arXiv subjects

A. Blanchard

Publications and source records attributed to A. Blanchard.

At least 55 records · Page 3Linked to original sources

arXiv2001

Cosmology from Cosmic Microwave Background and Galaxy Clusters

We present the results of analysis of constraints on cosmological parameters from cosmic microwave background (CMB) alone and in combination with galaxy cluster baryon fraction assuming inflation--generated adiabatic scalar fluctuations. The CMB constraints are obtained using our likelihood approximation method (Bartlett et al., 2000, Douspis et al., 2001). In the present analysis we use the new data coming from MAXIMA and BOOMERanG balloon borne experiments, the first results of the DASI interferometer together with the COBE/DMR data. The quality of these independent data sets implies that the $C_\ell$ are rather well known, and allow reliable constraints. We found that the constraints in the $Ω-H_0$ plane are very tightened, favouring a flat Universe, that the index of the primordial fluctuations is very close to one, that the primordial baryon density is now in good agreement with primordial nucleosynthesis. Nevertheless degeneracies between several parameters still exist, and for instance the constraint on the cosmological constant or the Hubble constant are very weak, preferred values being low. A way to break these degeneracies is to ``cross-constrain'' the parameters by combining with constraints from other independent data. We use the baryon fraction determination from X-ray clusters of galaxies as an additional constraint and show that the combined analysis leads to strong constraints on all cosmological parameters. Using a high baryon fraction ($\sim 15%$ for $h = 0.5$) we found a rather low Hubble constant, values around 80 km/s/Mpc being ruled out. Using a recent and low baryon fraction estimation ($\sim 10%$ for $h = 0.5$) we found a preferred model with a low Hubble constant and a high density content ($Ω_m$), an Einstein--de Sitter model being only weakly ruled out.

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arXiv2001

Bias in Matter Power Spectra ?

We review the constraints given by the linear matter power spectra data on cosmological and bias parameters, comparing the data from the PSCz survey (Hamilton et al., 2000) and from the matter power spectrum infered by the study of Lyman alpha spectra at z=2.72 (Croft et al., 2000). We consider flat--$Λ$ cosmologies, allowing $Λ$, $H_0$ and $n$ to vary, and we also let the two ratio factors $r_{pscz}$ and $r_{lyman}$ ($r^2_i = \frac{P_{i}(k)}{P_{CMB}(k)}$) vary independently. Using a simple $χ^2$ minimisation technique, we find confidence intervals on our parameters for each dataset and for a combined analysis. Letting the 5 parameters vary freely gives almost no constraints on cosmology, but requirement of a universal ratio for both datasets implies unacceptably low values of $H_0$ and $Λ$. Adding some reasonable priors on the cosmological parameters demonstrates that the power derived by the PSCz survey is higher by a factor $\sim 1.75$ compared to the power from the Lyman $α$ forest survey.

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arXiv2001

The XMM-Newton $Ω$ Project

The abundance of high-redshift galaxy clusters depends sensitively on the matter density $\OmM$ and, to a lesser extent, on the cosmological constant $Λ$. Measurements of this abundance therefore constrain these fundamental cosmological parameters, and in a manner independent and complementary to other methods, such as observations of the cosmic microwave background and distance measurements. Cluster abundance is best measured by the X-ray temperature function, as opposed to luminosity, because temperature and mass are tightly correlated, as demonstrated by numerical simulations. Taking advantage of the sensitivity of XMM-Newton, our Guaranteed Time program aims at measuring the temperature of the highest redshift (z>0.4) SHARC clusters, with the ultimate goal of constraining both $\OmM$ and $Λ$.

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arXiv2001

The baryon fraction distribution in X-rays galaxy clusters

The baryon fraction in galaxy clusters is one of the most direct way to constrain $\omo$. The baryonic fraction is estimated to be in the range fb = (0.15 - 0 .20) h^{-3/2} which is several times higher than expected from the observed light element abundances in $\omo=1$ universe, leading to the conclusion that we live in low density universe. In this work I will first present the various steps which lead to these results and then foccus on the distribution of the baryon fraction inside clusters. I will show that while the baryon fraction in clusters follows a scaling law, its theoretical expected shape does not agree with what is inferred from observations. I will show that when various systematics entering into the determination of clusters gas fraction are taken into account the observed baryon fraction profile matches better the predictions, provided that the average baryon fraction at the virial radius is lower than previously found fb~10%.

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arXiv2001

Cosmological Constraints from the Cosmic Microwave Background

Using an approximate likelihood method adapted to band--power estimates, we analyze the ensemble of first generation cosmic microwave background anisotropy experiments to deduce constraints over a six--dimensional parameter space describing Inflation--generated adiabatic, scalar fluctuations. The basic preferences of simple Inflation scenarios are consistent with the data set: flat geometries $(\OmT \equiv 1-\Omk \sim 1)$ and a scale--invariant primeval spectrum ($n\sim 1$) are favored. Models with significant negative curvature ($\OmT < 0.7$) are eliminated, while constraints on postive curvature are less stringent. Degeneracies among the parameters prevent independent determinations of the matter density $\OmM$ and the cosmological constant $Λ$, and the Hubble constant $\Ho$ remains relatively unconstrained. We also find that the height of the first Doppler peak relative to the amplitude suggested by data at larger $l$ indicates a high baryon content ($\Omb h^2$), almost independently of the other parameters. Besides the overall qualitative advance expected of the next generation experiments, their improved dipole calibrations will be particularly useful for constraining the peak height. Our analysis includes a {\em Goodness--of--Fit} statistic applicable to power estimates and which indicates that the maximum likelihood model provides an acceptable fit to the data set.

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arXiv2001

Concerning Parameter Estimation Using the Cosmic Microwave Background

Most parameter constraints obtained from cosmic microwave background (CMB) anisotropy data are based on power estimates and rely on approximate likelihood functions; computational difficulties generally preclude an exact analysis based on pixel values. With the specific goal of testing this kind of approach, we have performed a complete (un-approximated) likelihood analysis combining the COBE, Saskatoon and MAX data sets. We examine in detail the ability of certain approximate techniques based on band-power estimates to recover the full likelihood constraints. The traditional $χ^2$-method does not always find the same best-fit model as the likelihood analysis (a bias), due mainly to the false assumption of Gaussian likelihoods that makes the method overly sensitive to data outliers. Although an improvement, other approaches employing non-Gaussian flat-band likelihoods do not always faithfully reproduce the complete likelihood constraints either; not even when using the exact flat-band likelihood curves. We trace this to the neglect of spectral information by simple flat band-power estimates. A straightforward extension incorporating a local effective slope (of the power spectrum, $C_l$) provides a faithful representation of the likelihood surfaces without significantly increasing computing cost. Finally, we also demonstrate that the best-fit model to this particular data set is a {\em good fit}, or that the observations are consistent with Gaussian sky fluctuations, according to our statistic.

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arXiv2001

An Approximation to the Likelihood Function for Band-Power Estimates of CMB Anisotropies

Band-power estimates of cosmic microwave background fluctuations are now routinely used to place constraints on cosmological parameters. For this to be done in a rigorous fashion, the full likelihood function of band-power estimates must be employed. Even for Gaussian theories, this likelihood function is not itself Gaussian, for the simple reason that band-powers measure the {\em variance} of the random sky fluctuations. In the context of Gaussian sky fluctuations, we use an ideal situation to motivate a general form for the full likelihood function from a given experiment. This form contains only two free parameters, which can be determined if the 68% and 95% confidence intervals of the true likelihood function are known. The ansatz works remarkably well when compared to the complete likelihood function for a number of experiments. For application of this kind of approach, we suggest that in the future both 68% and 95% (and perhaps also the 99.7%) confidence intervals be given when reporting experimental results.

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arXiv2001

New Light on the Baryon Fraction in Galaxy Clusters

Baryon fraction in clusters, combined with constraints from primordial nucleosynthesis is currently used to provide a robust upper limit on the cosmological density parameter $Ω_0$. Current analyses lead to gas fractions at virial radius which are typically of the order of 0.20$h^{-3/2}_{50}$, favoring a low density universe. In this work, we examine critically this issue through the analysis of the baryon distribution in clusters. We find that the currently derived gas fraction profile, increases regularly from the inner part to the outer part, up to the virial radius, and beyond. Such a shape contrasts with what is expected from numerical hydro-dynamical simulations, in which the gas fraction is more or less constant in the outer region, reaching a plateau when the contrast density falls off below $10^4$. We argue that such a difference is hardly explained by reheating effects, while taking into account various factors entering into the determination of clusters gas fraction may erase such a difference. Indeed, using recent estimates on gas content in the outer part of clusters (Vikhlinin et al., 1999) and applying the correction factor due to the effect of gas clumping, we find that the gas fraction shape over the range $ 200< δ< 10^5$ is roughly consistent with hydro-dynamical simulations for an universal gas fraction in the range 8-11% (for $h_{50} =1.$), the mass estimators being calibrated from numerical simulations. In contrast, values of the order of 20% do not give acceptable fit to the data on any scale. We conclude, that high values of $Ω_0$ can not be ruled out on the basis of the baryon fraction argument.

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arXiv2000

A New Local Temperature Distribution Function for X-ray Clusters: Cosmological Applications

(abridged) We present a new determination of the local temperature function of X-ray clusters. We use a new sample comprising fifty clusters for which temperature information is now available, making it the largest complete sample of its kind. It is therefore expected to significantly improve the estimation of the temperature distribution function of moderately hot clusters. We find that the resulting temperature function is higher than previous estimations, but agrees well with the temperature distribution function inferred from the BCS and RASS luminosity function. We have used this sample to constrain the amplitude of the matter fluctuations on cluster's scale of $8\sqrt[3]{Ω_0}^{-1}h^{-1}$Mpc, assuming a mass-temperature relation based on recent numerical simulations. We find $σ_8 = 0.6\pm 0.02$ for an $Ω_0 = 1$ model. Our sample provides an ideal reference at $z \sim 0$ to use in the application of the cosmological test based on the evolution of X-ray cluster abundance (Oukbir & Blanchard 1992, 1997). Using Henry's sample, we find that the abundance of clusters at $z = 0.33$ is significantly smaller, by a factor larger than 2, which shows that the EMSS sample provides strong evidence for evolution of the cluster abundance. A likelihood analysis leads to a rather high value of the mean density parameter of the universe: $Ω=0.92 \pm 0.22$ (open case) and $Ω=0.86 \pm 0.25$ (flat case), which is consistent with a previous, independent estimation based on the full EMSS sample by Sadat et al.(1998). Some systematic uncertainties which could alter this result are briefly discussed.

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arXiv2000

The Baryon Content of Groups and Clusters of Galaxies

We have analyzed the properties of a sample of 33 groups and clusters of galaxies and examine the baryon content to check for trends over a decade in temperature down to 1 keV. We examine the relative contribution of galaxies and ICM to baryons in clusters through the gas-to-stellar mass ratio and find that the typical stellar contribution to the baryonic mass is between 5 and 20%. This ratio is found to be roughly independent of temperature. Therefore, we do not confirm the trend of increasing gas-to-stellar mass ratio with increasing temperature as previously claimed. We also determine distribution of the baryon fraction with the density contrast. Virial masses are estimated from two different mass estimators: one based on the isothermal hydrostatic equation (IHE), the other based on scaling law models (SLM). Comparing the two methods, we find that SLM lead to less dispersed baryon fractions over all density contrasts and that the derived mean absolute valuesare significantly lower than IHE mean values. We show that this is not due to the uncertainties on the outer slope $β$ of the gas density profile but is rather indicating that IHE masses are less reliable. Examining the shape of the baryon fraction profiles we find that cluster baryon fractions estimated from SLM follow a scaling law. Moreover, we do not find any strong evidence of increasing baryon (gas) fraction with temperature, neither do we find the slope $β$ to increase with temperature. The absence of clear trends between $f_{b}$ and $M_{gas}/M_{*}$ with temperature is consistent with the similarity of baryon fraction profiles and suggests that non-gravitational processes such as galaxy feedback, do not play a dominant rôle in heating the intra-cluster gas on the virial scale.

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arXiv2000

CMB Cosmological Parameter Estimation: Methods and Current Results

The majority of present efforts to constrain cosmological parameters with cosmic microwave background (CMB) anisotropy data employ approximate likelihood functions, the time consuming nature of a complete analysis being a major obstacle. We have performed a full (unapproximated) likelihood analysis on several experiments that allows us to examine the various assumptions made in these approximate methods and to evaluate their performance. Our results indicate that care should be taken when using such approaches. With an improved approximate method, we present some constraints on cosmological parameters using the entire present CMB data set.

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arXiv1999

XEUS - The X-ray Evolving Universe Spectroscopy Mission

XEUS is under study by ESA as part of the Horizon 2000+ program to utilize the International Space Station (ISS) for astronomical applications. XEUS will be a long-term X-ray observatory with an initial mirror area of 6m2 at 1 keV that will be grown to 30m2 following a visit to the ISS. The 1 keV spatial resolution is expected to be 2-5'' HEW. XEUS will consist of separate detector and mirror spacecraft aligned by active control to provide a focal length of 50m. A new detector spacecraft, complete with the next generation of instruments, will also be added after visiting the ISS. The limiting sensitivity will then be ~4 10-18 erg/cm2/s - around 250 times better than XMM, allowing XEUS to study the properties of the hot baryons and dark matter at high redshift.

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arXiv1999

Warm photo-ionized IGM: a Clue for galaxy and cluster formation history?

In this paper we investigate the overcooling problem and propose some possible solutions. We show that the overcooling problem is generic to the hierarchical picture of structure formation, as long as the cooling is actually possible in small halos at high redshift. Solutions to this problem are likely to be associated with some feedback mechanism, and probably imply the existence of a warm IGM containing most of the cosmological baryons. We concentrate on some possible solutions, mainly photoionization and bulk-heating of the IGM. We show photoionization can act as a significant feedback mechanism but is not strong enough to solve the entire overcooling problem. We therefore assume that the IGM is maintained hot by some energy injection provided by supernova and galaxy formation is then limited by this feedback mechanism. Such a self regulated scheme allows us to compute the thermal history of the IGM... (abridged)

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arXiv1999

The ESO Slice Project (ESP) Galaxy Redshift Survey. VII. The Redshift and Real-Space Correlation Functions

We present analyses of the two-point correlation properties of the ESP galaxy redshift survey. From the redshift-space correlation function xi(s), we see positive clustering out to separations ~50/h Mpc, with a smooth break on larger scales and zero-crossing between 60 and 80/h Mpc. xi(s) is reasonably well described by a shallow power law with γ~1.5 between 3 and 50/h Mpc, while on smaller scales (0.2-2/h Mpc) it has a shallower slope (γ~ 1). We examine the full effect of redshift-space distortions through the two-dimensional correlation function xi(rp,pi), from which we project out the real-space xi(r) below 10/h Mpc. This function is well described by a power-law model (r/r_o)^{-γ}, with r_o=4.15^{+0.20}_{-0.21} h^{-1} Mpc and γ=1.67^{+0.07}_{-0.09}. Comparison to other redshift surveys shows a consistent picture in which clustering remains positive out to separations of 50/h Mpc or larger, in substantial agreement with the results obtained from angular surveys like the APM and EDSGC. Also the shape of the two-point correlation function is remarkably unanimous among these data sets, in all cases requiring more power above 5/h Mpc (a `shoulder'), than a simple extrapolation of the canonical xi(r)=(r/5)^{-1.8}. xi(s) for volume-limited subsamples shows evidence of luminosity segregation only for the most luminous sample with M_{b_J}\le -20.5. When redshift-space distortions are removed through projection of xi(rp,pi), however, a weak dependence on luminosity is seen at small separations also at fainter magnitudes. This effect is masked in redshift space, as the mean pairwise velocity dispersion experiences a parallel increase, basically erasing the effect of the clustering growth on xi(s).

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arXiv1998

The ESO Slice Project (ESP) galaxy redshift survey: VI Groups of Galaxies

In this paper we identify objectively and analyze groups of galaxies in the recently completed ESP survey. We find 231 groups above the number overdensity threshold delta(rho)/rho=80 in the redshift range 5000<cz <60000 km/s. These groups contain 1250 members, 40.5% of the 3085 ESP galaxies within the same redshift range. The median velocity dispersion (corrected for measurement errors and computed at the redshift of the group) is sigma_{ESP,median} = 194 km/s. We show that our result is reliable in spite of the particular geometry of the ESP survey (two rows of tangent circular fields of radius 15 arcmin), which causes most systems to be only partially surveyed. In general, we find that the properties of ESP groups are consistent with those of groups in shallower (and wider) catalogs (e.g. CfA2N and SSRS2). As in shallower catalogs, ESP groups trace very well the geometry of the large scale structure. Our results are of particular interest because the depth of the ESP survey allows us to sample group properties over a large number of structures. We also compare luminosity function and spectral properties of galaxies that are members of groups with those of isolated galaxies. We find that galaxies in groups have a brighter M* with respect to non--member galaxies; the slope αis the same, within the errors, in the two cases. We find that 34% of ESP galaxies with detectable emission lines are members of groups. The fraction of galaxies without detectable emission lines in groups is significantly higher: 45%. More generally, we find a gradual decrease of the fraction of emission line galaxies among members of systems of increasing richness. This result confirms that the morphology-density relation found for clusters also extends toward systems of lower density.

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arXiv1998

Approximating the Likelihood Function of CMB Experiments

We discuss the problem of constraining cosmological parameters with cosmic microwave background band--power estimates. Because these latter are variances, they do not have gaussian distribution functions and, hence, the standard $χ^2$--approach is not strictly applicable. A general purpose approximation to experimental band--power likelihood functions is proposed, which requires only limited experimental details. Comparison with the full likelihood function calculated for several experiments shows that the approximation works well.

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arXiv1998

Constraints on Cosmological Parameters from Current CMB Data

We discuss the constraints one can place on cosmological parameters using current cosmic microwave background data. A standard $χ^2$--minimization over band--power estimates is first presented, followed by a discussion of the more correct likelihood approach. We propose an approximation to the complete likelihood function of an arbitrary experiment requiring only limited and easily found information about the observations. Examination of both open models -- $(Ω,h,Q,n)$ -- and flat models ($Ω+Ω_Λ=1$) -- $(Ω,Ω_b,h,Q,n)$ -- leaves one rather robust result: models with small curvature are favored.

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arXiv1998

Galactic H alpha emission and the Cosmic Microwave Background

We present observations of Galactic H alpha emission along two declination bands where the South Pole cosmic microwave background experiment reports temperature fluctuations. The high spectral resolution of our Fabry-Perot system allows us to separate the Galactic signal from the much larger local sources of H alpha emission, such as the Earth's geocorona. For the two bands (at declination -62 and -63 degrees), we find a total mean emission of about 1 Rayleigh with variations of about 0.3 R. The variations are within the estimated uncertainty of our total intensity determinations. For an ionized gas at T around 10**4 K, this corresponds to a maximum free-free brightness temperature of less than 10 microK at 30 GHz (K-band). Thus, unless there is a hot gas component with T around 10**6 K, our results imply that there is essentially no free-free contamination of the SP91 (Schuster et al. 1993) and SP94 (Gunderson et al.1995) data sets.

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