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ASTR 130
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Astronomy
Date
Apr 3, 2024
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Uploaded by BrigadierSteel392
Characterizing the diversity of galaxies with Galaxy Zoo
Through binoculars the spiral nebula was
a smudged white thumbprint on the night sky.
Stories said it was a mark left by the hand
of Night, that old she, easily weaving
the universe out of milky strings of chaos.
-- Minnie Bruce Pratt, The Blue Cup
Overview
●
Become familiar with the basic structural properties of galaxies
●
Understand the relationship between galaxy structures, and the types of stars in galaxies
●
Participate in scientific research using the ‘Galaxy Zoo’
Introduction
Fig 1: the two main types of modern galaxies
You have seen in the lectures that galaxy structures come in two
main flavors: galaxies with a dominant
bulge
(left), and galaxies
with dominant
disks
(right). The structures of these galaxies are a
rich source of information about how the galaxy evolved: galaxies
with dominant disks are thought to have evolved in relative
isolation; galaxies with dominant bulges may have undergone interactions with other galaxies that randomized
and disrupted the disks (or formed at earlier times, when interactions were more common).
In this lab, we will explore the structures of galaxies from a large sky survey, compare other properties with
their structures, and contribute to an international research project. Also, we’ll look at pretty pictures.
The Sloan Digital Sky Survey (SDSS) uses a dedicated telescope to perform an optical and spectroscopic
survey of much of the sky. SDSS-IV, which wrapped up in early 2020, included the Mapping Nearby Galaxies
at APO (MaNGA) project, which captured images and spectra of ~10,000 nearby galaxies. During the original
project, the SDSS imaged over 930,000 galaxies. Classification of these galaxies to learn about their structures
(bulges, disks, bars and spiral arms, and interacting galaxies), is important, but impossible for small teams of
astronomers to do alone.
In 2007, an online project called the Galaxy Zoo was created to allow members of the public to help classify
galaxies from the SDSS. Users of the site are presented with an image of a galaxy and are asked a series of
questions, designed to differentiate smooth round galaxies from structured disk galaxies, estimate the
prominence of a bulge component, and characterize any features such as spiral arms or bars. The initial phase
was so successful that they have expanded to include new data sets, including images from the Dark Energy
Camera Legacy Survey (DECaLS.) UofM is an institutional member of the Dark Energy Survey.
Fig. 2: “Green peas” (top row) were spotted by citizen scientists
participating in Galaxy Zoo.
In the first few years, citizen scientists participating in
Galaxy Zoo actually identified two completely new objects.
One was a new class of compact galaxies called “Green
Peas” for their tendency to appear small, round, and
green in the SDSS images. Another was “Hanny's
Voorwerp”, which appears to be a light echo from a
quasar. Their success encouraged scientists to develop
more citizen science projects. There are now dozens of
projects you can get involved with, from distant galaxies to
the bugs in your backyard, cutting edge physics to ancient
manuscripts. There’s even a mobile app for many of the projects,
including Galaxy Zoo!
Fig. 3: Hanny's Voorwerp, the green wispy structure, was first identified
by Dutch schoolteacher Hanny van Arkel.
One thing to note, Galaxy Zoo asks you to classify galaxies as
either smooth or as having a disk or feature. In general, all
galaxies that are smooth will be bulge galaxies, however, you
could also have a bulge type galaxy that appears to have a dust
lane or other feature. These are somewhat rare, but you could
come across one or two while doing your classifications, so
be
careful about taking notes as you go
!
Page 2 of 7
Before Class
You must register in order to
save
your work. If you want to register, it is strongly recommended that you
register before class. If you don’t register, you’ll have to be careful about recording your data as you go,
because you won’t be able to go back later and look at it.
Please register before class at:
http://galaxyzoo.org
and click the signup button in the upper right. All that is needed is a username, e-mail
address and password.
You may wish to try out a few classifications before class so you know how it works and can dive right into the
activity. The tutorial is strongly recommended. The Field Guide on the right edge will help you decide how to
make your selections.
Answer the questions in part 1 for yourself before class so you’re ready to discuss them when you get to class.
In class procedure:
Note the procedure is included here because it is long, and putting it here makes the worksheet portion you’ll
need to upload to Gradescope more manageable.
1.
Discuss the three questions in part 1 with your group and record your answers in the worksheet.
2.
For part 2, you will classify 10 galaxies using the Galaxy Zoo website and record your observations in
table 1. IMPORTANT: the information you record for the table is not exactly the same as the information
Galaxy Zoo asks about, so read through the list below before you start!
3.
Go to
http://www.galaxyzoo.org/
. If someone in your group created an account, have them log in.
4.
Click on “Classify” near the top to get your first galaxy image.
Fig. 4: useful buttons in the galaxy zoo interface
5.
Click the “favorite” button, which looks like a heart below the
galaxy picture, and is indicated by the red arrow in fig. 4. The
button next to it allows you to create a collection, which can be
shared with group members.
6.
Decide on the color of the galaxy. Color is sometimes hard to
determine – many ‘red’ galaxies appear yellow or orange. The
core may be yellow and the outer parts blue. For the purposes
of this exercise, any galaxy without an obvious blue portion is
‘red’, and galaxies with an obvious blue/green/violet portion are
‘blue’.
a.
In the color column of the spreadsheet, record the color
as R for red, B for blue, or F for obviously false color, or if it isn’t a galaxy. If you record an F, add
a comment in the notes column about why.
7.
The blue box in figure 4 indicates the "invert galaxy image" button. If you toggle this on and off, it will
toggle you between the positive and negative images. Features may be more apparent in one than the
other so you can use this to help with the remaining classification questions.
8.
The field guide on the right can provide clarification.
Page 3 of 7
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9.
Decide whether the galaxy is smooth, has features or a disk, or is a star or artifact. The field guide on
the right can provide clarification.
a.
In the type column of the spreadsheet, record an S for “smooth”, D for “features or disk” or A for
“Star or artifact”
b.
Click Next to go on to the next question in Galaxy zoo.
10. The next steps depend on what you answered in the previous step:
a.
If you answered A, add a note about what it seems to be.
b.
If you said it was smooth (S):
i.
Leave the “Edge or face-on Disk” column blank
ii.
Enter B in the Bulge or Disk column of the spreadsheet.
iii.
Finish the classification steps in Galaxy Zoo. If you note and disturbances, mergers, or
rare features, include that in the notes.
c.
If you answered D (features or disk):
i.
In the Edge or Face On Disk column: record E if it is an edge on disk, F if it appears to
be a face on disk (e.g. if it has spiral arms), or N if it is not actually a disk (e.g. it is
smooth except for a dust lane, or it is a lumpy blob)
ii.
If you recorded E or F in the previous step, record D in the Bulge or Disk column. If you
recorded N in the previous step, record a B in this column.
iii.
Finish the classification steps in Galaxy Zoo. If you note any disturbances, mergers, or
rare features, include that in the notes.
Additional Resources
●
The research behind Galaxy Zoo
https://www.zooniverse.org/projects/zookeeper/galaxy-zoo/about/research
●
The Sloan Digital Sky Survey
https://www.sdss.org/
●
The Dark Energy Camera Legacy Survey (DECaLS)
https://www.legacysurvey.org/decamls/
●
Green Peas:
https://blog.galaxyzoo.org/tag/green-peas/
●
Hanny’s Voorwerp
https://apod.nasa.gov/apod/ap110210.html
Page 4 of 7
Worksheet
Part 1: Background and Preparation
In general, galaxies with a dominant bulge have less cold gas and fewer blue stars than galaxies with a dominant disk.
1.
Based on your recollection of stellar populations from earlier in the semester, which color of stars will have a
shorter main sequence lifetime – blue or red?
Blue stars would have shorter main sequence lifetimes due to higher temperatures, resulting in a faster
rate of nuclear fusion burning through their supply of hydrogen faster.
2.
In the light of your answer, does it make sense that one type of galaxy would have both cold gas and blue stars,
but another type would have neither? Explain your reasoning, and check your answer with your neighbor.
Yes this makes sense because in order for blue stars to be born, the gas clouds have to be cold and dense
which allows for the formation of more massive stars, aka blue stars. On the other hand, the opposite is
true as well. A galaxy can have neither cold gas nor blue stars, this would be a less active star-birthing
galaxy with warm gas which is less likely to birth massive blue stars but rather less massive red stars.
The Galaxy Zoo effort has involved over 250,000 volunteers so far. Having such a large number of users can be an
advantage for several reasons.
3.
Why is it important to have more than one person classify the shape and features of each galaxy in Galaxy Zoo?
Mistakes or misclassification can occur, so having more people classify the shape and features of each
galaxy will ensure a more accurate result of the features that each galaxy has in Galaxy Zoo.
Page 5 of 7
Part 2: Classify some galaxies
Record everything for this part in the class spreadsheet. Be sure to use the single letter indicated in the top row so the
analysis sheet can automatically do the calculations for you.
Copy your group’s portion of the spreadsheet here
:
13
Justin
Chen
Hawaii
Two-O
1
R
S
F
D
2
R
S
F
D
3
R
D
F
B
4
R
S
F
D
5
R
D
E
B
6
R
D
E
D
7
R
S
F
D
8
R
D
F
B
Moderate Bulge, bright clumps, minor disturbances
9
R
D
E
B
Two very close to each other, merging
10
R
D
F
B
Very bright medium circular object next to it
Part 3: Analyze Your Results
When analyzing your results, don’t forget that Type D could actually be either bulge or disk, so use the bulge or disk
column, not the type column.
1.
Add up the following FOR YOUR GALAXIES:
a.
Total number of galaxies: ___
10
____
b.
Number of bulge galaxies: ___
5
____
c.
Number of red bulge galaxies: ___
5
___
d.
Number of disk galaxies: ___
5
___
e.
Number of red disk galaxies:_____
5
_____
f.
Number of red disk galaxies that are face on: ___
4
___
2.
Calculate the following:
a.
What percentage of the galaxies are bulge type? _____
50%
______
b.
What percentage of the galaxies are disks? ____
50%
____
c.
What percentage of your bulge galaxies are red? ______
100%
_______
d.
What percentage of your disk galaxies are red? ______
100%
______
e.
What percentage of red disks are face-on?___
80%
____
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Concluding Questions:
1.
Discuss if YOUR results support each of the following statements:
a.
the majority of galaxies imaged by the SDSS are bulge type galaxies;
i.
Half of my galaxies are bulge while the other half are disk galaxies.
b.
the majority of bulge galaxies are red;
i.
This is true, all of my bulge galaxies are red
c.
disk galaxies generally contain younger stars (hint: consider Part 1 question 2)
i.
My results do not support this.
d.
the color of disk galaxies depends on their inclination (edge-on vs. face-on.)
i.
My results don’t necessarily reflect this very well but this is true as edge-on the disk galaxies look
redder due to the dust and clouds absorbing blue light leaving only the red light to pass through.
Face-on this would not be an issue so it can look blue.
2.
The analysis tab of the spreadsheet has the class totals. If you consider the results from the whole class, do your
answers to the items in question 1 change? If so, which ones change, and how?
The class analysis mostly agrees with my results except for part c. I happen to not have results supporting
that disk galaxies are mostly young stars but the class analysis has much more red edge-on disk galaxies
which can mean they are actually blue if seen face on due to the gas and clouds absorbing the blue light.
This would change my part c into agreeing that disk galaxies mostly contain younger stars.
3.
Do you believe members of the public can make a meaningful contribution to astronomy through projects like
Galaxy Zoo? Why or why not?
I think that the public can definitely make a meaningful contribution through projects like Galaxy Zoo
through helping astronomers do generic classifications of galaxies or other such work. This would greatly
help astronomers decrease the work they have as well as allowing them to work on more technical
classifications. This could also cause more widespread interest in astronomy which would be meaningful
in acknowledging the work that all astronomers do.
4.
Would you consider continuing your involvement with this project outside of this assignment? What about other
citizen science projects (see for example https://www.zooniverse.org/projects) Why or why not?
I would definitely consider participating in a few projects as astronomy has always interested me and I
would love to learn more whether that be Galaxy Zoo or other projects.
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