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1.
I study the evolution of halo density profiles as a function of time in the SCDM and ΛCDM cosmologies. Following Del Popolo, I calculate the concentration parameter c = r v / a and study its time evolution. For a given halo mass, I find that c ( z ) ∝ 1/(1+ z ) in both the ΛCDM and SCDM cosmology, in agreement with the analytic model of Bullock et al. and N -body simulations. In both models, a ( z ) is roughly constant. The present model predicts a stronger evolution of c ( z ) with respect to the Navarro, Frenk & White model. Finally I show some consequences of the results on galaxy modelling.  相似文献   

2.
We study triple systems of galaxies with mean projected harmonic separation ≃0.6  h −1 Mpc     We call the systems 'wide triplets', in contrast to compact triplets with mean projected harmonic separation ≃0.04  h −1 Mpc, studied by Karachentsev et al. Data are taken for 108 wide triplets from a list compiled by Trofimov & Chernin; at least one-third of them are considered to be probably isolated physical systems. With typical crossing times of about the Hubble time, the wide triplets seem to be in a state of ongoing collapse. This is confirmed by a set of computer models which simulate well the observational characteristics of the ensemble of wide triplets. The simulations also give a statistical estimate of the total mass of a typical wide triplet: it proves to be ≃1013 M⊙. This figure indicates that the dark matter mass is 15–30 times the mass of baryonic matter in the systems. The dynamics of wide triplets, as well as their dark matter content, provide new direct cosmological constraints by establishing that hierarchical evolution is occurring on a mass scale of ∼1013 M⊙ and a spatial scale of ∼1 Mpc.  相似文献   

3.
Gravitational lensing magnifies the observed flux of galaxies behind the lens. We use this effect to constrain the total mass in the cluster Abell 1689 by comparing the lensed luminosities of background galaxies with the luminosity function of an undistorted field. Under the assumption that these galaxies are a random sample of luminosity space, this method is not limited by clustering noise. We use photometric redshift information to estimate galaxy distance and intrinsic luminosity. Knowing the redshift distribution of the background population allows us to lift the mass/background degeneracy common to lensing analysis. In this paper we use nine filters observed over 12 h with the Calar Alto 3.5-m telescope to determine the redshifts of 1000 galaxies in the field of Abell 1689. Using a complete sample of 146 background galaxies we measure the cluster mass profile. We find that the total projected mass interior to 0.25  h −1 Mpc is M 2D(<0.25  h −1 Mpc)=(0.48±0.16)×1015  h −1 M⊙, where our error budget includes uncertainties from the photometric redshift determination, the uncertainty in the offset calibration and finite sampling. This result is in good agreement with that found by number-count and shear-based methods and provides a new and independent method to determine cluster masses.  相似文献   

4.
We measure the autocorrelation function, ξ , of galaxies in the IRAS Point Source Catalogue galaxy redshift (PSC z ) survey and investigate its dependence on the far-infrared colour and absolute luminosity of the galaxies. We find that the PSC z survey correlation function can be modelled out to a scale of 10  h −1 Mpc as a power law of slope 1.30±0.04 and correlation length 4.77±0.20 . At a scale of 75  h −1 Mpc we find the value of J 3 to be 1500±400 .
We also find that galaxies with higher 100 μm/60 μm flux ratio, corresponding to cooler dust temperatures, are more strongly clustered than warmer galaxies. Splitting the survey into three colour subsamples, we find that, between 1 and 10  h −1 Mpc, the ratio of ξ is a factor of 1.5 higher for the cooler galaxies compared with the hotter galaxies. This is consistent with the suggestion that hotter galaxies have higher star formation rates, and correspond to later-type galaxies which are less clustered than earlier types.
Using volume-limited subsamples, we find a weak variation of ξ as a function of absolute luminosity, in the sense that more luminous galaxies are less clustered than fainter galaxies. The trend is consistent with the colour dependence of ξ and the observed colour–luminosity correlation, but the large uncertainties mean that it has a low statistical significance.  相似文献   

5.
We analyse a high-redshift sample (0.4 < z < 0.5) of luminous red galaxies (LRGs) extracted from the Sloan Digital Sky Survey data release 4 and their surrounding structures to explore the presence of alignment effects of these bright galaxies with neighbour objects. In order to avoid projection effects, we compute photometric redshifts for galaxies within 3  h −1 Mpc in projection of LRGs and calculate the relative angle between the LRG major axis and the direction to neighbours within 1000 km s−1. We find a clear signal of alignment between LRG orientations and the distribution of galaxies within 1.5  h −1 Mpc. The alignment effects are present only for the red population of tracers; LRG orientation is uncorrelated to the blue population of neighbour galaxies. These results add evidence to the alignment effects between primaries and satellites detected at low redshifts. We conclude that such alignments were already present at z ∼ 0.5.  相似文献   

6.
The alignment of clusters of galaxies with their nearest neighbours and between clusters within a supercluster is investigated using simulations of 5123 dark matter particles for ΛCDM and τ CDM cosmological models. Strongly significant alignments are found for separations of up to 15  h −1 Mpc in both cosmologies, but for the ΛCDM model the alignments extend up to separations of 30  h −1 Mpc. The effect is strongest for nearest neighbours, but is not significant enough to be useful as an observational discriminant between cosmologies. As a check of whether this difference in alignments is present in other cosmologies, smaller simulations with 2563 particles are investigated for four different cosmological models. Because of poor number statistics, only the standard CDM model shows indications of having different alignments from the other models.  相似文献   

7.
We study the size and shape of low-density regions in the local Universe, which we identify in the smoothed density field of the PSCz flux-limited IRAS galaxy catalogue. After quantifying the systematic biases that enter the detection of voids using our data set and method, we identify, using a smoothing length of 5  h −1 Mpc, 14 voids within 80  h −1 Mpc (having volumes 103  h −3 Mpc3) and, using a smoothing length of 10  h −1 Mpc, eight voids within 130  h −1 Mpc (having volumes  8×103 h−3 Mpc3)  . We study the void size distribution and morphologies and find that there is roughly an equal number of prolate and oblate-like spheroidal voids. We compare the measured PSCz void shape and size distributions with those expected in six different cold dark matter (CDM) models and find that only the size distribution can discriminate between models. The models preferred by the PSCz data are those with intermediate values of   σ 8(≃0.83)  , independent of cosmology.  相似文献   

8.
A total of 235 active galactic nuclei (AGN) from two different soft X-ray surveys [the ROSAT Deep Survey (DRS) and the ROSAT International X-ray Optical Survey (RIXOS)] with redshifts between 0 and 3.5 are used to study the clustering of X-ray selected AGN and its evolution. A 2σ significant detection of clustering of such objects is found on scales < 40–80 h −1 Mpc in the RIXOS sample, while no clustering is detected on any scales in the DRS sample. Assuming a single power-law model for the spatial correlation function (SCF), quantitative limits on the AGN clustering have been obtained: a comoving correlation length 1.5 ≲  r 0 ≲ 3.3  h −1 Mpc is implied for comoving evolution, while 1.9 ≲  r 0 ≲ 4.8 for stable clustering and 2.2 ≲  r 0 ≲ 5.5 for linear evolution. These values are consistent with the correlation lengths and evolutions obtained for galaxy samples, but imply smaller amplitude or faster evolution than recent ultraviolet and optically selected AGN samples. We also constrain the ratio of bias parameters between X-ray selected AGN and IRAS galaxies to be ≲ 1.7 on scales ≲ 10  h −1 Mpc, a somewhat smaller value than is inferred from local large-scale dynamical studies.  相似文献   

9.
We present an analysis of the relative bias between early- and late-type galaxies in the Two-degree Field Galaxy Redshift Survey (2dFGRS) – as defined by the η parameter of Madgwick et al., which quantifies the spectral type of galaxies in the survey. We calculate counts in cells for flux-limited samples of early- and late-type galaxies, using approximately cubical cells with sides ranging from 7 to  42 h −1 Mpc  . We measure the variance of the counts in cells using the method of Efstathiou et al., which we find requires a correction for a finite volume effect equivalent to the integral constraint bias of the autocorrelation function. Using a maximum-likelihood technique we fit lognormal models to the one-point density distribution, and develop methods of dealing with biases in the recovered variances resulting from this technique. We then examine the joint density distribution function,   f (δE, δL)  , and directly fit deterministic bias models to the joint counts in cells. We measure a linear relative bias of ≈1.3, which does not vary significantly with ℓ. A deterministic linear bias model is, however, a poor approximation to the data, especially on small scales  (ℓ≤ 28  h −1 Mpc)  where deterministic linear bias is excluded at high significance. A power-law bias model with index   b 1≈ 0.75  is a significantly better fit to the data on all scales, although linear bias becomes consistent with the data for  ℓ≳ 40  h −1 Mpc  .  相似文献   

10.
The origin of the magnetic field in galaxies is an open question in astrophysics. Several mechanisms have been proposed related, in general, to the generation of small seed fields amplified by a dynamo mechanism. In general, these mechanisms have difficulty in satisfying both the requirements of a sufficiently high strength for the magnetic field and the necessary large coherent scales. We show that the formation of dense and turbulent shells of matter, in the multiple explosion scenario of Miranda &38; Opher for the formation of the large-scale structures of the Universe, can naturally act as a seed for the generation of a magnetic field. During the collapse and explosion of Population III objects, a temperature gradient not parallel to a density gradient can naturally be established, producing a seed magnetic field through the Biermann battery mechanism. We show that seed magnetic fields ∼ 10−12–10−14 G can be produced in this multiple explosion scenario on scales of the order of clusters of galaxies (with coherence length L  ∼ 1.8 Mpc) and up to ∼ 4.5 × 10−10 G on scales of galaxies ( L  ∼ 100 kpc).  相似文献   

11.
We present measurements of the clustering properties of galaxies in the field of redshift range 0.5 ≲ z ≲ 1.5 Ultra Steep Spectrum radio sources selected from the Sydney University Molonglo Sky Survey and the National Radio Astronomy Observatories Very Large Array Sky Survey. Galaxies in these USS fields were identified in deep near-infrared observations, complete down to   K s= 20  , using the IRIS2 instrument at the Anglo-Australian Telescope. We used the redshift distribution of   K s < 20  galaxies taken from Cimatti et al. (2002) to constrain the correlation length r 0. We find a strong correlation signal of galaxies with   K s < 20  around our USS sample. A comoving correlation length   r 0= 14.0 ± 2.8  h −1 Mpc  and γ= 1.98 ± 0.15 are derived in a flat cosmological model universe.
We compare our findings with those obtained in a cosmological N -body simulation populated with galform semi-analytic galaxies. We find that clusters of galaxies with masses in the range   M = 1013.4–14.2  h −1 M⊙  have a cluster–galaxy cross-correlation amplitude comparable to those found between the USS hosts and galaxies. These results suggest that distant radio galaxies are excellent tracers of galaxy overdensities and pinpoint the progenitors of present day rich clusters of galaxies.  相似文献   

12.
We investigated the influence of environment on cluster morphology by examining the alignment of the cluster position angle with respect to the nearest neighbour cluster. The cluster position angle and ellipticity were measured using high spatial resolution X-ray data taken from the Chandra archive, while the nearest neighbour sample was extracted from the Abell cluster catalogue. We found high confidence for an alignment when neighbour distance (Dn) was less than  70  h −1  Mpc, while we found no significant variations of the confidence level at various values of Dn less than  70  h −1  Mpc.  相似文献   

13.
We present a new determination of the local volume-averaged star formation rate from the 1.4-GHz luminosity function of star forming galaxies. Our sample, taken from the   B ≤12  Revised Shapley–Ames catalogue (231 normal spiral galaxies over an effective area of 7.1 sr) has ≃100 per cent complete radio detections and is insensitive to dust obscuration and cirrus contamination. After removal of known active galaxies, the best-fitting Schechter function has a faint-end slope of  −1.27±0.07  in agreement with the local H α luminosity function, characteristic luminosity   L ∗=(2.6±0.7)×1022 W Hz−1  and density   φ ∗=(4.8±1.1)×10−4 Mpc−3.  The inferred local radio luminosity density of  (1.73±0.37±0.03)×1019 W Hz−1 Mpc−3  (Poisson noise, large-scale structure fluctuations) implies a volume-averaged star formation rate ∼2 times larger than the Gallego et al. H α estimate, i.e.   ρ 1.4 GHz=(2.10±0.45±0.04)×10−2 M⊙ yr−1 Mpc−3  for a Salpeter initial mass function from  0.1–125 M⊙  and Hubble constant of 50 km s−1 Mpc−1. We demonstrate that the Balmer decrement is a highly unreliable extinction estimator, and argue that optical–ultraviolet (UV) star formation rates (SFRs) are easily underestimated, particularly at high redshift.  相似文献   

14.
We use semi-analytic models of galaxy formation combined with high-resolution N -body simulations to make predictions for galaxy–dark matter correlations and apply them to galaxy–galaxy lensing. We analyse cross-power spectra between the dark matter and different galaxy samples selected by luminosity, colour or star formation rate. We compare the predictions with the recent detection by the Sloan Digital Sky Survey (SDSS). We show that the correlation amplitude and the mean tangential shear depend strongly on the luminosity of the sample on scales below 1  h −1 Mpc, reflecting the correlation between the galaxy luminosity and the halo mass. The cross-correlation cannot, however, be used to infer the halo profile directly because different halo masses dominate on different scales and because not all galaxies are at the centres of the corresponding haloes. We compute the redshift evolution of the cross-correlation amplitude and compare it with those of galaxies and dark matter. We also compute the galaxy–dark matter correlation coefficient and show that it is close to unity on scales above 1  h −1 Mpc for all considered galaxy types. This would allow one to extract the bias and the dark matter power spectrum on large scales from the galaxy and galaxy–dark matter correlations.  相似文献   

15.
Comparing the gravitational acceleration induced on the Local Group of galaxies by different tracers of the underlying density field we estimate, within the linear gravitational instability theory and the linear biasing ansatz, their relative bias factors. Using optical SSRS2 galaxies, IRAS (PSC z ) galaxies and Abell/ACO clusters, we find b O,I≈1.21±0.06 and b C,I≈4.3±0.8, in agreement with other recent studies. Finally, there is an excellent one-to-one correspondence of the PSC z and Abell/ACO cluster dipole profiles, once the latter is rescaled by b C,I, out to at least ∼150  h −1 Mpc.  相似文献   

16.
We have reanalysed and compared the dipoles of the 1.2-Jy and 0.6-Jy (QDOT) IRAS galaxy samples. We find strong indications from both samples for (a) significant contributions to the gravitational field that shapes the Local Group motion from depths up to ∼ 170 h −1 Mpc and (b) a large-scale coherence of the dipole anisotropy, indications provided mainly by the fact that the differential dipoles of large equal-volume shells are aligned with the CMB dipole and exhibit significant dipole signals. The two IRAS dipoles are indistinguishable within 50 h −1 Mpc and beyond ∼ 130 h −1 Mpc while the QDOT dipole, having a lower flux limit, continues growing with respect to the 1.2-Jy sample up to ∼ 100 h −1 Mpc in agreement with Rowan-Robinson et al.  相似文献   

17.
We present a semi-analytic treatment of galactic winds within high-resolution, large-scale cosmological N -body simulations of a Λ cold dark matter (ΛCDM) universe. The evolution of winds is investigated by following the expansion of supernova-driven superbubbles around the several hundred thousand galaxies that form in an approximately spherical region of space with diameter 52  h −1 Mpc and mean density close to the mean density of the universe. We focus our attention on the impact of winds on the diffuse intergalactic medium. Initial conditions for mass loss at the base of winds are taken from Shu, Mo & Mao. Results are presented for the volume filling factor and the mass fraction of the intergalactic medium (IGM) affected by winds, and their dependence on the model parameters is carefully investigated. The mass-loading efficiency of bubbles is a key factor to determine the evolution of winds and their global impact on the IGM: the higher the mass loading, the later the IGM is enriched with metals. Galaxies with 109 < M ★ < 1010 M⊙ are responsible for most of the metals ejected into the IGM at   z = 3  , while galaxies with   M ★ < 109 M⊙   give a non-negligible contribution only at higher redshifts, when larger galaxies have not yet assembled. We find a higher mean IGM metallicity than Lyα forest observations suggest, and we argue that the discrepancy may be explained by the high temperatures of a large fraction of the metals in winds, which may not leave detectable imprints in absorption in the Lyα forest.  相似文献   

18.
Observations of turbulent velocity dispersions in the H  i component of galactic discs show a characteristic floor in galaxies with low star formation rates and within individual galaxies the dispersion profiles decline with radius. We carry out several high-resolution adaptive mesh simulations of gaseous discs embedded within dark matter haloes to explore the roles of cooling, star formation, feedback, shearing motions and baryon fraction in driving turbulent motions. In all simulations the disc slowly cools until gravitational and thermal instabilities give rise to a multiphase medium in which a large population of dense self-gravitating cold clouds are embedded within a warm gaseous phase that forms through shock heating. The diffuse gas is highly turbulent and is an outcome of large-scale driving of global non-axisymmetric modes as well as cloud–cloud tidal interactions and merging. At low star formation rates these processes alone can explain the observed H  i velocity dispersion profiles and the characteristic value of  ∼10 km s−1  observed within a wide range of disc galaxies. Supernovae feedback creates a significant hot gaseous phase and is an important driver of turbulence in galaxies with a star formation rate per unit area  ≳10−3 M⊙ yr−1 kpc−2  .  相似文献   

19.
We re-examine the existence and extent of the planar structure in the local galaxy density field, the so-called supergalactic plane (SGP). This structure is studied here in three dimensions using both the new Optical Redshift Survey (ORS) and the IRAS 1.2-Jy redshift survey. The density contrast in a slab of thickness 20  h −1 Mpc and diameter 80 Mpc aligned with the standard de Vaucouleurs supergalactic coordinates is δ sgp∼0.5 for both ORS and IRAS . The structure of the SGP is not well described by a homogeneous ellipsoid, although it does appear to be a flattened structure, which we quantify by calculating the moment of inertia tensor of the density field. The directions of the principal axes vary with radius, but the minor axis remains within θ z ∼30° of the standard SGP Z -axis, out to a radius of 80  h −1 Mpc, for both ORS and IRAS . However, the structure changes shape with radius, varying between a flattened pancake and a dumbbell, the latter at a radius of ∼50  h −1 Mpc, where the Great Attractor and Perseus–Pisces superclusters dominate the distribution. This calls to question the connectivity of the 'plane' beyond ∼40  h −1 Mpc. The configuration found here can be viewed as part of a web of filaments and sheets, rather than as an isolated pancake-like structure. An optimal minimum variance reconstruction of the density field using Wiener filtering, which corrects for both redshift distortion and shot noise, yields a similar misalignment angle and behaviour of axes. The background-independent statistic of axes proposed here can be best used for testing cosmological models by comparison with N -body simulations.  相似文献   

20.
We calculate the statistical clustering of Lyman-break galaxies predicted in a selection of currently fashionable structure formation scenarios. These models are all based on the cold dark matter model, but vary in the amount of dark matter, the initial perturbation spectrum, the background cosmology and the presence or absence of a cosmological constant term. If Lyman-break galaxies form as a result of hierarchical merging, the amplitude of clustering depends quite sensitively on the minimum halo mass that can host such a galaxy. Interpretation of the recent observations by Giavalisco et al. would therefore be considerably clarified by a direct determination of the relevant halo properties. For a typical halo mass around 1011  h −1 M⊙ the observations do not discriminate strongly between cosmological models, but if the appropriate mass is larger, say 1012  h −1 M⊙ (which seems likely on theoretical grounds), then the data strongly favour models with a low matter density.  相似文献   

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