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Unformatted text preview: ich the ﬁeld has approximately 5 minutes of arc size (verify that the label
in the lower right hand corner of the Sky window shows a number close to 0◦ 05 ). Please do
not zoom in or zoom out after that. If you do or if you decided to choose your own random
ﬁeld (feel free to do so), please zoom again to the point when the label is as close to 0◦ 05
as possible. This is because we want to compare counts of galaxies within the same area of
the sky in diﬀerent places, which requires that the size of the patch we use is the same (5
arcminutes in our case). Also do not resize the Google Earth window as you count galaxies
in diﬀerent ﬁelds, your counts may be aﬀected if you resize the window.
Once the Sky ﬁnishes its zoom, wait until the SDSS Galaxy Query layer identiﬁes galaxies
brighter than apparent magnitude in r band of mr = 21 in the SDSS catalog. Once the
layer queries the SDSS database over the internet, the galaxies in the ﬁeld will be marked
by circles. Count the number of marked galaxies in the ﬁeld and then click on field 2 and
repeat the exercise.
For this part of the lab we need to discuss a little bit of statistics. As you know, statistics is
a big part of analysis of data in various ﬁelds: physics, biology, economics, etc. The statistics
of counts of relatively small random samples of objects is typically governed by what is called
the Poisson distribution. For example, if galaxies were distributed completely uniformly on
Google Sky is an extension of Google Earth software into the sky (the simple version of which, Google
Maps, you probably have used to get directions, view a map, etc.). See the description of Google Sky that will
be distributed to you, as well as video introduction to Google Sky at earth.google.com/sky/skyedu.html
4 PHSC12000 V1.0 4 the sky, we would expect that the probability to ﬁnd a given number of galaxies in a ﬁeld of
a given size is given by the Poisson distribution. The key feature of this distribution is that
if the average number of galaxies per ﬁeld of...
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