arXiv subjects

M. Fumagalli

Publications and source records attributed to M. Fumagalli.

At least 37 records · Page 2Linked to original sources

arXiv2022

CLusters in the Uv as EngineS (CLUES): I. Survey presentation \& FUV spectral analysis of the stellar light

The CLusters in the Uv as EngineS (CLUES) survey is a Cosmic Origins Spectrograph (COS) campaign aimed at acquiring the 1130 to 1770 Å, restframe spectroscopy of very young (<20 Myr) and massive (>10^4 solar masses) star clusters in galaxies that are part of the Hubble treasury program Legacy ExtraGalactic Uv Survey (LEGUS). In this first paper of a series, we describe the CLUES sample consisting of 20 young star clusters and report their physical properties as derived by both multi-wavelength photometry and far-UV (FUV) spectroscopy with Hubble Space Telescope (HST). Thanks to the synergy of the two different datasets we build a coherent picture of the diverse stellar populations found in each region (with sizes of 40 to 160 pc). We associate the FUV-brightest stellar population to the central targeted star cluster and the other modeled population to the diffuse stars that are included in the COS aperture. We observe better agreement between photometric and spectroscopic ages for star clusters younger than 5 Myr. For clusters older than 5 Myr, photometry and spectroscopy measurements deviate, with the latter producing older ages, due to the degeneracy of photometric models. FUV spectroscopy enables us to better constrain the stellar metallicities, a parameter that optical colors are insensitive to. Finally, the derived E(B-V) are quite similar, with a tendency for FUV spectroscopy to favor solutions with higher extinctions. The recovered masses are in agreement within a factor of 2 for all the clusters.

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arXiv2021

Productive Multitasking for Industrial Robots

The application of robotic solutions to small-batch production is challenging: economical constraints tend to dramatically limit the time for setting up new batches. Organizing robot tasks into modular software components, called skills, and allowing the assignment of multiple concurrent tasks to a single robot is potentially game-changing. However, due to cycle time constraints, it may be necessary for a skill to take over without waiting on another to terminate, and the available literature lacks a systematic approach in this case. In the present article, we fill the gap by (a) establishing the specifications of skills that can be sequenced with partial executions, (b) proposing an implementation based on the combination of finite-state machines and behavior trees, and (c) demonstrating the benefits of such skills through extensive trials in the environment of ARIAC (Agile Robotics for Industrial Automation Competition).

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arXiv2020

Shaping the structure of a GMC with radiation and winds

We study the effect of stellar feedback (photodissociation/ionization, radiation pressure and winds) on the evolution of a Giant Molecular Cloud (GMC), by means of a 3D radiative transfer, hydro-simulation implementing a complex chemical network featuring ${\rm H}_2$ formation and destruction. We track the formation of individual stars with mass $M>1\,{\rm M}_\odot$ with a stochastic recipe. Each star emits radiation according to its spectrum, sampled with 10 photon bins from near-infrared to extreme ultra-violet bands; winds are implemented by energy injection in the neighbouring cells. We run a simulation of a GMC with mass $M=10^5\,{\rm M}_\odot$, following the evolution of different gas phases. Thanks to the simultaneous inclusion of different stellar feedback mechanisms, we identify two stages in the cloud evolution: (1) radiation and winds carve ionized, low-density bubbles around massive stars, while FUV radiation dissociates most ${\rm H}_2$ in the cloud, apart from dense, self-shielded clumps; (2) rapid star formation (SFR$\simeq 0.1\,{\rm M}_\odot\,{\rm yr}^{-1}$) consumes molecular gas in the dense clumps, so that UV radiation escapes and ionizes the remaining HI gas in the GMC. ${\rm H}_2$ is exhausted in $1.6$ Myr, yielding a final star formation efficiency of 36 per cent. The average intensity of FUV and ionizing fields increases almost steadily with time; by the end of the simulation ($t=2.5$ Myr) we find $\langle G_0 \rangle \simeq 10^3$ (in Habing units), and a ionization parameter $\langle U_{\rm ion} \rangle \simeq 10^2$, respectively. The ionization field has also a more patchy distribution than the FUV one within the GMC. Throughout the evolution, the escape fraction of ionizing photons from the cloud is $f_{\rm ion, esc} < 0.03$.

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arXiv2020

Quasar Sightline and Galaxy Evolution (QSAGE) survey -- II. Galaxy overdensities around UV luminous quasars at z=1-2

We demonstrate that the UV brightest quasars at z=1-2 live in overdense environments. This is based on an analysis of deep Hubble Space Telescope WFC3 G141 grism spectroscopy of the galaxies along the lines-of-sight to UV luminous quasars in the redshift range z=1-2. This constitutes some of the deepest grism spectroscopy performed by WFC3, with 4 roll angles spread over a year of observations to mitigate the effect of overlapping spectra. Of the 12 quasar fields studied, 8 display evidence for a galaxy overdensity at the redshift of the quasar. One of the overdensities, PG0117+213 at z=1.50, has potentially 36 spectroscopically confirmed members, consisting of 19 with secure redshifts and 17 with single-line redshifts, within a cylinder of radius ~700 kpc. Its halo mass is estimated to be log (M/Msol)=14.7. This demonstrates that spectroscopic and narrow-band observations around distant UV bright quasars may be an excellent route for discovering protoclusters. Our findings agree with previous hints from statistical observations of the quasar population and theoretical works, as feedback regulated black hole growth predicts a correlation between quasar luminosity and halo mass. We also present the high signal-to-noise rest-frame optical spectral and photometric properties of the quasars themselves.

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arXiv2019

Gas filaments of the cosmic web located around active galaxies in a proto-cluster

Cosmological simulations predict the Universe contains a network of intergalactic gas filaments, within which galaxies form and evolve. However, the faintness of any emission from these filaments has limited tests of this prediction. We report the detection of rest-frame ultraviolet Lyman-alpha radiation from multiple filaments extending more than one megaparsec between galaxies within the SSA 22 proto-cluster at a redshift of 3.1. Intense star formation and supermassive black-hole activity is occurring within the galaxies embedded in these structures, which are the likely sources of the elevated ionizing radiation powering the observed Lyman-alpha emission. Our observations map the gas in filamentary structures of the type thought to fuel the growth of galaxies and black holes in massive proto-clusters.

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arXiv2019

The MUSE Ultra Deep Field (MUDF). II. Survey design and the gaseous properties of galaxy groups at 0.5 < z < 1.5

We present the goals, design, and first results of the MUSE Ultra Deep Field (MUDF) survey, a large programme using the Multi Unit Spectroscopic Explorer (MUSE) instrument at the ESO Very Large Telescope. The MUDF survey is collecting ~ 150 hours on-source of integral field optical spectroscopy in a 1.5 x 1.2 square arcmin region which hosts several astrophysical structures along the line of sight, including two bright z ~ 3.2 quasars with close separation (~ 500 kpc). Following the description of the data reduction procedures, we present the analysis of the galaxy environment and gaseous properties of seven groups detected at redshifts 0.5 < z < 1.5, spanning a large dynamic range in halo mass, log(Mh/Msun) ~ 11 - 13.5. For four of the groups, we find associated MgII absorbers tracing cool gas in high-resolution spectroscopy of the two quasars, including one case of correlated absorption in both sightlines at distance ~ 480 kpc. The absorption strength associated with the groups is higher than what has been reported for more isolated galaxies of comparable mass and impact parameters. We do not find evidence for widespread cool gas giving rise to strong absorption within these groups. Combining these results with the distribution of neutral and ionised gas seen in emission in lower-redshift groups, we conclude that gravitational interactions in the group environment strip gas from the galaxy haloes into the intragroup medium, boosting the cross section of cool gas and leading to the high fraction of strong MgII absorbers that we detect.

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arXiv2019

The MUSE Ultra Deep Field (MUDF). I. Discovery of a group of Ly$α$ nebulae associated with a bright $z\approx 3.23$ quasar pair

We present first results from Multi Unit Spectroscopic Explorer (MUSE) observations at the Very Large Telescope in the MUSE Ultra Deep Field (MUDF), a $\approx 1.2\times 1.4$ arcmin$^2$ region for which we are collecting $\approx$200 hours of integral field spectroscopy. The $\approx 40$-hour observation completed to date reveals the presence of a group of three Ly$α$ nebulae associated with a bright quasar pair at $z\simeq3.23$ with projected separation of $\approx 500\rm~kpc$. Two of the nebulae are physically associated with the quasars which are likely powering the Ly$α$ emission, and extend for $\gtrsim 100~\rm kpc$ at a surface brightness level of $\approx 6\times 10^{-19}~\rm erg~s^{-1}~cm^{-2}~arcsec^{-2}$. A third smaller ($\approx$35 kpc) nebula lies at a velocity offset of $\approx 1550$ km s$^{-1}$. Despite their clustered nature, the two large nebulae have properties similar to those observed in isolated quasars and exhibit no sharp decline in flux at the current depth, suggesting an even more extended distribution of gas around the quasars. We interpret the shape and the alignment of the two brighter nebulae as suggestive of the presence of an extended structure connecting the two quasar host galaxies, as seen for massive galaxies forming within gas-rich filaments in cosmological simulations.

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arXiv2019

Star Cluster Catalogs for the LEGUS Dwarf Galaxies

We present the star cluster catalogs for 17 dwarf and irregular galaxies in the $HST$ Treasury Program "Legacy ExtraGalactic UV Survey" (LEGUS). Cluster identification and photometry in this subsample are similar to that of the entire LEGUS sample, but special methods were developed to provide robust catalogs with accurate fluxes due to low cluster statistics. The colors and ages are largely consistent for two widely used aperture corrections, but a significant fraction of the clusters are more compact than the average training cluster. However, the ensemble luminosity, mass, and age distributions are consistent suggesting that the systematics between the two methods are less than the random errors. When compared with the clusters from previous dwarf galaxy samples, we find that the LEGUS catalogs are more complete and provide more accurate total fluxes. Combining all clusters into a composite dwarf galaxy, we find that the luminosity and mass functions can be described by a power law with the canonical index of $-2$ independent of age and global SFR binning. The age distribution declines as a power law, with an index of $\approx-0.80\pm0.15$, independent of cluster mass and global SFR binning. This decline of clusters is dominated by cluster disruption since the combined star formation histories and integrated-light SFRs are both approximately constant over the last few hundred Myr. Finally, we find little evidence for an upper-mass cutoff ($<2σ$) in the composite cluster mass function, and can rule out a truncation mass below $\approx10^{4.5}$M$_{\odot}$ but cannot rule out the existence of a truncation at higher masses.

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arXiv2018

The Spatial Relation between Young Star Clusters and Molecular Clouds in M 51 with LEGUS

We present a study correlating the spatial locations of young star clusters with those of molecular clouds in NGC~5194, in order to investigate the timescale over which clusters separate from their birth clouds. The star cluster catalogues are from the Legacy ExtraGalactic UV Survey (LEGUS) and the molecular clouds from the Plateau de Bure Interefrometer Arcsecond Whirpool Survey (PAWS). We find that younger star clusters are spatially closer to molecular clouds than older star clusters. The median ages for clusters associated with clouds is 4~Myr whereas it is 50~Myr for clusters that are sufficiently separated from a molecular cloud to be considered unassociated. After $\sim$6~Myr, the majority of the star clusters lose association with their molecular gas. Younger star clusters are also preferentially located in stellar spiral arms where they are hierarchically distributed in kpc-size regions for 50-100~Myr before dispersing. The youngest star clusters are more strongly clustered, yielding a two-point correlation function with $α=-0.28\pm0.04$, than the GMCs ($α=-0.09\pm0.03$) within the same PAWS field. However, the clustering strength of the most massive GMCs, supposedly the progenitors of the young clusters for a star formation efficiency of a few percent, is comparable ($α=-0.35\pm0.05$) to that of the clusters. We find a galactocentric-dependence for the coherence of star formation, in which clusters located in the inner region of the galaxy reside in smaller star-forming complexes and display more homogeneous distributions than clusters further from the centre. This result suggests a correlation between the survival of a cluster complex and its environment.

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arXiv2018

Overdensity of submillimeter galaxies around the $z\simeq 2.3$ MAMMOTH-1 nebula

In the hierarchical model of structure formation, giant elliptical galaxies form through merging processes within the highest density peaks known as protoclusters. While high-redshift radio galaxies usually pinpoint the location of these environments, we have recently discovered at z~2-3 three Enormous (>200 kpc) Lyman-Alpha Nebulae (ELANe) that host multiple AGN and that are surrounded by overdensities of Lyman-alpha Emitters (LAE). These regions are prime candidates of massive protoclusters in the early stages of assembly. To characterize the star-forming activity within these rare structures - both on ELAN and protocluster scales - we have initiated an observational campaign with the JCMT and the APEX telescopes. In this paper we report on sensitive SCUBA-2/JCMT 850 and 450 $μ$m observations of a 128 arcmin$^2$ field comprising the ELAN MAMMOTH-1, together with the peak of the hosting BOSS1441 LAE overdensity at z=2.32. These observations unveil $4.0\pm1.3$ times higher source counts at 850 $μ$m with respect to blank fields, likely confirming the presence of an overdensity also in obscured tracers. We find a strong detection at 850 $μ$m associated with the continuum source embedded within the ELAN MAMMOTH-1, which - together with the available data from the literature - allow us to constrain the spectral energy distribution of this source to be of an ULIRG with a far-infrared luminosity of $L_{\rm FIR}^{\rm SF}=2.4^{+7.4}_{-2.1}\times10^{12}$ L$_{\odot}$, and hosting an obscured AGN. Such a source is thus able to power the hard photoionization plus outflow scenario depicted in Cai et al. (2017b) to explain the extended Lyman-alpha, HeII$\lambda1640$ and CIV$\lambda1549$ emission, and their kinematics. In addition, the two brightest detections at 850 $μ$m ($f_{850}>18$ mJy) sit at the density peak of the LAEs overdensity, likely pinpointing the core of the protocluster.

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arXiv2018

A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE).IV. A tail of Ionised Gas in the Merger Remnant NGC 4424

We have observed the late-type peculiar galaxy NGC 4424 during VESTIGE, a blind narrow-band Halpha[NII] imaging survey of the Virgo cluster carried out with MegaCam at the CFHT. The presence of a 110 kpc long HI tail in the S direction indicates that this galaxy is undergoing a ram pressure stripping event. The deep narrow-band image revealed the presence of a low surface brightness ionised gas tail ~10 kpc long extending from the centre of the galaxy to the NW direction, in the direction opposite to the HI tail. Chandra and XMM X-rays data do not show any compact source in the nucleus nor the presence of an extended tail of hot gas, while MUSE spectroscopy indicates that the gas is photo-ionised in the inner regions and shock-ionised in the outer parts. IFU spectroscopy confirms that the ionised gas is kinematically decoupled from the stellar component and indicates the presence of two kinematically distinct structures in the stellar disc. The analysis of the SED of the galaxy indicates that the activity of star formation has been totally quenched in the outer disc ~ 250-280 Myr ago, while only reduced by ~80% in the central regions. All this observational evidence suggests that NGC 4424 is the remnant of an unequal-mass merger occurred <= 500 Myr ago, when the galaxy was already a member of the Virgo cluster, now undergoing a ram pressure stripping event which has removed the gas and quenched the activity of star formation in the outer disc. The tail of ionised gas probably results from the outflow produced by a central starburst fed by the collapse of gas induced by the merging episode. This outflow is sufficiently powerful to overcome the ram pressure induced by the intracluster medium on the disc of the galaxy crossing the cluster. This analysis thus suggests that feedback can participate in the quenching process of galaxies in high-density regions.

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arXiv2018

Quasar Sightline and Galaxy Evolution (QSAGE) Survey - I. The Galaxy Environment of OVI Absorbers up to z=1.4 around PKS 0232-04

We present the first results from a study of OVI absorption around galaxies at $z<1.44$ using data from a near-infrared grism spectroscopic Hubble Space Telescope Large Program, the Quasar Sightline and Galaxy Evolution (QSAGE) survey. QSAGE is the first grism galaxy survey to focus on the circumgalactic medium at $z\sim1$, providing a blind survey of the galaxy population. Using the first of 12 fields, we provide details of the reduction methods, in particular the handling of the deep grism data which uses multiple position angles to minimise the effects of contamination from overlapping traces. The resulting galaxy sample is H$α$ flux limited ($f({\rm Hα}) > 2\times10^{-17}$ erg s$^{-1}$ cm$^{-2}$) at 0.68 0.68, we find 5 OVI absorption systems along the line of sight with identified galaxies lying at impact parameters of $b\approx100-350$ kpc (proper), whilst we find a further 13 galaxies with no significant associated OVI absorption (i.e. $N({\rm OVI})<10^{13.5-14}$ cm$^{-2}$) in the same impact parameter and redshift range. We find a large scatter in the stellar mass and star-formation rates of the closest galaxies with associated OVI. Whilst one of the OVI absorber systems is found to be associated with a low mass galaxy group at $z\approx1.08$, we infer that the detected OVI absorbers typically lie in the proximity of dark matter halos of masses $10^{11.5} {\rm M_\odot}\lesssim M_{\rm halo}\lesssim10^{12} {\rm M_\odot}$.

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arXiv2018

Dissecting cold gas in a high-redshift galaxy using a lensed background quasar

We present a study of cold gas absorption from a damped Lyman-$α$ absorber (DLA) at redshift $z_{\rm abs}=1.946$ towards two lensed images of the quasar J144254.78+405535.5 at redshift $z_{\rm QSO} = 2.590$. The physical separation of the two lines of sight at the absorber redshift is $d_{\rm abs}=0.7$~kpc based on our lens model. We observe absorption lines from neutral carbon and H$_2$ along both lines of sight indicating that cold gas is present on scales larger than $d_{\rm abs}$. We measure column densities of HI to be $\log N(\rm H\,ı) = 20.27\pm0.02$ and $20.34\pm0.05$ and of H$_2$ to be $\log N(\rm H_2) = 19.7\pm0.1$ and $19.9\pm0.2$. The metallicity inferred from sulphur is consistent with Solar metallicity for both sightlines: $[{\rm S/H}]_A = 0.0\pm0.1$ and $[{\rm S/H}]_B = -0.1\pm0.1$. Based on the excitation of low rotational levels of H$_2$, we constrain the temperature of the cold gas phase to be $T=109\pm20$ and $T=89\pm25$ K for the two lines of sight. From the relative excitation of fine-structure levels of CI, we constrain the hydrogen volumetric densities in the range of $40-110$ cm$^{-3}$. Based on the ratio of observed column density and volumetric density, we infer the average individual `cloud' size along the line of sight to be $l\approx0.1$ pc. Using the transverse line-of-sight separation of 0.7 kpc together with the individual cloud size, we are able to put an upper limit to the volume filling factor of cold gas of $f_{\rm vol} < 0.2$ %. Nonetheless, the projected covering fraction of cold gas must be large (close to unity) over scales of a few kpc in order to explain the presence of cold gas in both lines of sight. Compared to the typical extent of DLAs (~10-30 kpc), this is consistent with the relative incidence rate of CI absorbers and DLAs.

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arXiv2018

Connecting Young Star Clusters to CO Molecular Gas in NGC 7793 with ALMA-LEGUS

We present an investigation of the relationship between giant molecular cloud (GMC) properties and the associated stellar clusters in the nearby flocculent galaxy NGC 7793. We combine the star cluster catalog from the HST LEGUS (Legacy ExtraGalactic UV Survey) program with the 15 parsec resolution ALMA CO(2-1) observations. We find a strong spatial correlation between young star clusters and GMCs such that all clusters still associated with a GMC are younger than 11 Myr and display a median age of 2 Myr. The age distribution increases gradually as the cluster-GMC distance increases, with star clusters that are spatially unassociated with molecular gas exhibiting a median age of 7 Myr. Thus, star clusters are able to emerge from their natal clouds long before the timescale required for clouds to disperse. To investigate if the hierarchy observed in the stellar components is inherited from the GMCs, we quantify the amount of clustering in the spatial distributions of the components and find that the star clusters have a fractal dimension slope of $-0.35 \pm 0.03$, significantly more clustered than the molecular cloud hierarchy with slope of $-0.18 \pm 0.04$ over the range 40-800 pc. We find, however, that the spatial clustering becomes comparable in strength for GMCs and star clusters with slopes of $-0.44\pm0.03$ and $-0.45\pm0.06$ respectively, when we compare massive ($>$10$^5$ M$_{\odot}$) GMCs to massive and young star clusters. This shows that massive star clusters trace the same hierarchy as their parent GMCs, under the assumption that the star formation efficiency is a few percent.

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arXiv2018

The young star cluster population of M51 with LEGUS - II. Testing environmental dependencies

It has recently been established that the properties of young star clusters (YSCs) can vary as a function of the galactic environment in which they are found. We use the cluster catalogue produced by the Legacy Extragalactic UV Survey (LEGUS) collaboration to investigate cluster properties in the spiral galaxy M51. We analyse the cluster population as a function of galactocentric distance and in arm and inter-arm regions. The cluster mass function exhibits a similar shape at all radial bins, described by a power law with a slope close to $-2$ and an exponential truncation around $10^5\ \rm{M}_{\odot}$ . While the mass functions of the YSCs in the spiral arm and inter-arm regions have similar truncation masses, the inter-arm region mass function has a significantly steeper slope than the one in the arm region; a trend that is also observed in the giant molecular cloud mass function and predicted by simulations. The age distribution of clusters is dependent on the region considered, and is consistent with rapid disruption only in dense regions, while little disruption is observed at large galactocentric distances and in the inter-arm region. The fraction of stars forming in clusters does not show radial variations, despite the drop in the $H_2$ surface density measured as function of galactocentric distance. We suggest that the higher disruption rate observed in the inner part of the galaxy is likely at the origin of the observed flat cluster formation efficiency radial profile.

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arXiv2018

Extinction Maps and Dust-to-Gas Ratios in Nearby Galaxies with LEGUS

We present a study of the dust-to-gas ratios in five nearby galaxies NGC 628 (M74), NGC 6503, NGC 7793, UGC 5139 (Holmberg I), and UGC 4305 (Holmberg II). Using Hubble Space Telescope broad band WFC3/UVIS UV and optical images from the Treasury program LEGUS (Legacy ExtraGalactic UV Survey) combined with archival HST/ACS data, we correct thousands of individual stars for extinction across these five galaxies using an isochrone-matching (reddening-free Q) method. We generate extinction maps for each galaxy from the individual stellar extinctions using both adaptive and fixed resolution techniques, and correlate these maps with neutral HI and CO gas maps from literature, including The HI Nearby Galaxy Survey (THINGS) and the HERA CO-Line Extragalactic Survey (HERACLES). We calculate dust-to-gas ratios and investigate variations in the dust-to-gas ratio with galaxy metallicity. We find a power law relationship between dust-to-gas ratio and metallicity, consistent with other studies of dust-to-gas ratio compared to metallicity. We find a change in the relation when H$_2$ is not included. This implies that underestimation of $N_{H_2}$ in low-metallicity dwarfs from a too-low CO-to-H$_2$ conversion factor $X_{CO}$ could have produced too low a slope in the derived relationship between dust-to-gas ratio and metallicity. We also compare our extinctions to those derived from fitting the spectral energy distribution (SED) using the Bayesian Extinction and Stellar Tool (BEAST) for NGC 7793 and find systematically lower extinctions from SED-fitting as compared to isochrone matching.

astro-ph.GA↗

arXiv2018

A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE).III. Star formation in the stripped gas of NGC 4254

During pilot observations of the Virgo Environmental Survey Tracing Galaxy Evolution (VESTIGE), a blind narrow-band Halpha+[NII] imaging survey of the Virgo cluster carried out with MegaCam at the CFHT, we have observed the spiral galaxy NGC 4254 (M99). Deep Halpha+[NII] narrow-band and GALEX UV images revealed the presence of 60 compact (70-500 pc radius) star forming regions up to ~ 20 kpc outside the optical disc of the galaxy. These regions are located along a tail of HI gas stripped from the disc of the galaxy after a rapid gravitational encounter with another Virgo cluster member that simulations indicate occurred 280-750 Myr ago. We have combined the VESTIGE data with multifrequency data from the UV to the far-infrared to characterise the stellar populations of these regions and study the star formation process in an extreme environment such as the tails of stripped gas embedded in the hot intracluster medium. The colour, spectral energy distribution (SED), and linear size consistently indicate that these regions are coeval and have been formed after a single burst of star formation that occurred ~< 100 Myr ago. These regions might become free floating objects within the cluster potential well, and be the local analogues of compact sources produced after the interaction of gas-rich systems that occurred during the early formation of clusters.

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arXiv2018

Two more, bright, z > 6 quasars from VST ATLAS and WISE

Recently, Carnall et al. discovered two bright high redshift quasars using the combination of the VST ATLAS and WISE surveys. The technique involved using the 3-D colour plane i-z:z-W1:W1-W2 with the WISE W1 (3.4 micron) and W2 (4.5 micron) bands taking the place of the usual NIR J band to help decrease stellar dwarf contamination. Here we report on our continued search for 5.7 6 quasars, VST-ATLAS J158.6938-14.4211 at z=6.07 and J332.8017-32.1036 at z=6.32 with magnitudes of z_AB=19.4 and 19.7 mag respectively. J158.6938-14.4211 was confirmed by Keck LRIS observations and J332.8017-32.1036 was confirmed by ESO NTT EFOSC-2 observations. Here we present VLT X-shooter Visible and NIR spectra for the four ATLAS quasars. We have further independently rediscovered two z>5.7 quasars previously found by the VIKING/KiDS and PanSTARRS surveys. This means that in ATLAS we have now discovered a total of six quasars in our target 5.7<z<6.4 redshift range. Making approximate corrections for incompleteness, we find that our quasar space density agrees with the SDSS results of Jiang et al. at M_1450A~-27mag. Preliminary virial mass estimates based on the CIV and MIII emission lines give black hole masses in the range M_BH~1-6x10e9 M_solar for the four ATLAS quasars.

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