arXiv subjects

Christopher J. Conselice

Publications and source records attributed to Christopher J. Conselice.

At least 271 records · Page 15Linked to original sources

arXiv2011

Breaking down the link between luminous and dark matter in massive galaxies

We present a study on the clustering of a stellar mass selected sample of galaxies with stellar masses M*>10^10Msol at redshifts 0.4<z<2.0, taken from the Palomar Observatory Wide-field Infrared Survey. We examine the clustering properties of these stellar mass selected samples as a function of redshift and stellar mass, and find that galaxies with high stellar masses have a progressively higher clustering strength than galaxies with lower stellar masses. We also find that galaxies within a fixed stellar mass range have a higher clustering strength at higher redshifts. We further estimate the average total masses of the dark matter haloes hosting these stellar-mass selected galaxies. For all galaxies in our sample the stellar-mass-to-total-mass ratio is always lower than the universal baryonic mass fraction and the stellar-mass-to-total-mass ratio is strongly correlated with the halo masses for central galaxies, such that more massive haloes contain a lower fraction of their mass in the form of stars. The remaining baryonic mass is included partially in stars within satellite galaxies in these haloes, and as diffuse hot and warm gas. We also find that, at a fixed stellar mass, the stellar-to-total-mass ratio increases at lower redshifts. This suggests that galaxies at a fixed stellar mass form later in lower mass dark matter haloes, and earlier in massive haloes. We interpret this as a `halo downsizing' effect.

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arXiv2011

How do galaxies get their baryons?

Understanding how galaxies obtain baryons, their stars and gas, over cosmic time is traditionally approached in two different ways - theoretically and observationally. In general, observational approaches to galaxy formation include measuring basic galaxy properties, such as luminosities, stellar masses, rotation speeds, star formation rates and how these features evolve through time. Theoretically, cosmologically based models collate the physical effects driving galaxy assembly - mergers of galaxies, accretion of gas, star formation, and feedback, amongst others, to form predictions which are matched to galaxy observables. An alternative approach is to examine directly, in an observational way, the processes driving galaxy assembly, including the effects of feedback. This is a new `third way' towards understanding how galaxies are forming from gas accretion and mergers, and directly probes these effects instead of relying on simulations designed to reproduce observations. This empirical approach towards understanding galaxy formation, including the acquisition history of baryons, displays some significant differences with the latest galaxy formation models, in addition to directly demonstrating the mechanisms by which galaxies form most of their baryonic mass.

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arXiv2011

A Deep Probe of the Galaxy Stellar Mass Functions at z~1-3 with the GOODS NICMOS Survey

We use a sample of 8298 galaxies observed in the HST GOODS NICMOS Survey (GNS) to construct the galaxy stellar mass function as a function of both redshift and stellar mass up to z=3.5 and down to masses of Mstar=10^8.5 Msun at z~1. We discover that a significant fraction of all massive Mstar>10^11 Msun galaxies are in place up to the highest redshifts we probe, with a decreasing fraction of lower mass galaxies present at all redshifts. This is an example of `galaxy mass downsizing', and is the result of massive galaxies forming before lower mass ones, and not just simply ending their star formation earlier as in traditional downsizing scenarios. We find that the faint end slope is significantly steeper than what is found in previous investigations. We demonstrate that this steeper mass function better matches the stellar mass added due to star formation, thereby alleviating some of the mismatch between these two measures of the evolution of galaxy mass. We furthermore examine the stellar mass function divided into blue/red systems, as well as for star forming and non-star forming galaxies. We find a similar mass downsizing present for both blue/red and star-forming/non-star forming galaxies, and that the low mass galaxies are mostly all blue, and are therefore creating the steep mass functions. We furthermore show that, although there is a downsizing such that high mass galaxies are nearer their z=0 values at high redshift, this turns over at masses Mstar~10^10 Msun, such that the lowest mass galaxies are more common than galaxies at slight higher masses, creating a `dip' in the observed galaxy mass function. We argue that the galaxy assembly process may be driven by different mechanisms at low and high masses, and that the efficiency of the galaxy formation process is lowest at masses Mstar~10^10 Msun at 1<z<3. (Abridged)

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arXiv2010

Galaxy properties in different environments at z > 1.5 in the GOODS-NICMOS Survey

We present a study of the relationship between galaxy colour, stellar mass, and local galaxy density in a deep near-infrared imaging survey up to a redshift of z~3 using the GOODS NICMOS Survey (GNS). The GNS is a very deep, near-infrared Hubble Space Telescope survey imaging a total of 45 arcmin^2 in the GOODS fields, reaching a stellar mass completeness limit of M* = 10^9.5 M_sun at z=3. Using this data we measure galaxy local densities based on galaxy counts within a fixed aperture, as well as the distance to the 3rd, 5th and 7th nearest neighbour. We find a strong correlation between colour and stellar mass at all redshifts up to z~3. We do not find a strong correlation between colour and local density, however, the highest overdensities might be populated by a higher fraction of blue galaxies than average or underdense areas, indicating a possible reversal of the colour-density relation at high redshift. Our data suggests that the possible higher blue fraction at extreme overdensities might be due to a lack of massive red galaxies at the highest local densities.

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arXiv2010

Galaxy properties in different environments up to z ~ 3 in the GOODS NICMOS Survey

We study the relationship between galaxy colour, stellar mass, and local galaxy density in a deep near-infrared imaging survey up to a redshift of z~3 using the GOODS NICMOS Survey (GNS). The GNS is a deep near-infrared Hubble Space Telescope survey imaging a total of 45 square arcminutes of the GOODS fields, reaching a stellar mass completeness limit of log M* = 9.5 M_sun at z=3. Using this data we measure galaxy local densities based on galaxy counts within a fixed aperture, as well as the distance to the 3rd, 5th and 7th nearest neighbour. We compare the average rest-frame (U-B) colour and fraction of blue galaxies in different local densities and at different stellar masses. We find a strong correlation between colour and stellar mass at all redshifts up to z~3. Massive red galaxies are already in place at z~3 at the expected location of the red-sequence in the colour-magnitude diagram, although they are star forming. We do not find a strong correlation between colour and local density, however, there may be evidence that the highest overdensities are populated by a higher fraction of blue galaxies than average or underdense areas. This could indicating that the colour-density relation at high redshift is reversed with respect to lower redshifts (z<1), where higher densities are found to have lower blue fractions. Our data suggests that the possible higher blue fraction at extreme overdensities might be due to a lack of massive red galaxies at the highest local densities.

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arXiv2010

The mass-L_x relation for moderate luminosity X-ray clusters

We present measurements of the masses of a sample of 25 moderate X-ray luminosity clusters of galaxies from the 160 square degree ROSAT survey. The masses were obtained from a weak lensing analysis of deep F814W images obtained using the Advanced Camera for Surveys (ACS). We present an accurate empirical correction for the effect of charge transfer (in)efficiency on the shapes of faint galaxies. A significant lensing signal is detected around most of the clusters. The lensing mass correlates tightly with the cluster richness. We measured the intrinsic scatter in the scaling relation between M_2500 and L_X and find the best fit power law slope and normalisation to be alpha=0.68+-0.07 and M_X=(1.2+-0.12)10^14M_sun (for L_X=2x10^44 erg/s). These results agree well with a number of recent studies, but the normalisation is lower compared to the study of Rykoff et al. (2008b). One explanation for this difference may be the fact that (sub)structures projected along the line-of-sight boost both the galaxy counts and the lensing mass. Such superpositions lead to an increased mass at a given L_X when clusters are binned by richness.

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arXiv2010

z~7 galaxy candidates from NICMOS observations over the HDF South and the CDF-S and HDF-N GOODS fields

We use ~88 arcmin**2 of deep (>~26.5 mag at 5 sigma) NICMOS data over the two GOODS fields and the HDF South to conduct a search for bright z>~7 galaxy candidates. This search takes advantage of an efficient preselection over 58 arcmin**2 of NICMOS H-band data where only plausible z>~7 candidates are followed up with NICMOS J-band observations. ~248 arcmin**2 of deep ground-based near-infrared data (>~25.5 mag, 5 sigma) is also considered in the search. In total, we report 15 z-dropout candidates over this area -- 7 of which are new to these search fields. Two possible z~9 J-dropout candidates are also found, but seem unlikely to correspond to z~9 galaxies. The present z~9 search is used to set upper limits on the prevalence of such sources. Rigorous testing is undertaken to establish the level of contamination of our selections by photometric scatter, low mass stars, supernovae (SNe), and spurious sources. The estimated contamination rate of our z~7 selection is ~24%. Through careful simulations, the effective volume available to our z>~7 selections is estimated and used to establish constraints on the volume density of luminous (L*(z=3), or -21 mag) galaxies from these searches. We find that the volume density of luminous star-forming galaxies at z~7 is 13_{-5}^{+8}x lower than at z~4 and >25x lower (1 sigma) at z~9 than at z~4. This is the most stringent constraint yet available on the volume density of >~L* galaxies at z~9. The present wide-area, multi-field search limits cosmic variance to <20%. The evolution we find at the bright end of the UV LF is similar to that found from recent Subaru Suprime-Cam, HAWK-I or ERS WFC3/IR searches. The present paper also includes a complete summary of our final z~7 z-dropout sample (18 candidates) identified from all NICMOS observations to date (over the two GOODS fields, the HUDF, galaxy clusters).

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arXiv2010

How does galaxy environment matter? The relationship between galaxy environments, colour and stellar mass at 0.4 < z < 1 in the Palomar/DEEP2 survey

We present a study characterizing the environments of galaxies in the redshift range of 0.4 < z < 1 based on data from the POWIR near infrared imaging and DEEP2 spectroscopic redshift surveys, down to a stellar mass of log M* = 10.25 M_sun. Galaxy environments are measured in terms of nearest neighbour densities as well as fixed aperture densities and kinematical and dynamical parameters of neighbour galaxies within a radius of 1 Mpc. We disentangle the correlations between galaxy stellar mass, galaxy colour and galaxy environment, using not only galaxy number densities, but also other environmental characteristics such as velocity dispersion, mean harmonic radius, and crossing time. We find that galaxy colour and the fraction of blue galaxies depends very strongly on stellar mass at 0.4 < z < 1, while a weak additional dependence on local number densities is in place at lower redshifts (0.4 < z < 0.7). This environmental influence is most visible in the colours of intermediate mass galaxies (10.5 < log M* < 11), whereas colours of lower and higher mass galaxies remain largely unchanged with redshift and environment. At a fixed stellar mass, the colour-density relation almost disappears, while the colour-stellar mass relation is present at all local densities. We find a weak correlation between stellar mass and environment at intermediate redshifts, which contributes to the overall colour-density relation. We furthermore do not find a significant correlation between galaxy colour and virial mass, i.e., parent dark matter halo mass. Galaxy stellar mass thus appears to be the crucial defining parameter for intrinsic galaxy properties such as ongoing star formation and colour.

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arXiv2010

Dwarf Galaxies in Clusters as Probes of Galaxy Formation and Dark Matter

We present the results of a Hubble Space Telescope (HST) ACS and WFPC2 study of dwarf galaxies in the nearby Perseus Cluster, down to M_V = -12, spanning the core and outer regions of this cluster. We examine how properties such as the colour magnitude relation, structure and morphology are affected by environment for the lowest mass galaxies. The low masses of dwarf galaxies allow us to determine their environmentally driven based galaxy evolution, the effects of which are harder to examine in massive galaxies. The structures of our dwarfs in both the core and outer regions of the cluster are quantified using the concentration, asymmetry and clumpiness (CAS) parameters. We find that, on average, dwarfs in the outer regions of Perseus are more disturbed than those in the cluster core, with higher asymmetries and clumpier light distributions. We measure the (V-I)_0 colours of the dEs, and find that dwarfs in both the inner and outer regions of the cluster lie on the same colour magnitude relation. Based on these results, we infer that the disturbed dwarfs in the cluster outskirts are likely "transition dwarfs", with their colours transforming before their structures. Finally, we infer from the smoothness of the cluster core population that dwarfs in the inner regions of the cluster must be highly dark matter dominated to prevent their disruption by the cluster potential. We derive a new method to determine the minimum mass the dwarfs must have to prevent this disruption without the need for resolved spectroscopy, and determine their mass-to-light ratios. At their orbit pericentre, dwarfs in the core of Perseus require mass-to-light ratios between 1 and 120 to prevent their disruption, comparable to those found for the Local Group dSphs.

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arXiv2010

Origins of the extragalactic background at 1mm from a combined analysis of the AzTEC and MAMBO data in GOODS-N

We present a study of the cosmic infrared background, which is a measure of the dust obscured activity in all galaxies in the Universe. We venture to isolate the galaxies responsible for the background at 1mm; with spectroscopic and photometric redshifts we constrain the redshift distribution of these galaxies. We create a deep 1.16mm map (sigma ~ 0.5mJy) by combining the AzTEC 1.1mm and MAMBO 1.2mm datasets in GOODS-N. This combined map contains 41 secure detections, 13 of which are new. By averaging the 1.16mm flux densities of individually undetected galaxies with 24um flux densities > 25uJy, we resolve 31--45 per cent of the 1.16mm background. Repeating our analysis on the SCUBA 850um map, we resolve a higher percentage (40--64 per cent) of the 850um background. A majority of the background resolved (attributed to individual galaxies) at both wavelengths comes from galaxies at z > 1.3. If the ratio of the resolved submillimeter to millimeter background is applied to a reasonable scenario for the origins of the unresolved submillimeter background, 60--88 per cent of the total 1.16mm background comes from galaxies at z > 1.3.

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arXiv2010

Galaxy and Mass Assembly (GAMA): FUV, NUV, ugrizYJHK Petrosian, Kron and Sèrsic photometry

In order to generate credible 0.1-2 μm SEDs, the GAMA project requires many Gigabytes of imaging data from a number of instruments to be re-processed into a standard format. In this paper we discuss the software infrastructure we use, and create self-consistent ugrizYJHK photometry for all sources within the GAMA sample. Using UKIDSS and SDSS archive data, we outline the pre-processing necessary to standardise all images to a common zeropoint, the steps taken to correct for seeing bias across the dataset, and the creation of Gigapixel-scale mosaics of the three 4x12 deg GAMA regions in each filter. From these mosaics, we extract source catalogues for the GAMA regions using elliptical Kron and Petrosian matched apertures. We also calculate Sérsic magnitudes for all galaxies within the GAMA sample using SIGMA, a galaxy component modelling wrapper for GALFIT 3. We compare the resultant photometry directly, and also calculate the r band galaxy LF for all photometric datasets to highlight the uncertainty introduced by the photometric method. We find that (1) Changing the object detection threshold has a minor effect on the best-fitting Schechter parameters of the overall population (M* +/- 0.055mag, α +/- 0.014, Φ* +/- 0.0005 h^3 Mpc^{-3}). (2) An offset between datasets that use Kron or Petrosian photometry regardless of the filter. (3) The decision to use circular or elliptical apertures causes an offset in M* of 0.20mag. (4) The best-fitting Schechter parameters from total-magnitude photometric systems (such as SDSS modelmag or Sérsic magnitudes) have a steeper faint-end slope than photometry dependent on Kron or Petrosian magnitudes. (5) Our Universe's total luminosity density, when calculated using Kron or Petrosian r-band photometry, is underestimated by at least 15%.

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arXiv2010

On the co-evolution of supermassive black holes and their host galaxies since z = 3

[Abridged] To investigate the evolution in the relation between galaxy stellar and central black hole mass we construct a volume limited complete sample of 85 AGN with host galaxy stellar masses M_{*} > 10^{10.5} M_{sol}, and specific X-ray luminosities L_{X} > 2.35 x 10^{43} erg s^{-1} at 0.4 < z < 3. We calculate the Eddington limiting masses of the supermassive black holes residing at the centre of these galaxies, and observe an increase in the average Eddington limiting black hole mass with redshift. By assuming that there is no evolution in the Eddington ratio (μ) and then that there is maximum possible evolution to the Eddington limit, we quantify the maximum possible evolution in the M_{*} / M_{BH} ratio as lying in the range 700 < M_{*}/M_{BH} < 10000, compared with the local value of M_{*}/M_{BH} ~ 1000. We furthermore find that the fraction of galaxies which are AGN (with L_{X} > 2.35 x 10^{43} erg s^{-1}) rises with redshift from 1.2 +/- 0.2 % at z = 0.7 to 7.4 +/- 2.0 % at z = 2.5. We use our results to calculate the maximum timescales for which our sample of AGN can continue to accrete at their observed rates before surpassing the local galaxy-black hole mass relation. We use these timescales to calculate the total fraction of massive galaxies which will be active (with L_{X} > 2.35 x 10^{43} erg s^{-1}) since z = 3, finding that at least ~ 40% of all massive galaxies will be Seyfert luminosity AGN or brighter during this epoch. Further, we calculate the energy density due to AGN activity in the Universe as 1.0 (+/- 0.3) x 10^{57} erg Mpc^{-3} Gyr^{-1}, potentially providing a significant source of energy for AGN feedback on star formation. We also use this method to compute the evolution in the X-ray luminosity density of AGN with redshift, finding that massive galaxy Seyfert luminosity AGN are the dominant source of X-ray emission in the Universe at z < 3.

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arXiv2010

Hubble Space Telescope survey of the Perseus Cluster -III: The effect of local environment on dwarf galaxies

We present the results of a Hubble Space Telescope (HST) study of dwarf galaxies in the outer regions of the nearby rich Perseus Cluster, down to M_V = -12, and compare these with the dwarf population in the cluster core from our previous HST imaging. In this paper we examine how properties such as the colour magnitude relation, structure and morphology are affected by environment for the lowest mass galaxies. Dwarf galaxies are excellent tracers of the effects of environment due to their low masses, allowing us to derive their environmentally based evolution, which is more subtle in more massive galaxies. We identify 11 dwarf elliptical (dE) and dwarf spheroidal (dSph) galaxies in the outer regions of Perseus, all of which are previously unstudied. We measure the (V-I)_0 colours of our newly discovered dEs, and find that these dwarfs lie on the same red sequence as those in the cluster core. The morphologies of these dwarfs are examined by quantifying their light distributions using CAS parameters, and we find that dEs in the cluster outskirts are on average more disturbed than those in the core, with =0.13\pm0.09 and =0.18\pm0.08, compared to =0.02\pm0.04, =0.01\pm0.07 for those in the core. Based on these results, we infer that these objects are ``transition dwarfs'', likely in the process of transforming from late-type to early type galaxies as they infall into the cluster, with their colours transforming before their structures. When we compare the number counts for both the core and outer regions of the cluster, we find that below M_V = -12, the counts in the outer regions of the cluster exceed those in the core. This is evidence that in the very dense region of the cluster, dwarfs are unable to survive unless they are sufficiently massive to prevent their disruption by the cluster potential and interactions with other galaxies.

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arXiv2010

A Spectroscopic Search for Leaking Lyman Continuum at z~0.7

We present the results of rest-frame, UV slitless spectroscopic observations of a sample of 32 z~0.7 Lyman break galaxy (LBG) analogs in the COSMOS field. The spectroscopic search was performed with the Solar Blind Channel (SBC) on Hubble Space Telescope. We report the detection of leaking Lyman continuum (LyC) radiation from an AGN-starburst composite. While we find no direct detections of LyC emission in the remainder of our sample, we achieve individual lower limits (3 sigma) of the observed non-ionizing UV to LyC flux density ratios, f_{nu}(1500A)/f_{nu}(830A) of 20 to 204 (median of 73.5) and 378.7 for the stack. Assuming an intrinsic Lyman break of 3.4 and an intergalactic medium (IGM) transmission of LyC photons along the line of sight to the galaxy of 85% we report an upper limit for the relative escape fraction in individual galaxies of 0.02 - 0.19 and a stacked 3 sigma upper limit of 0.01. We find no indication of a relative escape fraction near unity as seen in some LBGs at z~3. Our UV spectra achieve the deepest limits to date at any redshift for the escape fraction in individual sources. The contrast between these z~0.7 low escape fraction LBG analogs with z~3 LBGs suggests that either the processes conducive to high escape fractions are not being selected for in the z<1 samples or the average escape fraction is decreasing from z~3 to z~1. We discuss possible mechanisms which could affect the escape of LyC photons.

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arXiv2010

Absence of Evidence Is Not Evidence of Absence: The Color-Density Relation at Fixed Stellar Mass Persists to z ~ 1

We use data drawn from the DEEP2 Galaxy Redshift Survey to investigate the relationship between local galaxy density, stellar mass, and rest-frame galaxy color. At z ~ 0.9, we find that the shape of the stellar mass function at the high-mass (log (M*/Msun) > 10.1) end depends on the local environment, with high-density regions favoring more massive systems. Accounting for this stellar mass-environment relation (i.e., working at fixed stellar mass), we find a significant color-density relation for galaxies with 10.6 < log(M*/Msun) < 11.1 and 0.75 < z < 0.95. This result is shown to be robust to variations in the sample selection and to extend to even lower masses (down to log(M*/Msun) ~ 10.4). We conclude by discussing our results in comparison to recent works in the literature, which report no significant correlation between galaxy properties and environment at fixed stellar mass for the same redshift and stellar mass domain. The non-detection of environmental dependence found in other data sets is largely attributable to their smaller samples size and lower sampling density, as well as systematic effects such as inaccurate redshifts and biased analysis techniques. Ultimately, our results based on DEEP2 data illustrate that the evolutionary state of a galaxy at z ~ 1 is not exclusively determined by the stellar mass of the galaxy. Instead, we show that local environment appears to play a distinct role in the transformation of galaxy properties at z > 1.

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arXiv2010

Gemini K-band NIRI Adaptive Optics Observations of Massive Galaxies at 1 < z < 2

We present deep K-band adaptive-optics observations of eight very massive (M* ~ 4 x 10^11 Msun) galaxies at 1 < z < 2 utilizing the Gemini NIRI/Altair Laser Guide System. These systems are selected from the Palomar Observatory Wide-Field Infrared (POWIR) survey, and are amongst the most massive field galaxies at these epochs. The depth and high spatial resolution of our images allow us to explore for the first time the stellar mass surface density distribution of massive distant galaxies from 1 to 15 kpc on an individual galaxy basis, rather than on stacked images. We confirm that some of these massive objects are extremely compact with measured effective radii between 0."1 - 0."2, giving sizes which are < 2 kpc, a factor of ~ 7 smaller in effective radii than similar mass galaxies today. Examining stellar mass surface densities as a function of fixed physical aperture, we find an over-density of material within the inner profiles, and an under-density in the outer profile, within these high-z galaxies compared with similar mass galaxies in the local universe. Consequently, massive galaxies should evolve in a way to decrease the stellar mass density in their inner region, and at the same time creating more extensive outer light envelopes. We furthermore show that ~ 38% +- 20% of our sample contains evidence for a disturbed outer stellar matter distribution suggesting that these galaxies are undergoing a recent dynamical episode, such as a merger or accretion event. We calculate that massive galaxies at z < 2 will undergo on the order of five of these events, a much higher rate than observed for major mergers, suggesting that these galaxies are growing in size and stellar mass in part through minor mergers during this epoch.

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arXiv2010

A Deep HST Search for Escaping Lyman Continuum Flux at z~1.3: Evidence for an Evolving Ionizing Emissivity

We have obtained deep Hubble Space Telescope far-UV images of 15 starburst galaxies at z~1.3 in the GOODS fields to search for escaping Lyman continuum photons. These are the deepest far-UV images m_{AB}=28.7, 3σ, 1" diameter) over this large an area (4.83 arcmin^2) and provide the best escape fraction constraints for any galaxy at any redshift. We do not detect any individual galaxies, with 3σlimits to the Lyman Continuum (~700 Å) flux 50--149 times fainter (in f_nu) than the rest-frame UV (1500 Å) continuum fluxes. Correcting for the mean IGM attenuation (factor ~2), as well as an intrinsic stellar Lyman Break (~3), these limits translate to relative escape fraction limits of f_{esc,rel}<[0.03,0.21]. The stacked limit is f_{esc,rel}(3σ)<0.02. We use a Monte Carlo simulation to properly account for the expected distribution of IGM opacities. When including constraints from previous surveys at z~1.3 we find that, at the 95% confidence level, no more than 8% of star--forming galaxies at z~1.3 can have relative escape fractions greater than 0.50. Alternatively, if the majority of galaxies have low, but non-zero, escaping Lyman Continuum, the escape fraction can not be more than 0.04. Both the stacked limits, and the limits from the Monte Carlo simulation suggest that the average ionizing emissivity (relative to non-ionizing UV emissivity) at z~1.3 is significantly lower than has been observed in Lyman Break Galaxies (LBGs) at z~3. If the ionizing emissivity of star-forming galaxies is in fact increasing with redshift, it would help to explain the high photoionization rates seen in the IGM at z>4 and reionization of the intergalactic medium at z>6. [Abridged]

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arXiv2009

Smooth, undisturbed dwarf spheroidal galaxies in the Perseus Cluster core: Implications for dark matter content

Using deep HST/ACS observations of the core of the Perseus Cluster, we identify a large population of dwarf elliptical galaxies down to M_V = -12. All these dwarfs are remarkably smooth in appearance, showing no evidence for internal features that could be the result of tidal processes or star formation induced by the cluster potential. Based on these observations and the relatively large sizes of these dwarfs, we argue that at least some must have a large dark matter component to prevent their disruption by the cluster potential. We further derive a new method to quantify the dark matter content of cluster dSphs without the use of kinematics, which are impossible to obtain at these distances. We find that mass-to-light ratios for dwarfs in the core of the Perseus Cluster are comparable to those found for Local Group dSphs, ranging between M/L ~ 1 and 120. This is evidence that dwarf spheroidals reside in dark matter subhalos that protect them from tidal processes in the cores of dense clusters.

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