientists determined that it has a stellar mass
of density of over one hundred stars per cubic light-year, making it the
densest known galaxy.
The astronomers used the Chandra X-ray Observatory to reveal an X-ray
source at the dwarf galaxy's center. The source could be either a
nuclear supermassive black hole or perhaps a pair of binary stars
emitting X-rays - additional observations are needed to resolve the
issue. The authors point out that if the central object is a nuclear
black hole, it is likely that this dwarf galaxy was once a much larger
galaxy that collided with another galaxy. It was stripped of most of
its matter in the interaction but its nucleus, left relatively
unaffected, remained behind to consolidate the remnants into the dense
system seen today. While it is always fascinating to discover new
extrema in the cosmic zoo, these results also shed light on processes
that affect galaxy evolution and the properties of dense stellar
systems.
"The Densest Galaxy," Jay Strader, Anil C. Seth, Duncan A. Forbes,
Giuseppina Fabbiano, Aaron J. Romanowsky, Jean P. Brodie, Charlie
Conroy, Nelson Caldwell, Vincenzo Pota, Christopher Usher, and Jacob A.
Arnold, ApJLett 775, L6, 2013
