Search

The INSPIRE project has built the largest sample of ultra-compact massive galaxies (UCMGs) at 0.12, and the other stellar population parameters.We use the Full-Index-Fitting method to fit the INSPIRE spectra to single stellar population (SSP) models. This allows us to measure, for the first time, the low-mass end slope of the IMF, as well as stellar metallicity [M/H], [Mg/Fe], [Ti/Fe] and [Na/Fe] ratios, and study correlations between them and the DoR. Similarly to normal-sized galaxies, UCMGs with larger stellar masses have overall higher metallicities. We found a correlation between the low-mass end of the IMF slope and the DoR, that, however, breaks down for systems with a more extended SFH. An even stronger dependency is found between the IMF and the fraction of mass formed at high-z. At equal velocity dispersion and metallicity, galaxies with a higher DoR have a dwarf-richer IMF than that of low-DoR counterparts. This might indicate that the cosmic epoch and formation mechanisms influence the fragmentation of the star formation cloud and hence might be the explanation for IMF variations detected in massive ETGs., Comment: 16 pages, 11 figures, submitted to MNRAS

Aims. We aim to investigate the stellar population properties, ages, and metal content of the globular clusters (GCs) in NGC 3311, the central galaxy of the Hydra I cluster, to better constrain its evolution history. Methods. We used integral-field spectroscopic data from the Multi-Unit Spectroscopic Explorer (MUSE) to identify 680 sources in the central region of NGC 3311 and extract their 1D spectra. An analysis of these sources in terms of morphologies, radial velocities, and emission lines allowed us to narrow down our selection to 49 bona fide GC candidates. We split these candidates into two groups depending on their projected distance to the galaxy center (R), namely inner (R<20arcsec) and outer (R>20arcsec) GCs. We stacked the extracted 1D spectra of the inner and outer GC populations to increase the signal-to-noise ratios (S/Ns) of the resulting spectra and hence allow full-spectrum fitting. In addition, we also created a stacked spectrum of all GCs in NGC 3311 and one of the two most central GC candidates. Using the code pPXF, we performed a stellar population analysis on the four stacked 1D spectra, obtaining mass-weighted integrated ages, metallicities, and [$\alpha$/Fe] abundances. Results. All GCs are old, with ages >13.5 Gyr, and they have super-solar metallicities. Looking at the color distribution, we find that the inner ones tend to be redder and more metal rich than the outer ones. This is consistent with the two-phase formation scenario. Looking at the full-spectral fitting results, at face value the outer GCs have a larger [$\alpha$/Fe] ratio, which is in line with what is found for the stars that dominate the surface brightness profile at the same radii. However, the values for outer and inner GCs are consistent within the uncertainties., Comment: Accepted for publication by A&A on November 15th 2023

Planetary nebulae (PNe) are essential tracers of the kinematics of the diffuse halo and intracluster light where stellar spectroscopy is unfeasible, due to their strong emission lines. However, that is not all they can reveal about the underlying stellar population. In recent years, it has also been found that PNe in the metal-poor halos of galaxies have different properties (specific frequency, luminosity function), than PNe in the more metal-rich galaxy centers. A more quantitative understanding of the role of age and metallicity in these relations would turn PNe into valuable stellar-population tracers. In order to do that, a full characterization of PNe in regions where the stellar light can also be analysed in detail is necessary. In this work, we make use of integral-field spectroscopic data covering the central regions of galaxies, which allow us to measure both stellar ages and metallicities as well as to detect PNe. This analysis is fundamental to calibrate PNe as stellar population tracers and to push our understanding of galaxy properties at unprecedented galactocentric distances., Comment: Submitted to the Proceedings of IAU Symposium 384: Planetary Nebulae: a Universal Toolbox in the Era of Precision Astrophysics. Eds: O. De Marco, A. Zijlstra, R. Szczerba

During the early stages of galaxy evolution, a significant fraction of galaxies undergo a transitional phase between the ``blue nugget" systems, which arise from the compaction of large, active star-forming disks, and the ``red nuggets", which are red and passive compact galaxies. These objects are typically only observable with space telescopes, and detailed studies of their size, mass, and stellar population parameters have been conducted on relatively small samples. Strong gravitational lensing can offer a new opportunity to study them in detail, even with ground-based observations. In this study, we present the first 6 \textit{bona fide} sample of strongly lensed post-blue nugget (pBN) galaxies, which were discovered in the Kilo Degree Survey. By using the lensing-magnified luminosity from optical and near-infrared bands, we have derived robust structural and stellar population properties of the multiple images of the background sources. \rui{The pBN galaxies have very small sizes of $\rm R_{eff}<1.3$ kpc, high mass density inside 1 kpc of $ \rm \log (\Sigma_1/M_{\odot} \mathrm{kpc}^{-2})>9.3$, and low specific star formation rates of $\log (\mathrm{sSFR/Gyr})\lesssim0$, The size-mass and $\Sigma_1$-mass relations of this sample are consistent with those of the red nuggets, while their sSFR is close to the lower end of compact star-forming blue nugget systems at the same redshift, suggesting a clear evolutionary link between them., Comment: Accepted for publication in APJ

Strong gravitational lenses provide unique laboratories for cosmological and astrophysical investigations, but they must first be discovered - a task that can be met with significant contamination by other astrophysical objects and asterisms. Here we review strong lens searches, covering various sources (quasars, galaxies, supernovae, FRBs, GRBs, and GWs), lenses (early- and late-type galaxies, groups, and clusters), datasets (imaging, spectra, and lightcurves), and wavelengths. We first present the physical characteristics of the lens and source populations, highlighting relevant details for constructing targeted searches. Search techniques are described based on the main lensing feature that is required for the technique to work, namely one of: (i) an associated magnification, (ii) multiple spatially-resolved images, (iii) multiple redshifts, or (iv) a non-zero time delay between images. To use the current lens samples for science, and for the design of future searches, we list several selection biases that exist due to these discovery techniques. We conclude by discussing the future of lens searches in upcoming surveys and the new population of lenses that will be discovered., Comment: 54 pages, 15 figures, submitted to Space Science Reviews, Topical Collection "Strong Gravitational Lensing", eds. J. Wambsganss et al

Looking into the faintEst WIth MUSE (LEWIS) is an ESO large observing programme aimed at obtaining the first homogeneous integral-field spectroscopic survey of 30 extremely low-surface brightness (LSB) galaxies in the Hydra I cluster of galaxies, with MUSE at ESO-VLT. The majority of LSB galaxies in the sample (22 in total) are ultra-diffuse galaxies (UDGs). The distribution of systemic velocities Vsys ranges between 2317 km/s and 5198 km/s and is centred on the mean velocity of Hydra I (Vsys = 3683 $\pm$ 46 km/s). Considering the mean velocity and the velocity dispersion of the cluster, 17 out of 20 targets are confirmed cluster members. To assess the quality of the data and demonstrate the feasibility of the science goals, we report the preliminary results obtained for one of the sample galaxies, UDG11. For this target, we derived the stellar kinematics, including the 2-dimensional maps of line-of-sight velocity and velocity dispersion, constrained age and metallicity, and studied the globular cluster (GC) population hosted by the UDG. Results are compared with the available measurements for UDGs and dwarf galaxies in literature. By fitting the stacked spectrum inside one effective radius, we find that UDG11 has a velocity dispersion $\sigma = 20 \pm 8$ km/s, it is old ($10\pm1$ Gyr), metal-poor ([M/H]=-1.17$\pm$0.11 dex) and has a total dynamical mass-to-light ratio M$/L_V\sim 14$, comparable to those observed for classical dwarf galaxies. The spatially resolved stellar kinematics maps suggest that UDG11 does not show a significant velocity gradient along either major or minor photometric axes. We find two GCs kinematically associated with UDG11. The estimated total number of GCs in UDG11, corrected for the spectroscopic completeness limit, is $N_{GC}= 5.9^{+2.2}_ {-1.8}$, which corresponds to a GC specific frequency of $S_N = 8.4^{+3.2}_{-2.7}$., Comment: Accepted for publication in Astronomy and Astrophysics

Although ultra diffuse galaxies (UDGs) are found in large numbers in clusters of galaxies, the role of the cluster environment in shaping their low surface brightness and large sizes is still uncertain. Here we examine a sample of UDGs in the Hydra I cluster (D = 51 Mpc) with new radial velocities obtained as part of the LEWIS (Looking into the faintest with MUSE) project using VLT/MUSE data. Using a phase-space, or infall diagnostic, diagram we compare the UDGs to other known galaxies in the Hydra I cluster and to UDGs in other clusters. The UDGs, along with the bulk of regular Hydra I galaxies, have low relative velocities and are located near the cluster core, and thus consistent with very early infall into the cluster. Combining with literature data, we do not find the expected trend of GC-rich UDGs associated with earlier infall times. This result suggests that quenching mechanisms other than cluster infall should be further considered, e.g. quenching by strong feedback or in cosmic sheets and filaments. Tidal stripping of GCs in the cluster environment also warrants further modelling., Comment: 6 pages, 2 figures, MNRAS, 525, 93

We measure the [${\alpha}$/Fe] abundances for 183 quiescent galaxies at z = 0.60 - 0.75 with stellar masses ranging 10.4 \leq log10 10.4 $\leq$ log10 (M$_*$ /M$_\odot$) $\leq$ 11.6 selected from the LEGA-C survey. We estimate [${\alpha}$/Fe] from the ratio of the spectral indices Mgb (${\lambda} \sim 5177$ {\AA}) and Fe4383, compared to predictions of simple stellar population models. We find that 91% of quiescents in our sample have supersolar [${\alpha}$/Fe], with an average value of [${\alpha}$/Fe] = +0.24 $\pm$ 0.01. We find no significant correlation between [${\alpha}$/Fe] and stellar metallicity, mass, velocity dispersion, and average formation time. Galaxies that formed the bulk of their stellar mass on time scales shorter than 1 Gyr follow the same [${\alpha}$/Fe] distribution as those which formed on longer time scales. In comparison to local early-type galaxies and to stacked spectra of quiescent galaxies at z = 0.38 and z = 0.07, we find that the average [${\alpha}$/Fe] has not changed between z = 0.75 and the present time. Our work shows that the vast majority of massive quiescent galaxies at z $\sim$ 0.7 are ${\alpha}$-enhanced, and that no detectable evolution of the average [${\alpha}$/Fe] has taken place over the last $\sim$ 6.5 Gyr., Comment: 12 pages, 10 figures, 2 tables, 1 appendix

We derived stellar ages and metallicities [Z/H] for $\sim$70 passive early type galaxies (ETGs) selected from VANDELS survey over the redshift range 1.0$<$$z$$<$1.4 and stellar mass range 10$<$log(M$_*$/M$_\odot$)$<$11.6. We find significant systematics in their estimates depending on models and wavelength ranges considered. Using the full-spectrum fitting technique, we find that both [Z/H] and age increase with mass as for local ETGs. Age and metallicity sensitive spectral indices independently confirm these trends. According to EMILES models, for 67 per cent of the galaxies we find [Z/H]$>$0.0, a percentage which rises to $\sim$90 per cent for log(M$_*$/M$_\odot$)$>$11 where the mean metallicity is [Z/H]=0.17$\pm$0.1. A comparison with homogeneous measurements at similar and lower redshift does not show any metallicity evolution over the redshift range 0.0$11.0 host stellar populations with [Z/H]>0.05, formed over short timescales ($\Delta{t50}$$<$1 Gyr) at early epochs (t$_{form}$$<$2 Gyr), implying high star formation rates (SFR$>$100 M$_\odot$/yr) in high mass density regions (log($\Sigma_{1kpc}$)$>$10 M$_\odot$/kpc$^2$). This sharp picture tends to blur at lower masses: log(M$_*$/M$_\odot$)$\sim$10.6 galaxies can host either old stars with [Z/H]$<$0.0 or younger stars with [Z/H]$>$0.0, depending on the duration ($\Delta{t50}$) of the SF. The relations between galaxy mass, age and metallicities are therefore largely set up ab initio as part of the galaxy formation process. Mass, SFR and SF time-scale all contribute to shape up the stellar mass-metallicity relation with the mass that modulates metals retention., Comment: 18 pages + Appendix, 16 figures, 5 tables. Accepted for publication in MNRAS

In this work, we extend the catalog of low-surface brightness (LSB) galaxies, including Ultra-Diffuse Galaxy (UDG) candidates, within $\approx 0.4R_{vir}$ of the Hydra I cluster of galaxies, based on deep images from the VST Early-type GAlaxy Survey (VEGAS). The new galaxies are found by applying an automatic detection tool and carrying out additional visual inspections of $g$ and $r$ band images. This led to the detection of 11 UDGs and 8 more LSB galaxies. For all of them, the cluster membership has been assessed using the color-magnitude relation derived for early-type giant and dwarf galaxies in Hydra I. The UDGs and new LSB galaxies found in Hydra I span a wide range of central surface brightness ($22.7 \lesssim \mu_{0,g} \lesssim 26.5$ mag/arcsec$^2$), effective radius ($0.6 \lesssim R_e \lesssim 4.0$ kpc) and color ($0.4 \leq g-r \leq 0.9$ mag), and have stellar masses in the range $\sim 5\times 10^6 - 2\times 10^8$M$_{\odot}$. The 2D projected distribution of both galaxy types is similar to the spatial distribution of dwarf galaxies, with over-densities in the cluster core and north of the cluster centre. They have similar color distribution and comparable stellar masses to the red dwarf galaxies. Based on photometric selection, we identify a total of 9 globular cluster candidates associated to the UDGs and 4 to the LSB galaxies, with the highest number of candidates in an individual UDG being three. We find that there are no relevant differences between dwarfs, LSB galaxies and UDGs: the structural parameters (that is surface brightness, size, colors, n-index) and GCs content of the three classes have similar properties and trends. This finding is consistent with UDGs being the extreme LSB tail of the size-luminosity distribution of dwarfs in this environment., Comment: Accepted for publication in A&A. 30 pages, 22 figures

At ~50 Mpc, the Hydra I cluster of galaxies is among the closest cluster in the z=0 Universe, and an ideal environment to study dwarf galaxy properties in a cluster environment. We exploit deep imaging data of the Hydra I cluster to construct a new photometric catalog of dwarf galaxies in the cluster core, which is then used to derive properties of the Hydra I cluster dwarf galaxies population as well as to compare with other clusters. Moreover, we investigate the dependency of dwarf galaxy properties on their surrounding environment. The new Hydra I dwarf catalog contains 317 galaxies with luminosity between -18.5<$M_r$<-11.5 mag, a semi-major axis larger than ~200 pc (a=0.84 arcsec), of which 202 are new detections, previously unknown dwarf galaxies in the Hydra I central region. We estimate that our detection efficiency reaches 50% at the limiting magnitude $M_r$=-11.5 mag, and at the mean effective surface brightness $\overline{\mu}_{e,r}$=26.5 mag/$arcsec^2$. We present the standard scaling relations for dwarf galaxies and compare them with other nearby clusters. We find that there are no observational differences for dwarfs scaling relations in clusters of different sizes. We study the spatial distribution of galaxies, finding evidence for the presence of substructures within half the virial radius. We also find that mid- and high-luminosity dwarfs ($M_r$<-14.5 mag) become on average redder toward the cluster center, and that they have a mild increase in $R_e$ with increasing clustercentric distance, similar to what is observed for the Fornax cluster. No clear clustercentric trends are reported with surface brightness and S\'ersic index. Considering galaxies in the same magnitude-bins, we find that for high and mid-luminosity dwarfs ($M_r$<-13.5 mag) the g-r color is redder for the brighter surface brightness and higher S\'ersic n index objects., Comment: Accepted for publication in A&A. 25 pages, 21 figures

The Fornax cluster provides an unparalleled opportunity to investigate in detail the formation and evolution of early-type galaxies in a dense environment. We aim at kinematically characterizing photometrically detected globular cluster (GC) candidates in the core of the cluster. We used the VLT/VIMOS spectroscopic data from the FVSS survey in the Fornax cluster, covering one square degree around the central massive galaxy NGC 1399. We confirmed a total of 777 GCs, almost doubling the previously detected GCs, using the same dataset by Pota et al. (2018). Combined with previous literature radial velocity measurements of GCs in Fornax, we compiled the most extensive spectroscopic GC sample of 2341 objects in this environment. We found that red GCs are mostly concentrated around major galaxies, while blue GCs are kinematically irregular and are widely spread throughout the core region of the cluster. The velocity dispersion profiles of blue and red GCs show a quite distinct behaviour. Blue GCs exhibit a sharp increase in the velocity dispersion profile from 250 to 400km/s within 5 arcminutes (29 kpc/1 reff of NGC 1399) from the central galaxy. The velocity dispersion profile of red GCs follows a constant value in between 200-300km/s until 8 arcminutes (46 kpc/1.6reff), and then rises to 350km/s at 10 arcminutes (58 kpc/2 reff). Beyond 10 arcminutes and out to 40 arcminutes (230 kpc/8 reff), blue and red GCs show a constant velocity dispersion of 300+/-50km/s, indicating that both GC populations are tracing the cluster potential. We have kinematically confirmed and characterized the previously photometrically discovered overdensities of intra-cluster GCs. We found that those substructured intra-cluster regions in Fornax are dominated mostly by blue GCs., Comment: 22 Pages, 22 Figures. Accepted for publication in Astronomy and Astrophysics

The VEGAS imaging survey of the Hydra I cluster reveals an extended network of stellar filaments to the south-west of the spiral galaxy NGC3314A. Within these filaments, at a projected distance of ~40 kpc from the galaxy, we discover an ultra-diffuse galaxy (UDG) with a central surface brightness of $\mu_{0,g}\sim26$ mag arcsec$^{-2}$ and effective radius $R_e\sim3.8$ kpc. This UDG, named UDG32, is one of the faintest and most diffuse low-surface brightness galaxies in the Hydra~I cluster. Based on the available data, we cannot exclude that this object is just seen in projection on top of the stellar filaments, thus being instead a foreground or background UDG in the cluster. However, the clear spatial coincidence of UDG32 with the stellar filaments of NGC3314A suggests that it might have formed from the material in the filaments, becoming a detached, gravitationally bound system. In this scenario, the origin of UDG32 depends on the nature of the stellar filaments in NGC3314A, which is still unknown. They could result from ram-pressure stripping or have a tidal origin. In this letter, we focus on the comparison of the observed properties of the stellar filaments and UDG32, and speculate about their possible origin. The relatively red color ($g-r=0.54 \pm 0.14$~mag) of the UDG, similar to that of the disk in NGC3314A, combined with an age older than 1Gyr, and the possible presence of a few compact stellar systems, all point towards a tidal formation scenario inferred for the UDG32., Comment: Accepted for publication in Astronomy & Astrophysics Letter

We present a sample of 16 likely strong gravitational lenses identified in the VST Optical Imaging of the CDFS and ES1 fields (VOICE survey) using Convolutional Neural Networks (CNNs). We train two different CNNs on composite images produced by superimposing simulated gravitational arcs on real Luminous Red Galaxies observed in VOICE. Specifically, the first CNN is trained on single-band images and more easily identifies systems with large Einstein radii, while the second one, trained on composite RGB images, is more accurate in retrieving systems with smaller Einstein radii. We apply both networks to real data from the VOICE survey, taking advantage of the high limiting magnitude (26.1 in the r-band) and low PSF FWHM (0.8" in the r-band) of this deep survey. We analyse $\sim21,200$ images with $mag_r<21.5$, identifying 257 lens candidates. To retrieve a high-confidence sample and to assess the accuracy of our technique, nine of the authors perform a visual inspection. Roughly 75% of the systems are classified as likely lenses by at least one of the authors. Finally, we assemble the LIVE sample (Lenses In VoicE) composed by the 16 systems passing the chosen grading threshold. Three of these candidates show likely lensing features when observed by the Hubble Space Telescope. This work represents a further confirmation of the ability of CNNs to inspect large samples of galaxies searching for gravitational lenses. These algorithms will be crucial to exploit the full scientific potential of forthcoming surveys with the Euclid satellite and the Vera Rubin Observatory, Comment: 15 pages, 12 figures, submitted to MNRAS

Ultra-compact massive galaxies UCMGs, i.e. galaxies with stellar masses $M_{*} > 8 \times 10^{10} M_{\odot}$ and effective radii $R_{e} < 1.5$ kpc, are very rare systems, in particular at low and intermediate redshifts. Their origin as well as their number density across cosmic time are still under scrutiny, especially because of the paucity of spectroscopically confirmed samples. We have started a systematic census of UCMG candidates within the ESO Kilo Degree Survey, together with a large spectroscopic follow-up campaign to build the largest possible sample of confirmed UCMGs. This is the third paper of the series and the second based on the spectroscopic follow-up program. Here, we present photometrical and structural parameters of 33 new candidates at redshifts $0.15 \lesssim z \lesssim 0.5$ and confirm 19 of them as UCMGs, based on their nominal spectroscopically inferred $M_{*}$ and $R_{e}$. This corresponds to a success rate of $\sim 58\%$, nicely consistent with our previous findings. The addition of these 19 newly confirmed objects, allows us to fully assess the systematics on the system selection, and finally reduce the number density uncertainties. Moreover, putting together the results from our current and past observational campaigns and some literature data, we build the largest sample of UCMGs ever collected, comprising 92 spectroscopically confirmed objects at $0.1 \lesssim z \lesssim 0.5$. This number raises to 116, allowing for a $3\sigma$ tolerance on the $M_{*}$ and $R_{e}$ thresholds for the UCMG definition. For all these galaxies we have estimated the velocity dispersion values at the effective radii which have been used to derive a preliminary mass-velocity dispersion correlation.

The KiDS Strongly lensed QUAsar Detection project (KiDS-SQuaD) aims at finding as many previously undiscovered gravitational lensed quasars as possible in the Kilo Degree Survey. This is the second paper of this series where we present a new, automatic object classification method based on machine learning technique. The main goal of this paper is to build a catalogue of bright extragalactic objects (galaxies and quasars), from the KiDS Data Release 4, with a minimum stellar contamination, preserving the completeness as much as possible, to then apply morphological methods to select reliable gravitationally lensed (GL) quasar candidates. After testing some of the most used machine learning algorithms, decision trees based classifiers, we decided to use CatBoost, that was specifically trained with the aim of creating a sample of extragalactic sources as clean as possible from stars. We discuss the input data, define the training sample for the classifier, give quantitative estimates of its performances, and finally describe the validation results with Gaia DR2, AllWISE, and GAMA catalogues. We have built and make available to the scientific community the KiDS Bright EXtraGalactic Objects catalogue (KiDS-BEXGO), specifically created to find gravitational lenses. This is made of $\approx6$ millions of sources classified as quasars ($\approx 200\,000$) and galaxies ($\approx 5.7$M), up to $r<22^m$. From this catalog we selected 'Multiplets': close pairs of quasars or galaxies surrounded by at least one quasar, presenting the 12 most reliable gravitationally lensed quasar candidates, to demonstrate the potential of the catalogue, which will be further explored in a forthcoming paper. We compared our search to the previous one, presented in the first paper from this series, showing that employing a machine learning method decreases the stars-contaminators within the GL candidates., Comment: Submitted to Astronomy and Astrophysics, 19 pages, 12 figures, 4 tables

We have scanned 5000 deg$^{2}$ of Southern Sky to search for strongly lensed quasars with five methods, all source-oriented, but based on different assumptions and selection criteria. We analyse morphological searches based on Gaia multiplet detection and chromatic offsets, fibre-spectroscopic preselection, and X-ray and radio preselection. The performance and complementarity of the methods are evaluated on a common sample of known lenses in the Dark Energy Survey public DR1 footprint. We recovered in total 13 known lenses, of which 8 quadruplets. The method that found the largest number of known lenses is the one based on morphological and colour selection of objects from the WISE and Gaia-DR2 Surveys. We finally present a list of high-grade candidates from each method, to facilitate follow-up spectroscopic campaigns, including two previously unknown quadruplets: WG210014.9-445206.4 and WG021416.37-210535.3., Comment: MNRAS subm., 9 pages, 4 figures, 3 tables

Based on MUSE integral-field data we present evidence for a radial variation at the low-mass end of the stellar initial-mass function (IMF) in the central regions of the giant early-type galaxy NGC4486 (M87). We used state-of-the-art stellar population models and the observed strength of various IMF-sensitive absorption-line features to solve for the best low-mass tapered "bimodal" form of the IMF, while accounting also for radial variations in stellar metallicity, the overall $\alpha$-elements abundance, and the abundance of individual elements such as Ti, O, Na and Ca. Our analysis reveals a strong IMF gradient in M87, corresponding to an exceeding fraction of low-mass stars compared to the case of the Milky Way toward the center of M87 that drops to nearly Milky-way levels by 0.4 $R_e$. This IMF gradient is found to correlate well with both the radial profile for stellar metallicity and for $\alpha$-elements abundance but not with stellar velocity dispersion. Such IMF variations correspond to over a factor two increase in stellar mass-to-light M/L ratio compared to the case of a Milky-way like IMF, consistent with other investigations into IMF gradients in early-type galaxies, including recent dynamical constraints on M/L radial variations in M87 by Oldham & Auger. In addition to constraining the IMF in M87 we also looked into the abundance of Sodium, which turned up to be super-Solar over the entire radial range of our MUSE observations and to exhibit a considerable negative gradient. These findings suggest an additional role of metallicity in boosting the Na-yields in the central, metal-rich regions of M87 during its early and brief star-formation history. Our work adds the case of M87 to the few objects that as of today have radial constraints on their IMF or [Na/Fe] abundance, while also illustrating the accuracy that MUSE could bring to this kind of investigations., Comment: 17 pages, 13 figures, re-submitted for publication on MNRAS following the referee's comments

The combination of strong gravitational lensing and stellar kinematics provides a powerful and robust method to investigate the mass and dynamical structure of early-type galaxies. We demonstrate this approach by analysing two massive ellipticals from the XLENS Survey for which both high-resolution HST imaging and X-Shooter spectroscopic observations are available. We adopt a flexible axisymmetric two-component mass model for the lens galaxies, consisting of a generalised NFW dark halo and a realistic self-gravitating stellar mass distribution. For both systems, we put constraints on the dark halo inner structure and flattening, and we find that they are dominated by the luminous component within one effective radius. By comparing the tight inferences on the stellar mass from the combined lensing and dynamics analysis with the values obtained from stellar population studies, we conclude that both galaxies are characterised by a Salpeter-like stellar initial mass function., Comment: Proceedings of the IAU Symposium 309, Contributed Talk, Vienna, July 2014; 4 pages, 2 figures

We perform a direct comparison of two state-of-the art single stellar population (SSP) models that have been used to demonstrate the non-universality of the low-mass end of the Initial Mass Function (IMF) slope. The two public versions of the SSP models are restricted to either solar abundance patterns or solar metallicity, too restrictive if one aims to disentangle elemental enhancements, metallicity changes and IMF variations in massive early-type galaxies (ETGs) with star formation histories different from the solar neighborhood. We define response functions (to metallicity and \alpha-abundance) to extend the parameter space of each set of models. We compare these extended models with a sample of Sloan Digital Sky Survey (SDSS) ETGs spectra with varying velocity dispersions. We measure equivalent widths of optical IMF-sensitive stellar features to examine the effect of the underlying model assumptions and ingredients, such as stellar libraries or isochrones, on the inference of the IMF slope down to ~0.1 solar masses. We demonstrate that the steepening of the low-mass end of the Initial Mass Function (IMF) based on a non-degenerate set of spectroscopic optical indicators is robust against the choice of the stellar population model. Although the models agree in a relative sense (i.e. both imply more bottom-heavy IMFs for more massive systems), we find non-negligible differences on the absolute values of the IMF slope inferred at each velocity dispersion by using the two different models. In particular, we find large inconsistency in the quantitative predictions of IMF slope variations and abundance patterns when sodium lines are used. We investigate the possible reasons for these inconsistencies., Comment: 16 pages, 9 figures, 2 tables, accepted for publication on ApJ

We conduct a detailed investigation of the properties of the stellar initial mass function (IMF) in two massive early-type lens galaxies with velocity dispersions of sigma ~245 km/s and sigma ~325 km/s, for which both HST imaging and X-Shooter spectra are available. We compare the inferences obtained from two fully independent methods: (i) a combined gravitational lensing and stellar dynamics (L&D) analysis of the data sets employing self-consistent axisymmetric models, and (ii) a spectroscopic simple stellar population (SSP) analysis of optical line-strength indices, assuming single power-law IMFs. The results from the two approaches are found to be in agreement within the 1-sigma uncertainties. Both galaxies are consistent with having a Salpeter IMF (power-law slope of x = 2.35), which is strongly favoured over a Chabrier IMF (x = 1.8), with probabilities inferred from the joint analysis of 89% and 99%, respectively. Bottom-heavy IMFs significantly steeper than Salpeter (x >= 3.0) are ruled out with decisive evidence (Bayes factor B > 1000) for both galaxies, as they exceed the total mass derived from the L&D constraints. Our analysis allows, for the first time, the inference of the low-mass cut-off of the IMF (M_low). Combining the joint L&D and SSP analyses of both galaxies, we infer an IMF slope of x = 2.22 +/- 0.14, consistent with Salpeter IMF, and a low-mass limit M_low = 0.13 +/- 0.03 M_sun, just above the hydrogen burning limit., Comment: 6 pages, 2 figures. Accepted for publication in MNRAS

We investigate the optical spectral region of spectra of 1000 stars searching for IMF-sensitive features to constrain the low-mass end of the initial mass function (IMF) slope in elliptical galaxies. We use the MILES stellar library in the wavelength range [3500-7500] {\AA} to select indices that are sensitive to cool dwarf stars and that only weakly depend on age and metallicity. In this wavelength range, the response of a change in the effective temperature of the cool red giant population is similar to the response of a change in the number of dwarf stars in the galaxy. We investigate the degeneracy between IMF variation and Teff,RGB and show that it is possible to break it with the new IMF indicators defined here. In particular, we define a CaH1 index around {\lambda}6380 {\AA} that arises purely from cool dwarfs, does not strongly depend on age and is anti-correlated with [a/Fe]. This index allows the determination of the the IMF slope when combined with TiO lines and age- and metallicity-dependent features. The use of several indicators is crucial to break degeneracies between IMF, age, abundance pattern and Teff of the RGB population. We measure line-index strengths of the optical IMF indicators in the SSP models and compare these with index strengths of the same features in stacked SSDSS early-type galaxy spectra with varying velocity dispersions. Using 10 different indicators, we find a clear trend of a steepening IMF with increasing velocity dispersion described by the equation x=(2.3+-0.1)*log{\sigma}200+(2.13+-0.15), where x is the IMF slope and {\sigma}200 is the central stellar velocity dispersion measured in units of 200km/s. We compute the 'IMF mismatch' parameter as the ratio of stellar mass-to-light ratio predicted from the x-{\sigma}200 relation to that inferred from SSP models assuming a Salpeter IMF and find good agreement with independent published results., Comment: 19 pages, 12 figures, accepted for publication on MNRAS