首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Using the method of searching for arbitrary shaped voids in the distribution of volume-limited samples of galaxies from the DR5 SDSS survey, we have identified voids and investigated their characteristics and the change in these characteristics with decreasing M lim (from ?19.7 to ?21.2, H 0 = 100 km s?1 Mpc?1)—the upper limit on the absolute magnitude of the galaxies involved in the construction of voids. The total volume of the 50 largest voids increases with decreasing M lim with a break near M* = ?20.44—the characteristic value of the luminosity function for SDSS galaxies. The mean overdensity in voids increases with decreasing M lim also with a weak break near M*. The exponent of the dependence of the volume of a void on its rank increases significantly with decreasing M lim starting from M lim ～ ?20.4 in the characteristic range of volumes, which reflects the tendency for greater clustering of brighter galaxies. The averaged profile of the galaxy overdensity in voids has a similar pattern almost at all M lim. The galaxies mostly tend to gravitate toward the void boundaries and to avoid the central void regions; the overdensity profile is flat in the intermediate range of distances from the void boundaries. The axial ratios of the ellipsoids equivalent to the voids are, on average, retained with changing M lim and correspond to elongated and nonoblate void shapes, but some of the voids can change their shape significantly. The directions of the greatest void elongations change chaotically and are distributed randomly at a given M lim. The void centers show correlations reflecting the correlations of the galaxy distribution on scales (35–70)h ?1 Mpc. The galaxy distribution in the identified voids is nonrandom—groups and filaments can be identified. We have compared the properties of the galaxies in voids (in our case, the voids are determined by the galaxies with absolute magnitudes M abs < M lim = ?20.44, except for the isolated galaxies) and galaxies in structures identified using the minimum spanning tree. A bimodal color distribution of the galaxies in voids has been obtained. A noticeable difference is observed in the mean color indices and star formation rates per unit stellar mass of the galaxies in dense regions (structures)—as expected, the galaxies in voids are, on average, bluer and have higher log (SFR/M star). These tendencies become stronger toward the central void regions.  相似文献   

2.
One of the basic assumptions in cosmology is that the universe is homogeneous and isotropic on large scales. This assumption is the most important keystone of modern cosmology. In order to verify the homogeneity of galaxy distribution on large scales, we have computed the fractal dimensionality of the galaxy distribution in SDSS-DR4. The fractal dimensionality of the observed spatial geometric bodies is determined with random samples. The redshifts of sample galaxies are in the range 0.01～0.26. When the scale grows continuously to dozens of Mpc, the fractal dimensionality of the galaxy distribution approaches to 3 consistently. All the 6 samples exhibit obviously a transition scale. For scales larger than the transition scale, the fractal dimensionality DG of the galaxy distribution is very close to 3, the galaxy distribution is homogeneous. This result supports the assumption that the universe is homogeneous on large scales. The transition scale of the sample increases with the luminosity of the sample. This means that the galaxy distribution on small scales is not a of simple fractal distribution, but a multi-fractal distribution. The transition scale of high-luminosity galaxies is very large, the distribution will not become homogeneous till about 100 h?1Mpc.  相似文献   

3.
A Fourier analysis on galaxy number counts from redshift data of both the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey indicates that galaxies have preferred periodic redshift spacings of Δz= 0.0102, 0.0246, and 0.0448 in the SDSS and strong agreement with the results from the 2dF GRS. The redshift spacings are confirmed by the mass density fluctuations, the power spectrum P(z) and N pairs calculations. Application of the Hubble law results in galaxies preferentially located on co-moving concentric shells with periodic spacings. The combined results from both surveys indicate regular co-moving radial distance spacings of 31.7±1.8 h?1?Mpc, 73.4±5.8 h?1?Mpc and 127±21 h?1? Mpc. The results are consistent with oscillations in the expansion rate of the universe over past epochs.  相似文献   

4.
The apparatus of correlation gamma function (Γ*(r)) is used to analyze volume-limited samples from the DR4 Main Galaxy Sample of the SDSS survey with the aim of determining the characteristic scales of galaxy clustering. Up to 20h ?1 Mpc (H 0 = 65 km s?1 Mpc?1), the distribution of galaxies is described by a power-law density—distance dependence, Γ*(r) ∝ r ?γ , with an index γ ≈ 1.0. A change in the state of clustering (a significant deviation from the power law) was found on a scale of (20–25) h ?1 Mpc. The distribution of SDSS galaxies becomes homogeneous (γ ～ 0) from a scale of ～60h ?1 Mpc. The dependence of γ on the luminosity of galaxies in volume-limited samples was obtained. The power-law index γ increases with decreasing absolute magnitude of sample galaxies M abs. At M abs ～ ?21.4, which corresponds to the characteristic value M r * of the SDSS luminosity function, this dependence exhibits a break followed by a more rapid increase in γ.  相似文献   

5.
We present the results of the search for spherical volumes containing no galaxies with luminosities brighter than the Magellanic Clouds in the Local Supercluster and its vicinity. Within a distance of 40 Mpc from us, 89 cosmic voids were discovered with the diameters of 24 to 12 Mpc, containing no galaxies with absolute magnitudes brighter than M K < ?18.4. A list of these voids and the sky distribution maps are given. It was found that 93% of spherical voids overlap, forming three more extended percolated voids (hypervoids). The largest of them, HV1, has 56 initial spherical cells and extends in a horseshoe shape, enveloping the Local Volume and the Virgo cluster. The Local Void (Tully, 1988) in the Hercules-Aquila region is the closest part of the HV1. Another hypervoid, HV2, contains 22 spherical voids in the Eridanus constellation, and the third compact hypervoid (HV3) comprises 6 spherical cells in the Bootes. The total volume of these voids incorporates about 30% of the Local Universe. Among 2906 dwarf galaxies excluded from the original sample (n = 10502) in the search for spherical volumes, only 68 are located in the voids we have discovered. They are characterized by late morphological types (85% are Ir, Im, BCD, Sm), absolute magnitudes MB ranging from ?13.0 to ?16.7, moderate star formation rates (log SSFR ～ ?10 M ⊙ yr?1 L ?1 ⊙ ) and gas reserves per luminosity unit twice to three times larger than in the other dwarf galaxies located in normal environments. The dwarf population of the voids shows a certain tendency to sit shallow near the surfaces of cosmic voids.  相似文献   

6.
The apparatus of a correlation gamma function is used to analyze a sample of 2dFGRS galaxies. We suggest a modified gamma-function algorithm with nonintersecting working spheres, which ensures that the counts are independent at each step. The method of nonintersecting spheres has revealed a change in the regime of clustering (a break of the gamma function) on a scale of 10–16h?1 Mpc (H0 = 65 km s?1 Mpc?1). The standard method yields a scale of 30h?1 Mpc. An artificial distribution is used as an example to show that, in some cases, the modified algorithm is more responsive to a mixture of distributions with various properties than the standard method.  相似文献   

7.
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 MHI >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.  相似文献   

8.
We analyze the statistical properties of normal galaxies to be detected in the all-sky survey by the eROSITA X-ray telescope of the Spectrum-X-Gamma observatory. With the current configuration and parameters of the eROSITA telescope, the sensitivity of a 4-year-long all-sky survey will be ≈10?14 erg s?1 in the 0.5–2 keV band. This will allow ～(1.5–2) × 104 normal galaxies with approximately the same contribution of star-forming and elliptical galaxies to be detected. All galaxies of the X-ray survey are expected to enter into the existing far-infrared (IRAS) or near-infrared (2MASS) catalogs; the sample of star-forming galaxies will be approximately equivalent in sensitivity to the sample of star-forming galaxies in the IRAS catalog of infrared sources. Thus, a large homogeneous sample of normal galaxies with measured X-ray, near-infrared, and far-infrared fluxes will be formed. About 90% of the galaxies in the survey are located within ～200–400 Mpc. A typical (most probable) galaxy will have a luminosity log L X ～ 40.5–41.0, will be located at a distance of ～70–90 Mpc, and will be either a star-forming galaxy with a star formation rate of ～20M ⊙ yr?1 whose X-ray emission is produced by ultraluminous X-ray sources (ULXs) or an elliptical galaxy with amass log M * ～ 11.3 emitting through to a hot interstellar gas. The galaxies within 35 Mpc will collectively contain ～102 ULXs with luminosities log L X > 40, ～80% of whichwill be the only luminous source in the galaxy. Thus, although the angular resolution of the eROSITA telescope is too low for the luminosity function of compact sources in galaxies to be studied in detail, the survey data will allow one to investigate its bright end and, possibly, to impose constraints on the maximum luminosity of ULXs.  相似文献   

9.
The structure of the A1035 cluster of galaxies (10h32m + 40°13′, cz ～ 22000 kms?1), which exhibits a bimodal distribution of galaxy radial velocities (ΔV ≈ 3000 kms?1), is analyzed using three methods of determining the relative distances to clusters from early-type galaxies: the Kormendy relation corrected for the dependence of residuals on galaxy magnitude, the photometric plane, and the fundamental plane. We use the data obtained with the l-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences and SDSS (DR5) data to show that A1035 consists of two gravitationally unbound independent clusters. These clusters with the velocity dispersions of 566 km s?1 and 610 km s?1 and masses within R 200 equal to 2.7 · 1014 and 3.5 · 1014 M ⊙, respectively, obey the Hubble law.  相似文献   

10.
We present properties of the low-surface-brightness galaxy KDG218 observed with the HST/ACS. The galaxy has a half-light (effective) diameter of a e = 47″ and a central surface brightness of SB V (0) = 24.m4/□″. The galaxy remains unresolved with the HST/ACS, which implies its distance of D > 13.1 Mpc and linear effective diameter of A e > 3.0 kpc. We notice that KDG218 is most likely associated with a galaxy group around the massive lenticular NGC4958 galaxy at approximately 22 Mpc, or with the Virgo Southern Extension filament at approximately 16.5 Mpc. At these distances, the galaxy is classified as an ultra-diffuse galaxy (UDG) similar to those found in the Virgo, Fornax, and Coma clusters. We also present a sample of 15 UDG candidates in the Local Volume. These sample galaxies have the following mean parameters: 〈D〉 = 5.1 Mpc, 〈A e 〉 = 4.8 kpc, and 〈SB B (e)〉 = 27.m4/□″. All the local UDG candidates reside near massive galaxies located in the regions with the mean stellar mass density (within 1 Mpc) about 50 times greater than the average cosmic density. The local fraction of UDGs does not exceed 1.5% of the Local Volume population. We notice that the presented sample of local UDGs is a heterogeneous one containing irregular, transition, and tidal types, as well as objects consisting of an old stellar population.  相似文献   

11.
We calculate the parameters of the two-point correlation function of quasars w(r) = (r c /r) γ on the basis of the SDSS DR3 data. The correlation functions are first determined from projected distances with the use of a special technique for compiling randomized catalogs. Next the parameters of the spatial correlation function are obtained with the assumption of local isotropy. For the quasars with redshifts z = 0.8–2.1, we obtained the estimates γ = 1.76 ± 0.14, r c = 6.60 ± 0.85 h ?1 Mpc in the comoving distance range 2–30 Mpc and γ = 1.90 ± 0.11, r c = 6.95±0.57 h ?1 Mpc in the range 2–50 Mpc. These estimates agree, within the limits of errors, with the estimates obtained for the redshifts 0.4 < z < 2.1. The original catalog shows some deficit of pairs with separations less than 1 Mpc.  相似文献   

12.
Based on archival Hubble Space Telescope images, we have performed stellar photometry for the galaxy M 101 and other neighboring galaxies located at a small angular distance from M 101 and having radial velocities similar to that of M 101: M 51, M 63, NGC 5474, NGC 5477, UGC 9405, Ho IV, KUG1413+573, and others. Based on the TRGB method, we have determined the distances to these galaxies. We have found that the M 101 group lies at a distance of 6.8 Mpc and is a small compact galaxy group consisting of four galaxies: NGC 5474, NGC 5477, UGC 9405, and Ho IV. The bright massive galaxies M 51 and M 63 are considerably farther (D = 9.0 and 9.3 Mpc, respectively) than the M 101 group and do not belong to it. Applying the virial theorem to 27 objects (H II regions and galaxies),M 101 satellites located at different distances from the galaxy, has revealed an increase in the dynamical mass of M 101 with increasing sizes of the system of satellites used in calculating the mass. The maximum calculated mass of M 101 is 7.5 × 1011 M ⊙. The dynamical mass of M 101 calculated on the basis of the four galaxies constituting the group is 6.2 × 1011 M ⊙. The mass-to-light ratio for this mass is M/L = 18 (at the adopted luminosity of M 101, M B = ?20.8).  相似文献   

13.
  相似文献   

14.
The peculiarities of non-Hubble bulk motions of galaxies are studied by analyzing a sample of 1271 thin edge-on spirals with distances determined using a multiparametric Tully-Fisher relation that includes the amplitude of the galaxy rotation, the blue and red diameters, surface brightness, and morphological type. In the purely dipole approximation, the bulk motion of galaxies relative to the cosmic microwave background frame can be described by the velocity of 336±96 km s?1 in the direction l=321°, b=?1° within radius R max =10000 km s?1. An analysis of more complex velocity field models shows that the anisotropy of the Hubble expansion described by the quadrupole term is equal to ～5% on scale lengths R max=6000–10000 km s?1. The amplitude within the Local Supercluster (R max=3000 km s?1) is as high as ～20%. The inclusion of the octupole component reduces the dipole amplitude to 134±111 km s?1 on scale lengths of ～8000 km s?1. The most remarkable feature of the galaxy velocity field within R max=8000 km s?1 is the zone of minimum centered on l=80°, b=0° (the constellation of Cygnus) whose amplitude reaches 18% of the mean Hubble velocity.  相似文献   

15.
We report the results of a statistical analysis of the space distribution of galaxies of the 2MRS catalog, which contains redshifts of 43533 galaxies of the 2MASS all-sky IR survey. Because of the unique features of the 2MRS survey, such as its 90% sky coverage, galaxy selection in the IR, the complete incorporation of the old stellar population of galaxies, weakness of the dust extinction effects, and the smallness of the k- and e-corrections allowed us to determine the statistical properties of the global distribution of galaxies in the Local Universe. We took into account the main methodological factors that distort the theoretically expected relations compared to those actually observed. We construct the radial galaxy number counts N(R), SL(R, r) statistics, and the complete correlation function (conditional density) Γ(r) for volume-limited (VL) galaxy samples. The observed conditional density Γ(r) in the redshift space is independent of the luminosity of galaxies and has the form of a power-law function with exponent γ ≈ 1.0 over a large range scale-length spanning from 0.1 to 100 Mpc. We compare the statistical properties of the space distribution of galaxies of the 2MRS catalog with the corresponding properties of simulated catalogs: stochastic fractal distributions and galaxies of the Millennium catalog.  相似文献   

16.
We analyse the topology of density surfaces drawn from the recently completed southern galactic strip (SGP) of the 2-degree field galaxy redshift survey (2dFGRS). We calculate the genus curves of the smoothed density fields drawn from the 2dFGRS SGP on scales ranging from 5h-1 Mpc to 50h-1 Mpc and compare them with the genus curves of the density fields with the same power spectrum but random phases, therefore measuring the level of phase correlation present in 2dF. We find that on scales larger than 15h-1 Mpc the measured genus curves of 2dF are very close to those expected for gaussian random fields. Below this scale, however, we can observe non-gaussianity which we believe originates from non-linear gravitational evolution. The unprecedented number of resolution elements in 2dF allows us to measure non-gaussianity with great accuracy.  相似文献   

17.
The peculiar galaxy NGC 4650 A (α=12h 42m. 1; = δ—40° 26′; 1950·0) has been studied by means of direct and spectral observations with the ESO 3·6-m telescope. It is interpreted as a prolate, elliptical galaxy surrounded by a warped ring of H II regions, dust and stars. The distance is 47 Mpc (H 0=55 km s−1 Mpc−1). The ring is seen nearly edge-on (inclination 85°) and it rotates. It has a diameter of about 21 kpc and is bluer than the elliptical galaxy for which the (M/L v) ratio is ∼12 in solar units. The observed configuration may be the result of interaction with the nearby galaxy, NGC 4650.  相似文献   

18.
A statistical analysis of radial distributions of Luminous Red Galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS DR7) catalogue within an interval 0.16≤z≤0.47 is carried out. We found that the radial distribution of ～ 106,000 LRGs incorporates a few quasi-periodical components relatively to a variable η, dimensionless line-of-sight comoving distance calculated for the ΛCDM cosmological model. The most significant peaks of the power spectra are obtained for two close periodicities corresponding to the spatial comoving scales (135±12) h?1?Mpc and (101±6) h?1?Mpc. The latter one is dominant and consistent with the characteristic scale of the baryon acoustic oscillations. We analyse also the radial distributions of two other selected LRG samples: ～33,400 bright LRGs (?23.2<M≤?21.8) and ～60,300 all LRGs within a rectangle region on the sky, and show differences of the quasi-periodical features characteristic for different samples. Being confirmed the results would allow to give preference of the spatial against temporal models which could explain the quasi-periodicities discussed here. As a caveat we show that estimations of the significance levels of the peaks strongly depend on a smoothed radial function (trend) as well as characteristics of random fluctuations.  相似文献   

19.
We analyzed the X-ray data obtained by the Chandra telescope for the galaxy cluster CL0024+17 (z = 0.39). The mean temperature of the cluster is estimated (kT = 4.35 ?0.44 +0.51 keV) and the surface brightness profile is derived. We generated the mass and density profiles for dark matter and gas using numerical simulations and the Navarro-Frenk-White dark matter density profile (Navarro et al., 1995) for a spherically symmetric cluster in which gas is in hydrostatic equilibrium with the cluster field. The total mass of the cluster is estimated to be M 200 = 3.51 ?0.47 +0.38 × 10 Sun 14 within a radius of R 200 = 1.24 ?0.17 +0.12 Mpc of the cluster center. The contribution of dark matter to the total mass of the cluster is estimated as ${{M_{200_{DM} } } \mathord{\left/ {\vphantom {{M_{200_{DM} } } {M_{tot} }}} \right. \kern-0em} {M_{tot} }} = 0.89$ .  相似文献   

20.
A distribution of matter up to 100 h -1 Mpc could be obtained from the large-scale galaxy peculiar motions. The best picture of such motion is given for today by two galaxy catalogs, namely MarkIII and RFGC. We compare their velocity fields and show they are very similar and have not only practically the same multipole structure but also a corresponding small-scale motions . We estimate a mean radial component of this small-scale motion as 500–700 kms. An additional comparison of distances to common galaxies is made. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号