arXiv subjects

K. Boutsia

Publications and source records attributed to K. Boutsia.

At least 55 records · Page 3Linked to original sources

arXiv2014

The galaxy stellar mass function at 3.5<z<7.5 in the CANDELS/UDS, GOODS-South, and HUDF fields

The galaxy stellar mass function (GSMF) at high-z provides key information on star-formation history and mass assembly in the young Universe. We aimed to use the unique combination of deep optical/NIR/MIR imaging provided by HST, Spitzer and the VLT in the CANDELS-UDS, GOODS-South, and HUDF fields to determine the GSMF over the redshift range 3.5 4. These results confirm the unique synergy of the CANDELS+HUDF, HUGS, and SEDS surveys for the discovery and study of moderate/low-mass galaxies at high redshifts.

astro-ph.GA↗

arXiv2014

The Hawk-I UDS and GOODS Survey (HUGS): Survey design and deep K-band number counts

We present the results of a new, ultra-deep, near-infrared imaging survey executed with the Hawk-I imager at the ESO VLT, of which we make all the data public. This survey, named HUGS (Hawk-I UDS and GOODS Survey), provides deep, high-quality imaging in the K and Y bands over the CANDELS UDS and GOODS-South fields. We describe here the survey strategy, the data reduction process, and the data quality. HUGS delivers the deepest and highest quality K-band images ever collected over areas of cosmological interest, and ideally complements the CANDELS data set in terms of image quality and depth. The seeing is exceptional and homogeneous, confined to the range 0.38"-0.43". In the deepest region of the GOODS-S field, (which includes most of the HUDF) the K-band exposure time exceeds 80 hours of integration, yielding a 1-sigma magnitude limit of ~28.0 mag/sqarcsec. In the UDS field the survey matches the shallower depth of the CANDELS images reaching a 1-sigma limit per sq.arcsec of ~27.3mag in the K band and ~28.3mag in the Y-band, We show that the HUGS observations are well matched to the depth of the CANDELS WFC3/IR data, since the majority of even the faintest galaxies detected in the CANDELS H-band images are also detected in HUGS. We present the K-band galaxy number counts produced by combining the HUGS data from the two fields. We show that the slope of the number counts depends sensitively on the assumed distribution of galaxy sizes, with potential impact on the estimated extra-galactic background light (abridged).

astro-ph.GA↗

arXiv2014

Scaling relations of cluster elliptical galaxies at z~1.3. Distinguishing luminosity and structural evolution

[Abridged] We studied the size-surface brightness and the size-mass relations of a sample of 16 cluster elliptical galaxies in the mass range 10^{10}-2x10^{11} M_sun which were morphologically selected in the cluster RDCS J0848+4453 at z=1.27. Our aim is to assess whether they have completed their mass growth at their redshift or significant mass and/or size growth can or must take place until z=0 in order to understand whether elliptical galaxies of clusters follow the observed size evolution of passive galaxies. To compare our data with the local universe we considered the Kormendy relation derived from the early-type galaxies of a local Coma Cluster reference sample and the WINGS survey sample. The comparison with the local Kormendy relation shows that the luminosity evolution due to the aging of the stellar content already assembled at z=1.27 brings them on the local relation. Moreover, this stellar content places them on the size-mass relation of the local cluster ellipticals. These results imply that for a given mass, the stellar mass at z~1.3 is distributed within these ellipticals according to the same stellar mass profile of local ellipticals. We find that a pure size evolution, even mild, is ruled out for our galaxies since it would lead them away from both the Kormendy and the size-mass relation. If an evolution of the effective radius takes place, this must be compensated by an increase in the luminosity, hence of the stellar mass of the galaxies, to keep them on the local relations. We show that to follow the Kormendy relation, the stellar mass must increase as the effective radius. However, this mass growth is not sufficient to keep the galaxies on the size-mass relation for the same variation in effective radius. Thus, if we want to preserve the Kormendy relation, we fail to satisfy the size-mass relation and vice versa.

astro-ph.GA↗

arXiv2014

Deep R-band counts of z~3 Lyman break galaxy candidates with the LBT

Aims. We present a deep multiwavelength imaging survey (UGR) in 3 different fields, Q0933, Q1623, and COSMOS, for a total area of ~1500arcmin^2. The data were obtained with the Large Binocular Camera on the Large Binocular Telescope. Methods. To select our Lyman break galaxy (LBG) candidates, we adopted the well established and widely used color-selection criterion (U-G vs. G-R). One of the main advantages of our survey is that it has a wider dynamic color range for U-dropout selection than in previous studies. This allows us to fully exploit the depth of our R-band images, obtaining a robust sample with few interlopers. In addition, for 2 of our fields we have spectroscopic redshift information that is needed to better estimate the completeness of our sample and interloper fraction. Results. Our limiting magnitudes reach 27.0(AB) in the R band (5σ) and 28.6(AB) in the U band (1σ). This dataset was used to derive LBG candidates at z~3. We obtained a catalog with a total of 12264 sources down to the 50% completeness magnitude limit in the R band for each field. We find a surface density of ~3 LBG candidates arcmin^2 down to R=25.5, where completeness is >=95% for all 3 fields. This number is higher than the original studies, but consistent with more recent samples.

astro-ph.CO↗

arXiv2013

Diffuse optical intracluster light as a measure of stellar tidal stripping: the cluster CL0024+17 at $z\sim $0.4 observed at LBT

We have evaluated the diffuse intracluster light (ICL) in the central core of the galaxy cluster CL0024+17 at $z\sim 0.4$ observed with the prime focus camera (LBC) at LBT. The measure required an accurate removal of the galaxies light within $\sim 200$ kpc from the center. The residual background intensity has then been integrated in circular apertures to derive the average ICL intensity profile. The latter shows an approximate exponential decline as expected from theoretical cold dark matter models. The radial profile of the ICL over the galaxies intensity ratio (ICL fraction) is increasing with decreasing radius but near the cluster center it starts to bend and then decreases where the overlap of the halos of the brightest cluster galaxies becomes dominant. Theoretical expectations in a simplified CDM scenario show that the ICL fraction profile can be estimated from the stripped over galaxy stellar mass ratio in the cluster. It is possible to show that the latter quantity is almost independent of the properties of the individual host galaxies but mainly depends on the average cluster properties. The predicted ICL fraction profile is thus very sensitive to the assumed CDM profile, total mass and concentration parameter of the cluster. Adopting values very similar to those derived from the most recent lensing analysis in CL0024+17 we find a good agreement with the observed ICL fraction profile. The galaxy counts in the cluster core have then been compared with that derived from composite cluster samples in larger volumes, up to the clusters virial radius. The galaxy counts in the CL0024+17 core appear flatter and the amount of bending respect to the average cluster galaxy counts imply a loss of total emissivity in broad agreement with the measured ICL fraction.

astro-ph.CO↗

arXiv2012

LBT observations of the HR 8799 planetary system: First detection of HR8799e in H band

We have performed H and Ks band observations of the planetary system around HR 8799 using the new AO system at the Large Binocular Telescope and the PISCES Camera. The excellent instrument performance (Strehl ratios up to 80% in H band) enabled detection the inner planet HR8799e in the H band for the first time. The H and Ks magnitudes of HR8799e are similar to those of planets c and d, with planet e slightly brighter. Therefore, HR8799e is likely slightly more massive than c and d. We also explored possible orbital configurations and their orbital stability. We confirm that the orbits of planets b, c and e are consistent with being circular and coplanar; planet d should have either an orbital eccentricity of about 0.1 or be non-coplanar with respect to b and c. Planet e can not be in circular and coplanar orbit in a 4:2:1 mean motion resonances with c and d, while coplanar and circular orbits are allowed for a 5:2 resonance. The analysis of dynamical stability shows that the system is highly unstable or chaotic when planetary masses of about 5 MJup for b and 7 MJup for the other planets are adopted. Significant regions of dynamical stability for timescales of tens of Myr are found when adopting planetary masses of about 3.5, 5, 5, and 5 Mjup for HR 8799 b, c, d, and e respectively. These masses are below the current estimates based on the stellar age (30 Myr) and theoretical models of substellar objects.

astro-ph.EP↗

arXiv2012

High Resolution Images of Orbital Motion in the Orion Trapezium Cluster with the LBT Adaptive Optics System

The new 8.4m LBT adaptive secondary AO system, with its novel pyramid wavefront sensor, was used to produce very high Strehl (75% at 2.16 microns) near infrared narrowband (Br gamma: 2.16 microns and [FeII]: 1.64 microns) images of 47 young (~1 Myr) Orion Trapezium theta1 Ori cluster members. The inner ~41x53" of the cluster was imaged at spatial resolutions of ~0.050" (at 1.64 microns). A combination of high spatial resolution and high S/N yielded relative binary positions to ~0.5 mas accuracies. Including previous speckle data, we analyse a 15 year baseline of high-resolution observations of this cluster. We are now sensitive to relative proper motions of just ~0.3 mas/yr (0.6 km/s at 450 pc) this is a ~7x improvement in orbital velocity accuracy compared to previous efforts. We now detect clear orbital motions in the theta1 Ori B2/B3 system of 4.9+/-0.3 km/s and 7.2+/-0.8 km/s in the theta1 Ori A1/A2 system (with correlations of PA vs. time at >99% confidence). All five members of the theta1 Ori B system appear likely as a gravitationally bound "mini-cluster". The very lowest mass member of the theta1 Ori B system (B4; mass ~0.2 Msun) has, for the first time, a clearly detected motion (at 4.3+/-2.0 km/s; correlation=99.7%) w.r.t B1. However, B4 is most likely in an long-term unstable (non-hierarchical) orbit and may "soon" be ejected from this "mini-cluster". This "ejection" process could play a major role in the formation of low mass stars and brown dwarfs.

astro-ph.SR↗

arXiv2012

The blue UV slopes of z~4 Lyman break galaxies: implications for the corrected star formation rate density

We plan to analyse dust extinction in Lyman Break Galaxies (LBGs) by introducing a new and more reliable approach to their selection and to the characterization of their distribution of UV slopes beta, using deep IR images from HST. We exploit deep WFC3 IR observations of the ERS and HUDF fields over GOODS-South, combined with HST-ACS optical data, to select z~4 LBGs through a new (B-V) vs. (V-H) colour diagram. The UV slope of the selected galaxies is robustly determined by a linear fit over their observed I, Z, Y, J magnitudes, coherently with the original definition of beta. The same fit is used to determine their rest-frame UV magnitudes M1600 through a simple interpolation. We estimate the effect of observational uncertainties with detailed simulations that we also exploit, under a parametric maximum-likelihood approach, to constrain the probability density function of UV slopes PDF(beta) as a function of rest-frame magnitude. We find 142 and 25 robust LBGs in the ERS and HUDF fields respectively, limiting our sample to S/N(H)>10 objects. Our newly defined criteria improve the selection of z~4 LBGs and allow us to exclude red interlopers at lower redshift, especially z~3-3.5 objects. We find that z~4 LBGs are characterized by blue UV slopes, suggesting a low dust extinction: all L \simeq-2.1, while brighter objects only are slightly redder ( -1.9). We find an intrinsic dispersion ~ 0.3 for PDF(beta) at all magnitudes. The SFRD at z~4 corrected according to these estimates turns out to be lower than previously found: log(SFRD)\simeq-1.09 M_sun/yr/Mpc^3. Finally, we discuss how the UV slope of z~4 galaxies changes as a function of the dust-corrected UV magnitude (i.e. SFR) showing that most galaxies with a high SFR (> 80 M_sun/yr) are highly extincted objects. [Abridged]

astro-ph.CO↗

arXiv2012

The evolving slope of the stellar mass function at 0.6 <= z < 4.5 from deep WFC3 data

We used Early Release Science (ERS) observations taken with the Wide Field Camera 3 (WFC3) in the GOODS-S field to study the galaxy stellar mass function (GSMF) at 0.6<=z<4.5. Deep WFC3 near-IR data (for Y as faint as 27.3, J and H as faint as 27.4 AB mag at 5 sigma), as well as deep Ks (as faint as 25.5 at 5 sigma) Hawk-I band data, provide an exquisite data set with which determine in an unprecedented way the low-mass end of the GSMF, allowing an accurate probe of masses as low as M~7.6 10^9 Msun at z~3. Although the area used is relatively small (~33 arcmin^2), we found generally good agreement with previous studies on the entire mass range. Our results show that the slope of the faint-end increases with redshift, from alpha=-1.44+/-0.03 at z~0.8 to alpha=-1.86+/-0.16 at z~3, although indications exist that it does not steepen further between z~3 and z~4. This result is insensitive to any uncertainty in the M* parameter. The steepness of the GSMF faint-end solves the well-known disagreement between the stellar mass density (SMD) and the integrated star formation history at z>2. However, we confirm the that there appears to be an excess of integrated star formation with respect to the SMD at z<2, by a factor of ~2-3. Our comparison of the observations with theoretical predictions shows that the models forecast a greater abundance of low mass galaxies, at least up to z~3, as well as a dearth of massive galaxies at z~4 with respect to the data, and that the predicted SMD is generally overestimated at z<~2.

astro-ph.CO↗

arXiv2011

Faint high-redshift AGN in the Chandra Deep Field South: the evolution of the AGN luminosity function and black hole demography

We present detection and analysis of faint X-ray sources in the Chandra deep field south (CDFS) using the 4 Msec Chandra observation and adopting a new detection algorithm, based on a targeted search at the position of known high-z galaxies. This optimized technique results in the identification of 54 z>3 AGNs, 29 of which are new detections. Applying stringent completeness criteria, we derive AGN luminosity functions in the redshift bins 3-4, 4-5 and >5.8 and for 42.75 3 (18+17-10%). Their optical counterparts are not strongly reddened and we thus conclude that the size of the X-ray absorber is likely smaller than the dust sublimation radius. We finally report the discovery of a highly star-forming galaxy at z=3.47. If confirmed, this would be one of the farthest objects in which stellar sources are detected in X-rays.

astro-ph.CO↗

arXiv2011

Spectroscopic confirmation of z~7 LBGs: probing the earliest galaxies and the epoch of reionization

We present the final results from our ultra-deep spectroscopic campaign with FORS2 at the ESO/VLT for the confirmation of z~7 "z--band dropout" candidates selected from our VLT/Hawk-I imaging survey over three independent fields. In particular we report on two newly discovered galaxies at redshift ~6.7 in the NTT deep field: both galaxies show a Ly-alpha emission line with rest-frame EWs of the order 15-20 A and luminosities of 2-4 X 10^{42} erg/s. We also present the results of ultra-deep observations of a sample of i-dropout galaxies, from which we set a solid upper limit on the fraction of interlopers. Out of the 20 z-dropouts observed we confirm 5 galaxies at 6.7 < z < 7.1. This is systematically below the expectations drawn on the basis of lower redshift observations: in particular there is a significant lack of objects with intermediate Ly-alpha EWs (between 20 and 55 A). We conclude that the trend for the fraction of Ly-alpha emission in LBGs that is constantly increasing from z~3 to z~6 is most probably reversed from z~6 to z~7. Explaining the observed rapid change in the LAE fraction among the drop-out population with reionization requires a fast evolution of the neutral fraction of hydrogen in the Universe. Assuming that the Universe is completely ionized at z=6 and adopting the semi-analytical models of Dijkstra et al. (2011), we find that our data require a change of the neutral hydrogen fraction of the order Delta chi_{HI} ~ 0.6 in a time Delta z ~ 1, provided that the escape fraction does not increase dramatically over the same redshift interval.

astro-ph.CO↗

arXiv2011

A low escape fraction of ionizing photons of L>L* Lyman break galaxies at z=3.3

We present an upper limit for the relative escape fraction (f_{esc}^{rel}) of ionizing radiation at z~3.3 using a sample of 11 Lyman Break Galaxies (LBGs) with deep imaging in the U band obtained with the Large Binocular Camera, mounted on the prime focus of the Large Binocular Telescope. We selected 11 LBGs with secure redshift in the range 3.27 =30.7(AB)mag at 1 sigma. We derive a 1 sigma upper limit of f_{esc}^{rel}~5%, which is one of the lowest values found in the literature so far at z~3.3. Assuming that the upper limit for the escape fraction that we derived from our sample holds for all galaxies at this redshift, the hydrogen ionization rate that we obtain (Gamma_{-12}<0.3 s^{-1}) is not enough to keep the IGM ionized and a substantial contribution to the UV background by faint AGNs is required. Since our sample is clearly still limited in size, larger z~3 LBG samples, at similar or even greater depths are necessary to confirm these results on a more firm statistical basis.

astro-ph.GA↗

arXiv2011

A critical analysis of the UV Luminosity Function at redshift~7 from deep WFC3 data

The study of the Luminosity Function (LF) of Lyman Break Galaxies (LBGs) at z=7 is important for ascertaining their role in the reionization of the Universe. We perform a detailed and critical analysis of the statistical and systematic errors in the z~7 LF determination: we have assembled a large sample of candidate LBGs at z~7 from different surveys, spanning a large variety of areas and depths. In particular, we have combined data from the deep (J<27.4) and ultradeep (J<29.2) surveys recently acquired with the new WFC3 NIR camera on HST, over the GOODS-ERS and the HUDF fields, with ground based surveys in wide and shallow areas from VLT and Subaru. We have used public ACS images in the z-band to select z-dropout galaxies, and other public data both in the blue (BVI) and in the red bands to reject possible low-redshift interlopers. We have compared our results with extensive simulations to quantify the observational effects of our selection criteria as well as the effects of photometric scatter, color selections or the morphology of the candidates. We have found that the number density of faint LBGs at z~7 is only marginally sensitive to the color selection adopted, but it is strongly dependent from the assumption made on the half light distributions of the simulated galaxies, used to correct the observed sample for incompleteness. The slope of the faint end of the LBGs LF has thus a rather large uncertainty, due to the unknown distribution of physical sizes of the z~7 LBGs. We conclude that galaxies at z~7 are unable to reionize the Universe unless there is a significant evolution in the clumpiness of the IGM or in the escape fraction of ionising photons or, alternatively, there is a large population of z~7 LBGs with large physical dimensions but still not detected by the present observations.

astro-ph.CO↗

arXiv2011

Spectroscopic confirmation of two Lyman break galaxies at redshift beyond 7

We report the spectroscopic confirmation of two Lyman break galaxies at redshift > 7. The galaxies were observed as part of an ultra-deep spectroscopic campaign with FORS2 at the ESO/VLT for the confirmation of z~7 ``z--band dropout'' candidates selected from our VLT/Hawk-I imaging survey. Both galaxies show a prominent emission line at 9735A and 9858A respectively: the lines have fluxes of ~ 1.6-1.2 x 10^(-17) erg/s/cm2 and exhibit a sharp decline on the blue side and a tail on the red side. The asymmetry is quantitatively comparable to the observed asymmetry in z ~ 6 Ly-alpha lines, where absorption by neutral hydrogen in the IGM truncates the blue side of the emission line profile. We carefully evaluate the possibility that the galaxies are instead at lower redshift and we are observing either [OII], [OIII] or H-alpha emission: however from the spectroscopic and the photometric data we conclude that there are no other plausible identifications, except for Ly-alpha at redshift >7, implying that these are two of the most robust redshift determination for galaxies in the reionization epoch. Based on their redshifts and broad--band photometry, we derive limits on the star formation rate and on the ultraviolet spectral slopes of the two galaxies. We argue that these two galaxies alone are unlikely to have ionized the IGM in their surroundings.

astro-ph.CO↗

arXiv2010

The lack of intense Lyman~alpha in ultradeep spectra of z=7 candidates in GOODS-S: imprint of reionization?

We present ultradeep optical spectroscopy obtained with FORS2 on VLT of seven Lyman-break galaxy (LBG) candidates at z>6.5 selected in the GOODS-S field from Hawk-I/VLT and WFC3/HST imaging. For one galaxy we detect a low significance emission line (S/N< 7), located at 9691.5 +/- 0.5A and with flux 3.4 x 10^(-18)erg/cm^2/s. If identified as Lyman alpha, it places the LBG at redshift z=6.972+/- 0.002, with a rest-frame equivalent width EW}=13A. Using Monte Carlo simulations and conservative EW distribution functions at 2 10 is ~ 2%, and that of observing only one galaxy out of seven with S/N=5 is ~4%, but these can be as small as ~1E-3, depending on the details of the EW distribution. We conclude that either a significant fraction of the candidates is not at high redshift or that some physical mechanism quenches the Lyman alpha emission emerging from the galaxies at z>6.5, abruptly reversing the trend of the increasing fraction of strong emitters with increasing redshift observed up to z~ 6.5. We discuss the possibility that an increasingly neutral intergalactic medium is responsible for such quenching.

astro-ph.CO↗

arXiv2010

The Great Observatories Origins Deep Survey: Constraints on the Lyman Continuum Escape Fraction Distribution of Lyman--Break Galaxies at 3.4<z<4.5

We use ultra-deep ultraviolet VLT/VIMOS intermediate-band and VLT/FORS1 narrow-band imaging in the GOODS Southern field to derive limits on the distribution of the escape fraction (f_esc) of ionizing radiation for L >~ L*(z=3) Lyman Break Galaxies (LBGs) at redshift 3.4--4.5. Only one LBG, at redshift z=3.795, is detected in its Lyman continuum (LyC; S/N~5.5), the highest redshift galaxy currently known with a direct detection. Its ultraviolet morphology is quite compact (R_eff=0.8, kpc physical). Three out of seven AGN are also detected in their LyC, including one at redshift z=3.951 and z850 = 26.1. From stacked data (LBGs) we set an upper limit to the average f_esc in the range 5%--20%, depending on the how the data are selected (e.g., by magnitude and/or redshift). We undertake extensive Monte Carlo simulations that take into account intergalactic attenuation, stellar population synthesis models, dust extinction and photometric noise in order to explore the moments of the distribution of the escaping radiation. Various distributions (exponential, log-normal and Gaussian) are explored. We find that the median f_esc is lower than ~6% with an 84% percentile limit not larger than 20%. If this result remains valid for fainter LBGs down to current observational limits, then the LBG population might be not sufficient to account for the entire photoionization budget at the redshifts considered here, with the exact details dependent upon the assumed ionizing background and QSO contribution thereto. It is possible that f_esc depends on the UV luminosity of the galaxies, with fainter galaxies having higher f_esc, and estimates of f_esc from a sample of faint LBG from the HUDF (i775<28.5) are in broad quantitative agreement with such a scenario.

astro-ph.CO↗

arXiv2010

The bright end of the z ~ 7 UV Luminosity Function from a wide and deep HAWK-I survey

(Abridged) We present here the second half of an ESO Large Programme, which exploits the unique combination of area and sensitivity provided in the near-IR by the camera Hawk-I at the VLT. We have obtained - 30 observing hours with Hawk-I in the Y-band of two high galactic latitude fields. We combined the Y-band data with deep J and K Hawk-I observations, and with FORS1/FORS2 U, B, V, R, I, and Z observations to select z-drop galaxies having Z - Y > 1, no optical detection and flat Y - J and Y - K colour terms. We detect 8 high-quality candidates in the magnitude range Y = 25.5 - 26.5 that we add to the z-drop candidates selected in two Hawk-I pointings over the GOODS-South field. We use this full sample of 15 objects found in -161 arcmin^2 of our survey to constrain the average physical properties and the evolution of the number density of z ~ 7 LBGs. A stacking analysis yields a best-fit SED with photometric redshift z= 6.85 +0.20 -0.15 and an E(B-V)=0.05 +0.15 -0.05. We compute a binned estimate of the z ~ 7 LF and explore the effects of photometric scatter and model uncertainties on the statistical constraints. After accounting for the expected incompleteness through MonteCarlo simulations, we strengthen our previous finding that a Schechter luminosity function constant from z=6 to z=7 is ruled out at a >99% confidence level, even including the effects of cosmic variance. For galaxies brighter than M_1500= -19.0, we derive a luminosity density rho_UV = 1.5^{+2.1}{-0.8} x 10^25 erg/s/Hz/Mpc^3, implying a decrease by a factor 3.5 from z=6 to z=6.8. We find that, under standard assumptions, the emission rate of ionizing photons coming from UV bright galaxies is lower by at least a factor of two than the value required for reionization. Finally, we exploit deep Hawk-I J and K band observations to derive an upper limit on the number density of M1500<~ -22.0 LBGs at z-8 (Y-dropouts).

astro-ph.CO↗

arXiv2010

Selecting AGN through variability in SN datasets

Variability is a main property of active galactic nuclei (AGN) and it was adopted as a selection criterion using multi epoch surveys conducted for the detection of supernovae (SNe). We have used two SN datasets. First we selected the AXAF field of the STRESS project, centered in the Chandra Deep Field South where, besides the deep X-ray surveys also various optical catalogs exist. Our method yielded 132 variable AGN candidates. We then extended our method including the dataset of the ESSENCE project that has been active for 6 years, producing high quality light curves in the R and I bands. We obtained a sample of ~4800 variable sources, down to R=22, in the whole 12 deg^2 ESSENCE field. Among them, a subsample of ~500 high priority AGN candidates was created using as secondary criterion the shape of the structure function. In a pilot spectroscopic run we have confirmed the AGN nature for nearly all of our candidates.

astro-ph.GA↗