Data

In the figure below, the dashed circle represents the orbit of the Sun (orange circle) around the galactic center. Galactic longitudes are labeled, with 0 degrees being directed toward the center of the galaxy. The white lines emanating from our Sun show the observational lines-of-sight. The yellow curves represent the outer arms, Perseus (PER) and Cygnus (CYG). For our model, we assumed that the emitting hydrogen along a particular line-of-sight is located where this line intersects the yellow curve. The orange oval represents the region where the tangent method was applied. For this method, the radius of the emitting hydrogen is calculated from the galactic longitude.

Corresponding data scans are shown below, plotted as intensity (Kelvins) versus relative velocity (km/s.) Velocities have been corrected for the Earth's orbital motion around the Sun, as well as the Earth's rotation around its own axis. Negative velocities correspond to approaching regions of the galaxy ("blueshifted") and positive velocities correspond to receding regions ("redshifted.") The main peak, near v=0 km/s, is due to neutral hydrogen in our local neighborhood of the galaxy; this peak is sometimes shifted due to the Sun's motion in our local standard of rest (LSR) Data was collected and processed using software from West Virginia University Radio Instrumentation Laboratory (WVU RAIL), courtesy Glen Langston.

Frequency Plot

Signal intensity is recorded by the SDR module as a function of frequency, in MHz. The center frequency evident in scans is always near 1420.4 MHz, which is the frequency emitted by the single electron in neutral hydrogen, undergoing the spin flip.

Velocity Plot

Horizontal axis is relative velocity (km/s.) This is done using the doppler shift formula for electromagnetic radiation. Note that this plot is a mirror image of the frequency plot: negative velocities are APPROACHING (bluehshifted) and positive velocities are RECEDING (redshifted) Velocities are corrected for the Earth's orbital motion around the Sun, as well as the Earth's rotation.

11 degrees

Looking toward the center of the galaxy. There is very little relative motion along this line of sight.

36 degrees

As we observe along directions moving away from the galactic center, a diffuse red-shifted wing appears, in this case extending to +90 km/s. This wing is due to the relatively diffuse gas clouds in the inner galactic regions, which are moving away from us. The threshold velocity of +90 km/s gives as estimate of the tangential velocity for an inner orbit (this is used in the 'tangent-point' method for estimating galactic velocities for radii less than the radius of our Sun around the GC.) The blue-shifted peak at -60 km/s is due to signal from outer arms on the opposite side of the galactic center, most likely Scutum and Cygnus.

45 degrees

Again, the broad red-shifted wing is visible. However, note that the threshold has decreased to +75 km/s. This is expected since relative motion of the inner orbiting material becomes smaller as its motion becomes more aligned with the our line of observation.

75 degrees

Although we are still in quadrant I, the redshifted wing has disappeared, as the inner orbital tangents are becoming more aligned with our line of sight. The blue-shifted peak around -100 km/s is a combination of signals from gas clouds in the Cygnus and Perseus arms.

102 degrees

Now we're in quadrant II of the galactic plane. All peaks our due to arms at larger radii than our Sun. Other than the central peak at 0 km/s (due to local hydrogen in our Orion spur) the other peaks are both blue-shifted, indicating outer arms that we are "catching up" with. The peaks at -107 km/s and -65 km/s are most likely due to the Cygnus and Perseus arms, respectively.

150 degrees

Blue-shifted peaks are moving back to center as line-of-sight approaches 180 degrees.

176 degrees

Again, there is very little relative motion when looking radially outward from the galactic center.

210 degrees

Now we're in quadrant III of the galactic plane. Features growing in at positive velocities (redshifted) indicating that we are seeing outer arms (Perseus and Cygnus) and that we are moving away from them.

235 degrees

Again, we are "getting ahead" of the outer arms as we rotated clockwise in the galactic plane. The peaks at +22, +38 and +80 km/s are most likely due to signal from our own Orion Spur, and the Perseus and Cygnus arms, respectively.

In addition to observations made within the galactic plane, I have also started a whole sky survey. By collecting 24 hour "drift scans" (telescope elevation and azimuth remain unchanged) I can collect hydrogen line data across the portion of the sky that is accessible from the Northern hemisphere (approximately 41 degrees latitude.) The figure below summarizes data collected to date. Each point on the graph represents a 10-minute averaged scan (data file.) The bottom-most curve delineates the lower limit for observable galactic latitude.

As one might expect, as the Earth rotates through a 24-hour period, two different portions of the galactic plane will pass overhead. The figure below shows a set of scans collected over a 24 hour period (Elevation=75 degrees, Azimuth=180 degrees.) The galactic plane passes through the telescope field of view at 11 and 23 hours (times are relative to the when the scans started.)

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