cD Envelope Research Articles

Sloshing in its cD halo: MUSE kinematics of the central galaxy NGC 3311 in the Hydra I cluster

Context.Early-type galaxies (ETGs) show a strong size evolution with redshift. This evolution is explained by fast “in-situ” star formation at high-zfollowed by a late mass assembly mostly driven by minor mergers that deposit stars primarily in the outer halo.Aims.We aim to identify the main structural components of the Hydra I cD galaxy NGC 3311 to investigate the connection between the central galaxy and the surrounding stellar halo.Methods.We produce maps of the line-of-sight velocity distribution (LOSVD) moments from a mosaic of MUSE pointings covering NGC 3311 out to 25 kpc. Combining deep photometric and spectroscopic data, we model the LOSVD maps using a finite mixture distribution, including four non-concentric components that are nearly isothermal spheroids, with different line-of-sight systemic velocitiesV, velocity dispersionsσ, and small (constant) values of the higher order Gauss-Hermite momentsh3andh4.Results.The kinemetry analysis indicates that NGC 3311 is classified as a slow rotator, although the galaxy shows a line-of-sight velocity gradient along the photometric major axis. The comparison of the correlations betweenh3andh4withV/σwith simulated galaxies indicates that NGC 3311 assembled mainly through dry mergers. Theσprofile rises to ≃ 400 km s-1at 20 kpc, a significant fraction (0.55) of the Hydra I cluster velocity dispersion, indicating that stars there were stripped from progenitors orbiting in the cluster core. The finite mixture distribution modeling supports three inner components related to the central galaxy and a fourth component with large effective radius (51 kpc) and velocity dispersion (327 km s-1) consistent with a cD envelope. We find that the cD envelope is offset from the center of NGC 3311 both spatially (8.6 kpc) and in velocity (ΔV= 204 km s-1), but coincides with the cluster core X-ray isophotes and the mean velocity of core galaxies. Also, the envelope contributes to the broad wings of the LOSVD measured by largeh4values within 10 kpc.Conclusions.The cD envelope of NGC 3311 is dynamically associated with the cluster core, which in Hydra I is in addition displaced from the cluster center, presumably due to a recent subcluster merger.

Presence of neutrons in the low-level background environment estimated by the analysis of the 595.8 keV gamma peak

In order to explore possible improvements of the existing techniques developed to estimate the neutron fluence in low-background Ge-spectroscopy systems, gamma spectra were collected by a HPGe detector in the presence of the 252Cf spontaneous fission neutron source. The spectra were taken with and without a Cd envelope on the detector dipstick, with different thicknesses of plastic used to slow down neutrons. We have analyzed the complex 595.8keV gamma peak, as well as several more gamma peaks following the neutron interactions in the detector itself and surroundings materials. The investigation shows that some changes of the initial neutron spectra can be monitored by the analysis of the 595.8keV gamma peak. We have found good agreement in the intensity changes between the long-tail component of the 595.8keV and the 691keV gamma peak (72Ge(n,n′)72Ge reaction), usually used for the estimation of the fast neutron fluence. Results also suggest that the thermal neutrons can have a stronger influence on creation of the Gaussian-like part of 595.8keV peak, than on the 139keV one following 74Ge(n,γ)75mGe reaction and used in the standard methods (Škoro et al., 1992) [8] for determination of the thermal neutron flux.

Evolution of the brightest cluster galaxies: the influence of morphology, stellar mass and environment

Using a sample of 425 nearby brightest cluster galaxies (BCGs) from von der Linden et al., we study the relationship between their internal properties (stellar masses, structural parameters and morphologies) and their environment. More massive BCGs tend to inhabit denser regions and more massive clusters than lower mass BCGs. Furthermore, cDs, which are BCGs with particularly extended envelopes, seem to prefer marginally denser regions and tend to be hosted by more massive haloes than elliptical BCGs. cD and elliptical BCGs show parallel positive correlations between their stellar masses and environmental densities. However, at a fixed environmental density, cDs are, on average, ∼40 per cent more massive. Our results, together with the findings of previous studies, suggest an evolutionary link between elliptical and cD BCGs. We suggest that most present-day cDs started their life as ellipticals, which subsequently grew in stellar mass and size due to mergers. In this process, the cD envelope developed. The large scatter in the stellar masses and sizes of the cDs reflects their different merger histories. The growth of the BCGs in mass and size seems to be linked to the hierarchical growth of the structures they inhabit: as the groups and clusters became denser and more massive, the BCGs at their centres also grew. This process is nearing completion since the majority (∼60 per cent) of the BCGs in the local Universe have cD morphology. However, the presence of galaxies with intermediate morphological classes (between ellipticals and cDs) suggests that the growth and morphological transformation of some BCGs is still ongoing.

Surface photometry of brightest cluster galaxies and intracluster stars in ΛCDM

We simulate the phase-space distribution of stellar mass in nine massive Λ cold dark matter galaxy clusters by applying the semi-analytic particle tagging method of Cooper et al. to the Phoenix suite of high-resolution N-body simulations (M200 ≈ 7.5–33 × 1014 M⊙). The resulting surface brightness (SB) profiles of brightest cluster galaxies (BCGs) match well to observations. On average, stars formed in galaxies accreted by the BCG account for ≳90 per cent of its total mass (the remainder is formed in situ). In circular BCG-centred apertures, the superposition of multiple debris clouds (each ≳10 per cent of the total BCG mass) from different progenitors can result in an extensive outer diffuse component, qualitatively similar to a ‘cD envelope'. These clouds typically originate from tidal stripping at z ≲ 1 and comprise both streams and the extended envelopes of other massive galaxies in the cluster. Stars at very low SB contribute a significant fraction of the total cluster stellar mass budget: in the central 1 Mpc2 of a z ∼ 0.15 cluster imaged at SDSS-like resolution, our fiducial model predicts 80–95 per cent of stellar mass below a SB of μV ∼ 26.5 mag arcsec−2 is associated with accreted stars in the envelope of the BCG. The ratio of BCG stellar mass (including this diffuse component) to total cluster stellar mass is ∼30 per cent.

Galactic accretion and the outer structure of galaxies in the CDM model

We have combined the semi-analytic galaxy formation model of Guo et al. (2011) with the particle-tagging technique of Cooper et al. (2010) to predict galaxy surface brightness profiles in a representative sample of ~1900 massive dark matter haloes (10^12--10^14 M_sol) from the Millennium II Lambda-CDM N-body simulation. Here we present our method and basic results focusing on the outer regions of galaxies, consisting of stars accreted in mergers. These simulations cover scales from the stellar haloes of Milky Way-like galaxies to the 'cD envelopes' of groups and clusters, and resolve low surface brightness substructure such as tidal streams. We find that the surface density of accreted stellar mass around the central galaxies of dark matter haloes is well described by a Sersic profile, the radial scale and amplitude of which vary systematically with halo mass (M_200). The total stellar mass surface density profile breaks at the radius where accreted stars start to dominate over stars formed in the galaxy itself. This break disappears with increasing M_200 because accreted stars contribute more of the total mass of galaxies, and is less distinct when the same galaxies are averaged in bins of stellar mass, because of scatter in the relation between M_star and M_200. To test our model we have derived average stellar mass surface density profiles for massive galaxies at z~0.08 by stacking SDSS images. Our model agrees well with these stacked profiles and with other data from the literature, and makes predictions that can be more rigorously tested by future surveys that extend the analysis of the outer structure of galaxies to fainter isophotes. We conclude that it is likely that the outer structure of the spheroidal components of galaxies is largely determined by collisionless merging during their hierarchical assembly

SERENDIPITOUS DISCOVERY OF A MASSIVE cD GALAXY ATz= 1.096: IMPLICATIONS FOR THE EARLY FORMATION AND LATE EVOLUTION OF cD GALAXIES

We have made a serendipitous discovery of a massive cD galaxy at z=1.096 in a candidate rich cluster in the HUDF area of GOODS-South. This brightest cluster galaxy is the most distant cD galaxy confirmed to date. Ultra-deep HST/WFC3 images reveal an extended envelope starting from ~10 kpc and reaching ~70 kpc in radius along the semi-major axis. The spectral energy distributions indicate that both its inner component and outer envelope are composed of an old, passively-evolving stellar population. The cD galaxy lies on the same mass-size relation as the bulk of quiescent galaxies at similar redshifts. The cD galaxy has a higher stellar mass surface density but a similar velocity dispersion to those of more-massive, nearby cDs. If the cD galaxy is one of the progenitors of today's more massive cDs, its size and stellar mass have had to increase on average by factors of $3.4\pm1.1$ and $3.3\pm1.3$ over the past ~8 Gyrs, respectively. Such increases in size and stellar mass without being accompanied by significant increases in velocity dispersion are consistent with evolutionary scenarios driven by both major and minor dry mergers. If such cD envelopes originate from dry mergers, our discovery of even one example proves that some BCGs entered the dry merger phase at epochs earlier than z=1. Our data match theoretical models which predict that the continuance of dry mergers at z<1 can result in structures similar to those of massive cD galaxies seen today. Moreover, our discovery is a surprise given that the extreme depth of the HUDF is essential to reveal such an extended cD envelope at z>1 and, yet, the HUDF covers only a minuscule region of sky. Adding that cDs are rare, Our serendipitous discovery hints that such cDs may be more common than expected. [Abridged]

Using the Millennium II simulation to test CDM predictions for the structure of massive galaxies

AbstractWe have combined the semi-analytic galaxy formation model of Guo et al. (2011) with a novel particle-tagging technique to predict galaxy surface brightness profiles in a representative sample of ~1900 massive dark matter haloes (1012–1014 M⊙) from the Millennium II ΛCDM N body simulation. We focus on the outer regions of galaxies and stars accreted in mergers. Our simulations cover scales from the stellar haloes of Milky Way-like galaxies to the ‘cD envelopes’ of groups and clusters, and resolve low surface brightness substructure such as the tidal streams of dwarf galaxies. We find that the spatial distribution of stars in low surface brightness regions is tightly correlated with DM halo mass and that collisionless merging during the hierarchical assembly of galaxies largely determines the structure of spheroidal stellar components. Our ΛCDM model agrees well with the available data.

Neutron dose equivalent measured at the maze door with various openings for the jaws and MLC

This study was undertaken to explore the effects of the jaws and the MLC openings on the neutron dose equivalent (DE) at the maze door and neutron flux at the patient plane. The neutron dose equivalent was measured at the maze entrance door of a 15 MV therapy linear accelerator room. All measurements were performed using various field sizes up to 40 cm × 40 cm. Activation detectors constructed from natural Indium (In) were exposed at Cd envelope to neutrons in order to estimate relative changes of epithermal neutron fluences in the patient plane. Our study showed that the dose equivalent at the maze door is at the highest when the jaw are closed and that maximal jaws opening reduces the DE by more than 20%. The neutron dose equivalent at the maze door measured for radiation fields defined by jaws do not differ significantly from the DE measured when MLC determines the same size radiation field. The epithermal capture reaction rate measured using different jaw openings differs by approximately 10%. When an MLC leaf is inserted into a fixed geometry for one opening of the jaws, an increase of the epithermal neutron capture reaction rate in Indium activation detectors was observed. There is no significant difference in the neutron DE when MLC defines radiation field instead of jaws. This leads to the conclusion that the overall number of neutrons remains similar and it does not depend on how primary photon beam was stopped-by the jaws or the MLC. An increase of the fast neutron capture reaction rate when MLC leaves are inserted probably originates from the neutron scattering.

Single Domain Metallothioneins: Supermetalation of Human MT 1a

Metallothioneins are a family of small, cysteine rich proteins that have been implicated in a range of roles including toxic metal detoxification, protection against oxidative stress, and as metallochaperones involved in the homeostasis of both essential zinc and copper. We report that human metallothionein 1a, well-known to coordinate 7 Zn(2+) or Cd(2+) ions with 20 cysteinyl thiols, will bind 8 structurally significant Cd(2+) ions, leading to the formation of the supermetalated Cd(8)-βα-rhMT 1a species, for which the structure is a novel single domain. ESI-mass spectrometry was used to determine the exact metalation status of the βα-rhMT. The derivative-shaped CD envelope of Cd(7)-βα-rhMT [peak extrema (+) 260 and (-) 239 nm] changed drastically upon formation of the Cd(8)-βα-rhMT with the appearance of a sharp monophasic CD band centered on 252 nm, a feature indicative of the loss of cluster symmetry. The structural significance of the eighth Cd(2+) ion was determined from a combination of direct and indirect (113)Cd nuclear magnetic resonance (NMR) spectra. In the case of Cd(8)-βα-rhMT, only four peaks were observed in the direct (113)Cd NMR spectrum. Significantly, while both of the isolated domains can be supermetalated forming Cd(4)-β-rhMT and Cd(5)-α-rhMT, Cd(8)-βα-rhMT and not Cd(9)-βα-rhMT was observed following addition of excess Cd(2+). We propose that both domains act in concert to coordinate the eighth Cd(2+) atom, and furthermore that this interaction results in a coalescence of the two domains leading to collapse of the two-domain structure. This is the first report of a possible single-"superdomain" metallothionein structure for Zn(2+) and Cd(2+) binding mammalian proteins. A computational model of a possible single-domain structure of Cd(8)-βα-rhMT is described.

SU‐E‐T‐297: Neutron Fluxes at the Patient Plane and Dosages Measured at the Maze Door with Various Openings for the Jaws and MLC

Purpose: This study was undertaken to study the effects of the jaws and MLC openings on the neutron flux and dosage. Methods: Activation detectors constructed from natural Indium (In) were exposed with and without the Cd envelope to neutrons created in the head of a 15 MV therapy linear accelerator for the purpose of estimating thermal and epithermal neutron fluences at the patient plane. All measurements were performed using openings for the jaws which defined various field sizes up to 40 cm × 40 cm. The neutron dose was measured at the maze entrance door of a therapy room. Results: Our studies determined that the epithermal neutron flux as measured using different jaws openings can differ by approximately 10%, the dose at the door is highest when the jaws are closed, and the maximal jaws opening reduces the dosage about 20%. When an MLC is inserted into a fixed geometry for the one opening of the jaws, the epithermal neutron flux can increase by up to 10%, while the neutron dose at the maze entrance door increases by approximately 6%. Conclusions: As expected, the epithermal neutron flux, as well as dosage measured at the maze door is lowest when the largest width is used for the jaw and MLC opening. The range of changes of flux and dose is determined for several different field geometries.

Surface photometry of cD envelopes

We have carried out deep BVR surface photometry of 6 cD and cD-like galaxies using the 2.1-m telescope at San Pedro Mártir, cD galaxies are supergiant galaxies (M&gt;1013M⊙) with enormous halos (&gt;100 kpc in radius) surrounding a giant elliptical galaxy, found generally at the centre of rich clusters (Oemler 1976, Schombert 1988). The surface brightness profiles of their halos (envelopes) break from the r1/4 law, containing more light at large radii (Schombert (1988)), although a detailed 1 and 2 dimensional analysis of their morphology has yet to be carried out. There have been four main theories as to the origin of cD envelopes (Schombert (1988) and references within), a) stripping of stars from other cluster member galaxies, b) formation of galaxy and envelope at the same time during the formation of the cluster, c) mergers of cluster members, which do not easily explain the high velocity dispersions in the envelopes (~ 1000 km s−1), d) cooling flows: accumulation of cooling X-ray-emitting ICM gas around the central galaxy. Very red envelopes have been found around some cD's, and star formation biased towards lowmass stars in cooling flows were invoked to explain these red halos, but the expected very bright near-IR halos were not detected (Joy et al. 1995 and references within). Previous detailed studies of cD galaxies (e.g. Mackie 1992) found a range of colour gradients.

Velocity Dispersion, Mass, and the Luminosity Function of the Fossil Cluster RX J1416.4+2315

We study the properties of the fossil cluster RX J1416.4+2315 through g' and i'-band imaging and spectroscopy of 25 member galaxies. The system is at a mean redshift of 0.137 and has a velocity dispersion of 584 km s^-1. Superimposed onto one quadrant of the cluster field there is a group of five galaxies at a mean redshift of 0.131, which, if included as part of the cluster, increases the velocity dispersion to 846 km/s. The central object of RX J1416.4+2315 is a normal elliptical galaxy, with no cD envelope. The luminosity function of the system, estimated by the number counts, and statistical background correction, in the range -22.6< M_g'< -16.6, is well fitted by a Schechter function with M_g'^* = -21.2 +/- 0.8 and alpha = -1.2 +/- 0.2 (H_0 = 70 km s^-1 Mpc^-1, Omega_M=0.3, Omega_Lambda=0.7). The luminosity function obtained from the spectroscopically confirmed members in both g' and i' bands agrees with the photometric results. The mass of the system, M 0.9 \times 10^14 h^-1_70 M_sun, its M/L of 445 h_70 M_sun/L_B_sun and L_X of 11 10^43 h^-2_70 ergs s^-1 (bolometric) suggest that this system is the second example of known fossil cluster, after RX J1552.2+2013, confirmed in the literature.

A Deep Radio Survey of Abell 2125. III. The Cluster Core: Merging and Stripping

We use radio, near-IR, optical, and X-ray observations to examine dynamic processes in the central region of the rich galaxy cluster Abell 2125. In addition to the central triple of E and cD galaxies, including members of both major dynamical subsystems identified from a redshift survey, this region features a galaxy showing strong evidence for ongoing gas stripping during a high-velocity passage through the gas in the cluster core. The disk galaxy C153 exhibits a plume stretching toward the cluster center seen in soft X-rays by Chandra, parts of which are also seen in [O II] emission and near-UV continuum light. Hubble Space Telescope imaging shows a distorted disk with star-forming knots asymmetrically distributed and remnant spiral structure possibly defined by dust lanes. The stars and ionized gas in this disk are kinematically decoupled, demonstrating that pressure stripping (possibly turbulent, as well as ram) must be important, and that tidal disruption is not the only mechanism at work. Comparison of the gas properties seen in the X-ray and optical data on the plume highlight significant and poorly known features of the history of stripped gas in the intracluster medium that could be clarified through further observations of this system. The nucleus of C153 also hosts an active galactic nucleus, shown by the weak and distorted extended radio emission and a radio-compact core. The unusual strength of the stripping signatures in this instance is likely related to the very high relative velocity of the galaxy with respect to the intracluster medium during a cluster-cluster merger and its passage very near the core of the cluster. An additional sign of recent dynamical events is the diffuse starlight asymmetrically placed about the central triple in a cD envelope. Transient and extreme dynamical events as seen in A2125 may be important drivers of galaxy evolution in the cores of rich clusters.

Deep Surface Photometry of M87 with 13 Optical Bands

Multicolor surface photometry of M87 was undertaken with the Beijing-Arizona-Taiwan-Connecticut photometric system, which contains 13 optical bands covering a range from 3800 to 10000 A. Radial profiles have been derived for surface brightness, ellipticity, position angle, and the residuals from the fitted ellipses. The results show a good agreement with pure elliptical isophotes for M87, and no obvious substructure or dust is detected out to r = 500'', corresponding to 40 kpc from the center. The surface brightness profile at r < 190'' is well fitted by the de Vaucouleurs law. Moreover, a cD envelope is detected outside that radius. Given the surface brightness profiles of the 13 bands, the spectral energy distribution (SED) of M87 is obtained as a function of semimajor axis (SMA). By fitting the observed SEDs to synthetic ones derived from the PEGASE model, we explored the star formation history (SFH) of this giant elliptical galaxy. In the hypothesis that the evolution time T of M87 is the same as the mean age of its globular clusters, about 14 Gyr, we find that the characteristic timescale τ of the SFH increases monotonically from 2 to 7 Gyr as a function of SMA, while the luminosity-weighted mean stellar ages range from about 11 to 9 Gyr in a mild decrease, and the mean stellar metallicity Z drops from 0.017 to 0.014 as the SMA increases to r = 500''. In the range 40'' < r < 190'', the metallicity gradient Δ[Fe/H]/Δ[log r] = -0.04, while in the range 190'' < r < 400'', the metallicity gradient Δ[Fe/H]/Δ[log r] = -0.17.

Photometric Properties of Low-Redshift Galaxy Clusters

A recent comprehensive photometric survey of 45 low-z X-ray selected Abell clusters (López-Cruz 1997) has measured significant variations in the faint end slope of the luminosity function (LF). This result indicates that dwarf galaxies (dGs) comprise a different fraction of the cluster population as a function cluster environment. Clusters having a central “cD-like” galaxy have a flatter faint end slope than non-cD clusters. Also, cD clusters were found to have a dwarf-to-giant ratio (D/ G) which was smaller than non-cD clusters. López-Cruz et al. (1997) has suggested that the light contained in cD envelopes can be accounted for by assuming that it is produced from stars that originally formed dGs. In this simple model, the D/ G would be expected to increase with radial distance from the cluster centre due to the decrease in the disruptive forces.

A new insight into the Ag + and Cu + binding sites in the metallothionein β domain

The copper( I) and silver( I) binding properties of the β fragment of recombinant mouse metallothionein 1 have been studied by electronic absorption and circular dichroism spectroscopy. When possible, the stoichiometry of the species formed was confirmed by electrospray mass spectrometry. The behaviour observed differs from that reported for the native protein. Titration of either Zn 3-βMT at pH 7 or apo-βMT at pH 3 with Cu + leads to the formation of species having the same stoichiometry and structure: Cu 6-βMT, Cu 7-βMT and Cu 10-βMT. In the first stage of the titration of Zn 3-βMT with Cu + at pH 7 one additional species of formula Cu 4Zn 1-βMT was detected. In contrast, the titration of Zn 3-βMT at pH 7.5 and of apo-βMT at pH 2.5 with Ag + proceeds through different reaction pathways, affording Zn x Ag 3-βMT, Ag 6-βMT and Ag 9-βMT or Ag 3-βMT, Ag 6-βMT and Ag 9-βMT, respectively. The CD envelope corresponding to species with the same stoichiometric ratio, Ag 6-βMT and Ag 9-βMT, indicates that they have a different structure at each pH value. On the basis of the differences observed, the postulated similarity between copper and silver binding to metallothionein may be questioned.

Metal abundances for a large sample of globular clusters in M87

We present deep integrated Washington CCD photometry of the extremely populous globular cluster system of NGC 4486 (M87) in Virgo. The metallicities of 407 of the best candidates with T_1_ &lt; 22 have been estimated from the (C - T_1_) color. These values are internally precise to ~0.15 dex and accurate to ~0.25 dex. The mean metallicity of the globular clusters is [Fe/H] = -0.86 dex, with a formal standard error of the mean of 0.03 dex but a more realistic uncertainty of about 0.2 dex. This value is higher than that of the corresponding means of the globular clusters in the Galaxy and M 31, but similar to those of the globular clusters in NGC 1399 and NGC 5128, as expected from the relative parent galaxy luminosities. ApproximatelY 10% of the clusters exceed solar abundance, similar to the case for the globular clusters in NGC 1399 and NGC 5128. As found by Couture et al. [ApJS, 73,671(1990)], there is a large metallicity dispersion at any radius, with a σ ~ 0.65 dex. However, in contrast to their result and that of previous small-field CCD studies, the globular clusters display a marked gradient in both mean (M- T_1_) and (C - T_1_) color with galactocentric distance. The mean metallicity decreases by some 0.6 dex over the radial extent of the cluster sample. This result is consistent with the early photographic study by Strom et al. [ApJ, 245,416 (1981)]. The metallicity distribution function is remarkably similar to that of NGC 1399 found by Ostrov et al. [AJ, 105,1762(1993)] using the same technique. The two central cluster ellipticals show significant peaks at very similar metallicities, indicating similar formation and enrichment histories. We also find that this structure in the metallicity distribution function persists to radii where the halo light is dominated by the cD envelope.

Globular clusters and cD formation - The case of M87

New wide-field CCD photometry around M87 is used to define the radial profile for its globular cluster system over the region 1'.2≤R≤9'.2. The best-fit de Vaucouleurs profile clearly shows an excess of clusters beginning at R∼4'.5, just where the cD envelope of the M87 halo begins. We find that this extra cD component of the globular cluster population has the same high specific frequency as the main body of M87 does; this result is used to argue for an epoch of cD envelope formation which precedes both astration of the envelope and relaxation of the Virgo Cluster itself

The stellar content of central dominant galaxies. II - Colors of cD envelopes

The study presents deep g- and r-band CCD imaging of three rich clusters, A2589, A2634, and A407, containing cD galaxies or structures possibly relating to cD envelopes. Scattered light r exp -2 halos were modeled and subtracted from the images. The (g-r) color profiles of the cDs NGC 7647 (A2589) and NGC 7720 (A2634) down to the start of the envelopes are found to be quite flat. There is no evidence of a change in color at the surface brightness break that signifies the start of the cD envelope, nor are the colors of the envelope dramatically blue. The color profile of the D galaxy NGC 7728 also in A2634 displays a steeper blueward color gradient than NGC 7720. A407, which has nine nuclei in a common envelope, may also possess a red envelope in comparison to its higher surface brightness region, and thus may evolve into a cD similar to NGC 7647. The implications for cD envelope formation and evolution are discussed.

Discovery of structure in the envelope of the cD galaxy in the cluster Abell 1795

The discovery of structure in the cD envelope of the central galaxy in the rich cluster A1795 is reported. The envelope extends predominatly in the north-south direction to at least 400 kpc from the center of the galaxy. At the largest radii, there is a region to the south of low surface brightness that turns slightly to the east. The total I-band magnitude of the galaxy and envelope is at least -26.6, indicating that it is one of the most luminous galaxies known. It is suggested that the structure in the envelope has been caused by a past merger of the central galaxy with another giant galaxy, probably in a subcluster that merged with the cluster. This would explain the alignment of both cD envelope and cluster.