Extended Red Emission in the “ Evil Eye ” Galaxy ( NGC 4826 )

NGC4826 (M64) is a nearby Sab galaxy with an outstanding, absorbing dust lane (called the " Evil Eye ") asymmetrically placed across its prominent bulge. In addition, its central region is associated with several regions of ongoing star formation activity. We obtained accurate low-resolution (4.3 ˚ A/pixel) long-slit spectroscopy (KPNO 4-Meter) of NGC4826 in the 5300 – 9100Å spectral range, with a slit of 4.4 arcmin length, encompassing the galaxy's bulge size, positioned – 2 – across its nucleus. The wavelength-dependent effects of absorption and scattering by the dust in the Evil Eye are evident when comparing the observed stellar spectral energy distributions (SEDs) of pairs of positions symmetrically located with respect to the nucleus, one on the dust lane side and one on the symmetrically opposite side of the bulge, under the assumption that the intrinsic (i.e. unobscured) radiation field is to first order axi-symmetric. We analyzed the SED ratios for a given number of pairs of positions through the multiple-scattering ra-diative transfer model of Witt & Gordon. As a main result, we discovered strong residual Extended Red Emission (ERE) from a region of the Evil Eye within a projected distance of about 13 arcsec from the nucleus, adjacent to a broad, bright HII region, intercepted by the spectrograph slit. ERE is an established phenomenon well-covered in the literature and interpreted as originating from photoluminescence by nanometer-sized clusters, illuminated by UV/optical pho-tons of the local radiation field. In the innermost part of the Evil Eye, the ERE band extends from about 5700 to 9100Å, with an estimated peak intensity of ∼ 3.7 × 10 −6 ergs s −1Å −1 cm −2 sr −1 near 8300Å and with an ERE-to-scattered light band-integrated intensity ratio, I(ERE)/I(sca), of about 0.7. At farther distances, approaching the broad, bright HII region, the ERE band and peak intensity shift toward longer wavelengths, while the ERE band-integrated intensity , I(ERE), diminishes and, eventually, vanishes at the inner edge of this HII region. The radial variation of I(ERE) and I(ERE)/I(sca) does not match that of the optical depth of the model derived for the dust lane. By contrast, the radial variation of I(ERE), I(ERE)/I(sca) and of the ERE spectral domain seems to depend strongly on the strength and hardness of the illuminating radiation field. In fact, I(ERE) and I(ERE)/I(sca) diminish and the ERE band shifts toward longer wavelengths when both the total integrated Lyman continuum photon rate, …