Overview

At one-hundred thousand light years in diameter, the Milky Way Galaxy is so large that we can never hope to "get outside of it" to take a picture. It has a complex structure, obscured with light-absorbing dust, and it has a giant, opaque core blocking much of our view of the other side of the street.   Indeed, we know more about the structure and behavior of galaxies other than our own, than we know about our own Milky Way!

Understanding the rotational behavior of our galaxy is an important goal of modern astronomy.  We believe that this rotation is governed by the laws of gravitation and angular momentum.  However, it appears that models based on these laws point to something that is "missing" in our galaxy (or in our model) - the mass required to create a gravitational field to explain the observed rotation.  This is a big deal which appears to apply to other galaxies as well.   It is like missing the two cups of flour called for in the recipe for a cake!

The scientific goal of this project is to collect experimental evidence in support of a model of galactic rotation.   Unlike a bicycle wheel or a set of fan blades, the arms of our galaxy rotate such that their angular speeds change with distance from the galactic core.  This is akin to the way that planets have different angular velocities around the Sun, but the physics is much more complex because of the complex distribution of matter within the galaxy.  Since the 1950s, astronomers have been collecting data to construct a galactic 'rotation curve' that characterizes the mean linear velocity of matter in the galaxy, as a function of radius (distance from the core.)  The speeds are fast (measured in kilometers per second)  However, even at such speeds, it still takes our Sun over 200 million years to revolve around the galactic core!  There is plenty of backstory here about how we know such things, and the reader is encouraged to learn more about this on their own!

Artist's conception of the Milky Way (courtesy NASA.)  https://www.nasa.gov/jpl/charting-the-milky-way-from-the-inside-out  The Sun's position in this diagram - about 8 kpc below the galactic center as represented above, constitutes the origin of what is called the galactic coordinate system.  In this coordinate system,  a galactic longitude of zero degrees is the direction pointing directly toward the galactic center.  Longitudinal angles increase in the counterclockwise direction to 360 degrees.  Galactic latitude is the angular measure of the direction above or below the galactic plane, and is not shown in this perspective.  This image is represented with galactic 'north' pointing out of the plane of the diagram, roughly in the same direction of our solar system's 'north' (although these do not line up exactly.)  Observations reveal that the galaxy rotates in the clockwise direction, from this perspective.  By contrast, the planets of our solar system rotate in the counterclockwise direction around the Sun.

This gif animates a sequence of scans I recorded through the galactic plane.  The range is limited by my observing location in the Northern hemisphere, and covers galactic longitudes ranging from about 10-240 degrees.

There is a wide array of experimental techniques used to learn about the structure and motion of the galaxy.   The radio telescope used in the present study detects signal from neutral atomic hydrogen that is present in vast clouds within the spiral arms.  This gas is exceedingly cold, and emits a microwave signal at a frequency of 1420.4 MHz.  The origin of this signal is quantum mechanical:  that is, it is due to the energy that is emitted when the single electron changes its spin state (which is about all it can do at such low temperatures.)  Fortunately, this signal is capable of travelling unimpeded through the dust and gas of the galaxy, and even travels through the Earth's atmosphere easily, being unaffected even by the clouds.  It is a weak signal, and the only reason we can detect it is due to the vast quantities of hydrogen in the spiral arms.  This is a fascinating example of how we learn about the structure and motion of something that is astronomically large by understanding and utilizing a phenomenon that is based on something sub-atomically small.

Measurements of this signal reveal that the frequency is often shifted to larger values (blue-shifted) and smaller values (red-shifted.)  These shifts indicate that there are discreet parts of our galaxy that are moving at different speeds relative to the Earth. (see the gif above)  The shifts follow the physics of the Doppler Effect, known to most of us as being responsible for the subtle shift in pitch of a passing ambulance siren, or the tool that weather forecasters use to measure the velocities of rain clouds.  The radio telescope should be capable of detecting these frequencies (sufficient gain) and resolving the directions from which they originate (i.e. galactic longitudes.)  By themselves, the Doppler measurements of the hydrogen line frequency only indicate relative velocities; however, a few methods can yield "absolute" velocities tangent to the galactic center. For observations of galactic longitudes less than ~75 degrees, the tangent-point method can be applied (assumes radius of Sun around galactic center.)  A trigonometric method can be applied to determine velocities  across a wide range of longitudes  extending into quadrants II and III (but with this method, the user must estimate the galactic radii of emitting regions.)