arXiv subjects

S. Lopez

Publications and source records attributed to S. Lopez.

53 records · Page 3Linked to original sources

arXiv2008

Galaxy Clusters in the Line of Sight to Background Quasars: I. Survey Design and Incidence of MgII Absorbers at Cluster Redshifts

We describe the first optical survey of absorption systems associated with galaxy clusters at z= 0.3-0.9. We have cross-correlated SDSS DR3 quasars with high-redshift cluster/group candidates from the Red-Sequence Cluster Survey. We have found 442 quasar-cluster pairs for which the MgII doublet might be detected at a transverse (physical) distance d<2 Mpc from the cluster centers. To investigate the incidence (dN/dz) and equivalent-width distribution n(W) of MgII systems at cluster redshifts, two statistical samples were drawn out of these pairs: one made of high-resolution spectroscopic quasar observations (46 pairs), and one made of quasars used in MgII searches found in the literature (375 pairs). The results are: (1) the population of strong MgII systems (W_0>2.0 Ang.) near cluster redshifts shows a significant (>3 sigma) overabundance (up to a factor of 15) when compared with the 'field' population; (2) the overabundance is more evident at smaller distances (d<1 Mpc) than larger distances (d<2 Mpc) from the cluster center; and, (3) the population of weak MgII systems (W_0<0.3 Ang.) near cluster redshifts conform to the field statistics. Unlike in the field, this dichotomy makes n(W) in clusters appear flat and well fitted by a power-law in the entire W-range. A sub-sample of the most massive clusters yields a stronger and still significant signal. Since either the absorber number density or filling-factor/cross-section affects the absorber statistics, an interesting possibility is that we have detected the signature of truncated halos due to environmental effects. Thus, we argue that the excess of strong systems is due to a population of absorbers in an overdense galaxy environment, and the lack of weak systems to a different population, that got destroyed in the cluster environment. (Abridged)

astro-ph↗

arXiv2007

A High Quality/Low Computational Cost Technique for Block Matching Motion Estimation

Motion estimation is the most critical process in video coding systems. First of all, it has a definitive impact on the rate-distortion performance given by the video encoder. Secondly, it is the most computationally intensive process within the encoding loop. For these reasons, the design of high-performance low-cost motion estimators is a crucial task in the video compression field. An adaptive cost block matching (ACBM) motion estimation technique is presented in this paper, featuring an excellent tradeoff between the quality of the reconstructed video sequences and the computational effort. Simulation results demonstrate that the ACBM algorithm achieves a slight better rate-distortion performance than the one given by the well-known full search algorithm block matching algorithm with reductions of up to 95% in the computational load.

cs.MM↗

arXiv2007

Clues to the nature of high-redshift OVI absorption systems from their (lack of) small-scale structure

We present results of the first survey of high-redshift ( ~ 2.3) OVI absorption systems along parallel lines of sight toward two lensed QSOs. After a careful and well-defined search, we find ten intervening OVI systems. Within the errors, all OVI systems appear at the same redshift and have similar line strengths in front of both QSO images, whereas in most cases CIV or SiIV show more differences across the lines of sight, either in radial velocity or line strength. We conclude that (1) the coherence length of OVI must be much larger than ~ 1 kpc, and (2) an important fraction of the CIV absorbers may not reside in the same volume as OVI. Since Doppler parameters are consistent with photoionization, we propose a model in which CIV occurs in two different photoionized phases, one large, with characteristic sizes of a few hundred kpc and bearing OVI, and another one a factor of ten smaller and containing CIII. This model is able to explain the various transverse differences observed in column density and kinematics. We apply the model successfully to 2 kinds of absorbers, with low and high metallicity. In the low-metallicity regime, [C/H] \~ -2, we find that [C/O] ~ -0.7 is required to explain the observations, which hints at late (z < 6) rather than early metal enrichment. In the high-metallicity regime, the observed dissociation between OVI and CIV gas might be produced by galactic outflows. Altogether, the relative abundances, inhomogeneous CIV and featureless OVI are consistent with gas that has been processed recently before the absorption occurred (thus close to star-forming regions). Finally, we discuss briefly three associated systems (z_abs ~ z_em) that also show OVI. (abridged)

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arXiv2007

A new measure of Delta alpha/alpha at redshift z = 1.84 from very high resolution spectra of Q1101-264

We probe the evolution of the fine-structure constant, alpha, with cosmic time. Accurate positions of the FeII lines 1608, 2382,and 2600 A are measured in the z = 1.84 absorption system from a high-resolution (FWHM = 3.8 km/s) and high signal-to-noise (S/N >= 100) spectrum of the quasar Q1101-264 (z_em = 2.15, V = 16.0), integrated for 15.4 hours. The Single Ion Differential alpha Measurement (SIDAM) procedure and the Delta chi^2 method are used to set constraints on Delta alpha/alpha. We have found a relative radial velocity shift between the 1608 A and 2382,2600 A lines of Delta v = -180 +/- 85 m/s (both random and systematic errors are included), which, if real, would correspond to Delta alpha/alpha = (5.4 +/- 2.5) 10^{-6} (1sigma C.L.). Considering the strong implications of a such variability, additional observations with comparable accuracy at redshift z ~ 1.8 are required to confirm this result.

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arXiv2006

Tomography of the intergalactic medium with Ly-alpha forests in close QSO pairs

We study the three-dimensional distribution of non virialised matter at z~2 using high resolution spectra of QSO pairs and simulated spectra drawn from cosmological hydrodynamical simulations. We have collected the largest sample of QSO pairs ever observed with UVES at the ESO-VLT, with angular separations between ~1 and 14 arcmin. The observed correlation functions of the transmitted flux in the HI Lyman alpha forest along and transverse to the lines of sight are in good agreement implying that the distortions in redshift space due to peculiar velocities are small. The clustering signal is significant up to velocity separations of ~200 km/s, or about 3 h^{-1} comoving Mpc. The regions at lower overdensity (rho/ < 6.5) are still clustered but on smaller scales (Delta v < 100 km/s). The observed and simulated correlation functions are compatible at the 3 sigma level. A better concordance is obtained when only the low overdensity regions are selected for the analysis or when the effective optical depth of the simulated spectra is increased artificially, suggesting a deficiency of strong lines in the simulated spectra. We found that also a lower value of the power-law index of the temperature-density relation for the Lyman alpha forest gas improves the agreement between observed and simulated results. If confirmed, this would be consistent with other observations favouring a late HeII reionization epoch (at z~3). We remark the detection of a significant clustering signal in the cross correlation coefficient at a transverse velocity separation Delta v_{\perp} ~500 km/s whose origin needs further investigation.

astro-ph↗

arXiv2006

The Evolution of Lyman alpha Absorbers in the Redshift Range 0.5<z<1.9

We investigate the evolution and the statistical properties of the Lya absorbers of the intergalactic medium (IGM) in the largely unexplored redshift range z=0.5-1.9. We use high-resolution (R > 30,000) UV (STIS) and optical (VLT/UVES and Keck/HIRES) spectra of nine bright quasars with z_em < 1.94. The main results for the combined Lya line sample are summarized as follows: 1. The evolution of the number density of the absorbers can be described by the power law dn/dz ~ (1+z)^gamma. The number density of the low column density lines decreases with decreasing z with gamma=0.74+-0.31 in the interval z=0.7-1.9. A comparison with results at higher redshifts shows that it is decelerated in the explored redshift range and turns into a flat evolution for z -> 0. The stronger absorbers thin out faster (gamma=1.50+-0.45). The break in their evolution predicted for z=1.5-1.7 cannot be seen down to z=0.7. On the other hand, a comparison with values from the literature for the local number density gives a hint that this break occurs at lower redshift. 2. The distribution of the column densities of the absorbers is complete down to N_HI=10^12.90 cm^-2. It can be approximated by a single power law with the exponent beta=1.60+-0.03 over almost three orders of magnitude. beta is redshift independent. 3. The Lya lines with lower column densities as well as the higher column density lines show marginal clustering with a 2 sigma significance over short distances (Delta v < 200 km/s and Delta v < 100 km/s, respectively). We do not see any difference in the clustering with either column density or redshift.

astro-ph↗

arXiv2006

On the Incidence of Strong MgII Absorbers Along GRB Sightlines

We report on a survey for strong (rest equivalent width W_r >= 1A), intervening MgII systems along the sightlines to long-duration gamma-ray bursts (GRBs). The GRB spectra which comprise the survey have a heterogeneous mix of resolution and wavelength coverage, but we implement a strict, uniform set of search criteria to derive a well-defined statistical sample. We identify 15 strong MgII absorbers along 14 GRB sightlines (nearly every sightline exhibits at least one absorber) with spectra covering a total pathlength Delta z = 15.5 at a mean redshift = 1.1. In contrast, the predicted incidence of such absorber systems along the same path length to quasar sightlines is only 3.8. The roughly four times higher incidence along GRB sightlines is inconsistent with a statistical fluctuation at greater than 99.9% c.l. Several effects could explain the result: (i) dust within the MgII absorbers obscures faint quasars giving a lower observed incidence along quasar sightlines; (ii) the gas is intrinsic to the GRB event; (iii) the GRB are gravitationally lensed by these absorbers. We present strong arguments against the first two effects and also consider lensing to be an unlikely explanation. The results suggest that at least one of our fundamental beliefs on absorption line research is flawed.

astro-ph↗

arXiv2005

Spectral shape of the UV ionizing background and OVI absorbers at z ~ 1.5 towards HS0747+4259

We report on high resolution spectra of the bright QSO HS0747+4259 (zem = 1.90, V = 15.8) observed to search for intermediate redshift OVI absorption systems. The spectra were obtained by means of the Space Telescope Imaging Spectrograph (STIS) at the Hubble Space Telescope (HST) and the High Resolution Echelle Spectrometer (HIRES) at the W. M. Keck telescope. We identify 16 OVI systems in the range 1.07 <= z <= 1.87. Among them, six systems with zabs = 1.46-1.8 exhibit a sufficient number of lines of different ionic transitions to estimate the shape of the ionizing radiation field in the range 1 Ryd < E < 10 Ryd. All recovered UV ionizing spectra are characterized by the enhanced intensity at E > 3 Ryd compared to the model spectrum of Haardt and Madau (1996). This is in line with the observational evidence of a deficiency of strong Ly-alpha absorbers with N(HI) > 10^{15} cm^{-2}, at z < 2. The UV background shows significant local variations: the spectral shape estimated at z = 1.59 differs from that obtained at z = 1.81 and 1.73. A possible cause of these variations is the presence of a QSO/AGN at z ~= 1.54-1.59 close to the line of sight. No features favoring the input of stellar radiation to the ionizing background are detected, limiting the escape fraction of the galactic UV photons to f_esc < 0.05.

astro-ph↗

arXiv2005

The Multiwavelength Survey by Yale-Chile (MUSYC): Survey Design and Deep Public UBVRIz' Images and Catalogs of the Extended Hubble Deep Field South

We present UBVRIz' optical images of the 0.32 square degree Extended Hubble Deep Field South. This is one of four fields comprising the MUSYC survey, which is optimized for the study of galaxies at z=3, AGN demographics, and Galactic structure. We calculate corrected aperture photometry and its uncertainties and find through tests that these provide a significant improvement upon standard techniques. Our photometric catalog of 62968 objects is complete to a total magnitude of R_AB=25. We select z=3 Lyman break galaxy (LBG) candidates from their UVR colors and find a sky surface density of 1.4/sq. arcmin and an angular correlation function w(theta) = 2.3+-1.0 theta^{-0.8}. (Abridged)

astro-ph↗

arXiv2005

Nitrogen-deficient and iron-rich associated absorbers with oversolar metallicities towards the quasar HE0141-3932

HE0141-3932 (zem=1.80) is a bright blue radio-quite quasar which reveals an emission line spectrum with an unusually weak Ly-alpha line. In addition, large redshift differences (Delta z=0.05) are observed between high ionization and low ionization emission lines. Absorption systems identified at z=1.78, 1.71, and 1.68 show mild oversolar metallicities (Z ~= 1-2Zsolar) and can be attributed to the associated gas clouds ejected from the circumnuclear region. The joint analysis of the emission and absorption lines leads to the conclusion that this quasar is seen almost pole-on. Its apparent luminosity may be Doppler boosted by ~10 times. The absorbing gas shows high abundance of Fe, Mg, and Al ([Fe, Mg, Al/C] ~= 0.15+/-0.10) along with underabundance of N ([N/C]<=-0.5). This abundance pattern is at variance with current chemical evolution models of QSOs predicting [N/C]>0 and [Fe/C]<0 at Z ~ Zsolar. Full details of this work are given in Reimers et al. (2005).

astro-ph↗

arXiv2005

Probing 3-D matter distribution at z~2 with QSO multiple lines of sight

We investigate the 3-D matter distribution at z~2 with high resolution (R ~ 40000) spectra of QSO pairs and groups obtained with the UVES spectrograph at ESO VLT. Our sample is unique for the number density of objects and the variety of separations, between 0.5 and 7 proper Mpc. We compute the real space cross-correlation function of the Lyman-alpha forest transmitted fluxes. There is a significant clustering signal up to ~2 proper Mpc, which is still present when absorption lines with high column density (log N > 13.8) are excluded.

astro-ph↗

arXiv2005

Evidence for a magnitude-dependent bias in the Hamburg/ESO Survey for Damped Lyman-alpha Systems

We present preliminary results from the Hamburg/ESO survey for Damped Ly-alpha (hereafter, DLA) Systems. This survey is characterized by (i) the good knowledge of the biases affecting the parent QSO survey, (ii) the brightness and (iii) the relatively wide magnitude distribution of the background QSOs. Therefore, it is well-suited to study possible magnitude-dependent biases in DLA surveys, such as the one expected from dust obscuration. We have systematically searched for damped Lyman-alpha line candidates in 5 A resolution spectra of the 188 QSOs that constitute our statistical sample. These candidates have later been reobserved with UVES at the ESO--Very Large Telescope (VLT) for confirmation and accurate N(HI) measurements. In the redshift range covered by the survey, 19 DLA systems have been discovered. Over the whole survey, we find that the number density, n(z), and cosmological density of gas, Omega_gas, have comparable values to the ones obtained by CORALS (Ellison et al. 2001). However, the number densities of DLA systems n(z) in two sub-samples of equal absorption distance path defined by the magnitude of the background QSOs differ by a factor of about 5. We estimate that the probability that n(z) is equal in the two sub-samples is < 0.003. A similar, only slightly less significant difference is found for Omega_gas.

astro-ph↗

arXiv2005

Metal Abundances in a Damped Lyman-alpha System Along Two Lines of Sight at z=0.93

We study metal abundances in the z=0.9313 damped Lya system observed in the two lines-of-sight, A and B, toward the gravitationally-lensed double QSO HE0512-3329. Spatially resolved STIS spectra constrain the neutral-gas column density to be LogN(HI)=20.5 in both Aand B. UVES spectra (spectral resolution FWHM=9.8 km/s) show, in contrast, significant line-of-sight differences in the column densities of MnII and FeII; these are not due to observational systematics. We find that [Mn/H]=-1.44 and [Fe/H]=-1.52 in damped Lya system A, while [Mn/H]=-0.98 and [Fe/H]>-1.32, and possibly as high as [Fe/H] approx. -1 in damped Lya system B. A careful assessment of possible systematic errors leads us to conclude that these transverse differences are significant at a 5 sigma level or greater. Although nucleosynthesis effects may also be at play, we favor differential dust-depletion as the main mechanism producing the observed abundance gradient. The transverse separation is 5 kpc at the redshift of the absorber, which is also likely to be the lensing galaxy. The derived abundances therefore probe two opposite sides of a single galaxy hosting both damped Lya systems. This is the first time firm abundance constraints have been obtained for a single damped system probed by two lines-of-sight. The significance of this finding for the cosmic evolution of metals is discussed.

astro-ph↗

arXiv2005

Cosmochemistry, Cosmology and Fundamental Constants: High-Resolution Spectroscopy of Damped Lyman-Alpha Systems

Spectroscopy of QSO absorption lines provides essential observational input for the study of nucleosynthesis and chemical evolution of galaxies at high redshift. But new observations may indicate that present chemical abundance data are biased due to deficient spectral resolution and unknown selection effects: Recent high-resolution spectra reveal the hitherto unperceived chemical nonuniformity of a molecule-bearing damped Lyman-alpha (DLA) system, and the still ongoing H/ESO DLA survey produces convincing evidence for the effect of dust attenuation. We present a revised analysis of the moleculecular hydrogen-bearing DLA complex toward the QSO HE 0515-4414 showing nonuniform differential depletion of chemical elements onto dust grains, and introduce the H/ESO DLA survey and its implications. Conclusively, we aim at starting an unbiased chemical abundance database established on high-resolution spectroscopic observations. New data to probe the temperature-redshift relation predicted by standard cosmology and to test the constancy of fundamental constants will be potential spin-offs.

astro-ph↗

arXiv2005

HE 0141-3932: a bright QSO with an unusual emission line spectrum and associated absorption

HE 0141-3932 (zem = 1.80) is a bright blue radio-quite quasar with an unusually weak Ly-alpha emission line. Large redshift differences (Delta z = 0.05) are observed between high ionization and low ionization emission lines. Absorption systems identified at zabs = 1.78, 1.71, and 1.68 show mild oversolar metallicities (Z ~= 1-2Zsolar) and can be attributed to the associated gas clouds ejected from the circumnuclear region. The joint analysis of the emission and absorption lines leads to the conclusion that this quasar is seen almost pole-on. Its apparent luminosity may be Doppler boosted by ~10 times. The absorbing gas shows a high abundance of Fe, Mg and Al ([Fe, Mg, Al/C] ~= 0.15 +/- 0.10) along with underabundance of N ([N/C] <= -0.5). This abundance pattern is at variance with current chemical evolution models of QSOs predicting [N/C] >= 0$ and [Fe/C] < 0 at Z ~ Zsolar.

astro-ph↗

arXiv2003

Clues to the origin of metals in the Ly-alpha forest

We analyze the z=2 Ly-alpha and CIV forest along two adjacent lines-of-sight at high S/N and spectral resolution to assess the question of the origin of metals in the intergalactic medium. We find that the gas containing detectable amounts of metals (as seen in absorption by CIV) is less quiescent on scales of a few kpc than the gas without metals. Although it is difficult to discriminate unequivocally between possible sources of enrichment and the extent to which each contributes to line-of-sight differences, those suggest that the metal-enriched systems are bound to galaxies (i.e., they do not arise in the IGM nor in galactic winds).

astro-ph↗

arXiv2003

Disentangling microlensing and differential extinction in the double QSO HE0512-3329

We present the first separate spectra of both components of the small-separation double QSO HE0512-3329 obtained with HST/STIS in the optical and near UV. The similarities especially of the emission line profiles and redshifts strongly suggest that this system really consists of two lensed images of one and the same source. The emission line flux ratios are assumed to be unaffected by microlensing and are used to study the differential extinction effects caused by the lensing galaxy. Fits of empirical laws show that the extinction properties seem to be different on both lines of sight. With our new results, HE0512-3329 becomes one of the few extragalactic systems which show the 2175 AA absorption feature, although the detection is only marginal. We then correct the continuum flux ratio for extinction to obtain the differential microlensing signal. Since this may still be significantly affected by variability and time-delay effects, no detailled analysis of the microlensing is possible at the moment. This is the first time that differential extinction and microlensing could be separated unambiguously. We show that, at least in HE0512-3329, both effects contribute significantly to the spectral differences and one cannot be analysed without taking into account the other. For lens modelling purposes, the flux ratios can only be used after correcting for both effects.

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