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D. Pierini, A. Majeed (UT), T.A. Boroson (NOAO), A.N. Witt (UT)
The Evil Eye Galaxy (NGC 4826) is a nearby galaxy with an asymmetrically placed, strongly absorbing dust lane across its prominent bulge, associated to an active star formation (SF) region. We obtained accurate low--resolution (\rm 4.2~Å/pixel) spectroscopy (KPNO 4-m) of NGC 4826 in the wavelength range 5300--9100\rm ~Å with a slit of \rm 4.4\prime length, positioned across the nucleus of the galaxy and encompassing its bulge size. We were able to study the wavelength dependent effects of absorption and scattering by the dust by comparing the stellar SEDs at corresponding positions on the bulge, symmetrically placed with respect to the nucleus, under the assumption that the intrinsic (i.e. unobscured by the dust lane) ISRF is radially symmetric, except for the ongoing SF region.
We report on the detection of strong extended red emission (ERE) from the dust lane of NGC 4826 within a radial distance of about \rm 15\prime \prime from its nucleus, adjacent to the active SF region. At the nucleus, the ERE band extends from about \rm 5800~Å to \rm 9100~Å, with peak near \rm 8300~Å, and the ERE-to-scattered light integrated intensity ratio is about 0.7. At farther distances, approaching the ongoing SF region, the ERE band and peak shift to longer wavelengths, while the integrated ERE intensity diminishes and, finally, vanishes there. The \rm H \alpha line intensity and the index [NII]\rm \lambda 6583/\rm H \alpha constrain the Lyman continuum photon rate and the effective temperatures of the OB association stars. The ERE-to-scattered light ratio decreases as well but shows a secondary maximum where the opacity of the dust lane peaks.
We interpret the ERE nature as photoluminescence by nanometer--sized clusters, illuminated by UV/visible photons of the local radiation field. When examined within the context of ERE observations in the diffuse ISM of our Galaxy and in a variety of other dusty environments, we conclude that the ERE photon conversion efficiency in NGC 4826 is as high as found elsewhere, but that the characteristic size of the nanoparticles there is about twice as large as that inferred in the Galactic diffuse ISM.
The author(s) of this abstract have provided an email address for comments about the abstract: pierini@ancona.astro.utoledo.edu