arXiv subjects

O. Lahav

Publications and source records attributed to O. Lahav.

At least 433 records · Page 24Linked to original sources

arXiv2001

The z ~ 1.2 Galaxy Luminosity Function from The LCIR Survey

We present results from the Las Campanas Infrared Survey, designed to identify a statistically significant sample of z>=1 galaxies using photometric redshift techniques. Here we summarize the design and strategies of the survey and present the first estimate of the galaxy luminosity function at z>=1 based on H-band selected galaxies identified in our survey. Results of number count studies and luminosity function measurements indicate that most early-type galaxies were already in place by z~1.2 with a modest space density evolution and a mild luminosity evolution over that expected from passive evolution.

astro-ph↗

arXiv2001

On the 1/3 factor in the CMB Sachs-Wolfe effect

We point out that a pseudo-Newtonian interpretations of the $(1/3)$ factor in the Sachs-Wolfe effect, which relates the fluctuations in temperature and potential, $(δT/ T) = (1/ 3) δΦ$, is not supported by the General Relativistic analysis. Dividing the full gravitational effect into separate parts depends on the choice of time slicing (gauge) and there exist infinitely many different choices. More importantly, interpreting the parts as being due to the gravitational redshift and the time dilation is not justified in the rigorous relativistic perturbation theory. We suggest to regard the $(1/3)$ factor as the General Relativistic result which applies in a restricted situation of adiabatic perturbation in the $K = 0 = Λ$ model with the last scattering occuring in the matter dominated era. For an isocurvature initial condition the corresponding result, $(δT/ T) = 2 δΦ$, has a different numerical coefficient.

astro-ph↗

arXiv2001

The 2dF Galaxy Redshift Survey: Luminosity dependence of galaxy clustering

We investigate the dependence of the strength of galaxy clustering on intrinsic luminosity using the Anglo-Australian two degree field galaxy redshift survey (2dFGRS). The 2dFGRS is over an order of magnitude larger than previous redshift surveys used to address this issue. We measure the projected two-point correlation function of galaxies in a series of volume-limited samples. The projected correlation function is free from any distortion of the clustering pattern induced by peculiar motions and is well described by a power-law in pair separation over the range 0.1 < r /h Mpc < 10. The clustering of L* galaxies in real space is well fit by a correlation length r0 = 4.9 +/- 0.3 /h Mpc and power-law slope gamma = 1.71 +/- 0.06. The clustering amplitude increases slowly with absolute magnitude for galaxies fainter than M*, but rises more strongly at higher luminosities. At low luminosities, our results agree with measurements from the SSRS2 by Benoist et al. However, we find a weaker dependence of clustering strength on luminosity at the highest luminosities. The correlation function amplitude increases by a factor of 4.0 between $M_{b_{J}} -5\log_{10}h = -18$ and -22.5, and the most luminous galaxies are 3.0 times more strongly clustered than L* galaxies. The power-law slope of the correlation function shows remarkably little variation for samples spanning a factor of 20 in luminosity. Our measurements are in very good agreement with the predictions of the hierarchical galaxy formation models of Benson et al.

astro-ph↗

arXiv2001

The Las Campanas IR Survey: Early Type Galaxy Progenitors Beyond Redshift One

(Abridged) We have identified a population of faint red galaxies from a 0.62 square degree region of the Las Campanas Infrared Survey whose properties are consistent with their being the progenitors of early-type galaxies. The optical and IR colors, number-magnitude relation and angular clustering together indicate modest evolution and increased star formation rates among the early-type field population at redshifts between one and two. The counts of red galaxies with $H$ magnitudes between 17 and 20 rise with a slope that is much steeper than that of the total H sample. The surface density of red galaxies drops from roughly 3000 per square degree at H = 20.5, I-H > 3 to ~ 20 per square degree at H = 20, I-H > 5. The V-I colors are approximately 1.5 magnitudes bluer on average than a pure old population and span a range of more than three magnitudes. The colors, and photometric redshifts derived from them, indicate that the red galaxies have redshift distributions adequately described by Gaussians with sigma_z ~ 0.2$ centered near redshift one, with the exception that galaxies having $V-I<1.6$ and $I-H>3$ are primarily in the 1.5 < z < 2 range. We find co-moving correlation lengths of 9-10 Mpc at z ~ 1, comparable to, or larger than, those found for early-type galaxies at lower redshifts. A simple photometric evolution model reproduces the counts of the red galaxies, with only a ~ 30% decline in the underlying space density of early-type galaxies at z ~ 1.2. We suggest on the basis of the colors, counts, and clustering that these red galaxies are the bulk of the progenitors of present day early-type galaxies.

astro-ph↗

arXiv2001

A measurement of the cosmological mass density from clustering in the 2dF Galaxy Redshift Survey

The large-scale structure in the distribution of galaxies is thought to arise from the gravitational instability of small fluctuations in the initial density field of the universe. A key test of this hypothesis is that superclusters of galaxies in the process of formation should generate systematic infall of other galaxies. This would be evident in the pattern of recessional velocities, causing an anisotropy in the inferred spatial clustering of galaxies. Here we report a precise measurement of this clustering, using the redshifts of more than 141,000 galaxies from the two-degree-field galaxy redshift survey. We determine the parameter beta = Omega^{0.6}/b = 0.43 +- 0.07, where Omega is the total mass-density parameter and b is a measure of the `bias' of the luminous galaxies in the survey. Combined with the anisotropy of the cosmic microwave background, our results favour a low-density universe with Omega approximately 0.3.

astro-ph↗

arXiv2000

Parameterisation of Galaxy Spectra in the 2dF Galaxy Redshift Survey

The 2dF Galaxy Redshift Survey has already yielded over 130,000 galaxy spectra (out of the planned 250,000). We present a method for the parameterisation of galaxy spectra in the 2dF and other large redshift surveys using a new continuous parameter eta=0.44pc1 - pc2, based on a Principal Components Analysis (where pc1 and pc2 are the projections onto the first and second principal components respectively). This parameter is designed to be robust to instrumental uncertainties and it represents the absorption/emission strength of a galaxy.

astro-ph↗

arXiv2000

The 2-10 keV XRB dipole and its cosmological implications

The hard X-ray (>2 keV) emission of the local and distant Universe as observed with the HEAO1-A2 experiment is reconsidered in the context of large scale cosmic structure. Using all-sky X-ray samples of AGN and galaxy clusters we remove the dominant local X-ray flux from within a redshift of ~ 0.02. We evaluate the dipolar and higher order harmonic structure in 4 X-ray colours. The estimated dipole anisotropy of the unresolved flux appears to be consistent with a combination of the Compton-Getting effect due to the Local Group motion (dipole amplitude Delta = 0.0042) and remaining large scale structure (0.0023 <~ Delta <~ 0.0085), in good agreement with the expectations of Cold Dark Matter models. The observed anisotropy does however also suggest a non-negligible Galactic contribution which is more complex than current, simple models of >2 keV Galactic X-ray emission. Comparison of the soft and hard colour maps with a harmonic analysis of the 1.5 keV ROSAT all-sky data qualitatively suggests that at least a third of the faint, unresolved ~ 18 deg scale structure in the HEAO1-A2 data may be Galactic in origin. However, the effect on measured flux dipoles is small (<~3%). We derive an expression for dipole anisotropy and acceleration and demonstrate how the dipole anisotropy of the distant X-ray frame can constrain the amplitude of bulk motions of the universe. From observed bulk motions over a local ~ 50 Mpc/h radius volume we determine 0.14 <~ Omega^0.6/b_x(0) <~ 0.59.

astro-ph↗

arXiv2000

Cosmological Parameters from Velocities, CMB and Supernovae

We compare and combine likelihood functions of the cosmological parameters Omega_m, h and sigma_8, from peculiar velocities, CMB and type Ia supernovae. These three data sets directly probe the mass in the Universe, without the need to relate the galaxy distribution to the underlying mass via a "biasing" relation. We include the recent results from the CMB experiments BOOMERANG and MAXIMA-1. Our analysis assumes a flat Lambda CDM cosmology with a scale-invariant adiabatic initial power spectrum and baryonic fraction as inferred from big-bang nucleosynthesis. We find that all three data sets agree well, overlapping significantly at the 2 sigma level. This therefore justifies a joint analysis, in which we find a joint best fit point and 95 per cent confidence limits of Omega_m=0.28 (0.17,0.39), h=0.74 (0.64,0.86), and sigma_8=1.17 (0.98,1.37). In terms of the natural parameter combinations for these data sigma_8 Omega_m^0.6 = 0.54 (0.40,0.73), Omega_m h = 0.21 (0.16,0.27). Also for the best fit point, Q_rms-ps = 19.7 muK and the age of the universe is 13.2 Gyr.

astro-ph↗

arXiv2000

Objective Classification of Galaxy Spectra using the Information Bottleneck Method

A new method for classification of galaxy spectra is presented, based on a recently introduced information theoretical principle, the `Information Bottleneck'. For any desired number of classes, galaxies are classified such that the information content about the spectra is maximally preserved. The result is classes of galaxies with similar spectra, where the similarity is determined via a measure of information. We apply our method to approximately 6000 galaxy spectra from the ongoing 2dF redshift survey, and a mock-2dF catalogue produced by a Cold Dark Matter-based semi-analytic model of galaxy formation. We find a good match between the mean spectra of the classes found in the data and in the models. For the mock catalogue, we find that the classes produced by our algorithm form an intuitively sensible sequence in terms of physical properties such as colour, star formation activity, morphology, and internal velocity dispersion. We also show the correlation of the classes with the projections resulting from a Principal Component Analysis.

astro-ph↗

arXiv1999

The Observed Evolution of Galaxy Clustering Vs. Epoch-Dependent Biasing Models

We study the observed evolution of galaxy clustering as a function of redshift. We find that the clustering of galaxies decreases as we go from observations of the local Universe to $z \sim 2$. On the other hand, clustering of the Lyman break galaxies at $z \sim 3$ is very strong, comparable to the clustering of present day galaxies. However there are three major factors to take into account while comparing clustering measurements coming from various surveys: the so-called ``scale-dependence'' effect, due to measurements being made at different scales; the ``type-selection'' effect introduced by the fact that different galaxy surveys select different populations which do not have the same clustering amplitudes; and the Malmquist bias. We correct for the first two effects and discuss the implications of Malmquist bias on the interpretation of the data at different $z$. Then we compare the observed galaxy clustering with models for the evolution of clustering in some fixed cosmologies. Correcting for the scale-dependence effect significantly reduces the discrepancies amongst different measurements. We interpret the observed clustering signal at high redshift as coming from objects which are highly biased with respect to the underlying distribution of mass; this is not the case for $z\simlt 2$ where measurements are compatible with the assumption of a much lower biasing level which only shows a weak dependence on $z$. Finally we propose a model-independent test that can be used to place a lower limit on the density parameter $Ω_0$.

astro-ph↗

arXiv1999

Bayesian `Hyper-Parameters' Approach to Joint Estimation: The Hubble Constant from CMB Measurements

Recently several studies have jointly analysed data from different cosmological probes with the motivation of estimating cosmological parameters. Here we generalise this procedure to take into account the relative weights of various probes. This is done by including in the joint χ^2 function a set of `Hyper-Parameters', which are dealt with using Bayesian considerations. The resulting algorithm (in the case of uniform priors on the log of the Hyper-Parameters) is very simple: instead of minimising \sum χ_j^2 (where χ_j^2 is per data set j) we propose to minimise \sum N_j \ln (χ_j^2) (where N_j is the number of data points per data set j). We illustrate the method by estimating the Hubble constant H_0 from different sets of recent CMB experiments (including Saskatoon, Python V, MSAM1, TOCO and Boomerang).

astro-ph↗

arXiv1999

Cosmological parameters from cluster abundances, CMB and IRAS

We combine information on cosmological parameters from cluster abundances, CMB primordial anisotropies and the IRAS 1.2 Jy galaxy redshift survey. We take as free parameters the present values of the total matter density of the universe, Omega_m, the Hubble parameter, h, sigma_8, and the IRAS biasing factor, b_IRAS. We assume that the universe is spatially flat, with a cosmological constant, and that structure formed from adiabatic initial fluctuations with a Harrison-Zel'dovich power spectrum. The nucleosynthesis value Omega_b=0.019/h^2 is adopted. We use the full three- and four- dimensional likelihood functions for each data set and marginalise these to two- and one- dimensional distributions in a Bayesian way, integrating over the other parameters. It is shown that the three data sets are in excellent agreement, with a best fit point of Omega_m= 1-Omega_Lambda=0.36, h=0.54, sigma_8=0.74, and b_IRAS=1.08. This point is within one sigma of the minimum for each data set alone. Pairs of these data sets have their degeneracies in sufficiently different directions that using only two data sets at a time is sufficient to place good constraints on the cosmological parameters. We show that the results from each of the three possible pairings of the data are also in good agreement. Finally, we combine all three data sets to obtain marginalised 68 per cent confidence intervals of 0.30<Omega_m<0.43, 0.48<h<0.59, 0.69<sigma_8<0.79, and 1.01<b_IRAS<1.16. For the best fit parameters the CMB quadrupole is Q_rms-ps=18.0 uK, Gamma=0.15, Omega_b=0.066 and the age of the universe is 16.7 Gyr.

astro-ph↗

arXiv1999

The 2dF Galaxy Redshift Survey: Spectral Types and Luminosity Functions

We describe the 2dF Galaxy Redshift Survey (2dFGRS), and the current status of the observations. In this exploratory paper, we apply a Principal Component Analysis to a preliminary sample of 5869 galaxy spectra and use the two most significant components to split the sample into five spectral classes. These classes are defined by considering visual classifications of a subset of the 2dF spectra, and also by comparing to high quality spectra of local galaxies. We calculate a luminosity function for each of the different classes and find that later-type galaxies have a fainter characteristic magnitude, and a steeper faint-end slope. For the whole sample we find M*=-19.7 (for Omega=1, H_0=100 km/sec/Mpc), alpha=-1.3, phi*=0.017. For class 1 (`early-type') we find M*=-19.6, alpha=-0.7, while for class 5 (`late-type') we find M*=-19.0, alpha=-1.7. The derived 2dF luminosity functions agree well with other recent luminosity function estimates.

astro-ph↗

arXiv1999

Constraints on the Clustering, Biasing and Redshift Distribution of Radio Sources

We discuss how different theoretical predictions for the variance $σ^2$ of the distribution of radio sources can be matched to measurements from the FIRST survey at different flux limits. The predictions are given by the integration of models for the angular correlation function $w(θ)$ for three different functional forms of the redshift distribution $N(z)$, different spatial correlation functions and by different evolutions of the bias $b(z)$ with redshift. We also consider the two cases of open and flat Universes. Although the predicted $w(θ)$ show substantial differences due to differences in the $N(z)$'s, these differences are not significant compared to the uncertainties in the current observations. It turns out that the best fit is provided by models with constant biasing at all times, although the difference between models with epoch-independent bias and models with bias that evolves linearly with redshift is not very large. All models with strong evolution of bias with epoch are ruled out. As a further step we directly calculated $w_{obs}(θ)$ at 3mJy from the catalogue and matched it with our models for the angular correlation function in the hypothesis that the clustering signal comes from two different populations, namely AGN-powered sources and starbursting galaxies. The results are consistent with a scenario for hierarchical clustering where the fainter starbursting galaxies trace the mass at all epochs, while brighter AGN's are strongly biased, with $b(z)$ evolving linearly with redshift, as suggested by some theories of galaxy formation and evolution.

astro-ph↗

arXiv1998

The Supergalactic Plane revisited with the Optical Redshift Survey

We re-examine the existence and extent of the planar structure in the local galaxy density field, the so-called Supergalactic Plane (SGP). This structure is studied here in three dimensions using both the new Optical Redshift Survey (ORS) and the IRAS 1.2 Jy redshift survey. The density contrast in a slab of thickness of 20 Mpc/h and diameter of 80 Mpc/h aligned with the standard de Vaucouleurs' Supergalactic coordinates, is delta_sgp =0.5 for both ORS and IRAS. The structure of the SGP is not well described by a homogeneous ellipsoid, although it does appear to be a flattened structure, which we quantify by calculating the moment of inertia tensor of the density field. The directions of the principal axes vary with radius, but the minor axis remains within 30 deg of the standard SGP Z-axis, out to a radius of 80 Mpc/h, for both ORS and \iras. However, the structure changes shape with radius, varying between a flattened pancake and a dumbbell, the latter at a radius of ~50 Mpc/h, where the Great Attractor and Perseus-Pisces superclusters dominate the distribution. This calls to question the connectivity of the `plane' beyond ~40 Mpc/h. The configuration found here can be viewed as part of a web of filaments and sheets, rather than as an isolated pancake-like structure. An optimal minimum variance reconstruction of the density field using Wiener filtering which corrects for both redshift distortion and shot noise, yields a similar misalignment angle and behaviour of axes. The background-independent statistic of axes proposed here can be best used for testing cosmological models by comparison with N-body simulations.

astro-ph↗

arXiv1998

Variance and Skewness in the FIRST survey

We investigate the large-scale clustering of radio sources in the FIRST 1.4-GHz survey by analysing the distribution function (counts in cells). We select a reliable sample from the the FIRST catalogue, paying particular attention to the problem of how to define single radio sources from the multiple components listed. We also consider the incompleteness of the catalogue. We estimate the angular two-point correlation function $w(θ)$, the variance $Ψ_2$, and skewness $Ψ_3$ of the distribution for the various sub-samples chosen on different criteria. Both $w(θ)$ and $Ψ_2$ show power-law behaviour with an amplitude corresponding a spatial correlation length of $r_0 \sim 10 h^{-1}$Mpc. We detect significant skewness in the distribution, the first such detection in radio surveys. This skewness is found to be related to the variance through $Ψ_3=S_3(Ψ_2)^α$, with $α=1.9\pm 0.1$, consistent with the non-linear gravitational growth of perturbations from primordial Gaussian initial conditions. We show that the amplitude of variance and skewness are consistent with realistic models of galaxy clustering.

astro-ph↗

arXiv1998

Principal Component Analysis Of Synthetic Galaxy Spectra

We analyse synthetic galaxy spectra from the evolutionary models of Bruzual&Charlot and Fioc&Rocca-Volmerange using the method of Principal Component Analysis (PCA). We explore synthetic spectra with different ages, star formation histories and metalicities, and identify the Principal Components (PCs) of variance in the spectra due to these different model parameters. The PCA provides a more objective and informative alternative to diagnostics by individual spectral lines. We discuss how the PCs can be used to estimate the input model parameters and explore the impact of noise in this inverse problem. We also discuss how changing the sampling of the ages and other model parameters affects the resulting PCs. Our first two synthetic PCs agree with a similar analysis on observed spectra obtained by Kennicutt and the 2dF redshift survey. We conclude that with a good enough signal-to-noise (S/N>> 10) it is possible to derive age, star formation history and metallicity from observed galaxy spectra using PCA.

astro-ph↗

arXiv1997

Galaxies Discovered Behind the Milky Way by the Dwingeloo Obscured Galaxies Survey

Our Galaxy blocks a significant portion of the extragalactic sky from view, hampering studies of large-scale structure. This produces an incomplete knowledge of the distribution of galaxies, and, assuming galaxies trace mass, of the gravity field. Further, just one unrecognized, nearby massive galaxy could have large influence over the Milky Way's motion with respect to the Cosmic Microwave Background. Diligent surveys in the optical and infrared wavebands can find galaxies through moderate Galactic gas and dust, but close to the Galactic Plane, only radio surveys are effective. The entire northern Zone of Avoidance is being searched at 21 cm for galaxies using the Dwingeloo 25-m telescope. A shallow search for nearby, and/or massive galaxies has been completed, yielding five objects. Two of these galaxies were previously unknown, and although they are not likely members of the Local Group, are part of the nearby Universe. A deeper search continues, which will produce a flux-limited catalog of hidden galaxies. This portion of the survey is one-third complete, and has detected about 40 objects to date. Based on present understanding of the HI mass function, the complete survey should uncover 50 - 100 galaxies.

astro-ph↗