The document discusses simulations of galaxy interactions using the Galaxy Crash Java applet. It explores how varying simulation parameters like galaxy mass, distance of closest approach (peri), and inclination angle affect the formation of tidal tails. Increasing peri decreases tidal tail size, while larger companion galaxy mass and smaller inclination angles increase the other galaxy's tidal tail. The minimum peri to avoid disturbance was around 35-40 kpc.

1. Comenius Dive in the Sky
Interacting Galaxies:
Making Tails
Ana Raquel Rodrigues
Lukas Schmutzler

2. Introduction
How structure formed in the Universe is a very active area of astronomy research and still generates a
huge amount of debate between astronomers. The generally accepted model at present is that smaller
objects formed first in the Universe, and that through a series of mergers, began forming the larger
galaxies which we see today. It is thought that the elliptical galaxies which are observed in galaxy
clusters, formed through the long process of two galaxies of similar mass, merging together. Even now,
our Galaxy, the Milky Way, is in the process of tearing apart some of the smaller galaxies which
surround us, leaving faint trails of stars for astronomers to study.
To realize this project we used the Java applet, Galaxy Crash (written by Chris Mihos at Case Western
University, USA - http://burro.astr.cwru.edu/JavaLab), to simulate interactions of galaxies. With this
applet you can model galaxy interactions and collisions, and see what parameters affect how the
interacting galaxies appear in the sky. By changing the parameters of each galaxy we created elliptical
galaxies, make tidal tails, and reproduce the types of galaxies which we observe today. In this project we
produced tidal tails for two interacting galaxies, and investigated what affects their formation.
Every object with mass in the Universe attracts objects through gravity. Newton’s Law of Gravitation
states that ‘any 2 point masses attract each other with a force that is proportional to each of their
masses and inversely proportional to the square of the distance between them’. In equation form, this is
given as:
𝐹 =
−𝐺𝑚2 𝑚2
𝑟2
As can be seen from the equation above, the force of gravity, F, on an object depends on how close the
objects are to each other (r). As an object moves away or towards another object, the pull of gravity
which the objects feel changes. This change in the gravitational pull creates a pressure on the bodies,
known as the gravitational tidal force. When galaxies collide, or pass close to each other, long streams of
stars are pulled out of the galaxies by the gravitational tidal force of each galaxy. These streams of stars
can extend hundreds of thousands of light years from the galaxy, and are given the name “tidal tails”.

3. 1.
a) When do the galaxies come closest together?
The collision between galaxies is a phenomenon that happens in the universe, where
different galaxies shock. The collisions last for a long time, lasting even for billions of years.
b) What happens to the relative velocities of the galaxies as they reach their
point of closest approach (perigalacticon, or peri for short)?
The relative velocity of the galaxies starts slow, approximately 260 km/s, and as they
collapse together, they accelerate until they reach a velocity of 600 km/s (when they are
side by side, as we can observe in the picture). As they drift apart, their relative velocity
slows down.

4. c) As time increases, do the galaxies come together again?
No. As time passes, the galaxies drift apart more and more, until they reach a point where
they're no longer on the screen.
d) What was the highest relative velocity for these galaxies, and when did this
occur?
The highest relative velocity was after the galaxies already collided, reaching a velocity of
approximately 600 km/s, as we can see in the picture on question b.

5. 2.
a) When do the galaxies come closest together? How does this compare to the
previous simulation?
b) What peak relative velocity do the galaxies reach in this case? How does this
compare to the previous simulation? Explain why you think these values are
different.
In this case, the relative velocity is 830 km/s, which is much faster than the example above,
because we changed the mass of the red galaxy, which consequently drives different
results.
c) Predict what would happen if you increase the mass of the companion galaxy.
If we increased the mass of the other galaxy, the tail of the smaller galaxy would be longer,
and the interaction between them would be slower. The damage of their collision would be
bigger, as opposed to the example above, because even though the red galaxy had bigger
mass, there are more damages to the other one, which doesn't disturb the one with bigger
mass, but if we increase the mass of the other galaxy, the damages would rise; if both had
the same mass, but the red galaxy had less mass, that would be the one more damaged.

6. 3.
a) Do your observations from these simulations match your predictions from
Question 2c?
In part, yes. The observation confirmed what we said about the tail of the smaller galaxy,
but it was much faster. Regarding the damage, we got it right, because the smaller galaxy
appeared much less damaged after the collision with the bigger one.
b) How does the development of the tidal tails for both galaxies change as you
increase the companion (red galaxy) mass?
4.
a) How do you think the formation of the tidal tails would change as you
increase the value for Peri?
The tidal tails of the galaxies will decrease as Peri grows, since gravity decreases as the
galaxies drift apart. As such, they can't steal each other's stars. If Peri were very big, the
galaxies would not collide completely.

7. 5.
a) How does the formation of tidal tails change as you increase the distance of
closest approach (Peri)? Is this what you predicted in Question 4a above?
When Peri is 15 kpc, a tail grew in both galaxies, reaching a velocity of approximately
520 km/s.
When Peri is 20 kpc, the tail created upon collision is far smaller than the one above,
reaching a velocity of approximately 457 km/s.

8. When Peri is 25 kpc, a tail is practically non-existent, reaching a velocity of 410 km/s.
In conclusion, with the increase of Peri the tail gets smaller.
b) What is the minimum distance for the galaxies not to disturb each other as
they pass by?
To know the minimum distance between galaxies, we gradually increased Peri:
 Peri 30 kpc - some disturbance
 Peri 35 kpc - no disturbance
We therefore conclude that the minimum distance for the galaxies not to disturb each other
as they pass by will be about 35/40 kpc.

9. 6.
a) Describe what you see as the simulation runs.
The galaxies collide in the begging of the simulation, reaching approximately 600 km/s.
A tail formed in one of the galaxies, and a collection of stars occupy the empty space
between both galaxies.
b) How do you think the formation of tidal tails for each galaxy will change as
you increase the inclination angle of the green galaxy?
When the inclination angle of the green galaxy increases above 90°, the tail will not appear,
whereas if the angle is smaller the tail will increase in size as it get closer to 0°.

10. c) Investigate how the inclination angle of the green galaxy affects the formation
of tidal tails by varying theta for the green galaxy from 45 to 180 degrees in
steps of 45 degrees. Describe below what happens as theta changes,
including when theta is 0 degrees, as investigated in Question 6a and how
this compares to your predictions. (Hint: the galaxies may rotate in different
directions in some simulations, which may affect the tidal tail formation).
0 degrees:
 Maximum velocity: 490 km/s
 Equal size in both tails from each galaxy.
 Both rotate counter clockwise.
 The stars between galaxies will disappear as time passes.
45 degrees:
 Maximum velocity: 490 km/s
 The green galaxy has a smaller tail than the red galaxy.
 Both rotate counter clockwise.
 The stars between galaxies will start disappearing after 500 Myrs.
90 degrees:
 Maximum velocity: 580 km/s
 The green galaxy tail is much bigger than the red one.
 Both rotate counter clockwise.
 The stars between galaxies will start disappearing after 700 Myrs.
135 degrees:
 Maximum velocity: 600 km/s
 Green galaxy's tail is practically non-existent and the red galaxy grows one as they drift
apart.
 The green galaxy rotates clockwise, while the red one rotates counter clockwise.
 The stars between galaxies will start disappearing after 600 Myrs.
 The green galaxy creates a halo.
180 degrees:
 Maximum velocity: 590 km/s
 Green galaxy's tail is non-existent, while the red galaxy had one.
 The green galaxy rotates clockwise, while the red one rotates counter clockwise.
 The stars between galaxies will start disappearing after 550 Myrs.
 The green galaxy creates a halo.
 Similar results with 135 degrees.

11. Here is a table to help compare results:
Maximum
Velocity
Tail Formation Orientation
Disappearance
of the stars
between
galaxies
Halo Formation
Red
galaxy
Green
galaxy
Red galaxy
Green
galaxy Red
galaxy
Green
galaxy
0º 490 km/s Yes Yes
Counter
clockwise
Counter
clockwise
As time goes by No No
45º 490 km/s Yes
Yes, but
shorter
Counter
clockwise
Counter
clockwise
500 Myrs No No
90º 580 km/s Yes
Yes, but
much
shorter
Counter
clockwise
Counter
clockwise
700 Myrs No No
135º 600 km/s Yes
Yes, but
practically
non-
existent
Counter
clockwise
Clockwise 600 Myrs No Yes
180º 590 km/s Yes No
Counter
clockwise
Clockwise 550 Myrs No Yes

12. References
http://resources.faulkes-telescope.com/course/view.php?id=63
http://burro.astr.cwru.edu/JavaLab