Detecting the Magellanic Stream

Report by Jonathan Kao.1

May 8, 2015

ABSTRACT

This report summarizes our attempt to detect the Magellanic Stream and

the results we obtained. The telescope we used was the 4.5m-diameter dish at

the Leuschner Observatory. The region of l = 61 to 110 and b = −30 to

−90 in galactic coordinates was observed with a 2 resolution, 380 profiles total.

Resulting images showed high-velocity gases of −100 to −350km/s, which we

believe were signals from the Magellanic Stream.

1. Introduction

1.1. About the Magellanic Stream

Our Milky Way Galaxy belongs to a cluster of galaxies often referred to as the Local

Group, which contains over 50 galaxies. Many of the dwarf galaxies surrounding the Milky

Way are known as the ”satellite galaxies” of the Milky Way, and among them two of the

closest to the Milky Way (visible with the naked eye) are the Large Magellanic Cloud (LMC)

and Small Magellanic Cloud (SMC). LMC and SMC are connected by a bridge of gas referred

to as the Magellanic Bridge (MBR) due to the strong tidal interaction between them, and

the two bodies travel together as a system, as shown in figure 1.

Due to the relatively short distance from the Milky Way, the tidal interaction between

the Milky Way and the Magellanic system is also significant. Some argue that this might have

caused the formation of the Magellanic Stream. Results announced in 2010 suggest that it

might have formed 2.5 billion years ago when LMC and SMC passed close to each other and

caused large-scale star formation and supernovae explosions, of which the release of energy

could have blown some gas out of the system, which then got caught by the gravitational

pull of the Milky Way and started flowing towards us.2

1Experiments done by Team HAJ: Han Aung, Ankit Patel and Jonathan Kao.

2Emily Baldwin, ”Giant Intergalactic Gas Streamer Gets Longer,” Astronomy Now. URL: http://

www.astronomynow.com/news/n1001/05streamer/

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Fig. 1.— An illustration of LMC, SMC, MBR and the Magellanic Stream relative to

the Milky Way. Picture credit: Harvard-Smithsonian Center for Astrophysics, URL:

www.cfa.harvard.edu/news/2010-18

1.2. Challenges We Expect

We attempt to detect the stream by sampling HI signals from the hydrogen gas. Unlike

in the projects our peers have been working on, in this project we are trying to detect a

stream of gas outside of our own Milky Way Galaxy, which means that the incoming signal

from our target is extremely weak compared to other sources and can easily get dominated

by signals from other bodies within the Milky Way and any source of noise, including our

own equipments. This means we need much longer integration times for each point we

are observing, and developing programs to extract information from our data can be very

challenging. Additionally, the relative location of the Magellanic Stream to the Milky Way

makes it much more easily observable from the southern hemisphere of Earth, so the time

in a day when we can observe it is very limited.

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2. Methods

2.1. The Telescope Setup

The telescope we use is the 4.5m-diameter radio dish at the Leuschner Observatory,

and we sample at 24MHz but only use 12MHz on the positive side. The received signal

goes through a series of amplifiers, band-pass filters and mixers before being integrated and

sampled. See figure 2 for a detailed block diagram of the setup.

Fig. 2.— The Telescope Setup: block diagram with filters and local oscillator frequencies

marked. Illustration by Jonathan Kao.

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2.2. Taking and Analyzing the Data

In terms of galactic coordinates (l, b), the area we want to cover is a pie-shaped area

bounded by l = 60 to 110 and b = −90 to −30, covering about 1250 square degrees. At

about 2 spacing, we determined the need of about 380 profiles, with an integration time of

above 10 minutes per profile. There are 31 values of b to map while the number of points for

each b varied, so we take the ”row-by-row” approach by fixing b and scanning through all

the l for that b. The pointing of the telescope is done with the provided ”follow” procedure

and the data recording using the ”leuschner rx” procedure, both in IDL.

To perform the intensity calibration, before and after taking each set of data we take

the ”noise-on datum” by turning the noise diode on. We calibrate the system by assuming

that the noise diode has temperature of 300K, and extract information about the velocity

from the data. For details on the process, refer to the report ”21-cm Line, Coax Cables and

Waveguides” by Jonathan Kao, with the major difference being using a smoothed spectrum

for calibration instead of the ratios of upper and lower sidebands.

Another piece of information we can get is the relative intensity of the HI line at each

point. The mass seen by the telescope can be represented with the formula MHI(v) =

1.8× 1018∆vd2mHTAΩb (grams), where ∆v is the velocity interval, d is the distance, mH is

the mass of each HI atom, TA is the antenna temperature, and Ωb is the solid angle of the

beam. Since everything other than TA and ∆v is constant, we get MHI ∝ TA∆v, so we can

use TA∆v to represent intensity.

2.3. Creating the Image

We present our resulting map in two color images. With the goal of mapping the

expanding gas of the Magellanic Stream, we plot the data in the 3D space of (l, b, v), which

are the galactic coordinates and the measured velocity at each point. The data points are

reorganized into a grid, and the 1D image is produced using the IDL procedure ”display”

provided. Additionally, since we can get information about intensity, we plot the same data

in the 4D space of (l, b, v, I) with I being the measured intensity at each point. This 2D

image is produced using the IDL procedure ”display 2d” provided.3

3The procedure ”display 2d” had a fixed layout for the image and the colorbar, which did not meet our

needs. We created a slightly altered version from it to create the layout we needed.

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3. Results

3.1. The Spectra

Figure 3 shows an example of the resulting spectrum for a certain point. The resulting

spectrum is a sum of all the separate spectra we took for the same point. The plot on the

top shows the full spectrum with temperature plotted against velocity. We believe that the

largest peak to the right represents the signals we receive from bodies within the Milky Way,

while the smallest peak to the left could be noise or signals from other bodies. The peak

in the middle at around −140km/s was what we were looking for, so we zoom in on that

section and fit a Gaussian to the shape, as shown in the plot on the bottom. On the other

hand, figure 4 shows a bad example in which a Gaussian cannot be fitted to the spectrum.

In this case we discard the data values at the point.

Fig. 3.— ”Good Data”: this figure shows the

full spectrum at b = −90 and the nicely fitted

Gaussian. Plots by Han Aung.

Fig. 4.— ”Bad Data”: this figure shows the

full spectrum at l = 61 and b = −66 and the

difficulty in Gaussian fit. Plots by Han Aung.

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3.2. The Images

Figures 5 and 6 shows the resulting 1D images of our data. First we note that since

the coordinates are in galactic coordinates, the more negative the b the less profiles (points),

meaning that it is a pie-shaped region. In our images we stretched it out to a rectangle,

therefore the top of the images are the least deformed while the bottom of the images are the

most stretched out. In figure 5 we can see LSR velocities ranging from around −100km/s

to −350km/s, with negative velocity meaning the HI gas we observe is traveling towards us.

In figure 6 we see that the relative intensity is higher towards the pole. Figure 7 shows the

two 1D images combined, in which we can see both of the trends.

Fig. 5.— 1D Image - Velocity: this figure

shows the LSR velocities of the gases we ob-

served. We can see higher-velocity gas clus-

tering at around b = −45 and the velocities

approaching 0km/s as we approach the pole

(b = −90). Image by Jonathan Kao.

Fig. 6.— 1d Image - Intensity: this figure

shows the relative intensities (normalized to

a scale of 0 to 1) of the gases we observed.

We can see that the intensities are higher as

we approach the pole (b = −90). Image by

Jonathan Kao.

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Fig. 7.— 2D Image: this figure shows the resulting image produced by combining the two

1D images into a 2D image. The color indicates the velocity and the brightness indicates

the relative intensity. Image by Jonathan Kao.

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4. Discussion

The most important thing to ask now is whether what we detected was in fact the

Magellanic Stream. Our 1D velocity map is again shown in figure 9, and we compare with

previous studies, figure 8 shows a velocity map from a study by Fox, Andrew J. et. al.. Since

the region we covered was approximately the upper half of the map in figure 8, comparing

the velocities we can say that the results are fairly close. In both figures we see that the

highest velocities occur at around b = −40 to −50 with peak values of −350km/s or above,

and velocity approaches 0km/s as we get closer to the pole at b = −90. With similar values

in velocity and similar distributions, we would argue that even though the measurements

may not be accurate, what we detected was indeed the Magellanic Stream.

Fig. 8.— Velocity Map of the Magellanic

Stream. Figure credit: Fox, Andrew J. et al.,

”Exploring the Origin and Fate of the Magel-

lanic Stream with Ultraviolet and Optical Ab-

sorption,” Astrophys.J. 718 (2010) 1046-1061.

Fig. 9.— 1D Image - Velocity: this figure

shows our 1D velocity map of the region ob-

served. It is the same as figure 5.

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However, there were some problems with the data we obtained that could have made our

relative intensity map inaccurate. While the setup of the telescope at Leuschner Observatory

sampled data with x-polarization and y-polarization, for unknown reasons the y-polarization

in the data we took did not reveal any useful information regarding the HI line. Therefore

we had to use the x-polarization only for calculating the intensity, and we believe that the

results are off. This was also why we made our images using relative intensities rather than

absolute values.

Lastly we would like to discuss whether the two trends we see make sense or not. As

described in section 3, the two trends we see are ”lower velocities” and ”higher intensities”

as we get closer to the pole at b = −90. If we base the discussion on the understanding

we got from the background information described in section 1.1, since the gases get caught

by the Milky Way’s gravitational pull, the older the age (age meaning how long since the

gas escaped from the Magellanic System) the higher the velocity (negative). Additionally,

we might also expect the regions closer to the Magellanic System having higher intensities.

Knowing that b = −30 is the furthest and b = −90 is the closest to the current position

of the Magellanic Clouds, it seems reasonable to see higher velocities towards the b = −30

end and higher intensities towards the b = −90 end.

5. Conclusion

There were a few things that could definitely have been improved, and a few difficulties

that could be solved under different circumstances. With the radio dish at Leuschner being

brand new (for a nice photo see figure 10), we could not be sure whether the telescope would

work as expected, and the intensity calibrations we did within this short period of time were

probably not accurate enough. Also, the signal from the Magellanic Stream was so weak

that we had to take a total of more than 60 hours of data to cover the whole region, which

meant that there were no chances to closely examine the data and retake data to replace

the bad ones. However, even though we could not say for sure that we accurately observed

the Magellanic Stream, we are proud to say that we did actually detect something from the

Stream. We could definitely make more accurate measurements if we had more time.

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6. Appendix - Photos

Fig. 10.— Professor Carl Heiles with the new 4.5m-diameter dish at Leuschner Observatory.

We can see the old dish down on the ground next to the new dish. Photo by Jonathan Kao.

Fig. 11.— Team HAJ presents: a scenic shot of the famous water tank with no water at

Leuschner Observatory. Photo by Jonathan Kao.