Progenitors Of Galaxies Research Articles

THE EVOLUTION OF CENTRAL GROUP GALAXIES IN HYDRODYNAMICAL SIMULATIONS

We trace the evolution of central galaxies in three ~10^13 M_sun galaxy groups simulated at high resolution in cosmological hydrodynamical simulations. The evolution in the group potential leads, at z=0, to central galaxies that are massive, gas-poor early-type systems supported by stellar velocity dispersion resembling either elliptical or S0 galaxies. Their z~2-2.5 main progenitors are massive M* ~ 3-10 x 10^10 M_sun, star forming (20-60 M_sun/yr) galaxies which host substantial reservoirs of cold gas (~5 x 10^9 M_sun) in extended gas disks. Our simulations thus show that star forming galaxies observed at z~2 are likely the main progenitors of central galaxies in galaxy groups at z=0. Their central stellar densities stay approximately constant from z~1.5 down to z=0. Instead, the galaxies grow inside-out, by acquiring a stellar envelope outside the innermost ~2 kpc. Consequently the density within the effective radius decreases by up to two orders of magnitude. Both major and minor mergers contribute to most of the mass accreted outside the effective radius and thus drive the evolution of the half-mass radii. In one of the three simulated groups the short central cooling time leads to a dramatic rejuvenation of the central group galaxy at z<1, affecting its morphology, kinematics and colors. This episode is eventually terminated by a group-group merger. Our simulations demonstrate that, in galaxy groups, the interplay between halo mass assembly, galaxy merging and gas accretion has a substantial influence on the star formation histories and z=0 morphologies of central galaxies.[Abridged]

CHEMICAL EVOLUTION IN HIERARCHICAL MODELS OF COSMIC STRUCTURE. II. THE FORMATION OF THE MILKY WAY STELLAR HALO AND THE DISTRIBUTION OF THE OLDEST STARS

This paper presents theoretical star formation and chemical enrichment histories for the stellar halo of the Milky Way based on new chemodynamical modeling. The goal of this study is to assess the extent to which metal-poor stars in the halo reflect the star formation conditions that occurred in halo progenitor galaxies at high redshift, before and during the epoch of reionization. Simple prescriptions that translate dark-matter halo mass into baryonic gas budgets and star formation histories yield models that resemble the observed Milky Way halo in its total stellar mass, metallicity distribution, and the luminosity function and chemical enrichment of dwarf satellite galaxies. These model halos in turn allow an exploration of how the populations of interest for probing the epoch of reionization are distributed in physical and phase space, and of how they are related to lower-redshift populations of the same metallicity. The fraction of stars dating from before a particular time or redshift depends strongly on radius within the galaxy, reflecting the "inside-out" growth of cold-dark-matter halos, and on metallicity, reflecting the general trend toward higher metallicity at later times. These results suggest that efforts to discover stars from z > 6 - 10 should select for stars with [Fe/H] <~ -3 and favor stars on more tightly bound orbits in the stellar halo, where the majority are from z > 10 and 15 - 40% are from z > 15. The oldest, most metal-poor stars - those most likely to reveal the chemical abundances of the first stars - are most common in the very center of the Galaxy's halo: they are in the bulge, but not of the bulge. These models have several implications for the larger project of constraining the properties of the first stars and galaxies using data from the local Universe.

STRUCTURE AND POPULATION OF THE ANDROMEDA STELLAR HALO FROM A SUBARU/SUPRIME-CAM SURVEY

We present a photometric survey of the stellar halo of the nearest giant spiral galaxy, Andromeda (M31), using Suprime-Cam on the Subaru Telescope. A detailed analysis of VI color-magnitude diagrams of the resolved stellar population is used to measure properties such as line-of-sight distance, surface brightness, metallicity, and age. These are used to isolate and characterize different components of the M31 halo: (1) the giant southern stream (GSS); (2) several other substructures; and (3) the smooth halo. First, the GSS is characterized by a broad red giant branch (RGB) and a metal-rich/intermediate-age red clump (RC). The I magnitude of the well-defined tip of the RGB suggests that the distance to the observed GSS field is (m – M)0 = 24.73 ± 0.11 (883 ± 45 kpc) at a projected radius of R ~ 30 kpc from M31's center. The GSS shows a high metallicity peaked at [Fe/H]–0.5 with a mean (median) of –0.7 (–0.6), estimated via comparison with theoretical isochrones. Combined with the luminosity of the RC, we estimate the mean age of its stellar population to be ~8 Gyr. The mass of its progenitor galaxy is likely in the range of 107-109 M ☉. Second, we study M31's halo substructure along the northwest/southeast minor axis out to R ~ 100 kpc and the southwest major-axis region at R ~ 60 kpc. We confirm two substructures in the southeast halo reported by Ibata et al. and discover two overdense substructures in the northwest halo. We investigate the properties of these four substructures as well as other structures including the western shelf and find that differences in stellar populations among these systems, thereby suggesting each has a different origin. Our statistical analysis implies that the M31 halo as a whole may contain at least 16 substructures, each with a different origin, so its outer halo has experienced at least this many accretion events involving dwarf satellites with mass 107-109 M ☉ since a redshift of z ~ 1. Third, we investigate the properties of an underlying, smooth, and extended halo component out to R>100 kpc. We find that the surface density of this smooth halo can be fitted to a Hernquist model of scale radius ~17 kpc or a power-law profile with Σ(R) ∝ R –2.17±0.15. In contrast to the relative smoothness of the halo density profile, its metallicity distribution appears to be spatially non-uniform with non-monotonic variations with radius, suggesting that the halo population has not had sufficient time to dynamically homogenize the accreted populations. Further implications for the formation of the M31 halo are discussed.

Early‐type dwarf galaxies with spiral structure

AbstractA significant fraction of Virgo cluster early‐type dwarf (dE) galaxies exhibit disk substructure, like spiral arms and bars. Are these disks the still visible remnants of late‐type progenitor galaxies that harassment transformed into dEs? Simulations of harassment predict tidal debris arcs around the remnants, as well as a distorted outer structure. We aimed at detecting such signatures by means of very deep ESO 2.2m/WFI images of dEs with weak spiral structure. Here, we present a preliminary analysis of these images and their implications (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The properties of fossil groups of galaxies

AbstractNumerical simulations as well as optical and X‐ray observations over the last few years have shown that poor groups of galaxies can evolve to what is called a fossil group. Dynamical friction as the driving process leads to the coalescence of individual galaxies in ordinary poor groups leaving behind nothing more than a central, massive elliptical galaxy supposed to contain the merger history of the whole group. Due to merging timescales for less‐massive galaxies and gas cooling timescales of the X‐ray intragroup medium exceeding a Hubble time, a surrounding faint‐galaxy population having survived this galactic cannibalism as well as an extended X‐ray halo similar to that found in ordinary groups, is expected. Recent studies suggest that fossil groups are very abundant and could be the progenitors of brightest cluster galaxies (BCGs) in the centers of rich galaxy clusters. However, only a few objects are known to the literature. This article aims to summarize the results of observational fossil group research over the last few years and presents ongoing work by the authors. Complementary to previous research, the SDSS and RASS surveys have been cross‐correlated to identify new fossil structures yielding 34 newly detected fossil group candidates. Observations with ISIS at the 4.2 m William Herschel Telescope on La Palma have been carried out to study the stellar populations of the central ellipticals of 6 fossil groups. In addition multi‐object spectroscopy with VLTs VIMOS has been performed to study the shape of the OLF of one fossil system (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

THE SIZE-STAR FORMATION RELATION OF MASSIVE GALAXIES AT 1.5 &lt;z&lt; 2.5

We study the relation between size and star formation activity in a complete sample of 225 massive (M > 5 x 10^10 Msun) galaxies at 1.5<z<2.5, selected from the FIREWORKS UV-IR catalog of the CDFS. Based on stellar population synthesis model fits to the observed restframe UV-NIR SEDs, and independent MIPS 24 micron observations, 65% of galaxies are actively forming stars, while 35% are quiescent. Using sizes derived from 2D surface brightness profile fits to high resolution (FWHM_{PSF}~0.45 arcsec) groundbased ISAAC data, we confirm and improve the significance of the relation between star formation activity and compactness found in previous studies, using a large, complete mass-limited sample. At z~2, massive quiescent galaxies are significantly smaller than massive star forming galaxies, and a median factor of 0.34+/-0.02 smaller than galaxies of similar mass in the local universe. 13% of the quiescent galaxies are unresolved in the ISAAC data, corresponding to sizes <1 kpc, more than 5 times smaller than galaxies of similar mass locally. The quiescent galaxies span a Kormendy relation which, compared to the relation for local early types, is shifted to smaller sizes and brighter surface brightnesses and is incompatible with passive evolution. The progenitors of the quiescent galaxies, were likely dominated by highly concentrated, intense nuclear star bursts at z~3-4, in contrast to star forming galaxies at z~2 which are extended and dominated by distributed star formation.

A Population of Compact Elliptical Galaxies Detected with the Virtual Observatory

Compact elliptical galaxies are characterized by small sizes and high stellar densities. They are thought to form through tidal stripping of massive progenitors. However, only a handful of them were known, preventing us from understanding the role played by this mechanism in galaxy evolution. We present a population of 21 compact elliptical galaxies gathered with the Virtual Observatory. Follow-up spectroscopy and data mining, using high-resolution images and large databases, show that all the galaxies exhibit old metal-rich stellar populations different from those of dwarf elliptical galaxies of similar masses but similar to those of more massive early-type galaxies, supporting the tidal stripping scenario. Their internal properties are reproduced by numerical simulations, which result in compact, dynamically hot remnants resembling the galaxies in our sample.

The origin of failed subhaloes and the common mass scale of the Milky Way satellite galaxies

Abstract We study the formation histories and present-day structure of satellite galaxies formed in a high-resolution hydrodynamic simulation of a Milky Way like galaxy. The simulated satellites span nearly four orders of magnitude in luminosity but have a very similar mass within their inner 600 pc, ∼3 × 107M⊙, with very little scatter. This result is in agreement with the recent measurements for dwarf spheroidal galaxies (dSphs) in the Milky Way by Strigari et al. In our simulations, a preferred mass scale arises naturally from the effects of the early re-ionization of gas. These impose a sharp threshold of ∼12kms−1 on the circular velocity of haloes which can cool gas and make stars. At the present day, subhaloes that host satellites as luminous as the classical Milky Way dwarfs (LV≥ 2.6 × 105L⊙) have typically grown to have circular velocities of ≳20 km s−1. There are, however, subhaloes with similar circular velocities today which were, nevertheless, below threshold at re-ionization and thus remain dark. Star formation in above-threshold haloes is truncated when the halo is accreted into the main galaxy progenitor. Thus, many properties of today's dwarf satellites such as their luminosity and star formation rate are set by their accretion time.

LENTICULAR GALAXIES AND THEIR ENVIRONMENTS

It is widely believed that lenticular (S0) galaxies were initially spirals from which the gas has been removed by interactions with hot cluster gas, or by ram-pressure stripping of cool gas from spirals that are orbiting within rich clusters of galaxies. However, problems with this interpretation are that: (1) Some lenticulars, such as NGC 3115, are isolated field galaxies rather than cluster members. (2) The distribution of flattening values of S0 galaxies in clusters, in groups and in the field are statistically indistinguishable. This is surprising because one might have expected most of the progenitors of field S0 galaxies to have been flattened late-type galaxies, whereas lenticulars in clusters are thought to have mostly been derived from bulge-dominated early-type galaxies. (3) It should be hardest for ram-pressure to strip massive luminous galaxies with deep potential wells. However, no statistically significant differences are seen between the luminosity distributions of early-type Shapley-Ames galaxies in clusters, groups and in the field. (4) Finally, both ram-pressure stripping and evaporation by hot intra-cluster gas would be most efficient in rich clusters. However, the small number of available data in the Shapley-Ames sample appears to show no statistically significant differences between the relative frequencies of dust-poor S0_1 and dust-rich S0_3 galaxies in clusters, groups and in the field. It is tentatively concluded that ram-pressure stripping, and heating by intra-cluster gas, may not be the only evolutionary channels that lead to the formation of lenticular galaxies. It is speculated that gas starvation, or gas ejection by active nuclei, may have play a major role in the formation of a significant fraction of all S0 galaxies.

On the origin of globular cluster bimodality

AbstractGlobular cluster systems in most large galaxies display bimodal color and metallicity distributions, which are frequently interpreted as indicating two distinct modes of cluster formation. The metal-rich (red) and metal-poor (blue) clusters have systematically different locations and kinematics in their host galaxies. However, the red and blue clusters have similar internal properties, such as their masses, sizes, and ages. It is therefore interesting to explore whether both metal-rich and metal-poor clusters could form by a common mechanism and still be consistent with the bimodal distribution. We show that if all globular clusters form only during mergers of massive, gas-rich protogalactic disks, their metallicity distribution could be statistically consistent with that of the Galactic globulars. We take the galaxy assembly history from cosmological dark-matter simulations and couple it with the observed scaling relations for the amount of cold gas available for star formation. In the best-fitting model, early mergers of smaller hosts create exclusively blue clusters, while subsequent mergers of progenitor galaxies with a range of masses create both red and blue clusters. Thus, bimodality arises naturally as the result of a small number of late, massive merger events. We calculate cluster mass loss, including the effects of two-body scattering and stellar evolution, and find that more blue than red clusters are disrupted by the present time because of their lower initial masses and older ages. The present-day mass function in the best-fitting model is consistent with the Galactic distribution. However, the spatial distribution of model clusters is much more extended than observed and is independent of the parameters of our model.

GEMINI SPECTROSCOPIC SURVEY OF YOUNG STAR CLUSTERS IN MERGING/INTERACTING GALAXIES. III. THE ANTENNAE

We present optical spectroscopy of 16 star clusters in the merging galaxies NGC 4038/39 ("The Antennae") and supplement this dataset with HST imaging. The age and metallicity of each cluster is derived through a comparison between the observed Balmer and metal line strengths with simple stellar population models. We then estimate extinctions and masses using the photometry. We find that all but three clusters have ages between ~3-200 Myr, consistent with the expected increase in the star-formation rate due to the merger. Most of the clusters have velocities in agreement with nearby molecular and HI gas that has been previously shown to be rotating within the progenitor galaxies, hence star/cluster formation is still taking place within the galactic disks. However, three clusters have radial velocities that are inconsistent with being part of the rotating gas disks, which is surprising given their young (200-500Myr) ages. Interestingly, we find a stellar association with the same colors (V-I) near one of these three clusters, suggesting that the cluster and association were formed concurrently and have remained spatially correlated. We find evidence for spatially distributed cluster formation throughout the duration of the merger. The impact of various assumptions about the star/cluster formation rate on the interpretation of the cluster age distribution are explored, and we do not find evidence for long term "infant mortality" as has been previously suggested. Models of galaxy mergers that include a prescription for star formation can provide an overall good fit to the observed cluster age distribution.

Generation of rotationally dominated galaxies by mergers of pressure-supported progenitors

Through the analysis of a set of numerical simulations of major mergers between initially non-rotating, pressure-supported progenitor galaxies with a range of central mass concentrations, we have shown that: (1) it is possible to generate elliptical-like galaxies, with outside one effective radius, as a result of the conversion of orbital- into internal-angular momentum; (2) the outer regions acquire part of the angular momentum first; (3) both the baryonic and the dark matter components of the remnant galaxy acquire part of the angular momentum, the relative fractions depending on the initial concentration of the merging galaxies. For this conversion to occur the initial baryonic component must be sufficiently dense and/or the encounter should take place on an orbit with high angular momentum. Systems with these hybrid properties have recently been observed through a combination of stellar absorption lines and planetary nebulae for kinematic studies of early-type galaxies. Our results are in qualitative agreement with these observations and demonstrate that even mergers composed of non rotating, pressure-supported progenitor galaxies can produce early-type galaxies with significant rotation at large radii.

Identifying the progenitor set of present-day early-type galaxies: a view from the standard model

We present a comprehensive theoretical study, using a semi-analytical model within the standard LCDM framework, of the photometric properties of the progenitors of present-day early-type galaxies in the redshift range 0<z<1. We explore progenitors of all morphologies and study their characteristics as a function of the luminosity and local environment of the early-type remnant at z=0. In agreement with previous studies, we find that, while larger early-types are generally assembled later, their luminosity-weighted stellar ages are typically older. In dense cluster-like environments, 70% of early-type systems are `in place' by z=1 and evolve without interactions thereafter, while in the field the corresponding value is 30%. Averaging across all environments at z~1, less than 50% of the stellar mass which ends up in early-types today is actually in early-type progenitors at this redshift, in agreement with recent observational work. We develop probabilistic prescriptions which provide a means of including spiral (i.e. non early-type) progenitors at intermediate and high redshifts, based on their luminosity and optical colours. For example, we find that, at intermediate redshifts (z~0.5), large (M_V<-21.5), red (B-V>0.7) spirals have 75-95% chance of being an early-type progenitor, while the corresponding probability for large blue spirals (M_B<-21.5, B-V<0.7) is 50-75%. The prescriptions developed here can be used to address, from the perspective of the standard model, the issue of `progenitor bias', whereby the exclusion of late-type progenitors in observational studies can lead to inaccurate conclusions regarding the evolution of the early-type population over cosmic time. (abridged)

THE RELATION BETWEEN COMPACT, QUIESCENT HIGH-REDSHIFT GALAXIES AND MASSIVE NEARBY ELLIPTICAL GALAXIES: EVIDENCE FOR HIERARCHICAL, INSIDE-OUT GROWTH

Recent studies have shown that massive quiescent galaxies at high redshift are much more compact than present-day galaxies of the same mass. Here we compare the radial stellar density profiles and the number density of a sample of massive galaxies at z ~ 2.3 to nearby massive elliptical galaxies. We confirm that the average stellar densities of the z ~ 2.3 galaxies within the effective radius, rho(<r_e), are two orders of magnitude higher than those of local elliptical galaxies of the same stellar mass. However, we also find that the densities measured within a constant physical radius of 1 kpc, rho(<1 kpc), are higher by a factor of 2-3 only. This suggests that inside-out growth scenarios are plausible, in which the compact high redshift galaxies make up the centers of normal nearby ellipticals. The compact galaxies are common at high redshift, which enables us to further constrain their evolution by requiring that the number density of their descendants does not exceed constraints imposed by the z=0 galaxy mass function. We infer that size growth must be efficient, with (r_{1+2}/r_1) ~ (M_{1+2}/M_1)^2. A simple model where compact galaxies with masses ~ 10^11 Msun primarily grow through minor mergers produces descendants with the approximate sizes, stellar densities, and number density of elliptical galaxies with masses 2-3 x10^11 Msun in the local Universe. We note that this model also predicts evolution in the M_BH - sigma relation, such that the progenitors of elliptical galaxies have lower black hole masses at fixed velocity dispersion. The main observational uncertainty is the conversion from light to mass; measurements of kinematics are needed to calibrate the masses and stellar densities of the high redshift galaxies.

DID THE MILKY WAY DWARF SATELLITES ENTER THE HALO AS A GROUP?

The dwarf satellite galaxies in the Local Group are generally considered to be hosted in dark matter subhalos that survived the disruptive processes during infall onto their host halos. It has recently been argued that if the majority of satellites entered the Milky Way halo in a group rather than individually, this could explain the spatial and dynamical peculiarities of its satellite distribution. Such groups were identified as dwarf galaxy associations that are found in the nearby Universe. In this paper we address the question whether galaxies in such associations can be the progenitors of the Milky Way satellite galaxies. We find that the dwarf associations are much more extended than would be required to explain the disk-like distribution of the Milky Way and Andromeda satellite galaxies. We further identify a possible minor filamentary structure, perpendicular to the supergalactic plane, in which the dwarf associations are located, that might be related to the direction of infall of a progenitor galaxy of the Milky Way satellites, if they are of tidal origin.

MODELING MOLECULAR HYDROGEN AND STAR FORMATION IN COSMOLOGICAL SIMULATIONS

We describe a phenomenological model for molecular hydrogen formation suited for applications in galaxy formation simulations, which includes on-equilibrium formation of molecular hydrogen on dust and approximate treatment of both its self-shielding and shielding by dust from the dissociating UV radiation. The model is applicable in simulations in which individual star forming regions - the giant molecular complexes - can be identified (resolution of tens of pc) and their mean internal density estimated reliably, even if internal structure is not resolved. In agreement with previous studies, calculations based on our model show that the transition from atomic to fully molecular phase depends primarily on the metallicity, which we assume is directly related to the dust abundance, and clumpiness of the interstellar medium. The clumpiness simply boosts the formation rate of molecular hydrogen, while dust serves both as a catalyst of molecular hydrogen formation and as an additional shielding from dissociating UV radiation. The upshot is that it is difficult to form fully-shielded giant molecular clouds while gas metallicity is low. However, once the gas is enriched to Z ~ 0.01-0.1 solar, the subsequent star formation and enrichment can proceed at a much faster rate. This may keep star formation efficiency in the low-mass, low-metallicity progenitors of galaxies very low for a certain period of time with the effect similar to a strong "feedback" mechanism. [abridged]

DISSIPATION AND EXTRA LIGHT IN GALACTIC NUCLEI. II. “CUSP” ELLIPTICALS

We study the origin and properties of or excess central in the surface brightness profiles of cusp or power-law elliptical galaxies. Dissipational mergers give rise to two-component profiles: an outer profile established by violent relaxation acting on stars already present in the progenitor galaxies prior to the final stages of the merger, and an inner stellar population comprising the light, formed in a compact central starburst. By combining a large set of hydrodynamical simulations with data that span a broad range of profiles at various masses, we show that observed cusp ellipticals appear consistent with the predicted extra light structure, and we use our simulations to motivate a two-component description of the observations that allows us to examine how the properties and mass of this component scale with, e.g., the mass, gas content, and other properties of the galaxies. We show how to robustly separate the physically meaningful and outer, violently relaxed profile, and demonstrate that the observed cusps and extra light are reliable tracers of the degree of dissipation in the spheroid-forming merger. We show that the typical degree of dissipation is a strong function of stellar mass, roughly tracing the observed gas fractions of disks of the same mass over the redshift range z ~ 0-2. We demonstrate a correlation between the strength of this component and effective radius at fixed mass, in the sense that systems with more dissipation are more compact, sufficient to explain the discrepancy in the maximum phase-space and mass densities of ellipticals and their progenitor spirals. We show that the outer shape of the profile in simulated and observed systems (when fit to properly account for the central light) does not depend on mass, with a mean outer Sersic index ~2.5. We also explore how this relates to, e.g., the shapes, kinematic properties, and stellar population gradients of ellipticals. Extra contributes to making remnants rounder and diskier, and imprints stellar population gradients. Simulations with the gas content needed to match observed surface brightness profiles reproduce the observed age, metallicity, and color gradients of cusp ellipticals, and we make predictions for how these can be used as tracers of the degree of dissipation in spheroid formation.

ARE DISK GALAXIES THE PROGENITORS OF GIANT ELLIPTICALS?

A popular formation scenario for giant elliptical galaxies proposes that they might have formed from binary mergers of disk galaxies. Difficulties with the scenario that emerged from earlier studies included providing the necessary stellar mass and metallicity, maintaining the tight color-magnitude relation, and avoiding phase-space limits. In this paper we revisit the issue and put constraints on the binary disk merger scenario based on the stellar populations of disk galaxies. We draw the following conclusions: low-redshift collisionless or gaseous mergers of present-day Milky-Way-like disk galaxies do not form present-day elliptical galaxies. Binary mergers of the progenitors of present-day Milky-Way-like disk galaxies could have evolved into intermediate-mass elliptical galaxies (M < M *) if they merged earlier than ≈ 3-4 Gyr ago. Assuming that most present-day disk galaxies formed in a way similar to the Milky Way model presented here, more massive giant ellipticals in general could not have formed from binary mergers of the progenitors of present-day disk galaxies. A major reason for these conclusions is that the mass in metals of a typical disk galaxy is approximately a factor of 4-8 smaller than the mass in metals of a typical early-type galaxy, and this ratio grows to larger values with increasing redshift.

GLOBULAR CLUSTER POPULATIONS IN FOUR EARLY-TYPE POSTSTARBURST GALAXIES

We present a study of the globular cluster (GC) systems of four early-type poststarburst galaxies using deep g- and I-band images from the Advanced Camera for Surveys aboard the Hubble Space Telescope. All the galaxies feature shells distributed around their main bodies and are thus likely merger remnants. The color distribution of the GCs in all four galaxies shows a broad peak centered on g − I ≈ 1.4, while PGC 6240 and PGC 42871 show a significant number of GCs with g − I ≈ 1.0. The latter GCs are interpreted as being of age ∼500 Myr and likely having been formed in the merger. The color of the redder peak is consistent with that expected for an old metal-poor population that is very commonly found around normal galaxies. However, all galaxies except PGC 10922 contain several GCs that are significantly brighter than the maximum luminosity expected of a single old metal-poor population. To test for multiple-age populations of overlapping g − I color, we model the luminosity functions of the GCs as composites of an old metal-poor subpopulation with a range of plausible specific frequencies and an intermediate-age subpopulation of solar metallicity. We find that three of the four sample galaxies show evidence for the presence of an intermediate-age (∼1 Gyr) GC population, in addition to the old metal-poor GC population seen in normal early-type galaxies. None of the galaxies show a significant population of clusters consistent with an old, metal-rich red cluster population that is typically seen in early-type galaxies. The presence of a substantial number of intermediate-age clusters and the absence of old, metal-rich clusters indicate that the progenitor galaxies which formed the resulting shell galaxy were gas rich and did not host significant bulges. Late-type spirals seem to be the most plausible progenitors. These results lend credence to the "merger scenario" in which the red, metal-rich GCs observed in normal ellipticals are formed during a dissipative merger event that also forms the elliptical itself.

On the origin of dwarf elliptical galaxies: the fundamental plane

Context. Early-type dwarf (dE) galaxies are the most common type of galaxies observed in the Universe. Their study has important cosmological implications because according to hierarchical galaxy evolution theories they are the progenitors of brighter galaxies. Nevertheless, the origin of this kind of system is still not well understood. Aims. The aim of the present work is to investigate whether the different locations of dwarf galaxies with respect to ellipticals in the face-on view of the fundamental plane could be due to the transformation of bright disc galaxies in low-mass systems by harassment. Methods. We have run high-resolution N-body numerical simulations to test the tidal stripping scenario of dE galaxies. The present simulations modelled several individual tidal stripping events in initial disc-like galaxy models with different bulge-to-disc mass ratios. Results. The models have shown that tidal stripping is a very efficient mechanism for removing stars and dark matter particles from galaxies, specially from their outer parts. The particles of the disc and halo components were easily stripped, while the bulge particles were not. Thus, the scale length of the discs were 40−50% shorter than the initial ones. Prograde tidal interactions create tidal features like stable bars in the discs of the galaxies. In contrast, bars are inhibited in retrograde encounters. After several tidal interactions the galaxy remnants looks like a dwarf spheroidal system. The final position of the low-mass systems in the face-on view of the fundamental plane (FP) depends on the initial conditions of the simulations. Thus, simulated galaxies with initial large B/D ratios are closer to the face-on view of the fundamental plane defined by bright E and bulges of early-type galaxies. Nevertheless, galaxies with initially small B/D ratio are located, after four fast tidal encounters, at the position of dE galaxies in the face-on view of the fundamental plane.The final position of the remnants in the FP do not depend on the orbital configuration of the encounters. Conclusions. We conclude that fast galaxy-galaxy interactions are efficient mechanisms of transforming bright galaxies into dwarf ones. Indeed, the different location observed between Es and dEs in the face-on view of the fundamental plane can be explained by the formation of dwarf galaxies by harassment of late-type bright ones.