I-GALFA Reveals Dynamic Events in Our Galaxy

S. J. Gibson, J.-h. Kang, & B.-C. Koo, June 2009

Article featured in NAIC Newsletter #45

Figure 1. Negative-intensity image of I-GALFA H I brightness in and off the Sagittarius Arm (dist ~ 2.5 kpc), scaled by |sin(b)| to emphasize disk-halo structure. The white stripe marks the Galactic equator (b=0). Red lines show height above and below the plane along lines of constant longitude.
Figure 2. I-GALFA three-color H I image for FVW40.0+0.5, integrated over vlsr = -100 to -90 km/s (red), -90 to -80 km/s (green), and -80 to -70 km/s (blue). This image shows filamentary features forming an H I shell of unknown nature.
Figure 3. FVW40.0+0.5 H I data cube projections showing intensity integrated from -110 to -70 km/s (bottom left) and position-velocity cuts along the dotted lines in the integrated map (top, right). The p-v diagrams show velocity structure in the filaments and knots of the H I shell (Kang 2008).
The Inner Galaxy ALFA (I-GALFA) survey began in 2008 and is now starting a new observing season. I-GALFA is mapping all H I emission near the Galactic plane between longitudes of ~ 30o - 80o. The combined sensitivity, angular resolution, and sky coverage of this survey gives an exquisitely detailed view of gas dynamics within the Galactic disk and coupling into the halo.

Figure 1 shows H I emission in the first I-GALFA field we observed. The gas velocity places it in the Sagittarius spiral arm, the next arm inside the Sun's orbit around the Galactic center. The brightness temperature is multiplied by the sine of the latitude to reduce the contrast between low- and high-latitude features. I-GALFA data reveal a complex tracery of loops, filaments, and blobs hundreds of parsecs above and below the Galactic plane, with isolated compact clouds extending even higher. A number of these, particularly below the plane, include sharp, linear features resembling shock fronts that may be shaped by cloud infall or stellar energy sources closer to the plane. At such heights, the expected gravitational field (Boulares & Cox 1990) implies these clouds will take tens of Myr to fall back to the plane, during which time they may evolve considerably. The full survey will cover more than four times the area of this single field, providing a rich overview of disk-halo dynamics.

Some dynamical processes are subtle and easily overlooked. Forbidden-velocity wings (FVW) are faint H I line emission features appearing at velocities beyond those allowed by Galactic rotation, probably due to some energetic event forcing disk gas away from normal motions. FVWs are a separate population from high-velocity clouds (Kang & Koo 2007). Many are believed to arise from old, hidden supernova remnants, but this has been difficult to assess without adequate resolution to examine their spatial structure, leaving the nature of most FVWs unclear. ALFA aids this process considerably. Figures 2 & 3 show one example, FVW40.0+0.5, in glorious detail. This object was studied before with Arecibo (A2055; Kang 2008), but I-GALFA allows its full environmental context to be examined. FVW40.0+0.5 is a large H I shell with complex filaments and knots tracing its interaction with the ambient ISM. Of the visible portion expanding toward us, the shell cap (red) appears inside the boundaries of gas with less extreme velocities (blue). The low H I column density of 2.9 x 1019 cm-2 requires Arecibo's sensitivity for proper imaging. A possible association with a pulsar near the shell center, B1900+06, would give a distance of ~ 10 kpc, placing FVW40.0+0.5 on the far side of the Galactic disk, with diameter of ~ 350 pc, H I mass ~ 7300 Msun, expansion velocity ~ 85 km/s, and age ~ 1 Myr. We expect many more FVWs with smaller angular sizes will be identified in the I-GALFA data, extending the current census of these fascinating tracers of ISM energetics.

Boulares, A., & Cox, D. P. 1990, ApJ, 365, 544
Kang, J-h. 2008, PhD Thesis, Seoul Nat. Univ.
Kang, J-h. & Koo, B.-C. 2007, ApJS, 173, 85

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