Hercules Supercluster Research Articles

The Cosmic Large-Scale Structure in X-rays (CLASSIX) Cluster Survey

It is important to map the large-scale matter distribution in the local Universe for cosmological studies, such as the tracing of the large-scale peculiar velocity flow, the characterisation of the environment for different astronomical objects, and for precision measurements of cosmological parameters. We used X-ray luminous clusters to map this matter distribution and find that about 51% of the groups and clusters are members of superclusters which occupy only a few percent of the volume. In this paper we provide a detailed description of these large-scale structures. With a friends-to-friends algorithm, we find eight superclusters with a cluster overdensity ratio of at least two with five or more galaxy group and cluster members in the cosmic volume out to z = 0.03. The four most prominent ones are the Perseus-Pisces, the Centaurus, the Coma, and the Hercules supercluster, with lengths from about 40 to over 100 Mpc and estimated masses of 0.6 − 2.2 × 1016 M⊙. The largest of these structures is the Perseus-Pisces supercluster. The four smaller superclusters include the Local and the Abell 400 supercluster and two superclusters in the constellations Sagittarius and Lacerta. We provide detailed maps, member catalogues, and physical descriptions of the eight superclusters. By constructing superclusters with a range of cluster sub-samples with different lower X-ray luminosity limits, we show that the main structures are always reliably recovered.

Unveiling the internal structure of the Hercules supercluster

ABSTRACT We have investigated the structure of the Hercules supercluster (SCL160) based on data originally extracted from the Sloan Digital Sky Survey (SDSS-DR7). We have traced the mass distribution in the field through the numerical density, weighted by the r′ luminosity of the galaxies, and classified objects based on their spatial position and redshift. This has allowed us to address not only the kinematics of the supercluster as a whole, but also the internal kinematics of each cluster, which had not been explored further before. We have confirmed that the Hercules supercluster is composed of the galaxy clusters A2147, A2151, and A2152. A2151 consists of five subclusters, A2147 two, and A2152 at least two. They form the heart of the Hercules supercluster. We also have found two other gravitationally bound clusters, therefore increasing the known members of the supercluster. We have estimated a total mass of 2.1 ± 0.2 × 1015 M⊙ for the Hercules supercluster. To determine the dynamical masses in this work, we have resorted to the M200−σ scaling relation and the caustic technique. Comparing both methods with simulated data of bimodal merging clusters, we found that the caustic, as well as the σ-based masses, is biased through the merger age, showing a boost just after pericentric passage. This is not in line with the principle of the caustic method, which affirms it does not depend on the cluster dynamical state.

Outskirts of Galaxy Clusters A1139, A1314, A1656, A2040, A 2052, A2107: Star-Formation Rate

We investigate the variation of the fraction of galaxies with suppressed star formation (MK < −21 . m 5) and early-type galaxies (fracE) of the “red sequence” along the projected radius in six galaxy clusters:Coma (A1656), A1139, and A1314 in the Leo supercluster region (z ≈ 0.037) and A2040, A2052, A2107 in the Hercules supercluster region (z ≈ 0.036). According to SDSS (DR10) data, fracE is the highest in the central regions of galaxy clusters and it is, on the average, equal to 0.62 ± 0.03, whereas in the 2–3R/R200c interval and beyond the Rsp ≈ 0.95 ± 0.04 R200m radius that we inferred from the observed profile fracE is minimal and equal to 0.25 ± 0.02. This value coincides with the estimate fracE = 0.24 ± 0.01 that we inferred for field galaxies located between the Hercules and Leo superclusters at the same redshifts. We show that the fraction of galaxies with suppressed star formation decreases continuously with cluster radius from 0.87 ± 0.02 in central regions down to 0.43 ± 0.03 in the 2–3 R/R200c interval and beyond Rsp, but remains, on the average, higher than 26% than the corresponding fraction for field objects. This decrease is especially conspicuous in the galaxy mass interval log M* [M⊙] = 9.5–10. We found that galaxies with ongoing star formation have average clustercentric distances 1.5–2.5 R/R200c and that their radial-velocity dispersions are higher than those of galaxies with suppressed star formation.

Evolution of galaxy groups

We study the variations of the properties of groups of galaxies with dynamical masses of 1013 M ⊙<M 200<1014 M ⊙, represented by two samples: one has redshifts of z 0.027, and is located in the regions of the following superclusters of galaxies: Hercules, Leo, Bootes, Ursa Major, and Corona Borealis. Using the archived data of the SDSS and 2MASX catalogs, we determined the concentration of galaxies in the systems by measuring it as the inner density of the group within the distance of the fifth closest galaxy from the center brighter than M K = −23. m 3. We also measured the magnitude gap between the first and the fourth brightest galaxies ΔM 14 located within one half of the selected radius R 200, the fraction of early-type galaxies, and the ratio of bright dwarf galaxies (Mr = [−18. m 5,−16. m 5]) to giant galaxies (M r < −18. m 5) (DGR) within the radius R 200. The main aim of the investigation is to find among these characteristics the ones that reflect the evolution of groups of galaxies.We determined that the ratio of bright dwarf galaxies to early-type giant galaxies on the red sequence depends only on the x-ray luminosity: the DGR increases with luminosity. The fraction of early-type galaxies in the considered systems is equal, on average, to 0.65 ± 0.01, and varies significantly for galaxies with σ200 < 300 kms−1. Based on the luminosity of the brightest galaxy, the magnitude gap between the first and the fourth brightest galaxies in the groups, and on model computations of these parameters, we selected four fossil group candidates: AWM4, NGC0533, NGC0741, and NGC6098 (where the brightest galaxy is a double).We observe no increase in the number of faint galaxies (the α parameter of the Schechter function is less than 1) in our composite luminosity function (LF) for galaxy systems with z < 0.027 in the M K = [−26m,−21. m 5] range, whereas earlier we obtained α > 1 for the LF of the Hercules and Leo superclusters of galaxies.

Structure of galaxy groups and clusters and measurement of their masses

We report the results of measurement and comparison of masses for a sample of 29 groups and clusters of galaxies (z < 0.1). We use the SDSSDR7 archive data to determine dynamical masses from the one-dimensional dispersion of line-of-sight velocities for virialized regions of radii R200 and Re. Our method for determination of effective radii of galaxy systems fromthe cumulative distribution of the number of galaxies depending on squared clustercentric distance allowed us to estimate masses M1/2 (within Re), which are related to the masses contained inside R200: M200 ~ 1.65M1/2. A comparison of the inferred dynamic masses and the hydrostatic masses determined from the radiation of hot gas in galaxy groups and clusters (based on published data) led us to conclude that the inferred masses for the main sample of 21 groups and clusters agree to within 12%. These systems also obey the relation MX,200 ~ 1.65M1/2. For the remaining eight systems, which are mostly located in the Hercules supercluster, the discrepancy between the hydrostatic and the dynamic masses amounts to 2σ. This discrepancy is most likely due to the incompleteness of the formation processes of these clusters via hierarchical merger in the region of the rich Hercules supercluster.

Peculiar motions of galaxy clusters and groups in the Hercules and Leo supercluster regions

We present the results of our study of the peculiar motions for 41 galaxy clusters and groups in the regions of the Hercules and Leo superclusters (0.027 < z < 0.045). For this purpose, based on SDSS (Data Release 8) data, we compiled a sample of early-type galaxies in the investigated clusters and groups, constructed their fundamental planes (FPs), and determined independent distances and peculiar velocities. The Hubble law between the radial velocities galaxy clusters and the distances derived from the FPs holds for the rich Hercules supercluster as a whole. At the same time, however, significant peculiar motions along the line of sight with rms deviations of 736 ± 50 and 584 ± 50 km s−1 are observed within this supercluster and in its immediate neighborhood. In the poor Leo supercluster, the rms deviation is also large, 625 ± 70 km s−1. Its Hubble diagram exhibits the approach of galaxy clusters and groups along the line of sight relative to the most massive cluster A1185. In the immediate neighborhood of the Leo supercluster (virtually in its field), the rms deviation of the peculiar velocities is minimal, 287 ± 60 km s−1.

X-RAY ABSORPTION BY THE WARM-HOT INTERGALACTIC MEDIUM IN THE HERCULES SUPERCLUSTER

The "missing baryons", in the form of warm-hot intergalactic medium (WHIM), are expected to reside in cosmic filamentary structures that can be traced by signposts such as large-scale galaxy superstructures. The clear detection of an X-ray absorption line in the Sculptor Wall demonstrated the success of using galaxy superstructures as a signpost to search for the WHIM. Here we present an XMM-Newton Reflection Grating Spectrometer (RGS) observation of the blazar Mkn 501, located in the Hercules Supercluster. We detected an O VII K$\alpha$ absorption line at the 98.7 % level ($2.5\sigma$) at the redshift of the foreground Hercules Supercluster. The derived properties of the absorber are consistent with theoretical expectations of the WHIM. We discuss the implication of our detection for the search for the "missing baryons". While this detection shows again that using signposts is a very effective strategy to search for the WHIM, follow-up observations are crucial both to strengthen the statistical significance of the detection and to rule out other interpretations. A local, $z\sim0$ O VII K$\alpha$ absorption line was also clearly detected at the $4\sigma$ level, and we discuss its implications for our understanding of the hot gas content of our Galaxy.

Investigation of properties of galaxy clusters in the Hercules supercluster region

We investigated the properties of galaxy clusters in the region of the Hercules supercluster using observational data from the SDSS and 2MASS catalogs and the NED. We have selected 13 galaxy clusters with a total dynamical mass of 4.82 × 1015 M ⊙ in a 100 × 45 Mpc supercluster region in the plane of the sky (0.030 < z < 0.041). In addition, our sample includes eight clusters from the immediate neighborhoods of the superclusters and ten field clusters at the same z. The derived properties of the rich Hercules supercluster are shown in comparison with the data for the poor Leo supercluster. The main parameters of the virialized galaxy cluster regions in the near infrared (K s ) for the Hercules supercluster differ from those for the Leo supercluster: the number of galaxies and the total luminosity (to a limiting magnitude of −21 · 5) increase with cluster mass (L K,200 ∝ M 200 0.91±0.07 and N 200 ∝ M 200 0.94±0.07 ), but the dependences are steeper by 0.28 and 0.22. In the virialized cluster regions, the fraction of early-type galaxies selected by the bulge contribution, concentration index, and u t= r color is, on average, 66% (60% in Leo, 70% in the field) among the galaxies brighter than −23 · 3 and 54% (51% in Leo, 61% in the field) among the galaxies brighter than −22 · 3. The fraction of early-type galaxies in the superclusters does not change with galaxy cluster mass and luminosity. The composite luminosity function of the rich Hercules supercluster is described by a Schechter function and does not differ from the luminosity function of the poor Leo supercluster for the luminosity interval [−26 m , −21 · 5] but differs from the field luminosity function at the same z determined from ten galaxy clusters.

The discovery of a massive supercluster at z= 0.9 in the UKIDSS Deep eXtragalactic Survey

Abstract We analyse the first publicly released deep field of the UK Infrared Deep Sky Survey (UKIDSS) Deep eXtragalactic Survey to identify candidate galaxy overdensities at z∼ 1 across ∼1 deg2 in the ELAIS-N1 field. Using I−K, J−K and K− 3.6 μm colours, we identify and spectroscopically follow up five candidate structures with Gemini/Gemini Multi-Object Spectrograph and confirm that they are all true overdensities with between five and 19 members each. Surprisingly, all five structures lie in a narrow redshift range at z= 0.89 ± 0.01, although they are spread across 30 Mpc on the sky. We also find a more distant overdensity at z= 1.09 in one of the spectroscopic survey regions. These five overdense regions lying in a narrow redshift range indicate the presence of a supercluster in this field and by comparing with mock cluster catalogues from N-body simulations we discuss the likely properties of this structure. Overall, we show that the properties of this supercluster are similar to the well-studied Shapley and Hercules superclusters at lower redshift.

On the investigations of galaxy redshift periodicity

In this article we present a historical review of study of the redshift periodicity of galaxies, starting from the first works performed in the seventies of the twentieth century until the present day. We discuss the observational data and methods used, showing in which cases the discretization of redshifts was observed. We conclude that galaxy redshift periodisation is an effect which can really exist. We also discussed the redshift discretization in two different structures: the Local Group of galaxies and the Hercules Supercluster. Contrary to the previous studies we consider all galaxies which can be regarded as a structure member disregarding the accuracy of velocity measurements. We applied the power spectrum analysis using the Hann function for weighting, together with the jackknife error estimator. In both the structures we found weak effects of redshift periodisation.

ROSAT observations of the soft X-ray background and of the cluster soft excess emission in the Hercules supercluster

Recent observations with the XMM-Newton satellite confirmed the existence of the soft excess phenomenon in galaxy clusters, earlier discovered in several EUVE, ROSAT and BeppoSAX observations. Among the clusters for which XMM has reported detection of soft excess emission are MKW03s and A2052, two clusters in the Hercules concentration. The Hercules supercluster lies along the southern extension of the North Polar Spur, a region of bright soft X-ray emission clearly visible in ROSAT All-Sky Survey images. We analyze 11 pointed ROSAT PSPC observations toward 3 clusters in the Hercule concentration, MKW03s, A2052 and A2063, and 8 neighboring fields in order to investigate the soft X-ray emission in that region of the sky. We find that the soft X-ray emission varies by a factor of few on scales of few degrees, rendering the background subtraction a complex task. If the Noth Polar Spur emission is of local origin, we find that only A2052 and A2063 have evidence of cluster soft excess emission, and that the OVII emission lines detected in XMM observations of A2052 and MKW03s are not associated with the cluster. If part or all of the North Polar Spur soft X-ray enhancement is of extragalactic nature, the three clusters feature strong soft excess emission, and the OVII emission lines observed by XMM are genuinely associated with the clusters. We interpret the soft excess emission with the presence of warm gas, either intermixed with the hot intra-cluster medium or in filamentary structures located around the clusters, and estimate that the warm gas is approximately as massive as the hot intra-cluster medium.

A Tidal Dwarf Galaxy in the Hercules Cluster?

A candidate Tidal Dwarf Galaxy, ce-61, was identified in the merger system IC 1182 in the Hercules supercluster. The multi-wavelength data we obtained so far do not prove, however, that it is kinematically detached from the IC 1182 system and gravitationally bound.

Lensing in the Hercules Supercluster

We report Keck LRIS observations of an arclike background galaxy near the center of Abell 2152 (z = 0.043), one of the three clusters comprising the Hercules supercluster. The background object has a redshift z = 0.1423 and is situated 25'' north of the primary component of the A2152 brightest cluster galaxy (BCG). The object is about 15'' in total length and has a reddening-corrected R-band magnitude of mR = 18.55 ± 0.03. Its spectrum shows numerous strong emission lines, as well as absorption features. The strength of the Hα emission would imply a star formation rate SFR ≈ 3 h-2 M⊙ yr-1 in the absence of any lensing. However, the curved shaped of this object and its tangential orientation along the major axis of the BCG suggest lensing. We model the A2152 core mass distribution including the two BCG components and the cluster potential. We present velocity and velocity dispersion profile measurements for the two BCG components and use these to help constrain the potential. The lens modeling indicates a likely magnification factor of ~1.9 for the lensed galaxy, making A2152 the nearest cluster in which such significant lensing of a background source has been observed. Finally, we see evidence for a concentration of early-type galaxies at z = 0.13 near the centroid of the X-ray emission previously attributed to A2152. We suggest that emission from this background concentration is the cause of the offset of the X-ray center from the A2152 BCG. The background concentration and the dispersed mass of the Hercules supercluster could add further to the lensing strength of the A2152 cluster.

Globular Clusters in Dense Clusters of Galaxies

Deep imaging data from the Keck II Telescope are employed to study the globular cluster (GC) populations in the cores of six rich Abell clusters. The sample includes A754, A1644, A2124, A2147, A2151, and A2152 and covers the redshift range z = 0.035–0.066. These clusters also span a range in morphology from spiral-rich, irregular systems to centrally concentrated cD clusters rich in early-type galaxies. Globular cluster specific frequencies SN and luminosity function dispersions σLF are measured for a total of nine galaxies in six central fields. The measured values of SN for the six brightest cluster galaxies (BCGs) are all higher than typical values for giant elliptical galaxies, in accord with the SN-density correlations found by Blakeslee, Tonry, & Metzger. The three non-BCGs analyzed also have elevated values of SN, confirming that central location is a primary factor. Two different models are used to estimate ηGC, the number of GCs per unit mass, for these central cluster fields. The values for ηGC are consistent with those found in the earlier sample, again indicating that the number of GCs scales with mass and that the SN variations are due to a deficit of halo light, i.e., SN reflects mass-to-light ratio. The similarity of the GC color distributions of BCGs and more ordinary ellipticals further implies that no special mechanism is needed for explaining the properties of these GC populations. The discussion builds on an earlier suggestion that the GCs (both metal-rich and metal-poor) around the central cluster galaxies were assembled at early times, and that star formation halted prematurely in the central galaxies at the epoch of cluster collapse. It is argued that this saturation model is consistent with recent simulations of BCG/cluster formation. The subsequent addition of luminous material to the BCG through cluster dynamical evolution can cause SN to decrease while conserving ηGC, but both theory and observations indicate that the timescale for this is long. However, there may be some evidence of it in the present sample and elsewhere. Finally, the GC luminosity function measurements are used to constrain the relative distances of the three clusters that make up the Hercules supercluster.

Kinematics of the Hercules Supercluster

The Hercules Supercluster consists of the Abell clusters 2147, 2151, and 2152. Previous studies of the kinematics have been confounded by the difficulty of correctly assigning galaxies to the individual clusters, which are not well-separated. Our study has a total of 468 available velocities for galaxies in the region, 175 of them new. 414 galaxies are in the supercluster, about three times the number used in the previous supercluster study. We verify the existence of the three individual clusters and compute their individual dynamical parameters. We investigate several techniques for assigning galaxy membership to clusters in this crowded field. We use the KMM mixture-modeling algorithm to separate the galaxies into clusters; we find that A2152 has a higher mean velocity than previous studies have reported. A2147 and A2152 also have lower velocity dispersions: 821(+68)(-55) and 715(+81)(-61) km/s. The assignment of galaxies to either A2152 or A2147 requires velocity and position information. We study the kinematics of the supercluster using the two-body formalism of Beers, Geller, and Huchra (1982) and conclude that A2147 and A2151 are probably bound to each other, and that the supercluster as a whole may also be bound. The mass of the supercluster, if bound, is $(7.6\pm2.0)\times 10^{15} h^{-1} M_{\sun}$; with the supercluster luminosity, $(1.4\pm0.2) \times 10^{13} h^{-2} L_{\sun}$, this yields Omega = 0.34\pm0.1$.

An Obscured Galaxy Redshift Survey with FLAIR

A problem for studies of large scale structures in nearby space (cz &lt; 10,000 km s-1) is the presence of the Zone of Avoidance which is so large and wide on the sky that potentially important clusters and voids remain undetected. A prime example was the Ophiuchus cluster discovered by Wakamatsu and Malkan (1981) as a heavily obscured cD cluster close to the Galactic centre region (l = 0·5°, b = +9·5°). It is the second brightest X-ray cluster after Perseus. A hidden galaxy survey was performed by visually searching ESO/SERC Sky Survey (R and J) copy films of the region centred at l = 355°, b = +10° finding more than 4000 galaxies in six fields. Several irregular clusters adjacent to Ophiuchus were found forming a supercluster which may be connected to the Hercules supercluster by a wall structure parallel to the local supergalactic plane (Wakamatsu et al. 1994). In front of this supercluster, an 'Ophiuchus Void' is suggested (cz = 4,500 km s-1). The Ophiuchus supercluster at cz = 8,500 km s-1 is similar to the Hercules supercluster (cz = 11,000 km s-1), and extends north toward the latter supercluster.

The alignment of galaxies in superclusters

Abstract The orientation of galaxies in the Local Supercluster, Coma/A1367, Perseus and Hercules Superclusters was investigated. In each supercluster galaxy planes show a tendency to be perpendicular to the main plane of the parent supercluster.

Counts-in-cells comparisons of redshift surveys

A statistical technique is presented for comparing galaxy counts in cells in redshift surveys sampling the same volume of space. The technique can be used to recover the variances and covariances in the cell counts. It can be used to derive the relative amplitude of the clustering pattern traced by galaxies of different types, for example optical and infrared-selected galaxies, early- and late-type galaxies, or luminous and faint galaxies. The statistics can also be used to test for systematic errors in galaxy catalogues. We illustrate the technique with two applications. We compare the cell count statistics of the 1.2-Jy and QDOT redshift surveys of IRAS galaxies, and show that, although there are large differences in the variances measured in cells of size ℓ = 30 and 40 h− 1 Mpc,1 the two surveys are compatible with the hypothesis that they sample the same underlying density field. We show that the variances measured from the QDOT survey are extremely sensitive to a small number of galaxies in the region of the Hercules supercluster. We also compare the clustering of optically luminous and faint galaxies in the Stromlo–APM redshift survey and show, within large errors, that there is no firm evidence for any differences in the clustering properties of galaxies with different luminosities on scales ℓ ≳ 10 h− 1 Mpc.

The power spectrum of &lt;italic&gt;IRAS&lt;/italic&gt; galaxies

We estimate the three-dimensional power spectrum of IRAS galaxies from the QDOT and $1.2$Jy redshift surveys. We use identical estimators for both surveys and show how the results depend on the weights assigned to the galaxies. The power spectrum for the QDOT survey is steeper and has a higher amplitude at wavenumbers $k \sim 0.05\; h {\rm Mpc}^{-1}$ (where $h$ is Hubble's constant in units of $100 \kmsmpc$) than the power spectrum derived from the $1.2$Jy sample. However, the QDOT power spectrum is sensitive to a small number of galaxies in the Hercules supercluster, in agreement with a recent analysis of galaxy counts in cells in these surveys. We argue that the QDOT results are an upward fluctuation. We combine the two surveys to derive our best estimate of the power spectrum of IRAS galaxies. This is shallower and has a lower amplitude on scales $\simlt 0.1 h {\rm Mpc}^{-1}$ than the power spectrum derived by Feldman \et (1994) from the QDOT survey alone. The power spectrum of the combined surveys is well described by the linear theory power spectrum of a scale-invariant cold dark matter model with $\Omega h = 0.2$.

Subcluster mergers and galaxy infall in A2151

We have obtained a 12.5 ks image of the Hercules Cluster, A2151, with the ROSAT PSPC. Comparison of the optical and X-ray emission coincides with the highest-density peak in the distribution, and is bimodal. The northern subclummp, distinct in position and velocity, has no detectable X-ray gas. The eastern subclump, apparent in the optical contour map, is indistinguishable from the clump in velocity space, but is clearly visible in the X-ray image. X-ray spectra derived from the central peak of emission yield a best-fit temperature of 1.6 keV. The emission coincident with the eastern clump of galaxies is cooler, 0.8 keV, and is outside the 90% confidence intervals of the central peak temperature. We suggest that the eastern and central subclusters have recently undergone a merger event. The lack of X-ray emission to the north suggests that those galaxies do not form a physically distinct structure (i.e., they are not located within a distinct gravitational potential), but rather that they are falling into the cluster core along the filament defined by the Hercules Supercluster.