Observations and Reduction

WFPC2 images of NGC 1569 were taken 23 September 1999 for the Cycle 8 program, GO-8133. NGC 1569 was oriented in two of the wide-field chips (WF2 & WF3) of the camera with a nearby $10^{th}$ magnitude star placed out of the field of view (see Figure 1). The effective plate scale is 0$\farcs$0996 pixel$^{-1}$ (1.07 pc pixel$^{-1}$ at the adopted distance of 2.2 Mpc). The GO-8133 images used here were F469N (HeII), F502N ([OIII]), and F547M ($\sim$ Strömgren $y$). NGC 1569's radial velocity is $-104$ km s$^{-1}$. The shift of each emission line is $\sim$ 2 Å, and therefore, all emission lines were observed very near the center of the filter transmission curve. Observational parameters for these data are found in Table 1.

These data were recalibrated using the best reference files and the new STScI ``on-the-fly" calibration (OTFC) system. OTFC images gave the same image statistics over the wide-field chips compared to the manually calibrated ones. Additionally, we used OTFC processed archival WFPC2 imagery of NGC 1569 from the Cycle 6 programs GO-6111 and GO-6423, which were primarily broadband images in UBVR. Please refer to [Greggio et al.(1998)] for more details on the GO-6111 imagery and to [Hunter et al.(2000)] for discussion of the GO-6423 imagery. Table 1 has observational parameters for these data as well.

The GO-6111 data were dithered by a few pixels for a subset of four exposures. These images needed to be aligned with respect to one another, which was done using standard IRAF applications. For all images, cosmic rays were best removed by using the STSDAS package crrej. The cosmic ray removed images were then rotated (in the case of the Cycle 6 data) and aligned with respect to the Cycle 8 F547M image. The F502N and F469N images were continuum subtracted. The total counts of several bright field stars were determined in the F547M, F502N, and F469N images. Average ratios of the emission line to continuum total counts for these stars were found, scaled to the F547M image, and the new F547M image was subtracted from each interference-filter image. This process was iterated until the field stars were subtracted out, and the residuals were at average levels of the background noise.

Drissen et al. (1999) used a weighted average between F547M and F439W images for continuum subtraction of their F469N image of NGC 2403. They indicated that this would properly subtract red stars and not leave them as ``holes" if only the F547M were employed. We attempted this method and found that the weighting heavily favored F547M ($\sim$99%), and thus, only F547M was used. Furthermore, the only F439W (GO-6111) image covers one-half of the galaxy on our field.

Flux calibration of our continuum-subtracted, interference-filter images was done using the WFPC2 exposure time calculator found at STScI's home page. A generic flux of $1.0\times10^{-16}$ ergs cm$^{-2}$ s$^{-1}$, an exposure time of 1000 seconds, and the redshift velocity of NGC 1569 ($-104$ km s$^{-1}$) were inputted to determine the number of object electrons for the F469N and F502N filters. The number of electrons sec$^{-1}$ was converted to DN sec$^{-1}$, and setting the DN sec$^{-1}$ value equal to the generic flux of $1.0\times10^{-16}$ ergs cm$^{-2}$ s$^{-1}$, we solved for the flux in one DN sec$^{-1}$. These conversion numbers are in units of ergs cm$^{-2}$ s$^{-1}$ DN$^{-1}$ and are $1.084\times10^{-14}$ for F502N and $1.847\times10^{-14}$ for F469N.

With the calibrated He II image, we determined whether the He II emission was due to a WR star, nebular source, or associated with a stellar cluster. All pixels with counts over 3 times the deviation in the background (3$\sigma$) in the F469N image were noted as possible detections. Each pixel with apparent emission was carefully checked in the individual F469N exposures to see whether the location had been struck by a cosmic ray, warm pixel or defect which might account for it being high. There was also the possibility that the pixel had a blemish in the continuum image which also might account for it being high. If a pixel location was in any of these categories, it was thrown out. What remained after this rejection process was considered a ``good" detection.

Each pixel that met the above criteria was superimposed on the F502N and F555W images. We determined from the superposition whether a good detection's location corresponded to a bright, point-like source in one or both of the images. The detections meeting the requirement of being a point source in F502N but no corresponding point-source in F555W were not found in our search. If the good detection had a corresponding point source in F555W but not in F502N, it was labeled S (for stellar sources) or C (for cluster sources). If the good detection was not associated with any point source in either of the images, it was labeled U in the figures and tables below.

For the S and C sources, the He II flux and WFPC2 B and V magnitudes were calculated. WFPC2 B magnitudes were not measured for some sources because the GO-6111 images did not cover some He II locations, or the corresponding point in the F439W image had low signal-to-noise. Our magnitude and search criteria were based on the IRAF procedure apphot. The He II pixel location of each S source was chosen as the starting position for the brightest pixel search in the B and V images. The search radius was restricted to within 2 pixels of the He II location. The apertures for the stellar sources were 2 pixels because most of the He II pixels were concentrated in the crowded stellar region of the galaxy. Background counts were taken from an annulus that was one pixel outside the aperture. The number of pixel centers found within the 2 pixel aperture were counted and the total number of counts was computed. The He II flux was found by using the same IRAF program and parameters, and the total counts were converted to an absolute flux. The same procedure was used in the case of the C sources, except the aperture was set to a larger value. Looking at the F555W images, one visually inspected the maximum radius of each cluster, and the average radius was 5 pixels. SSC A was set at 10 pixels, and the background annulus was appropriately expanded. Only the He II flux was found for the U sources, although the procedures were the same as for the S sources.

The absolute emission line fluxes and BV magnitudes presented in Tables 2 through 4 have not been corrected for reddening. [Devost, Roy, & Drissen(1997)] infer line-of-sight extinction due to the Galaxy in the direction of NGC 1569 is E(B-V) = 0.50 (A$_V$ = 1.6 mag), and the mean intrinsic extinction of NGC 1569 is E(B-V) = 0.20 (A$_V$ = 0.6 mag). We will adopt these values for this study. Using these numbers along with the extinction curve from Seaton (1979), we found the f($\lambda$) value of 0.042 for He II and the reddening correction of 11.8. To correct the V and B magnitudes, a value of 28.9 mag must be subtracted to produce an absolute magnitude. This number was computed using the equation

$$(V-M_{V})_{\rm NGC 1569} = 5\log({\rm D})-5+{\rm A}_{V}$$ (1)

where D is the adopted distance to NGC 1569, A$_{V}$ is the visual extinction and $V$ is the WFPC2 F555W magnitude measured. Please note that the magnitudes are F555W and F439W magnitudes based on the STScI ``Vega" system and are not true Johnson B and V magnitudes.

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