Mapping the Ionized Component of High-Velocity Cloud Complex A

Allison Duncan
Rose-Hulman Institute of Technology

Astrophysics REU - Summer 2005
University of Wisconsin-Madison
Advisor: Matt Haffner






High Velocity Clouds

High velocity clouds (HVCs) are regions of gas moving at velocities anomalous with the rotation of the galactic disk. They are located at high latitudes, large distances from the galactic plane. HVCs are differentiated from intermediate-velocity clouds by velocities greater than ~90 km/s relative to the local standard of rest. Though they have been studied extensively in HI, still little is known about their nature and origin. One interesting aspect of HVCs that has not been well studied is their ionized component, observed via H-alpha emission. Studying the relationship between H-alpha and HI emission may provide insight into the ionization process and conditions in the galactic halo. My research this summer involved mapping and studying the H-alpha emission from a particular HVC structure labeled Complex A.

Complex A

Complex A is located in the approximate region 175 > l > 130 and 50 > b > 25. The distance to Complex A has been limited to a range from 4.0 to 9.9 kpc (Wakker 2001). Its metallicity has not been determined. The structure of Complex A is fairly linear and has several cores seen in HI emission. Two previous detections of H-alpha emission were found towards the AIII and AIV cores, with intensities of 0.08 and 0.09 R (Tufte et al. 1998). The complex is located near the large Complex C structure, part of which can be seen in the upper right corner of following HI map. In the lower right corner is an extension of the Outer Arm.

HI emission in the Complex A region, integrated over -220 km/s to -110 km/s LSR


The data used for my research was obtained from the Wisconsin Hydrogen-Alpha Mapper (WHAM). The WHAM telescope was developed to study ionized gas, specifically the warm ionized medium, through H-alpha emission. The WHAM instrument is a 15 cm aperture dual-etalon Fabry-Perot spectrometer. Spectral observations from WHAM have a resolution of 12 km/s over a 200 km/s window near H-alpha. The telescope was installed at Kitt Peak in late 1996. In 1997 and 1998, the telescope completed a survey of the entire northern sky in H-alpha. More information about WHAM and the Northern Sky Survey can be found at their website. The data used for my work this summer was obtained in the spring of 2005.


The WHAM instrument obtains spectra for each pointing of the 1 degree resolution beam. The pointings are arranged in groups called “blocks.” Each block contains up to 49 pointings, in a 7 x 7 grid. A total of 42 blocks were obtained for the area of Complex A. The spectra must be flat-fielded and cleaned of background, geocoronal, and atmospheric line emission through a fitting and subtraction procedure. This process is further described on the next page.

The 21 cm line data used to compare with the H-alpha data came from the Leiden/Dwingeloo survey (LDS; Hartmann & Burton 1997) with a resolution of 0.5 degrees and spectral resolution of 1 km/s.

Continue to my research


The Wisconsin Hydrogen-Alpha Mapper

NASA Astrophysics Data System

Research projects of other students

UW-Madison Astronomy Department

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Haffner, L.M., Reynolds, R.J., Tufte, S.L., Madsen, G.J., Jaehnig, K.P., Percival, J.W. 2003, ApJ, 149, 405
Hartmann, D., & Burton, W.B. 1997, Atlas of Galactic Neutral Hydrogen (Cambridge: Cambridge Univ. Press)
Tufte, S.L., Reynolds, R.J., & Haffner, L.M. 1998, ApJ, 504, 773
Wakker, B.P. 2001, ApJ, 136, 463