Gregory Antonio Topasna

An Imaging Survey of Northern Galactic Halpha Emission with Arcminute Resolution

We present preliminary results from a high-resolution, high-sensitivity imaging survey of the northern galactic Halpha emission. The survey is carried out using the Spectral Line Imaging Camera (SLIC) which incorporates a fast (f/1.2) lens attached to a cryogenic CCD in combination with a narrowband interference filter. The pixel size is 1.6 arcminutes and the diameter of each field is 10°. The fast optics, narrow bandpass (1.7 nm) filter, and high quantum-efficiency, low-noise CCD yield a high brightness sensitivity to Halpha emission on arcminute scales. This gives an equivalent sensitivity to emission measure structure below 1 pc cm^-6. Some faint features detected include a supershell connected with the star forming region W4 extending 7° above the galactic plane, and filaments possibly related to galactic loops II and III. In addition, we have carried out deep observations of fields in which anisotropies in the cosmic microwave background radiation have been detected. Our observations place stringent limits upon the contribution to the apparent microwave fluctuations from free-free emission in the galactic foreground.

Detection in Hα of a Supershell Associated with W4

From H I observations, Normandeau, Taylor, & Dewdney have identified a possible Galactic chimney emanating from W4. We observed a 10° diameter field centered on this region in the Hα line using a CCD camera sensitive to faint extended emission. Our image shows an apparent shell of H II, which we interpret as the ionized inner wall of a superbubble produced by stellar winds from the very young star cluster OCl 352. An analysis of the ionization balance indicates that much of the Lyman continuum radiation from the star cluster is absorbed and does not escape from the disk. The shell appears to close 6° (or about 230 pc) above the star cluster, and at a Galactic latitude of 7°. The shell is quite elongated, with its major axis approximately perpendicular to the Galactic plane, as predicted for a superbubble formed in a stratified Galactic disk. The large size of the shell leads to an estimated age between 6.4 and 9.6 Myr, which exceeds that of OCl 352 (2.5 Myr). The reason for this discrepancy is unclear, although it is possible that an earlier epoch of stellar outflow has contributed to the growth of the W4 superbubble.

The Contribution Of Galactic Free-Free Emission to Anisotropies in the Cosmic Microwave Background Found by the Saskatoon Experiment

We made a sensitive, wide-field Hα image of the north celestial polar region. Using this image, we constrain the contribution of irregularities in interstellar free-free emission to the degree-scale anisotropies in the cosmic microwave background detected in recent observations at Saskatoon by the Princeton group. The analysis of the Hα image mimics the Saskatoon data analysis: the resulting signal is the strength of irregularities sampled with the Saskatoon beam (i.e., degree-scale) along the 85° declination circle. We found no such irregularities that could be attributed to Hα emission. The implied upper bound on the rms variation in free-free brightness temperature is less than 4.6 μK at 27.5 GHz. The observed cosmic microwave background anisotropies are much larger. Therefore, the contribution of irregularities in interstellar free-free emission to the observed anisotropies is negligible.

An Imaging Survey of the Galactic H-Alpha Emission with Arcminute Resolution

We are presently carrying out a northern hemisphere survey of the Galactic Hα emission. Our instrument, the Virginia Tech Spectral Line Imaging Camera (SLIC) utilizes a fast objective lens (f/1.2) with a cryogenically-cooled TK 512 × 512 CCD. A filter wheel in front of the lens allows us to select interference filters, including a narrowband Hα filter and a broader bandpass continuum filter in a line free part of the spectrum. The fast optics in combination with the low noise CCD result in sub-Rayleigh sensitivity at confusion limited levels. (1 Rayleigh = 106/4π photons cm-2 s-1 sr-1.) This corresponds to an emission measure sensitivity of ≈ 1 pc cm-6. Parameters of our system are given in Table 1.