arXiv subjects

J. Silk

Publications and source records attributed to J. Silk.

At least 91 records · Page 5Linked to original sources

arXiv2007

The Evolving Faint-End of the Luminosity Function

We investigate the evolution of the faint-end slope of the luminosity function, $α$, using semi-analytical modeling of galaxy formation. In agreement with observations, we find that the slope can be fitted well by $α(z) =a+b z$, with a=-1.13 and b=-0.1. The main driver for the evolution in $α$ is the evolution in the underlying dark matter mass function. Sub-L_* galaxies reside in dark matter halos that occupy a different part of the mass function. At high redshifts, this part of the mass function is steeper than at low redshifts and hence $α$ is steeper. Supernova feedback in general causes the same relative flattening with respect to the dark matter mass function. The faint-end slope at low redshifts is dominated by field galaxies and at high redshifts by cluster galaxies. The evolution of $α(z)$ in each of these environments is different, with field galaxies having a slope b=-0.14 and cluster galaxies b=-0.05. The transition from cluster-dominated to field-dominated faint-end slope occurs roughly at a redshift $z_* \sim 2$, and suggests that a single linear fit to the overall evolution of $α(z)$ might not be appropriate. Furthermore, this result indicates that tidal disruption of dwarf galaxies in clusters cannot play a significant role in explaining the evolution of $α(z)$ at z< z_*. In addition we find that different star formation efficiencies a_* in the Schmidt-Kennicutt-law and supernovae-feedback efficiencies $ε$ generally do not strongly influence the evolution of $α(z)$.

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arXiv2007

Ages of Elliptical Galaxies: Single versus Multi Population Interpretation

New calibrations of spectrophotometric indices of elliptical galaxies as functions of spectrophotometric indices are presented, permitting estimates of mean stellar population ages and metallicities. These calibrations are based on evolutionary models including a two-phase interstellar medium, infall and a galactic wind.Free parameters were fixed by requiring that models reproduce the mean trend of data in the color-magnitude diagram as well as in the plane of indices Hbeta-Mg2 and Mg2- . To improve the location of faint ellipticals(MB > -20) in the Hbeta-Mg2 diagram, down-sizing was introduced. An application of our calibrations to a sample of ellipticals and a comparison with results derived from single stellar population models is given. Our models indicate that mean population ages span an interval of 7-12 Gyr and are correlated with metallicities, which range from approximately half up to three times solar.

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arXiv2007

Cluster abundances and S-Z power spectra: effects of non-Gaussianity and early dark energy

In the standard Lambda CDM cosmological model with a Gaussian primordial density fluctuation field, the relatively low value of the mass variance parameter (sigma_8=0.74{+0.05}{-0.06}, obtained from the WMAP 3-year data) results in a reduced likelihood that the measured level of CMB anisotropy on the scales of clusters is due to the Sunyaev-Zeldovich (S-Z) effect. To assess the feasibility of producing higher levels of S-Z power, we explore two alternative models which predict higher cluster abundance. In the first model the primordial density field has a chi^2_1 distribution, whereas in the second an early dark energy component gives rise to the desired higher cluster abundance. We carry out the necessary detailed calculations of the levels of S-Z power spectra, cluster number counts, and angular 2-point correlation function of clusters, and compare (in a self-consistent way) their predicted redshift distributions. Our results provide a sufficient basis upon which the viability of the three models may be tested by future high quality measurements.

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arXiv2007

Can large-scale structure probe CMB-constrained non-Gaussianity?

The first year Wilkinson Microwave Anisotropy Probe (WMAP) set quantitative constraints on the amplitude of any primordial non-Gaussianity. We run a series of dark matter-only N-body simulations with the WMAP constraints to investigate the effect of the presence of primordial non-Gaussianity on large scale structures. The model parameters can be constrained using the observations of protoclusters associated with Ly-$α$ emitters at high redshift ($2 \leq z \leq 4$), assuming the galaxy velocity bias can be modelled properly. High redshift structure formation potentially provides a more powerful test of possible primordial non-Gaussianity than does the CMB, albeit on smaller scales. Another constraint is given by the local galaxy density probability distribution function (PDF), as mapped by the 2 degree Field Galaxy Redshift Survey (2dFGRS). The PDF of 2dFGRS \lstar galaxies is substantially higher than the standard model predictions and requires either a non-negligible bias between galaxy and dark matter on $\sim 12$~\hmpc scales or a stronger non-Gaussianity than allowed by the WMAP year one data. The latter interpretation is preferred since second-order bias corrections are negative. With a lower normalisation of the power spectrum fluctuations, sigma_8=0.74, as favoured by the WMAP 3 year data, the discrepancy between the Gaussian model and the data is even larger.

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arXiv2006

Observables sensitive to absolute neutrino masses: A reappraisal after WMAP-3y and first MINOS results

In the light of recent neutrino oscillation and non-oscillation data, we revisit the phenomenological constraints applicable to three observables sensitive to absolute neutrino masses: The effective neutrino mass in single beta decay (m_beta); the effective Majorana neutrino mass in neutrinoless double beta decay (m_2beta); and the sum of neutrino masses in cosmology (Sigma). In particular, we include the constraints coming from the first Main Injector Neutrino Oscillation Search (MINOS) data and from the Wilkinson Microwave Anisotropy Probe (WMAP) three-year (3y) data, as well as other relevant cosmological data and priors. We find that the largest neutrino squared mass difference is determined with a 15% accuracy (at 2-sigma) after adding MINOS to world data. We also find upper bounds on the sum of neutrino masses Sigma ranging from ~2 eV (WMAP-3y data only) to ~0.2 eV (all cosmological data) at 2-sigma, in agreement with previous studies. In addition, we discuss the connection of such bounds with those placed on the matter power spectrum normalization parameter sigma_8. We show how the partial degeneracy between Sigma and sigma_8 in WMAP-3y data is broken by adding further cosmological data, and how the overall preference of such data for relatively high values of sigma_8 pushes the upper bound of Sigma in the sub-eV range. Finally, for various combination of data sets, we revisit the (in)compatibility between current Sigma and m_2beta constraints (and claims), and derive quantitative predictions for future single and double beta decay experiments.

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arXiv2006

Massive and Red Objects predicted by a semianalytical model of galaxy formation

We study whether hierarchical galaxy formation in a concordance $Λ$CDM universe can produce enough massive and red galaxies compared to the observations. We implement a semi-analytical model in which the central black holes gain their mass during major mergers of galaxies and the energy feedback from active galaxy nuclei (AGN) suppresses the gas cooling in their host halos. The energy feedback from AGN acts effectively only in massive galaxies when supermassive black holes have been formed in the central bulges. Compared with previous models without black hole formation, our model predicts more massive and luminous galaxies at high redshift, agreeing with the observations of K20 up to $z\sim 3$. Also the predicted stellar mass density from massive galaxies agrees with the observations of GDDS. Because of the energy feedback from AGN, the formation of new stars is stopped in massive galaxies with the termination of gas cooling and these galaxies soon become red with color $R-K>$5 (Vega magnitude), comparable to the Extremely Red Objects (EROs) observed at redshift $z\sim$1-2. Still the predicted number density of very EROs is lower than observed at $z\sim 2$, and it may be related to inadequate descriptions of dust extinction, star formation history and AGN feedback in those luminous galaxies.

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arXiv2006

On the Origin of Stars in Bulges and Elliptical Galaxies

We investigate the stellar composition of bulges and elliptical galaxies as predicted by the CDM paradigm using semi-analytical modelling. We argue that spheroid stars are built up of two main components, {\it merger} and {\it quiescent}, according to the origin of the stars. The merger component is formed during major mergers by gas driven to the centre, while the quiescent component is formed in gaseous discs and added later to the spheroid during major mergers. Galaxies more massive than $M_C=3 \times 10^{10}$ M$_{\odot}$ have on average only a 15% merger component in their spheroids, while smaller galaxies can have up to 30%. The merger component increases with redshift due to mergers involving more gas. However we do not find mergers with gas fraction above $ \sim 40 %$ of the remnants mass. Generally the gas fraction is a decreasing function of the redshift at which the merger occurs and the mass of the remnant, with more massive remnants having smaller gas fraction and hence smaller merger components. This trend is independent of the environment of the galaxy with the only impact of the environment being that galaxies less massive than $M_C$ have slightly larger merger components in dense environments. The fraction of stars in bulges for galaxies more massive than $M_C$ is larger than 50%. We find that the majority of stars in galaxies more massive than $M_C$ reside within bulges and ellipticals independent of redshift and that the fraction increases with redshift. The most massive galaxies at each redshift are elliptical galaxies.

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arXiv2006

Bursts from the very early universe

Bursts of weakly interacting particles such as neutrinos or even more weakly interacting particles such as wimps and gravitons from the very early universe would offer a much deeper ``look back time'' to early epochs than is possible with photons. We consider some of the issues related to the existence of such bursts and their detectability. Characterizing the burst rate by a probability $\cal P$ per Hubble four-volume we find, for events in the radiation-dominated era, that the natural unit of description is the present intensity of the CMB times $\cal P$. The existence of such bursts would make the observation of phenomena associated with very early times in cosmology at least conceptually possible. One might even hope to probe the transplanckian epoch if complexes more weakly interacting than the graviton can exist. Other conceivable applications include the potential detectability of the formation of "pocket" universes'' in a multiverse.

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arXiv2006

Impact of a non-Gaussian density field on Sunyaev-Zeldovich observables

The main statistical properties of the Sunyaev-Zeldovich (S-Z) effect - the power spectrum, cluster number counts, and angular correlation function - are calculated and compared within the framework of two density fields which differ in their predictions of the cluster mass function at high redshifts. We do so for the usual Press and Schechter mass function, which is derived on the basis of a Gaussian density fluctuation field, and for a mass function based on a chi^2 distributed density field. These three S-Z observables are found to be very significantly dependent on the choice of the mass function. The different predictions of the Gaussian and non-Gaussian density fields are probed in detail by investigating the behaviour of the three S-Z observables in terms of cluster mass and redshift. The formation time distribution of clusters is also demonstrated to be sensitive to the underlying mass function. A semi-quantitative assessment is given of its impact on the concentration parameter and the temperature of intracluster gas.

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arXiv2006

The progenitor set of present-day early-type galaxies

We present a comprehensive theoretical study, within a fully realistic semi-analytical framework, of the photometric properties of early-type progenitors in the redshift range 0 0.7) spirals have ~75-95 percent chance of being a progenitor, while the corresponding probability for large blue spirals (M_B<-21.5, B-V<0.7) is ~50-75 percent. Finally, we explore the correspondence between the true progenitor set of present-day early-types and the commonly used `red-sequence', defined as the set of galaxies within the part of the colour-magnitude space which is dominated by early-type objects. While large members (M_V<-22) of the `red sequence' trace the progenitor set accurately in terms of numbers and mass, the relationship breaks down severely at fainter luminosities (M_V>-21). Hence the red sequence is generally not a good proxy for the progenitor set of early-type galaxies.

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arXiv2005

Probing the origins of voids with the CMB

In this talk presented at the 5th Rencontres du Vietnam 2004, we discuss our preliminary investigations into voids of primordial origin. We show that if voids in the cold dark matter distribution existed at the epoch of decoupling, they could contribute significantly to the apparent rise in cosmic microwave background (CMB) power on small scales detected by the Cosmic Background Imager (CBI) Deep Field. Here we present the preliminary results of our improved method for predicting the effects of primordial voids on the CMB in which we treat the voids as an external source in the cold dark matter (CDM) distribution, employing a Boltzmann solver. Our improved predictions include the effects of a cosmological constant (Lambda) and acoustic oscillations generated by voids at early times. We find that models with relatively large voids on the last scattering surface predict too much CMB power in an Einstein-de Sitter background cosmology but could be consistent with the current CMB observations in a Lambda CDM universe.

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arXiv2004

Do the Unidentified EGRET Sources Trace Annihilating Dark Matter in the Local Group?

In a cold dark matter (CDM) framework of structure formation, the dark matter haloes around galaxies assemble through successive mergers with smaller haloes. This merging process is not completely efficient, and hundreds of surviving halo cores, or {\it subhaloes}, are expected to remain in orbit within the halo of a galaxy like the Milky Way. While the dozen visible satellites of the Milky Way may trace some of these subhaloes, the majority are currently undetected. A large number of high-velocity clouds (HVCs) of neutral hydrogen {\it are} observed around the Milky Way, and it is plausible that some of the HVCs may trace subhaloes undetected in the optical. Confirming the existence of concentrations of dark matter associated with even a few of the HVCs would represent a dramatic step forward in our attempts to understand the nature of dark matter. Supersymmetric (SUSY) extensions of the Standard Model of particle physics currently suggest neutralinos as a natural well-motivated candidate for the non-baryonic dark matter of the universe. If this is indeed the case, then it may be possible to detect dark matter indirectly as it annihilates into neutrinos, photons or positrons. In particular, the centres of subhaloes might show up as point sources in gamma-ray observations. In this work we consider the possibility that some of the unidentified EGRET $γ$-ray sources trace annihilating neutralino dark matter in the dark substructure of the Local Group. We compare the observed positions and fluxes of both the unidentified EGRET sources and the HVCs with the positions and fluxes predicted by a model of halo substructure, to determine to what extent any of these three populations could be associated.

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arXiv2004

A VLT spectroscopic survey of RX J0152.7-1357, a forming cluster of galaxies at z=0.837

We present the results of an extensive spectroscopic survey of RX J0152.7-1357, one of the most massive distant clusters of galaxies known. Multi-object spectroscopy, carried out with FORS1 and FORS2 on the ESO Very Large Telescope (VLT), has allowed us to measure more than 200 redshifts in the cluster field and to confirm 102 galaxies as cluster members. The mean redshift of the cluster is $z=0.837 \pm 0.001$ and we estimate the velocity dispersion of the overall cluster galaxy distribution to be $\sim 1600 \mathrm{km \ s^{-1}}$. The distribution of cluster members is clearly irregular, with two main clumps that follow the X-ray cluster emission mapped by Chandra. A third clump of galaxies to the east of the central structure and at the cluster redshift has also been identified. The two main clumps have velocity dispersions of $\sim919$ and $\sim737 \mathrm{km s^{-1}}$ respectively, and the peculiar velocity of the two clumps suggests that they will merge into a single more massive cluster. A segregation in the star formation activity of the member galaxies is observed. All star forming galaxies are located outside the high-density peaks, which are populated only by passive galaxies. A population of red galaxies (belonging to the cluster red sequence) with clear post-starburst spectral features and [OII] ($λ$3727) emission lines is observed in the outskirts of the cluster. Two AGNs, which were previously confused with the diffuse X-ray emission from the intracluster medium in ROSAT and BeppoSAX observations, are found to be cluster members.

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arXiv2004

Observables sensitive to absolute neutrino masses: Constraints and correlations from world neutrino data

In the context of three-flavor neutrino mixing, we present a thorough study of the phenomenological constraints applicable to three observables sensitive to absolute neutrino masses: The effective neutrino mass in Tritium beta decay (m_beta); the effective Majorana neutrino mass in neutrinoless double beta decay (m_2beta); and the sum of neutrino masses in cosmology (Sigma). We discuss the correlations among these variables which arise from the combination of all the available neutrino oscillation data, in both normal and inverse neutrino mass hierarchy. We set upper limits on m_beta by combining updated results from the Mainz and Troitsk experiments. We also consider the latest results on m_2beta from the Heidelberg-Moscow experiment, both with and without the lower bound claimed by such experiment. We derive upper limits on Sigma from an updated combination of data from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite and the 2 degrees Fields (2dF) Galaxy Redshifts Survey, with and without Lyman-alpha forest data from the Sloan Digital Sky Survey (SDSS), in models with a non-zero running of the spectral index of primordial inflationary perturbations. The results are discussed in terms of two-dimensional projections of the globally allowed region in the (m_beta,m_2beta,Sigma) parameter space, which neatly show the relative impact of each data set. In particular, the (in)compatibility between Sigma and m_2beta constraints is highlighted for various combinations of data. We also briefly discuss how future neutrino data (both oscillatory and non-oscillatory) can further probe the currently allowed regions.

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arXiv2004

The star formation history of intermediate redshift late-type galaxies

We combine the latest observations of disk galaxy photometry and rotation curves at moderate redshift from the FORS Deep Field (FDF) with simple models of chemical enrichment. Our method describes the buildup of the stellar component through infall of gas and allows for gas and metal outflows. In this framework, we keep a minimum number of constraints and we search a large volume of parameter space, looking for the models which best reproduce the photometric observations in the observed redshift range (0.5<z<1). We find the star formation efficiency to correlate well with vMAX so that massive disks are more efficient in the formation of stars and have a smaller spread in stellar ages. This trend presents a break at around vMAX 140km/s. Galaxies on either side of this threshold have significantly different age distributions. This break has been already suggested by several authors in connection with the contribution from either gravitational instabilities or supernova-driven turbulence to star formation. No clear trend is seen between galaxy mass and infall timescale or gas outflows. The model presented in this paper suggests massive disks have formation histories resembling those of early-type galaxies, with highly efficient and short-lived bursts, in contrast with low-mass disks, which have a more extended star formation history. One option to explain the observed shallow slope of the Tully-Fisher relation at intermediate redshift could be small episodes of star formation in low-mass disks.

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arXiv2004

On the excess of power in high resolution CMB experiments

We revisit the possibility that an excess in the CMB power spectrum at small angular scales (CBI, ACBAR) can be due to galaxy clusters (or compact sources in general). We perform a Gaussian analysis of ACBAR-like simulated data based on wavelets. We show how models with a significant excess should show a clear non-Gaussian signal in the wavelet space. In particular, a value of the normalization sigma_8 = 1 would imply a highly significant skewness and kurtosis in the wavelet coefficients at scales around 3 arcmin. Models with a more moderate excess also show a non-Gaussian signal in the simulated data. We conclude that current data (ACBAR) should show this signature if the excess is to be due to the SZ effect. Otherwise, the reason for that excess should be explained by some systematic effect. The significance of the non-Gaussian signal depends on the cluster model but it grows with the surveyed area. Non-Gaussianity test performed on incoming data sets should reveal the presence of a cluster population even for models with moderate-low sigma_8's.

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arXiv2004

Constraining CMB-consistent primordial voids with cluster evolution

Using cosmological simulations, we make predictions for the distribution of clusters in a plausible non-gaussian model where primordial voids nucleated during inflation act together with scale-invariant adiabatic gaussian fluctuations as seeds for the formation of large-scale structure. This model agrees with most recent observations of the anisotropies of the cosmic microwave background (CMB) and can account for the excess of power measured on cluster scales by the Cosmic Background Imager (CBI), the large empty regions apparent in nearby galaxy redshift surveys and the number of giant arcs measured in deep cluster lensing surveys. We show that the z=0 cluster mass function differs from predictions for a standard LCDM cosmology with the same sigma_8. Moreover, as massive clusters also form much earlier in the "void" scenario, we show that integrated number counts of SZ sources and simple statistics of strong lensing can easily falsify this model.

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arXiv2003

On the formation of cold fronts in massive mergers

Using adiabatic hydrodynamical simulations, we follow the evolution of two symmetric cold fronts developing in the remnant of a violent z=0.3 massive cluster merger. The structure and location of the simulated cold fronts are very similar to those recently found in X-ray cluster observations, supporting the merger hypothesis for the origin of at least some of the cold fronts. The cold fronts are preceded by an almost spherical bow shock which originates at the core and disappears after 1.6 Gyr. The cold fronts last longer and survive until z=0. We trace back the gas mass constituting the fronts and find it initially associated with the two dense cores of the merging clusters. Conversely, we follow how the energy of the gas of the initial merging cores evolves until z=0 from the merging and show that a fraction of this gas can escape from the local potential well of the sub-clumps. This release occurs as the sub-clumps reach their apocentre in an eccentric orbit and is due to decoupling between the dark matter and part of the gas in the sub-clump because of, first, heating of the gas at first close core passage and of, second, the effect of the global cluster pressure which peaks as the centrifugal acceleration of the sub-clump is maximal. The fraction of the gas of the sub-clump liberated in the outbound direction then cools as it expands adiabatically and constitutes the cold fronts.

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