When you look at the spectrum of a galaxy, you are really looking at
the combination of spectra from the millions of stars in the galaxy. So studying
the features of a galaxy spectrum tells you about the types of stars
the galaxy contains, and the relative abundances of each type of star.
Galaxy spectra also clearly show you whether a galaxy contains star-forming regions
called HII regions. HII is a spectral emission line that corresponds to
ionized hydrogen - a hydrogen atom that has lost its electron. HII regions are
areas of a galaxy where hydrogen nuclei and electrons are recombining
to form neutral hydrogen.
When an electron recombines with a hydrogen nucleus, it loses energy and
gives off a photon (you might recognize this as the opposite of what
happens in absorption lines from the Spectral Types project). The HII
spectral line is in the red part of the spectrum, so HII regions in galaxies
have a beautiful red or pink tint in visible photographs (NOTE: SDSS uses
the red filter for the green picture in a tri-color image. Therefore,
HII regions do no appear red in SDSS images). You can clearly see HII regions in the
photograph of M51 at the right.
Now, let's examine several galaxies both visually and spectroscopically.
Below is a table of galaxies. Look up each galaxy using the
Object Explorer. When you click on a galaxy's Object ID, the
Object Explorer for this galaxy will open in a new window.
Examine the picture of the galaxy and classify it
on the Hubble Tuning fork. Then scroll down and click on the galaxy's
spectrum. Study its spectrum - pay close attention to its pattern of
spectral lines. As you examine the galaxies, think about how to answer
questions 7 through 10.
What is the color of each galaxy? Color can be measured by u-r,
with lower values being blue and higher values being red.
What type of galaxy is each?
Question 9: What strong
emission or absorption lines do you see in each galaxy?
Question 10: Do you
notice a relationship between color, emission/absorption lines, and
Galaxies do change as they age. Older galaxies have few young
blue stars. Younger galaxies have lots of HII regions where stars