Our galaxy, the Milky Way, is large spiral galaxy. It is composed of
several distinct parts that formed at different times and thus with different
populations of stars. Here is a quick breakdown of those parts:
- The Bulge: The central spherical region of our galaxies, which
contains mostly old population II stars. In general, the stars have a
moderately low metallicity (-1 < [Fe/H] <0.5) compared to the Sun. It also is
thought to hold a massive black hole.
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- The Disk: The flat disk-shaped part of the Milky Way, which
contains the four spiral arms, and our Sun. It is dominated by population I
stars and is generally broken into two parts: the more metal-rich thin disk
and the more metal-poor thick disk.
- The Halo: The halo is a spherical distribution of old, population II stars that encompasses all the other parts of the galaxy. Recent work has
suggested that the halo may be made of two distinct parts:
- Inner Halo: The older more metal-rich part of the halo that has
little net rotation. It has a peak in it's metallicity around [Fe/H] = -1.6.
It is thought to have formed during the first few large mergers that formed
the Milky Way.
- Outer Halo: The younger and more metal-poor part of the halo that appears
to have a net retrograde motion. It has a metallicity that peaks around
[Fe/H] = -2.2. It is thought to have been formed by later accretions of dwarf
galaxies onto the Milky Way.
Inside of these main parts of the Milky Way there are smaller strucutres.
Here is a list of those:
- Dwarf Galaxies/Streams It has been known for a while now that
the Milky Way has a number of dwarf galaxy companions. These galaxies are
small and irregulaly shaped. Recent research has discovered that from time
to time the Milky Way eats it's companion galaxy by gravitationally pulling
them appart forming a trail of stars called a stream. Dwarf galaxies are by definition not part of the Milky Way, but when they become streams, they become part of the halo.
- Globular Clusters: These kinds of clusters are very dense
collections of 10,000 to a million stars. Like open clusters, the stars
share the same age and chemical composition. Unlike open clusters, globulars
tend to be old objects and thus show us a much more evolved state. These are
primarily found in the halo, although there are some the live in the bulge
and near the disk.
- Open Clusters: This is a loose collection of stars that were born
from the same dust cloud. The stars in these clusters have approximately the
same age and chemical composition. Due to their low density, most open
clusters break apart quickly, with few lasting to a billion years. In general,
these clusters are found in the disk of the galaxy.
My Projects:
My thesis project is specifically concerned with the nature of the halo.
In particular, trying to understand the difference, if any, between the inner
and outer halo. To do this, I'm using field RR Lyrae stars as tracers of the
kinematics of the halo. To learn more click here.
Another way to understand the halo is to look at the dwarf galaxies in the
Local Group, our local neighborhood. One such dwarf galaxies is know as the
Fornax Dwarf galaxy. I am looking at the RR Lyrae
population there to see if dwarf galaxies of that sort could have formed our
halo. A third project looks at a globular cluster in the bulge of our galaxiy
NGC 6304 is a relatively metal-rich globular cluster, which may contain a
very unusual poplulation of RR Lyrae.
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