Distribution by Scientific Domains
Distribution within Physics and Astronomy

Kinds of Universe

  • early universe
  • expanding universe
  • local universe
  • nearby universe

  • Selected Abstracts



    A view of extraterrestrial soils

    G. Certini
    Summary The nature of soils on celestial bodies other than Earth is a growing area of research in planetary geology. However, disagreement over the significance of these deposits arises, in part, through the lack of a unified concept and definition of soil in the literature. The pragmatic definition ,medium for plant growth' is taken by some to imply the necessity of biota for soil to exist, and has been commonly adopted in the planetary science community. In contrast, a more complex and informative definition is based on scientific theory: soil is the (bio)geochemically/physically altered material at the surface of a planetary body that encompasses surficial extraterrestrial telluric deposits. This definition is based on the premise that soil is a body that retains information about its environmental history and that it does not need the presence of life to form. Four decades of missions have gathered geochemical information regarding the surface of planets and bodies within the Solar System, and information is quickly increasing. Reviewing the current knowledge on properties of extraterrestrial regoliths, we conclude that the surficial deposits of Venus, Mars and our moon should be considered to be soils in a pedological sense, and that Mercury and some large asteroids are covered in mantles that are soil candidates. A key environmental distinction between Earth and other Solar System bodies is the presence of life, and because of this dissimilarity in soil-forming processes, it is reasonable to distinguish these (presently) abiotic soils as Astrosols. Attempts to provide detailed classifications of extraterrestrial soils are premature, given our poor current knowledge of the Universe, but they highlight the fact that Earth possesses almost-abiotic environments that lend themselves to providing more understanding about telluric bodies of the Solar System. "He found himself in the neighbourhood of the asteroids 325, 326, 327, 328, 329, and 330. He began, therefore, by visiting them, in order to add to his knowledge." (Excerpt from the The Little Prince by Antoine de Saint-Exupéry) [source]

    Books and Multimedia Reviews

    Article first published online: 4 FEB 2010
    Book reviewed in this article: Catalogue of Meteorites, Fifth Edition (Revised and Enlarged) by Monica M. Grady Extrasolar Planets: The Search for New Worlds by Stuart Clark The Moon: Resources, Future Development and Colonization by David Schrunk, Burton Sharpe, Bonnie Cooper and Madhu Thangavelu Rare Earth: Why Complex Life is Uncommon in the Universe by Peter D. Ward and Donald E. Brownlee [source]

    Pomponazzi: Moral Virtue in a Deterministic Universe

    John L. Treloar

    Hydrodynamical N -body simulations of coupled dark energy cosmologies

    Marco Baldi
    ABSTRACT If the accelerated expansion of the Universe at the present epoch is driven by a dark energy scalar field, there may well be a non-trivial coupling between the dark energy and the cold dark matter (CDM) fluid. Such interactions give rise to new features in cosmological structure growth, like an additional long-range attractive force between CDM particles, or variations of the dark matter particle mass with time. We have implemented these effects in the N -body code gadget-2 and present results of a series of high-resolution N -body simulations where the dark energy component is directly interacting with the CDM. As a consequence of the new physics, CDM and baryon distributions evolve differently both in the linear and in the non-linear regime of structure formation. Already on large scales, a linear bias develops between these two components, which is further enhanced by the non-linear evolution. We also find, in contrast with previous work, that the density profiles of CDM haloes are less concentrated in coupled dark energy cosmologies compared with ,CDM, and that this feature does not depend on the initial conditions setup, but is a specific consequence of the extra physics induced by the coupling. Also, the baryon fraction in haloes in the coupled models is significantly reduced below the universal baryon fraction. These features alleviate tensions between observations and the ,CDM model on small scales. Our methodology is ideally suited to explore the predictions of coupled dark energy models in the fully non-linear regime, which can provide powerful constraints for the viable parameter space of such scenarios. [source]

    Constraints on modified gravity from the observed X-ray luminosity function of galaxy clusters

    David Rapetti
    ABSTRACT We use measurements of the growth of cosmic structure, as inferred from the observed evolution of the X-ray luminosity function (XLF) of galaxy clusters, to constrain departures from general relativity (GR) on cosmological scales. We employ the popular growth rate parameterization, ,m(z),, for which GR predicts a growth index ,, 0.55. We use observations of the cosmic microwave background (CMB), type Ia supernovae (SNIa) and X-ray cluster gas mass fractions (fgas), to simultaneously constrain the expansion history and energy content of the Universe, as described by the background model parameters: ,m, w and ,k, i.e. the mean matter density, the dark energy equation of state parameter and the mean curvature, respectively. Using conservative allowances for systematic uncertainties, in particular for the evolution of the mass,luminosity scaling relation in the XLF analysis, we find ,= 0.51+0.16,0.15 and ,m= 0.27 ± 0.02 (68.3 per cent confidence limits), for a flat cosmological constant, cold dark matter (,CDM) background model. Allowing w to be a free parameter, we find ,= 0.44+0.17,0.15. Relaxing the flatness prior in the ,CDM model, we obtain ,= 0.51+0.19,0.16. When in addition to the XLF data we use the CMB data to constrain , through the ISW effect, we obtain a combined constraint of ,= 0.45+0.14,0.12 for the flat ,CDM model. Our analysis provides the tightest constraints to date on the growth index. We find no evidence for departures from GR on cosmological scales. [source]

    A QSO host galaxy and its Ly, emission at z= 6.43,

    Tomotsugu Goto
    ABSTRACT Host galaxies of highest redshift quasi-stellar objects (QSOs) are of interest; they provide us with a valuable opportunity to investigate physics relevant to the starburst,active galactic nuclei (AGN) connection at the earliest epoch of the Universe, with the most luminous black holes. Here, we report an optical detection of an extended structure around a QSO at z= 6.43 in deep z,- and zr -band images of the Subaru/Suprime-Cam. Our target is CFHQS J2329-0301 (z= 6.43), the highest redshift QSO currently known. We have carefully subtracted a point spread function (PSF) constructed using nearby stars from the images. After the PSF (QSO) subtraction, a structure in the z, band extends more than 4 arcsec on the sky (Re= 11 kpc), and, thus, is well resolved (16, detection). The PSF-subtracted zr -band structure is in a similar shape to that in the z, band, but less significant with a 3, detection. In the z, band, a radial profile of the QSO+host shows a clear excess over that of the averaged PSF in 0.8,3 arcsec radius. Since the z, band includes a Ly, emission at z= 6.43, we suggest the z, flux is a mixture of the host (continuum light) and its Ly, emission, whereas the zr -band flux is from the host. Through a SED modelling, we estimate 40 per cent of the PSF-subtracted z,-band light is from the host (continuum) and 60 per cent is from Ly, emission. The absolute magnitude of the host is M1450=,23.9 (cf. M1450=,26.4 for the QSO). A lower limit of the SFR(Ly,) is 1.6 M, yr,1 with stellar mass ranging from 6.2 × 108 to 1.1 × 1010 M, when 100 Myr of age is assumed. The detection shows that a luminous QSO is already harboured by a large, star-forming galaxy in the early Universe only after ,840 Myr after the big bang. The host may be a forming giant galaxy, co-evolving with a super-massive black hole. [source]

    On the interstellar medium and star formation demographics of galaxies in the local universe

    Matthew S. Bothwell
    ABSTRACT We present a demographic analysis of integrated star formation and gas properties for a sample of galaxies representative of the overall population at z, 0. This research was undertaken in order to characterize the nature of star formation and interstellar medium (ISM) behaviour in the local Universe, and test the extent to which global star formation rates (SFRs) can be seen as dependent on the interstellar gas content. Archival 21-cm derived H i data are compiled from the literature, and are combined with CO (J =1 , 0) derived H2 masses to calculate and characterize the total gas content for a large sample of local galaxies. The distribution in stellar mass-normalized H i content is found to exhibit the noted characteristic transition at stellar masses of ,3 × 1010 M,, turning off towards low values, but no such transition is observed in the equivalent distribution of molecular gas. H, based SFRs and specific star formation rates (SSFRs) are also compiled for a large (1110) sample of local galaxies. We confirm two transitions as found in previous work: a turnover towards low SFRs at high luminosities, indicative of the quenching of SF characteristic of the red sequence; and a broadening of the SF distribution in low-luminosity dwarf galaxies, again to extremely low SFRs of <10,3 M, yr,1. However, a new finding is that while the upper luminosity transition is mirrored by the turnover in H i content, suggesting that the low SFRs of the red sequence result from a lack of available gas supply, the transition towards a large spread of SFRs in the least luminous dwarf galaxies is not matched by a prominent increase in scatter in gas content. Possible mass-dependent quenching mechanisms are discussed, along with speculations that in low-mass galaxies, the H, luminosity may not faithfully trace the SFR. [source]

    Major dry mergers in early-type brightest cluster galaxies

    F. S. Liu
    ABSTRACT 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 ,mr < 1.5 and projected separation rp < 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 (tmerge/0.3 Gyr),1 major dry mergers and through this process increases their luminosity (mass) by 15 per cent (tmerge/0.3 Gyr),1 (fmass/0.5) on average since z= 0.7, where tmerge is the merging time-scale and fmass 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. [source]

    An implementation of radiative transfer in the cosmological simulation code gadget

    Margarita Petkova
    ABSTRACT We present a novel numerical implementation of radiative transfer in the cosmological smoothed particle hydrodynamics (SPH) simulation code gadget. It is based on a fast, robust and photon-conserving integration scheme where the radiation transport problem is approximated in terms of moments of the transfer equation and by using a variable Eddington tensor as a closure relation, following the Optically Thin Variable Eddington Tensor suggestion of Gnedin & Abel. We derive a suitable anisotropic diffusion operator for use in the SPH discretization of the local photon transport, and we combine this with an implicit solver that guarantees robustness and photon conservation. This entails a matrix inversion problem of a huge, sparsely populated matrix that is distributed in memory in our parallel code. We solve this task iteratively with a conjugate gradient scheme. Finally, to model photon sink processes we consider ionization and recombination processes of hydrogen, which is represented with a chemical network that is evolved with an implicit time integration scheme. We present several tests of our implementation, including single and multiple sources in static uniform density fields with and without temperature evolution, shadowing by a dense clump and multiple sources in a static cosmological density field. All tests agree quite well with analytical computations or with predictions from other radiative transfer codes, except for shadowing. However, unlike most other radiative transfer codes presently in use for studying re-ionization, our new method can be used on-the-fly during dynamical cosmological simulation, allowing simultaneous treatments of galaxy formation and the re-ionization process of the Universe. [source]

    The disc-dominated host galaxy of FR-I radio source B2 0722+30

    B. H. C. Emonts
    ABSTRACT We present new observational results that conclude that the nearby radio galaxy B2 0722+30 is one of the very few known disc galaxies in the low-redshift Universe that host a classical double-lobed radio source. In this paper, we use H i observations, deep optical imaging, stellar population synthesis modelling and emission-line diagnostics to study the host galaxy, classify the active galactic nucleus (AGN) and investigate environmental properties under which a radio-loud AGN can occur in this system. Typical for spiral galaxies, B2 0722+30 has a regularly rotating gaseous disc throughout which star formation occurs. Dust heating by the ongoing star formation is likely responsible for the high infrared luminosity of the system. The optical emission-line properties of the central region identify a Low Ionization Nuclear Emission-line Region (LINER)-type nucleus with a relatively low [O iii] luminosity, in particular when compared with the total power of the Fanaroff & Riley type-I radio source that is present in this system. This classifies B2 0722+30 as a classical radio galaxy rather than a typical Seyfert galaxy. The environment of B2 0722+30 is extremely H i -rich, with several nearby interacting galaxies. We argue that a gas-rich interaction involving B2 0722+30 is a likely cause for the triggering of the radio AGN and/or the fact that the radio source managed to escape the optical boundaries of the host galaxy. [source]

    Optimal integrated Sachs,Wolfe detection and joint likelihood for cosmological parameter estimation

    M. Frommert
    ABSTRACT We analyse the local variance effect in the standard method for detecting the integrated Sachs,Wolfe (ISW) effect via cross-correlating the cosmic microwave background (CMB) with the large-scale structure (LSS). Local variance is defined as the systematic noise in the ISW detection that originates in the realization of the matter distribution in the observed Universe. We show that the local variance contributes about 11 per cent to the total variance in the standard method if a perfect and complete LSS survey up to z, 2 is assumed. Due to local variance, the estimated detection significance and cosmological parameter constraints in the standard method are biased. In this work, we present an optimal method of how to reduce the local variance effect in the ISW detection by working conditional on the LSS data. The variance of the optimal method, and hence the signal-to-noise ratio, depends on the actual realization of the matter distribution in the observed Universe. We show that for an ideal galaxy survey, the average signal-to-noise ratio is enhanced by about 7 per cent in the optimal method, as compared to the standard method. In the framework of our method, it is straightforward to correct for the magnification bias coming from gravitational lensing effects. Furthermore, there is no need to estimate the covariance matrix by Monte Carlo simulations as in the standard method, which saves time and increases the accuracy. Finally, we derive the correct joint likelihood function for cosmological parameters given CMB and LSS data within the linear LSS formation regime, which includes a small coupling of the two data sets due to the ISW effect. [source]

    Non-local thermodynamic equilibrium dust nucleation in subsaturated vapours

    Davide Lazzati
    ABSTRACT We use the kinetic theory of nucleation to explore the properties of dust nucleation in subsaturated vapours. Due to radiation losses, the subcritical clusters have a smaller temperature compared to their vapour. This alters the dynamical balance between the attachment and detachment of monomers, allowing for stable nucleation of grains in vapours that are subsaturated for their temperature. We find this effect particularly important at low densities and in the absence of a strong background radiation field. We find new conditions for stable nucleation in the n,T phase diagram. The nucleation in the non-local thermodynamic equilibrium (non-LTE) regions is likely to be at much slower rate than in the supersaturated vapours. We evaluate the nucleation rate, warning the reader that it does depend on poorly substantiated properties of the macro-molecules assumed in the computation. On the other hand, the conditions for nucleation depend only on the properties of the large stable grains and are more robust. We finally point out that this mechanism may be relevant in the early Universe as an initial dust pollution mechanism, since once the interstellar medium is polluted with dust, mantle growth is likely to be dominant over non-LTE nucleation in the diffuse medium. [source]

    Observational biases in Lagrangian reconstructions of cosmic velocity fields

    G. Lavaux
    ABSTRACT Lagrangian reconstruction of large-scale peculiar velocity fields can be strongly affected by observational biases. We develop a thorough analysis of these systematic effects by relying on specially selected mock catalogues. For the purpose of this paper, we use the Monge,Ampère,Kantorovitch (MAK) reconstruction method, although any other Lagrangian reconstruction method should be sensitive to the same problems. We extensively study the uncertainty in the mass-to-light assignment due to incompleteness (missing luminous mass tracers), and the poorly determined relation between mass and luminosity. The impact of redshift distortion corrections is analysed in the context of MAK and we check the importance of edge and finite-volume effects on the reconstructed velocities. Using three mock catalogues with different average densities, we also study the effect of cosmic variance. In particular, one of them presents the same global features as found in observational catalogues that extend to 80 h,1 Mpc scales. We give recipes, checked using the aforementioned mock catalogues, to handle these particular observational effects, after having introduced them into the mock catalogues so as to quantitatively mimic the most densely sampled currently available galaxy catalogue of the nearby Universe. Once biases have been taken care of, the typical resulting error in reconstructed velocities is typically about a quarter of the overall velocity dispersion, and without significant bias. We finally model our reconstruction errors to propose an improved Bayesian approach to measure ,m in an unbiased way by comparing the reconstructed velocities to the measured ones in distance space, even though they may be plagued by large errors. We show that, in the context of observational data, it is possible to build a nearly unbiased estimator of ,m using MAK reconstruction. [source]

    A census of metals and baryons in stars in the local Universe

    Anna Gallazzi
    ABSTRACT We combine stellar metallicity and stellar mass estimates for a large sample of galaxies drawn from the Sloan Digital Sky Survey Data Release 2 (SDSS DR2) spanning wide ranges in physical properties, in order to derive an inventory of the total mass of metals and baryons locked up in stars in the local Universe. Physical parameter estimates are derived from galaxy spectra with high signal-to-noise ratio (S/N) (of at least 20). Co-added spectra of galaxies with similar velocity dispersions, absolute r -band magnitudes and 4000-Å break values are used for those regions of parameter space where individual spectra have lower S/N. We estimate the total density of metals ,Z and of baryons ,* in stars and, from these two quantities, we obtain a mass- and volume-averaged stellar metallicity of ,Z*,= 1.04 ± 0.14 Z,, i.e. consistent with solar. We also study how metals are distributed in galaxies according to different properties, such as mass, morphology, mass- and light-weighted age, and we then compare these distributions with the corresponding distributions of stellar mass. We find that the bulk of metals locked up in stars in the local Universe reside in massive, bulge-dominated galaxies, with red colours and high 4000-Å break values corresponding to old stellar populations. Bulge-dominated and disc-dominated galaxies contribute similar amounts to the total stellar mass density, but have different fractional contributions to the mass density of metals in stars, in agreement with the mass,metallicity relation. Bulge-dominated galaxies contain roughly 40 per cent of the total amount of metals in stars, while disc-dominated galaxies less than 25 per cent. Finally, at a given galaxy stellar mass, we define two characteristic ages as the median of the distributions of mass and metals as a function of age. These characteristic ages decrease progressively from high-mass to low-mass galaxies, consistent with the high formation epochs of stars in massive galaxies. [source]

    The UV properties of E+A galaxies: constraints on feedback-driven quenching of star formation

    S. Kaviraj
    ABSTRACT We present the first large-scale study of E+A galaxies that incorporates photometry in the ultraviolet (UV) wavelengths. E+A galaxies are ,post-starburst' systems, with strong Balmer absorption lines indicating significant recent star formation, but without [O ii] and H, emission lines which are characteristic of ongoing star formation. The starburst that creates the E+A galaxy typically takes place within the last Gyr and creates a high fraction (20,60 per cent) of the stellar mass in the remnant over a short time-scale (<0.1 Gyr). We find a tight correlation between the luminosity of our E+A galaxies and the implied star formation rate (SFR) during the starburst. While low-luminosity E+As [M(z) > ,20] exhibit implied SFRs of less than 50 M, yr,1, their luminous counterparts [M(z) < ,22] show SFRs greater than 300 and as high as 2000 M, yr,1, suggesting that luminous and ultra-luminous infrared galaxies in the low-redshift Universe could be the progenitors of massive nearby E+A galaxies. We perform a comprehensive study of the characteristics of the quenching that truncates the starburst in E+A systems. We find that, for galaxies less massive than 1010 M,, the quenching efficiency decreases as the galaxy mass increases. However, for galaxies more massive than 1010 M,, this trend is reversed and the quenching efficiency increases with galaxy mass. Noting that the mass threshold at which this reversal occurs is in excellent agreement with the mass above which active galactic nuclei (AGN) become significantly more abundant in nearby galaxies, we use simple energetic arguments to show that the bimodal behaviour of the quenching efficiency is consistent with AGN and supernovae (SN) being the principal sources of negative feedback above and below M, 1010 M,, respectively. The arguments assume that quenching occurs through the mechanical ejection or dispersal of the gas reservoir and that, in the high-mass regime (M > 1010 M,), the Eddington ratios in this sample of galaxies scale as M,, where 1 < , < 3. Finally, we use our E+A sample to estimate the time it takes for galaxies to migrate from the blue cloud to the red sequence. We find migration times between 1 and 5 Gyr, with a median value of 1.5 Gyr. [source]

    The baryonic and dark matter properties of high-redshift gravitationally lensed disc galaxies

    P. Salucci
    ABSTRACT We present a detailed study of the structural properties of four gravitationally lensed disc galaxies at z= 1. Modelling the rotation curves on sub-kpc scales, we derive the values for the disc mass, the reference dark matter density and core radius, and the angular momentum per unit mass. The derived models suggest that the rotation curve profile and amplitude are best fitted with a dark matter component similar to those of local spiral galaxies. The stellar component also has a similar length-scale, but with substantially smaller masses than similarly luminous disc galaxies in the local Universe. Comparing the average dark matter density inside the optical radius, we find that the disc galaxies at z= 1 have larger densities (by up to a factor of ,7) than similar disc galaxies in the local Universe. Furthermore, the angular momentum per unit mass versus reference velocity is well matched to the local relation, suggesting that the angular momentum of the disc remains constant between high redshifts and the present day. Though statistically limited, these observations point towards a spirals' formation scenario in which stellar discs are slowly grown by the accretion of angular momentum conserving material. [source]

    Delaunay Tessellation Field Estimator analysis of the PSCz local Universe: density field and cosmic flow

    Emilio Romano-Díaz
    ABSTRACT We apply the Delaunay Tessellation Field Estimator (DTFE) to reconstruct and analyse the matter distribution and cosmic velocity flows in the local Universe on the basis of the PSCz galaxy survey. The prime objective of this study is the production of optimal resolution 3D maps of the volume-weighted velocity and density fields throughout the nearby universe, the basis for a detailed study of the structure and dynamics of the cosmic web at each level probed by underlying galaxy sample. Fully volume-covering 3D maps of the density and (volume-weighted) velocity fields in the cosmic vicinity, out to a distance of 150 h,1 Mpc, are presented. Based on the Voronoi and Delaunay tessellation defined by the spatial galaxy sample, DTFE involves the estimate of density values on the basis of the volume of the related Delaunay tetrahedra and the subsequent use of the Delaunay tessellation as natural multidimensional (linear) interpolation grid for the corresponding density and velocity fields throughout the sample volume. The linearized model of the spatial galaxy distribution and the corresponding peculiar velocities of the PSCz galaxy sample, produced by Branchini et al., forms the input sample for the DTFE study. The DTFE maps reproduce the high-density supercluster regions in optimal detail, both their internal structure as well as their elongated or flattened shape. The corresponding velocity flows trace the bulk and shear flows marking the region extending from the Pisces,Perseus supercluster, via the Local Superclusters, towards the Hydra,Centaurus and the Shapley concentration. The most outstanding and unique feature of the DTFE maps is the sharply defined radial outflow regions in and around underdense voids, marking the dynamical importance of voids in the local Universe. The maximum expansion rate of voids defines a sharp cut-off in the DTFE velocity divergence probability distribution function. We found that on the basis of this cut-off DTFE manages to consistently reproduce the value of ,m, 0.35 underlying the linearized velocity data set. [source]

    Determining the optimum scan map strategy for Herschel -SPIRE using the SPIRE photometer simulator

    T. J. Waskett
    ABSTRACT The forthcoming Herschel space mission will provide an unprecedented view of the far-infrared/submillimetre Universe, with the SPIRE instrument covering the 200,670 ,m wavelength range. To obtain the best quality of astronomical data from such an expensive mission the observing modes must be optimized as far as possible. This paper presents the possible scanning strategies that can be utilized by the SPIRE photometer, within the limitations imposed by the Herschel spacecraft. Each strategy is investigated for effectiveness by performing simulated observations, using the SPIRE photometer simulator. By quantifying the data quality using a simple metric, we have been able to select the optimum scanning strategy for SPIRE when it begins taking science data within the next couple of years. Additionally, this work has led to the development of a specific SPIRE mapmaking algorithm, based on the CMB code MADmap, to be provided as part of the SPIRE data pipeline processing suite. This will allow every SPIRE user to take full advantage of the optimized scan map strategy, which requires the use of maximum likelihood mapmakers such as MADmap. [source]

    A highly obscured and strongly clustered galaxy population discovered with the Spitzer Space Telescope

    M. Magliocchetti
    ABSTRACT The ,800 optically unseen (R > 25.5) 24-,m selected sources in the complete Spitzer First Look Survey sample with F24 ,m, 0.35 mJy are found to be very strongly clustered. If, as indicated by several lines of circumstantial evidence, they are ultraluminous far-infrared galaxies at z, 1.6,2.7, the amplitude of their spatial correlation function is very high. The associated comoving clustering length is estimated to be r0= 14.0+2.1,2.4 Mpc, value which puts these sources amongst the most strongly clustered populations of our known Universe. Their 8,24 ,m colours suggest that the active galactic nucleus contribution dominates above F24 ,m, 0.8 mJy, consistent with earlier analyses. The properties of these objects (number counts, redshift distribution, clustering amplitude) are fully consistent with those of proto-spheroidal galaxies in the process of forming most of their stars and of growing their active nucleus, as described by the Granato et al. model. In particular, the inferred space density of such galaxies at z, 2 is much higher than what is expected from most semi-analytic models. Matches of the observed projected correlation function w(,) with models derived within the so-called halo occupation scenario show that these sources have to be hosted by haloes more massive than ,1013.4 M,. This value is significantly higher than that for the typical galactic haloes hosting massive elliptical galaxies, suggesting a duration of the starburst phase of massive high-redshift dusty galaxies of TB, 0.5 Gyr. [source]

    Cosmological hydrogen recombination: populations of the high-level substates

    J. Chluba
    ABSTRACT We present results for the spectral distortions of the cosmic microwave background (CMB) arising due to bound,bound transitions during the epoch of cosmological hydrogen recombination at frequencies down to ,,100 MHz. We extend our previous treatment of the recombination problem now including the main collisional processes and following the evolution of all the hydrogen angular momentum substates for up to 100 shells. We show that, due to the low baryon density of the Universe, even within the highest considered shell full statistical equilibrium (SE) is not reached and that at low frequencies the recombination spectrum is significantly different when assuming full SE for n > 2. We also directly compare our results for the ionization history to the output of the recfast code, showing that especially at low redshifts rather big differences arise. In the vicinity of the Thomson visibility function the electron fraction differs by roughly ,0.6 per cent which affects the temperature and polarization power spectra by , 1 per cent. Furthermore, we shortly discuss the influence of free,free absorption and line broadening due to electron scattering on the bound,bound recombination spectrum and the generation of CMB angular fluctuations due to scattering of photons within the high shells. [source]

    The stellar mass density at z, 6 from Spitzer imaging of i,-drop galaxies

    Laurence P. Eyles
    ABSTRACT We measure the ages, stellar masses, and star formation histories of z, 6 galaxies, observed within 1 Gyr of the big bang. We use imaging from the Hubble Space Telescope (HST) and the Spitzer Space Telescope from the public ,Great Observatories Origins Deep Survey' (GOODS), coupled with ground-based near-infrared imaging, to measure their spectral energy distributions (SEDs) from 0.8,5 ,m, spanning the rest-frame ultraviolet (UV) and optical. From our sample of ,50 ,i,-drop' Lyman-break star-forming galaxies in GOODS-South with z,AB < 27, we focus on ,30 with reliable photometric or spectroscopic redshifts. Half of these are confused with foreground sources at Spitzer resolution, but from the 16 with clean photometry we find that a surprisingly large fraction (40 per cent) have evidence for substantial Balmer/4000-Å spectral breaks. This indicates the presence of old underlying stellar populations that dominate the stellar masses. For these objects, we find ages of ,200,700 Myr, implying formation redshifts of 7 ,zf, 18, and large stellar masses in the range ,1,3 × 1010 M,. Analysis of seven i,-drops that are undetected at 3.6 ,m indicates that these are younger, considerably less massive systems. We calculate that emission line contamination should not severely affect our photometry or derived results. Using SED fits out to 8 ,m, we find little evidence for substantial intrinsic dust reddening in our sources. We use our individual galaxy results to obtain an estimate of the global stellar mass density at z, 6. Correcting for incompleteness in our sample, we find the z, 6 comoving stellar mass density to be 2.5 × 106 M, Mpc,3. This is a lower limit, as post-starburst and dust-obscured objects, and also galaxies below our selection thresholds, are not accounted for. From our results, we are able to explore the star formation histories of our selected galaxies, and we suggest that the past global star formation rate may have been much higher than that observed at the z, 6 epoch. The associated UV flux we infer at z > 7 could have played a major role in reionizing the Universe. [source]

    The halo mass function from the dark ages through the present day

    Darren S. Reed
    ABSTRACT We use an array of high-resolution N -body simulations to determine the mass function of dark matter haloes at redshifts 10,30. We develop a new method for compensating for the effects of finite simulation volume that allows us to find an approximation to the true ,global' mass function. By simulating a wide range of volumes at different mass resolution, we calculate the abundance of haloes of mass 105,12 h,1 M,. This enables us to predict accurately the abundance of the haloes that host the sources that reionize the Universe. In particular, we focus on the small mass haloes (,105.5,6 h,1 M,) likely to harbour Population III stars where gas cools by molecular hydrogen emission, early galaxies in which baryons cool by atomic hydrogen emission at a virial temperature of ,104K (,107.5,8 h,1 M,), and massive galaxies that may be observable at redshift ,10. When we combine our data with simulations that include high-mass haloes at low redshift, we find that the best fit to the halo mass function depends not only on the linear overdensity, as is commonly assumed in analytic models, but also on the slope of the linear power spectrum at the scale of the halo mass. The Press,Schechter model gives a poor fit to the halo mass function in the simulations at all epochs; the Sheth-Tormen model gives a better match, but still overpredicts the abundance of rare objects at all times by up to 50 per cent. Finally, we consider the consequences of the recently released WMAP 3-yr cosmological parameters. These lead to much less structure at high redshift, reducing the number of z= 10,mini-haloes' by more than a factor of two and the number of z= 30 galaxy hosts by nearly four orders of magnitude. Code to generate our best-fitting halo mass function may be downloaded from http://icc.dur.ac.uk/Research/PublicDownloads/genmf_readme.html. [source]

    The clustering of narrow-line AGN in the local Universe

    Cheng Li
    ABSTRACT We have analysed the clustering of ,90 000 narrow-line active galactic nuclei (AGN) drawn from the Data Release 4 (DR4) of the Sloan Digital Sky Survey. Our analysis addresses the following questions. (i) How do the locations of galaxies within the large-scale distribution of dark matter influence ongoing accretion on to their central black holes? (ii) Is AGN activity triggered by interactions or mergers between galaxies? We compute the cross-correlation between AGN and a reference sample of galaxies drawn from the DR4. We compare this to results for control samples of inactive galaxies matched simultaneously in redshift, stellar mass, concentration, velocity dispersion and mean stellar age, as measured by the 4000-Å break strength. We also compare near-neighbour counts around AGN and around the control galaxies. On scales larger than a few Mpc, AGN have almost the same clustering amplitude as the control sample. This demonstrates that AGN host galaxies and inactive control galaxies populate dark matter haloes of similar mass. On scales between 100 kpc and 1 Mpc, AGN are clustered more weakly than the control galaxies. We use mock catalogues constructed from high-resolution N -body simulations to interpret this antibias, showing that the observed effect is easily understood if AGN are preferentially located at the centres of their dark matter haloes. On scales less than 70 kpc, AGN cluster marginally more strongly than the control sample, but the effect is weak. When compared to the control sample, we find that only one in 100 AGN has an extra neighbour within a radius of 70 kpc. This excess increases as a function of the accretion rate on to the black hole, but it does not rise above the few per cent level. Although interactions between galaxies may be responsible for triggering nuclear activity in a minority of nearby AGN, some other mechanism is required to explain the activity seen in the majority of the objects in our sample. [source]

    The properties of galaxies in voids

    Santiago G. Patiri
    ABSTRACT We present a comparison of the properties of galaxies in the most underdense regions of the Universe, where the galaxy number density is less than 10 per cent of the mean density, with galaxies from more typical regions. We have compiled a sample of galaxies in 46 large nearby voids that were identified using the Sloan Digital Sky Survey DR4, which provides the largest coverage of the sky. We study the u,r colour distribution, morphology, specific star formation rate (SFR) and radial number density profiles for a total of 495 galaxies fainter than Mr=,20.4 + 5 log h located inside the voids and compare these properties with a control sample of field galaxies. We show that there is an excess of blue galaxies inside the voids. However, inspecting the properties of blue and red galaxies separately, we find that galaxy properties such as colour distribution, bulge-to-total ratios and concentrations are remarkably similar between the void and overall sample. The void galaxies also show the same specific SFR at fixed colour as the control galaxies. We compare our results with the predictions of cosmological simulations of galaxy formation using the Millennium Run semi-analytic galaxy catalogue. We show that the properties of the simulated galaxies in large voids are in reasonably good agreement with those found in similar environments in the real Universe. To summarize, in spite of the fact that galaxies in voids live in the least dense large-scale environment, this environment makes very little impact on the properties of galaxies. [source]

    Cosmic reionization constraints on the nature of cosmological perturbations

    Pedro P. Avelino
    ABSTRACT We study the reionization history of the Universe in cosmological models with non-Gaussian density fluctuations, taking them to have a renormalized ,2 probability distribution function parametrized by the number of degrees of freedom, ,. We compute the ionization history using a simple semi-analytical model, considering various possibilities for the astrophysics of reionization. In all our models we require that reionization is completed prior to z= 6, as required by the measurement of the Gunn,Peterson optical depth from the spectra of high-redshift quasars. We confirm previous results demonstrating that such a non-Gaussian distribution leads to a slower reionization as compared to the Gaussian case. We further show that the recent WMAP three-year measurement of the optical depth due to electron scattering, ,= 0.09 ± 0.03, weakly constrains the allowed deviations from Gaussianity on the small scales relevant to reionization if a constant spectral index is assumed. We also confirm the need for a significant suppression of star formation in minihaloes, which increases dramatically as we decrease ,. [source]

    galics, V: Low- and high-order clustering in mock Sloan Digital Sky Surveys

    Jérémy Blaizot
    ABSTRACT We use the galics hybrid model of galaxy formation to explore the nature of galaxy clustering in the local Universe. We bring the theoretical predictions of our model into the observational plane using the momaf software to build mock catalogues which mimic Sloan Digital Sky Survey (SDSS) observations. We measure low- and high-order angular clustering statistic from these mock catalogues, after selecting galaxies the same way as for observations, and compare them directly to estimates from the SDSS data. Note that we also present the first measurements of high-order statistics on the SDSS DR1. We find that our model is in general good agreement with observations in the scale/luminosity range where we can trust the predictions. This range is found to be limited (i) by the size of the dark matter simulation used , which introduces finite volume effects at large scales , and by the mass resolution of this simulation , which introduces incompleteness at apparent magnitudes fainter than r, 20. We then focus on the small-scale clustering properties of galaxies and investigate the behaviour of three different prescriptions for positioning galaxies within haloes of dark matter. We show that galaxies are poor tracers of either DM particles or DM substructures, within groups and clusters. Instead, SDSS data tells us that the distribution of galaxies lies somewhat in between these two populations. This confirms the general theoretical expectation from numerical simulations and semi-analytic modelling. [source]

    The circles-in-the-sky signature for three spherical universes

    R. Aurich
    ABSTRACT The mysteriously low cosmic microwave background (CMB) power on the largest scales might point to a Universe which consists of a multi-connected space. In addition to a suppression of large-scale power, a multi-connected space can be revealed by its circles-in-the-sky signature. In this paper, a detailed search for this signature is carried out for those three homogeneous multi-connected spherical space forms that lead to the smallest large-scale power. A simultaneous search for all occurring paired circles is made using filtered CMB sky maps which enhance the ordinary Sachs,Wolfe contribution. A marginal hint is found for the right-handed Poincaré dodecahedron at ,tot, 1.015 and for the right-handed binary tetrahedral space at ,tot, 1.068. However, due to the complicated noise and foreground structure of the available microwave sky maps, we cannot draw firm conclusions from our findings. [source]

    Cosmic evolution of metal densities: the enrichment of the intergalactic medium

    F. Calura
    ABSTRACT By means of chemo-photometric models for galaxies of different morphological types, we have carried out a detailed study of the history of element production by spheroidal and dwarf irregular galaxies. Spheroidal galaxies suffer a strong and intense star formation episode at early times. In dwarf irregulars, the star formation rate (SFR) proceeds at a low regime but continuously. Both galactic types enrich the intergalactic medium (IGM) with metals by means of galactic winds. We have assumed that the galaxy number density is fixed and normalized to the value of the optical luminosity function observed in the local Universe. Our models allow us to investigate in detail how the metal fractions locked up in stars in spheroids and dwarf irregulars, those present in the interstellar medium (ISM) and those ejected into the IGM have changed with cosmic time. By relaxing the instantaneous recycling approximation and taking into account stellar lifetimes, for the first time we have studied the evolution of the chemical abundance ratios in the IGM and compared our predictions with a set of observations by various authors. Our results indicate that the bulk of the IGM enrichment is due to spheroids, with dwarf irregular galaxies playing a negligible role. Our predictions grossly account for the [O/H] observed in the IGM at high redshift, but overestimate the [C/H]. Furthermore, it appears hard to reproduce the abundance ratios observed in the high-redshift IGM. Some possible explanations are discussed in the text. This is the first attempt to study the abundance ratios in the IGM by means of detailed chemical evolution models which take into account the stellar lifetimes. Numerical simulations adopting our chemical evolution prescriptions could be useful to improve our understanding of the IGM chemical enrichment. [source]

    Extracting star formation histories from medium-resolution galaxy spectra

    H. Mathis
    ABSTRACT We adapt an existing data compression algorithm, moped, to the extraction of median-likelihood star formation histories from medium-resolution galaxy spectra. By focusing on the high-pass components of galaxy spectra, we minimize potential uncertainties arising from the spectrophotometric calibration and intrinsic attenuation by dust. We validate our approach using model high-pass spectra of galaxies with different star formation histories covering the wavelength range 3650,8500 Å at a resolving power of ,2000. We show that the method can recover the full star formation histories of these models, without prior knowledge of the metallicity, to within an accuracy that depends sensitively on the signal-to-noise ratio. The investigation of the sensitivity of the flux at each wavelength to the mass fraction of stars of different ages allows us to identify new age-sensitive features in galaxy spectra. We also highlight a fundamental limitation in the recovery of the star formation histories of galaxies for which the optical signatures of intermediate-age stars are masked by those of younger and older stars. As an example of application, we use this method to derive average star formation histories from the highest-quality spectra of typical (in terms of their stellar mass), morphologically identified early- and late-type galaxies in the Early Data Release (EDR) of the Sloan Digital Sky Survey (SDSS). We find that, in agreement with the common expectation, early-type galaxies must have formed most of their stars over 8 Gyr ago, although a small fraction of the total stellar mass of these galaxies may be accounted for by stars with ages down to 4 Gyr. In contrast, late-type galaxies appear to have formed stars at a roughly constant rate. We also investigate the constraints set by the high-pass signal in the stacked spectra of a magnitude-limited sample of 20 623 SDSS-EDR galaxies on the global star formation history of the Universe and its distribution among galaxies in different mass ranges. We confirm that the stellar populations in the most massive galaxies today appear to have formed on average earlier than those in the least massive galaxies. Our results do not support the recent suggestion of a statistically significant peak in the star formation activity of the Universe at redshifts below unity, although such a peak is not ruled out. [source]