arXiv subjects

N. Benitez

Publications and source records attributed to N. Benitez.

87 records · Page 5Linked to original sources

arXiv2002

Cosmology with GTC. A combined mm-optical galaxy cluster survey

The CMB Planck satellite (ESA) will provide a catalogue of nearly 30000 galaxy clusters (Sunyaev-Zel'dovich effect) between redshift 0 and approx. 2-3 (depending on the cosmological model). This hughe catalogue of clusters will allow detailed studies of the evolution of the cluster population with redshift which will constrain the cosmological model. The identification of the high redshift clusters should be done with a large telescope (like GTC) since Planck data alone will not provide any clue about the redshift of the clusters. In this work we show that an optical follow up of only 300 clusters (randomly selected from the Planck catalogue) is needed in order to distinguish models with or without a cosmological constant.

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arXiv2001

A joint fit to the cosmological Model and the T-M and L_x-T relations using cluster data

In this work we show how well can the cosmological parameters be constrained using galaxy cluster data. We also show how in the process of fitting the model which attempts to describe the data it is possible to get some information about the cluster scaling relations T-M and L_x-T. Among other conclusions we found that only low density universes (Omega approx. 0.3 with or without cosmological constant) are compatible with recent data sets. These constraints will be improved with future SZ data.

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arXiv2001

The SZ effect as a cosmological discriminator

We show how future measurements of the SZ effect (SZE) can be used to constrain the cosmological parameters. We combine the SZ information expected from the Planck full-sky survey, N(S), where no redshift information is included, with the N(z) obtained from an optically-identified SZ-selected survey covering less than 1% of the sky. We demonstrate how with a small subsample (approx. 300 clusters) of the whole SZ catalogue observed optically it is possible to drastically reduce the degeneracy among the cosmological parameters. We have studied the requirements for performing the optical follow-up and we show the feasibility of such a project. Finally we have compared the cluster expectations for Planck with those expected for Newton-XMM during their lifetimes. It is shown that, due to its larger sky coverage, Planck will detect a factor approx. 5 times more clusters than Newton-XMM and also with a larger redshift coverage.

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arXiv2000

Quasar-galaxy associations revisited

Gravitational lensing predicts an enhancement of the density of bright, distant QSOs around foreground galaxies. We measure this QSO-galaxy correlation w_qg for two complete samples of radio-loud quasars, the southern 1Jy and Half-Jansky samples. The existence of a positive correlation between z~1 quasars and z~0.15 galaxies is confirmed at a p=99.0% significance level (>99.9%) if previous measurements on the northern hemisphere are included). A comparison with the results obtained for incomplete quasar catalogs (e.g. the Veron-Cetty and Veron compilation) suggests the existence of an `identification bias', which spuriously increases the estimated amplitude of the quasar-galaxy correlation for incomplete samples. This effect may explain many of the surprisingly strong quasar-galaxy associations found in the literature. Nevertheless, the value of w_qg that we measure in our complete catalogs is still considerably higher than the predictions from weak lensing. We consider two effects which could help to explain this discrepancy: galactic dust extinction and strong lensing.

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arXiv1999

Measuring Omega/b with weak lensing

A correlation between the surface density of foreground galaxies tracing the Large Scale Structure and the position of distant galaxies and quasars is expected from the magnification bias effect (Canizares 1981). We show that this foreground--background correlation function w_{fb} can be used as a straightforward and almost model-free method to measure the cosmological ratio Omega/b. For samples with appropriate redshift distributions, w_{fb} is proportional to Omega , where delta and g are respectively the foreground dark matter and galaxy surface density fluctuations. Therefore, Omega/b is proportional to the ratio w_{fb}/w, where w is equivalent to , the foreground galaxy angular two-point correlation function, b is the biasing factor, and the proportionality factor is independent of the dark matter power spectrum. Simple estimations show that the application of this method to the galaxy and quasar samples generated by the upcoming Sloan Sky Digital Survey will achieve a highly accurate and well resolved measurement of the ratio Omega/b.

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arXiv1998

Segregated Optical-NIR colour distributions of MDS galaxies

We present a K survey of 29 fields covering approximately 90 arcmin^2 from the Medium Deep Survey (MDS) catalogue down to a completeness magnitude of K=18.0 (limiting magnitude K=19.0). The morphology obtained by the MDS team using high resolution images from HST/WFPC2 along with our NIR observations allow a Colour-Magnitude and Colour-Colour analysis that agrees in general with spectral evolution models (Bruzual & Charlot 1998) especially if a reasonable range of metallicities for the Simple Stellar Populations used (0.2 < Z/Zsolar < 2.5) is considered. However, a significant population of spheroids was found, which appear bluer than expected, confirming previous observations (Forbes et al. 1996, Koo et al. 1996, Glazebrook et al. 1998). This blueness might possibly signal the existence of non-negligible star formation in ellipticals and bulges at medium redshift. A number counts calculation for different morphological types show disks become the dominant population at faint magnitudes. The median redshift of the sample is z~0.2, from a photometric redshift estimation using V-K and I-K. A search for EROs in the survey field was also performed, with no detection of objects having I-K > 4.5, setting an upper limit to the number density of EROs at dn_{EROs}/dΩ< 0.011 arcmin^{-2} (K < 18.0).

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arXiv1998

Imaging around PC1643+4631A at the Lyman limit

We present a list of candidates for high redshift late-type galaxies in the field around the z=3.79 quasar PC1643+4631A. Deep U,V and R imaging has been used to search for objects with a strong Lyman break between U and V, characteristic of galaxies with high hydrogen column densities at $z\sim 3$. A further study of the red objects detected by Hu & Ridgway (1994) has been done, allowing the temptative identification of many of them as low redshift ($z\sim 0.4$) elliptical galaxies.

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arXiv1998

Gravitational Lens Magnification and the Mass of Abell 1689

We present the first application of lens magnification to measure the absolute mass of a galaxy cluster; Abell 1689. The absolute mass of a galaxy cluster can be measured by the gravitational lens magnification of a background galaxy population by the cluster potential. The lensing signal is complicated by the variation in number counts due to galaxy clustering and shot-noise, and by additional uncertainties in relating magnification to mass in the strong lensing regime. Clustering and shot-noise can be dealt with using maximum likelihood methods. Local approximations can then be used to estimate the mass from magnification. Alternatively if the lens is axially symmetric we show that the amplification equation can be solved nonlocally for the surface mass density and the tangential shear. In this paper we present the first maps of the total mass distribution in Abell 1689, measured from the deficit of lensed red galaxies behind the cluster. Although noisier, these reproduce the main features of mass maps made using the shear distortion of background galaxies but have the correct normalisation, finally breaking the ``sheet-mass'' degeneracy that has plagued lensing methods based on shear. We derive the cluster mass profile in the inner 4' (0.48 Mpc/h). These show a profile with a near isothermal surface mass density κ= (0.5+/-0.1)(θ/1')^{-1} out to a radius of 2.4' (0.28Mpc/h), followed by a sudden drop into noise. We find that the projected mass interior to 0.24 h^{-1}$Mpc is M(<0.24 Mpc/h)=(0.50+/- 0.09) \times 10^{15} Msol/h. We compare our results with masses estimated from X-ray temperatures and line-of-sight velocity dispersions, as well as weak shear and lensing arclets and find all are in fair agreement for Abell 1698.

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arXiv1997

Are optically-selected QSO catalogs biased ?

A thorough study of QSO-galaxy correlations has been done on a region close to the North Galactic Pole using a complete subsample of the optically selected CFHT/MMT QSO survey and the galaxy catalog of Odewahn and Aldering (1995). Although a positive correlation between bright QSOs and galaxies is expected because of the magnification bias effect, none is detected. On the contrary, there is a significant (>99.6%) anticorrelation between z<1.6 QSOs and red galaxies on rather large angular distances. This anticorrelation is much less pronounced for high redshift z>1.6 QSOs, which seems to exclude dust as a cause of the QSO underdensity. This result suggests that the selection process employed in the CFHT/MMT QSO survey is losing up to 50% of low redshift z<1.6 QSOs in regions of high galaxy density. The incompleteness in the whole z<1.6 QSO sample may reach 10% and have important consequences in the estimation of QSO evolution and the QSO autocorrelation function.

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arXiv1997

Weak lensing correlations in open and flat universes

Correlations between the magnification or polarization of background sources, induced by gravitational lensing due to the large-scale structure, and the positions of foreground galaxies are investigated. We found that their amplitude is enhanced with respect to correlations for a single population. We analize the dependence of the correlations with the density parameter Ωconsidering a nonlinear evolution of the matter power spectrum. The contribution of the linear evolution is negligible at scales below several arcminutes. Detailed results on the dependence of the correlations on the redshift of the foreground and background populations for different cosmological models are presented. The effect reaches its maximum amplitude for foreground populations with relatively small redshifts due to the fast increase of the nonlinear matter power spectrum at recent times.

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arXiv1997

Statistical excess of foreground galaxies around high-z radiogalaxies

K-band imaging of a sample of 21 radiogalaxies with z \sim 1.5 reveals the existence of a statistical association of foreground galaxies with the positions of the radiosources. The excess is detected within a 1' radius at a high significance level (>99.8%). K-band light is a good tracer of stellar mass, so this result indicates the existence of an association between foreground mass perturbations and background radiosources, as expected from the magnification bias effect, and confirms previous results obtained for other high-z radio-loud AGN.

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arXiv1996

The absorbers towards Q0836+113

We have performed RIJHK_S imaging of the field around the z=2.67 quasar Q0836+113, which presents several metal line and a damped Lyαabsorption systems in its spectrum. The images reveal the existence of a red K_S=18.9 object \approx 11 arcsec from the quasar. On the basis of the empirical relationships between absorption radius and luminosity we conclude that this object may be the CIV absorber at z=1.82. This could be the first detection of a high redshift galaxy causing high-ionisation absorption. After carefully subtracting the QSO, we do not detect, up to a 3σlimiting magnitude for extended objects of K_S=20.8, the damped Lyαabsorber apparently detected as a Lyαemitter at z=2.47. It is also suggested, that object ``SW'' from Wolfe et al. (1992) could be the galaxy responsible for the claimed MgII absorption at z=0.37.

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arXiv1996

Large Scale QSO-galaxy correlations for radio loud and optically selected QSO samples

We have studied the distribution of $B_J<20.5$ galaxies from the ROE/NRL COSMOS/UKST catalogue around two samples of $z>0.3$ QSOs with similar redshift distributions. The first sample is formed by 144 radio-loud QSOs from the Parkes Catalogue, and the other contains 167 optically selected QSOs extracted from the Large Bright Quasar Survey. It is found that there is a $\approx 99.0%$ significance level excess of COSMOS/UKST galaxies around the PKS QSOs, whereas there is a marginal defect of galaxies around the LBQS QSOs. When the distribution of galaxies around both samples is compared, we found that there is an overdensity of galaxies around the PKS sample with respect to the LBQS sample anticorrelated with the distance from the QSOs at a $99.7%$ significance level. Although this result apparently agrees with the predictions of the double magnification bias, it is difficult to explain by gravitational lensing effects alone; dust in the foreground galaxies and selection effects in the detection of LBQS QSOs should be taken into account. It has been established that the lines of sight to PKS flat-spectrum QSOs go through significatively higher foreground galaxy densities than the directions to LBQS quasars, what may be partially related with the reported reddening of PKS QSOs.

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arXiv1995

High redshift AGNs from the 1Jy catalogue and the magnification bias

We have found a statistically significant (99.1 \%) excess of red ($O-E>2$) galaxies with photographic magnitudes $E<19.5$, $O< 21$ taken from the APM Sky Catalogue around $z \sim 1$ radiosources from the 1Jy catalogue. The amplitude, scale and dependence on galaxy colours of the observed overdensity are consistent with its being a result of the magnification bias caused by the weak gravitational lensing of large scale structures at redshift $z \approx 0.2-0.4$ and are hardly explained by other causes, as obscuration by dust.

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arXiv1994

R-Band Imaging of Fields Around 1<z<2 Radiogalaxies

We have taken deep $R$-band images of fields around five radiogalaxies: 0956+47, 1217+36, 3C256, 3C324 and 3C294 with $1<z<2$ . 0956+47 is found to show a double nucleus. Our data on 1217+36 suggest the revision of its classification as a radiogalaxy. We found a statistically significant excess of bright ($19.5<R<22$) galaxies on scales of 2 arcmin around the radiogalaxies (which have $R \approx 21.4$) in our sample. The excess has been determined empirically to be at $\gtrsim 99.5\%$ level. It is remarkable that this excess is not present for $22<R<23.75$ galaxies within the same area, suggesting that the excess is not physically associated to the galaxies but due to intervening groups and then related to gravitational lensing.

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