The advance in detector technology also spurred several groups to initiate deep, comprehensive surveys of H from the Milky Way. Table 1 compiles a list of these recent efforts with their characteristics. WHAM and SHASSA are the only all-sky surveys to date; the other four surveys listed concentrate on the Galactic plane. Finkbeiner (2003) has combined WHAM, SHASSA, and VTSS into a complete Galactic survey (Figure 2), although the resolution and sensitivity vary across the image. While most of these new surveys are imaging, our own efforts have been focused on providing a unique spectroscopic view.
|Wisconsin H-Alpha||complete||< 0.1 R||1deg||(1)|
|Southern H-Alpha Sky||< +15°||few R||few arcmin||(2)|
|Survey Atlas (SHASSA)|
|Virginia Tech Spectral-line||> -15°||few R||few arcmin||(3)|
|Survey (VTSS)|||b| < 30°|
|AAO/UKST SuperCOSMOS||> +2°||< 5 R||1 - 2 arcsec||(4)|
|H-Alpha Survey (SHS)|||b| < 10°|
|INT/WFC Photometric||> +10°||few R||< 2 arcsec||(5)|
|H-Alpha Survey (IPHAS)|||b| < 5°|
|VST/OMEGACAM Photometric||< -10°||few R||< 2 arcsec||(6)|
|H-Alpha Survey (VPHAS+)|||b| < 5°|
|(1) Haffner et al. 2003; http://www.astro.wisc.edu/wham/. Survey is spectral, not imaging. For details and southern survey progress, see Section 4. All-sky public release expected in 2011 or 2012.|
|(2) Gaustad et al. 2001; http://amundsen.swarthmore.edu/.||(3) Dennison et al. 1998; http://www.phys.vt.edu/~halpha/. About 50% of fields available.|
|(4) Parker et al. 2005; http://www-wfau.roe.ac.uk/sss/halpha/. Digitally scanned from originally photographic survey.|
|(5) Drew et al. 2005; http://www.iphas.org/. About 60% available in an initial data release; fully calibrated survey to be released near end of 2010.||(6) http://www.vphas.org/. Preliminarily approved for the VLT Survey Telescope with observations expected to start in 2011.|
Figure 2. A composite view of the H sky. WHAM, SHASSA, and VTSS surveys combine to provide a sensitive, all-sky picture of the diffuse ionized gas of the Milky Way. Data from Finkbeiner (2003).
Leveraging the success of the Fabry-Perot spectrometer for study of the WIM, in the early 1990s Reynolds proposed to build a dedicated observatory to undertake the first kinematic survey of the diffuse ionized gas in the Galaxy. With funding primarily from the National Science Foundation, he and collaborators built the Wisconsin H-Alpha Mapper (WHAM) to be highly optimized for observations of large-angular-scale optical emission (Figure 3). WHAM consists of an all-sky siderostat that feeds a 0.6-m primary lens that delivers a 1deg beam on the sky to a dual-etalon, 15-cm diameter Fabry-Perot spectrometer. The primary configuration produces a 200 km s-1 spectrum with 12 km s-1 resolution. Gap spacings are fixed in the etalons, but this spectral window can be tuned between about 4800 Å and 7300 Å by changing the gas pressure (SF6) in the etalon chambers. Considerable detail about WHAM's optical design and performance can be found in Tufte (1997).
Figure 3. The Wisconsin H-Alpha Mapper. WHAM observing the Chilean sky from Cerro Tololo in early 2009. Photo courtesy Alex Hill.
Commissioned at Pine Bluff Observatory (PBO) in Wisconsin during 1996, we installed WHAM at Kitt Peak National Observatory (KPNO) in November, 1996 and began the Northern Sky Survey (WHAM-NSS; Haffner et al. 2003) in early 1997. Two years of H survey observations were followed by an exploration of the physics of the WIM through other optical emission lines (e.g., Madsen et al. 2006, Madsen & Reynolds 2005, Hausen et al. 2002, Reynolds et al. 2001, Reynolds et al. 1999, Haffner et al. 1999, Reynolds et al. 1998) and attempts to detect and map faint H emission from high-velocity complexes (e.g., Haffner et al. 2009b, Haffner 2005, Tufte et al. 2002, Haffner et al. 2001, Tufte et al. 1998). After eleven years at KPNO, we decommissioned WHAM and moved it back to PBO for a year of maintenance. In early 2009, WHAM shipped to Chile and was installed at Cerro Tololo Inter-American Observatory (CTIO) in March. After a short period of recalibration and recommissioning, science observations resumed. At this point (mid-2010), more than 90% of the H data newly observable ( < -30°) and more than 70% of a full southern sky survey ( < +30°) has been obtained. We expect to release a fully calibrated, all-sky survey by late 2011 or early 2012.
While surveys of neutral hydrogen have typically enjoyed high spectral resolution (e.g., McClure-Griffiths et al. 2009, Kalberla et al. 2005, Bajaja et al. 2005, Hartmann & Burton 1997), WHAM delivers the first all-sky kinematic survey at H. Although the angular resolution is modest for optical wavelengths, its sensitivity and ability to separate terrestrial lines from Galactic components reveals emission that covers the sky. Spectral resolution also allows us to isolate structure along lines of sight where rotation separates spiral arms and provides new insight into the dynamics of large-scale discrete objects. An example of the power of the kinematic survey can be found in Haffner et al. (this volume, pg. xxx).