arXiv subjects

Simon J. Lilly

Publications and source records attributed to Simon J. Lilly.

At least 55 records · Page 3Linked to original sources

arXiv2016

Faraday Rotation Measure Synthesis of intermediate redshift quasars as a probe of intervening matter

There is evidence that magnetized material along the line of sight to distant quasars is detectable in the polarization properties of the background sources. The polarization properties appear to be correlated with the presence of intervening MgII absorption, which is thought to arise in outflowing material from star forming galaxies. In order to investigate this further, we have obtained high spectral resolution polarization measurements, with the VLA and ATCA, of a set of 49 unresolved quasars for which we have high quality optical spectra. These enable us to produce a Faraday Depth spectrum for each source, using Rotation Measure Synthesis. Our new independent radio data confirms that interveners are strongly associated with depolarization. We characterize the complexity of the Faraday Depth spectrum using a number of parameters and show how these are related, or not, to the depolarization and to the presence of MgII absorption along the line of sight. We argue that complexity and structure in the Faraday Depth distribution likely arise from both intervening material and intrinsically to the background source and attempt to separate these. We find that the strong radio depolarization effects associated with intervening material at redshifts out to $z \approx 1$ arise from inhomogeneous Faraday screens producing a dispersion in Rotation Measure across individual sources of around 10~rad/m$^2$. This is likely produced by disordered fields with strengths of at least $3\;μ$G.

astro-ph.GA↗

arXiv2016

Minor Mergers or Progenitor Bias? The Stellar Ages of Small and Large Quenched Early-Type Galaxies

We investigate the origin of the evolution of the population-averaged size of quenched galaxies (QGs) through a spectroscopic analysis of their stellar ages. The two most favoured scenarios for this evolution are either the size growth of individual galaxies through a sequence of dry minor merger events, or the addition of larger, newly quenched galaxies to the pre-existing population (i.e., a progenitor bias effect). We use the 20k zCOSMOS-bright spectroscopic survey to select bona fide quiescent galaxies at 0.2<z<0.8. We stack their spectra in bins of redshift, stellar mass and size to compute stellar population parameters in these bins through fits to the rest-frame optical spectra and through Lick spectral indices. We confirm a change of behaviour in the size-age relation below and above the ~10^11 MSun stellar mass scale: In our 10.5 < log M*/MSun < 11 mass bin, over the entire redshift window, the stellar populations of the largest galaxies are systematically younger than those of the smaller counterparts, pointing at progenitor bias as the main driver of the observed average size evolution at sub-10^11 MSun masses. In contrast, at higher masses, there is no clear trend in age as a function of galaxy size, supporting a substantial role of dry mergers in increasing the sizes of these most massive QGs with cosmic time. Within the errors, the [alpha/Fe] abundance ratios of QGs are (i) above-solar over the entire redshift range of our analysis, hinting at universally short timescales for the buildup of the stellar populations of QGs, and (ii) similar at all masses and sizes, suggesting similar (short) timescales for the whole QG population and strengthening the role of mergers in the buildup of the most massive QGs in the Universe.

astro-ph.GA↗

arXiv2016

Constraining the lifetime and opening angle of quasars using fluorescent Lyman $α$ emission: the case of Q0420-388

A toy model is developed to understand how the spatial distribution of fluorescent emitters in the vicinity of bright quasars could be affected by the geometry of the quasar bi-conical radiation field and by its lifetime. The model is then applied to the distribution of high equivalent width Lyman $α$ emitters (with rest-frame equivalent widths above 100 A, threshold used in e.g. Trainor & Steidel, 2013) identified in a deep narrow-band 36x36 arcmin$^2$ image centered on the luminous quasar Q0420-388. These emitters are found to the edge of the field and show some evidence of an azimuthal asymmetry on the sky of the type expected if the quasar is radiating in a bipolar cone. If these sources are being fluorescently illuminated by the quasar, the two most distant objects require a lifetime of at least 15 Myr for an opening angle of 60 degrees or more, increasing to more than 40 Myr if the opening angle is reduced to a minimum 30 degrees. However, few of the other expected signatures of boosted fluorescence are seen at the current survey limits, e.g. a fall off in Lyman $α$ brightness, or equivalent width, with distance. Furthermore, to have most of the Lyman $α$ emission of the two distant sources to be fluorescently boosted would require the quasar to have been significantly brighter in the past. This suggests that these particular sources may not be fluorescent, invalidating the above lifetime constraints. This would cast doubt on the use of this relatively low equivalent width threshold and thus on the lifetime analysis in Trainor & Steidel (2013).

astro-ph.GA↗

arXiv2016

Ubiquitous giant Ly $α$ nebulae around the brightest quasars at $z\sim3.5$ revealed with MUSE

Direct Ly $α$ imaging of intergalactic gas at $z\sim2$ has recently revealed giant cosmological structures around quasars, e.g. the Slug Nebula (Cantalupo et al. 2014). Despite their high luminosity, the detection rate of such systems in narrow-band and spectroscopic surveys is less than 10%, possibly encoding crucial information on the distribution of gas around quasars and the quasar emission properties. In this study, we use the MUSE integral-field instrument to perform a blind survey for giant Ly $α$ nebulae around 17 bright radio-quiet quasars at $3<z<4$ that does not suffer from most of the limitations of previous surveys. After data reduction and analysis performed with specifically developed tools, we found that each quasar is surrounded by giant Ly $α$ nebulae with projected sizes larger than 100 physical kpc and, in some cases, extending up to 320 kpc. The circularly averaged surface brightness profiles of the nebulae appear very similar to each other despite their different morphologies and are consistent with power laws with slopes $\approx-1.8$. The similarity between the properties of all these nebulae and the Slug Nebula suggests a similar origin for all systems and that a large fraction of gas around bright quasars could be in a relatively "cold" (T$\sim$10$^4$K) and dense phase. In addition, our results imply that such gas is ubiquitous within at least 50 kpc from bright quasars at $3<z<4$ independently of the quasar emission opening angle, or extending up to 200 kpc for quasar isotropic emission.

astro-ph.GA↗

arXiv2016

Large scale structure around a z=2.1 cluster

The most prodigious starburst galaxies are absent in massive galaxy clusters today, but their connection with large scale environments is less clear at $z\gtrsim2$. We present a search of large scale structure around a galaxy cluster core at $z=2.095$ using a set of spectroscopically confirmed galaxies. We find that both color-selected star-forming galaxies (SFGs) and dusty star-forming galaxies (DSFGs) show significant overdensities around the $z=2.095$ cluster. A total of 8 DSFGs (including 3 X-ray luminous active galactic nuclei, AGNs) and 34 SFGs are found within a 10 arcmin radius (corresponds to $\sim$15 cMpc at $z\sim2.1$) from the cluster center and within a redshift range of $Δz=0.02$, which leads to galaxy overdensities of $δ_{\rm DSFG}\sim12.3$ and $δ_{\rm SFG}\sim2.8$. The cluster core and the extended DSFG- and SFG-rich structure together demonstrate an active cluster formation phase, in which the cluster is accreting a significant amount of material from large scale structure while the more mature core may begin to virialize. Our finding of this DSFG-rich structure, along with a number of other protoclusters with excess DSFGs and AGNs found to date, suggest that the overdensities of these rare sources indeed trace significant mass overdensities. However, it remains puzzling how these intense star formers are triggered concurrently. Although an increased probability of galaxy interactions and/or enhanced gas supply can trigger the excess of DSFGs, our stacking analysis based on 850 $μ$m images and morphological analysis based on rest-frame optical imaging do not show such enhancements of merger fraction and gas content in this structure.

astro-ph.GA↗

arXiv2016

Surface density effects in quenching: cause or effect?

There are very strong observed correlations between the specific star-formation rates (sSFR) of galaxies and their mean surface mass densities, Σ, as well as other aspects of their internal structure. These strong correlations have often been taken to indicate that the internal structure of a galaxy must play a major physical role, directly or indirectly, in the control of star-formation. In this paper we show by means of a very simple toy model that these correlations can arise naturally without any such physical role once the observed evolution of the size-mass relation for star-forming galaxies is taken into account. In particular, the model reproduces the sharp threshold in Σ between galaxies that are star-forming and those that are quenched, and the evolution of this threshold with redshift. Similarly, it produces iso-quenched-fraction contours in the ${f_Q(m,R_e)}$ plane that are almost exactly parallel to lines of constant Σ for centrals and shallower for satellites. It does so without any dependence on quenching on size or Σ, and without invoking any differences between centrals and satellites, beyond the different mass-dependences of their quenching laws. The toy-model also reproduces several other observations, including the sSFR gradients within galaxies and the appearance of inside-out build-up of passive galaxies. Finally, it is shown that curvature in the Main Sequence sSFR-mass relation can produce curvature in the apparent B/T ratios with mass. Our analysis therefore suggests that many of the strong correlations that are observed between galaxy structure and sSFR may well be a consequence of things unrelated to quenching and should not be taken as evidence of the physical processes that drive quenching.

astro-ph.GA↗

arXiv2016

ZAP -- Enhanced PCA Sky Subtraction for Integral Field Spectroscopy

We introduce Zurich Atmosphere Purge (ZAP), an approach to sky subtraction based on principal component analysis (PCA) that we have developed for the Multi Unit Spectrographic Explorer (MUSE) integral field spectrograph. ZAP employs filtering and data segmentation to enhance the inherent capabilities of PCA for sky subtraction. Extensive testing shows that ZAP reduces sky emission residuals while robustly preserving the flux and line shapes of astronomical sources. The method works in a variety of observational situations from sparse fields with a low density of sources to filled fields in which the target source fills the field of view. With the inclusion of both of these situations the method is generally applicable to many different science cases and should also be useful for other instrumentation. ZAP is available for download at http://muse-vlt.eu/science/tools.

astro-ph.IM↗

arXiv2015

AGN evolution from a galaxy evolution viewpoint

We explore the connections between the evolving galaxy and AGN populations. We present a simple phenomenological model that links the evolving galaxy mass function and the evolving quasar luminosity function, which makes specific and testable predictions for the distribution of host galaxy masses for AGN of different luminosities. We show that the $ϕ^{*}$ normalisations of the galaxy mass function and of the AGN luminosity function closely track each other over a wide range of redshifts, implying a constant "duty cycle" of AGN activity. The strong redshift evolution in the AGN $L^*$ can be produced by either an evolution in the distribution of Eddington ratios, or in the $m_{bh}/m_{*}$ mass ratio, or both. To try to break this degeneracy we look at the distribution of AGN in the SDSS ($m_{bh},L$) plane, showing that an evolving ratio $m_{bh}/m_{*} \propto (1+z)^2$ reproduces the observed data and also reproduces the local relations which connect the black hole population with the host galaxies for both quenched and star-forming populations. We stress that observational studies that compare the masses of black holes in active galaxies at high redshift with those in quiescent galaxies locally will always see much weaker evolution. Evolution of this form would produce, or could be produced by, a redshift-independent $m_{bh} - σ$ relation and could explain why the local $m_{bh} - σ$ relation is tighter than $m_{bh} - m_{*}$ even if $σ$ is not directly linked to black hole growth. Irrespective of the evolution of $m_{bh}/m_{*}$, the model reproduces both the appearance of "downsizing" and the so-called "sub-Eddington boundary" without any mass-dependence in the evolution of black hole growth rates.

astro-ph.GA↗

arXiv2015

Quenching of Star Formation in SDSS Groups: Centrals, Satellites, and Galactic Conformity

We re-examine the fraction of low-redshift Sloan Digital Sky Survey satellites and centrals in which star formation has been quenched, using the environment quenching efficiency formalism that separates out the dependence of stellar mass. We show that the centrals of the groups containing the satellites are responding to the environment in the same way as their satellites (at least for stellar masses above 10^10.3 Msun), and that the well-known differences between satellites and the general set of centrals arise because the latter are overwhelmingly dominated by isolated galaxies. The widespread concept of "satellite quenching" as the cause of environmental effects in the galaxy population can therefore be generalized to "group quenching". We then explore the dependence of the quenching efficiency of satellites on overdensity, group-centric distance, halo mass, the stellar mass of the satellite, and the stellar mass and specific star formation rate (sSFR) of its central, trying to isolate the effect of these often interdependent variables. We emphasize the importance of the central sSFR in the quenching efficiency of the associated satellites, and develop the meaning of this "galactic conformity" effect in a probabilistic description of the quenching of galaxies. We show that conformity is strong, and that it varies strongly across parameter space. Several arguments then suggest that environmental quenching and mass quenching may be different manifestations of the same underlying process. The marked difference in the apparent mass dependencies of environment quenching and mass quenching which produces distinctive signatures in the mass functions of centrals and satellites will arise naturally, since, for satellites at least, the distributions of the environmental variables that we investigate in this work are essentially independent of the stellar mass of the satellite.

astro-ph.GA↗

arXiv2014

A Consistent Study of Metallicity Evolution at 0.8 < z < 2.6

We present the correlations between stellar mass, star formation rate (SFR) and [NII]/Ha flux ratio as indicator of gas-phase metallicity for a sample of 222 galaxies at 0.8 < z < 2.6 and log(M*/Msun)=9.0-11.5 from the LUCI, SINS/zC-SINF and KMOS3D surveys. This sample provides a unique analysis of the mass-metallicity relation (MZR) over an extended redshift range using consistent data analysis techniques and strong-line metallicity indicator. We find a constant slope at the low-mass end of the relation and can fully describe its redshift evolution through the evolution of the characteristic turnover mass where the relation begins to flatten at the asymptotic metallicity. At fixed mass and redshift, our data do not show a correlation between the [NII]/Ha ratio and SFR, which disagrees with the 0.2-0.3dex offset in [NII]/Ha predicted by the "fundamental relation" between stellar mass, SFR and metallicity discussed in recent literature. However, the overall evolution towards lower [NII]/Ha at earlier times does broadly agree with these predictions.

astro-ph.GA↗

arXiv2014

Mass and Environment as Drivers of Galaxy Evolution III: The constancy of the faint-end slope and the merging of galaxies

We explore using our continuity approach the underlying connections between the evolution of the faint-end slope of the stellar mass function of star-forming galaxies, the logarithmic slope of the sSFR-mass relation and the merging of galaxies. We derive analytically the consequences of the observed constancy of the faint-end slope since redshifts of at least z ~ 2. If the logarithmic slope of the sSFR-mass relation is negative, then the faint-end slope should quickly diverge due to the differential mass increase of galaxies on the star-forming main sequence, and this will also quickly destroy the Schechter form of the mass function. This problem can be solved by removing low mass galaxies by merging them into more massive galaxies. We quantify this process by introducing the specific merger mass rate (sMMR) as the specific rate of mass added to a given galaxy through mergers. For a modest negative value of the logarithmic slope of the sSFR-mass relation of beta ~ -0.1, an average sMMR ~ 0.1sSFR across the population is required to keep the faint-end slope constant with epoch, as observed. This in turn implies a merger rate of ~ 0.2sSFR for major mergers, which is consistent with the available observational estimates. More negative values of beta require higher sMMR and higher merger rates, and the steepening of the mass function becomes impossible to control for beta < ~ -0.6, for an observed value of the faint-end slope of ~ -1.4. The close link that is required between the in situ sSFR and the sMMR probably arises because both are closely linked to the buildup of dark matter haloes. These new findings further develop the formalism for the evolving galaxy population that we introduced earlier, and show how striking symmetries in the galaxy population can emerge as the result of deep links between the physical processes involved.

astro-ph.GA↗

arXiv2014

The X-Ray Zurich Environmental Study (X-ZENS). II. X-ray Observations of the Diffuse Intragroup Medium in Galaxy Groups

We present the results of a pilot XMM-$Newton$ and $Chandra$ program aimed at studying the diffuse intragroup medium (DIM) of optically-selected nearby groups from the Zurinch ENvironmental Study (ZENS) catalog. The groups are in a narrow mass range about $10^{13}M_\odot$, a mass scale at which the interplay between the DIM and the group member galaxies is still largely unprobed. X-ray emission from the DIM is detected in the energy band 0.5--2 keV with flux $\le 10^{-14}$ erg cm$^{-1}$ s$^{-1}$, which is one order of magnitude fainter than for typical ROSAT groups (RASS). For many groups we set upper limits to the X-ray luminosity, indicating that the detections are likely probing the upper envelope of the X-ray emitting groups. We find evidence for our optically selected groups to be under-luminous with respect to predictions from X-ray scaling relations. X-ray mass determinations are in best agreement with those based on the member galaxies bulge luminosity, followed by their total optical luminosity and velocity dispersion. We measure a stellar mass fraction with a median value of about 1$\%$, with a contribution from the most massive galaxies between 30 to 50 \%. Optical and X-ray data give often complementary answers concerning the dynamical state of the groups, and are essential for a complete picture of the system. Extending this pilot program to a larger sample of groups is mandatory to unveil any imprint of interaction between member galaxies and DIM in halo potentials of key importance for environmentally-driven galactic evolution.

astro-ph.CO↗

arXiv2013

The SINS/zC-SINF Survey of z~2 Galaxy Kinematics: The Nature of Dispersion Dominated Galaxies

We analyze the spectra, spatial distributions and kinematics of Ha, [NII] and [SII] emission in a sample of 42, z~2.2 UV/optically selected star forming galaxies (SFGs) from the SINS & zC-SINF surveys, 35 of which were observed in the adaptive optics mode of SINFONI. This is supplemented by kinematic data from 48 z~1-2.5 galaxies from the literature. We find that the kinematic classification of the high-z SFGs as `dispersion dominated' or `rotation dominated' correlates most strongly with their intrinsic sizes. Smaller galaxies are more likely `dispersion-dominated' for two main reasons: 1) The rotation velocity scales linearly with galaxy size but intrinsic velocity dispersion does not depend on size, and as such, their ratio is systematically lower for smaller galaxies, and 2) Beam smearing strongly decreases large-scale velocity gradients and increases observed dispersion much more for galaxies with sizes at or below the resolution. Dispersion dominated SFGs may thus have intrinsic properties similar to `rotation dominated' SFGs, but are primarily more compact, lower mass, less metal enriched and may have higher gas fractions, plausibly because they represent an earlier evolutionary state.

astro-ph.CO↗

arXiv2013

Gas-regulation of galaxies: the evolution of the cosmic sSFR, the metallicity-mass-SFR relation and the stellar content of haloes

A very simple physical model of galaxies, in which the formation of stars is instantaneously regulated by the mass of gas in a reservoir, links together three different aspects of the evolving galaxy population:(a) the cosmic time evolution of the specific star-formation rate sSFR relative to the growth of haloes, (b) the gas-phase metallicities across the galaxy population and over cosmic time, and (c) the ratio of the stellar to dark matter mass of haloes. If the SFR efficiency and wind mass loading are constant, the sSFR is set to the specific accretion rate of the galaxy: more realistic situations lead to an sSFR which is perturbed from this identity. The metallicity is set by the instantaneous operation of the regulator system rather than by the past history of the system. The regulator system naturally produces a Z(mstar, SFR) relation, with SFR as a second parameter in the mass-metallicity relation. This will be the same at all epochs unless the efficiency and mass-loading change with time, naturally producing a so-called "fundamental metallicity relation". The observed Z(mstar) relation of SDSS galaxies implies a strong dependence of stellar mass on halo mass that reconciles the different faint end slopes of the stellar and halo mass-functions in standard ΛCDM. It also boosts the sSFR relative to the specific accretion rate and produces a different dependence on mass, both of which are observed. The derived Z(mstar, SFR) relation for the regulator system is fit to published Z(mstar, SFR) data. The fitted efficiency is consistent with observed molecular gas-depletion timescales in galaxies while the fitted mass-loading is also plausible. The model also successfully reproduces the mass-metallicity relation of star-forming galaxies at z ~ 2.

astro-ph.CO↗

arXiv2013

A comparative analysis of virial black-hole mass estimates of moderate-luminosity active galactic nuclei using Subaru/FMOS

We present an analysis of broad emission lines observed in moderate-luminosity active galactic nuclei (AGNs), typical of those found in X-ray surveys of deep fields, with the aim to test the validity of single-epoch virial black hole mass estimates. We have acquired near-infrared (NIR) spectra of AGNs up to z ~ 1.8 in the COSMOS and Extended Chandra Deep Field-South Survey, with the Fiber Multi-Object Spectrograph (FMOS) mounted on the Subaru Telescope. These low-resolution NIR spectra provide a significant detection of the broad Halpha line that has been shown to be a reliable probe of black hole mass at low redshift. Our sample has existing optical spectroscopy which provides a detection of MgII, a broad emission line typically used for black hole mass estimation at z > 1. We carry out a spectral-line fitting procedure using both Halpha and MgII to determine the virial velocity of gas in the broad line region, the monochromatic continuum luminosity at 3000 A, and the total Halpha line luminosity. With a sample of 43 AGNs spanning a range of two decades in luminosity (i.e., L ~ 10^44-46 ergs/s), we find a tight correlation between the continuum and line luminosity with a distribution characterized by = 1.52 and a dispersion sigma = 0.16. There is also a close one-to-one relationship between the FWHM of Halpha and of MgII up to 10000 km/s with a dispersion of 0.14 in the distribution of the logarithm of their ratios. Both of these then lead to there being very good agreement between Halpha- and MgII-based masses over a wide range in black hole mass (i.e., M_BH ~ 10^7-9 M_sun). We do find a small offset in MgII-based masses, relative to those based on Halpha, of +0.17 dex and a dispersion sigma = 0.32. In general, these results demonstrate that local scaling relations, using MgII or Halpha, are applicable for AGN at moderate luminosities and up to z ~ 2.

astro-ph.CO↗

arXiv2012

The SINS/zC-SINF survey of z~2 galaxy kinematics: Outflow properties

Based on SINFONI Ha, [NII] and [SII] AO data of 30 z \sim 2 star-forming galaxies (SFGs) from the SINS and zcSINF surveys, we find a strong correlation of the Ha broad flux fraction with the star formation surface density of the galaxy, with an apparent threshold for strong outflows occurring at 1 Msun yr^-1 kpc^-2. Above this threshold, we find that SFGs with logm_\ast>10 have similar or perhaps greater wind mass loading factors (eta = Mdotout/SFR) and faster outflow velocities than lower mass SFGs. This trend suggests that the majority of outflowing gas at z \sim 2 may derive from high-mass SFGs, and that the z \sim 2 mass-metallicity relation is driven more by dilution of enriched gas in the galaxy gas reservoir than by the efficiency of outflows. The mass loading factor is also correlated with the SFR and inclination, such that more star-forming and face-on galaxies launch more powerful outflows. For galaxies that have evidence for strong outflows, we find that the broad emission is spatially extended to at least the half-light radius (\sim a few kpc). We propose that the observed threshold for strong outflows and the observed mass loading of these winds can be explained by a simple model wherein break-out of winds is governed by pressure balance in the disk. Using the ratio of the [SII] doublet in a broad and narrow component, we find that outflowing gas has a density of \sim10-100 cm^-3, significantly less than that of the star forming gas (600 cm^-3).

astro-ph.CO↗

arXiv2012

Mass and Environment as Drivers of Galaxy Evolution II: The quenching of satellite galaxies as the origin of environmental effects

We extend the phenomenological study of the evolving galaxy population of Peng et al (2010) to the central/satellite dichotomy in Yang et al. SDSS groups. We find that satellite galaxies are responsible for all the environmental effects in our earlier work. The fraction of centrals that are red does not depend on their environment but only on their stellar masses, whereas that of the satellites depends on both. We define a relative satellite quenching efficiency, which is the fraction of blue centrals that are quenched upon becoming the satellite of another galaxy. This is shown to be independent of stellar mass, but to depend strongly on local overdensity. The red fraction of satellites correlate much better with the local overdensity, a measure of location within the group, than with the richness of the group, i.e. dark matter halo mass. This, and the fact that satellite quenching depends on local density and not on either the stellar mass of the galaxy or the halo mass gives clues as to the nature of the satellite-quenching process. We furthermore show that the action of mass-quenching on satellite galaxies is also independent of the DM mass of the parent halo. We then apply the Peng et al (2010) approach to predict the mass functions of central and satellite galaxies, split into passive and active galaxies, and show that these match very well the observed mass functions from SDSS, further strengthening the validity of this phenomenological approach. We highlight the fact that the observed M* is the same for the star-forming centrals and satellites and the observed M* for the star-forming satellites is independent of halo mass above 10^12M\odot, which emphasizes the universality of the mass-quenching process that we identified in Peng et al (2010). Post-quenching merging modifies the mass function of the central galaxies but can increase the mass of typical centrals by only about 25%.

astro-ph.CO↗

arXiv2012

Detection of dark galaxies and circum-galactic filaments fluorescently illuminated by a quasar at z=2.4

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.

astro-ph.CO↗