Martin G. Haehnelt

Cancer Research UK Cambridge Institute, Cambridge, England, United Kingdom

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Publications (154)530.06 Total impact

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    Tiago Costa, Debora Sijacki, Martin Haehnelt
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    ABSTRACT: Observations of the emission from spatially extended cold gas around bright high-redshift QSOs reveal surprisingly large velocity widths exceeding 2000 km s^(-1), out to projected distances as large as 30 kpc. The high velocity widths have been interpreted as the signature of powerful AGN-driven outflows. Naively, these findings appear in tension with hydrodynamic models in which AGN-driven outflows are energy-driven and thus very hot with typical temperatures T = 10^6-7 K. Using the moving-mesh code Arepo, we perform 'zoom-in' cosmological simulations of a z = 6 QSO and its environment, following black hole growth and feedback via energy-driven outflows. In the simulations, the QSO host galaxy is surrounded by a clumpy circum-galactic medium pre-enriched with metals due to supernovae-driven galactic outflows. As a result, part of the AGN-driven hot outflowing gas can cool radiatively, leading to large amounts (> 10^9 M_sun) of cold gas comoving with the hot bipolar outflow. This results in velocity widths of spatially extended cold gas similar to those observed. We caution, however, that gas inflows, random motions in the deep potential well of the QSO host galaxy and cooling of supernovae-driven winds contribute significantly to the large velocity width of the cold gas in the simulations, complicating the interpretation of observational data.
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    ABSTRACT: We compare cosmological hydrodynamical simulations combined with the homogeneous metagalactic UV background (UVB) of Haardt & Madau (2012) (HM2012) to observations of the Lyman-alpha forest that are sensitive to the thermal and ionization state of the intergalactic medium (IGM). The transition from optically thick to thin photoheating predicted by the simple one-zone, radiative transfer model implemented by HM2012 predicts a thermal history that is in remarkably good agreement with the observed rise of the IGM temperature at z~3 if we account for the expected evolution of the volume filling factor of HeIII. Our simulations indicate that there may be, however, some tension between the observed peak in the temperature evolution and the rather slow evolution of the HeII opacities suggested by recent Hubble Space Telescope/COS measurements. The HM2012 UVB also underpredicts the metagalactic hydrogen photoionization rate required by our simulations to match the observed opacity of the forest at z>4 and z<2.
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    ABSTRACT: We examine the kinematic structure of Damped Lyman-alpha Systems (DLAs) in a series of cosmological hydrodynamic simulations using the AREPO code. We are able to match the distribution of velocity widths of associated low ionisation metal absorbers substantially better than earlier work. Our simulations produce a population of DLAs dominated by halos with virial velocities around 70 km/s, consistent with a picture of relatively small, faint objects. In addition, we reproduce the observed correlation between velocity width and metallicity and the equivalent width distribution of SiII. Some discrepancies of moderate statistical significance remain; too many of our spectra show absorption concentrated at the edge of the profile and there are slight differences in the exact shape of the velocity width distribution. We show that the improvement over previous work is mostly due to our strong feedback from star formation and our detailed modelling of the metal ionisation state.
  • Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 06/2014
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    Tiago Costa, Debora Sijacki, Martin G. Haehnelt
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    ABSTRACT: We employ hydrodynamical simulations using the moving-mesh code AREPO to investigate the role of energy and momentum input from Active Galactic Nuclei (AGN) in driving large-scale galactic outflows. We start by reproducing analytic solutions for both energy- and momentum-driven outflowing shells in simulations of a spherical isolated dark matter potential with gas in hydrostatic equilibrium and with no radiative cooling. We confirm that for this simplified setup, galactic outflows driven by a momentum input rate of order L_Edd/c can establish an M_BH - sigma relation with slope and normalisation similar to that observed. We show that momentum input at a rate of L_Edd/c is however insufficient to drive efficient outflows once cooling and gas inflows as predicted by cosmological simulations at resolved scales are taken into account. We argue that observed large-scale AGN-driven outflows are instead likely to be energy-driven and show that such outflows can reach momentum fluxes exceeding 10 L_Edd/c within the innermost 10 kpc of the galaxy. The outflows are highly anisotropic, with outflow rates and a velocity structure found to be inadequately described by spherical outflow models. We verify that the hot energy-driven outflowing gas is expected to be strongly affected by metal-line cooling, leading to significant amounts (>10^9 M_sun) of entrained cold gas.
    Monthly Notices of the Royal Astronomical Society 06/2014; 444(3). · 5.52 Impact Factor
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    ABSTRACT: We examine the abundance, clustering and metallicity of Damped Lyman-$\alpha$ Absorbers (DLAs) in hydrodynamic cosmological simulations using the moving mesh code AREPO. We incorporate models of supernova and AGN feedback, and molecular hydrogen formation. We compare our simulations to the column density distribution function at $z=3$, the total DLA abundance at $z=2-4$, the measured DLA bias at $z=2$ and DLA metallicities at $z=2-4$. Our preferred models produce populations of DLAs in good agreement with most of these observations, except the DLA abundance at $z < 3$, which we show requires stronger feedback in $10^{11-12} h^{-1} M_\odot$ mass halos. While the DLA population probes a range of halo masses, we find the cross-section is dominated by halos of mass $10^{10.5} h^{-1} M_\odot$ at $z=3$ and $10^{11} h^{-1} M_\odot$ at $z=2$. Simulations without feedback are in strong tension with all these observables, indicating a need for strong stellar feedback at $z=2-4$ independently of the star formation threshold. We demonstrate that DLAs are a powerful probe of the physical processes that shape galaxy formation, in particular supernova feedback processes. As DLAs arise from gas at a lower density than the star formation threshold, the information they provide is complementary to that contained in the stellar component of galaxies.
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    Luke A. Barnes, Martin G. Haehnelt
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    ABSTRACT: We discuss the recent BOSS measurement of a rather high bias factor for the host galaxies/haloes of Damped Lyman-alpha Absorbers (DLAs), in the context of our previous modelling of the physical properties of DLAs within the {\Lambda}CDM paradigm. Joint modelling of the column density distribution, the velocity width distribution of associated low ionization metal absorption, and the bias parameter suggests that DLAs are hosted by galaxies with dark matter halo masses in the range 10.5 < log Mv < 13, with a rather sharp cutoff at the lower mass end, corresponding to viral velocities of ~90 km/s. The observed properties of DLAs appear to suggest very efficient (stellar) feedback in haloes with masses/virial velocities below the cutoff and a large retained baryon fraction (> 35%) in haloes above the cutoff.
    03/2014; 440(3).
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    John A. Regan, Peter H. Johansson, Martin G. Haehnelt
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    ABSTRACT: We have performed high-resolution numerical simulations with the hydrodynamical AMR code Enzo to investigate the formation of massive seed black holes in a sample of six dark matter haloes above the atomic cooling threshold. The aim of this study is to illustrate the effects of varying the maximum refinement level on the final object formed. The virial temperatures of the simulated haloes range from $\rm{T} \sim 10000\ \rm{K} - 16000\ \rm{K}$ and they have virial masses in the range $\rm{M} \sim 2 \times 10^7 \rm{M_{\odot}}$ to $\rm{M} \sim 7 \times 10^7 \rm{M_{\odot}}$ at $z \sim 15$. The outcome of our six fiducial simulations is both generic and robust. A rotationally supported, marginally gravitationally stable, disk forms with an exponential profile. The mass and scale length of this disk depends strongly on the maximum refinement level used. Varying the maximum refinement level by factors between 1 / 64 to 256 times the fiducial level illustrates the care that must be taken in interpreting the results. The lower resolution simulations show tentative evidence that the gas may become rotationally supported out to 20 pc while the highest resolution simulations show only weak evidence of rotational support due to the shorter dynamical times for which the simulation runs. The higher resolution simulations do, however, point to fragmentation at small scales of the order of $\sim 100$ AU. In the highest resolution simulations a central object of a few times $10^2\ \rm{M_{\odot}}$ forms with multiple strongly bound, Jeans unstable, clumps of $\sim 10\ \rm{M_{\odot}}$ and radii of 10 - 20 AU suggesting the formation of dense star clusters in these haloes.
    12/2013; 439(1).
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    ABSTRACT: Low ionization metal absorption due to OI has been identified as an important probe of the inter-/circumgalactic medium at the tail-end of reionization. We use here high-resolution hydrodynamic simulations to interpret the incidence rate of OI absorbers at z~6. For plausible assumptions about the photo-ionization rate due to the meta-galactic UV background and the self-shielding of optically thick absorbers, our modelling reproduces the equivalent width distribution and incidence rate of the observations by Becker et al. (2011) with a model where the weak OI absorbers have typical HI column densities in the range of sub-DLAs, densities of 80 times the mean baryonic density and metallicities of about 1/500 th solar. This is similar to the metallicity inferred at similar overdensities at z~3, suggesting that the metal enrichment of the CGM has already progressed considerably by z~6. The apparently rapid evolution of the incidence rates for OI absorption over 5<z<6 mirrors that of LLSs at lower redshift and is mainly due to the rapid decrease of the meta-galactic photo-ionization rate at z>5. Assuming the same metallicity-density relation inferred at z~6, we predict the incidence rate of OI absorbers at z=7-8 to continue to rise rapidly with increasing redshift as the IGM becomes more neutral. If the distribution of metals extends to lower density regions, OI absorbers will allow the metal enrichment of the increasingly neutral filamentary structures of the cosmic web to be probed.
    Monthly Notices of the Royal Astronomical Society 10/2013; · 5.52 Impact Factor
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    ABSTRACT: Building on the experience of the high-resolution community with the suite of VLT high-resolution spectrographs, which has been tremendously successful, we outline here the (science) case for a high-fidelity, high-resolution spectrograph with wide wavelength coverage at the E-ELT. Flagship science drivers include: the study of exo-planetary atmospheres with the prospect of the detection of signatures of life on rocky planets; the chemical composition of planetary debris on the surface of white dwarfs; the spectroscopic study of protoplanetary and proto-stellar disks; the extension of Galactic archaeology to the Local Group and beyond; spectroscopic studies of the evolution of galaxies with samples that, unlike now, are no longer restricted to strongly star forming and/or very massive galaxies; the unraveling of the complex roles of stellar and AGN feedback; the study of the chemical signatures imprinted by population III stars on the IGM during the epoch of reionization; the exciting possibility of paradigm-changing contributions to fundamental physics. The requirements of these science cases can be met by a stable instrument with a spectral resolution of R~100,000 and broad, simultaneous spectral coverage extending from 370nm to 2500nm. Most science cases do not require spatially resolved information, and can be pursued in seeing-limited mode, although some of them would benefit by the E-ELT diffraction limited resolution. Some multiplexing would also be beneficial for some of the science cases. (Abridged)
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    ABSTRACT: We present new measurements of the thermal state of the intergalactic medium (IGM) at z~2.4 derived from absorption line profiles in the Lyman-alpha forest. We use a large set of high-resolution hydrodynamical simulations to calibrate the relationship between the temperature-density relation in the IGM and the distribution of HI column densities, N_HI, and velocity widths, b_HI, of discrete Lyman-alpha forest absorbers. This calibration is then applied to the measurement of the lower cut-off of the b_HI-N_HI distribution recently presented by Rudie et al. (2012). We infer a power-law temperature-density relation, T=T_0\Delta^(\gamma-1), with a temperature at mean density, T_0=[1.00^+0.32_-0.21] x10^4 K and slope (\gamma-1)=0.54\pm 0.11. The slope is fully consistent with that advocated by the analysis of Rudie et al. (2012); however, the temperature at mean density is lower by almost a factor of two, a difference due primarily to an adjustment in the relationship between column density and physical density assumed by these authors. These new results bring the b_HI-N_HI cut-off constraint into excellent agreement with the recent temperature measurements of Becker et al. (2011), based on the curvature of the transmitted flux in the Lyman-alpha forest. The fact that independent analyses using different techniques constrain IGM temperatures to less than 30 per cent and are mutually consistent suggests that the thermal state of the IGM at this redshift is now reasonably well characterised, at least over the range of densities probed by these methods. Future works using these and other techniques should be able to further clarify the thermal structure and evolution of the IGM at a level of precision sufficient to deliver new insights into cosmic reionisation and the properties of ionising sources.
    Monthly Notices of the Royal Astronomical Society 08/2013; · 5.52 Impact Factor
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    ABSTRACT: We employ cosmological hydrodynamical simulations to investigate models in which the supermassive black holes (BHs) powering luminous z ~ 6 QSOs grow from massive seeds. We simulate at high resolution 18 fields sampling regions with densities ranging from the mean cosmic density all the way to the highest sigma peaks in the Millennium simulation volume. Only in the most massive halos, BHs can grow to masses up to ~ 10^9 Msun by z ~ 6 without invoking super-Eddington accretion. Accretion onto the most massive BHs becomes limited by thermal AGN feedback by z ~ 9-8 with further BH growth proceeding in short Eddington limited bursts. Our modelling suggests that current flux-limited surveys of QSOs at high redshift preferentially detect objects at their peak luminosity and therefore miss a substantial population of QSOs powered by similarly massive BHs but with low accretion rates. To test whether the required host halo masses are consistent with the observed galaxy environment of z ~ 6 QSOs, we produce realistic rest-frame UV images of our simulated galaxies. Without strong stellar feedback, our simulations predict numbers of bright galaxies larger than observed by a factor ten or more. Supernova-driven galactic winds reduce the predicted numbers to a level consistent with observations indicating that stellar feedback was already very efficient at high redshifts. We have further investigated the effect of thermal AGN feedback on the surrounding gas. Our adopted AGN feedback prescription drives mostly energy-driven highly anisotropic outflows with gas speeds of >= 1000 km/s to distances of >= 10 kpc consistent with observations. The spatially extended thermal X-ray emission around bright QSOs powered by these outflows can exceed by large factors the emission expected without AGN feedback and is an important diagnostic of the mechanism whereby AGN feedback energy couples to surrounding gas.
    07/2013; 439(2).
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    M. Viel, G. D. Becker, J. S. Bolton, M. G. Haehnelt
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    ABSTRACT: We present updated constraints on the free-streaming of warm dark matter (WDM) particles derived from an analysis of the Lya flux power spectrum measured from high-resolution spectra of 25 z > 4 quasars obtained with the Keck High Resolution Echelle Spectrometer (HIRES) and the Magellan Inamori Kyocera Echelle (MIKE) spectrograph. We utilize a new suite of high-resolution hydrodynamical simulations that explore WDM masses of 1, 2 and 4 keV (assuming the WDM consists of thermal relics), along with different physically motivated thermal histories. We carefully address different sources of systematic error that may affect our final results and perform an analysis of the Lya flux power with conservative error estimates. By using a method that samples the multi-dimensional astrophysical and cosmological parameter space, we obtain a lower limit mwdm > 3.3 keV (2sigma) for warm dark matter particles in the form of early decoupled thermal relics. Adding the Sloan Digital Sky Survey (SDSS) Lya flux power spectrum does not improve this limit. Thermal relics of masses 1 keV, 2 keV and 2.5 keV are disfavoured by the data at about the 9sigma, 4sigma and 3sigma C.L., respectively. Our analysis disfavours WDM models where there is a suppression in the linear matter power spectrum at (non-linear) scales corresponding to k=10h/Mpc which deviates more than 10% from a LCDM model. Given this limit, the corresponding "free-streaming mass" below which the mass function may be suppressed is 2x10^8 Msun/h. There is thus very little room for a contribution of the free-streaming of WDM to the solution of what has been termed the small scale crisis of cold dark matter.
    Physical review D: Particles and fields 06/2013; 88(4).
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    ABSTRACT: A spectroscopically detected Lyman alpha emitting halo at redshift 3.216 in the GOODS-N field is found to reside at the convergence of several Lyman alpha filaments. HST images show that some of the filaments are inhabited by galaxies. Several of the galaxies in the field have pronounced head-tail structures, which are partly aligned with each other. The blue colors of most tails suggest the presence of young stars, with the emission from at least one of the galaxies apparently dominated by high equivalent width Lyman alpha. Faint, more diffuse, and similarly elongated, apparently stellar features, can be seen over an area with a linear extent of at least 90 kpc. The region within several arcseconds of the brightest galaxy exhibits spatially extended emission by HeII, NV and various lower ionization metal lines. The gas-dynamical features present are strongly reminiscent of ram-pressure stripped galaxies, including evidence for recent star formation in the stripped contrails. Spatial gradients in the appearance of several galaxies may represent a stream of galaxies passing from a colder to a hotter intergalactic medium. The stripping of gas from the in-falling galaxies, in conjunction with the occurrence of star formation and stellar feedback in the galactic contrails suggests a mechanism for the metal enrichment of the high redshift intergalactic medium that does not depend on long-range galactic winds, at the same time opening a path for the escape of ionizing radiation from galaxies.
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    ABSTRACT: In this third paper in a series on the nature of extended, asymmetric Lyman alpha emitters at z ~ 3 we report the discovery, in an ultra-deep, blind, spectroscopic long-slit survey, of a Lyman alpha emitting halo around a QSO at redshift 3.045. The QSO is a previously known, obscured AGN. The halo appears extended along the direction of the slit and exhibits two faint patches separated by 17 proper kpc in projection from the QSO. Comparison of the 2-dimensional spectrum with archival HST ACS images shows that these patches coincide spatially with emission from a peculiar, dumbbell-shaped, faint galaxy. The assumptions that the Lyman alpha emission patches are originating in the galaxy and that the galaxy is physically related to the QSO are at variance with photometric estimates of the galaxy redshift. We show, however, that a population of very young stars at the redshift of the QSO may fit the existing rest frame broad band UV photometry of the galaxy. If this scenario is correct then the symmetry of the galaxy in continuum and Lyman alpha emission, the extension of the QSO's Lyman alpha emission in its direction, and the likely presence of a young stellar population in close proximity to a (short-lived) AGN suggest that this may be an example of AGN feedback triggering external star formation in high redshift galaxies.
    Monthly Notices of the Royal Astronomical Society 02/2013; 431(1). · 5.52 Impact Factor
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    ABSTRACT: We present forecasts for the accuracy of determining the parameters of a minimal cosmological model and the total neutrino mass based on combined mock data for a future Euclid-like galaxy survey and Planck. We consider two different galaxy surveys: a spectroscopic redshift survey and a cosmic shear survey. We make use of the Monte Carlo Markov Chains (MCMC) technique and assume two sets of theoretical errors. The first error is meant to account for uncertainties in the modelling of the effect of neutrinos on the non-linear galaxy power spectrum and we assume this error to be fully correlated in Fourier space. The second error is meant to parametrize the overall residual uncertainties in modelling the non-linear galaxy power spectrum at small scales, and is conservatively assumed to be uncorrelated and to increase with the ratio of a given scale to the scale of non-linearity. It hence increases with wavenumber and decreases with redshift. With these two assumptions for the errors and assuming further conservatively that the uncorrelated error rises above 2% at k = 0.4 h/Mpc and z = 0.5, we find that a future Euclid-like cosmic shear/galaxy survey achieves a 1-sigma error on Mnu close to 32 meV/25 meV, sufficient for detecting the total neutrino mass with good significance. If the residual uncorrelated errors indeed rises rapidly towards smaller scales in the non-linear regime as we have assumed here then the data on non-linear scales does not increase the sensitivity to the total neutrino mass. Assuming instead a ten times smaller theoretical error with the same scale dependence, the error on the total neutrino mass decreases moderately from sigma(Mnu) = 18 meV to 14 meV when mildly non-linear scales with 0.1 h/Mpc < k < 0.6 h/Mpc are included in the analysis of the galaxy survey data.
    Journal of Cosmology and Astroparticle Physics 10/2012; 2013(01). · 6.04 Impact Factor
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    James S. Bolton, Martin G. Haehnelt
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    ABSTRACT: A variety of independent observational studies have now reported a significant decline in the fraction of Lyman-break galaxies which exhibit Ly-a emission over the redshift interval z=6-7. In combination with the strong damping wing extending redward of Ly-a in the spectrum of the bright z=7.085 quasar ULAS 1120+0641, this has strengthened suggestions that the hydrogen in the intergalactic medium (IGM) is still substantially neutral at z~7. Current theoretical models imply HI fractions as large as 40-90 per cent may be required to explain these data assuming there is no intrinsic evolution in the Ly-a emitter population. We propose that such large neutral fractions are not necessary. Based on a hydrodynamical simulation which reproduces the absorption spectra of high-redshift (z~6-7) quasars, we demonstrate that the opacity of the intervening IGM redward of rest-frame Ly-a can rise rapidly in average regions of the Universe simply because of the increasing incidence of absorption systems which are optically thick to Lyman continuum photons as the tail-end of reionisation is approached. Our simulations suggest these data do not require a large change in the IGM neutral fraction by several tens of per cent from z=6-7, but may instead be indicative of the rapid decrease in the typical mean free path for ionising photons expected during the final stages of reionisation.
    Monthly Notices of the Royal Astronomical Society 08/2012; 429(2). · 5.52 Impact Factor
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    ABSTRACT: Supermassive black holes (BH) are powerful sources of energy that are already in place at very early epochs of the Universe (by z=6). Using hydrodynamical simulations of the formation of a massive M_vir=5 10^11 M_sun halo by z=6 (the most massive progenitor of a cluster of M_vir=2 10^15 M_sun at z=0), we evaluate the impact of Active Galactic Nuclei (AGN) on galaxy mass content, BH self-regulation, and gas distribution inside this massive halo. We find that SN feedback has a marginal influence on the stellar structure, and no influence on the mass distribution on large scales. In contrast, AGN feedback alone is able to significantly alter the stellar-bulge mass content by quenching star formation when the BH is self-regulating, and by depleting the cold gas reservoir in the centre of the galaxy. The growth of the BH proceeds first by a rapid Eddington-limited period fed by direct cold filamentary infall. When the energy delivered by the AGN is sufficiently large to unbind the cold gas of the bulge, the accretion of gas onto the BH is maintained both by smooth gas inflow and clump migration through the galactic disc triggered by merger-induced torques. The feedback from the AGN has also a severe consequence on the baryon mass content within the halo, producing large-scale hot superwinds, able to blow away some of the cold filamentary material from the centre and reduce the baryon fraction by more than 30 per cent within the halo's virial radius. Thus in the very young universe, AGN feedback is likely to be a key process, shaping the properties of the most massive galaxies.
    Monthly Notices of the Royal Astronomical Society 06/2012; 428(4). · 5.52 Impact Factor
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    ABSTRACT: We report the observation of a further asymmetric, extended Lyman alpha emitting halo at z=2.63, from our ultra-deep, long-slit spectroscopic survey of faint high redshift emitters, undertaken with Magellan LDSS3 in the GOODS-S field. The Lya emission, detected over more than 30 kpc, is spatially coincident with a concentration of galaxies visible in deep broad-band imaging. While these faint galaxies without spectroscopic redshifts cannot with certainty be associated with one another or with the Lya emission, there are a number of compelling reasons why they very probably form a Milky Way halo-mass group at the Lya redshift. A filamentary structure, possibly consisting of Lya emission at very high equivalent width, and evidence for disturbed stellar populations, suggest that the properties of the emitting region reflect ongoing galaxy assembly, with recent galaxy mergers and star formation occurring in the group. Hence, the Lya provides unique insights into what is probably a key mode of galaxy formation at high redshifts. The Lya emission may be powered by cooling radiation or spatially extended star formation in the halo, but is unlikely to be fluorescence driven by either an AGN or one of the galaxies. The spatial profile of the emission is conspicuously different from that of typical Lya emitters or Lyman break galaxies, which is consistent with the picture that extended emission of this kind represents a different stage in the galaxy formation process. Faint, extended Lya emitters such as these may be lower-mass analogues of the brighter Lya blobs. Our observations provide further, circumstantial evidence that galaxy mergers may promote the production and / or escape of ionizing radiation, and that the halos of interacting galaxies may be significant sources for ionizing photons during the epoch of reionization.
    Monthly Notices of the Royal Astronomical Society 06/2012; 429(1). · 5.52 Impact Factor
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    Sebastiano Cantalupo, Simon J. Lilly, Martin G. Haehnelt
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    ABSTRACT: A deep narrow-band survey for Ly-alpha emission carried out on the VLT-FORS2 has revealed 98 Ly-alpha candidates down to a flux limit of 4.e-18 erg/s/cm^2 in a volume of 5500 comoving Mpc^3 at z=2.4 centered on the hyperluminous quasar HE0109-3518. The properties of the detected sources in terms of their i) equivalent width distribution, ii) luminosity function, and iii) the average luminosity versus projected distance from the quasar, all suggest that a large fraction of these objects have been fluorescently "illuminated" by HE0109-3518. This conclusion is supported by comparison with detailed radiative transfer simulations of the effects of the quasar illumination. 18 objects have a rest-frame Equivalent Width (EW0) larger than 240A, the expected limit for Ly-alpha emission powered by Population II star formation and 12 sources among these do not have any continuum counterpart in a deep V-band imaging of the same field. For these, a stacking analysis indicates EW0>800A, effectively ruling out Ly-alpha powered by internal star formation. These sources are thus the best candidates so far for proto-galactic clouds or "dark" galaxies at high-redshift, whose existence has recently been suggested by several theoretical studies. Assuming they are mostly ionized by the quasar radiation, we estimate that their gas masses would be about 10^9 Msun implying that their star formation efficiencies (SFE) are less than 1.e-11 yr^-1 several times below the SFE of the most gas-rich dwarf galaxies locally, and five hundred times lower than typical massive star-forming galaxies at z~2. We have also discovered extended, filamentary gas, also likely illuminated by the quasar, around some of the brightest continuum-detected sources with EW0>240A. This emission is compatible with the expectations for circum-galactic cold streams but other origins, including tidal stripping, are also possible.
    Monthly Notices of the Royal Astronomical Society 04/2012; 425(3). · 5.52 Impact Factor

Publication Stats

4k Citations
530.06 Total Impact Points


  • 2004–2014
    • Cancer Research UK Cambridge Institute
      Cambridge, England, United Kingdom
  • 1993–2013
    • University of Cambridge
      • Institute of Astronomy
      Cambridge, England, United Kingdom
  • 2012
    • Université de Savoie
      Chambéry, Rhône-Alpes, France
  • 2008–2012
    • The Astronomical Observatory of Brera
      Merate, Lombardy, Italy
  • 2011
    • INFN - Istituto Nazionale di Fisica Nucleare
      Frascati, Latium, Italy
  • 2002–2006
    • Imperial College London
      Londinium, England, United Kingdom
  • 2005
    • University of California, Santa Barbara
      • Kavli Institute for Theoretical Physics
      Santa Barbara, CA, United States
    • Cambridge Healthtech Institute
      Needham, Massachusetts, United States
  • 1995–2002
    • Max Planck Institute for Astrophysics
      Arching, Bavaria, Germany
  • 1999
    • University of Illinois, Urbana-Champaign
      Urbana, Illinois, United States
  • 1998
    • The University of Arizona
      Tucson, Arizona, United States
    • California Institute of Technology
      • Department of Astronomy
      Pasadena, California, United States