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Unformatted text preview: a given area is N , the typical variation from
ﬁeld to ﬁeld should be N .
3. Count the number of galaxies brighter than mr = 21 (the galaxies marked by circles)
and record the number for each ﬁeld.
4. Calculate the average number of galaxies per ﬁeld as well as the average number of
galaxies per arcminute squared using counts from all ten ﬁelds. You can choose more
ﬁelds of your own within the areas covered by the SDSS survey, just make sure that
they are all 5 arcminutes on a side.
5. Compare counts in each ﬁeld to the average and expected variation from ﬁeld to ﬁeld.
Does it look like the distribution of galaxy counts in these ﬁelds is close to the Poisson
distribution (i.e., is the distribution of galaxies on the sky uniform)?
Note: In a couple weeks, you will need the results from the above tasks in another
PHSC 12000 lab, so please be sure to keep a separate copy of your results in
addition to the lab write up that you turn in. Galaxy Spectra
In Google Sky activate the SDSS Galaxy Spectroscopic Layer, make sure the SDSS Galaxy
Photometric Layer is turned oﬀ. On the lowest zoom, ﬁnd the strips of sky that were observed by the SDSS. Now zoom in and look for some nearby (i.e., large and bright) galaxies
that have spectra available (circled in brown). Remember, circles are only placed around
the brightest galaxies in your ﬁeld of view, so if you’d like to check whether SDSS has a dim
galaxy’s spectrum, just zoom in on that galaxy until it is alone or is one of the brightest
galaxies. Find a few disk/spiral galaxies, and a few elliptical galaxies. You can mark galaxies
and go back to them later with the placemark tool (thumbtack in the top toolbar). To
view their spectra click on the desired circled galaxies, and follow the link at the bottom of
the popup bubble.
6. Review the spectra of six elliptical and six spiral galaxies, observe the general pattern
and similarities in the spectra of galaxies of either class. PHSC12000 V1.0 5 7. What is the relationship between the morphological class of a galaxy and its spectrum?
Sketch a typical spectrum characteristic for elliptical and spiral galaxies.
8. Discuss the main diﬀerences in the spectra of elliptical and spiral galaxies and give
your explanation for the origin of these diﬀerences....
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