I-GALFA image of hydrogen gas in a tangent region outside the Sagittarius arm. The intensity scale is negative, so dark areas contain more gas. White areas near the edges of the map have not been observed.
With the I-GALFA survey now completed and data processing at the refinement stage, we can begin to exploit the combined imaging power of ALFA and sensitivity of Arecibo to capture structures never before seen. Here is a view of gas thousands of kiloparsecs away (1 kpc = 3,300 light years), lying between our Sun's orbit around the Galactic center and the Sagittarius spiral arm, the next arm inward from our own. This is a similar but larger view than that shown in a previous single-field gallery image.
Because we are viewing this gas in a direction closely aligned with (tangent to) its orbital motion, it appears to be moving away from us (we are orbiting the Galactic center with a similar speed, but our own motion in a different direction). By using models of how gas moves around the Galaxy, we can exploit such velocity measurements to get a rough idea of where the gas is located, even if this cannot be measured directly.
The gas appears to end near a Galactic longitude of 60 degrees at the velocity shown, but this is an illusion. In reality there is gas all around us, at all Galactic longitudes, but the part of its motion that we see as radial velocity varies with position. The gas near 60 degrees longitude is being viewed tangent to its orbital motion. Gas at higher longitudes has a different (lower) radial velocity.
The many filaments and loops extending off the arm in all directions are indicators of the violent activity within the Galactic disk from massive star formation and supernovae that disturbs the gas in which the stars reside. Past surveys with lower angular resolution have given the impression that the gas is more smooth and less disturbed than we see here.
The logarithmic intensity scale of this image emphasizes the excellent fidelity of the Arecibo data over nearly 3 orders of magnitude in surface brightness, a rarity in radio observations of the hydrogen 21cm line. The intensity scale is also flexible enough to show a variety of exotic structures with different brightnesses to be seen over a large area away from the Galactic plane, including chimneys, worms, and halo clouds. Most gas in this picture is at least 3 kiloparsecs (10,000 light years) away. At a distance of 3 kpc, a Galactic latitude of +/- 10 degrees is equivalent to a displacement of about 500 parsecs, or 1600 light years, above or below the Galactic plane.