The evolution of substructure – III. The outskirts of clusters

We present an investigation of satellite galaxies in the outskirts of galaxy clusters taken from a series of high-resolution N -body simulations. We focus on the so-called backsplash population, i.e. satellite galaxies that once were inside the virial radius of the host but now reside beyond it. We find that this population is significant in number and needs to be appreciated when interpreting the various galaxy morphology environmental relationships and decoupling the degeneracy between nature and nurture. Specifically, we find that approximately half of the galaxies with current cluster-centric distance in the interval 1–2 virial radii of the host are backsplash galaxies that once penetrated deep into the cluster potential, with 90 per cent of these entering to within 50 per cent of the virial radius. These galaxies have undergone significant tidal disruption, losing on average 40 per cent of their mass. This results in a mass function for the backsplash population different from those galaxies infalling for the first time. We further show that these two populations are kinematically distinct and should be observable within existent spectroscopic surveys.     

Galaxy groups in the 2dF Galaxy Redshift Survey: effects of environment on star formation

We estimate the fraction of star-forming galaxies in a catalogue of groups, constructed from the 2dF Galaxy Redshift Survey by Merchán & Zandivarez. We use the η spectral type parameter of galaxies and subdivide the sample of galaxies in groups into four types depending on the values of the η parameter following Madgwick et al. We obtain a strong correlation between the relative fraction of galaxies with high star formation and the parent group virial mass. We find that even in the environment of groups with low virial mass   M ∼10¹³ M_⊙  the star formation of their member galaxies is significantly suppressed. The relation between the fraction of early-type galaxies and the group virial mass obeys a simple power law spanning over three orders of magnitude in virial mass. Our results show quantitatively the way that the presence of galaxies with high star formation rates is inhibited in massive galaxy systems.     

Stabilization of systems of galaxies by subclustering

We investigate the contribution of gravitationally bound subsystems to the potential energy of systems of galaxies. We derive a statistical correction factor to the virial mass using Holmberg's observed frequency function of multiple galaxies. It is of the order of ten for groups of galaxies and negligible for rich clusters. However, we emphasize that an individual treatment is necessary. As an illustration we calculate the contribution to the binding energy by observed binary galaxies in the de Vaucouleurs groups 48 and 49, and the Coma Cluster. In all three cases a single double galaxy removes the virial mass paradox. We conclude that corrections for subclustering are substantial in stability analyses of systems of galaxies, that they can remove easily mass discrepancies of the order of ten in groups and rich clusters, and that they should be made individually and not statistically.     

Collisions of galaxies of different masses, forms and sizes

We study the tidal effects of close collisions between two spherical galaxies of various mass and mass distributions by numerical simulations. The galaxies are represented by polytropes of indices n=4 and n=0 which denote cases of highcentral concentration and uniform mass distribution respectively.The initial relative velocity of the galaxies is chosen to be 700 km s^-1.The results indicate that the tidal effects are quite sensitive to both mass and mass distribution of the galaxies. The dependence oftidal capture and tidal disruption on the choice of the model and mass ratio is investigated. The classification of collisionis given for each simulation. The results also indicate that in a collision between two identical galaxies relatively more spin is imparted to the galaxies ifthey are centrally concentrated than if they are homogeneous.     

On the Andromeda to Milky Way mass ratio

We have explored the hypothesis that the total mass ratio of the two main galaxies of the Local Group, the Andromeda galaxy (M31) and the Milky Way (MW), can be constrained by measuring the tidal force induced by the surrounding mass distribution, M31 included, on the MW. We argue that the total mass ratio between the two groups can be approximated, at least qualitatively, by finding the tidal radius where the internal binding force of the MW balances the external tidal force acting on it. Since M31 is the massive tidal 'perturber' of the local environment, we have used a wide range of M31 to MW mass-ratio combinations to compute the corresponding tidal radii. Of these, only a few match the distance of the zero-tidal shell, i.e. the shell identified observationally by the outermost dwarf galaxies which do not show any sign of tidal effects. This is the key to constraining the best mass-ratio interval of the two galaxies. Our results favour a solution where the mass ratio ranges from 2 to 3, implying a massive predominance of M31.     

The basic properties of galaxy groups from the volume-limited main galaxy sample of SDSS DR6

Using the Berlind et al. algorithm and the Davis et al. algorithm, we find that the mean velocity dispersion, virial radius, and virial mass of the group catalogs identified in the volume-limited sample are much smaller than those identified in the flux-limited sample. Our study shows that these properties of groups are heavily influenced by the relative values of the linking parameters and that there may be no values for the linking lengths that will work perfectly for every sample. In addition, we note that the luminosity distribution of member galaxies of groups identified by different algorithms is nearly the same, but member galaxies of groups identified using the linking lengths b_⊥ = 0.14, b_∥ = 0.75 have a higher proportion of blue galaxies and a lower proportion of red galaxies than member galaxies of groups identified using the linking length b = 0.2, and the early-type fraction of member galaxies of groups identified using the linking length b = 0.2 is higher than that of member galaxies of groups identified using the linking lengths b_⊥ = 0.14, b_∥ = 0.75. Published in Astrofizika, Vol. 51, No. 4, pp. 555–566 (August 2008).     

Contribution of the tully groups to the soft X-ray background

We examine a model for the expected contributions to the soft X-ray background (SXRB) due to bremsstrahlung and re-combination radiation of gas, enriched with heavy elements, located in groups of galaxies taken from Tully'sCatalog of Nearby Groups of Galaxies. It is shown that the contribution to the SXRB of groups of galaxies can explain up to 20% of the observed flux depending on the chemical composition and ratio of emitting mass to assumed virial mass.     

Surveying the Local Supercluster Plane

We investigate the distribution and velocity field of galaxies situated in a band of 100 by 20 degrees centered on M87 and oriented along the Local supercluster plane. Our sample amounts 2158 galaxies with radial velocities less than 2000 km s^?1. Of them, 1119 galaxies (52%) have distance and peculiar velocity estimates. About 3/4 of early-type galaxies are concentrated within the Virgo cluster core, most of the late-type galaxies in the band locate outside the virial radius. Distribution of gas-rich dwarfs with M_HI >M_* looks to be insensitive to the Virgo cluster presence. Among 50 galaxy groups in the equatorial supercluster band 6 groups have peculiar velocities about 500–1000 km s^?1 comparable with virial motions in rich clusters. The most cryptic case is a flock of nearly 30 galaxies around NGC4278 (Coma I cloud), moving to us with the mean peculiar velocity of ?840 km s^?1. This cloud (or filament?) resides at a distance of 16.1 Mpc from us and approximately 5 Mpc away from the Virgo center. Galaxies around Virgo cluster exhibit Virgocentric infall with an amplitude of about 500 km s^?1. Assuming the spherically symmetric radial infall, we estimate the radius of the zero-velocity surface to be R₀ = (7.0±0.3) Mpc that yields the total mass of Virgo cluster to be (7.4 ± 0.9)× 10¹⁴M_⊙ in tight agreement with its virial mass estimates. We conclude that the Virgo outskirts does not contain significant amounts of dark mater beyond its virial core.     

The concentration–velocity dispersion relation in galaxy groups

Based on results from cold dark matter N -body simulations, we develop a dynamical model for the evolution of subhaloes within group-sized host haloes. Only subhaloes more massive than 5 × 10⁸ M_⊙ are considered, because they are massive enough to possibly host luminous galaxies. On their orbits within a growing host potential the subhaloes are subject to tidal stripping and dynamical friction. At the present time  ( z = 0)  , all model hosts have equal mass  ( M _vir= 3.9 × 10¹³ M_⊙)  but different concentrations associated with different formation times. We investigate the variation of subhalo (or satellite galaxy) velocity dispersion with host concentration and/or formation time. In agreement with the Jeans equation, the velocity dispersion of subhaloes increases with the host concentration. Between concentrations of ∼5 and ∼20, the subhalo velocity dispersions increase by a factor of ∼1.25. By applying a simplified tidal disruption criterion, that is, rejection of all subhaloes with a tidal truncation radius below 3  kpc at   z = 0  , the central velocity dispersion of the 'surviving' subhalo sample increases substantially for all concentrations. The enhanced central velocity dispersions in the surviving subhalo samples are caused by a lack of slow tangential motions. Additionally, we present a fitting formula for the anisotropy parameter which does not depend on concentration if the group-centric distances are scaled by r _s, the characteristic radius of the Navarro, Frenk & White profile. Since the expected loss of subhaloes and galaxies due to tidal disruption increases the velocity dispersion of surviving galaxies, the observed galaxy velocity dispersion can substantially overestimate the virial mass.     

The anatomy of the NGC 5044 group – II. Stellar populations and star formation histories

The distribution of galaxy properties in groups and clusters holds important information on galaxy evolution and growth of structure in the Universe. While clusters have received appreciable attention in this regard, the role of groups as fundamental to formation of the present-day galaxy population has remained relatively unaddressed. Here, we present stellar ages, metallicities and α-element abundances derived using Lick indices for 67 spectroscopically confirmed members of the NGC 5044 galaxy group with the aim of shedding light on galaxy evolution in the context of the group environment.
We find that galaxies in the NGC 5044 group show evidence for a strong relationship between stellar mass and metallicity, consistent with their counterparts in both higher and lower mass groups and clusters. Galaxies show no clear trend of age or α-element abundance with mass, but these data form a tight sequence when fitted simultaneously in age, metallicity and stellar mass. In the context of the group environment, our data support the tidal disruption of low-mass galaxies at small group-centric radii, as evident from an apparent lack of galaxies below  ∼10⁹ M_⊙  within ∼100 kpc of the brightest group galaxy. Using a joint analysis of absorption- and emission-line metallicities, we are able to show that the star-forming galaxy population in the NGC 5044 group appears to require gas removal to explain the ∼1.5 dex offset between absorption- and emission-line metallicities observed in some cases. A comparison with other stellar population properties suggests that this gas removal is dominated by galaxy interactions with the hot intragroup medium.     

Evolutionary approach to the missing mass problem of clusters of galaxies

In a gravitationally bound and stable cluster of galaxies the amount and distribution of matter determine both the velocity dispersion of the members and the type of evolution of the system. The use of the first of these physical connections — the application of virial theorem — led to the idea of missing mass in clusters, that of the second one seems to support this idea by an independent “rediscovery” and “redistribution” of missing mass. On the basis of this “evolutionary approach” to the missing mass problem — that is free from the uncertainties of measuring and interpreting red shifts — evidences are obtained for the existence of large amounts of discretely distributed dark circumgalactic or intragalactic matter in rich clusters of galaxies.     

Changes in the gravitational energy of galaxies during collisions

The problem of the change in gravitational energy of a colliding galaxy due to tidal effects is considered. The change in the internal energy, the mass of escaping matter and the change in the mean radius of the test galaxy have been estimated for a relative velocity of 1000 km s^–1 for three distances of closest approach for the following four cases: (a) both galaxies centrally concentrated, (b) both galaxies homogeneous, (c) test galaxy centrally concentrated, field galaxy homogeneous, and (d) test galaxy homogeneous, field galaxy centrally concentrated. The masses and radii of the two galaxies are taken as 10¹¹ M and 10 kpc respectively. For simplicity, the galaxies are assumed to be spherically symmetric and the distribution of mass within a centrally concentrated galaxy is assumed to be that of a polytrope of indexn=4. The results also provide estimates for the minimum relative velocity a galaxy must have in order that it may not be captured by another to form a double system. It has been found that normally a relative velocity of less than about 500 km s^–1 will lead to the formation of a double galaxy by tidal capture. In the case of a head-on collision between two centrally concentrated galaxies even a relative velocity of about 1000 km s^–1 is small enough for tidal capture. The changes in the structure of the galaxies for relative velocities equal to velocity of escape are also indicated. These results show that there is no escape of matter from the test galaxy in cases (b) and (c). In the case (a) the escape of matter can be as high as 4% of the total mass. The head-on collision between galaxies are normally not accompanied by any escape of matter. All the gain in the internal energy of galaxies during such collisions results in increase in their dimensions. The fractional increase in the mean radius of the test galaxy in the head-on collision is 1.5 in the case (a), 3.2 in the case (b) and 0.01 in the case (c). In the case (d) the test galaxy will be disrupted by the tidal forces.     

Mass distribution in nearby Abell clusters

We study the mass distribution in six nearby  ( z < 0.06)  relaxed Abell clusters of galaxies A0262, A0496, A1060, A2199, A3158 and A3558. Given the dominance of dark matter in galaxy clusters, we approximate their total density distribution by the Navarro, Frenk & White (NFW) formula characterized by virial mass and concentration. We also assume that the anisotropy of galactic orbits is reasonably well described by a constant and that galaxy distribution traces that of the total density. Using the velocity and position data for 120–420 galaxies per cluster we calculate, after removal of interlopers, the profiles of the lowest order even velocity moments, dispersion and kurtosis. We then reproduce the velocity moments by jointly fitting the moments to the solutions of the Jeans equations. Including the kurtosis in the analysis allows us to break the degeneracy between the mass distribution and anisotropy and constrain the anisotropy as well as the virial mass and concentration. The method is tested in detail on mock data extracted from the N -body simulations of dark matter haloes. We find that the best-fitting Galactic orbits are remarkably close to isotropic in most clusters. Using the fitted pairs of mass and concentration parameters for the six clusters, we conclude that the trend of decreasing concentration for higher masses found in the cosmological N -body simulations is consistent with the data. By scaling the individual cluster data by mass, we combine them to create a composite cluster with 1465 galaxies and perform a similar analysis on such sample. The estimated concentration parameter then lies in the range  1.5 < c < 14  and the anisotropy parameter in the range  −1.1 < β < 0.5  at the 95 per cent confidence level.     

On the dynamics of the satellite galaxies in NGC 5044

The NGC 5044 galaxy group is dominated by a luminous elliptical galaxy that is surrounded by ∼160 dwarf satellites. The projected number density profile of this dwarf population deviates within ∼1/3 of the virial radius from a projected Navarro, Frenk and White (NFW) profile, which is assumed to approximate the underlying total matter distribution. By means of a semi-analytic model, we demonstrate that the interplay between gravitation, dynamical friction and tidal mass loss and destruction can explain the observed number density profile. We use only two parameters in our models: the total to stellar mass fraction of the satellite haloes and the disruption efficiency. The disruption efficiency is expressed by a minimum radius. If the tidal radius of a galaxy (halo) falls below this radius, it is assumed to become unobservable. The preferred parameters are an initial total to stellar mass fraction of ∼20 and a disruption radius of  4 kpc  . In that model, about 20 per cent of all the satellites are totally disrupted on their orbits within the group environment. Dynamical friction is less important in shaping the inner slope of the number density profile because the reduction in mass by tidal forces lowers the impact of the friction term. The main destruction mechanism is tide. In the preferred model, the total B -band luminosity of all disrupted galaxies is about twice the observed luminosity of the central elliptical galaxy, indicating that a significant fraction of stars are scattered into the intragroup medium. Dwarf galaxy satellites closer to the centre of the NGC 5044 group may exhibit optical evidence of partial tidal disruption. If dynamical friction forces the satellite to merge with the central elliptical, the angular momentum of the satellite tends to be removed at the apocentre passage. Afterwards, the satellite drops radially towards the centre.     

Note on a polytropic β-model to fit the X-ray surface brightness of clusters of galaxies

I suggest that the β -model used to fit the X-ray surface brightness profiles of extended sources, like groups and clusters of galaxies, has to be corrected when the counts are collected in a wide energy band comparable to the mean temperature of the source, and a significant gradient in the gas temperature is observed. I present a revised version of the β -model for the X-ray brightness that applies to an intracluster gas with temperature and density related by a polytropic equation and extends the standard version that is strictly valid for an isothermal gas. Given a temperature gradient observed through an energy window of 1–10 keV typical for the new generation of X-ray observatories, the β parameter can change systematically by up to 20 per cent from the value obtained under isothermal assumption, i.e. by an amount larger than any statistical uncertainty obtained from the present data. Within the virial regions of typical clusters of galaxies, these systematic corrections affect the total gravitating mass estimate by 5–10 per cent, the gas mass by 10–30 per cent and the gas fraction value up to 50 per cent, when compared with the measurements obtained under the isothermal assumption.     

宇宙大尺度结构对星系形成和空间排列的影响

星系的空间排列以及指向并非随机的.星系的主轴和角动量与物质的大尺度分布存在一定的指向相关.这一方向的研究将可以促进我们对结构形成机制的认识,并提供一个独立的视角去检验星系形成过程中的环境效应.本文就相关问题做了详细的探讨.     

Motion of a Rigid Body in a Tidal Field

We investigate the motion, near the equilibrium configurations, of an initially spinless rigid body subject to an external tidal field. Two cases are considered: when the center of mass of the body is at rest at the equilibrium point of the field generated by a generic mass distribution, and when it is placed on a circular orbit subject to a spherically symmetric potential. A complete analysis of the equilibrium configurations is carried out for both cases. First, we derive the conditions for the stable equilibria, and then we analyze the frequencies of oscillations around the equilibrium positions. In view of these results, we consider the problem of alignment of galaxies in clusters. After estimating the period of the oscillations induced on the galaxies by the tidal field of the cluster, we discuss the possible effect of resonances between stellar orbits inside the galaxy and the oscillations of the galaxy as a whole; this may be a mechanism responsible for producing an intracluster stellar population. This revised version was published online in July 2006 with corrections to the Cover Date.     

Galaxy star formation in different environments

We use a semi-analytic model of galaxy formation to study signatures of large-scale modulations in the star formation (SF) activity in galaxies. In order to do this, we carefully define local and global estimators of the density around galaxies. The former are computed using a voronoi tessellation technique and the latter are parametrized by the normalized distance to haloes and voids, in terms of the virial and void radii, respectively. As a function of local density, galaxies show a strong modulation in their SF, a result that is in agreement with those from several authors. When taking subsamples of equal local density at different large-scale environments, we find relevant global effects whereby the fraction of red galaxies diminishes for galaxies in equal local density environments farther away from clusters and closer to voids. In general, the semi-analytic simulation is in good agreement with the available observational results, and offers the possibility to disentangle many of the processes responsible for the variation of galaxy properties with the environment; we find that the changes found in samples of galaxies with equal local environment but different distances to haloes or voids come from the variations in the underlying mass function of dark matter (DM) haloes. There is an additional possible effect coming from the host DM halo ages, indicating that halo assembly also plays a small but significant role (1.14σ) in shaping the properties of galaxies, and in particular, hints at a possible spatial correlation in halo/stellar mass ages. An interesting result comes from the analysis of the coherence of flows in different large-scale environments of fixed local densities; the neighbourhoods of massive haloes are characterized by lower coherences than control samples, except for galaxies in filament-like regions, which show highly coherent motions.     

Tidal tails in cold dark matter cosmologies

We study the formation of tidal tails in pairs of merging disc galaxies with structural properties motivated by current theories of cold dark matter (CDM) cosmologies. In a recent study, Dubinski, Mihos & Hernquist showed that the formation of prominent tidal tails can be strongly suppressed by massive and extended dark haloes. For the large halo-to-disc mass ratio expected in CDM cosmologies their sequence of models failed to produce strong tails like those observed in many well-known pairs of interacting galaxies. In order to test whether this effect can constrain the viability of CDM cosmologies, we construct N ‐body models of disc galaxies with structural properties derived in analogy to the recent analytical work of Mo, Mao & White. With a series of self-consistent collisionless simulations of galaxy–galaxy mergers we demonstrate that even the discs of very massive dark haloes have no problems developing long tidal tails, provided the halo spin parameter is large enough. For our class of models, the halo-to-disc mass ratio is not a good indicator of the ability to produce tails. Instead, the relative size of disc and halo or, alternatively, the ratio of circular velocity to local escape speed at the half mass radius of the disc is a more useful criterion. This result holds in all CDM models. While tidal tails can provide useful information on the structure of galaxies, it thus appears unlikely that they are able to constrain the values of the cosmological parameters within these models.     

TheHI content of the Eridanus group of galaxies

The HI content of galaxies in the Eridanus group is studied using the GMRT observations and the HIPASS data. A significant HI deficiency up to a factor of 2–3 is observed in galaxies in the high galaxy density regions. The HI deficiency in galaxies is observed to be directly correlated to the local projected galaxy density, and inversely correlated to the line-of-sight radial velocity. Furthermore, galaxies with larger optical diameters are predominantly in the lower galaxy density regions. It is suggested that the HI deficiency in Eridanus is due to tidal interactions. In some galaxies, evidences of tidal interactions are seen. An important implication is that significant evolution of galaxies can take place in the group environment. In the hierarchical way of formation of clusters via mergers of groups, a fraction of the observed HI deficiency in clusters could have originated in groups. The co-existence of S0s and severely HI deficient galaxies in the Eridanus group suggests that tidal interaction is likely to be an effective mechanism for transforming spirals to S0s.