Sten Hasselquist
Research Title:
Searching for Chemical Substructure in the Milky Way
Research Description:
Understanding the formation and evolution of our own Milky Way Galaxy (MW) is paramount if
we are to understand galaxy formation and evolution in general. Unlike in other galaxies we can
actually use large-scale surveys to study the detailed chemical composition of the individual
stars that make up our Galaxy. The second iteration of the Apache Point Observatory Galactic
Evolution Experiment (APOGEE-2), one such survey, consists of a near-infrared, high resolution
spectrograph on board the Sloan 2.5m telescope. The goal of APOGEE 2 is to pick up where
APOGEE left off and observe hundreds of thousands of red giant stars across the MW. Detailed
chemical abundances for over 15 elements are provided for each star. A twin instrument is
currently being constructed and will operate on board the 2.5m Du Pont telescope at Las
Campanas Observatory in Chile starting January 2017.
I propose to work with Professor Jon Holtzman from January 2017 to January 2018 to analyze
the data taken by APOGEE and APOGEE 2 to search for chemical substructure in our Galaxy. By
using the chemical abundances provided by APOGEE we will be able to determine which stars in
the MW were formed together and describe the environment in which they were formed. From
this information we can distinguish between stars born in the MW and stars born elsewhere
that have since merged to become part of our Galaxy. As a proof of concept we have already
begun to search debris leftover from the merging of the Sagittarius Dwarf Galaxy (Sgr) and find
that these stars do indeed exhibit similar chemical abundance patterns to Sgr, a galaxy I have
previously studied. Finding and analyzing chemical substructure in the MW will help constrain
formation and evolution models which can then be applied to the vast zoo of galaxies in the
Universe.
NASA Mission Directorate Alignment:
This project will support the NASA Science Mission Directorate Astrophysics Division as it will
help answer the question, “How did the Universe originate and evolve to produce galaxies,
stars, and planets we see today?” (NASA 2010 Science Plan). This research addresses this
question by investigating how our Galaxy formed and evolved. We will be able to discover and
trace the remnants of past mergers and how they contributed to the hierarchical buildup of our
Milky Way. We will be able to determine which parts of our Galaxy were formed by in situ
processes and what parts are of extragalactic origin. By better understanding our own Galaxy in
which we live we will be able to understand other galaxies that populate the Universe in such
mind-numbing quantities.
Career Interests:
I am most interested in looking at the light emitted from stars at different wavelengths
(spectrum) and using that light to determine what stars are made of. By analyzing the chemical
composition of groups of stars one can learn about the environments in which these stars were
born. This “Galactic Archaeology” technique can be applied to our own Milky Way Galaxy to
figure out how our own Galaxy formed and evolved over its lifetime. Even though we live in it,
we know very little about how exactly our Galaxy came to be. As we look more closely at the
Milky Way we reveal more questions about its history. I want to contribute to finding the
answers to these Milky Way formation questions.
Finishing my PhD in 1.5 years and continuing on in academia as a postdoctoral researcher are
my immediate career goals. Shortly after that I would like to get involved with NASA’s
incredible James Webb Space Telescope (JWST) and use data from this telescope to extend my
investigation of galaxy formation to other galaxies in general. This telescope will provide
detailed views of Milky Way-like galaxies in their infancy. A long term goal is to become a
tenured professor at a prominent research institution.
As a scientist I believe that public outreach is one of the most important aspects of the job. We
have the power to educate children and inspire the next generation of scientists. Throughout
my career I plan to remain heavily involved in public outreach. In particular, I want to reach out
to elementary and middle school students who don't nominally have astronomy as part of their
curriculum. I believe it is very important to understand where we came from and it is our duty
to learn all about the Universe we live in.