Astronomer's discovery poses challenge to galaxy
formation theories
A team led by an Indiana University astronomer has found a
sample of massive galaxies with properties that suggest that 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 that 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 that 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 findings 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.
###
Indiana University. Thanks to Indiana University for this
article.