From a quantitative analysis of 413 Virgo cluster early-type dwarf galaxies (dEs) with Sloan Digital Sky Survey imaging data, we find that the dE class can be divided into multiple subpopulations that differ significantly in their morphology and clustering properties. Three dE subclasses are shaped like thick disks and show no central clustering: (1) dEs with disk features like spiral arms or bars, (2) dEs with central star formation, and (3) ordinary, bright dEs that have no or only a weak nucleus. These populations probably formed from infalling progenitor galaxies. In contrast, ordinary nucleated dEs follow the picture of classical dwarf elliptical galaxies in that they are spheroidal objects and are centrally clustered like E and S0 galaxies, indicating that they have resided in the cluster since a long time, or were formed along with it. These results define a morphology-density relation within the dE class. We find that the difference in the clustering properties of nucleated dEs and dEs with no or only a weak nucleus is not caused by selection biases, as opposed to previously reported suggestions. The correlation between surface brightness and observed axial ratio favors oblate shapes for all subclasses, but our derivation of intrinsic axial ratios indicates the presence of at least some triaxiality. We discuss possible interrelations and formation mechanisms (ram-pressure stripping, tidally induced star formation, harassment) of these dE subpopulations.

Despite the common picture of an early-type dwarf (dE) as a quiescent galaxy with no star formation and little gas, we identify 23 dEs that have blue central colors caused by recent or ongoing star formation in our sample of 476 Virgo cluster dEs. In addition, 14 objects that were mostly classified as (candidate) BCDs have similar properties. Among the certain cluster members, the dEs with blue centers reach a fraction of more than 15% of the dE population at brighter (m B ≤ 16) magnitudes. A spectral analysis of the centers of 16 galaxies reveals in all cases an underlying old population that dominates the mass, with M old ≥ 90% for all but one object. Therefore the majority of these galaxies will appear like ordinary dEs within ∼ one Gigayear or less after the last episode of star formation. Their overall gas content is less than that of dwarf irregular galaxies, but higher than that of ordinary dEs. Their flattening distribution suggests the shape of a thick disk, similar to what has been found for dEs with disk features in Paper I of this series. Their projected spatial distribution shows no central clustering, and their distribution with projected local density follows that of irregular galaxies, indicative of an unrelaxed population. This is corroborated by their velocity distribution, which displays two side peaks characteristic of recent infall. We discuss possible formation mechanisms (ram-pressure stripping, tidally induced star formation, harassment) that might be able to explain both the disk shape and the central star formation of the dEs with blue centers.

We have started a new, deep multi-imaging survey of the Fornax cluster, dubbed the Fornax Deep Survey (FDS), at the VLT Survey Telescope (VST). In this paper we present the deep photometry inside two square degrees around the bright galaxy NGC1399 in the core of the cluster. We found that the core of the Fornax cluster is characterized by a very extended and diffuse envelope surrounding the luminous galaxy NGC1399: we map the surface brightness out to 33 arcmin (∼192 kpc) from the galaxy center and down to μ g ∼31 mag arcsec −2 in the g band. The deep photometry allows us to detect a faint stellar bridge in the intracluster region on the west side of NGC1399 and toward NGC1387. By analyzing the integrated colors of this feature, we argue that it could be due to the ongoing interaction between the two galaxies, where the outer envelope of NGC1387 on its east side is stripped away. By fitting the light profile, we found that there exists a physical break radius in the total light distribution at R=10 arcmin (∼58 kpc) that sets the transition region between the bright central galaxy and the outer exponential halo, and that the stellar halo contributes 60% of the total light of the galaxy (Section 3.5). We discuss the main implications of this work on the build-up of the stellar halo at the center of the Fornax cluster. By comparing with the numerical simulations of the stellar halo formation for the most massive bright cluster galaxies (i.e., M M 13 log 14, we find that the observed stellar halo mass fraction is consistent with a halo formed through the multiple accretion of progenitors with stellar mass in the range 10 8 -10 11 M e . This might suggest that the halo of NGC1399 has also gone through a major merging event. The absence of a significant number of luminous stellar streams and tidal tails out to 192 kpc suggests that the epoch of this strong interaction goes back to an early formation epoch. Therefore, different from the Virgo cluster, the extended stellar halo around NGC1399 is characterized by a more diffuse and well-mixed component, including the intracluster light.

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.

Context. The Fornax Deep Survey (FDS), an imaging survey in the u , g , r , and i -bands, has a supreme resolution and image depth compared to the previous spatially complete Fornax Cluster Catalog (FCC). Our new data allows us to study the galaxies down to r -band magnitude m r ≈ 21 mag (M r ≈ −10.5 mag), which opens a new parameter regime to investigate the evolution of dwarf galaxies in the cluster environment. After the Virgo cluster, Fornax is the second nearest galaxy cluster to us, and with its different mass and evolutionary state, it provides a valuable comparison that makes it possible to understand the various evolutionary effects on galaxies and galaxy clusters. These data provide an important legacy dataset to study the Fornax cluster. Aims. We aim to present the Fornax Deep Survey (FDS) dwarf galaxy catalog, focusing on explaining the data reduction and calibrations, assessing the quality of the data, and describing the methods used for defining the cluster memberships and first order morphological classifications for the catalog objects. We also describe the main scientific questions that will be addressed based on the catalog. This catalog will also be invaluable for future follow-up studies of the Fornax cluster dwarf galaxies. Methods. As a first step we used the SExtractor fine-tuned for dwarf galaxy detection, to find galaxies from the FDS data, covering a 26 deg 2 area of the main cluster up to its virial radius, and the area around the Fornax A substructure. We made 2D-decompositions of the identified galaxies using GALFIT, measure the aperture colors, and the basic morphological parameters like concentration and residual flux fraction. We used color-magnitude, luminosity-radius and luminosity-concentration relations to separate the cluster galaxies from the background galaxies. We then divided the cluster galaxies into early-and late-type galaxies according to their morphology and gave first order morphological classifications using a combination of visual and parametric classifications. Results. Our final catalog includes 14 095 galaxies. We classify 590 galaxies as being likely Fornax cluster galaxies, of which 564 are dwarfs (M r > −18.5 mag) consisting our Fornax dwarf catalog. Of the cluster dwarfs we classify 470 as early-types, and 94 as late-type galaxies. Our final catalog reaches its 50% completeness limit at magnitude M r = −10.5 mag and surface brightness µ e,r = 26 mag arcsec −2 , which is approximately three magnitudes deeper than the FCC. Based on previous works and comparison with a spectroscopically confirmed subsample, we estimate that our final Fornax dwarf galaxy catalog has 10% contamination from the background objects.The catalogs are only at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/620/A165Article published by EDP Sciences A165, page 1 of 31 A&A 620, A165 (2018)