arXiv subjects

C. Spiniello

Publications and source records attributed to C. Spiniello.

At least 37 records · Page 2Linked to original sources

arXiv2023

INSPIRE: INvestigating Stellar Population In RElics III. Second data release (DR2): testing the systematics on the stellar velocity dispersion

This is the second data release (DR2) of the INvestigating Stellar Population In RElics (INSPIRE) project, comprising 21 new systems with observations completed before March 2022. For each system, we release four one-dimensional (1D) spectra to the ESO Science Archive, one spectrum for each arm of the X-Shooter spectrograph. In this paper, we focus on the line-of-sight velocity distribution, measuring integrated stellar velocity dispersions from the spectra, and assessing their robustness and the associated uncertainties. For each of the 21 new systems, we systematically investigated the effect of the parameters and set-ups of the full spectral fitting on the stellar velocity dispersion ($σ$) measurements. In particular, we tested how $σ$ changes when several parameters of the fit as well as the resolution and spectral coverage of the input spectra are varied. We found that the effect that causes the largest systematic uncertainties on $σ$ is the wavelength range used for the fit, especially for spectra with a lower signal-to-noise ratio (S/N $\leq$ 30). When using blue wavelengths (UVB arm) one generally underestimates the velocity dispersion (by $\sim$15 km/s). The values obtained from the near-IR (NIR) arm present a larger scatter because the quality of the spectra is lower. We finally compared our results with those in literature, finding a very good agreement overall. Joining results obtained in DR1 with those presented here, INSPIRE contains 40 ultra-compact massive galaxies, corresponding to 75% of the whole survey. By plotting these systems in a stellar mass-velocity dispersion diagram, we identify at least four highly reliable relic candidates among the new systems. Their velocity dispersion is larger than that of normal-sized galaxies of similar stellar mass.

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arXiv2022

STRIDES: Automated uniform models for 30 quadruply imaged quasars

Gravitational time delays provide a powerful one step measurement of $H_0$, independent of all other probes. One key ingredient in time delay cosmography are high accuracy lens models. Those are currently expensive to obtain, both, in terms of computing and investigator time (10$^{5-6}$ CPU hours and $\sim$ 0.5-1 year, respectively). Major improvements in modeling speed are therefore necessary to exploit the large number of lenses that are forecast to be discovered over the current decade. In order to bypass this roadblock, building on the work by Shajib et al. (2019), we develop an automated modeling pipeline and apply it to a sample of 30 quadruply imaged quasars and one lensed compact galaxy, observed by the Hubble Space Telescope in multiple bands. Our automated pipeline can derive models for 30/31 lenses with few hours of human time and <100 CPU hours of computing time for a typical system. For each lens, we provide measurements of key parameters and predictions of magnification as well as time delays for the multiple images. We characterize the cosmography-readiness of our models using the stability of differences in Fermat potential (proportional to time delay) w.r.t. modeling choices. We find that for 10/30 lenses our models are cosmography or nearly cosmography grade (<3% and 3-5% variations). For 6/30 lenses the models are close to cosmography grade (5-10%). These results are based on informative priors and will need to be confirmed by further analysis. However, they are also likely to improve by extending the pipeline modeling sequence and options. In conclusion, we show that uniform cosmography grade modeling of large strong lens samples is within reach.

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arXiv2022

The halo of M105 and its group environment as traced by planetary nebula populations: II. Using kinematics of single stars to unveil the presence of intragroup light around the Leo I galaxies NGC 3384 and M105

M105 is an early-type galaxy in the nearby Leo I group, the closest galaxy group to contain all galaxy types and therefore an excellent environment to explore the low-mass end of intra-group light (IGL) assembly. We present a new extended kinematic survey of planetary nebulae (PNe) in M105 and the surrounding 30'x30' in the Leo I group with the Planetary Nebula Spectrograph. We use PNe as kinematic tracers of the diffuse stellar light in the halo and IGL and employ Gaussian mixture models to separate contributions from the companion galaxy NGC 3384 and associate PNe with halo and IGL components around M105. We present a catalogue of 314 PNe and firmly associate 93 with NGC 3384 and 169 with M105. The PNe in M105 are further associated with its halo and the surrounding exponential envelope. We construct smooth velocity and velocity dispersion fields and calculate projected rotation, velocity dispersion, and $λ_R$ profiles for each component. Halo PNe exhibit declining velocity dispersion and rotation profiles, while the velocity dispersion and rotation of the exponential envelope increase notably at large radii. We identify three regimes with distinct kinematics that are linked to distinct stellar population properties: (i) the rotating core (within $1~R_\mathrm{eff}$) formed in situ and dominated by metal-rich ([M/H]~0) stars likely formed in situ, (ii) the halo from 1 to $7.5~R_\mathrm{eff}$ consisting of intermediate-metallicity stars ([M/H]>-1), either formed in situ or brought in through major mergers, and (iii) the exponential envelope reaching beyond our farthest data point at 16 $R_\mathrm{eff}$, predominately composed of metal-poor ([M/H]<-1) stars. The high velocity dispersion and moderate rotation of the latter are consistent with that measured for dwarf satellite galaxies in the Leo I group, indicating that the exponential envelope traces the transition to the IGL.

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arXiv2021

High-quality strong lens candidates in the final Kilo Degree survey footprint

We present 97 new high-quality strong lensing candidates found in the final $\sim 350\,\rm deg^2$, that completed the full $\sim 1350\,\rm deg^2$ area of the Kilo-Degree Survey (KiDS). Together with our previous findings, the final list of high-quality candidates from KiDS sums up to 268 systems. The new sample is assembled using a new Convolutional Neural Network (CNN) classifier applied to $r$-band (best seeing) and $g,~r,~i$ color-composited images separately. This optimizes the complementarity of the morphology and color information on the identification of strong lensing candidates. We apply the new classifiers to a sample of luminous red galaxies (LRGs) and a sample of bright galaxies (BGs) and select candidates that received a high probability to be a lens from the CNN ($P_{\rm CNN}$). In particular, setting $P_{\rm CNN}>0.8$ for the LRGs, the $1$-band CNN predicts 1213 candidates, while the $3$-band classifier yields 1299 candidates, with only $\sim$30\% overlap. For the BGs, in order to minimize the false positives, we adopt a more conservative threshold, $P_{\rm CNN} >0.9$, for both CNN classifiers. This results in 3740 newly selected objects. The candidates from the two samples are visually inspected by 7 co-authors to finally select 97 "high-quality" lens candidates which received mean scores larger than 6 (on a scale from 0 to 10). We finally discuss the effect of the seeing on the accuracy of CNN classification and possible avenues to increase the efficiency of multi-band classifiers, in preparation of next-generation surveys from ground and space.

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arXiv2021

The Fornax Cluster VLT Spectroscopic Survey IV -- Cold kinematical substructures in the Fornax core from COSTA

The COld STream finder Algorithm (COSTA) is a novel algorithm to find streams in the phase space of planetary nebulae (PNe) and globular cluster (GCs) populations. COSTA isolates groups of particles with small velocity dispersion, using an iterative sigma-clipping over a defined number of neighbor particles. We have applied COSTA to a catalog of about 2000 PNe and GCs from the Fornax Cluster VLT Spectroscopic Survey (FVSS), within 200 kpc from the cluster core, to detect cold substructures and characterize their kinematics. We have found 13 cold substructures, with velocity dispersion ranging from 20 to 100 kms^{-1}, which are likely associated either to large galaxies or to ultra-compact dwarf (UCD) galaxies in the Fornax core. These streams show a clear correlation of their luminosity with the internal velocity dispersion, and their surface brightness with size and distance from the cluster center that are compatible with dissipative processes producing them. However, we cannot exclude that some of these substructures have formed by violent relaxation of massive satellites finally merged into the central galaxy. Among these substructures we have: a stream connecting NGC1387 to the central galaxy, NGC1399, previously reported in literature; a new giant stream produced by the interaction of NGC1382 with NGC1380 and possibly NGC1381; a series of streams kinematically connected to nearby ultra compact dwarf galaxies; clumps of tracers with no clear kinematical association to close cluster members. We show evidence for a variety of cold substructure predicted in simulations. Most of the streams are kinematically connected to UCDs, supporting the scenario that they can be remnants of disrupted dwarf systems. [abridged]

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arXiv2021

INSPIRE: INvestigating Stellar Population In RElics II. First Data Release (DR1)

The INvestigating Stellar Population In RElics is an on-going project targeting 52 ultra-compact massive galaxies at 0.1 2) through a short and intense star formation burst, that evolved passively and undisturbed until the present-day. Relics provide a unique opportunity to study the mechanisms of star formation at high-z. In this paper, we present the first INSPIRE Data Release, comprising 19 systems with observations completed in 2020. We use the methods already presented in the INSPIRE Pilot, but revisiting the 1D spectral extraction. For these 19 systems, we obtain an estimate of the stellar velocity dispersion, fitting separately the two UVB and VIS XSH arms at their original resolution. We estimate [Mg/Fe] abundances via line-index strength and mass-weighted integrated stellar ages and metallicities with full spectral fitting on the combined spectrum. Ages are generally old, in agreement with the photometric ones, and metallicities are almost always super-solar, confirming the mass-metallicity relation. The [Mg/Fe] ratio is also larger than solar for the great majority of the galaxies, as expected. We find that 10 objects have formed more than 75% of their stellar mass (M*) within 3 Gyr from the Big Bang and classify them as relics. Among these, we identify 4 galaxies which had already fully assembled their M* by that time. They are therefore `extreme relics' of the ancient Universe. The INSPIRE DR1 catalogue of 10 known relics to-date augment by a factor of 3.3 the total number of confirmed relics, also enlarging the redshift window. It is therefore the largest publicly available collection. Thanks to the larger number of systems, we can also better quantify the existence of a 'degree of relicness', already hinted at the Pilot Paper.

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arXiv2021

VEXAS: VISTA EXtension to Auxiliary Surveys -- Data Release 2: Machine-learning based classification of sources in the Southern Hemisphere

We present the second public data release (DR) of the VISTA EXtension to Auxiliary Surveys (VEXAS), where we classify objects into stars, galaxies and quasars based on an ensemble of machine learning algorithms. The aim of VEXAS is to build the widest multi-wavelength catalogue, providing reference magnitudes, colours and morphological information for a large number of scientific uses. We apply an ensemble of 32 different machine learning models, based on three different algorithms and on different magnitude sets, training samples and classification problems on the three VEXAS DR1 optical+infrared (IR) tables. The tables were created in DR1 cross-matching VISTA near-IR data with WISE far-IR data and with optical magnitudes from the Dark Energy Survey (VEXAS-DESW), the Sky Mapper Survey (VEXAS-SMW), and the PanSTARRS (VEXAS-PSW). We assemble a large table of spectroscopically confirmed objects (415 628 unique objects), based on the combination of 6 different spectroscopic surveys that we use for training. We develop feature imputation to classify also objects for which magnitudes in one or more bands are missing. We classify in total ~90 million objects in the Southern Hemisphere. Among these,~62.9M (~52.6M) are classified as 'high confidence' ('secure') stars, ~920k (~750k) as 'high confidence' ('secure') quasars and ~34.8M (~34.1M) as 'high confidence' ('secure') galaxies, with probabilities $p_{\rm class}\ge 0.7$ ($p_{\rm class}\ge 0.9$). The density of high-confidence extragalactic objects varies strongly with the survey depth: at $p_{\rm class}\ge 0.7$, there are 111/deg$^2$ quasars in the VEXAS-DESW footprint and 103/deg$^2$ in the VEXAS-PSW footprint, while only 10.7/deg$^2$ in the VEXAS-SM footprint. Improved depth in the midIR and coverage in the optical and nearIR are needed for the SM footprint that is not already covered by DESW and PSW.

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arXiv2021

A preserved high-z compact progenitor in the heart of NGC3311 revealed with MUSE 2D stellar population analysis

Massive early-type galaxies are believed to be the end result of an extended mass accretion history. The stars formed in situ very early on in the initial phase of the assembly might have originated from an extremely intense star formation burst, and may still be found within the cores of such galaxies today. We investigate the presence of a surviving high-$z$ compact progenitor component in the brightest galaxy of the Hydra I cluster, NGC 3311, by mapping its 2D kinematics and stellar population out to 2 effective radii, combining MUSE observations, extended EMILES models, and a newly developed parametric fully Bayesian framework using full-spectrum fitting. We present 2D maps and radial profiles of the stellar velocity dispersion, age, total metallicity, $α$-element, sodium abundance ([Na/Fe]), and the initial mass function (IMF) slope. All properties have significant gradients, confirming the existence of multiple structural components, including a young, metal-rich "blue spot". We find that the component dominating the light budget of NGC 3311 within $R\lesssim 2.0$ kpc is the surviving $z=0$ analog of a high-$z$ compact core. This concentrated structure has a relatively small velocity dispersion ($σ_*\approx 180$ km s$^{-1}$), is very old (ages$\gtrsim 11$ Gyr), metal-rich ([Z/H]$\sim0.2$ and [Na/Fe]$\sim0.4$), and has a bottom-heavy IMF (with slope $Γ_b\sim2.4$). In the outer region, stars become increasingly hotter, younger, metal and sodium poorer, $α$-element richer, and the IMF slope becomes Chabrier-like. The multiple structural components in NGC 3311 confirm the predictions from the two-phase formation scenario for NGC 3311. Interestingly, the outer stellar population has an overabundant [$α$/Fe], most likely because NGC 3311, located at the center of the galaxy cluster, accreted stars from rapidly quenched satellites.[Abridged]

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arXiv2020

TDCOSMO IV: Hierarchical time-delay cosmography -- joint inference of the Hubble constant and galaxy density profiles

The H0LiCOW collaboration inferred via gravitational lensing time delays a Hubble constant $H_0=73.3^{+1.7}_{-1.8}$ km s$^{-1}{\rm Mpc}^{-1}$, describing deflector mass density profiles by either a power-law or stars plus standard dark matter halos. The mass-sheet transform (MST) that leaves the lensing observables unchanged is considered the dominant source of residual uncertainty in $H_0$. We quantify any potential effect of the MST with flexible mass models that are maximally degenerate with H0. Our calculation is based on a new hierarchical approach in which the MST is only constrained by stellar kinematics. The approach is validated on hydrodynamically simulated lenses. We apply the method to the TDCOSMO sample of 7 lenses (6 from H0LiCOW) and measure $H_0=74.5^{+5.6}_{-6.1}$ km s$^{-1}{\rm Mpc}^{-1}$. In order to further constrain the deflector mass profiles, we then add imaging and spectroscopy for 33 strong gravitational lenses from the SLACS sample. For 9 of the SLAC lenses we use resolved kinematics to constrain the stellar anisotropy. From the joint analysis of the TDCOSMO+SLACS sample, we measure $H_0=67.4^{+4.1}_{-3.2}$ km s$^{-1}{\rm Mpc}^{-1}$, assuming that the TDCOSMO and SLACS galaxies are drawn from the same parent population. The blind H0LiCOW, TDCOSMO-only and TDCOSMO+SLACS analyses are in mutual statistical agreement. The TDCOSMO+SLACS analysis prefers marginally shallower mass profiles than H0LiCOW or TDCOSMO-only. While our new analysis does not statistically invalidate the mass profile assumptions by H0LiCOW, and thus their $H_0$ measurement relying on those, it demonstrates the importance of understanding the mass density profile of elliptical galaxies. The uncertainties on $H_0$ derived in this paper can be reduced by physical or observational priors on the form of the mass profile, or by additional data, chiefly spatially resolved kinematics of lens galaxies.

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arXiv2020

What does (not) drive the variation of the low-mass end of the stellar initial mass function of early-type galaxies

The stellar initial mass function (IMF) seems to be variable and not universal, as argued in the literature in the last three decades. Several relations among the low-mass end of the IMF slope and other stellar population, photometric or kinematic parameters of massive early-type galaxies (ETGs) have been proposed, but a consolidated agreement on a factual cause of the observed variations has not been reached yet. We investigate the relations between the IMF and other stellar population parameters in NGC 3311, the central galaxy of the Hydra I cluster. NGC 3311 is characterized by old and metal-rich stars, like other massive ETGs, but has unusual increasing stellar velocity dispersion and [$α/$Fe] profiles. We use spatially resolved MUSE observations to obtain stellar population properties using Bayesian full-spectrum fitting in the central part of NGC 3311 to compare the IMF slope against other stellar parameters with the goal of assessing their relations/dependencies. For NGC 3311, we unambiguously invalidate the previously observed direct correlation between the IMF slope and the local stellar velocity dispersion, confirming some doubts already raised in the literature. This relation may arise as a spatial coincidence only, between the region with the largest stellar velocity dispersion, with that where the oldest, $\textit{in situ}$ population is found and dominates. We also show robust evidence that the proposed IMF-metallicity relation is contaminated by the degeneracy between these two parameters. The tightest correlations we found are those between stellar age and IMF and between galactocentric radius and IMF. The variation of the IMF is not due to kinematical, dynamical, or global properties in NGC 3311. We speculate that IMF might be dwarf-dominated in the "red-nuggets" formed at high redshifts that ended up being the central cores of today's giant ellipticals. [Abridged]

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arXiv2020

INSPIRE: INvestigating Stellar Population In RElics -- I. Survey presentation and pilot program

Massive ETGs are thought to form through a two-phase process. At early times, an intense and fast starburst forms blue and disk-dominated galaxies. After quenching, the remaining structures become red, compact and massive, i.e., 'red nuggets'. Then, a time-extended second phase which is dominated by mergers, causes structural evolution and size growth. Given the stochastic nature of mergers, a small fraction of red nuggets survives, without any interaction, massive and compact until today: relic galaxies. Since this fraction depends on the processes dominating the size growth, counting relics at low-z is a valuable way to disentangle between different galaxy evolution models. In this paper, we introduce the INvestigating Stellar Population In RElics (INSPIRE) Project, that aims at spectroscopically confirming and fully characterizing a large number of relics at 0.1 80 % of their stellar mass within a short (0.5 - 1.0 Gyrs) initial star formation episode occurred only ~1 Gyr after the Big Bang. The third galaxy has a more extended star formation history and a lower velocity dispersion. Thus we confirm two out of three candidates as relics.

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arXiv2020

Discovery of two Einstein crosses from massive post--blue nugget galaxies at z>1 in KiDS

We report the discovery of two Einstein Crosses (ECs) in the footprint of the Kilo-Degree Survey (KiDS): KIDS J232940-340922 and KIDS J122456+005048. Using integral field spectroscopy from MUSE@VLT, we confirm their gravitational-lens nature. In both cases, the four spectra of the source clearly show a prominence of absorption features, hence revealing an evolved stellar population with little star formation. The lensing model of the two systems, assuming a singular isothermal ellipsoid (SIE) with external shear, shows that: 1) the two crosses, located at redshift $z=0.38$ and 0.24, have Einstein radius $R_{\rm E}=5.2$ kpc and 5.4 kpc, respectively; 2) their projected dark matter fractions inside the half effective radius are 0.60 and 0.56 (Chabrier IMF); 3) the sources are ultra-compact galaxies, $R_{\rm e}\sim0.9$ kpc (at redshift $z_{\rm s}=1.59$) and $R_{\rm e}\sim0.5$ kpc ($z_{\rm s}=1.10$), respectively. These results are unaffected by the underlying mass density assumption. Due to size, blue color and absorption-dominated spectra, corroborated by low specific star-formation rates derived from optical-NIR spectral energy distribution fitting, we argue that the two lensed sources in these ECs are blue nuggets migrating toward their quenching phase.

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arXiv2020

COSTA: the COld STream finder Algorithm. Searching for kinematical substructures in the phase space of discrete tracers

Context: We present COSTA (COld STream finder Algorithm), a novel algorithm to search for cold kinematical substructures in the phase space of planetary nebulae (PNe) and globular clusters (GCs) in the halo of massive galaxies and intracluster regions. Aims: COSTA aims at detecting small sized, low velocity dispersion streams, as the ones produced in recent interactions of dwarf galaxies with the halo of more massive galaxies, including the ones sitting in the central region of rich galaxy clusters. Methods: COSTA is based on a deep friend-of-friend procedure that isolates groups of N particles with small velocity dispersion (between 10 kms and $\sim$ 100 kms), using an iterative (n) sigma-clipping over a defined number of (k) neighbor particles. The algorithm has three parameters (k-n-N), plus a velocity dispersion cut-off, which defines the "coldness" of the stream, that are set using Montecarlo realizations of the sample under exam. Results: In this paper, we show the ability of COSTA to recover simulated streams on mock data-sets of discrete kinematical tracers with different sizes and measurement errors, from publicly available hydrodynamical simulations. We also show the best algorithm set-up for a realistic case of stream finding in the core of the Fornax cluster, for future applications of COSTA to real populations of PNe and GCs. Conclusions: COSTA can be generalized to all problems of finding small substructures in the phase space of a limited sample of discrete tracers, provided that the algorithm is trained on realistic mock observations reproducing the specific dataset under exam.

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arXiv2020

TDCOSMO. I. An exploration of systematic uncertainties in the inference of $H_0$ from time-delay cosmography

Time-delay cosmography of lensed quasars has achieved 2.4% precision on the measurement of the Hubble constant, $H_0$. As part of an ongoing effort to uncover and control systematic uncertainties, we investigate three potential sources: 1- stellar kinematics, 2- line-of-sight effects, and 3- the deflector mass model. To meet this goal in a quantitative way, we reproduced the H0LiCOW/SHARP/STRIDES (hereafter TDCOSMO) procedures on a set of real and simulated data, and we find the following. First, stellar kinematics cannot be a dominant source of error or bias since we find that a systematic change of 10% of measured velocity dispersion leads to only a 0.7% shift on $H_0$ from the seven lenses analyzed by TDCOSMO. Second, we find no bias to arise from incorrect estimation of the line-of-sight effects. Third, we show that elliptical composite (stars + dark matter halo), power-law, and cored power-law mass profiles have the flexibility to yield a broad range in $H_0$ values. However, the TDCOSMO procedures that model the data with both composite and power-law mass profiles are informative. If the models agree, as we observe in real systems owing to the "bulge-halo" conspiracy, $H_0$ is recovered precisely and accurately by both models. If the two models disagree, as in the case of some pathological models illustrated here, the TDCOSMO procedure either discriminates between them through the goodness of fit, or it accounts for the discrepancy in the final error bars provided by the analysis. This conclusion is consistent with a reanalysis of six of the TDCOSMO (real) lenses: the composite model yields $74.0^{+1.7}_{-1.8}$ $km.s^{-1}.Mpc^{-1}$, while the power-law model yields $H_0=74.2^{+1.6}_{-1.6}$ $km.s^{-1}.Mpc^{-1}$. In conclusion, we find no evidence of bias or errors larger than the current statistical uncertainties reported by TDCOSMO.

astro-ph.CO↗

arXiv2020

The first detection of ultra-diffuse galaxies in the Hydra I cluster from VEGAS survey

In this paper we report on the discovery of 27 low-surface brightness galaxies, of which 12 are candidate ultra-diffuse galaxy (UDG) in the Hydra I cluster, based on deep observations taken as part of the VST Early-type Galaxy Survey (VEGAS). This first sample of UDG candidates in the Hydra I cluster represents an important step in our project that aims to enlarge the number of confirmed UDGs and, through study of statistically relevant samples, constrain the nature and formation of UDGs. This study presents the main properties of this class of galaxies in the Hydra I cluster. For all UDGs, we analyse the light and colour distribution, and provide a census of the globular cluster (GC) systems around them. Given the limitations of a reliable GC selection based on two relatively close optical bands only, we find that half of the UDG candidates have a total GC population consistent with zero. Of the other half, two galaxies have a total population larger than zero at 2$σ$ level. We estimate the stellar mass, the total number of GCs and the GC specific frequency ($S_N$). Most of the candidates span a range of stellar masses of $10^7-10^8$~M$_{\odot}$. Based on the GC population of these newly discovered UDGs, we conclude that most of these galaxies have a standard or low dark matter content, with a halo mass of $\leq 10^{10}$~M$_{\odot}$.

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arXiv2020

Nature versus nurture: relic nature and environment of the most massive passive galaxies at $z < 0.5$

Relic galaxies are thought to be the progenitors of high-redshift red nuggets that for some reason missed the channels of size growth and evolved passively and undisturbed since the first star formation burst (at $z>2$). These local ultracompact old galaxies are unique laboratories for studying the star formation processes at high redshift and thus the early stage of galaxy formation scenarios. Counterintuitively, theoretical and observational studies indicate that relics are more common in denser environments, where merging events predominate. To verify this scenario, we compared the number counts of a sample of ultracompact massive galaxies (UCMGs) selected within the third data release of the Kilo Degree Survey, that is, systems with sizes $R_{\rm e} < 1.5 \, \rm kpc$ and stellar masses $M_{\rm \star} > 8 \times 10^{10}\, \rm M_{\odot}$, with the number counts of galaxies with the same masses but normal sizes in field and cluster environments. Based on their optical and near-infrared colors, these UCMGs are likely to be mainly old, and hence representative of the relic population. We find that both UCMGs and normal-size galaxies are more abundant in clusters and their relative fraction depends only mildly on the global environment, with denser environments penalizing the survival of relics. Hence, UCMGs (and likely relics overall) are not special because of the environment effect on their nurture, but rather they are just a product of the stochasticity of the merging processes regardless of the global environment in which they live.

astro-ph.GA↗

arXiv2020

New high-quality strong lens candidates with deep learning in the Kilo Degree Survey

We report new high-quality galaxy scale strong lens candidates found in the Kilo Degree Survey data release 4 using Machine Learning. We have developed a new Convolutional Neural Network (CNN) classifier to search for gravitational arcs, following the prescription by \cite{2019MNRAS.484.3879P} and using only $r-$band images. We have applied the CNN to two "predictive samples": a Luminous red galaxy (LRG) and a "bright galaxy" (BG) sample ($r<21$). We have found 286 new high probability candidates, 133 from the LRG sample and 153 from the BG sample. We have then ranked these candidates based on a value that combines the CNN likelihood to be a lens and the human score resulting from visual inspection (P-value) and we present here the highest 82 ranked candidates with P-values $\ge 0.5$. All these high-quality candidates have obvious arc or point-like features around the central red defector. Moreover, we define the best 26 objects, all with scores P-values $\ge 0.7$ as a "golden sample" of candidates. This sample is expected to contain very few false positives and thus it is suitable for follow-up observations. The new lens candidates come partially from the the more extended footprint adopted here with respect to the previous analyses, partially from a larger predictive sample (also including the BG sample). These results show that machine learning tools are very promising to find strong lenses in large surveys and more candidates that can be found by enlarging the predictive samples beyond the standard assumption of LRGs. In the future, we plan to apply our CNN to the data from next-generation surveys such as the Large Synoptic Survey Telescope, Euclid, and the Chinese Space Station Optical Survey.

astro-ph.GA↗

arXiv2020

STRIDES: a 3.9 per cent measurement of the Hubble constant from the strong lens system DES J0408-5354

We present a blind time-delay cosmographic analysis for the lens system DES J0408$-$5354. This system is extraordinary for the presence of two sets of multiple images at different redshifts, which provide the opportunity to obtain more information at the cost of increased modelling complexity with respect to previously analyzed systems. We perform detailed modelling of the mass distribution for this lens system using three band Hubble Space Telescope imaging. We combine the measured time delays, line-of-sight central velocity dispersion of the deflector, and statistically constrained external convergence with our lens models to estimate two cosmological distances. We measure the "effective" time-delay distance corresponding to the redshifts of the deflector and the lensed quasar $D_{Δt}^{\rm eff}=3382^{+146}_{-115}$ Mpc and the angular diameter distance to the deflector $D_{\rm d}=1711^{+376}_{-280}$ Mpc, with covariance between the two distances. From these constraints on the cosmological distances, we infer the Hubble constant $H_0 = 74.2^{+2.7}_{-3.0}$ km s$^{-1}$ Mpc$^{-1}$ assuming a flat $Λ$CDM cosmology and a uniform prior for $Ω_{\rm m}$ as $Ω_{\rm m} \sim \mathcal{U}(0.05, 0.5)$. This measurement gives the most precise constraint on $H_0$ to date from a single lens. Our measurement is consistent with that obtained from the previous sample of six lenses analyzed by the $H_0$ Lenses in COSMOGRAIL's Wellspring (H0LiCOW) collaboration. It is also consistent with measurements of $H_0$ based on the local distance ladder, reinforcing the tension with the inference from early Universe probes, for example, with 2.2$σ$ discrepancy from the cosmic microwave background measurement.

astro-ph.CO↗