A. F. M. Moorwood

European Southern Observatory, Arching, Bavaria, Germany

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Publications (353)826.51 Total impact

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    ABSTRACT: MATISSE is a mid-infrared spectro-interferometer combining the beams of up to four Unit Telescopes or Auxiliary Telescopes of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory. MATISSE will constitute an evolution of the two-beam interferometric instrument MIDI. New characteristics present in MATISSE will give access to the mapping and the distribution of the material, the gas and essentially the dust, in the circumstellar environments by using the mid-infrared band coverage extended to L, M and N spectral bands. The four beam combination of MATISSE provides an efficient uv-coverage: 6 visibility points are measured in one set and 4 closure phase relations which can provide aperture synthesis images in the mid-infrared spectral regime. We give an overview of the instrument including the expected performances and a view of the Science Case. We present how the instrument would be operated. The project involves the collaborations of several agencies and institutes: the Observatoire de la Co te d'Azur of Nice and the INSU-CNRS in Paris, the Max Planck Institut fur Astronomie of Heidelberg; the University of Leiden and the NOVA-ASTRON Institute of Dwingeloo, the Max Planck Institut fur Radioastronomie of Bonn, the Institut fur Theoretische Physik und Astrophysik of Kiel, the Vienna University and the Konkoly Observatory.
    No preview · Conference Paper · Sep 2012
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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.
    Full-text · Article · Nov 2011 · The Astrophysical Journal
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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.
    Full-text · Article · Aug 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.
    Full-text · Article · May 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.
    Full-text · Article · Dec 2010 · The Astrophysical Journal Letters
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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.
    Full-text · Article · Nov 2010 · The Astrophysical Journal Letters
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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
    Full-text · Article · Oct 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
    Full-text · Article · Jul 2010 · Astronomy and Astrophysics
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    ABSTRACT: Aims. We perform a deep search for galaxies in the redshift range 6.5 <= z <= 7.5, to measure the evolution of the number density of luminous galaxies in this redshift range and derive useful constraints on the evolution of their luminosity function. Methods. We present here the first results 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 two Hawk-I pointings on the GOODS South field for a total of similar to 32 observing hours, covering similar to 90 arcmin(2). The images reach Y = 26.7 mag for the two fields. We used public ACS images in the z band to select z-dropout galaxies with the colour criteria Z - Y >= 1, Y - J < 1.5, and Y - K < 2. The other public data in the UBVRIJK bands are used to reject possible low redshift interlopers. The output has been compared with extensive Monte Carlo simulations to quantify the observational effects of our selection criteria, as well as the effects of photometric errors. Results. We detect 7 high-quality candidates in the magnitude range Y = 25.5-26.7. This interval samples the critical range for M(*) at z > 6 (M(1500) similar or equal to -19.5 to -21.5). After accounting for the expected incompleteness, we rule out a luminosity function constant from z = 6 to z = 7 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(-0.9)(+2.0) x 10(25) erg s(-1) Hz(-1) Mpc(-3), implying a decrease by a factor 3.5 from z = 6 to z similar or equal to 6.8. On the basis of our findings, we make predictions for the surface densities expected in future surveys, based on ULTRA-VISTA, HST-WFC3, or JWST-NIRCam, evaluating the best observational strategy to maximise their impact.
    Full-text · Article · Feb 2010 · Astronomy and Astrophysics
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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.
    No preview · Chapter · Dec 2008
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    ABSTRACT: Using deep NIR VLT ISAAC and optical HST WFPC2 imaging in the HDF-S and MS 1054-03, we study the rest-frame UV-to-optical colors and magnitudes of galaxies to z ~ 3. Unlike the present day, there is no evidence for a red sequence at z ~ 3, but there does appear to be a well-defined CMR for blue galaxies at all redshifts, with more luminous galaxies having redder U - V colors. The slope of the blue CMR is independent of redshift δ(U - V)/δMV = -0.09 ± 0.01 and can be explained by a correlation of dust reddening with luminosity. The average color at fixed luminosity reddens strongly Δ(U - V) ≈ 0.75 from z ~ 3 to z = 0, much of which can be attributed to aging of stars. The color scatter of the blue sequence is relatively small σ(U - V) = 0.25 ± 0.03 and constant to z ~ 3, but notably asymmetrical, having a sharp blue ridge and a wing toward redder colors. We explore sets of SFHs to study the constraints placed by the shape of the scatter at z = 2-3. One particular set of models, episodic star formation, reproduces the detailed properties very well. For a two-state model with high and low star formation, the duty cycle is constrained to be >40% and the contrast in SFR between the states must be a factor >5 [or a scatter in log(SFR) of >0.35 dex around the mean]. Nevertheless, episodic models do not explain the observed tail of very red galaxies, primarily DRGs, which may have ceased star formation altogether or are more heavily obscured. Finally, the relative number density of red, luminous MV < -20.5 galaxies increases by a factor of ~6 from z = 2.7 to 0.5, as does their contribution to the total rest-frame V-band luminosity density. We are likely viewing the progressive formation of red, passively evolving galaxies.
    Preview · Article · Dec 2008 · The Astrophysical Journal
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    ABSTRACT: We recently identified a substantial population of galaxies at z‚‚gt;2 with comparatively red rest-frame optical colors. These distant red galaxies (DRGs) are efficiently selected by the simple observed color criterion Js-Ks‚‚gt;2.3. In this paper we present near-infrared spectroscopy with Keck NIRSPEC of six DRGs with previously measured redshifts 2.4‚‚lt;z‚‚lt;3.2, two of which were known to host an active nucleus. We detect continuum emission and emission lines of all observed galaxies. Equivalent widths of H‚‚alpha; in the nonactive galaxies are 20-30 ‚‚Aring;, smaller than measured for Lyman break galaxies (LBGs) and nearby luminous infrared galaxies and comparable to normal nearby galaxies. The modest equivalent widths imply that the galaxies either have a decreasing star formation rate or are very dusty. Fitting both the photometry and the H‚‚alpha; lines, we find continuum extinction AV=1-2 mag, ages 1-2.5 Gyr, star formation rates 200-400 Msolar yr-1, and stellar masses (1-5)‚‚times;1011 Msolar for models with constant star formation rates. Models with a declining star formation lead to significantly lower extinction, star formation rates, and ages but similar stellar masses. From [N II]/H‚‚alpha; ratios we infer that the metallicities are high, 1-1.5 times solar. For four galaxies we can determine line widths from the optical emission lines. The widths are high, ranging from 130 to 240 km s-1, and by combining data for LBGs and DRGs, we find significant correlations between line width and rest-frame U-V color and between line width and stellar mass. The latter correlation has a similar slope and offset as the ``baryonic Tully-Fisher relation'' for nearby galaxies. From the line widths and effective radii we infer dynamical masses and mass-to-light (M/L) ratios. The median dynamical mass is ~2‚‚times;1011 Msolar, supporting the high stellar masses inferred from the photometry. We find that the median M/LV~0.8(M/L)solar, a factor of ~5 higher than measured for LBGs. We infer from our small sample that DRGs are dustier, more metal-rich, and more massive and have higher ages than z~3 LBGs of the same rest-frame V-band luminosity. Although their volume density is still uncertain, their high M/L ratios imply that they contribute significantly to the stellar mass density at z~2.5. As their stellar masses are comparable to those of early-type galaxies, they may have already assembled most of their final mass.
    Full-text · Article · Dec 2008 · The Astrophysical Journal
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    ABSTRACT: The luminosity-size and mass-size distributions of galaxies out to z ~ 3 are presented. We use very deep near-infrared images of the Hubble Deep Field-South in the Js, H, and Ks bands, taken as part of FIRES at the VLT, to follow the evolution of the optical rest-frame sizes of galaxies. For a total of 168 galaxies with Ks,AB ≤ 23.5, we find that the rest-frame V-band sizes re,V of luminous galaxies ( ~ 2 × 1010 h-2 L☉) at 2 < z < 3 are 3 times smaller than for equally luminous galaxies today. In contrast, the mass-size relation has evolved relatively little: the size at mass ~ 2 × 1010 h-2 M☉ has changed by 20% (±20%) since z ~ 2.5. Both results can be reconciled by the fact that the stellar M/L ratio is lower in the luminous high-z galaxies than in nearby ones because they have young stellar populations. The lower incidence of large galaxies at z ~ 3 seems to reflect the rarity of galaxies with high stellar mass.
    Full-text · Article · Dec 2008 · The Astrophysical Journal
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    ABSTRACT: We investigate the nature of the substantial population of high-redshift galaxies with Js - Ks ≥ 2.3 colors recently discovered as part of our Faint Infrared Extragalactic Survey (FIRES). This color cut efficiently isolates galaxies at z > 2 with red rest-frame optical colors ("distant red galaxies" [DRGs]). We select Js - Ks ≥ 2.3 objects in both FIRES fields, the ≈25 × 25 Hubble Deep Field-South (HDF-S) and the ≈5' × 5' field around the MS 1054-03 cluster at z = 0.83; the surface densities at Ks,Vega < 21 mag are 1.6 ± 0.6 and 1.0 ± 0.2 arcmin-2, respectively. We here discuss a subsample of 34 DRGs at 2 ≤ z ≤ 3.5: 11 at Ks,Vega < 22.5 mag in HDF-S and 23 at Ks,Vega < 21.7 mag in the MS 1054-03 field. This sample enables for the first time a robust assessment of the population properties of DRGs. We analyze the λ = 0.3-2.2 μm spectral energy distributions (SEDs) constructed from our very deep near-infrared (NIR) and optical imaging collected at the ESO Very Large Telescope and from the Hubble Space Telescope. We develop diagnostics involving the I814 - Js, Js - H, and H - Ks colors to argue that the red NIR colors of our DRG sample cannot be attributed solely to interstellar dust extinction and require for many the presence of an evolved stellar population with a prominent Balmer/4000 Å break. In the rest frame, the optical colors of DRGs fall within the envelope of normal nearby galaxies and the ultraviolet colors suggest a wide range in star formation activity and/or extinction. This is in stark contrast with the much bluer and more uniform SEDs of Lyman break galaxies (LBGs). From evolutionary synthesis modeling assuming constant star formation (CSF), we derive for the DRGs old ages, large extinctions, and high stellar masses, mass-to-light ratios, and star formation rates (SFRs). For solar metallicity, a Salpeter initial mass function (IMF) between 0.1 and 100 M☉, and the Calzetti et al. extinction law, the median values for the HDF-S (MS 1054-03 field) sample are 1.7 (2.0) Gyr, AV = 2.7 (2.4) mag, M* = 0.8 (1.6) × 1011 M☉, M*/LV,* = 1.2 (2.3) M☉ L, and SFR = 120 (170) M☉ yr-1. Models assuming exponentially declining SFRs with e-folding timescales in the range from 10 Myr to 1 Gyr generally imply younger ages, lower extinction, and lower SFRs, but similar stellar masses within a factor of 2. Compared to LBGs at similar redshifts and rest-frame V-band luminosities, DRGs are older, more massive, and more obscured for any given star formation history. For the entire sample of Ks-band selected galaxies in the FIRES fields at 2 ≤ z ≤ 3.5 and to the above magnitude limits, we find that the derived ages, extinctions, and stellar masses increase with redder Js - Ks colors. Although the rest-frame optical colors of the DRGs are similar to those of local normal galaxies, the derived properties are quite different; detailed studies of this new z > 2 population may significantly enhance our understanding of how massive galaxies assembled their stellar mass.
    Preview · Article · Dec 2008 · The Astrophysical Journal
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    ABSTRACT: We present the evolution of the volume-averaged properties of the rest-frame optically luminous (LV > 3 × 1010 h L☉) galaxy population to z ~ 3, determined from four disjoint deep fields. We characterize their rest-frame UV through optical properties via the mean SED. To measure evolution, we apply the selection criteria to a sample of galaxies from the SDSS and COMBO-17 survey. The mean rest-frame 2200 Å through V-band SED becomes steadily bluer with increasing redshift, but at all redshifts z < 3 the mean SED falls within the range defined by ``normal'' galaxies in the nearby universe. We measure the mean stellar mass-to-light ratios (/L) and stellar mass densities (ρ) by fitting models to the mean rest-frame UV-optical SEDs. The ρ in galaxies selected at a fixed luminosity has increased by a factor of 3.5-7.9 from z = 3 to 0.1. If we instead use our observed /LV evolution to select galaxies at a fixed mass, ρ evolves by a factor of 5.3-16.7. After correcting to total, the measured ρ at z < 2 lie below the integral of the star formation rate density as a function of redshift as derived from UV-selected samples after a standard correction for extinction. We find large discrepancies between recent model predictions for the evolution of ρ and our results, even when our observational selection is applied to the models. Finally, we determine that distant red galaxies (selected to have Js - Ks > 2.3) in our L-selected samples contribute 30% and 64% of the stellar mass budget at z ~ 2 and z ~ 2.8, respectively. These galaxies are largely absent from UV surveys, and this result highlights the need for mass selection of high-redshift galaxies.
    Full-text · Article · Dec 2008 · The Astrophysical Journal
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    ABSTRACT: We analyze very deep HST, VLT, and Spitzer photometry of galaxies at 2 < z < 3.5 in the Hubble Deep Field-South. The sample is selected from the deepest public K-band imaging currently available. We show that the rest-frame U - V versus V - J color-color diagram is a powerful diagnostic of the stellar populations of distant galaxies. Galaxies with red rest-frame U - V colors are generally red in rest-frame V - J as well. However, at a given U - V color a range in V - J colors exists, and we show that this allows us to distinguish young, dusty galaxies from old, passively evolving galaxies. We quantify the effects of IRAC photometry on estimates of masses, ages, and the dust content of z > 2 galaxies. The estimated distributions of these properties do not change significantly when IRAC data are added to the UBVIJHK photometry. However, for individual galaxies the addition of IRAC can improve the constraints on the stellar populations, especially for red galaxies: uncertainties in stellar mass decrease by a factor of 2.7 for red [(U − V)rest > 1] galaxies, but only by a factor of 1.3 for blue [(U − V)rest < 1] galaxies. We find a similar color dependence of the improvement for estimates of age and dust extinction. In addition, the improvement from adding IRAC depends on the availability of full NIR JHK coverage; if only K band were available, the mass uncertainties of blue galaxies would decrease by a more substantial factor of 1.9. Finally, we find that a trend of galaxy color with stellar mass is already present at z > 2. The most massive galaxies at high redshift have red rest-frame U - V colors compared to lower mass galaxies, even when allowing for complex star formation histories.
    Preview · Article · Dec 2008 · The Astrophysical Journal
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    ABSTRACT: We present low-resolution mid-infrared (MIR) spectra of 16 ultraluminous infrared galaxies (ULIRGs) obtained with the circular variable filter (CVF) spectroscopy mode of ISOCAM on board the Infrared Space Observatory (ISO). Our sample completes previous ISO spectroscopy of ultra- and hyperluminous infrared galaxies toward higher luminosities. The combined samples cover an infrared luminosity range of ~1012-1013.1 L☉. To discriminate active galactic nucleus (AGN) and starburst activity, we use the AGN-related MIR continuum and the starburst-related 6.2, 7.7, 8.6, and 11.3 μm MIR emission bands attributed to aromatic carbonaceous material. For about half of the high-luminosity ULIRGs studied here, strong aromatic emission bands suggest starburst dominance. Other spectra are dominated by a strong AGN-related continuum with weak superposed emission features of uncertain nature. Our sample contains one unusual example, IRAS F00183-7111, of an AGN that is highly obscured even in the MIR. An improved method to characterize quantitatively the relative contribution of star formation and AGN activity to the MIR emission of ULIRGs is presented. The ULIRG spectra are fitted by a superposition of a starburst and an AGN spectrum, both of which may be obscured at different levels. Models in which starburst and AGN obscuration differ are significantly more successful than models with a single extinction. Previous results based on a simpler line-to-continuum measure of aromatic emission strength are confirmed, further supporting the robustness of the aromatic emission feature as a diagnostic of ULIRG power sources. As dominant sources of the bolometric luminosity, starbursts prevail at the lower end and AGNs at the higher end of this range. The transition between mostly starburst and mostly AGN powered occurs at ~1012.4-1012.5 L☉, and individual luminous starbursts are found up to ~1012.65 L☉.
    Full-text · Article · Dec 2008 · The Astrophysical Journal
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    ABSTRACT: We present deep 3.6-8 μm imaging of the Hubble Deep Field-South with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We study distant red galaxies (DRGs) at z > 2 selected by Js-Ks > 2.3 and compare them with a sample of Lyman break galaxies (LBGs) at z = 2-3. The observed UV-to-8 μm spectral energy distributions are fitted with stellar population models to constrain star formation histories and derive stellar masses. We find that 70% of the DRGs are best described by dust-reddened star-forming models and 30% are very well fitted with old and "dead" models. Using only the I-Ks and Ks-4.5 μm colors, we can effectively separate the two groups. The dead systems are among the most massive at z ~ 2.5 (mean stellar mass M* = 0.8 × 1011 M☉) and likely formed most of their stellar mass at z > 5. To a limit of 0.5 × 1011 M☉, their number density is ~10 times lower than that of local early-type galaxies. Furthermore, we use the IRAC photometry to derive rest-frame near-infrared J, H, and K fluxes. The DRGs and LBGs together show a large variation (a factor of 6) in the rest-frame K-band mass-to-light ratios (M/LK), implying that even a Spitzer 8 μm-selected sample would be very different from a mass-selected sample. The average M/LK of the DRGs is about 3 times higher than that of the LBGs, and DRGs dominate the high-mass end. The M/LK values and ages of the two samples appear to correlate with derived stellar mass, with the most massive galaxies being the oldest and having the highest mass-to-light ratios, similar to what is found in the low-redshift universe.
    Preview · Article · Dec 2008 · The Astrophysical Journal
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    ABSTRACT: We present the first results of a spectroscopic survey of Lyman break galaxies (LBGs) in the near-infrared aimed at detecting the emission lines of [O II], [O III], and Hβ from the H II regions of normal star-forming galaxies at z 3. From observations of 15 objects with the Keck telescope and the Very Large Telescope augmented with data from the literature for an additional four objects, we reach the following main conclusions. The rest-frame optical properties of LBGs at the bright end of the luminosity function are remarkably uniform, their spectra are dominated by emission lines, [O III] is always stronger than Hβ and [O II], and projected velocity dispersions are between 50 and 115 km s-1. Contrary to expectations, the star formation rates deduced from the Hβ luminosity are on average no larger than those implied by the stellar continuum at 1500 Å; presumably any differential extinction between rest-frame optical and UV wavelengths is small compared to the relative uncertainties in the calibrations of these two star formation tracers. For the galaxies in our sample, the abundance of oxygen can only be determined to within 1 order of magnitude without recourse to other emission lines ([N II] and Hα), which are generally not available. Even so, it seems well established that LBGs are the most metal-enriched structures at z 3, apart from quasi-stellar objects, with abundances greater than about 1/10 solar and generally higher than those of damped Lyα systems at the same epoch. They are also significantly overluminous for their metallicities; this is probably an indication that their mass-to-light ratios are low compared to present-day galaxies. At face value, the measured velocity dispersions imply virial masses of about 1010 M☉ within half-light radii of 2.5 kpc. The corresponding mass-to-light ratios, M/L ≈ 0.15 in solar units, are indicative of stellar populations with ages between 108 and 109 yr, consistent with the UV-optical spectral energy distributions. However, we are unable to establish conclusively whether or not the widths of the emission lines reflect the motions of the H II regions within the gravitational potential of the galaxies, even though in two cases we see hints of rotation curves. All 19 LBGs observed show evidence for galactic-scale superwinds; such outflows have important consequences for regulating star formation, distributing metals over large volumes, and allowing Lyman continuum photons to escape and ionize the intergalactic medium.
    Full-text · Article · Dec 2008 · The Astrophysical Journal
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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.
    Preview · Article · Dec 2008 · The Astrophysical Journal

Publication Stats

9k Citations
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Institutions

  • 1981-2011
    • European Southern Observatory
      Arching, Bavaria, Germany
  • 2008
    • Yale University
      • Department of Astronomy
      New Haven, CT, United States
  • 2007
    • Max Planck Institute for Astronomy
      Heidelburg, Baden-Württemberg, Germany
  • 2004
    • Cornell University
      Итак, New York, United States
  • 2002
    • Pontifical Catholic University of Chile
      CiudadSantiago, Santiago Metropolitan, Chile
  • 1998-2002
    • Tel Aviv University
      • Department of Physics and Astronomy
      Tell Afif, Tel Aviv, Israel
  • 2000
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, Maryland, United States
  • 1999
    • Cardiff University
      • School of Physics and Astronomy
      Cardiff, WLS, United Kingdom
  • 1992
    • South African Astronomical Observatory
      Kaapstad, Western Cape, South Africa
  • 1991
    • The Entomological Society of Ontario
      Canada
  • 1990
    • University of Santiago, Chile
      CiudadSantiago, Santiago Metropolitan, Chile
  • 1986
    • Observatoire de Haute-Provence
      Manosque, Provence-Alpes-Côte d'Azur, France
  • 1984
    • National Research Council
      Roma, Latium, Italy
  • 1976-1980
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 1975-1980
    • University College London
      • Department of Physics and Astronomy
      Londinium, England, United Kingdom