Virgo Cluster Early-Type Dwarf Galaxies with the Sloan Digital Sky Survey. I. On the Possible Disk Nature of Bright Early-Type Dwarfs

We present a systematic search for disk features in 476 Virgo Cluster early-type dwarf (dE) galaxies. This is the first such study of an almost-complete, statistically significant dE sample, which includes all certain or possible cluster members with m_B<=18 that are covered by the optical imaging data of the Sloan Digital Sky Survey Data Release 4. Disk features (spiral arms, edge-on disks, or bars) were identified by applying unsharp masks to a combined image from three bands (g, r, and i), as well as by subtracting the axisymmetric light distribution of each galaxy from that image. Fourteen objects are unambiguous identifications of disks, 10 objects show ``probable disk'' features, and 17 objects show ``possible disk'' features. The number fraction of these galaxies, for which we introduce the term ``dEdi,'' reaches more than 50% at the bright end of the dE population and decreases to less than 5% for magnitudes m_B>16. Although part of this observed decline might be due to the lower signal-to-noise ratio at fainter magnitudes, we show that it cannot be caused solely by the limitations of our detection method. The luminosity function of our full dE sample can be explained by a superposition of dEdis and ordinary dEs, strongly suggesting that dEdis are a distinct type of galaxy. This is supported by the projected spatial distribution: dEdis show basically no clustering and roughly follow the spatial distribution of spirals and irregulars, whereas ordinary dEs are distributed similarly to the strongly clustered E/S0 galaxies. While the flattening distribution of ordinary dEs is typical for spheroidal objects, the distribution of dEdis is significantly different and agrees with their being flat oblate objects. We therefore conclude that the dEdis are not spheroidal galaxies that just have an embedded disk component but are instead a population of genuine disk galaxies. Several dEdis display well-defined spiral arms with grand-design features that clearly differ from the flocculent, open arms typical for late-type spirals that have frequently been proposed as progenitors of dEs. This raises the question of what process is able to create such spiral arms-with pitch angles like those of Sab/Sb galaxies-in bulgeless dwarf galaxies.