Papers by Giovanni Covone
arXiv preprint astro-ph/0601291, Jan 13, 2006
Abstract: We present an integral field spectroscopy survey of rich clusters of galaxies aimed at ... more Abstract: We present an integral field spectroscopy survey of rich clusters of galaxies aimed at studying their lensing properties. Thanks to knowledge of the spectroscopic caracteristics of more than three families of multiple images in a single lens, one is able in principle to derive constraints on the geometric cosmological parameters. We show that this ambitious program is feasible and present some new results, in particular the redshift measurement of the giant arc in A2667 and the resdshift confirmation of the counter-image of the radial arc ...
N-body + hydrodynamical simulations of formation and evolution of galaxy groups in a ΛCDM cosmolo... more N-body + hydrodynamical simulations of formation and evolution of galaxy groups in a ΛCDM cosmology have been performed. The properties of the galaxy populations in 12 groups are here discussed, with focus on the colour-magnitude relation in both normal and fossil groups.
Astronomy and Astrophysics
The only way to detect planets around stars at distances several kpc is by (photometric or astrom... more The only way to detect planets around stars at distances several kpc is by (photometric or astrometric) microlensing (µL) observations. In this paper, we show that the capability of photometric µL extends to the detection of signals caused by planets around stars in nearby galaxies (e.g. M31) and that there is no other method that can achieve this. Due to the large crowding, µL experiments towards M31 can only observe the high-magnification part of a lensing light curve. Therefore, the dominating channel for µL signals by planets is in distortions near the peak of high-magnification events as discussed by ). We calculate the probability to detect planetary anomalies for µL experiments towards M31 and find that jupiter-like planets around stars in M31 can be detected. Though the characterization of the planet(s) involved in this signal will be difficult, the absence of such signals can yield strong constraints on the abundance of jupiter-like planets.
Monthly Notices of the Royal Astronomical Society, 2014
We investigate the evolution of dark and luminous matter in the central regions of early-type gal... more We investigate the evolution of dark and luminous matter in the central regions of early-type galaxies (ETGs) up to z ∼ 0.8. We use a spectroscopically selected sample of 154 cluster and field galaxies from the EDisCS survey, covering a wide range in redshifts (z ∼ 0.4-0.8), stellar masses (log M ⋆ /M ⊙ ∼ 10.5-11.5 dex) and velocity dispersions (σ ⋆ ∼ 100-300 km/s). We obtain central dark matter (DM) fractions by determining the dynamical masses from Jeans modelling of galaxy aperture velocity dispersions and the M ⋆ from galaxy colours, and compare the results with local samples. We discuss how the correlations of central DM with galaxy size (i.e. the effective radius, R e ), M ⋆ and σ ⋆ evolve as a function of redshift, finding clear indications that local galaxies are, on average, more DM dominated than their counterparts at larger redshift. This DM fraction evolution with z can be only partially interpreted as a consequence of the size-redshift evolution. We discuss our results within galaxy formation scenarios, and conclude that the growth in size and DM content which we measure within the last 7 Gyr is incompatible with passive evolution, while it is well reproduced in the multiple minor merger scenario. We also discuss the impact of the IMF on our DM inferences and argue that this can be non-universal with the lookback time. In particular, we find the Salpeter IMF can be better accommodated by low redshift systems, while producing stellar masses at high-z which are unphysically larger than the estimated dynamical masses (particularly for lower-σ ⋆ systems).
Astronomy & Astrophysics, 2015
Context. One of the most peculiar characteristics of Active Galactic Nuclei (AGN) is their variab... more Context. One of the most peculiar characteristics of Active Galactic Nuclei (AGN) is their variability over all wavelengths. This property has been used in the past to select AGN samples and is foreseen to be one of the detection techniques applied in future multi-epoch surveys, complementing photometric and spectroscopic methods. Aims. In this paper, we aim to construct and characterise an AGN sample using a multi-epoch dataset in the r band from the SUDARE-VOICE survey. Methods. Our work makes use of the VST monitoring program of an area surrounding the Chandra Deep Field South to select variable sources. We use data spanning a six month period over an area of 2 square degrees, to identify AGN based on their photometric variability.
Monthly Notices of the Royal Astronomical Society, 2015
The joint analysis of clustering and stacked gravitational lensing of galaxy clusters in large su... more The joint analysis of clustering and stacked gravitational lensing of galaxy clusters in large surveys can constrain the formation and evolution of structures and the cosmological parameters. On scales outside a few virial radii, the halo bias, b, is linear and the lensing signal is dominated by the correlated distribution of matter around galaxy clusters. We discuss a method to measure the power spectrum amplitude σ 8 and b based on a minimal modelling. We considered a sample of ∼ 120000 clusters photometrically selected from the Sloan Digital Sky Survey in the redshift range 0.1 < z < 0.6. The auto-correlation was studied through the two-point function of a subsample of ∼ 70000 clusters; the matter-halo correlation was derived from the weak lensing signal of the subsample of ∼ 1200 clusters with Canada-France-Hawaii Lensing Survey data. We obtained a direct measurement of b, which increases with mass in agreement with predictions of the ΛCDM paradigm. Assuming Ω M = 0.3, we found σ 8 = 0.78 ± 0.17. We used the same clusters for measuring both lensing and clustering and the estimate of σ 8 did require neither the mass-richness relation, nor the knowledge of the selection function, nor the modelling of b. With an additional theoretical prior on the bias, we obtained σ 8 = 0.80 ± 0.10.
Astronomy & Astrophysics, 2015
Context. The statistics of peaks in weak lensing convergence maps is a promising tool to investig... more Context. The statistics of peaks in weak lensing convergence maps is a promising tool to investigate both the properties of dark matter haloes and constrain the cosmological parameters. Aims. We study how the number of detectable peaks and its scaling with redshift depend upon the cluster dark matter halo profiles and use peak statistics to constrain the parameters of the mass -concentration (MC) relation. We investigate which constraints the Euclid mission can set on the MC coefficients also taking into account degeneracies with the cosmological parameters. Methods. To this end, we first estimate the number of peaks and its redshift distribution for different MC relations. We find that the steeper the mass dependence and the larger the normalisation, the higher is the number of detectable clusters, with the total number of peaks changing up to 40% depending on the MC relation. We then perform a Fisher matrix forecast of the errors on the MC relation parameters as well as cosmological parameters. Results. We find that peak number counts detected by Euclid can determine the normalization A v , the mass B v and redshift C v slopes and intrinsic scatter σ v of the MC relation to an unprecedented accuracy being
Proceedings of the International Astronomical Union, 2006
We present a study of the morphologies and stellar content of the galaxies in the central ∼ 74" ×... more We present a study of the morphologies and stellar content of the galaxies in the central ∼ 74" × 64" region of the galaxy cluster Abell 2218 (see ). The spectroscopic data were obtained with the integral field unit PMAS in the PPAK mode , at the 3.5 m telescope of the Calar Alto Observatory. The covered wavelength range was 4650-8000Å, with a spectral resolution FWHM ∼ 10Å. These data were combined with with deep HST/ACS F475W, F555W, F625W and F850LP imaging, and additional data from the literature.
Monthly Notices of the Royal Astronomical Society, 2014
Hubble Space Telescope imaging of the galaxy cluster Abell 1689 has revealed an exceptional numbe... more Hubble Space Telescope imaging of the galaxy cluster Abell 1689 has revealed an exceptional number of strongly lensed multiply-imaged galaxies, including high-redshift candidates. Previous studies have used this data to obtain the most detailed dark matter reconstructions of any galaxy cluster to date, resolving substructures 25 kpc across. We examine Abell 1689 (hereafter, A1689) non-parametrically, combining strongly lensed images and weak distortions from wider field Subaru imaging, and we incorporate member galaxies to improve the lens solution. Strongly lensed galaxies are often locally affected by member galaxies, however, these perturbations cannot be recovered in grid based reconstructions because the lensing information is too sparse to resolve member galaxies. By adding luminosity-scaled member galaxy deflections to our smooth grid we can derive meaningful solutions with sufficient accuracy to permit the identification of our own strongly lensed images, so our model becomes self consistent. We identify 11 new multiply lensed system candidates and clarify previously ambiguous cases, in the deepest optical and NIR data to date from Hubble and Subaru. Our improved spatial resolution brings up new features not seen when the weak and strong lensing effects are used separately, including clumps and filamentary dark matter around the main halo. Our treatment means we can obtain an objective mass ratio between the cluster and galaxy components, for examining the extent of tidal stripping of the luminous member galaxies. We find a typical massto-light ratios of M/L B = 21 ± 14 inside the r < 1 arcminute region that drops to M/L B = 17 ± 8 inside the r < 40 arcsecond region. Our model independence means we can objectively evaluate the competitiveness of stacking cluster lenses for defining the geometric lensing-distance-redshift relation in a model independent way.
Astronomy & Astrophysics, 2015
Context. Active galaxies are characterized by variability at every wavelength, with timescales fr... more Context. Active galaxies are characterized by variability at every wavelength, with timescales from hours to years depending on the observing window. Optical variability has proven to be an effective way of detecting AGNs in imaging surveys, lasting from weeks to years. Aims. In the present work we test the use of optical variability as a tool to identify active galactic nuclei in the VST multiepoch survey of the COSMOS field, originally tailored to detect supernova events. Methods. We make use of the multiwavelength data provided by other COSMOS surveys to discuss the reliability of the method and the nature of our AGN candidates. Results. The selection on the basis of optical variability returns a sample of 83 AGN candidates; based on a number of diagnostics, we conclude that 67 of them are confirmed AGNs (81% purity), 12 are classified as supernovae, while the nature of the remaining 4 is unknown. For the subsample of AGNs with some spectroscopic classification, we find that Type 1 are prevalent (89%) compared to Type 2 AGNs (11%). Overall, our approach is able to retrieve on average 15% of all AGNs in the field identified by means of spectroscopic or X-ray classification, with a strong dependence on the source apparent magnitude (completeness ranging from 26% to 5%). In particular, the completeness for Type 1 AGNs is 25%, while it drops to 6% for Type 2 AGNs. The rest of the X-ray selected AGN population presents on average a larger r.m.s. variability than the bulk of non-variable sources, indicating that variability detection for at least some of these objects is prevented only by the photometric accuracy of the data. The low completeness is in part due to the short observing span: we show that increasing the temporal baseline results in larger samples as expected for sources with a red-noise power spectrum. Our results allow us to assess the usefulness of this AGN selection technique in view of future wide-field surveys. We thank Marcella Brusa, Francesca Civano and Stefano Marchesi for providing access to updated versions of the COSMOS catalogs. We also thank the anonymous referee for helpful comments and suggestions.
We present the on-going observational program of a VIMOS Integral Field Unit survey of the centra... more We present the on-going observational program of a VIMOS Integral Field Unit survey of the central regions of massive, gravitational lensing galaxy clusters at redshift z ≃ 0.2. We have observed six clusters using the low-resolution blue grism (R ≃ 200), and the spectroscopic survey is complemented by a wealth of photometric data, including Hubble Space Telescope optical data and near infrared VLT data. The principal scientific aims of this project are: the study of the high-z lensed galaxies, the transformation and evolution of galaxies in cluster cores and the use of multiple images to constrain cosmography. We briefly report here on the first results from this project on the clusters Abell 2667 and Abell 68.
The Astronomical Journal, 2014
We present a new analysis of HST, Spitzer telescope imaging and VLT imaging and spectroscopic dat... more We present a new analysis of HST, Spitzer telescope imaging and VLT imaging and spectroscopic data of a bright lensed galaxy at z=1.0334 in the lensing cluster Abell 2667. Using this high-resolution imaging we present an updated lens model that allows us to fully understand the lensing geometry and reconstruct the lensed galaxy in the source plane. This giant arc gives a unique opportunity to peer into the structure of a high-redshift disk galaxy. We find that the lensed galaxy of Abell 2667 is a typical spiral galaxy with morphology similar to the structure of its counterparts at higher redshift z ∼ 2. The surface brightness of the reconstructed source galaxy in the z 850 band reveals the central surface brightness I(0) = 20.28 ± 0.22 mag arcsec −2 and the characteristic radius r s = 2.01±0.16 kpc at redshift z ∼ 1. The morphological reconstruction in different bands shows obvious negative radial color gradients for this galaxy. Moreover, the redder central bulge tends to contain a metal-rich stellar population, rather than being heavily reddened by dust due to high and patchy obscuration. We analyze the VIMOS/IFU spectroscopic data and find that, in the given wavelength range (∼ 1800 − 3200Å), the combined arc spectrum of the source galaxy is characterized by a strong continuum emission with strong UV absorption lines (FeII and MgII) and shows the features of a typical starburst Wolf-Rayet galaxy
The Astrophysical Journal, 2007
We have carried out an extensive spectroscopic survey with the Keck and VLT telescopes, targeting... more We have carried out an extensive spectroscopic survey with the Keck and VLT telescopes, targeting lensed galaxies in the background of the massive cluster Abell 68. Spectroscopic measurements are obtained for 26 lensed images, including a distant galaxy at z = 5.4. Redshifts have been determined for 5 out of 7 multiply-image systems. Through a careful modeling of the mass distribution in the strongly-lensed regime, we derive a mass estimate of 5.3×10 14 M ⊙ within 500 kpc. Our mass model is then used to constrain the redshift distribution of the remaining multiply-imaged and singly-imaged sources. This enables us to examine the physical properties for a subsample of 7 Lyman-α emitters at 1.7 z 5.5, whose unlensed luminosities of ≃ 10 41 ergs s −1 are fainter than similar objects found in blank fields. Of particular interest is an extended Lyman-α emission region surrounding a highly magnified source at z = 2.6, detected in VIMOS Integral Field Spectroscopy data. The physical scale of the most distant lensed source at z = 5.4 is very small (< 300 pc), similar to the lensed z ∼ 5.6 emitter reported by in Abell 2218. New photometric data available for Abell 2218 allow for a direct comparison between these two unique objects. Our survey illustrates the practicality of using lensing clusters to probe the faint end of the z ∼ 2 − 5 Lyman-α luminosity function in a manner that is complementary to blank field narrow-band surveys.
The Astrophysical Journal, 2009
We study the correlation between the locations of galaxy-galaxy strong lensing candidates and tra... more We study the correlation between the locations of galaxy-galaxy strong lensing candidates and tracers of large-scale structure from both weak lensing or X-ray emission. The COSMOS survey is a unique data set, combining deep, high resolution and contiguous imaging in which strong lenses have been discovered, plus unparalleled multiwavelength coverage. To help interpret the COSMOS data, we have also produced mock COSMOS strong and weak lensing observations, based on ray-tracing through the Millenium simulation. In agreement with the
The Astrophysical Journal, 2005
This is a report of Chandra, XMM-Newton, HST and ARC observations of an extended Xray source at z... more This is a report of Chandra, XMM-Newton, HST and ARC observations of an extended Xray source at z = 0.59. The apparent member galaxies range from spiral to elliptical and are all relatively red (i ′ -K s about 3). We interpret this object to be a fossil group based on the difference between the brightness of the first and second brightest cluster members in the i ′band, and because the rest-frame bolometric X-ray luminosity is about 9.2 × 10 43 h −2 70 erg s −1 . This makes Cl 1205+44 the highest redshift fossil group yet reported. The system also contains a central double-lobed radio galaxy which appears to be growing via the accretion of smaller galaxies. We discuss the formation and evolution of fossil groups in light of the high redshift of Cl 1205+44.
The Astrophysical Journal, 2012
We study the redshift distribution of two samples of early-type gravitational lenses, extracted f... more We study the redshift distribution of two samples of early-type gravitational lenses, extracted from a larger collection of 122 systems, to constrain the cosmological constant in the ΛCDM model and the parameters of a set of alternative dark energy models (XCDM, Dvali-Gabadadze-Porrati and Ricci dark energy models), in a spatially flat universe. The likelihood is maximized for Ω Λ = 0.70 ± 0.09 when considering the sample excluding the Sloan Lens ACS systems (known to be biased toward large image-separation lenses) and no-evolution, and Ω Λ = 0.81 ± 0.05 when limiting to gravitational lenses with image separation ∆θ > 2 ′′ and no-evolution. In both cases, results accounting for galaxy evolution are consistent within 1σ. The present test supports the accelerated expansion, by excluding the null hypothesis (i.e., Ω Λ = 0) at more than 4σ, regardless of the chosen sample and assumptions on the galaxy evolution. A comparison between competitive world models is performed by means of the Bayesian information criterion. This shows that the simplest cosmological constant model that has only one free parameter is still preferred by the available data on the redshift distribution of gravitational lenses. We perform an analysis of the possible systematic effects, finding that the systematic errors due to sample incompleteness, galaxy evolution and model uncertainties approximately equal the statistical errors, with present-day data. We find that the largest sources of systemic errors are the dynamical normalization and the high-velocity cutoff factor, followed by the faint-end slope of the velocity dispersion function.
The Astrophysical Journal, 2011
Understanding the mechanisms that lead dense environments to host galaxies with redder colors, mo... more Understanding the mechanisms that lead dense environments to host galaxies with redder colors, more spheroidal morphologies, and lower star formation rates than field populations remains an important problem. As most candidate processes ultimately depend on host halo mass, accurate characterizations of the local environment, ideally tied to halo mass estimates and spanning a range in halo mass and redshift are needed. In this work, we present and test a rigorous, probabalistic method for assigning galaxies to groups based on precise photometric redshifts and X-ray selected groups drawn from the COSMOS field. The groups have masses in the range 10 13 M 200c /M 10 14 and span redshifts 0 < z < 1. We characterize our selection algorithm via tests on spectroscopic subsamples, including new data obtained at the VLT, and by applying our method to detailed mock catalogs. We find that our group member galaxy sample has a purity of 84% and completeness of 92% within 0.5R 200c . We measure the impact of uncertainties in redshifts and group centering on the quality of the member selection with simulations based on current data as well as future imaging and spectroscopic surveys. As a first application of our new group member catalog which will be made publicly available, we show that member galaxies exhibit a higher quenched fraction compared to the field at fixed stellar mass out to z ∼ 1, indicating a significant relationship between star formation and environment at group scales. We also address the suggestion that dusty star forming galaxies in such groups may impact the high-power spectrum of the cosmic microwave background and find that such a population cannot explain the low power seen in recent SZ measurements.
Research in Astronomy and Astrophysics, 2013
We present a multi-wavelength study of the gravitational lens COSMOS J095930+023427 (z l = 0.892)... more We present a multi-wavelength study of the gravitational lens COSMOS J095930+023427 (z l = 0.892), together with the associated galaxy group located at z ∼ 0.7 along the line of sight and the lensed background galaxy. The source redshift is currently unknown, but estimated to be at z s ∼ 2. The analysis is based on the available public HST, Subaru, Chandra imaging data, and VLT spectroscopy. The lensing system is an early-type galaxy showing a strong [OII] emission line, and produces 4 bright images of the distant background source. It has an Einstein radius of 0.79 ′′ , about 4 times large than the effective radius. We perform a lensing analysis using both a Singular Isothermal Ellipsoid (SIE) and a Peudo-Isothermal Elliptical Mass Distribution (PIEMD) for the lensing galaxy, and find that the final results on the total mass, the dark matter (DM) fraction within the Einstein radius and the external shear due to a foreground galaxy group are robust with respect of the choice of the parametric model and the source redshift (yet unknown). We measure the luminous mass from the photometric data, and find the DM fraction within the Einstein radius f DM to be between 0.71 ± 0.13 and 0.79 ± 0.15, depending on the unknown source redshift. Meanwhile, the non-null external shear found in our lensing models supports the presence and structure of a galaxy group at z ∼ 0.7, and an independent measurement of the 0.5-2 keV X-ray luminosity within 20" around the X-ray centroid provides a group mass of M = (3 − 10) × 10 13 M ⊙ , in good agreement with the previous estimate derived through weak lensing analysis. Finally, by inverting the HST/ACS I 814 image with the lensing equation, we obtain the reconstructed image of the magnified source galaxy, which has a scale of about 3.3 kpc at z s = 2 (2.7 kpc at z s = 4) and the typical disturbed disk-like appearance observed in low-mass star-forming galaxies at z ∼ 3. However, deep, spatially resolved spectroscopic data for similar lensed sources are still required to detected the first stage of galaxy evolution, since the available spectrum shows no clear feature due to the background source.
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Papers by Giovanni Covone