A. Moorwood

European Southern Observatory, Arching, Bavaria, Germany

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Publications (61)88.69 Total impact

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    ABSTRACT: We present the final results from our ultra-deep spectroscopic campaign with FORS2 at the ESO Very Large Telescope (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 New Technology Telescope Deep Field. Both galaxies show an Lyα emission line with rest-frame equivalent widths (EWs) of the order of 15-20 Å and luminosities of (2-4) × 1042 erg s–1. 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 6.6 < 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 Lyα EWs (between 20 and 55 Å). We conclude that the observed trend for the rising fraction of Lyα emission in Lyman break galaxies from z ~ 3 to z ~ 6 is most probably reversed from z ~ 6 to z ~ 7. Explaining the observed rapid change in the Lyα emitter fraction among the dropout 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 a set of semi-analytical models, we find that our data require a change of the neutral hydrogen fraction of the order of in a time Δz ~ 1, provided that the escape fraction does not increase dramatically over the same redshift interval.
    The Astrophysical Journal 11/2011; 743(2):132. · 6.73 Impact Factor
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    ABSTRACT: 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 Lyα emission line with rest-frame EWs of the order 15-2 A and luminosities of 2-4×10 42 ergs −1 . 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 Lyα EWs (between 20 and 5 A). We conclude that the trend for the fraction of Lyα 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 ∆χ HI ∼ 0.6 in a time ∆z ∼ 1, provided that the escape fraction does not increase dramatically over the same redshift interval.
    08/2011;
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    ABSTRACT: Aims. Ly-alpha emitters (LAEs) can be detected out to very high redshifts during the epoch of reionization. The evolution of the LAE luminosity function with redshift is a direct probe of the Ly-alpha transmission of the intergalactic medium (IGM), and therefore of the IGM neutral-hydrogen fraction. Measuring the Ly-alpha luminosity function (LF) of LAEs at redshift z = 7.7 therefore allows us to constrain the ionizing state of the Universe at this redshift. Methods. We observed three 7.5'x7.5' fields with the HAWK-I instrument at the VLT with a narrow band filter centred at 1.06 $\mu$m and targeting LAEs at redshift z ~ 7.7. The fields were chosen for the availability of multiwavelength data. One field is a galaxy cluster, the Bullet Cluster, which allowed us to use gravitational amplification to probe luminosities that are fainter than in the field. The two other fields are subareas of the GOODS Chandra Deep Field South and CFHTLS-D4 deep field. We selected z=7.7 LAE candidates from a variety of colour criteria, in particular from the absence of detection in the optical bands. Results. We do not find any LAE candidates at z = 7.7 in ~2.4 x 10^4 Mpc^3 down to a narrow band AB magnitude of ~ 26, which allows us to infer robust constraints on the Ly-alpha LAE luminosity function at this redshift. Conclusions. The predicted mean number of objects at z = 6.5, derived from somewhat different LFs of Hu et al. (2010), Ouchi et al. (2010), and Kashikawa et al. (2011) are 2.5, 13.7, and 11.6, respectively. Depending on which of these LFs we refer to, we exclude a scenario with no evolution from z = 6.5 to z = 7.7 at 85% confidence without requiring a strong change in the IGM Ly-alpha transmission, or at 99% confidence with a significant quenching of the IGM Ly-alpha transmission, possibly from a strong increase in the high neutral-hydrogen fraction between these two redshifts.
    05/2011;
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    ABSTRACT: We present ultradeep optical spectroscopy obtained with FORS2 on the Very Large Telescope (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 λ = 9691.5 ± 0.5 Å and with flux 3.4 × 10–18 erg cm–2 s–1. If identified as Lyα, it places the LBG at redshift z = 6.972 ± 0.002, with a rest-frame equivalent width EWrf = 13 Å. Using Monte Carlo simulations and conservative EW distribution functions at 2 < z < 6, we estimate that the probability of observing no galaxies in our data with S/N>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 ~10–3, 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 Lyα 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.
    The Astrophysical Journal Letters 12/2010; 725(2):L205. · 6.35 Impact Factor
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    ABSTRACT: 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.
    The Astrophysical Journal Letters 11/2010; 730(2). · 6.35 Impact Factor
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    ABSTRACT: 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<z<6, we estimate that the probability of observing no galaxies in our data with S/N>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. Comment: Submitted to ApJL: re-submitted version after modification responding to the referee's report
    10/2010;
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    ABSTRACT: (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). Comment: 12 pages, 8 figures. Accepted for publication in Astronomy & Astrophysics
    Astronomy and Astrophysics 07/2010; · 5.08 Impact Factor
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    ABSTRACT: Aims: We perform a deep search for galaxies in the redshift range 6.5
    Astronomy and Astrophysics 01/2010; · 5.08 Impact Factor
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    ABSTRACT: HAWK-I, the High Accuity Wide-field K-band Imager, will be a near-optimum camera for the VLT. The 7.5 arcminute square field is practically the largest IR field possible at Nasmyth while keeping reasonably uniform sensitivity in all bands. The mosaic of four 2k×2k detectors which fills this field has a pixel scale of 0.1 arcsec/pixel, which is sufficiently small to adequately sample the best seeing at Paranal, even with future ground layer adaptive optics correction. An all-reflective optical design gives very high throughput. The end result is an imager with the best possible performance, limited predominantly by the telescope design and atmospheric seeing conditions.
    12/2008: pages 315-318;
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    ABSTRACT: We present a study of the submillimeter emission of distant red galaxies (DRGs). The DRGs are selected by the criterion J - K > 2.3, and are generally massive galaxies at redshifts higher than 2, with red rest-frame optical colors. Using a deep SCUBA submillimeter image of a field centered on the cluster MS 1054-03, we obtain a statistical detection of the DRGs at redshift z = 2-3.5, with an average 850 μm flux density of 1.11 ± 0.28 mJy. The detection implies an average star formation rate (SFR) of 127 ± 34 M☉ yr-1 (lensing corrected), assuming that the far-infrared (FIR) spectral energy distribution (SED) is well described by a modified blackbody. The SFR derived from the submillimeter agrees well with SFRs derived from SED fitting of optical-near-infrared data and average X-ray emission. Constant star formation models imply ages of 2 Gyr, extinction AV = 2.4 mag, which is consistent with the FIR to rest-frame optical luminosity ratio of ~15. DRGs are older and have lower SFRs relative to optical luminosity than (ultra-) luminous infrared galaxies, although their FIR luminosities are similar. The DRGs at 2 < z < 3.5 and the extremely red objects (I - K > 4) at 1 < z < 2, which were also investigated, contribute 5.7 and 5.9 Jy deg-2, respectively, to the submillimeter background. Simple estimates suggest that these populations contribute ~50% of the flux from sources with 0.5 < f850 < 5 mJy, which is where the peak of energy is produced. We have therefore uncovered one of the most important populations of galaxies contributing to the submillimeter background.
    The Astrophysical Journal 12/2008; 632(1):L9. · 6.73 Impact Factor
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    ABSTRACT: Dust enshrouded activity can ideally be studied by mid-infrared (MIR) observations. In order to explore the AGN versus star forming origin of the nuclear MIR emission of galaxies, observations of high spatial resolution are required. Here we report on 11.3mic. observations with VISIR at the VLT, reaching 0.35'' spatial resolution (FWHM). During the scientific verification of VISIR we have observed a sample of 36 nearby galaxies having a variety of optically classified nuclear activity: 17 black hole driven active galactic nuclei (AGN), 10 starbursts (SBs) and 9 quiet spirals. 16/17 AGN are detected and unresolved, 5/10 SBs are detected and resolved with structured emission up to a few arcsec, while for 5/10 SB and all 9 quiet nuclei low upper limits are provided. The morphology of the resolved SB nuclei follows that seen at radio frequencies. The compactness of AGN and the extent of the SB nuclei is consistent with predictions from radiative transfer models and with MIR spectra of lower spatial resolution. We explore the nuclear MIR surface brightness as a quantitative measure. While AGN and SB cannot be distinguished with MIR data from 4m class telescopes, our data provide evidence that, up to a distance of 100 Mpc, AGN and SB can well be separated by means of MIR surface brightness when using 8m class telescopes. Comment: accepted by A&A: 14 pages, 5 Figures; rsiebenm@eso.org
    Astronomy and Astrophysics 05/2008; · 5.08 Impact Factor
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    ABSTRACT: We describe the design, development, and performance of HAWK-I, the new High-Acuity Wide-field K-band Imager for ESO's Very Large Telescope, which is equipped with a mosaic of four 2 k × 2 k arrays and operates from 0.9-2.4 μm over 7.5 arcmin × 7.5 arcmin with 0.1´´ pixels. A novel feature is the use of all reflective optics that, together with filters of excellent throughput and detectors of high quantum efficiency, has yielded an extremely high throughput. Commissioning and science verification observations have already delivered a variety of excellent and deep images that demonstrate its high scientific potential for addressing important astrophysical questions of current interest.
    Astronomy and Astrophysics 01/2008; 491(3):941-950. · 5.08 Impact Factor
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    ABSTRACT: We present a study of the submillimeter emission of distant red galaxies (DRGs). The DRGs are selected by the criterion J-K>2.3 red rest-frame optical colours, and are generally massive galaxies at redshifts >2. Using a deep SCUBA submm image of a field centered on the cluster MS 1054-03, we obtain a statistical detection of the DRGs at z = 2 - 3.5, with an average 850mum flux density of 1.11±0.28 mJy. The detection implies an average star formation rate (SFR) of 127±34 M_&sun; yr^{-1} (lensing corrected), assuming that the far-infrared (FIR) spectral energy distribution (SED) is well described by a modified blackbody. The SFR derived from the submm agrees well with SFRs derived from SED fitting of optical/near-IR data and average X-ray emission. The DRGs at 2
    12/2007;
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    ABSTRACT: We present HST NICMOS+ACS and Spitzer IRAC+MIPS observations of 41 galaxies at 2<z<3.5 in the FIRES MS 1054 field with red and blue rest-frame optical colors. About half of the galaxies are very compact (effective radii re<1 kpc) at rest-frame optical wavelengths; the others are extended (1 kpc<re<10 kpc). For reference, 1 kpc corresponds to 0.12" at z=2.5 in the adopted cosmology. We separate actively star-forming galaxies from quiescent galaxies by modeling their rest-frame UV-NIR SEDs. The star-forming galaxies span the full range of sizes, while the quiescent galaxies all have re<2 kpc. In the redshift range where MIPS 24 mum imaging is a sensitive probe of reradiated dust emission (z<2.5), the 24 mum fluxes confirm that the light of the small quiescent galaxies is dominated by old stars, rather than dust-enshrouded star formation or AGN activity. The inferred surface mass densities and velocity dispersions for the quiescent galaxies are very high compared to those in local galaxies. The galaxies follow a Kormendy relation (between surface brightness and size) with approximately the same slope as locally, but shifted to brighter surface brightnesses, consistent with a mean stellar formation redshift of zf~5. This paper demonstrates a direct relation between star formation activity and size at z~2.5 and the existence of a significant population of massive, extremely dense, old stellar systems without readily identifiable counterparts in the local universe. Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555 observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407; and observations collected at the European Southern Observatory, Paranal, Chile (ESO Program 164.O-0612).
    The Astrophysical Journal 01/2007; · 6.73 Impact Factor
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    ABSTRACT: Aims:The first star forming galaxies in the early universe should be copious Lyalpha emitters and may play a significant role in ionizing the intergalactic medium (IGM). It has been proposed that the luminosity function of Lyalpha emitting galaxies beyond z ~ 6 may be used to constrain the neutral fraction of the IGM during this epoch. In this work we report on a search for Lyalpha emitters at redshift 8.8. Methods: We performed a narrow band imaging programme using ISAAC at the ESO VLT. Seven fields, covering a total area of 31 sq. arcmin and for which optical and broad band infra-red images have been obtained in the GOODS survey, were imaged to a limiting flux (respectively luminosity) of ~1.3 × 10-17 erg s-1 cm-2 (respectively ~1.3 × 1043 erg s-1) in a narrow band filter centered in a region of low OH sky emission at 1.19 mum. Candidate Lyalpha emitters are objects that are detected in the ISAAC NB images and undetected in the visible broad band images. Results: No z=8.8 Lyalpha emitting galaxies were detected to a limit approaching recent estimates of the luminosity function at z˜ 6. Our results do suggest, however, that detections or substantial constraints could be achieved by this method in the near future with larger field instruments planned for various telescopes.
    Astronomy and Astrophysics 01/2007; 461(3):911-916. · 5.08 Impact Factor
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    ABSTRACT: CRIRES is a cryogenic, pre-dispersed, infrared echelle spectrograph designed to provide a nominal resolving power λ/Δλ of 105 between 1000 and 5000 nm for a nominal slit width of 0.2'. The CRIRES installation at the Nasmyth focus A of the 8-m VLT UT1 (Antu) marks the completion of the original instrumentation plan for the VLT. A curvature sensing adaptive optics system feed is used to minimise slit losses and to provide 0.2' spatial resolution along the slit. A mosaic of four Aladdin InSb-arrays packaged on custom-fabricated ceramic boards has been developed. It provides for an effective 4096 × 512 pixel focal plane array to maximise the free spectral range covered in each exposure. Insertion of gas cells is possible in order to measure radial velocities with high precision. Measurement of circular and linear polarisation in Zeeman sensitive lines for magnetic Doppler imaging is foreseen but not yet fully implemented. A cryogenic Wollaston prism on a kinematic mount is already incorporated. The retarder devices are located close to the Unit Telescope focal plane. Here we briefly recall the major design features of CRIRES and describe the commissioning of the instrument including a report of extensive laboratory testing and a preview of astronomical results. Thanks to the strong efforts of the CRIRES commissioning team and all other ESO staff involved, it was possible to include the instrument in the general ESO call for proposals for Period 79.
    The Messenger. 11/2006; 126:32-36.
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    ABSTRACT: Deep multicolor fields are an important tool to explore the formation and evolution of galaxies out to the highest redshifts and faintest magnitude limits (Ferguson et al. [11]). This is also the aim of the FORS Deep Field (FDF), which has about 5 times the field-size of the two Hubble Deep Fields together and, in addition, contains a relatively bright quasar of 18.5mag at z = 3.36 at its center. A full exploration of the FDF multicolor data requires good photometric redshifts. In this paper we provide an overview of the data, describe our photometric redshift technique and present the redshift distribution in the FDF.
    02/2006: pages 96-101;
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    N. Ageorges, A. Moorwood
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    ABSTRACT: This paper presents all ESO infrared instruments offering a polarimetric facility. It is not a detailed review but more an overview and appetizer of the existing possibilities.
    11/2005; 343:15-19.
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    ABSTRACT: We investigate the z˜1.5 rest-frame B-band Tully-Fisher relation, using a sample of Hα-emitting galaxies and find a ˜ 2 mag offset from the local Tully-Fisher relation. We demonstrate why the interpretation of this result is not straightforward: analysis of high-redshift Tully-Fisher relations is plagued by selection effects, biases, observational challenges, and other effects.
    04/2005; 329:81P.
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    ABSTRACT: After more than two years of regular operation, a number of upgrades have recently been installed for NACO and offered to the community. The article describes the new observing modes and provides examples of astronomical applications in high-contrast imaging, spectroscopy and polarimetry.
    The Messenger. 02/2005; 119:9-13.