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1.
We confirm and extend the recent finding that the central surface density  μ0D≡ r 0ρ0  of galaxy dark matter haloes, where r 0 and  ρ0  are the halo core radius and central density, is nearly constant and independent of galaxy luminosity. Based on the co-added rotation curves (RCs) of ∼1000 spiral galaxies, the mass models of individual dwarf irregular and spiral galaxies of late and early types with high-quality RCs, and the galaxy–galaxy weak-lensing signals from a sample of spiral and elliptical galaxies, we find that  log μ0D= 2.15 ± 0.2  in units of  log(M⊙ pc−2)  . We also show that the observed kinematics of Local Group dwarf spheroidal galaxies are consistent with this value. Our results are obtained for galactic systems spanning over 14 mag, belonging to different Hubble types and whose mass profiles have been determined by several independent methods. In the same objects, the approximate constancy of  μ0D  is in sharp contrast to the systematical variations, by several orders of magnitude, of galaxy properties, including  ρ0  and central stellar surface density.  相似文献   

2.
Motivated by recent observational studies of the environment of   z ∼ 6  QSOs, we have used the Millennium Run (MR) simulations to construct a very large  (∼4°× 4°)  mock redshift survey of star-forming galaxies at   z ∼ 6  . We use this simulated survey to study the relation between density enhancements in the distribution of i 775-dropouts and Lyα emitters, and their relation to the most massive haloes and protocluster regions at   z ∼ 6  . Our simulation predicts significant variations in surface density across the sky with some voids and filaments extending over scales of 1°, much larger than probed by current surveys. Approximately one-third of all   z ∼ 6  haloes hosting i -dropouts brighter than   z = 26.5  mag  (≈ M *UV, z =6)  become part of   z = 0  galaxy clusters. i -dropouts associated with protocluster regions are found in regions where the surface density is enhanced on scales ranging from a few to several tens of arcminutes on the sky. We analyse two structures of i -dropouts and Lyα emitters observed with the Subaru Telescope and show that these structures must be the seeds of massive clusters in formation. In striking contrast, six   z ∼ 6  QSO fields observed with Hubble Space Telescope show no significant enhancements in their i 775-dropout number counts. With the present data, we cannot rule out the QSOs being hosted by the most massive haloes. However, neither can we confirm this widely used assumption. We conclude by giving detailed recommendations for the interpretation and planning of observations by current and future ground- and space-based instruments that will shed new light on questions related to the large-scale structure at   z ∼ 6  .  相似文献   

3.
We investigate the large-scale clustering of radio sources in the FIRST 1.4-GHz survey by analysing the distribution function ( counts in cells ). We select a reliable sample from the the FIRST catalogue, paying particular attention to the problem of how to define single radio sources from the multiple components listed. We also consider the incompleteness of the catalogue. We estimate the angular two-point correlation function w (θ), the variance Ψ2 and skewness Ψ3 of the distribution for the various subsamples chosen on different criteria. Both w (θ) and Ψ2 show power-law behaviour with an amplitude corresponding to a spatial correlation length of r 0 ∼ 10  h −1Mpc. We detect significant skewness in the distribution, the first such detection in radio surveys. This skewness is found to be related to the variance through Ψ3 =  S 3(Ψ2)α, with α = 1.9 ± 0.1, consistent with the non-linear gravitational growth of perturbations from primordial Gaussian initial conditions. We show that the amplitude of variance and the skewness are consistent with realistic models of galaxy clustering.  相似文献   

4.
We consider a situation where the density and peculiar velocities in real space are linear, and we calculate ξ s , the two-point correlation function in redshift space, incorporating all non-linear effects which arise as a consequence of the map from real to redshift space. Our result is non-perturbative and it includes the effects of possible multi-streaming in redshift space. We find that the deviations from the predictions of the linear redshift distortion analysis increase for the higher spherical harmonics of ξ s . While the deviations are insignificant for the monopole ξ 0, the hexadecapole ξ 4 exhibits large deviations from the linear predictions. For a COBE normalized     ,     cold dark matter (CDM) power spectrum, our results for ξ 4 deviate from the linear predictions by a factor of two on the scale of ∼10  h −1 Mpc. The deviations from the linear predictions depend separately on f (Ω) and b . This holds the possibility of removing the degeneracy that exists between these two parameters in the linear analysis of redshift surveys which yields only     .
We also show that the commonly used phenomenological model, where the non-linear redshift two-point correlation function is calculated by convolving the linear redshift correlation function with an isotropic pair velocity distribution function, is a limiting case of our result.  相似文献   

5.
The stochasticity in the distribution of dark haloes in the cosmic density field is reflected in the distribution function   P V ( N h| δ m)  , which gives the probability of finding N h haloes in a volume V with mass density contrast δ m. We study the properties of this function using high-resolution N -body simulations, and find that   P V ( N h| δ m)  is significantly non-Poisson. The ratio between the variance and the mean goes from ∼1 (Poisson) at  1+ δ m≪1  to <1 (sub-Poisson) at  1+ δ m∼1  to >1 (super-Poisson) at  1+ δ m≫1  . The mean bias relation is found to be well described by halo bias models based on the Press–Schechter formalism. The sub-Poisson variance can be explained as a result of halo exclusion, while the super-Poisson variance at high δ m may be explained as a result of halo clustering. A simple phenomenological model is proposed to describe the behaviour of the variance as a function of δ m. Galaxy distribution in the cosmic density field predicted by semi-analytic models of galaxy formation shows similar stochastic behaviour. We discuss the implications of the stochasticity in halo bias to the modelling of higher order moments of dark haloes and of galaxies.  相似文献   

6.
We present a correlation function analysis for the catalogue of photometric redshifts obtained from the Hubble Deep Field image by Fernandez-Soto, Lanzetta & Yahil. By dividing the catalogue into redshift bins of width Δ z =0.4 we measured the angular correlation function w ( θ ) as a function of redshift up to z ∼4.8. From these measurements we derive the trend of the correlation length r 0. We find that r 0( z ) is roughly constant with look-back time up to z ≃2, and then increases to higher values at z ≳2.4. We estimate the values of r 0, assuming ξ ( r , z )=[ r r 0( z )]− γ , γ =1.8 and various geometries. For Ω0=1 we find r 0( z =3)≃7.00±4.87  h −1 Mpc, in good agreement with the values obtained from analysis of the Lyman break galaxies.  相似文献   

7.
We investigate the cross-talk between the two key components of tidal-torque theory, the inertia ( I ) and shear ( T ) tensors, using a cosmological N -body simulation with thousands of well-resolved haloes. We find that the principal axes of I and T are strongly aligned , even though I characterizes the protohalo locally while T is determined by the large-scale structure. Thus, the resultant galactic spin, which plays a key role in galaxy formation, is only a residual due to ∼10 per cent deviations from the perfect alignment of T and I . The   T – I   correlation induces a weak tendency for the protohalo spin to be perpendicular to the major axes of T and I , but this correlation is erased by non-linear effects at late times, making the observed spins poor indicators of the initial shear field.
However, the   T – I   correlation implies that the shear tensor can be used for identifying the positions and boundaries of protohaloes in cosmological initial conditions – a missing piece in galaxy formation theory. The typical configuration is of a prolate protohalo lying perpendicular to a large-scale high-density ridge, with the surrounding voids inducing compression along the major and intermediate inertia axes of the protohalo. This leads to a transient sub-halo filament along the large-scale ridge, whose subclumps then flow along the filament and merge into the final halo.
The centres of protohaloes tend to lie in ∼1 σ overdensity regions, but their association with linear density maxima smoothed on galactic scales is vague: only ∼40 per cent of the protohaloes contain peaks. Several other characteristics distinguish protohaloes from density peaks, e.g. they tend to compress along two principal axes while many peaks compress along three axes.  相似文献   

8.
We use the Hubble Ultra Deep Field to study the galaxy luminosity–size  ( M – R e )  distribution. With a careful analysis of selection effects due to both detection completeness and measurement reliability, we identify bias-free regions in the   M – R e   plane for a series of volume-limited samples. By comparison to a nearby survey also having well-defined selection limits, namely the Millennium Galaxy Catalogue, we present clear evidence for evolution in surface brightness since   z ∼ 0.7  . Specifically, we demonstrate that the mean, rest-frame B -band  〈μ〉 e   for galaxies in a sample spanning 8 mag in luminosity between   M B =−22  and −14 mag increases by ∼1.0 mag arcsec−2 from   z ∼ 0.1  to 0.7. We also highlight the importance of considering surface brightness-dependent measurement biases in addition to incompleteness biases. In particular, the increasing, systematic underestimation of Kron fluxes towards low surface brightnesses may cause diffuse, yet luminous, systems to be mistaken for faint, compact objects.  相似文献   

9.
We study the distribution function (DF) of dark matter particles in haloes of mass range  1014–1015 M⊙  . In the numerical part of this work we measure the DF for a sample of relaxed haloes formed in the simulation of a standard Λ cold dark matter (ΛCDM) model. The DF is expressed as a function of energy E and the absolute value of the angular momentum L , a form suitable for comparison with theoretical models. By proper scaling we obtain the results that do not depend on the virial mass of the haloes. We demonstrate that the DF can be separated into energy and angular momentum components and propose a phenomenological model of the DF in the form     . This formulation involves three parameters describing the anisotropy profile in terms of its asymptotic values (β0 and  β∞  ) and the scale of transition between them ( L 0). The energy part   f E ( E )  is obtained via inversion of the integral for spatial density. We provide a straightforward numerical scheme for this procedure as well as a simple analytical approximation for a typical halo formed in the simulation. The DF model is extensively compared with the simulations: using the model parameters obtained from fitting the anisotropy profile, we recover the DF from the simulation as well as the profiles of the dispersion and kurtosis of radial and tangential velocities. Finally, we show that our DF model reproduces the power-law behaviour of phase-space density   Q =ρ( r )/σ3( r )  .  相似文献   

10.
Using cosmological hydrodynamic simulations, we measure the mean transmitted flux in the Lyα forest for quasar sightlines that pass near a foreground quasar. We find that the trend of absorption with pixel quasar separation distance can be fitted using a simple power-law form including the usual correlation function parameters r 0 and γ, so that     . From the simulations, we find the relation between r 0 and quasar host mass, and formulate this as a way to estimate quasar host dark matter halo masses, quantifying uncertainties due to cosmological and IGM parameters, and redshift errors. With this method, we examine data for ∼9000 quasars from the Sloan Digital Sky Survey (SDSS) Data Release 5, assuming that the effect of ionizing radiation from quasars (the so-called transverse proximity effect) is unimportant (no evidence for it is seen in the data). We find that the best-fitting host halo mass for SDSS quasars with mean redshift z = 3 and absolute G -band magnitude −27.5 is  log  M /M⊙= 12.68+0.81−0.67  . We also use the Lyman-Break Galaxy (LBG) and Lyα forest data of Adelberger et al. in a similar fashion to constrain the halo mass of LBGs to be  log10  M /M⊙= 11.41+0.54−0.59  , a factor of ∼20 lower than the bright quasars. In addition, we study the redshift distortions of the Lyα forest around quasars, using the simulations. We use the quadrupole to monopole ratio of the quasar Lyα forest correlation function as a measure of the squashing effect. We find its dependence on halo mass difficult to measure, but find that it may be useful for constraining cosmic geometry.  相似文献   

11.
We suggest a new way to establish the relation between the electron temperature t 3 within the [O  iii ] zone and the electron temperature t 2 within the [O  ii ] zone in high-metallicity  (12 + log(O/H) > 8.25)  H  ii regions. The   t 2– t 3  diagram is constructed by applying our method to a sample of 372 H  ii regions. We find that the correlation between t 2 and t 3 is tight and can be approximated by a linear expression. The new   t 2– t 3  relation can be used to determine t 2 and accurate abundances in high-metallicity H  ii regions with a measured t 3. It can also be used in conjunction with the ff relation for the determination of t 3 and t 2 and oxygen abundances in high-metallicity H  ii regions, where the [O  iii ]λ4363 auroral line is not detected. The derived   t 2– t 3  relation is independent of photoionization models of H  ii regions.  相似文献   

12.
We have carried out the harmonic analysis of the atomic hydrogen (H  i ) surface density maps and the velocity fields for 11 galaxies belonging to the Ursa Major group, over a radial range of 4–6 disc scalelengths in each galaxy. This analysis gives the radial variation of spatial lopsidedness, quantified by the Fourier amplitude A 1 of the   m = 1  component normalized to the average value. The kinematical analysis gives a value for the elongation of the potential to be ∼10 per cent. The mean amplitude of spatial lopsidedness is found to be ∼0.14 in the inner disc, similar to the field galaxies, and is smaller by a factor of ∼2 compared to the Eridanus group galaxies. It is also shown that the average value of A 1 does not increase with the Hubble type, contrary to what is seen in field galaxies. We argue that the physical origin of lopsidedness in the Ursa Major group of galaxies is tidal interactions, albeit weaker and less frequent than in Eridanus. Thus systematic studies of lopsidedness in groups of galaxies can provide dynamical clues regarding the interactions and evolution of galaxies in a group environment.  相似文献   

13.
Using simple dimensional arguments for both spiral and elliptical galaxies, we present formulae to derive an estimate of the halo spin parameter λ for any real galaxy, in terms of common observational parameters. This allows a rough estimate of λ, which we apply to a large volume-limited sample of galaxies taken from the Sloan Digital Sky Survey data base. The large numbers involved (11 597) allow the derivation of reliable λ distributions, as signal adds up significantly in spite of the errors in the inferences for particular galaxies. We find that if the observed distribution of λ is modelled with a lognormal function, as often done for this distribution in dark matter haloes that appear in cosmological simulations, we obtain parameters  λ0= 0.04 ± 0.005  and  σλ= 0.51 ± 0.05  , interestingly consistent with values derived from simulations. For spirals, we find a good correlation between empirical values of λ and visually assigned Hubble types, highlighting the potential of this physical parameter as an objective classification tool.  相似文献   

14.
We search for ongoing major dry mergers in a well-selected sample of local brightest cluster galaxies (BCGs) from the C4 cluster catalogue. 18 out of 515 early-type BCGs with redshift between 0.03 and 0.12 are found to be in major dry mergers, which are selected as pairs (or triples) with r -band magnitude difference  δ m r < 1.5  and projected separation   r p < 30 kpc  , and showing signatures of interaction in the form of significant asymmetry in residual images. We find that the fraction of BCGs in major dry mergers increases with the richness of the clusters, consistent with the fact that richer clusters usually have more massive (or luminous) BCGs. We estimate that present-day early-type BCGs may have experienced on average  ∼0.6 ( t merge/0.3 Gyr)−1  major dry mergers and through this process increases their luminosity (mass) by 15 per cent  ( t merge/0.3 Gyr)−1 ( f mass/0.5)  on average since   z = 0.7  , where t merge is the merging time-scale and f mass is the mean mass fraction of companion galaxies added to the central ones. We also find that major dry mergers do not seem to elevate radio activities in BCGs. Our study shows that major dry mergers involving BCGs in clusters of galaxies are not rare in the local Universe, and they are an important channel for the formation and evolution of BCGs.  相似文献   

15.
We present the rest-frame optical and infrared colours of a complete sample of  1114 z < 0.3  galaxies from the Spitzer Wide-Area Infrared Extragalactic (SWIRE) Legacy Survey and the Sloan Digital Sky Survey (SDSS). We discuss the optical and infrared colours of our sample and analyse in detail the contribution of dusty star-forming galaxies and active galactic nuclei (AGN) to optically selected red sequence galaxies.
We propose that the optical  ( g − r )  colour and infrared  log( L 24/ L 3.6)  colour of galaxies in our sample are determined primarily by a bulge-to-disc ratio. The  ( g − r )  colour is found to be sensitive to the bulge-to-disc ratio for disc-dominated galaxies, whereas the  log( L 24/ L 3.6)  colour is more sensitive for bulge-dominated systems.
We identify ∼18 per cent (195 sources) of our sample as having red optical colours and infrared excess. Typically, the infrared luminosities of these galaxies are found to be at the high end of star-forming galaxies with blue optical colours. Using emission-line diagnostic diagrams, 78 are found to have an AGN contribution and 117 are identified as star-forming systems. The red  ( g − r )  colour of the star-forming galaxies could be explained by extinction. However, their high optical luminosities cannot. We conclude that they have a significant bulge component.
The number densities of optically red star-forming galaxies are found to correspond to ∼13 per cent of the total number density of our sample. In addition, these systems contribute ∼13 per cent of the total optical luminosity density, and 28 per cent of the total infrared luminosity density of our SWIRE/SDSS sample. These objects may reduce the need for 'dry mergers'.  相似文献   

16.
We calculate analytically and numerically the distance–redshift equation in perfect fluid quintessence models and give an accurate fit to the numerical solutions for all the values of the density parameter and the quintessence equation of state. Then we apply our solutions to the estimation of H 0 from multiple image time delays and find that the inclusion of quintessence modifies significantly the likelihood distribution of H 0, generally reducing the best estimate with respect to a pure cosmological constant. Marginalizing over the other parameters (Ω m and the quintessence equation of state) we obtain H 0=71±6 km s−1 Mpc−1 for an empty beam and H 0=64±4 km s−1 Mpc−1 for a filled beam. These errors, however, do not take into account the uncertainty on the modelling of the lens. We also discuss the future prospects for distinguishing quintessence from a cosmological constant with time delays.  相似文献   

17.
We present 21-cm H  i line observations of the blue compact dwarf galaxy NGC 1705. Previous optical observations show a strong outflow powered by an ongoing starburst dominating the H  ii morphology and kinematics. In contrast, most of the H  i lies in a rotating disc. An extraplanar H  i spur accounts for ∼8 per cent of the total H  i mass, and is possibly associated with the H  ii outflow. The inferred mass loss rate out of the core of the galaxy is significant, ∼0.2 − 2 M⊙ yr−1, but does not dominate the H  i dynamics. Mass model fits to the rotation curve show that the dark matter (DM) halo is dominant at nearly all radii and has a central density ρ0 ≈ 0.1 M⊙ pc−3: ten times higher than typically found in dwarf irregular galaxies, but similar to the only other mass-modelled blue compact dwarf, NGC 2915. This large difference strongly indicates that there is little evolution between dwarf irregular and blue compact dwarf types. Instead, dominant DM haloes may regulate the morphology of dwarf galaxies by setting the critical surface density for disc star formation. Neither our data nor catalogue searches reveal any likely external trigger to the starburst in NGC 1705.  相似文献   

18.
The H  i surface density maps for a sample of 18 galaxies in the Eridanus group are Fourier analysed. This analysis gives the radial variation of the lopsidedness in the H  i spatial distribution. The lopsidedness is quantified by the Fourier amplitude A 1 of the m = 1 component normalized to the average value. It is also shown that in the radial region where the stellar disc and H  i overlap, their A 1 coefficients are comparable. All the galaxies studied show significant lopsidedness in H  i . The mean value of A 1 in the inner regions of the galaxies (1.5–2.5 scalelengths) is ≥ 0.2. This value of A 1 is twice the average value seen in the field galaxies. Also, the lopsidedness is found to be smaller for late-type galaxies; this is opposite to the trend seen in the field galaxies. These two results indicate a different physical origin for disc lopsidedness in galaxies in a group environment compared to the field galaxies. Further, a large fraction (∼30 per cent) shows a higher degree of lopsidedness ( A 1≥ 0.3). It is also seen that the disc lopsidedness increases with the radius as demonstrated in earlier studies, but over a radial range that is two times larger than done in the previous studies. The average lopsidedness of the halo potential is estimated to be ∼10 per cent, assuming that the lopsidedness in H  i disc is due to its response to the halo asymmetry.  相似文献   

19.
We have conducted a submillimetre mapping survey of faint, gravitationally lensed sources, where we have targeted 12 galaxy clusters and additionally the New Technology Telescope (NTT) Deep Field. The total area surveyed is 71.5 arcmin2 in the image plane; correcting for gravitational lensing, the total area surveyed is 40 arcmin2 in the source plane for a typical source redshift z ≈ 2.5. In the deepest maps, an image plane depth of 1σ rms ∼0.8 mJy is reached. This survey is the largest survey to date to reach such depths. In total 59 sources were detected, including three multiply imaged sources. The gravitational lensing makes it possible to detect sources with flux density below the blank field confusion limit. The lensing-corrected fluxes range from 0.11 to 19 mJy. After correcting for multiplicity, there are 10 sources with fluxes <2 mJy of which seven have submJy fluxes, doubling the number of such sources known. Number counts are determined below the confusion limit. At 1 mJy, the integrated number count is  ∼104 deg−2  , and at 0.5 mJy it is  ∼2 × 104 deg−2  . Based on the number counts, at a source plan flux limit of 0.1 mJy, essentially all of the 850-μm background emission has been resolved. The dominant contribution (>50 per cent) to the integrated background arises from sources with fluxes S 850 between 0.4 and 2.5 mJy, while the bright sources S 850 > 6 mJy contribute only 10 per cent.  相似文献   

20.
We present CCD photometry in the Washington system C and T 1 passbands down to   T 1∼ 22  in the fields of L35, L45, L49, L50, L62, L63 and L85, seven poorly studied star clusters in the inner region of the Small Magellanic Cloud (SMC). We measured T 1 magnitudes and   C − T 1  colours for a total of 114 826 stars distributed throughout cluster areas of 13.7 × 13.7 arcmin2 each. Cluster radii were estimated from star counts distributed throughout the entire observed fields. The seven clusters are generally characterized by a relatively small angular size and by a high field star contamination. We performed an in-depth analysis of the field star contamination of the colour–magnitude diagrams (CMDs), and statistically cleaned the cluster CMDs. Based on the best fits of isochrones computed by the Padova group to the  ( T 1,  C − T 1)  CMDs, we derive ages for the sample, assuming Z = 0.004, finding ages between 25 Myr and 1.2 Gyr. We then examined different relationships between positions in the SMC, age and metallicity of a larger sample of clusters including our previous work whose ages and metallicities are on the same scale used in this paper. We confirm previous results in the sense that the further a cluster is from the centre of the galaxy, the older and more metal poor it is, with some dispersion; although clusters associated with the Magellanic Bridge clearly do not obey the general trend. The number of clusters within ∼ 2° of the SMC centre appears to have increased substantially after ∼2.5 Gyr ago, hinting at a burst.  相似文献   

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