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 ~ { } \AA /pixel ) long-slit spectroscopy ( KPNO 4-Meter ) of NGC4826 in the 5300 – 9100 ~ { } \AA spectral range , with a slit of 4.4 arcmin length , encompassing the galaxy ’ s bulge size , positioned 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 radiative 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 photons of the local radiation field .
In the innermost part of the Evil Eye , the ERE band extends from about 5700 to 9100 ~ { } \AA , with an estimated peak intensity of \sim 3.7 \times 10 ^ { -6 } ~ { } ergs~ { } s ^ { -1 } ~ { } \AA ^ { -1 } ~ { } cm ^ { -2 } ~ { } sr ^ { -1 } near 8300 ~ { } \AA 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 , Q ( H ^ { 0 } ) _ { TOT } , and the characteristic effective temperature , T _ { eff } , of the illuminating OB-stars increase . Q ( H ^ { 0 } ) _ { TOT } and T _ { eff } are estimated from the extinction-corrected H \alpha~ { } ( \lambda = 6563 ~ { } \AA ) line intensity and line intensity ratios [ NII ] ( \lambda 6583 ) / H \alpha and [ SII ] ( \lambda \lambda 6716 + 6731 ) / H \alpha , respectively , and are consistent with model and observed values typical of OB-associations .
Unfortunately , we do not have data shortward of 5300 ~ { } \AA , so that the census of the UV/optical flux is incomplete .
The complex radial variation of the ERE peak intensity and peak wavelength , of I ( ERE ) and I ( ERE ) / I ( sca ) with optical depth and strength of the UV/optical radiation field is reproduced in a consistent way through the theoretical interpretation of the photophysics of the ERE carrier by Smith & Witt , which attributes a key-role to the experimentally established recognition that photoionization quenches the luminescence of nanoparticles .
When examined within the context of ERE observations in the diffuse ISM of our Galaxy and in a variety of other dusty environments such as reflection nebulae , planetary nebulae and the Orion Nebula , we conclude that the ERE photon conversion efficiency in NGC4826 is as high as found elsewhere , but that the size of the actively luminescing nanoparticles in NGC4826 is about twice as large as those thought to exist in the diffuse ISM of our Galaxy .