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 by its
Object ID using the Navigation Tool, then look up its spectrum
by plate and fiber number using the Get Spectra Tool, then answer
questions 7 through 10.
Click on the Object ID of each galaxy to launch the Navigation tool
centered on the galaxy. Click the red Zoom button to bring the Zoom window
to the front, then use the scrollbars to locate the green circle that marks
the galaxy. Click on the green circle to see the galaxy's data.
Click on the Fiber number of each galaxy to see its spectrum in the
Get Spectra tool.
Launch the Navigation tool
Launch the Get Spectra tool
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 (you may need to set the Zoom window size to get a better
Question 9: What strong
emission or absorption lines do you see in each galaxy?
Question 10: Do you
notice a relationship between color, emission/absorbtion lines, and
Galaxies do change as they age. Older galaxies have few young
blue stars. Younger galaxies have lots of HII regions where stars