On the Origin of Nebular He II in NGC 1569

The presence of nebular He II outside of planetary nebulae is rare because few thermal sources produce enough photons with energies $>$ 54 eV. [Garnett et al.(1991)] stated three alternative ionizing mechanisms which could account for this nebular He II. Hot stellar ionizing continua is a definite possibility for this galaxy. The starburst which produced the WR stars in SSC A occurred only 2-3 Myr ago [González-Delgado et al.(1997)] or 4-5 Myr ago [Hunter et al.(2000)]. [Schaerer & Vacca(1998)] modeled strong nebular He II due to massive stars in the early starburst phases. These models have been subsequently updated [Schaerer(2000)]. Using the I(He II 4686)/I(H$\beta$) ratios from KS97 (their Table 2; values of 8$\times$10$^{-3}$ and 1.2$\times$10$^{-2}$) and Figure 8 from [Schaerer(2000)], we conclude that the burst age must be 3-4 Myr old. This is in fair agreement with the younger age of the two-stage burst proposed by [González-Delgado et al.(1997)] and the recently calculated ages of SSC A and surrounding clusters [Hunter et al.(2000)].

The second mechanism for the formation of He II is shock excitation such that the strength of the nebular He II depends on the velocity of the shock: ( $V_{\rm shock} \approx 120$ km s$^{-1}$; Garnett et al. 1991). If shocks were present, bright [O III] emission would be at the same location which may have been seen for C3. At this time, based on our limited imagery, shocks cannot be considered a viable source. Photoionization by X-rays is the final, alternative explanation for nebular He II. [Pakull & Angebault(1986)] found that nebular He II was produced around the black hole candidate binary LMC X-1. [Della Ceca et al.(1996)] state that NGC 1569's hard X-ray spectra is due to the two bright X-ray point sources located in the ROSAT high resolution imager data. These sources could be interpreted as low-mass X-ray binaries or young supernova remnants. Future X-ray images would resolve whether these point sources are X-ray binaries and coincident with some of the He II sources stated here. Chandra observations of this object were taken May 2000, and we will have to wait and see whether this hypothesis is valid. Preliminary findings of the Chandra data do not show X-ray binaries coincident with any of our He II sources [Kobulnicky(2000)].