arXiv subjects

Carlton M. Baugh

Publications and source records attributed to Carlton M. Baugh.

123 records · Page 7Linked to original sources

arXiv2004

The abundance of Lyman-(α) emitters in hierarchical models

We present predictions for the abundance of Ly-$α$ emitters in hierarchical structure formation models. We use the {\tt GALFORM} semi-analytical model to explore the impact on the predicted counts of varying assumptions about the escape fraction of Ly-$α$ photons, the redshift at which the universe reionised and the cosmological density parameter. A model with a fixed escape fraction gives a remarkably good match to the observed counts over a wide redshift interval. We present predictions for the expected counts in a typical observation with the Multi Unit Spectroscopic Explorer instrument proposed for the Very Large Telescope.

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arXiv2004

The 2dF Galaxy Redshift Survey: Spherical Harmonics analysis of fluctuations in the final catalogue

We present the result of a decomposition of the 2dFGRS galaxy overdensity field into an orthonormal basis of spherical harmonics and spherical Bessel functions. Galaxies are expected to directly follow the bulk motion of the density field on large scales, so the absolute amplitude of the observed large-scale redshift-space distortions caused by this motion is expected to be independent of galaxy properties. By splitting the overdensity field into radial and angular components, we linearly model the observed distortion and obtain the cosmological constraint Omega_m^{0.6} sigma_8=0.46+/-0.06. The amplitude of the linear redshift-space distortions relative to the galaxy overdensity field is dependent on galaxy properties and, for L_* galaxies at redshift z=0, we measure beta(L_*,0)=0.58+/-0.08, and the amplitude of the overdensity fluctuations b(L_*,0) sigma_8=0.79+/-0.03, marginalising over the power spectrum shape parameters. Assuming a fixed power spectrum shape consistent with the full Fourier analysis produces very similar parameter constraints.

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arXiv2003

The 2dF Galaxy Redshift Survey: Wiener Reconstruction of the Cosmic Web

We reconstruct the underlying density field of the 2 degree Field Galaxy Redshift Survey (2dFGRS) for the redshift range 0.035<z<0.200 using the Wiener Filtering method. The Wiener Filter suppresses shot noise and accounts for selection and incompleteness effects. The method relies on prior knowledge of the 2dF power spectrum of fluctuations and the combination of matter density and bias parameters however the results are only slightly affected by changes to these parameters. We present maps of the density field in two different resolutions: 5 Mpc/h and 10 Mpc/h. We identify all major superclusters and voids in the survey. In particular, we find two large superclusters and two large local voids. A version of this paper with full set of colour maps can be found at http://www.ast.cam.ac.uk/~pirin.

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arXiv2003

The 2dF Galaxy Redshift Survey: Final Data Release

The 2dF Galaxy Redshift Survey (2dFGRS) has obtained spectra for 245591 sources, mainly galaxies, brighter than a nominal extinction-corrected magnitude limit of b_J=19.45. Reliable redshifts were measured for 221414 galaxies. The galaxies are selected from the extended APM Galaxy Survey and cover an area of approximately 1500 square degrees in three regions: an NGP strip, an SGP strip and random fields scattered around the SGP strip. This paper describes the 2dFGRS final data release of 30 June 2003 and complements Colless et al. (2001), which described the survey and the initial 100k data release. The 2dFGRS database and full documentation are available on the WWW at http://www.mso.anu.edu.au/2dFGRS/

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arXiv2003

The Halo Occupation Distribution and the Physics of Galaxy Formation

The halo occupation distribution (HOD) describes the bias between galaxies and dark matter by specifying (a) the probability P(N|M) that a halo of virial mass M contains N galaxies of a particular class and (b) the relative distributions of galaxies and dark matter within halos. We calculate predicted HODs for a Lambda-CDM cosmological model using an SPH hydrodynamic simulation and a semi-analytic (SA) galaxy formation model. Although the two methods predict different galaxy mass functions, their HOD predictions agree remarkably well. For mass-selected samples, the mean occupation exhibits a sharp cutoff at low halo masses, a slowly rising plateau for ~1-2, and a more steeply rising high occupancy regime. At low , the mean pair and triple counts are well below Poisson expectations, with important consequences for small scale behavior of 2- and 3-point correlation functions. The HOD depends strongly on galaxy age, with high mass halos populated mainly by old galaxies and low mass halos by young galaxies. The SPH simulation supports several simplifying assumptions about HOD bias: the most massive galaxy in a halo usually lies close to the center and moves near the halo's mean velocity; satellite galaxies have the same radial profile and velocity dispersion as the dark matter; and the mean occupation at fixed halo mass is independent of the halo's larger scale environment. By applying the SPH and SA HODs to a large volume N-body simulation, we show that both methods predict slight, observable departures from a power-law galaxy correlation function. The predicted HODs are closely tied to the underlying galaxy formation physics, they offer useful guidance to theoretical models of galaxy clustering, and they will be tested empirically by ongoing analyses of galaxy redshift surveys. (Shortened)

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arXiv2003

The Properties of Spiral Galaxies: Confronting Hierarchical Galaxy Formation Models with Observations

We compare the properties of local spiral galaxies with the predictions of the Cole et al. semi-analytic model of hierarchical galaxy formation, in order to gain insight into the baryonic processes that were responsible for shaping these galaxies. On the whole, the models reproduce the properties of present-day spirals (such as SFRs, SFHs, scale sizes, metallicities, and gas fractions) rather well. In particular, we find that once the effects of dust and variations in stellar populations have been taken into account, published spiral galaxy scale-size distributions derived from optical data (with logarithmic widths ~0.3) can be reconciled with the width of the stellar mass scale-size distribution predicted by the semi-analytic model (~0.5). There are some illuminating discrepancies between the observations and the model predictions. The model colours of luminous spiral galaxies are somewhat too blue and those of faint galaxies somewhat too red, most likely indicating shortcomings in the way that gas is accreted by spiral galaxies. Furthermore, the model produces too few luminous spiral galaxies. These difficulties could be resolved by altering the way in which gas cooling is treated or, perhaps, by adopting a higher baryon fraction and invoking galactic `superwinds.' Secondly, stellar M/Ls are found to be as high as observations permit. Yet, typically 60% of the mass in the inner half-light radius of the model galaxies is dark. This results in an offset between the model and observed spiral galaxy luminosity-linewidth relation. This could be resolved by substantially reducing the mass of baryons which make it into a galaxy disc (with an attendant decrease in stellar M/L), or by modifying the assumed dark matter profile to include less dark matter in the inner parts. [Abridged]

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arXiv2002

Forming Globular Cluster Systems in a Semi-analytic Scheme

We apply the semi-analytical galaxy formation code of Cole et al. to investigate the formation of globular cluster (GC) systems in hierarchical clustering scenarios. The nature of the model allows us to investigate the properties of GC systems and their parent galaxies within a cosmological framework, over a wide dynamic range of mass and time resolution. Assuming GCs form during mergers of gaseous systems, the metal-rich peak of the classical 'bimodal' metallicity distribution of GCs naturally falls out of our model, where such merging occurs over a wide range of redshifts. The physical origin of old, metal-poor GCs (the metal-poor peak) is harder to understand, since their formation must be decoupled from the ongoing star formation in these systems at high redshift (z~5). Within the context of semi-analytic models in general, a possible solution lies in a cut-off in the GC formation efficiency at a characteristic local star formation rate.

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arXiv2002

Galaxy formation using halo merger histories taken from N-body simulations

We develop a hybrid galaxy formation model which uses outputs from an N-body simulation to follow the merger histories (or ``merger trees'') of dark matter halos and treats baryonic processes, such as the cooling of gas within halos and subsequent star formation, using the semi-analytic model of Cole et al. We compare this hybrid model to an otherwise identical model which utilises merger tree realisations generated by a Monte-Carlo algorithm and find that, apart from the limited mass resolution imposed by the N-body particle mass, the only significant differences between the models are due to the known discrepancy between the distribution of halo progenitor masses predicted by extended Press Schechter theory and that found in N-body simulations. We investigate the effect of limited mass resolution on the hybrid model by comparing to a purely semi-analytic model with greatly improved mass resolution. We find that the mass resolution of the simulation we use, which has a particle mass of 1.4x10^10 Msolar/h, is insufficient to produce a reasonable luminosity function for galaxies with magnitudes in the b_J band fainter than -17.

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arXiv2002

A Comparison of Gas Dynamics in SPH and Semi-analytic Models of Galaxy Formation

We compare the results of two techniques used to calculate the evolution of cooling gas during galaxy formation: Smooth Particle Hydrodynamics (SPH) simulations and semi-analytic modelling. We improve upon the earlier statistical comparison of Benson et al. by taking halo merger histories from the dark matter component of the SPH simulation, which allows us to compare the evolution of galaxies on an object-by-object basis in the two treatments. We use a ``stripped-down'' version of the semi-analytic model described by Helly et al. which includes only shock heating and radiative cooling of gas and which is adjusted to mimic the resolution and other parameters of a comparison SPH simulation as closely as possible. We compare the total mass of gas that cools in halos of different mass as a function of redshift as well as the masses and spatial distribution of individual ``galaxies.'' At redshift z=0, the cooled gas mass in well-resolved halos agrees remarkably well (to better than ~20%) in the SPH simulation and stripped-down semi-analytic model. At high redshift, resolution effects in the simulation become increasingly important and, as a result, more gas tends to cool in low mass halos in the SPH simulation than in the semi-analytic model. The cold gas mass function of individual galaxies in the two treatments at z=0 also agrees very well and, when the effects of mergers are accounted for, the masses of individual galaxies and their 2-point correlation functions are also in excellent agreement in the two treatments. Thus, our comparison confirms and extends the earlier conclusion of Benson et al. that SPH simulations and semi-analytic models give consistent results for the evolution of cooling galactic gas.

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arXiv2002

Parameter constraints for flat cosmologies from CMB and 2dFGRS power spectra

We constrain flat cosmological models with a joint likelihood analysis of a new compilation of data from the cosmic microwave background (CMB) and from the 2dF Galaxy Redshift Survey (2dFGRS). Fitting the CMB alone yields a known degeneracy between the Hubble constant h and the matter density Omega_m, which arises mainly from preserving the location of the peaks in the angular power spectrum. This `horizon-angle degeneracy' is considered in some detail and shown to follow a simple relation Omega_m h^{3.4} = constant. Adding the 2dFGRS power spectrum constrains Omega_m h and breaks the degeneracy. If tensor anisotropies are assumed to be negligible, we obtain values for the Hubble constant h=0.665 +/- 0.047, the matter density Omega_m=0.313 +/- 0.055, and the physical CDM and baryon densities Omega_c h^2 = 0.115 +/- 0.009, Omega_b h^2 = 0.022 +/- 0.002 (standard rms errors). Including a possible tensor component causes very little change to these figures; we set a upper limit to the tensor-to-scalar ratio of r<0.7 at 95% confidence. We then show how these data can be used to constrain the equation of state of the vacuum, and find w<-0.52 at 95% confidence. The preferred cosmological model is thus very well specified, and we discuss the precision with which future CMB data can be predicted, given the model assumptions. The 2dFGRS power-spectrum data and covariance matrix, and the CMB data compilation used here, are available from http://www.roe.ac.uk/~wjp/

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arXiv2002

The 2dF Galaxy Redshift Survey: The amplitudes of fluctuations in the 2dFGRS and the CMB, and implications for galaxy biasing

We compare the amplitudes of fluctuations probed by the 2dF Galaxy Redshift Survey and by the latest measurements of the Cosmic Microwave Background anisotropies. By combining the 2dFGRS and CMB data we find the linear-theory rms mass fluctuations in 8 Mpc/h spheres to be sigma_8 = 0.73 +-0.05 (after marginalization over the matter density parameter Omega_m and three other free parameters). This normalization is lower than the COBE normalization and previous estimates from cluster abundance, but it is in agreement with some revised cluster abundance determinations. We also estimate the scale-independent bias parameter of present-epoch L_s = 1.9L_* APM-selected galaxies to be b(L_s,z=0) = 1.10 +- 0.08 on comoving scales of 0.02 < k < 0.15 h/Mpc. If luminosity segregation operates on these scales, L_* galaxies would be almost un-biased, b(L_*,z=0) = 0.96. These results are derived by assuming a flat Lambda-CDM Universe, and by marginalizing over other free parameters and fixing the spectral index n=1 and the optical depth due to reionization tau=0. We also study the best fit pair (Omega_m,b), and the robustness of the results to varying n and tau. Various modelling corrections can each change the resulting b by 5-15 per cent. The results are compared with other independent measurements from the 2dFGRS itself, and from the SDSS, cluster abundance and cosmic shear.

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arXiv2002

The 2dF Galaxy Redshift Survey: The Number and Luminosity Density of Galaxies

We present the bivariate brightness distribution (BBD) for the 2dF Galaxy Redshift Survey (2dFGRS) based on a preliminary subsample of 45,000 galaxies. The BBD is an extension of the galaxy luminosity function incorporating surface brightness information. It allows the measurement of the local luminosity density, j_B, and the galaxy luminosity and surface brightness distributions while accounting for surface brightness selection biases. The recovered 2dFGRS BBD shows a strong surface brightness-luminosity relation (M_B~2.4μ_e). The luminosity-density is dominated by normal galaxies and the luminosity-density peak lies away from the selection boundaries implying that the 2dFGRS is complete and that luminous low surface brightness galaxies are rare. The final value we derive for the local luminosity-density, inclusive of surface brightness corrections, is: j_B=2.49+/-0.20x10^8 h L_solar Mpc^-3. Representative Schechter function parameters are: M*=-19.75+/-0.05, phi*=2.02+/-0.02x10^-2 and alpha=-1.09+/-0.03. Extending the conventional methodology to incorporate surface brightness selection effects has resulted in an increase in the luminosity-density of 37%.

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arXiv2001

Cluster correlations in redshift space

We test an analytic model for the two-point correlations of galaxy clusters in redshift space using the Hubble Volume N-body simulations. The correlation function of clusters shows no enhancement along the line of sight, due to the lack of any virialised structures in the cluster distribution. However, the distortion of the clustering pattern due to coherent bulk motions is clearly visible. The distribution of cluster peculiar motions is well described by a Gaussian, except in the extreme high velocity tails. The simulations produce a small but significant number of clusters with large peculiar motions. The form of the redshift space power spectrum is strongly influenced by errors in measured cluster redshifts in extant surveys. When these errors are taken into account, the model reproduces the power spectrum recovered from the simulation to an accuracy of 15% or better over a decade in wavenumber. We compare our analytic predictions with the power spectrum measured from t! he! APM cluster redshift survey. The cluster power spectrum constrains the amplitude of density fluctuations, as measured by the linear rms variance in spheres of radius 8 h^(-1) Mpc, denoted by sigma_8. When combined with the constraints on sigma_8 and the density parameter Omega derived from the local abundance of clusters, we find a best fitting cold dark matter model with sigma_8~1.25 and Omega~0.2, for a power spectrum shape that matches that measured for galaxies. However, for the best fitting value of Omega and given the value of Hubble's constant from recent measurements, the assumed shape of the power spectrum is incompatible with the most readily motivated predictions from the cold dark matter paradigm.

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arXiv2001

The 2dF Galaxy Redshift Survey: The power spectrum and the matter content of the universe

The 2dF Galaxy Redshift Survey has now measured in excess of 160000 galaxy redshifts. This paper presents the power spectrum of the galaxy distribution, calculated using a direct FFT-based technique. We argue that, within the k-space region 0.02<k<0.15 h Mpc^-1, the shape of this spectrum should be close to that of the linear density perturbations convolved with the window function of the survey. This window function and its convolving effect on the power spectrum estimate are analyzed in detail. By convolving model spectra, we are able to fit the power-spectrum data and provide a measure of the matter content of the universe. Our results show that models containing baryon oscillations are mildly preferred over featureless power spectra. Analysis of the data yields 68% confidence limits on the total matter density times the Hubble parameter Ω_m h = 0.20 +/- 0.03, and the baryon fraction Ω_b/Ω_m = 0.15 +/- 0.07, assuming scale-invariant primordial fluctuations.

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arXiv1999

The morphological evolution of galaxy satellites

We study the evolution of galaxy satellites with high resolution N-body simulations. Satellites are modeled as replicas of typical low and high surface brightness galaxies (LSBs and HSBs). Encounters on high eccentricity orbits (as typical in hierarchical models of galaxy formation) strip LSBs of most of their stars and tend to decrease their surface brightness. On the contrary, bar instability in HSBs leads to substantial loss of angular momentum of the stellar component and to an increase of central surface brightness. In both cases the remnant resembles a spheroidal galaxy with an exponential surface brightness profile. A simple modeling of color evolution and interactions driven star formation gives M/L ratios for the remnants that are roughly consistent with observations. These results suggest an evolutionary scenario for the dwarf galaxies in our Local Group, faint dSphs being the descendents of LSBs and brighter dSphs/dEs being the final state of HSB satellites.

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