Intracluster Stars Research Articles

Missing thermal energy of the intracluster medium

The Sunyaev‐Zel’dovich (SZ) effect is a direct probe of thermal energy content of the Universe, induced in the cosmic microwave background (CMB) sky through scattering of CMB photons off hot electrons in the intracluster medium (ICM). We report a 9σ detection of the SZ signal in the CMB maps of Wilkinson Microwave Anisotropy Probe (WMAP) three-year data, through study of a sample of 193 massive galaxy clusters with observed X-ray temperatures greater than 3 keV. For the first time, we make a model-independent measurement of the pressure profile in the outskirts of the ICM, and show that it closely follows the profiles obtained by X-ray observations and numerical simulations. We find that our measurements of the SZ effect would account for only half of the thermal energy of the cluster, if all the cluster baryons were in the hot ICM phase. Our measurements indicate that a significant fraction, 35 ± 8 per cent, of baryonic mass is missing from the hot ICM, and thus must have cooled to form galaxies, intracluster stars, or an unknown cold phase of the ICM. There does not seem to be enough mass in the form of stars or cold gas in the cluster galaxies or intracluster space, signalling the need for a yet-unknown baryonic component (at 3σ level), or otherwise new astrophysical processes in the ICM.

The kinematics of intracluster planetary nebulae and the on-going subcluster merger in the Coma cluster core

Aims. The Coma cluster is the richest and most compact of the nearby clusters, yet there is growing evidence that its formation is still on-going. A sensitive probe of this evolution is the dynamics of intracluster stars, which are unbound from galaxies while the cluster forms, according to cosmological simulations. Methods. With a new multi-slit imaging spectroscopy technique pioneered at the 8.2 m Subaru telescope and FOCAS, we have detected and measured the line-of-sight velocities of 37 intracluster planetary nebulae associated with the diffuse stellar population of stars in the Coma cluster core, at 100 Mpc distance. Results. We detect clear velocity substructures within a 6 arcmin diameter field. A substructure is present at ~5000${\rm\,km\,s^{-1}}$, probably from in-fall of a galaxy group, while the main intracluster stellar component is centered around ~6500${\rm\,km\,s^{-1}}$, ~700${\rm\,km\,s^{-1}}$ offset from the nearby cD galaxy NGC 4874. The kinematics and morphology of the intracluster stars show that the cluster core is in a highly dynamically evolving state. In combination with galaxy redshift and X-ray data this argues strongly that the cluster is currently in the midst of a subcluster merger, where the NGC 4874 subcluster core may still be self-bound, while the NGC 4889 subcluster core has probably dissolved. The NGC 4889 subcluster is likely to have fallen into Coma from the eastern A2199 filament, in a direction nearly in the plane of the sky, meeting the NGC 4874 subcluster arriving from the west. The two inner subcluster cores are presently beyond their first and second close passage, during which the elongated distribution of diffuse light has been created. We predict the kinematic signature expected in this scenario, and argue that the extended western X-ray arc recently discovered traces the arc shock generated by the collision between the two subcluster gas halos. Any preexisting cooling core region would have been heated by the subcluster collision.

The Metallicity Distribution of Intracluster Stars in Virgo

We have used the Hubble Space Telescope's Advanced Camera for Surveys (ACS) to detect and measure ~5300 stars in a single intracluster field in the Virgo Cluster. By performing F606W and F814W photometry on these stars, we have determined their metallicity distribution function and constrained the types of stars present in this portion of Virgo's intracluster space. Based on the small number of stars detected that were brighter than the red giant branch (RGB) tip, we suggest that in this region, Virgo's intracluster stars are mostly old (10 Gyr). Through analysis of the RGB stars themselves, we determine that the population contains the full range of metallicities probed (-2.3 ? [M/H] ? 0.0). We also present evidence that the younger (?10 Gyr) component of the population is more metal-rich, with [M/H] > -0.5. The spatial distribution of the most metal-poor stars in the field shows significantly more structure than that of the metal-rich stars, indicating that the intracluster population is not well mixed. We discuss the implications that these observations have for the production of intracluster stars and the dynamical evolution of the Virgo Cluster.

The importance of mergers for the origin of intracluster stars in cosmological simulations of galaxy clusters

We study the origin of the diffuse stellar component (DSC) in 117 galaxy clusters extracted from a cosmological hydrodynamical simulation. We identify all galaxies present in the simulated clusters at 17 output redshifts, starting with z= 3.5, and then build the family trees for all the z= 0 cluster galaxies. The most massive cluster galaxies show complex family trees, resembling the merger trees of dark matter haloes, while the majority of other cluster galaxies experience only one or two major mergers during their entire life history. Then, for each diffuse star particle identified at z= 0, we look for the galaxy to which it once belonged at an earlier redshift, thus linking the presence of the DSC to the galaxy formation history. The main results of our analysis are as follows. (i) On average, half of the DSC star particles come from galaxies associated with the family tree of the most massive galaxy (bright cluster galaxy – hereafter BCG), one quarter comes from the family trees of other massive galaxies and the remaining quarter from dissolved galaxies. That is, the formation of the DSC is parallel to the build-up of the BCG and other massive galaxies. (ii) Most DSC star particles become unbound during mergers in the formation history of the BCGs and of other massive galaxies, independent of cluster mass. Our results suggest that the tidal stripping mechanism is responsible only for a minor fraction of the DSC. (iii) At cluster radii larger than 250 h−1 kpc, the DSC fraction from the BCG is reduced and the largest contribution comes from the other massive galaxies; in the cluster outskirts, galaxies of all masses contribute to the DSC. (iv) The DSC does not have a preferred redshift of formation: however, most DSC stars are unbound at z < 1. (v) The amount of DSC stars at z= 0 does not correlate strongly with the global dynamical history of clusters, and increases weakly with cluster mass.

Virgo’s Intracluster Globular Clusters as Seen by the Advanced Camera for Surveys

We report the discovery of four candidate intracluster globular clusters (IGCs) in a single deep HST ACS field of the Virgo Cluster. We show that each cluster is roughly spherical, has a magnitude near the peak of the Virgo globular cluster luminosity function, has a radial profile that is best fitted by a King model, and is surrounded by an excess of point sources that have the colors and magnitudes of cluster red giant stars. Despite the fact that two of our IGC candidates have integrated colors redder than the mean of the M87 globular cluster system, we propose that all of the objects are metal-poor, with [M/H] < -1. We show that the tidal radii of our intracluster globular clusters are all larger than the mean for Milky Way clusters and suggest that the clusters have undergone less tidal stress than their Galactic counterparts. Finally, we normalize our globular cluster observations to the luminosity of intracluster stars and derive a value of SN ~ 6 for the specific frequency of Virgo intracluster globular clusters. We use these data to constrain the origins of Virgo's intracluster population and suggest that globular clusters in our intracluster field have a different origin than globular clusters in the vicinity of M87. In particular, we argue that dwarf elliptical galaxies may be an important source of intracluster stars.

Color‐Magnitude Diagrams of Resolved Stars in Virgo Cluster Dwarf Galaxies

The Advanced Camera for Surveys (ACS) on HST has been used to image two fields in the core of the Virgo cluster that contain a number of dwarf elliptical galaxies. The combined F555W and F814W images have resolved red giant stars in these galaxies, down to 1 mag below the giant branch tip. Two of the galaxies were targeted because of their extremely low central surface brightnesses (Bo > 27.0), thus the successful resolution into stars confirms the existence of such tenuous galaxies. Red giant stars were also found that are not ostensibly associated with any galaxy. Color-magnitude diagrams in V and I have been derived and used to derive distances and metallicities via the magnitude and mean color of the red giant branch tip. The mean abundances of stars in the dwarfs range from -1.2 [Fe/H] < -2.4, and fall along the relation between galaxy luminosity and metallicity found for Local Group and M81 group dwarf ellipticals. The mean distance modulus of the six Virgo galaxies is 31.0 +/- 0.05, or 16.1 +/- 0.4 Mpc, whereas that for the intracluster stars in those fields is 31.2 +/- 0.09 (17.4 +/- 0.7 Mpc).

Kinematic Substructures in the Coma Cluster Core as traced by Intracluster Planetary Nebulae

The Coma cluster is the richest and most compact of the nearby clusters, yet there is growing evidence that its formation is still on-going. A sensitive probe of this evolution is the dynamics of intracluster stars, which are unbound from galaxies while the cluster forms, according to cosmological simulations. With a new multi-slit imaging spectroscopy technique pioneered at the 8.2 m Subaru telescope and FOCAS, we can now detect and measure the line-of-sight velocities of the intracluster planetary nebulae which are associated with the diffuse stellar population of stars, at 100 Mpc distance. We detect significant velocity substructures within a 6 arcmin diameter field, centred on the Coma X-ray cluster emission. One substructure is present at $\sim $ 5000 km s $^{-1}$ , probably from infall of a galaxy group, while the main intracluster stellar component moves at $\sim $ 6500 km s $^{-1}$ . Hence the ICPNs associated with the diffuse light at the position of the MSIS field are not bound to the nearby cD galaxy NGC 4874, whose radial velocity is $\sim $ 700 km s $^{-1}$ higher.

Intracluster stars tracing motions in nearby clusters

Cosmological simulations of structure formation predict that galaxies are dramatically modified by galaxy harassment during the assembly of galaxy clusters, losing a substantial fraction of their stellar mass which today must be in the form of intracluster stars. Simulations predict non-uniform spatial and radial velocity distributions for these stars. Intracluster planetary nebulae are the only abundant component of the intracluster light whose kinematics can be measured at this time. Comparing these velocity distributions with simulations will provide a unique opportunity to investigate the hierarchical cluster formation process as it takes place in the nearby universe.

The Survival of Planetary Nebulae in the Intracluster Medium

The stellar population stripped from galaxies in clusters evolves under the extreme conditions imposed by the intracluster (IC) medium. Intracluster stars generally suffer very high systemic velocities moving through a rarefied and extremely hot IC medium. We present numerical simulations that aim to explore the evolution and survival of IC asymptotic giant branch (AGB) envelopes and planetary nebula (PN) shells. Our models reflect the evolution of a low-mass star under the observed conditions in the Virgo IC medium. We find that the integrated hydrogen-recombination line emission of a PN is dominated by the inner dense shell, whose evolution is unaffected by the environment. Ram pressure stripping mainly affects the outermost IC PN shell, which hardly influences the emission when the PN is observed as a point source. More importantly, we find that a PN with progenitor mass of 1 M☉ fades to ~30% and ~10% of its maximum emission in 5000 and 10,000 yr, respectively, disclosing an actual PN lifetime (tPN) several times shorter than what is usually adopted (25,000 yr). This result affects the theoretical calculation of the luminosity-specific density of IC PNe, which scales with tPN. For tPN = 10,000 yr, our more conservative estimate, we obtain that the luminosity-specific density of PNe is in fair agreement with the value obtained from red giants. With our more realistic PN lifetime, we infer a fraction of IC starlight in the Virgo core of above 15%, which is higher than current estimates.

Simulating galaxy clusters - III. Properties of the intracluster stars

Cosmological (� -cold dark matter) TREESPH simulations of the formation and evolution of galaxy groups and clusters have been performed. The simulations invoke star formation, chemical evolution with non-instantaneous recycling, metallicity-dependent radiative cooling, strong starburst, and (optionally) active galactic nuclei, driven galactic superwinds, effects of a metagalactic ultraviolet field and thermal conduction. Results for two clusters, one Virgo-like (T � 3k eV) and one (sub-) Coma-like (T � 6k eV), are presented. At z = 0 the stellar con- tents of both clusters consist of a central dominant (cD) galaxy surrounded by cluster galaxies and intracluster (IC) stars. The IC stars are found to contribute 20-40 per cent of the total cluster B-band luminosity at z = 0 and to form at a mean redshiftf ∼ 3, which is on average approximately 0.5 Gyr older than the stars in cluster galaxies. UBVRIJHK surface brightness profiles of the IC star populations are presented; the profile of the larger cluster matches the observed V-band profile of the cD in Abell 1413 (T � 8k eV) quite well. The typical colour of the IC stellar population is B − R = 1.4-1.5, comparable to the colour of sub-L ∗ E and S0 galaxies. The mean iron abundance of the IC stars is approximately solar in the central part of the cluster (r ∼ 100 kpc) decreasing to approximately half solar at the virial radius. The IC stars are α-element enhanced with a weak trend of (O/Fe) increasing with r and an overall (O/Fe) ∼ 0.4 dex, indicative of dominant enrichment from Type II supernovae. The IC stars are kinematically significantly colder than the cluster galaxies: the velocity dispersions of the IC stars are in the inner parts of the clusters (r ∼ 100-500 kpc) only approximately half of those of the cluster galaxies, increasing slightly outward to approximately 70 per cent at r = 1-2 Mpc. The typical projected velocity dispersion in the Virgo-like cluster at R 50 kpc is 300-600 km s −1 , depending on orientation and projected distance from the cluster centre. Rotation is found to be dynamically insignificant for the IC stars. The velocity distributions of IC stars and clusters galaxies are highly radially anisotropic in one cluster and in the other close to being isotropic. Ke yw ords: galaxies: clusters: general - galaxies: evolution - galaxies: formation - cosmology: theory.

Intracluster Light in Nearby Galaxy Clusters: Relationship to the Halos of Brightest Cluster Galaxies

We present a detailed analysis of the surface brightness distribution of the brightest cluster galaxy (BCG) in each of 24 galaxy clusters at 0.03<z<0.13. We use two-dimensional profile fitting to model the surface brightness out to r=300 kpc for each BCG, comparing r^{1/4}, r^{1/n}, and double r^{1/4} models. We obtain statistically superior fits using a two component model consisting of a pair of r^{1/4} profiles with independent scale lengths, ellipticities, and orientations. The two component model can simply reproduce the observed position angle and ellipticity gradients, which cannot generally be explained purely by triaxiality. The inner component of our two component model has properties similar to a typical massive elliptical galaxy and is clearly associated with the BCG. The outer component is 10-40 times larger in scale, has ~10 times the total luminosity of the inner component, and exhibits a steeper <\mu>-r_e relation than that of the elliptical fundamental plane. We interpret this outer component as a population of intracluster stars tracing the cluster potential. The two components are strongly aligned (|\Delta\theta|<10\degr) in roughly 40% of the clusters. When they are not aligned, the components tend toward high degrees of misalignment, suggesting that accretion of infalling material may change the orientation of some BCGs for a time. The extent of the outer component and its similar elongation to published cluster galaxy distributions indicates that the evolution of the intracluster light is tied to the cluster as a whole rather than to the BCG.

Intracluster Planetary Nebulae in the Virgo Cluster. III. Luminosity of the Intracluster Light and Tests of the Spatial Distribution

Intracluster planetary nebulae are a useful tracer of the evolution of galaxies and galaxy clusters. We analyze our catalog of 318 intracluster planetary nebulae candidates found in 0.89 deg-2 of the Virgo Cluster. We give additional evidence for the great depth of the Virgo Cluster's intracluster stellar population, which implies that the bulk of the intracluster stars come from late-type galaxies and dwarfs. We also provide evidence that the intracluster stars are clustered on the sky on arcminute scales, in agreement with tidal-stripping scenarios of intracluster star production. Although significant systematic uncertainties exist, we find that the average fraction of intracluster starlight in the Virgo Cluster is 15.8% ± 3.0% (statistical) ± 5.0% (systematic) and may be higher if the intracluster stars have a large spatial line-of-sight depth. We find that the intracluster star density changes little with radius or projected density over the range surveyed. These results, along with other intracluster star observations, imply that intracluster star production in Virgo is ongoing and consistent with the cluster's known dynamical youth.

The origin and properties of intracluster stars in a rich cluster

We use a multimillion particle N‐body + smooth particle hydrodynamics (SPH) simulation to follow the formation of a rich galaxy cluster in a Λ‐cold dark matter (ΛCDM) cosmology, with the goal of understanding the origin and properties of intracluster stars. The simulation includes gas cooling, star formation, the effects of a uniform ultraviolet background and feedback from supernovae. Haloes that host galaxies as faint as MR=−19.0 are resolved by this simulation, which includes 85 per cent of the total galaxy luminosity in a rich cluster. We find that the accumulation of intracluster light (ICL) is an ongoing process, linked to infall and stripping events. The unbound star fraction varies with time between 10 and 22 per cent of the total amount of cluster stars, with an overall trend to increase with time. The fraction is 20 per cent at z= 0, consistent with observations of galaxy clusters. The surface brightness profile of the cD galaxy shows an excess compared with a de Vaucouleur profile near 200 kpc, which is also consistent with observations. Both massive and small galaxies contribute to the formation of the ICL, with stars stripped preferentially from the outer, lower‐metallicity parts of their stellar distributions. Simulated observations of planetary nebulae (PNe) show significant substructure in velocity space, tracing separate streams of stripped intracluster stars. Despite an unrelaxed distribution, individual intracluster PNe might be useful mass tracers if more than five fields at a range of radii have measured line‐of‐sight velocities, where an accurate mass calculation depends more on the number of fields than the number of PNe measured per field. However, the orbits of intracluster stars are more anisotropic than those of galaxies or dark matter, which leads to a systematic underestimate of cluster mass relative to that calculated with galaxies, if not accounted for in dynamical models. Overall, the properties of ICL formed in a hierarchical scenario are in good agreement with current observations, supporting a model where ICL originates from the dynamical evolution of galaxies in dense environments. ICL should thus be ubiquitous in galaxy clusters.

Intracluster Stars and the Chemical Enrichment of the Intracluster Medium

We explore the contribution of intracluster stars (ICS) to the chemical enrichment history of the intracluster medium (ICM). In contrast to scenarios in which all the metals originate in cluster galaxies and are then transported into the ICM, intracluster stars enrich the ICM in situ, thereby contributing 100% of their supernovae ejecta directly into the ICM. Modeling the ICS as an ancient, single burst stellar population with a normal initial mass function, we generate ICM iron abundances in the range of the observed values of several tenths solar. Large observational and theoretical uncertainties preclude us from concluding that the intracluster stars are the primary contributor of metals to the ICM in general. However, for the two clusters in our sample, and one from the literature, for which all of the required observational constraints exist, we are able to reproduce between half and all of the ICM iron. Due to the ubiquity of intracluster stars in clusters and their direct connection with the ICM, we conclude that all models of the chemical enrichment history and energy budget of the ICM should account for the impact of the ICS.

Galaxy Threshing and the Origin of Intracluster Stellar Objects

We numerically investigate the dynamical evolution of non-nucleated dwarf elliptical/spiral galaxies (dE) and nucleated ones (dE,Ns) in clusters of galaxies in order to understand the origin of intracluster stellar objects, such as intracluster stars (ICSs), GCs (ICGCs), and “ultra-compact dwarfs” (UCDs) recently discovered by all-object spectroscopic survey centred on the Fornax cluster of galaxies. We find that the outer stellar components of a nucleated dwarf are removed by the strong tidal field of the cluster, whereas the nucleus manages to survive as a result of its initially compact nature. The developed naked nucleus is found to have physical properties (e.g., size and mass) similar to those observed for UCDs. We also find that the UCD formation process does depend on the radial density profile of the dark halo in the sense that UCDs are less likely to be formed from dwarfs embedded in dark matter halos with central ‘cuspy’ density profiles. Our simulations also suggest that very massive and compact stellar systems can be rapidly and efficiently formed in the central regions of dwarfs through the merging of smaller GCs. GCs initially in the outer part of dE and dE,Ns are found to be stripped to form ICGCs.

Intracluster Planetary Nebulae in Clusters and Groups

We present the results from multiple surveys for intracluster planetary nebulae (IPNe) in nearby galaxy clusters and groups. We find that in the case of clusters, our observations imply: 1) the amount of intracluster starlight is significant, up to 20% of the total starlight, 2) in Virgo, is elongated along our line of sight, and 3) is clustered on the sky, implying ongoing tidal stripping. In contrast, searches for IPNe in groups have found little or no intra-group population, implying there may be something in the cluster environment that significantly enhances intracluster star production. from high-resolution N-body simulations, we find that the IPNe should create observable features in position-velocity space, and that these features may eventually allow us to place limits on the dynamics of galaxy clusters.

The Systematics of Intracluster Starlight

Surveys for intracluster planetary nebulae and red giants have been performed in a variety of cluster environments, from the well-mixed regions of the Virgo and Fornax Clusters, to the low-density areas of the M81 Group. When coupled with the results of Abell Cluster surface photometry and intracluster supernova detections, these surveys provide clues to the systematics of the intracluster light phenomenon. We analyze the results of the intracluster surveys performed to date and show how the fraction of intracluster stars depends on parameters such as cluster richness and rate of galaxy interaction. We also compare our results to numerical predictions and discuss avenues for future research.

A class of compact dwarf galaxies from disruptive processes in galaxy clusters.

Dwarf galaxies have attracted increased attention in recent years, because of their susceptibility to galaxy transformation processes within rich galaxy clusters. Direct evidence for these processes, however, has been difficult to obtain, with a small number of diffuse light trails and intra-cluster stars being the only signs of galaxy disruption. Furthermore, our current knowledge of dwarf galaxy populations may be very incomplete, because traditional galaxy surveys are insensitive to extremely diffuse or compact galaxies. Aware of these concerns, we recently undertook an all-object survey of the Fornax galaxy cluster. This revealed a new population of compact members, overlooked in previous conventional surveys. Here we demonstrate that these 'ultra-compact' dwarf galaxies are structurally and dynamically distinct from both globular star clusters and known types of dwarf galaxy, and thus represent a new class of dwarf galaxy. Our data are consistent with the interpretation that these are the remnant nuclei of disrupted dwarf galaxies, making them an easily observed tracer of galaxy disruption.

Cluster‐Cluster Microlensing as a Probe of Intracluster Stars, MACHOs, and Remnants of the First‐Generation Stars

An Abell cluster, A2152, was recently found to be forming a cluster-cluster system with another, more distant cluster whose core is almost perfectly aligned with that of A2152. We discuss the detectability of microlensing events where a single star in the source cluster behind A2152 is extremely magnified by an intracluster compact object in A2152. We show that a search with an 8 m class telescope with a wide field of view, such as the Subaru/Suprime-Cam, can probe intracluster compact objects with a wide mass range of mco ~ 10-5 to 1010 M☉, including ranges that have not yet been constrained by any past observations. We expect that the event rate is more biased than the foreground cluster (A2152), which would be a unique signature of microlensing, making this experiment particularly powerful. The sensitivity of this experiment for the mass fraction of compact objects would be 1%-10% in the total dark matter of the cluster, which is roughly constant against mco, with a reasonable telescope time for large telescopes (~10 nights). Therefore, any compact objects in this mass range can be detected or rejected as the dominant component of the dark matter. About 10 events are expected if 20% of the cluster mass is in a form of compact objects with M ~ 1 M☉, as claimed by the MACHO collaboration for the Milky Way halo. Other possibly detectable targets include intracluster stars stripped by galaxy interactions, and hypothetical very massive black holes (M 100 M☉) produced as remnants of the first-generation stars, which might be responsible for the recently reported excess of the cosmic infrared background radiation that seems impossible to explain by normal galactic light.

Intracluster Planetary Nebulae in the Virgo Cluster. II. Imaging Catalog

Intracluster stars—stars outside of individual galaxies—are a sensitive measure of the poorly understood processes of galactic mergers, cluster accretion, and tidal stripping that occur in galaxy clusters. In particular, intracluster planetary nebulae are a useful probe of intracluster light as a whole. We present a catalog of 318 intracluster planetary nebula candidates in the nearby Virgo Cluster of galaxies, taken with the Kitt Peak National Observatory 4 m telescope. We detail the automated detection routines developed to search for these candidates and discuss the routines' strengths and weaknesses. We discuss the importance of contamination in the catalog, and the likely causes. We present magnitudes and positions of these candidates, suitable for spectroscopic follow-up observations. Analyses of these candidates are presented in other papers of this series.