arXiv subjects

M. Owers

Publications and source records attributed to M. Owers.

At least 19 recordsLinked to original sources

arXiv2021

The SAMI Galaxy Survey: the drivers of gas and stellar metallicity differences in galaxies

The combination of gas-phase oxygen abundances and stellar metallicities can provide us with unique insights into the metal enrichment histories of galaxies. In this work, we compare the stellar and gas-phase metallicities measured within a 1$R_{e}$ aperture for a representative sample of 472 star-forming galaxies extracted from the SAMI Galaxy Survey. We confirm that the stellar and interstellar medium (ISM) metallicities are strongly correlated, with scatter $\sim$3 times smaller than that found in previous works, and that integrated stellar populations are generally more metal-poor than the ISM, especially in low-mass galaxies. The ratio between the two metallicities strongly correlates with several integrated galaxy properties including stellar mass, specific star formation rate, and a gravitational potential proxy. However, we show that these trends are primarily a consequence of: (a) the different star formation and metal enrichment histories of the galaxies, and (b) the fact that while stellar metallicities trace primarily iron enrichment, gas-phase metallicity indicators are calibrated to the enrichment of oxygen in the ISM. Indeed, once both metallicities are converted to the same `element base' all of our trends become significantly weaker. Interestingly, the ratio of gas to stellar metallicity is always below the value expected for a simple closed-box model, which requires that outflows and inflows play an important role in the enrichment history across our entire stellar mass range. This work highlights the complex interplay between stellar and gas-phase metallicities and shows how care must be taken in comparing them to constrain models of galaxy formation and evolution.

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arXiv2019

4MOST Consortium Survey 7: Wide-Area VISTA Extragalactic Survey (WAVES)

WAVES is designed to study the growth of structure, mass and energy on scales of ~1 kpc to ~10 Mpc over a 7 Gyr timeline. On the largest length scales (1-10 Mpc) WAVES will measure the structures defined by groups, filaments and voids, and their emergence over recent times. Comparisons with bespoke numerical simulations will be used to confirm, refine or refute the Cold Dark Matter paradigm. At intermediate length scales (10 kpc-1 Mpc) WAVES will probe the size and mass distribution of galaxy groups, as well as the galaxy merger rates, in order to directly measure the assembly of dark matter halos and stellar mass. On the smallest length scales (1-10 kpc) WAVES will provide accurate distance and environmental measurements to complement high-resolution space-based imaging to study the mass and size evolution of galaxy bulges, discs and bars. In total, WAVES will provide a panchromatic legacy dataset of ~1.6 million galaxies, firmly linking the very low ($z < 0.1$) and intermediate ($z \sim 0.8$) redshift Universe.

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arXiv2019

Galaxy And Mass Assembly (GAMA): Environmental Quenching of Centrals and Satellites in Groups

Recently a number of studies have found a similarity between the passive fraction of central and satellite galaxies when controlled for both stellar and halo mass. These results suggest that the quenching processes that affect galaxies are largely agnostic to central/satellite status, which contradicts the traditional picture of increased satellite quenching via environmental processes such as stripping, strangulation and starvation. Here we explore this further using the Galaxy And Mass Assembly (GAMA) survey which extends to ~2dex lower in stellar mass than SDSS, is more complete for closely-separated galaxies (>95% compared to >70%), and identifies lower-halo-mass groups outside of the very local Universe (M$_{\mathrm{halo}}\sim10^{12}$M$_{\odot}$ at $0.1<z<0.2$). As far as possible we aim to replicate the selections, completeness corrections and central/satellite division of one of the previous studies but find clear differences between passive fractions of centrals and satellites. We also find that our passive fractions increase with both halo-to-satellite mass ratio and central-to-second rank mass ratio. This suggests that quenching is more efficient in satellites that are low-mass for their halo ($i.e$ at high halo-to-satellite mass ratio in comparison to low halo-to-satellite mass ratio) and are more likely to be passive in older groups - forming a consistent picture of environmental quenching of satellites. We then discuss potential explanations for the previously observed similarity, such as dependence on the group-finding method.

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arXiv2018

Galaxy And Mass Assembly (GAMA): The sSFR-M* relation part I - $σ_{\mathrm{sSFR}}$-M* as a function of sample, SFR indicator and environment

Recently a number of studies have proposed that the dispersion along the star formation rate - stellar mass relation ($σ_{\mathrm{sSFR}}$-M$_{*}$) is indicative of variations in star-formation history (SFH) driven by feedback processes. They found a 'U'-shaped dispersion and attribute the increased scatter at low and high stellar masses to stellar and active galactic nuclei feed-back respectively. However, measuring $σ_{\mathrm{sSFR}}$ and the shape of the $σ_{\mathrm{sSFR}}$-M$_{*}$ relation is problematic and can vary dramatically depending on the sample selected, chosen separation of passive/star-forming systems, and method of deriving star-formation rates ($i.e.$ H$α$ emission vs spectral energy distribution fitting). As such, any astrophysical conclusions drawn from measurements of $σ_{\mathrm{sSFR}}$ must consider these dependencies. Here we use the Galaxy And Mass Assembly survey to explore how $σ_{\mathrm{sSFR}}$ varies with SFR indicator for a variety of selections for disc-like `main sequence' star-forming galaxies including colour, star-formation rate, visual morphology, bulge-to-total mass ratio, Sérsic index and mixture modelling. We find that irrespective of sample selection and/or SFR indicator, the dispersion along the sSFR-M$_{*}$ relation does follow a 'U'-shaped distribution. This suggests that the shape is physical and not an artefact of sample selection or method. We then compare the $σ_{\mathrm{sSFR}}$-M$_{*}$ relation to state-of-the-art hydrodynamical and semi-analytic models and find good agreement with our observed results. Finally, we find that for group satellites this 'U'-shaped distribution is not observed due to additional high scatter populations at intermediate stellar masses.

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arXiv2018

The XXL Survey XX: The 365 cluster catalogue

In the currently debated context of using clusters of galaxies as cosmological probes, the need for well-defined cluster samples is critical. The XXL Survey has been specifically designed to provide a well characterised sample of some 500 X-ray detected clusters suitable for cosmological studies. The main goal of present article is to make public and describe the properties of the cluster catalogue in its present state, as well as of associated catalogues as super-clusters and fossil groups. We release a sample containing 365 clusters in total. We give the details of the follow-up observations and explain the procedure adopted to validate the cluster spectroscopic redshifts. Considering the whole XXL cluster sample, we have provided two types of selection, both complete in a particular sense: one based on flux-morphology criteria, and an alternative based on the [0.5-2] keV flux within one arcmin of the cluster centre. We have also provided X-ray temperature measurements for 80$\%$ of the clusters having a flux larger than 9$\times$10$^{-15}$$\rm \thinspace erg \, s^{-1} \, cm^{-2}$. Our cluster sample extends from z$\sim$0 to z$\sim$1.2, with one cluster at z$\sim$2. Clusters were identified through a mean number of six spectroscopically confirmed cluster members. Our updated luminosity function and luminosity-temperature relation are compatible with our previous determinations based on the 100 brightest clusters, but show smaller uncertainties. We also present an enlarged list of super-clusters and a sample of 18 possible fossil groups. This intermediate publication is the last before the final release of the complete XXL cluster catalogue when the ongoing C2 cluster spectroscopic follow-up is complete. It provides a unique inventory of medium-mass clusters over a 50~\dd\ area out to z$\sim$1.

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arXiv2018

The XXL Survey: XXVII. The 3XLSS point source catalogue

We present the version of the point source catalogue of the XXL Survey that was used, in part, in the first series of XXL papers. In this paper we release, in our database in Milan and at CDS: (i) the X-ray source catalogue with 26056 objects in two areas of 25 deg2; (ii) the associated multiwavelength catalogues with candidate counterparts of the X-ray sources in the infrared, near-infrared, optical, and ultraviolet (plus spectroscopic redshift when available); and (iii) a catalogue of spectroscopic redshifts recently obtained in the southern XXL area. We also present the basic properties of the X-ray point sources and their counterparts. Other catalogues described in the second series of XXL papers will be released contextually, and will constitute the second XXL data release.

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arXiv2018

The SAMI Galaxy Survey: embedded discs and radial trends in outer dynamical support across the Hubble sequence

We study the balance in dynamical support of 384 galaxies with stellar kinematics out to >1.5R_e in the Sydney AAO Multi-object Integral Field (SAMI) Galaxy Survey. We present radial dynamical profiles of the local rotation dominance parameter, V/sigma, and local spin, lambda_loc. Although there is a broad range in amplitude, most kinematic profiles monotonically increase across the probed radial range. We do not find many galaxies with kinematic transitions such as those expected between the inner in-situ and outer accreted stars within the radial range probed. We compare the V/sigma gradient and maximum values to the visual morphologies of the galaxies to better understand the link between visual and kinematic morphologies. We find that the radial distribution of dynamical support in galaxies is linked to their visual morphology. Late-type systems have higher rotational support at all radii and steeper V/sigma gradients compared to early-type galaxies. We perform a search for embedded discs, which are rotationally supported discy structures embedded within large scale slowly or non-rotating structures. Visual inspection of the kinematics reveals at most three galaxies (out of 384) harbouring embedded discs. This is more than an order of magnitude fewer than the observed fraction in some local studies. Our tests suggest that this tension can be attributed to differences in the sample selection, spatial sampling and beam smearing due to seeing.

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arXiv2018

The XXL Survey: XXX. Characterisation of the XLSSsC N01 supercluster and analysis of the galaxy stellar populations

Superclusters form from the largest enhancements in the primordial density perturbation field and extend for tens of Mpc, tracing the large-scale structure of the Universe. We characterise XLSSsCN01, a rich supercluster at z~0.3 detected in the XXL Survey, composed of X-ray clusters of different virial masses and luminosities. As one of the first studies on this topic, we investigate the stellar populations of galaxies in different environments in the supercluster region. We study a magnitude-limited (r<=20) and a mass-limited sample (log(M*/M_sun)>10.8) of galaxies in the virialised region and in the outskirts of 11 XLSSsCN01 clusters, in high- and low-density field. We compute the stellar population properties of galaxies using spectral energy distribution and spectral fitting techniques, and study the dependence of star formation rates (SFR), colours, and stellar ages on environment. For r<20, the fraction of SFing/blue galaxies, computed either from the specific-SFR (sSFR) or rest-frame(rf) colour, shows depletion within the cluster virial radii, where the number of galaxies with log(sSFR/yr^-1)>-12 and with (g-r)_rf<0.6 is lower than in the field. For log(M*/M_sun)>10.8, no trends with environment emerge, as massive galaxies are mostly already passive in all environments. No differences among low- and high-density field members and cluster members emerge in the sSFR-mass relation. The luminosity-weighted age-mass relation of the passive populations within cluster virial radii show signatures of recent environmental quenching. The study of luminous and massive galaxies in this supercluster shows that while environment has a prominent role in determining the fractions of SFing/blue galaxies, its effects on the star formation activity in SFing galaxies are negligible.

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arXiv2017

The XXL Survey: XXIII. The Mass Scale of XXL Clusters from Ensemble Spectroscopy

An X-ray survey with the XMM-Newton telescope, XMM-XXL, has identified hundreds of galaxy groups and clusters in two 25 deg$^2$ fields. Combining spectroscopic and X-ray observations in one field, we determine how the kinetic energy of galaxies scales with hot gas temperature and also, by imposing prior constraints on the relative energies of galaxies and dark matter, infer a power-law scaling of total mass with temperature. Our goals are: i) to determine parameters of the scaling between galaxy velocity dispersion and X-ray temperature, $T_{\rm 300kpc}$, for the halos hosting XXL-selected clusters, and; ii) to infer the log-mean scaling of total halo mass with temperature, $\langle \ln M_{200} \, | \, T, z \rangle$. We apply an ensemble velocity likelihood to a sample of $> 1500$ spectroscopic redshifts within $132$ spectroscopically confirmed clusters with redshifts $z < 0.6$ to model, $\langle \ln σ_{\rm gal}\,|\,T,z\rangle$, where $σ_{\rm gal}$ is the velocity dispersion of XXL cluster member galaxies and $T$ is a 300 kpc aperture temperature. To infer total halo mass we use a precise virial relation for massive halos calibrated by N-body simulations along with a single degree of freedom summarizing galaxy velocity bias with respect to dark matter. For the XXL-N cluster sample, we find $σ_{\rm gal} \propto T^{0.63\pm0.05}$, a slope significantly steeper than the self-similar expectation of $0.5$. Assuming scale-independent galaxy velocity bias, we infer a mean logarithmic mass at a given X-ray temperature and redshift, $\langle\ln (E(z) M_{200}/10^{14}\,{\rm M}_{\odot})|T,z\rangle=π+α\ln(T/T_p )+β\ln (E(z)/E(z_p) )$ using pivot values ${\rm k}T_{p}=2.2\,{\rm keV}$ and $z_p=0.25$, with normalization $π=0.45\pm0.24$ and slope $α=1.89\pm0.15$. We obtain only weak constraints on redshift evolution, $β=-1.29\pm1.14$.

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arXiv2017

The XXL Survey: XXII. The XXL-North spectrophotometric sample and galaxy stellar mass function in X-ray detected groups and clusters

The fraction of galaxies bound in groups in the nearby Universe is high (50% at z~0). Systematic studies of galaxy properties in groups are important in order to improve our understanding of the evolution of galaxies and of the physical phenomena occurring within this environment. We have built a complete spectrophotometric sample of galaxies within X-ray detected, optically spectroscopically confirmed groups and clusters (G&C), covering a wide range of halo masses at z<= 0.6. In the context of the XXL survey, we analyse a sample of 164 G&C in the XXL-North region (XXL-N), at z <= 0.6, with a wide range of virial masses (1.24 x 10^13 <=M_500 M_sun <= 6.63 x 10^14) and X-ray luminosities ( 2.27 x 10^41 <= L^XXL_500 (erg/s)<= 2.15 x10^44). The G&C are X-ray selected and spectroscopically confirmed. We describe the membership assignment and the spectroscopic completeness analysis, and compute stellar masses. As a first scientific exploitation of the sample, we study the dependence of the galaxy stellar mass function (GSMF) on global environment. We present a spectrophotometric characterisation of the G&C and their galaxies. The final sample contains 132 G&C, 22111 field galaxies and 2225 G&C galaxies with r-band magnitude <20. Of the G&C, 95% have at least three spectroscopic members, and 70% at least ten. The shape of the GSMF seems not to depend on environment (field versus G&C) or X-ray luminosity ( used as a proxy for the virial mass of the system). These results are confirmed by the study of the correlation between mean stellar mass of G&C members and L^XXL_500.We release the spectrophotometric catalogue of galaxies with all the quantities computed in this work. As a first homogeneous census of galaxies within X-ray spectroscopically confirmed G&C at these redshifts, this sample will allow environmental studies of the evolution of galaxy properties.

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arXiv2017

Using dust, gas and stellar mass selected samples to probe dust sources and sinks in low metallicity galaxies

We combine samples of nearby galaxies with Herschel photometry selected on their dust, metal, HI, and stellar mass content, and compare these to chemical evolution models in order to discriminate between different dust sources. In a companion paper, we used a HI-selected sample of nearby galaxies to reveal a sub-sample of very gas rich (gas fraction > 80 per cent) sources with dust masses significantly below predictions from simple chemical evolution models, and well below $M_d/M_*$ and $M_d/M_{gas}$ scaling relations seen in dust and stellar-selected samples of local galaxies. We use a chemical evolution model to explain these dust-poor, but gas-rich, sources as well as the observed star formation rates (SFRs) and dust-to-gas ratios. We find that (i) a delayed star formation history is required to model the observed SFRs; (ii) inflows and outflows are required to model the observed metallicities at low gas fractions; (iii) a reduced contribution of dust from supernovae (SNe) is needed to explain the dust-poor sources with high gas fractions. These dust-poor, low stellar mass galaxies require a typical core-collapse SN to produce 0.01 - 0.16 $M_{\odot}$ of dust. To match the observed dust masses at lower gas fractions, significant grain growth is required to counteract the reduced contribution from dust in SNe and dust destruction from SN shocks. These findings are statistically robust, though due to intrinsic scatter it is not always possible to find one single model that successfully describes all the data. We also show that the dust-to-metals ratio decreases towards lower metallicity.

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arXiv2017

The two-component giant radio halo in the galaxy cluster Abell 2142

We report on a spectral study at radio frequencies of the giant radio halo in A2142 (z=0.0909), which we performed to explore its nature and origin. A2142 is not a major merger and the presence of a giant radio halo is somewhat surprising. We performed deep radio observations with the GMRT at 608 MHz, 322 MHz, and 234 MHz and with the VLA in the 1-2 GHz band. We obtained high-quality images at all frequencies in a wide range of resolutions. The radio halo is well detected at all frequencies and extends out to the most distant cold front in A2142. We studied the spectral index in two regions: the central part of the halo and a second region in the direction of the most distant south-eastern cold front, selected to follow the bright part of the halo and X-ray emission. We complemented our observations with a preliminary LOFAR image at 118 MHz and with the re-analysis of archival VLA data at 1.4 GHz. The two components of the radio halo show different observational properties. The central brightest part has higher surface brightess and a spectrum whose steepness is similar to those of the known radio halos, i.e. $α^{\rm 1.78~GHz}_{\rm 118~MHz}=1.33\pm 0.08$. The ridge, which fades into the larger scale emission, is broader in size and has considerably lower surface brightess and a moderately steeper spectrum, i.e. $α^{\rm 1.78~GHz}_{\rm 118~MHz}\sim 1.5$. We propose that the brightest part of the radio halo is powered by the central sloshing in A2142, similar to what has been suggested for mini-halos, or by secondary electrons generated by hadronic collisions in the ICM. On the other hand, the steeper ridge may probe particle re-acceleration by turbulence generated either by stirring the gas and magnetic fields on a larger scale or by less energetic mechanisms, such as continuous infall of galaxy groups or an off-axis merger.

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arXiv2017

Galaxy And Mass Assembly: The 1.4GHz SFR indicator, SFR-M* relation and predictions for ASKAP-GAMA

We present a robust calibration of the 1.4GHz radio continuum star formation rate (SFR) using a combination of the Galaxy And Mass Assembly (GAMA) survey and the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey. We identify individually detected 1.4GHz GAMA-FIRST sources and use a late-type, non-AGN, volume-limited sample from GAMA to produce stellar mass-selected samples. The latter are then combined to produce FIRST-stacked images. This extends the robust parametrisation of the 1.4GHz-SFR relation to faint luminosities. For both the individually detected galaxies and our stacked samples, we compare 1.4GHz luminosity to SFRs derived from GAMA to determine a new 1.4GHz luminosity-to-SFR relation with well constrained slope and normalisation. For the first time, we produce the radio SFR-M* relation over 2 decades in stellar mass, and find that our new calibration is robust, and produces a SFR-M* relation which is consistent with all other GAMA SFR methods. Finally, using our new 1.4GHz luminosity-to-SFR calibration we make predictions for the number of star-forming GAMA sources which are likely to be detected in the upcoming ASKAP surveys, EMU and DINGO.

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arXiv2016

GAMA/H-ATLAS: A meta-analysis of SFR indicators - comprehensive measures of the SFR-M* relation and Cosmic Star Formation History at z < 0.4

We present a meta-analysis of star-formation rate (SFR) indicators in the GAMA survey, producing 12 different SFR metrics and determining the SFR-M* relation for each. We compare and contrast published methods to extract the SFR from each indicator, using a well-defined local sample of morphologically-selected spiral galaxies, which excludes sources which potentially have large recent changes to their SFR. The different methods are found to yield SFR-M* relations with inconsistent slopes and normalisations, suggesting differences between calibration methods. The recovered SFR-M* relations also have a large range in scatter which, as SFRs of the targets may be considered constant over the different timescales, suggests differences in the accuracy by which methods correct for attenuation in individual targets. We then recalibrate all SFR indicators to provide new, robust and consistent luminosity-to-SFR calibrations, finding that the most consistent slopes and normalisations of the SFR-M* relations are obtained when recalibrated using the radiation transfer method of Popescu et al. These new calibrations can be used to directly compare SFRs across different observations, epochs and galaxy populations. We then apply our calibrations to the GAMA II equatorial dataset and explore the evolution of star-formation in the local Universe. We determine the evolution of the normalisation to the SFR-M* relation from 0 < z < 0.35 - finding consistent trends with previous estimates at 0.3 < z < 1.2. We then provide the definitive z < 0.35 Cosmic Star Formation History, SFR-M* relation and its evolution over the last 3 billion years.

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arXiv2016

The Extraordinary Amount of Substructure in the Hubble Frontier Fields Cluster Abell 2744

We present a joint optical/X-ray analysis of the massive galaxy cluster Abell 2744 (z=0.308). Our strong- and weak-lensing analysis within the central region of the cluster, i.e., at R<1Mpc from the brightest cluster galaxy, reveals eight substructures, including the main core. All of these dark-matter halos are detected with a significance of at least 5sigma and feature masses ranging from 0.5 to 1.4x10^{14}Msun within R<150kpc. Merten et al. (2011) and Medezinski et al. (2016) substructures are also detected by us. We measure a slightly higher mass for the main core component than reported previously and attribute the discrepancy to the inclusion of our tightly constrained strong-lensing mass model built on Hubble Frontier Fields data. X-ray data obtained by XMM-Newton reveal four remnant cores, one of them a new detection, and three shocks. Unlike Merten et al. (2011), we find all cores to have both dark and luminous counterparts. A comparison with clusters of similar mass in the MXXL simulations yields no objects with as many massive substructures as observed in Abell 2744, confirming that Abell 2744 is an extreme system. We stress that these properties still do not constitute a challenge to $Λ$CDM, as caveats apply to both the simulation and the observations: for instance, the projected mass measurements from gravitational lensing and the limited resolution of the sub-haloes finders. We discuss implications of Abell 2744 for the plausibility of different dark-matter candidates and, finally, measure a new upper limit on the self-interaction cross-section of dark matter of sigma_{DM}<1.28cm2/g(68\% CL), in good agreement with previous results from Harvey et al. (2015).

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arXiv2016

The XXL Survey: VI. The 1000 brightest X-ray point sources

X-ray extragalactic surveys are ideal laboratories for the study of the evolution and clustering of active galactic nuclei (AGN). The XXL Survey spans two fields of a combined 50 $deg^2$ observed for more than 6Ms with XMM-Newton, occupying the parameter space between deep surveys and very wide area surveys; at the same time it benefits from a wealth of ancillary data. This paper marks the first release of the XXL point source catalogue selected in the 2-10 keV energy band with limiting flux $F_{2-10keV}=4.8\cdot10^{-14}\rm{erg\,s^{-1}\,cm^{-2}}$. We use both public and proprietary data sets to identify the counterparts of the X-ray point-like sources and improved upon the photometric redshift determination for AGN by applying a Random Forest classification trained to identify for each object the optimal photometric redshift model library. We also assign a probability to each source to be a star or an outlier. We model with Bayesian analysis the X-ray spectra assuming a power-law model with the presence of an absorbing medium. We find an average unabsorbed photon index of $Γ=1.85$ and average hydrogen column density $\log{N_{H}}=21.07 cm^{-2}$. We find no trend of $Γ$ or $N_H$ with redshift and a fraction of 26% absorbed sources ($\log N_{H}>22$). We show that the XXL-1000-AGN number counts extended the number counts of the COSMOS survey to higher fluxes and are fully consistent with the Euclidean expectation. We constrain the intrinsic luminosity function of AGN in the 2-10 keV energy band where the unabsorbed X-ray flux is estimated from the X-ray spectral fit up to z=3. Finally, we demonstrate the presence of a supercluster size structure at redshift 0.14, identified by means of percolation analysis of the XXL-1000-AGN sample. The XXL survey, reaching a medium flux limit and covering a wide area is a stepping stone between current deep fields and planned wide area surveys.

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arXiv2015

The XXL survey XIV. AAOmega redshifts for the southern XXL field

We present a catalogue containing the redshifts of 3,660 X-ray selected targets in the XXL southern field. The redshifts were obtained with the AAOmega spectrograph and 2dF fibre positioner on the Anglo-Australian Telescope. The catalogue contains 1,515 broad line AGN, 528 stars, and redshifts for 41 out of the 49 brightest X-ray selected clusters in the XXL southern field.

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arXiv2015

The XXL Survey. II. The bright cluster sample: catalogue and luminosity function

Context. The XXL Survey is the largest survey carried out by the XMM-Newton satellite and covers a total area of 50 square degrees distributed over two fields. It primarily aims at investigating the large-scale structures of the Universe using the distribution of galaxy clusters and active galactic nuclei as tracers of the matter distribution. Aims. This article presents the XXL bright cluster sample, a subsample of 100 galaxy clusters selected from the full XXL catalogue by setting a lower limit of $3\times 10^{-14}\,\mathrm{erg \,s^{-1}cm^{-2}}$ on the source flux within a 1$^{\prime}$ aperture. Methods. The selection function was estimated using a mixture of Monte Carlo simulations and analytical recipes that closely reproduce the source selection process. An extensive spectroscopic follow-up provided redshifts for 97 of the 100 clusters. We derived accurate X-ray parameters for all the sources. Scaling relations were self-consistently derived from the same sample in other publications of the series. On this basis, we study the number density, luminosity function, and spatial distribution of the sample. Results. The bright cluster sample consists of systems with masses between $M_{500}=7\times 10^{13}$ and $3\times 10^{14} M_\odot$, mostly located between $z=0.1$ and 0.5. The observed sky density of clusters is slightly below the predictions from the WMAP9 model, and significantly below the predictions from the Planck 2015 cosmology. In general, within the current uncertainties of the cluster mass calibration, models with higher values of $σ_8$ and/or $Ω_m$ appear more difficult to accommodate. We provide tight constraints on the cluster differential luminosity function and find no hint of evolution out to $z\sim1$. We also find strong evidence for the presence of large-scale structures in the XXL bright cluster sample and identify five new superclusters.

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