Professor Esther L. Zirbel
-- Astronomy Actvities
General Research Synopsis
Cosmology is one of the
oldest sciences of humanity yet we are still working on fundamental principles
today. Only seventy years ago we discovered galaxies outside our own and the
expansion of the universe. Our horizons (our minds as well as our view of the
universe) have broadened in every sense. Only in the past ten years we were
able to see galaxies that we (currently) believe to be at the edge of the
universe. Ironically, we are still asking the same questions as astronomers of
the antiquity – questions about our own existence, the meaning of it all, and
how and why our universe formed, and what will become of it in the future (we
cannot yet distinguish between totally opposing theories of whether the
universe will expand forever or collapse into a single point in space and
time). Although we are still far from answering many of those ultimate
questions, we can, nevertheless, study how our universe – with all of its stars
and galaxies – has evolved since its formation – and that is what I am
researching.
Since the light emitted from
distant galaxies takes a relatively longer time to reach us than light from
nearby galaxies, this light must have been emitted a relatively longer time
ago, during an epoch when those galaxies were younger. Therefore, by observing
galaxies at various distances (or epochs), we can map out how they have
evolved. Pictures of galaxies (many of which can be found on magazine covers,
postcards, and in fashion stores) then tell us story of the evolution of the
universe. Apparently galaxies come a variety of shapes, colors, and sizes; and
they can be classified into two broad categories: galaxies that have flat disks
with spiral structures (called “spiral” galaxies) and galaxies that look like
footballs in space (“elliptical” galaxies). The shapes of galaxies (their
morphologies) seem to be correlated with their colors: elliptical galaxies tend
to be red while spiral galaxies tend to be bluer. Blue galaxies contain newly
formed massive stars, which radiate most of their light at blue and
ultra-violet wavelengths. Redder galaxies no longer have this population of
young, blue and massive stars. As galaxies evolve, their stars age too. Since
massive stars evolve quicker than the lower mass stars, the only stars left
over in “older” galaxies are the lower mass stars which have redder colors.
Therefore, since elliptical galaxies have redder colors, they are believed to
be older than spiral galaxies. In my research I study the shapes, colors and
stellar populations of galaxies as a we look further and further away at
younger and younger galaxies.
Galaxies do not seem to have
evolved at the same rate everywhere. Nearby dense clusters tend to be dominated
by red elliptical galaxies (galaxies in the field are blue spirals) while
clusters that lived about 5 billion years ago (assuming the universe is about
15 billion years old) had significantly more blue galaxies. These blue galaxies
could either be elliptical galaxies with a population of young blue stars, or
they could be spiral galaxies that have transformed into elliptical galaxies by
today. Recently pictures taken by the Hubble Space Telescope demonstrated that
those blue galaxies are spiral galaxies. Since this evolution is not seen among
galaxies in the field, the general evolution of galaxies must depend on the
environment (as also demonstrated in my Ph.D. thesis) – it is either faster
and/or different in dense regions. To study this further I am now inspecting
Hubble Space Telescope images of galaxies in “groups” that bridge the density
gap between dense clusters and field galaxies. I am analyzing galaxy shapes,
morphological peculiarities of galaxies, their colors, stellar populations and
velocities, and hope to identify those mechanisms that transform galaxies.
Ultimately this may shed light on how galaxies formed in the early universe.
Here
is a 100-level laboratory exercise that explains galaxy morphologies in more
detail. Also the reader gets to design his/her own model of how galaxies have
evolved... (you'll need the Adobe Reader to open this document)
Another line of my research
involves radio galaxies. These are believed to be powered by a central black
hole, which in turn gives rise to jets that can be observed with radio
telescopes. The groups I use to study the described evolution of galaxies were
selected to contain powerful radio sources (due to their high radio
luminosities these galaxies can be pinpointed even at large cosmological
distances) – thus I use the same data for two different projects. Although this
was originally a side topic I ended up devoting a significant fraction of time
analyzing the origin of the radio-activity and published several papers on that
topic. As far as the above project is concerned I proved that groups containing
radio galaxies are typical of “ordinary” groups and can therefore be used to
study the general evolution of galaxies.
