Ursa Major Cluster Research Articles

The Cepheid Distance to the Narrow-line Seyfert 1 Galaxy NGC 4051

Abstract We derive a distance of D = 16.6 ± 0.3 Mpc (μ = 31.10 ± 0.04 mag) to the archetypal narrow-line Seyfert 1 galaxy NGC 4051 based on Cepheid period–luminosity relations and new Hubble Space Telescope multiband imaging. We identify 419 Cepheid candidates and estimate the distance at both optical and near-infrared wavelengths using subsamples of precisely photometered variables (123 and 47 in the optical and near-infrared subsamples, respectively). We compare our independent photometric procedures and distance-estimation methods to those used by the Supernovae, H0, for the Equation of State team and find agreement to 0.01 mag. The distance we obtain suggests an Eddington ratio of for NGC 4051, typical of narrow-line Seyfert 1 galaxies, unlike the seemingly odd value implied by previous distance estimates. We derive a peculiar velocity of −490 ± 34 km s−1 for NGC 4051, consistent with the overall motion of the Ursa Major Cluster in which it resides. We also revisit the energetics of the NGC 4051 nucleus, including its outflow and mass accretion rates.

Guessing the Riddle of a Black Hole

A static structure of matter, extremely compressed to the state of a Bose–Einstein condensate by its own gravitational field, is considered. Instead of the widely spread restriction detgik<0, I used a weaker condition of regularity: all invariants of gik are finite. This makes it possible to find regular static solutions to Einstein equations for a spherically symmetric distribution of matter with no restriction on total mass. In these regular static solutions, the metric component grr changes its sign twice: grr(r)=0 at r=rg and at r=rh>rg. The signature of the metric tensor is changed to (+,+,−,−) within the spherical layer rg<r<rh. Though the gravitation dominates at extremely high density, I assume that it does not violate the exchange interaction of elementary particles of the Standard Model. The found regular static solution to Einstein equations, having no limitation on mass, pretends to describe the state of a black hole to which the gravitational collapse leads. The features of a collapsed black hole, its internal composition depending on total mass and the relation with surrounding dark matter, are considered. An astrophysical application: The pressure balance at the interface between a black hole and dark matter determines the plateau velocity of a galaxy rotation curve as a function of the black hole mass. The plateau velocity is inversely proportional to the black hole mass. The speed of rotation of a star at the periphery of a galaxy is proportional to the square root of the black hole mass (direct attraction to the center) and inversely proportional to the mass of the same black hole (as the influence of dark matter). For a condensate of massive bosons in the Standard Model, the direct attraction to the black hole and the influence of dark matter are equal if the black hole mass is about M˜ ∼ 4.24×1037 g. In galaxies with black hole masses M≳M⊙=1.989×1033 g (like UMa: NGC 3726 and UMa: NGC 3769 of the Ursa Major cluster), the motion of stars is driven by dark matter. Their rotation curves should have a well-defined plateau. On the contrary, in galaxies with black hole masses M>>M˜ (like in our Milky Way with the black hole mass M=8.6×1039 g), the motion of stars is regulated by the black hole in the center. Dark matter does not play a significant role in our Milky Way Galaxy.

Mass of Spiral Galaxies by Means of a Maximum Disc Model

Maximum disc mass models for a set of spiral galaxies from the Ursa Major Cluster are presented. We construct the models using the Hunther method and the particular solutions are chosen in such away that the circular velocities are adjusted very accurately to the observed rotation curves of some specific spiral galaxies. Under the maximum disc hypothesis, we consider that the rotation curves of the analyzed galaxies can be modeled with only the contribution of the disc. This implies that it is not necessary to consider the contribution of the dark matter halo in the inner part of the spiral. In this way, the models reproduce the global behavior of the rotation curves in the great majority of galaxies. Producing good adjustments to calculate the total mass of these galaxies, and yielding values of the order of 1010M0. Based on the verticalstability criterion presented by Viera & Ramos-Caro(2016), we find that all the galaxies analyzed present a vertically stable behavior.On the other hand, from the analysis of the epicyclic frequency we find that al lthe models exhibit mainly a radial stable behaviour except at the edge of the disc.

MCG+08-22-082: A DOUBLE CORE AND BOXY APPEARANCE DWARF LENTICULAR GALAXY SUSPECTED TO BE A MERGER REMNANT

ABSTRACT We present a study on the dwarf lenticular galaxy MCG+08-22-082 (U141), located in the Ursa Major cluster, blue-centered, double-cored, and having a boxy appearance. Using publicly available data from the Sloan Digital Sky Survey (SDSS), we perform an analysis of the structural and stellar population properties of the galaxy and the cores. We find that the light profile of U141 follows an exponential law. U141 has a brightness of M r = − 16.01 mag, and an effective radius of R e = 1.7 kpc . The boxiness parameter a 4 / a is mostly between 0 and −0.05 in the inner parts, reaching an extreme of about −0.1. Double cores are seen at the center of U141; each of these cores has a stellar mass of ∼106 M ☉ and the separation between them is ∼300 pc. Optical spectroscopy of these cores shows prominent emission in Hα, suggesting ongoing star-forming activities. We interpret these morphological properties and speculate that U141 is a merger remnant of two disk galaxies. Thus, we might have discovered an intermediate stage of merging, providing possible evidence of double cores in the center of the galaxy.

The Ursa Major Cluster Redefined as a ‘Supergroup’

AbstractWe identify gravitationally bound structures in the Ursa Major region using positions, velocities and photometry from the Sloan Digital Sky Survey (SDSS DR7) and the Third Reference Catalogue of Bright Galaxies (RC3). A friends-of-friends algorithm is extensively tested on mock galaxy lightcones and then implemented on the real data to determine galaxy groups whose members are likely to be physically and dynamically associated with one another. We find several galaxy groups within the region that are likely bound to one another and in the process of merging. We classify 6 galaxy groups as the Ursa Major ‘supergroup’, which are likely to merge and form a poor cluster with a mass of ~ 8 × 1013 M⊙. Furthermore, the Ursa Major supergroup as a whole is likely bound to the Virgo cluster, which will eventually form an even larger system in the context of hierarchical structure formation. We investigate the evolutionary state of the galaxy groups in the Ursa Major region and conclude that these groups are in an early evolutionary state and the properties of their member galaxies are similar to those in the field.

The properties of early-type galaxies in the Ursa Major cluster

Using SDSS-DR7 and NASA/IPAC Extragalactic Database spectroscopic data, we identify 166 galaxies as members of the Ursa Major cluster with Mr < −13.5 mag. We morphological classify all galaxies by means of carefully inspecting g-, r-, i-band colour and monochromatic images. We show that the Ursa Major cluster is dominated by late-type galaxies, but also contains a significant number of early-type galaxies, particularly in the dwarf regime. We present further evidence for the existence of several subgroups in the cluster, consistent with previous findings. The early-type fraction is found to correlate with the mass of the subgroup. We also investigate environmental effects by comparing the properties of the Ursa Major early-type dwarf galaxies to those of the Virgo cluster. In contrast to the Virgo, the red sequence of the Ursa Major cluster is only sparsely populated in the optical and ultraviolet colour–magnitude relations. It also shows a statistically significant gap between −18 < Mr < −17 mag, i.e. the Ursa Major cluster lacks early-type dwarf galaxies at the bright end of their luminosity function. We discover that the majority of early-type dwarf galaxies in the Ursa Major cluster have blue cores with hints of recent or ongoing star formation. We suggest that gravitational tidal interactions can trigger central blue star-forming regions in early-type dwarfs. After that, star formation would only fade completely when the galaxies experience ram-pressure stripping or harassment, both of which are nearly absent in the Ursa Major cluster.

Ultraviolet Properties of Dwarf Galaxies in the Fornax and Ursa Major Clusters

We present ultraviolet (UV) properties of dwarf galaxies in the Fornax and Ursa Major clusters in comparison with the Virgo cluster. Most of the dwarf galaxies in the Fornax cluster show red UV colors which is consistent with that the Fornax is dynamically evolved cluster with large fraction of early-type galaxies. Meanwhile, majority of dwarf galaxies of Ursa Major cluster are late-type with blue UV colors, indicating prevalence of strong star formation activities in the low density environment.

Finite thin disc models of four galaxies in the Ursa Major cluster: NGC 3877, NGC 3917, NGC 3949 and NGC 4010

Finite thin disc models of four galaxies in the Ursa Major cluster are presented. The models are obtained by means of the Hunter method and the particular solutions are choosen in such a way that the circular velocities are adjusted very accurately to the observed rotation curves of some specific spiral galaxies. We present particular models for the four galaxies NGC3877, NGC3917, NGC3949 and NGC4010 with data taken from the recent paper by Verheijen & Sancici (2001). By integrating the corresponding surface mass densities, we obtain the total mass M of these four galaxies, all of them being of the order of 10^10 solar masses. These obtained values for M may be taken as a quite accurately estimative of the mass upper bound of these galaxies, since in the model was considered that all their mass was concentrated at the galactic disc. The models can be consider as a first approximation to the obtaining of quite realistic models of spiral galaxies

DISCOVERY OF A FAINT COMPANION TO ALCOR USING MMT/AO 5 μm IMAGING

We report the detection of a faint stellar companion to the famous nearby A5V star Alcor (80 UMa). The companion has M-band ($\lambda$ = 4.8 $\mu$m) magnitude 8.8 and projected separation 1".11 (28 AU) from Alcor. The companion is most likely a low-mass ($\sim$0.3 \msun) active star which is responsible for Alcor's X-ray emission detected by ROSAT (L$_{\rm X}$ $\simeq$ 10$^{28.3}$ erg/s). Alcor is a nuclear member of the Ursa Major star cluster (UMa; d $\simeq$ 25 pc, age $\simeq$ 0.5 Gyr), and has been occasionally mentioned as a possible distant (709") companion of the stellar quadruple Mizar ($\zeta$ UMa). Comparing the revised Hipparcos proper motion for Alcor with the mean motion for other UMa nuclear members shows that Alcor has a peculiar velocity of 1.1 km/s, which is comparable to the predicted velocity amplitude induced by the newly-discovered companion ($\sim$1 km/s). Using a precise dynamical parallax for Mizar and the revised Hipparcos parallax for Alcor, we find that Mizar and Alcor are physically separated by 0.36 $\pm$ 0.19 pc (74 $\pm$ 39 kAU; minimum 18 kAU), and their velocity vectors are marginally consistent ($\chi^2$ probability 6%). Given their close proximity and concordant motions we suggest that the Mizar quadruple and the Alcor binary be together considered the 2nd closest stellar sextuplet. The addition of Mizar-Alcor to the census of stellar multiples with six or more components effectively doubles the local density of such systems within the local volume (d $<$ 40 pc).

The near-IR luminosity function and bimodal surface brightness distributions of Virgo cluster galaxies

We have acquired deep, H-band imaging for a sample of 286 Virgo cluster galaxies with B T ≤ 16 mag and extracted surface photometry from optical g, r, i, z Sloan Digital Sky Survey images of 742 Virgo Cluster Catalogue galaxies, including those with H-band images. We confirm the detection of a dip in the luminosity function indicative of a discontinuity in the cluster galaxy population; the dip is more pronounced at redder wavelengths. We find, in agreement with earlier works of Tully and Verheijen and ours for Ursa Major cluster galaxies, a clear dichotomy between high and low surface brightness (HSB and LSB) galaxy discs. The difference between the low and high brightness peaks of Virgo disc galaxies is ∼2 H-mag arcsec -2 , significantly larger than any systematic errors. The high surface brightness disc galaxies have two distinct classes of high and low concentration bulges, while LSB galaxies have only low concentration bulges. Early-type galaxies exhibit a similar structural bimodality though offset from that of the spiral galaxies towards higher surface brightnesses. Both the early- and late-type structural bimodalities are uncorrelated with colour or any other structural parameter except, possibly, circular velocity. Random realizations of realistic surface brightness profiles suggest that a bimodal distribution of effective surface brightness is unexpected based on normal distributions of bulge and disc parameters. Rather, the structural bimodality may be linked to dynamic properties of galaxies. Low angular momentum systems may collapse to form dynamically important discs with high surface brightness, while high angular momentum systems would end up as low surface brightness galaxies dominated by the dark halo at all radii. The confirmation of structural bimodality for gas-rich and gas-poor galaxies in the high-density Virgo cluster as well as the low-density UMa cluster suggests that this phenomenon is independent of environment.

Bulge-disc decompositions and structural bimodality of Ursa Major cluster spiral galaxies

We present bulge and disc (B/D) decompositions of existing K' surface brightness profiles for 65 Ursa Major (UMa) cluster spiral galaxies. This improves upon the disc-only fits of Tully et al. The 1996 disc fits were used by Tully & Verheijen for their discovery of the bimodality of structural parameters in the UMa cluster galaxies. It is shown that our new one-dimensional B/D decompositions yield disc structural parameters that differ only slightly from the basic fits of Tully et al. and evidence for structural bimodality of UMa galaxies is maintained. Our B/D software for the decomposition of one-dimensional surface brightness profiles of galaxies uses a non-linear minimization scheme to recover the best-fitting Sersic bulge and the exponential disc while accounting for the possible presence of a compact nucleus and spiral arms and for the effects of seeing and disc truncations. In agreement with Tully & Verheijen, we find that the distribution of near-infrared disc central surface brightnesses is bimodal with an F-test confidence of 80 per cent. There is also strong evidence for a local minimum in the luminosity function at M K' ≃ -22. A connection between the brightness bimodality and a dynamical bimodality, based on new H i linewidths, is identified. The B/D parameters are presented in Table 1.

Analysis of the properties of clusters of galaxies in the region of the Ursa Major supercluster

We analyze the properties of the clusters of galaxies in the region of the Ursa Major (UMa) supercluster using observational data from SDSS and 2MASS catalogs. The region studied includes a supercluster (with a galaxy and cluster overdensity of 3 and 15, respectively) and field clusters inside the 150-Mpc diameter surrounding region. The total dynamical mass of 10 clusters of galaxies in UMa is equal to 2.25 × 1015 M ⊙, and the mass of 11 clusters of galaxies in the UMa neighborhood is equal to 1.70 × 1015 M ⊙. The fraction of early-type galaxies brighter than M * K + 1 in the virialized regions of clusters is, on the average, equal to 70%, and it is virtually independent on the mass of the cluster. The fraction of these galaxies and their average photometric parameters are almost the same both for UMa clusters and for the clusters located in its surroundings. Parameters of the clusters of galaxies, such as infrared luminosities up to a fixed magnitude, the mass-to-luminosity ratio, and the number of galaxies have almost the same correlations with the cluster mass as in other samples of galaxies clusters. However, the scatter of these parameters for UMa member clusters is twice smaller than the corresponding scatter for field clusters, possibly, due to the common origin of UMa clusters and synchronized dynamical evolution of clusters in the supercluster.

The dwarf low surface brightness galaxy population of the Virgo cluster - III. Comparisons with different environments

We have previously described the results of a search for low surface brightness (LSB) objects in data drawn from an east–west strip of the Virgo cluster and have compared this to a large-area strip outside the cluster. In this paper, we compare the cluster east–west data with new data along a cluster north–south strip and with data obtained for the Ursa Major (UMa) cluster and fields around the spiral galaxy M101. Our intention is to look for differences in the nature of the dwarf galaxy population with location in the cluster and within other environments. These uniform data sets reach central surface brightness values of ∼26 Bμ, and an apparent B magnitude of 21 (M(B)=−10 for a Virgo cluster distance of 16 Mpc). Over a north–south strip area of ∼15 deg(2), we find ∼22 LSB objects per square degree. This compares with ∼4 deg(−2) in both the UMa and the M101 fields. These results are very similar to what we have previously found for the east–west cluster strip and for other fields outside the cluster. There are far more small LSB features detected in Virgo cluster fields and we associate them with an extensive cluster dwarf galaxy population. In the cluster north–south strip there is an average dwarf-to-giant number ratio of 31 ± 9, which is much higher than would be expected from observations of galaxies in the Local Group, but significantly less than that required by Λ cold dark matter galaxy formation models. The dwarf-to-giant ratios in UMa and M101 are consistent with those in the Local Group. Comparing the cluster east–west and north–south data, we find clear differences in the way the number density of dwarfs decreases with cluster radius and the distributions of galaxy magnitudes and sizes. The north–south strip covers a region of the cluster that contains not only the main cluster region associated with M87, but also two more distant infalling clouds. Additionally, the north–south strip lies approximately along the extended filamentary structure associated with Virgo while the east–west strip is perpendicular to it. We interpret the differences between the two strips as being due to these different environments. We suggest that the Virgo cluster is assembling itself out of subclusters and clouds that are already rich in dwarf galaxies compared to the environment of the general field. There is no evidence for any systematic difference in dwarf galaxy (B−I) colour between dwarf galaxies of different magnitudes or in different parts of the Virgo cluster. A typical value of (B−I)= 1.8 is consistent with the colours of a wide range of stellar systems including the globular clusters of the central elliptical galaxy M87, but is bluer than is typical for giant ellipticals. We discuss and interpret our results in the context of various physical processes that are thought to act on galaxies as they form and evolve. Additionally, we give images and positions of possible new dwarf galaxy companions of M101.

The Dwarf Low Surface Brightness Population in Different Environments of the Local Universe

AbstractThe nature of the dwarf galaxy population as a function of location in the cluster and within different environments is investigated. We have previously described the results of a search for low surface brightness objects in data drawn from an East-West strip of the Virgo cluster (Sabatini et al(2004)) and have compared this to a large area strip outside of the cluster (Roberts et al (2004)). In this talk I compare the East-West data (sampling sub-cluster A and outward) to new data along a North-South cluster strip that samples a different region (part of sub-cluster A, and the N, M clouds) and with data obtained for the Ursa Major cluster and fields around the spiral galaxy M101. The sample of dwarf galaxies in different environments is obtained from uniform datasets that reach central surface brightness values of 26 B mag/arcsec2 and an apparent B magnitude of 21 (MB =−10 for a Virgo Cluster distance of 16 Mpc). We discuss and interpret our results on the properties and distribution of dwarf low surface brightness galaxies in the context of variuos physical processes that are thought to act on galaxies as they form and evolve.

Peculiar motions in the region of the Ursa Major supercluster of galaxies

We have investigated the peculiar motions of clusters of galaxies in the Ursa Major (UMa) supercluster and its neighborhood. Based on SDSS (Sloan Digital Sky Survey) data, we have compiled a sample of early-type galaxies and used their fundamental plane to determine the cluster distances and peculiar velocities. The samples of early-type galaxies in the central regions (within R 200) of 12 UMa clusters of galaxies, in three main subsystems of the supercluster—the filamentary structures connecting the clusters, and in nine clusters from the nearest UMa neighborhood have similar parameters. The fairly high overdensity (3 by the galaxy number and 15 by the cluster number) suggests that the supercluster as a whole is gravitationally bound, while no significant peculiar motions have been found: the peculiar velocities do not exceed the measurement errors by more than a factor of 1.5–2. The mean random peculiar velocities of clusters and the systematic deviations from the overall Hubble expansion in the supercluster are consistent with theoretical estimates. For the possible approach of the three UMa subsystems to be confirmed, the measurement accuracy must be increased by a factor of 2–3.

Detailed study of the Ursa Major supercluster of galaxies using the 2MASS and SDSS Catalogs

We study the infrared (K s band) properties of clusters of galaxies in the Ursa Major supercluster using data from 2MASS (Two-Micron All-Sky Survey) and SDSS (Sloan Digital Sky Survey). We identified three large filaments with mean redshifts of z = 0.051, 0.060, and 0.071. All clusters of the supercluster are located in these filaments. We determined the total K s-band luminosities and masses for 11 clusters of galaxies within comparable physical regions (within a radius R 200 close to the virial radius) using a homogeneous method. We constructed a combined luminosity function for the supercluster in this region, which can be described by the Schechter function with the following parameters: M K * = −24.50 and α = −0.98. The infrared luminosities of the clusters of galaxies correlate with their masses; the M/L K ratios of the systems increase with their masses (luminosities), with most of the Ursa Major clusters of galaxies (particularly the richer ones) closely following the relations derived previously for a large sample of clusters and groups of galaxies. The total mass-to-infrared-luminosity ratio is 52 M ⊙/L ⊙ for six Abell clusters and 49M ⊙/L ⊙ for all of the clusters, except Anon2.

Structure and kinematics of edge-on galaxy discs - V. The dynamics of stellar discs

In earlier papers in this series we determined the intrinsic stellar disc kinematics of 15 intermediate- to late-type edge-on spiral galaxies using a dynamical modelling technique. The sample covers a substantial range in maximum rotation velocity and deprojected face-on surface brightness, and contains seven spirals with either a boxy or peanut-shaped bulge. Here we discuss the structural, kinematical and dynamical properties. From the photometry we find that intrinsically more flattened discs tend to have a lower face-on central surface brightness and a larger dynamical mass-to-light ratio. This observation suggests that, at a constant maximum rotational velocity, lower surface brightness discs have smaller vertical stellar velocity dispersions. Although the individual uncertainties are large, we find from the dynamical modelling that at least 12 discs are submaximal. The average disc contributes 53 ± 4 per cent to the observed rotation at 2.2 disc scalelengths (hR), with a 1σ scatter of 15 per cent. This percentage becomes somewhat lower when effects of finite disc flattening and gravity by the dark halo and the gas are taken into account. Since boxy and peanut-shaped bulges are probably associated with bars, the result suggests that at 2.2hR the submaximal nature of discs is independent of barredness. The possibility remains that very high surface brightness discs are maximal, as these discs are underrepresented in our sample. We confirm that the radial stellar disc velocity dispersion is related to the galaxy maximum rotational velocity. The scatter in this σ versus vmax relation appears to correlate with the disc flattening, face-on central surface brightness and dynamical mass-to-light ratio. Low surface brightness discs tend to be more flattened and have smaller stellar velocity dispersions. The findings are consistent with the observed correlation between disc flattening and dynamical mass-to-light ratio and can generally be reproduced by the simple collapse theory for disc galaxy formation. Finally, the disc mass Tully–Fisher relation is offset from the maximum-disc scaled stellar mass Tully–Fisher relation of the Ursa Major cluster. This offset, −0.3 dex in mass, is naturally explained if the discs of the Ursa Major cluster spirals are submaximal.

Breakup of Moving Clusters in the Galactic Disk

Numerical simulations are used to analyze the breakup of moving groups in the Galactic disk through the internal velocity dispersion of the group components and the tidal effect of the external regular gravitational Galactic field and giant molecular clouds. The initial locations of the group centroids correspond to well-known moving streams: the Hyades, the Pleiades, the Ursa Major cluster, and the group HR 1614. The mean group breakup times have been found as a function of the adopted limiting group size. The interactions of stream stars with giant molecular clouds reduce significantly the group lifetime.

A search for low surface brightness dwarf galaxies in different environments

Current theories of large scale structure and galaxy formation predict the existence of numerous low mass dark matter haloes in the Universe today. If these haloes contain sufficient stars they should be detectable as low luminosity stellar systems or dwarf galaxies. We have searched for these objects in four regions of increasing density - the general field, the area around a giant spiral galaxy, the low density Ursa Major cluster, and the high density Virgo cluster. Using identical deep optical data covering a total of 602 and probing fainter magnitudes than has been done previously, we used identical selection and detection methods to compare the dwarf galaxy populations in these different environments. We found substantially more dwarfs per giant galaxy in the Virgo cluster (20:1) compared to the field environment (6:1 max). A comparison of the HI properties and (B-I) colours for the objects for which we had additional data also showed that in general, the cluster objects are redder and gas poor compared to the objects in the field. We discuss the possible mechanisms which may have resulted in creating a population of cluster dwarf galaxies, which would explain the high number density which we found in our data. It is likely that a combination of tidal interactions and transformation of infalling dlrrs into dEs will result in the large population of cluster dwarfs. Conclusive evidence regarding their formation must now be obtained by a more detailed investigation of their stellar populations. The lack of dwarf galaxies in the field region is likely to be due to the effect of inefficient star formation in the field environment compared to the cluster. Thus the low mass dark matter haloes predicted by CDM models must still be 'dark' and can only be identified by further deep HI studies of the field environment, and future gravitational lensing studies of substructure.

Dwarf LSB galaxies and their environment: The Virgo Cluster, the Ursa Major Cluster, isolated galaxies and voids

The widely varying dwarf galaxy counts in different environments provide a strong challenge to standard hierarchical clustering models. The luminosity function is not universal, but seems to be strongly dependent upon environment. In this paper we describe an automated procedure for detecting and measuring very Low Surface Brightness (LSB) features in deep CCD data. We apply this procedure to large area CCD survey fields of the Virgo cluster and find 105 dwarf LSB galaxies previously uncatalogued over an area of ∼14 sq deg. We show that there are many more faint (14≤MB≤–10) LSB galaxies than would be predicted from extrapolation of the Virgo cluster catalogue luminosity function. Over our limited range of measurement the faint end slope of the luminosity function becomesα=–1.6. Although these galaxies contribute a small fraction of the total stellar light of the cluster, they may contribute significantly to the mass in galaxies if they have large mass-to-light ratios similar to those recently measured for Local Group dwarf galaxies. By a comparison with different environments, the Virgo Cluster seems to be highly `dwarf rich' if considering the dwarf to giant ratio or it's luminosity function faint end slope.