Early-type Dwarf Galaxies Research Articles

STRUCTURAL PROPERTIES OF ULTRA-COMPACT DWARF GALAXIES IN THE FORNAX AND VIRGO CLUSTERS

We present a detailed analysis of two-band HST/ACS imaging of 21 ultra-compact dwarf (UCD) galaxies in the Virgo and Fornax Clusters. The aim of this work is to test two formation hypotheses for UCDs--whether they are bright globular clusters (GCs) or threshed'' early-type dwarf galaxies--by comparison of UCD structural parameters and colors with GCs and galaxy nuclei. We find that the UCD surface brightness profiles can be described by a range of models and that the luminous UCDs can not be described by standard King models with tidal cutoffs as they have extended outer halos. This is not expected from traditional King models of GCs, but is consistent with recent results for massive GCs. The total luminosities, colors and sizes of the UCDs are consistent with them being either luminous GCs or threshed nuclei of both early-type and late-type galaxies (not just early-type dwarfs). For the most luminous UCDs we estimate color gradients over a limited range of radius. These are systematically positive in the sense of getting redder outwards: mean Delta(F606W-F814W)=0.14 mag per 100 pc with rms=0.06 mag per 100 pc. The positive gradients found in the bright UCDs are consistent with them being either bright GCs or threshed early-type dwarf galaxies (except VUCD3). In contrast to the above results we find a very significant difference in the sizes of UCDs and early-type galaxy nuclei: the effective radii of UCDs are 2.2 times larger than those of early-type galaxy nuclei at the same luminosity. This result suggests an important test can be made of the threshing hypothesis by simulating the process and predicting what size increase is expected.

The early-type dwarf galaxy population of the Hydra I cluster

Aims. We analyse the properties of the early-type dwarf galaxy population ( mag) in the Hydra I cluster. We investigate the galaxy luminosity function (LF), the colour-magnitude relation (CMR), and the magnitude-surface brightness relation down to MV ~ -10 mag. Another goal of this study is to find candidates for ultra-compact dwarf galaxies (UCDs) in Hydra I.

Photometric properties of the Local Volume dwarf galaxies

Abstract We present surface photometry and metallicity measurements for 104 nearby dwarf galaxies imaged with the Advanced Camera for Surveys and Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope. In addition, we carried out photometry for 26 galaxies of the sample and for Sextans B on images of the Sloan Digital Sky Survey. Our sample comprises dwarf spheroidal, irregular and transition type galaxies located within ∼10 Mpc in the field and in nearby groups: M81, Centaurus A, Sculptor and Canes Venatici I cloud. It is found that the early-type galaxies have on average higher metallicity at a given luminosity in comparison to the late-type objects. Dwarf galaxies with MB≥−12 to −13 mag deviate towards larger scalelengths from the scalelength–luminosity relation common for spiral galaxies, h∝L0.5B. The following correlations between fundamental parameters of the galaxies are consistent with expectations if there is pronounced gas loss through galactic winds: (1) between the luminosity of early-type dwarf galaxies and the mean metallicity of constituent red giant branch stars, Z∼L0.4, (2) between mean surface brightness within the 25 mag arcsec−2 isophote and the corresponding absolute magnitude in the V and I bands, SB 25∼ 0.3M25 and (3) between the central surface brightness (or effective surface brightness) and integrated absolute magnitude of galaxies in the V and I bands, SB0∼ 0.5ML, SB e∼ 0.5 Me. The knowledge of basic photometric parameters for a large sample of dwarf galaxies is essential for a better understanding of their evolution.

VIRGO CLUSTER EARLY-TYPE DWARF GALAXIES WITH THE SLOAN DIGITAL SKY SURVEY. IV. THE COLOR-MAGNITUDE RELATION

We present an analysis of the optical colors of 413 Virgo cluster early-type dwarf galaxies (dEs), based on Sloan Digital Sky Survey imaging data. Our study comprises (1) a comparison of the color-magnitude relation (CMR) of the different dE subclasses that we identified in Paper III of this series, (2) a comparison of the shape of the CMR in low and high-density regions, (3) an analysis of the scatter of the CMR, and (4) an interpretation of the observed colors with ages and metallicities from population synthesis models. We find that the CMRs of nucleated (dE(N)) and non-nucleated dEs (dE(nN)) are significantly different from each other, with similar colors at fainter magnitudes (r > 17 mag), but increasingly redder colors of the dE(N)s at brighter magnitudes. We interpret this with older ages and/or higher metallicities of the brighter dE(N)s. The dEs with disk features have similar colors as the dE(N)s and seem to be only slightly younger and/or less metal-rich on average. Furthermore, we find a small but significant dependence of the CMR on local projected galaxy number density, consistently seen in all of u-r, g-r, and g-i, and weakly i-z. We deduce that a significant intrinsic color scatter of the CMR is present, even when allowing for a distance spread of our galaxies. No increase of the CMR scatter at fainter magnitudes is observed down to r = 17 mag (Mr = -14 mag). The color residuals, i.e., the offsets of the data points from the linear fit to the CMR, are clearly correlated with each other in all colors for the dE(N)s and for the full dE sample. We conclude that there must be at least two different formation channels for early-type dwarfs in order to explain the heterogeneity of this class of galaxy. (Abridged)

Radio Continuum and Star Formation in CO-rich Early-Type Galaxies

In this paper we present new high-resolution Very Large Array 1.4 GHz radio continuum observations of five FIR-bright CO-rich early-type galaxies and two dwarf early-type galaxies. The position on the radio-FIR correlation combined with striking agreements in morphology between high-resolution CO and radio maps show that the radio continuum is associated with star formation in at least four of the eight galaxies. The average star formation rate for the sample galaxies detected in radio is ∼2 M⊙ yr-1. There is no evidence of a luminous active galactic nucleus in any of our sample galaxies. We estimate Toomre Q-values and find that the gas disks may well be gravitationally unstable, consistent with the above evidence for star formation activity. The radio continuum emission thus corroborates other recent suggestions that star formation in early-type galaxies may not be uncommon.

A Flat Faint End of the Fornax Cluster Galaxy Luminosity Function

AbstractWe analyse the photometric properties of the early-type Fornax cluster dwarf galaxy population (MV > −17 mag), based on a wide field imaging study of the central cluster area in V and I band-passes with IMACS/Magellan at Las Campanas Observatory. We create a fiducial sample of ~ 100 Fornax cluster dwarf ellipticals (dEs) with −16.6 < MV < −8.8 mag in the following three steps: (1) To verify cluster membership, we measured I-band surface brightness fluctuations (SBF) distances to candidate dEs known from previous surveys; (2) We re-assessed morphological classifications for those candidate dEs that are too faint for SBF detection; and (3) We searched for new candidate dEs in the size-luminosity regime close to the resolution limit of previous surveys. The resulting fiducial dE sample follows a well-defined surface brightness – magnitude relation, showing that Fornax dEs are about 40% larger than Local Group dEs. The sample also defines a colour-magnitude relation similar to that of Local Group dEs. The early-type dwarf galaxy luminosity function in Fornax has a very flat faint end slope α ≃ −1.1 ± 0.1. We compare the number of dwarfs per unit mass with those in other environments and find that the Fornax cluster fits well into a general trend of a lack of high-mass dwarfs in more massive environments.

Virgo Cluster Early‐Type Dwarf Galaxies with the Sloan Digital Sky Survey. III. Subpopulations: Distributions, Shapes, Origins

From a quantitative analysis of 413 Virgo Cluster early-type dwarf galaxies (dEs) with SDSS 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 for 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.

The many faces of early-type dwarf galaxies

We present a study of 413 early-type dwarf (dE) galaxies in the Virgo Cluster with SDSS multicolour imaging. Several dE subclasses show systematic differences in shape, clustering properties, and stellar content. We find different colour-magnitude relations for nucleated and non-nucleated dEs, as well as for dE samples populating regions of different densities.

The early-type dwarf galaxy population of the Fornax cluster

AIMS: We analyse the photometric properties of the early-type Fornax cluster dwarf galaxy population, based on a wide field imaging study of the central cluster area in V and I. We used the instrument/telescope combination IMACS/Magellan at Las Campanas Observatory, providing much larger light collecting area and better image resolution than previous surveys. METHODS: We create a fiducial sample of Fornax cluster dwarf ellipticals (dEs) in the following three steps: (1) To verify cluster membership, we measured I-band surface brightness fluctuation (SBF) distances to candidate dEs; (2) We re-assessed morphological classifications for candidate dEs too faint for SBF detection; and (3) We searched for new candidate dEs in the size-luminosity regime close to the resolution limit of previous surveys. RESULTS: (1) We confirm cluster membership for 28 candidate dEs in the range -16.6<M_V<-10.1 mag by means of SBF distances. We find no SBF background galaxy. (2) Of 51 further candidate dEs in the range -13.2<M_V<-8.6 mag, 2/3 are confirmed as probable cluster members by morphological re-assessment, while 1/3 are re-classified as probable background objects. (3) We find 12 new dE candidates in the range -12.3<M_V<-8.8 mag. The surface brightness-magnitude relation defined by the resulting fiducial dE sample shows that Fornax dEs are about 40% larger than Local Group dEs. The Fornax dE sample furthermore defines a colour-magnitude relation that appears slightly shallower than that of Local Group dEs. The early-type dwarf galaxy luminosity function in Fornax has a faint end slope alpha = -1.1 +/- 0.1. We discuss these findings in the context of structure formation theories. (ABRIDGED)

The Binary Nucleus in VCC 128: A Candidate Supermassive Black Hole in a Dwarf Elliptical Galaxy

{\it Hubble Space Telescope} (\hst) Wide Field Planetary Camera 2 ({\it WFPC2}) images of the Virgo Cluster dwarf elliptical galaxy VCC 128 reveal an apparently double nucleus. The two components, which are separated by $\sim 32$ pc in projection, have the same magnitude and color. We present a spectrum of this double nucleus and show that it is inconsistent with one or both components being emission-line background objects or foreground stars. The most likely interpretation is that, as suggested by \citet{lauer_etal_96} for the double nucleus in NGC 4486B, we are seeing a nuclear disk surrounding a supermassive black hole. This is only the second time an early-type dwarf (dE/dSph) galaxy has been suggested to host a SMBH.

The Last Stages of Star Formation in dEs?

AbstractA significant fraction of Virgo cluster early-type dwarf galaxies have blue central colours caused by recent or ongoing star formation. A spectral analysis shows that even in their centers, stellar mass is dominated by an old population. These galaxies are an unrelaxed cluster population that possibly formed from morphological transformation of late-type galaxies.

The Early-Type Dwarf Galaxy Population of the Fornax Cluster

AbstractAIMS: We analyse the photometric properties of the early-type Fornax cluster dwarf galaxy population (MV&gt; − 17 mag), based on a wide field imaging study of the central cluster area in V and I bandpasses. We used the instrument/telescope combination IMACS/Magellan at Las Campanas Observatory, providing much larger light collecting area and better image resolution than previous wide field imaging surveys.METHODS: We create a fiducial sample of Fornax cluster dwarf ellipticals (dEs) in the following three steps: (1) To verify cluster membership, we measured I-band surface brightness fluctuations (SBF) distances to candidate dEs known from previous surveys; (2) We re-assessed morphological classifications for those candidate dEs that are too faint for SBF detection; and (3) We searched for new candidate dEs in the size-luminosity regime close to the resolution limit of previous surveys.RESULTS: (1) We confirm cluster membership for 28 candidate dEs in the range −16.6 &lt; MV &lt; −10.1 mag by means of SBF measurement. We find no SBF background galaxy. (2) Of 51 further candidate dEs in the range −13.2 &lt; MV &lt; −8.6 mag, 2/3 are confirmed as probable cluster members by morphological re-assessment, while 1/3 are re-classified as probable background objects. (3) We find 12 new dE candidates in the range −12.3 &lt; MV &lt; −8.8 mag, two of which are directly confirmed via SBF measurement. The resulting fiducial dE sample follows a well-defined surface brightness – magnitude relation, showing that Fornax dEs are about 40% larger than Local Group dEs. The sample also defines a colour-magnitude relation that appears slightly shallower than that of Local Group dEs. The early-type dwarf galaxy luminosity function in Fornax has a very flat faint end slope α ≃ −1.1 ± 0.1. We discuss these findings in the context of structure formation theories.CONCLUSIONS: The SBF method is a very powerful tool to help constrain the faint end of the galaxy luminosity function in nearby galaxy clusters. For the Fornax cluster, morphological cluster memberships – if performed at sufficient resolution – are very reliable.This contribution is based on Mieske et al. (2006).

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 disks in 476 Virgo cluster early-type dwarf (dE) galaxies. Disk features (spiral arms, edge-on disks, or bars) were identified by applying unsharp masks to a combined image from three bands (g, r, i), as well as by subtracting the axisymmetric light distribution of each galaxy from that image. 14 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 B>16. 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 early-type dwarfs. 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. (Abridged)

Neutral Hydrogen Clouds Near Early-Type Dwarf Galaxies of the Local Group

Parkes neutral hydrogen 21 cm line (H I) observations of the surroundings of nine early-type Local Group dwarfs are presented. We detected numerous H I clouds in the general direction of those dwarfs, and these clouds are often offset from the optical center of the galaxies. Although all the observed dwarfs, except Antlia, occupy phase-space regions where the high-velocity cloud (HVC) density is well above average, the measured offsets are smaller than one would expect from a fully random cloud distribution. Possible association is detected for 11 of the 16 investigated clouds, while for two galaxies, Sextans and Leo I, no H I was detected. The galaxies in which H I clouds were found not to coincide with the optical yet have a significant probability of being associated are the Sculptor dwarf, Tucana, LGS 3, Cetus, and Fornax. If the clouds are indeed associated, these galaxies have H I masses of M = 2 × 105, 2 × 106, 7 × 105, 7 × 105, and 1 × 105 M⊙, respectively. However, neither ram pressure nor tidal stripping can easily explain the offsets. In some cases, large offsets are found where ram pressure should be the least effective.

Globular clusters in early-type dwarf galaxies

One of the main aims of modern astrophysics is to understand the role of dwarf galaxies and their globular cluster (GC) systems as building blocks of massive galaxies. Dwarf spheroidal (dSph) and dwarf elliptical (dE) galaxies reveal unusual properties. Recent studies of dSph galaxies in the Local Group and other nearby groups within 10 Mpc show that these galaxies are composed mainly of old stars, contribute the faintest end of the galaxy luminosity function, are repositories of dark matter and often contain bright globular clusters near their centres. Studying the abundances and colour–magnitude diagrams of GCs in nearby dSph and dE galaxies can shed light on their evolutionary histories. GCs in the early-type satellites of the Andromeda galaxy, NGC 147, NGC 185 and NGC 205, are close enough to be resolved into single stars by the Hubble Space Telescope (HST). We discuss the results of our high-quality HST–WFPC2 photometry of stars in these GCs and the measurements of ages, metallicities and [α/Fe] rati...

Optical/near-infrared colours of early-type galaxies and constraints on their star formation histories

We introduce and discuss the properties of a theoretical (B−K)(J−K) integrated colour diagram for single-age, single-metallicity stellar populations. We show how this combination of integrated colours is able to largely disentangle the well-known age–metallicity degeneracy when the age of the population is greater than ∼300 Myr, and thus provides valuable estimates of both age and metallicity of unresolved stellar systems. We discuss in detail the effect on this colour–colour diagram of α-enhanced metal abundance ratios (typical of the oldest populations in the Galaxy), the presence of blue horizontal branch stars unaccounted for in the theoretical calibration and of statistical colour fluctuations in low-mass stellar systems. In the case of populations with multiple stellar generations, the luminosity-weighted mean age obtained from this diagram is shown to be heavily biased towards the youngest stellar components. We then apply this method to several data sets for which optical and near-infrared photometry are available in the literature. We find that Large Magellanic Cloud and M31 clusters have colours which are consistent with the predictions of the models, but these do not provide a sensitive test due to the fluctuations which are predicted by our modelling of the Poisson statistics in such low-mass systems. For the two Local Group dwarf galaxies NGC 185 and 6822, the mean ages derived from the integrated colours are consistent with the star formation histories inferred independently from photometric observations of their resolved stellar populations. The methods developed here are applied to samples of nearby early-type galaxies with high-quality aperture photometry in the literature. A sample of bright field and Virgo cluster elliptical galaxies is found to exhibit a range of luminosity-weighted mean ages from 3 to 14 Gyr, with a mean of ∼8 Gyr, independent of environment, and mean metallicities at or just above the solar value. Colour gradients are found in all of the galaxies studied, in the sense that central regions are redder. Apart from two radio galaxies, where the extreme central colours are clearly driven by the active galactic nucleus, and one galaxy which also shows a radial age gradient, these colour changes appear consistent with metallicity changes at a constant mean age. Finally, aperture data for five Virgo early-type dwarf galaxies show that these galaxies appear to be shifted to lower mean metallicities and lower mean ages (range 1–6 Gyr) than their higher luminosity counterparts.

Photometric scaling relations for bulges of galaxies

We describe the photometric parameters of the bulges of galaxies of different Hubble types including ellipticals, lenticulars, early and late type spirals and early type dwarf galaxies. Analyzing the distributions of various photometric parameters and two- and three-dimensional correlations between them, we find that there is a difference in the correlations exhibited by bright ($M_{\rm K} < -22$) and faint bulges, irrespective of their Hubble type. Importantly, the bright bulges, which include typically E/S0 galaxies and bulges of early type spirals, are tightly distributed around a common photometric plane (PP), while their fainter counterparts, mainly bulges of late type spirals and dwarf galaxies show significant deviation from the planar distribution. We show that the specific entropy, determined from the bulge structural parameters, systematically increases as we move from late to early Hubble types. We interpret this as evidence for hierarchical merging and passive evolution scenarios for bright and faint bulges respectively.

Detection of Neutral Hydrogen in Early-Type Dwarf Galaxies of the Sculptor Group

We present results of deep 21 cm neutral hydrogen (H I) line observations of five early- and mixed-type dwarf galaxies in the nearby Sculptor group using the Australia Telescope National Facility 64 m Parkes Radio Telescope. Four of these objects, ESO 294-G010, 410-G005, 540-G030, and 540-G032, were detected in H I with neutral hydrogen masses in the range × 105 M⊙ (M/LB = 0.08, 0.13, 0.16, and 0.18 M⊙ L, respectively). These H I masses are consistent with the gas mass expected from stellar outflows over a large period of time. Higher spatial resolution H I data from the Australia Telescope Compact Array interferometer were further analyzed to measure more accurate positions and the distribution of the H I gas. In the cases of the dwarfs ESO 294-G010 and ESO 540-G030, we find significant offsets of 290 and 460 pc, respectively, between the position of the H I peak flux and the center of the stellar component. These offsets are likely to have internal causes such as the winds from star-forming regions. The fifth object, the spatially isolated dwarf elliptical galaxy Scl-dE1, remains undetected at our 3 σ limit of 22.5 mJy km s-1 and thus must contain less than 105 M⊙ of neutral hydrogen. This leaves Scl-dE1 as the only Sculptor group galaxy known in which no interstellar medium has been found to date. The object joins a list of similar systems, including the Local Group dwarfs Tucana and Cetus, that do not fit into the global picture of the morphology-density relation in which gas-rich dwarf irregular galaxies are in relative isolation and gas-deficient dwarf elliptical galaxies are satellites of more luminous galaxies.

New distances of unresolved dwarf elliptical galaxies in the vicinity of the Local Group

We present Surface Brightness Fluctuation distances of nine early-type dwarf galaxies and the SO galaxy NGC 4150 in the Local Volume based on deep B- and R-band CCD images obtained with the 2.56 m Nordic Optical Telescope. Typically, six stellar fields at various galactocentric distances have been chosen for each galaxy as appropriately free of foreground stars and other contaminants, and Fourier analysed to determine the distances, which are found to lie in the range of 3 to 16 Mpc. The SBF method is thus demonstrated to efficiently measure distances from the ground with mid-aperture telescopes for galaxies for which only the tip of the red giant branch method in combination with the Hubble Space Telescope has been available until now. We obtained the following distance moduli: 28.11 ± 0.15 mag (or 4.2 ± 0.3 Mpc) for UGC 1703, 27.61 ± 0.17 mag for 3.3 ± 0.3 Mpc) for KDG 61, 29.00 ± 0.27 mag (or 6.3 ± 0.8 Mpc) for UGCA 200, 27.74 ± 0.18 mag (or 3.5 ± 0.3 Mpc) for UGC 5442, 30.22 ± 0.17 mag (or 11.1 ± 0.9 Mpc) for UGC 5944, 30.79 ± 0.11 mag (or 14.4 ± 0.7 Mpc) for NGC 4150, 31.02 ± 0.25 mag (or 16.0 ± 1.9 Mpc) for BTS 128, 29.27 ± 0.16 mag (or 7.1 ± 0.6 Mpc) for UGC 7639, 30.19 ± 0.23 mag (or 10.9 ± 1.2 Mpc) for UGC 8799 with an alternative distance of 30.61 ± 0.26 mag (or 13.2 ± 1.7 Mpc), and 29.60 ± 0.20 mag (or 8.3 ± 0.8 Mpc) for UGC 8882.

The Advanced Camera for Surveys Virgo Cluster Survey. V. Surface Brightness Fluctuation Calibration for Giant and Dwarf Early‐Type Galaxies

As part of the Advanced Camera for Surveys (ACS) Virgo Cluster Survey, we have measured surface brightness fluctuations (SBFs) in a sample of 100 early-type Virgo galaxies. Distances derived from these measurements are needed to explore the three-dimensional structure of the Virgo Cluster, study the intrinsic parameters of globular clusters associated with the program galaxies, and compare with the galaxy distances derived from globular cluster luminosity functions. Our SBF measurements have been performed in the F850LP bandpass of the Wide Field Channel of the ACS on the Hubble Space Telescope. These are the first measurements of this kind, and we present the first SBF calibration for this bandpass. The measured fluctuations depend on galaxy stellar population variations, which we quantify by galaxy color (g475 - z850)0, where g475 and z850 are the galaxy magnitudes in the F475W and F850LP ACS filters, respectively. We derive the calibration for the absolute SBF magnitude 850 = -2.06 ± 0.04 + (2.0 ± 0.2)[(g475 - z850)0 - 1.3] in the range 1.3 < (g475 - z850)0 ≤ 1.6, and 850 = -2.06 ± 0.04 + (0.9 ± 0.2)[(g475 - z850)0 - 1.3] in the range 1.0 ≤ (g475 - z850)0 ≤ 1.3. The quoted zero-point uncertainty here includes all sources of internal error; there is an additional systematic uncertainty of ~0.15 mag, due to the uncertainty in the distance scale calibration. Physically, the two different color regimes correspond to different galaxy types: giant ellipticals and S0s at the red end, and early-type dwarfs at the blue end. For the first time in SBF studies, we are able to provide a firm empirical calibration of SBF in early-type dwarf galaxies. Our results agree with stellar population model predictions from Bruzual & Charlot in the range 1.3 < (g475 - z850)0 ≤ 1.6, while our empirical slope is somewhat steeper than the theoretical prediction in the range 0.9 ≤ (g475 - z850)0 ≤ 1.3.