A team led by an Indiana University astronomer
has found a sample of massive galaxies with properties that suggest
they may have formed relatively recently. This would run counter to
the widely-held belief that massive, luminous galaxies (like our own
Milky Way Galaxy) began their formation and evolution shortly after
the Big Bang, some 13 billion years ago. Further research into the
nature of these objects could open new windows into the study of the
origin and early evolution of galaxies.
John Salzer, principal investigator for the study published today in
Astrophysical Journal Letters, said that the 15 galaxies in the sample
exhibit luminosities (a measure of their total light output) that
indicate that they are massive systems like the Milky Way and other
so-called “giant” galaxies. However, these particular galaxies are
unusual because they have chemical abundances that suggest very little
stellar evolution has taken place within them. Their relatively low
abundances of “heavy” elements (elements heavier than helium, called
“metals” by astronomers) imply the galaxies are cosmologically young
and may have formed recently.
The chemical abundances of the galaxies, combined with some simple
assumptions about how stellar evolution and chemical enrichment
progress in galaxies in general, suggest that they may only be 3 or 4
billion years old, and therefore formed 9 to 10 billion years after
the Big Bang. Most theories of galaxy formation predict that massive,
luminous systems like these should have formed much earlier.
If this overall interpretation proves correct, the galaxies may allow
astronomers to investigate phases of the galaxy formation and
evolution process that have been difficult to study because they
normally occur at such early times in the Universe, and therefore at
very large distances from us. “These objects may represent a unique
window on the process of galaxy formation, allowing us to study
relatively nearby systems that are undergoing a phase in their
evolution that is analogous to the types of events that, for most
galaxies, typically occurred much earlier in the history of the
Universe,” Salzer said.
The discoveries are the result of a multi-year survey of more than
2,400 star-forming galaxies called the Kitt Peak National Observatory
International Spectroscopic Survey (KISS). The survey was designed to
collect basic observational data for a large number of extragalactic
emission-line sources. Additional rounds of follow-up spectroscopy for
the sources discovered in the initial survey led to the discovery of
the 15 luminous, low-abundance systems.
“The reason we found these types of galaxies has to do with the unique
properties of the KISS survey method,” Salzer said. “Galaxies were
selected via their strong emission lines, which is the only way to
detect these specific galaxies.” Previous surveys done by others have
largely missed finding these unusual galaxies.
While the hypothesis that these galaxies are cosmologically young is
provocative, it is not the only possible explanation for these
enigmatic systems. An alternative explanation proposes that the
galaxies are the result of a recent merger between two smaller
galaxies. Such a model might explain these objects, since the two-fold
result of such a merger might be the reduction of metal abundances due
to dilution from unprocessed gas and a brief but large increase in
luminosity caused by rampant star formation. As a way to distinguish
between these two scenarios, Salzer and his team intend to request
observing time on NASA’s Hubble Space Telescope to use high-resolution
imaging to determine whether or not the systems might be products of
merging.
A National Science Foundation Presidential Faculty Award to Salzer, as
well as continued NSF support cumulatively totaling $1.2 million,
funded the KISS survey and supporting work.
Also contributing to the Astrophysical Journal Letters paper were
astronomers Anna Williams of Wesleyan University in Middletown, Conn.
and Caryl Gronwall of Pennsylvania State University. Salzer is at IU
while on leave from his position of professor of astronomy at
Wesleyan, but expects to formally join the faculty at IU in the coming
year. The authors also recognized KISS team members Gary Wegner, Drew
Phillips, Jessica Werk, Laura Chomiuk, Kerrie McKinstry, Robin
Ciardullo, Jeffrey Van Duyne and Vicki Sarajedini for their
participation in the follow-up spectroscopic observations over the
past several years.
To speak with Salzer, please contact Steve Chaplin, University
Communications, at 812-856-1896, or stjchap@indiana.edu.
