Two undergraduates at San José State University have discovered two galaxies that are the densest known. Similar to ordinary globular star clusters but a hundred to a thousand times brighter, the new systems have properties intermediate in size and luminosity between galaxies and star clusters.
The first system discovered by the investigators, M59-UCD3, has a
width two hundred times smaller than our own Milky Way Galaxy and a
stellar density 10,000 times larger than that in the neighborhood of the
Sun. For an observer in the core of M59-UCD3, the night sky would be a
dazzling display, lit up by a million stars. The stellar density of the
second system, M85-HCC1, is higher still: about a million times that of
the Solar neighborhood. Both systems belong to the new class of galaxies
known as ultracompact dwarfs (UCDs).
The study, led by undergraduates Michael Sandoval and Richard Vo,
used imaging data from the Sloan Digital Sky Survey, the Subaru
Telescope, and Hubble Space Telescope, as well as spectroscopy from the
Goodman Spectrograph on the Southern Astrophysical Research Telescope
(SOAR), located on the Cerro Tololo Inter-American Observatory site. The
National Optical Astronomy Observatory (NOAO) is a SOAR partner. The
SOAR spectrum was used to show that M59-UCD3 is associated with a larger
host galaxy, M59, and to measure the age and elemental abundances of
the galaxy’s stars.
“Ultracompact stellar systems like these are easy
to find once you know what to look for. However, they were overlooked
for decades because no one imagined such objects existed: they were
hiding in plain sight”, said Richard Vo. “When we discovered one UCD
serendipitously, we realized there must be others, and we set out to
find them.”
The students were motivated by the idea that all it takes to initiate
a discovery is a good idea, archival data, and dedication. The last
element was critical, because the students worked on the project on
their own time. Aaron Romanowsky, the faculty mentor and coauthor on the
study, explained, “The combination of these elements and the use of
national facilities for follow up spectroscopy is a great way to engage
undergraduates in frontline astronomical research, especially for
teaching universities like San José State that lack large research
budgets and their own astronomical facilities.”
The nature and origins of UCDs are mysterious – are they the remnant
nuclei of tidally stripped dwarf galaxies, merged stellar
super-clusters, or genuine compact dwarf galaxies formed in the smallest
peaks of primordial dark matter fluctuations?
Michael Sandoval favors the tidally stripped hypothesis. “One of the
best clues is that some UCDs host overweight supermassive black holes.
This suggests that UCDs were originally much bigger galaxies with normal
supermassive black holes, whose fluffy outer parts were stripped away,
leaving their dense centers behind. This is plausible because the known
UCDs are found near massive galaxies that could have done the
stripping.”
An additional line of evidence is the high abundance of heavy
elements such as iron in UCDs. Because large galaxies are more efficient
factories to make these metals, a high metal content may indicate that
the galaxy used to be much larger.
To test this hypothesis, the team will investigate the motions of
stars in the center of M59-UCD3 to look for a supermassive black hole.
They are also on the hunt for more UCDs, to understand how commonly they
occur and how diverse they are.
Reference:
“Hiding in plain sight: record-breaking compact
stellar systems in the Sloan Digital Sky Survey,” Michael A. Sandoval,
Richard P. Vo, Aaron J. Romanowsky et al. 2015, Astrophysical Journal Letters, 808, L32. (Preprint: http://arxiv.org/abs/1506.08828)
NOAO is operated by Association of Universities for Research in
Astronomy Inc. (AURA) under a cooperative agreement with the National
Science Foundation.
Science Contact
Dr. Aaron Romanowsky
Department of Physics and Astronomy
San José State University
One Washington Square
San Jose, CA 95192 USA
408-924-5225
E-mail: aaron.romanowsky@sjsu.edu
