arXiv subjects

Stefano Zibetti

Publications and source records attributed to Stefano Zibetti.

At least 37 records · Page 2Linked to original sources

arXiv2024

On the maximum age resolution achievable through stellar population synthesis models

As the reconstruction of the star-formation histories (SFH) of galaxies from spectroscopic data becomes increasingly popular, we explore the best age resolution achievable with stellar population synthesis (SPS) models, relying on different constraints: broad-band colours, absorption indices, a combination of the two, and the full spectrum. We perform idealized experiments on SPS models and show that the minimum resolvable relative duration of a star-formation episode (time difference between 10% and 90% of the stellar mass formed divided by the median age) is never better than 0.4, even when using spectra with signal-to-noise ratio (SNR) larger than 100 per AA. Typically, the best relative age resolution ranges between 0.4 and 0.7 over most of the age-metallicity plane, corresponding to minimum bin sizes for SFH sampling between 0.15 and 0.25 dex. This resolution makes the spectroscopic exploration of distant galaxies mandatory in order to reconstruct the early phases of galaxies' SFHs. We show that spectroscopy with SNR $\gtrsim$ 2/AA is essential for good age resolution. Remarkably, using the full spectrum does not prove significantly more effective than relying on absorption indices, especially at SNR $\lesssim$ 20/AA. We discuss the physical origins of the age resolution trends as a function of age and metallicity, and identify the presence of maxima in age resolution (i.e. minima in measurable relative time duration) at the characteristic ages that correspond to quick time variations in spectral absorption features. We connect these maxima to bumps commonly observed in reconstructed SFHs.

astro-ph.GA↗

arXiv2023

A census of star formation histories of massive galaxies at 0.6 < z < 1 from spectro-photometric modeling using Bagpipes and Prospector

We present individual star-formation histories of $\sim3000$ massive galaxies (log($\mathrm{M_*/M_{\odot}}$) > 10.5) from the Large Early Galaxy Astrophysics Census (LEGA-C) spectroscopic survey at a lookback time of $\sim$7 billion years and quantify the population trends leveraging 20hr-deep integrated spectra of these $\sim$ 1800 star-forming and $\sim$ 1200 quiescent galaxies at 0.6 < $z$ < 1.0. Essentially all galaxies at this epoch contain stars of age < 3 Gyr, in contrast with older massive galaxies today, facilitating better recovery of previous generations of star formation at cosmic noon and earlier. We conduct spectro-photometric analysis using parametric and non-parametric Bayesian SPS modeling tools - Bagpipes and Prospector to constrain the median star-formation histories of this mass-complete sample and characterize population trends. A consistent picture arises for the late-time stellar mass growth when quantified as $t_{50}$ and $t_{90}$, corresponding to the age of the universe when galaxies formed 50\% and 90\% of their total stellar mass, although the two sets of models disagree at the earliest formation times (e.g. $t_{10}$). Our results reveal trends in both stellar mass and stellar velocity dispersion as in the local universe - low-mass galaxies with shallower potential wells grow their stellar masses later in cosmic history compared to high-mass galaxies. Unlike local quiescent galaxies, the median duration of late-time star-formation ($τ_{SF,late}$ = $t_{90}$ - $t_{50}$) does not consistently depend on the stellar mass. This census sets a benchmark for future deep spectro-photometric studies of the more distant universe.

astro-ph.GA↗

arXiv2023

Less is less: photometry alone cannot predict the observed spectral indices of $z\sim1$ galaxies from the LEGA-C spectroscopic survey

We test whether we can predict optical spectra from deep-field photometry of distant galaxies. Our goal is to perform a comparison in data space, highlighting the differences between predicted and observed spectra. The Large Early Galaxy Astrophysics Census (LEGA-C) provides high-quality optical spectra of thousands of galaxies at redshift $0.6<z<1$. Broad-band photometry of the same galaxies, drawn from the recent COSMOS2020 catalog, is used to predict the optical spectra with the spectral energy distribution (SED) fitting code Prospector and the MILES stellar library. The observed and predicted spectra are compared in terms of two age and metallicity-sensitive absorption features (H$δ_\mathrm{A}$ and Fe4383). The global bimodality of star-forming and quiescent galaxies in photometric space is recovered with the model spectra. But the presence of a systematic offset in the Fe4383 line strength and the weak correlation between the observed and modeled line strength imply that accurate age or metallicity determinations cannot be inferred from photometry alone. For now we caution that photometry-based estimates of stellar population properties are determined mostly by the modeling approach and not the physical properties of galaxies, even when using the highest-quality photometric datasets and state-of-the-art fitting techniques. When exploring a new physical parameter space (i.e. redshift or galaxy mass) high-quality spectroscopy is always needed to inform the analysis of photometry.

astro-ph.GA↗

arXiv2023

Evolution in the orbital structure of quiescent galaxies from MAGPI, LEGA-C and SAMI surveys: direct evidence for merger-driven growth over the last 7 Gy

We present the first study of spatially integrated higher-order stellar kinematics over cosmic time. We use deep rest-frame optical spectroscopy of quiescent galaxies at redshifts z=0.05, 0.3 and 0.8 from the SAMI, MAGPI and LEGA-C surveys to measure the excess kurtosis $h_4$ of the stellar velocity distribution, the latter parametrised as a Gauss-Hermite series. Conservatively using a redshift-independent cut in stellar mass ($M_\star = 10^{11}\,{\rm M}_\odot$), and matching the stellar-mass distributions of our samples, we find 7 $σ$ evidence of $h_4$ increasing with cosmic time, from a median value of 0.019$\pm$0.002 at z=0.8 to 0.059$\pm$0.004 at z=0.06. Alternatively, we use a physically motivated sample selection, based on the mass distribution of the progenitors of local quiescent galaxies as inferred from numerical simulations; in this case, we find 10 $σ$ evidence. This evolution suggests that, over the last 7 Gyr, there has been a gradual decrease in the rotation-to-dispersion ratio and an increase in the radial anisotropy of the stellar velocity distribution, qualitatively consistent with accretion of gas-poor satellites. These findings demonstrate that massive galaxies continue to accrete mass and increase their dispersion support after becoming quiescent.

astro-ph.GA↗

arXiv2023

Different higher-order kinematics between star-forming and quiescent galaxies based on the SAMI, MAGPI and LEGA-C surveys

We present the first statistical study of spatially integrated non-Gaussian stellar kinematics spanning 7 Gyr in cosmic time. We use deep, rest-frame optical spectroscopy of massive galaxies (stellar mass $M_\star > 10^{10.5} {\rm M}_\odot$) at redshifts z = 0.05, 0.3 and 0.8 from the SAMI, MAGPI and LEGA-C surveys, to measure the excess kurtosis $h_4$ of the stellar velocity distribution, the latter parametrised as a Gauss-Hermite series. We find that at all redshifts where we have large enough samples, $h_4$ anti-correlates with the ratio between rotation and dispersion, highlighting the physical connection between these two kinematic observables. In addition, and independently from the anti-correlation with rotation-to-dispersion ratio, we also find a correlation between $h_4$ and $M_\star$, potentially connected to the assembly history of galaxies. In contrast, after controlling for mass, we find no evidence of independent correlation between $h_4$ and aperture velocity dispersion or galaxy size. These results hold for both star-forming and quiescent galaxies. For quiescent galaxies, $h_4$ also correlates with projected shape, even after controlling for the rotation-to-dispersion ratio. At any given redshift, star-forming galaxies have lower $h_4$ compared to quiescent galaxies, highlighting the link between kinematic structure and star-forming activity.

astro-ph.GA↗

arXiv2023

Nonparametric galaxy morphology from stellar and nebular emission with the CALIFA sample

We present a nonparametric morphology analysis of the stellar continuum and nebular emission lines for a sample of local galaxies. We explore the dependence of the various morphological parameters on wavelength and morphological type. Our goal is to quantify the difference in morphology between the stellar and nebular components. We derive the nonparametric morphological indicators of 364 galaxies from the CALIFA Survey. To calculate those indicators, we apply the StatMorph package on the high-quality integral field spectroscopic data cubes, as well as to the most prominent nebular emission-line maps, namely [OIII]$λ$5007, H$α$, and [NII]$λ$6583. We show that the physical size of galaxies, M$_{20}$ index, and concentration have a strong gradient from blue to red optical wavelengths. We find that the light distribution of the nebular emission is less concentrated than the stellar continuum. A comparison between the nonparametric indicators and the galaxy physical properties revealed a very strong correlation of the concentration with the specific star-formation rate and morphological type. Furthermore, we explore how the galaxy inclination affects our results. We find that edge-on galaxies show a more rapid change in physical size and concentration with increasing wavelength due to the increase in optical free path. We conclude that the apparent morphology of galaxies originates from the pure stellar distribution, but the morphology of the ISM presents differences with respect to the morphology of the stellar component. Our analysis also highlights the importance of dust attenuation and galaxy inclination in the measurement of nonparametric morphological indicators, especially in the the wavelength range 4000-5000 Å.

astro-ph.GA↗

arXiv2022

Maximally informed Bayesian modelling of disc galaxies

Dissecting the underlying structure of galaxies is of main importance in the framework of galaxy formation and evolution theories. While a classical bulge+disc decomposition of disc galaxies is usually taken as granted, this is only rarely solidly founded upon the full exploitation of the richness of data arising from spectroscopic studies with integral field units. In this work we describe a fully Bayesian estimation method of the global structure of disc galaxies which makes use of the wealth of photometric, kinematic, and mass-to-light ratio data, and that can be seen as a first step towards a machine-learning approach, certainly needed when dealing with larger samples of galaxies. Ours is a novel, hybrid line of action in tackling the problem of galactic parameter estimation, neither purely photometric nor orbit-based. Being rooted on a nested sampler, our code, which is available publicly as an online repository, allows for a statistical assessment of the need for multiple components in the dissecting process. As a first case-study the GPU-optimized code is applied to the S0 galaxy NGC-7683, finding that in this galaxy a pseudo-bulge, possibly the remnant of a bar-like structure, does exist in the center of the system. These results are then tested against the publicly available, orbit-based code DYNAMITE, finding substantial agreement.

astro-ph.GA↗

arXiv2022

Stellar mass as the "glocal" driver of galaxies' stellar population properties

The properties of the stellar populations in a galaxy are known to correlate with the amount and the distribution of stellar mass. We take advantage of the maps of light-weighted mean stellar age Agewr and metallicity Z*wr for a sample of 362 galaxies from the integral-field spectroscopic survey CALIFA (summing up to >600,000 individual regions of approximately 1 kpc linear size), produced in our previous works, to investigate how these local properties react to the local stellar-mass surface density mu* and to the global total stellar mass M* and mean stellar-mass surface density e. We establish the existence of i) a dual mu*-Agewr relation, resulting in a young sequence and an old ridge, and ii) a mu*-Z*wr relation, overall independent of the age of the regions. The global mass parameters (M* and, possibly secondarily, e) determine the distribution of mu* in a galaxy and set the maximum attainable mu*, which increases with M*. M* affects the shape and normalization of the local relations up to a threshold mass of $\sim 10^{10.3}$ MSun, above which they remain unchanged. We conclude that stellar mass is a "glocal" (i.e. simultaneously global and local) driver of the stellar population properties. We consider how the local and global mass-age and mass-metallicity relations are connected, and in particular discuss how it is possible, from a single local relation, to produce two different global mass-metallicity relations for quiescent and star-forming galaxies respectively, as reported in the literature. Structural differences in these two classes of galaxies are key to explain the duality in global scaling relations and appear as essential in modelling the baryonic cycle of galaxies.

astro-ph.GA↗

arXiv2021

Introducing ADAPTSMOOTH, a new code for the adaptive smoothing of astronomical images

We introduce and publicly release a new code, ADAPTSMOOTH, which serves to smooth astronomical images in an adaptive fashion, in order to enhance the signal-to-noise ratio (S/N). The adaptive smoothing scheme allows to take full advantage of the spatially resolved photometric information contained in an image in that at any location the minimal smoothing is applied to reach the requested S/N. Support is given to match more images on the same smoothing length, such that proper estimates of local colours can be done, with a big potential impact on multi-wavelength studies of extended sources (galaxies, nebulae). Different modes to estimate local S/N are provided. In addition to classical arithmetic-mean averaging mode, the code can operate in median averaging mode, resulting in a significant enhancement of the final image quality and very accurate flux conservation. To this goal also other code options are implemented and discussed in this paper. Finally, we analyze in great detail the effect of the adaptive smoothing on galaxy photometry, in particular in terms of surface brightness (SB) profiles and aperture photometry: deviations in SB with respect to the original image can be limited to <0.01 mag, with flux difference in apertures of less than 0.001 mag.

astro-ph.IM↗

arXiv2021

Towards precise galaxy evolution: a comparison between spectral indices of $z\sim1$ galaxies in the IllustrisTNG simulation and the LEGA-C survey

We present the first comparison of observed stellar continuum spectra of high-redshift galaxies and mock galaxy spectra generated from hydrodynamical simulations. The mock spectra are produced from the IllustrisTNG TNG100 simulation combined with stellar population models and take into account dust attenuation and realistic observational effects (aperture effects and noise). We compare the simulated $D_n4000$ and EW(H$δ$) of galaxies with $10.5 \leq \log(M_\ast/M_\odot) \leq 11.5$ at $0.6 \leq z \leq 1.0$ to the observed distributions from the LEGA-C survey. TNG100 globally reproduces the observed distributions of spectral indices, implying that the age distribution of galaxies in TNG100 is generally realistic. Yet there are small but significant differences. For old galaxies, TNG100 shows small $D_n4000$ when compared to LEGA-C, while LEGA-C galaxies have larger EW(H$δ$) at fixed $D_n4000$. There are several possible explanations: 1) LEGA-C galaxies have overall older ages combined with small contributions (a few percent in mass) from younger ($<1$~Gyr) stars, while TNG100 galaxies may not have such young sub-populations; 2) the spectral mismatch could be due to systematic uncertainties in the stellar population models used to convert stellar ages and metallicities to observables. In conclusion, the latest cosmological galaxy formation simulations broadly reproduce the global age distribution of galaxies at $z\sim1$ and, at the same time, the high quality of the latest observed and simulated datasets help constrain stellar population synthesis models as well as the physical models underlying the simulations.

astro-ph.GA↗

arXiv2021

The Large Early Galaxy Astrophysics Census (LEGA-C) Data Release 3: 3000 High-Quality Spectra of $K_s$-selected galaxies at $z>0.6$

We present the third and final data release of the Large Early Galaxy Astrophysics Census (LEGA-C), an ESO/VLT public spectroscopic survey targeting $0.6 < z < 1.0$, Ks-selected galaxies. The data release contains 3528 spectra with measured stellar velocity dispersions and stellar population properties, a 25-fold increase in sample size compared to previous work. This $K_s$-selected sample probes the galaxy population down to $\sim0.3 L^*$, for all colors and morphological types. Along with the spectra we publish a value-added catalog with stellar and ionized gas velocity dispersions, stellar absorption line indices, emission line fluxes and equivalent widths, complemented with structural parameters measured from HST/ACS imaging. With its combination of high precision and large sample size, LEGA-C provides a new benchmark for galaxy evolution studies.

astro-ph.GA↗

arXiv2021

An exquisitely deep view of quenching galaxies through the gravitational lens: Stellar population, morphology, and ionized gas

This work presents an in-depth analysis of four gravitationally lensed red galaxies at z = 1.6-3.2. The sources are magnified by factors of 2.7-30 by foreground clusters, enabling spectral and morphological measurements that are otherwise challenging. Our sample extends below the characteristic mass of the stellar mass function and is thus more representative of the quiescent galaxy population at z > 1 than previous spectroscopic studies. We analyze deep VLT/X-SHOOTER spectra and multi-band Hubble Space Telescope photometry that cover the rest-frame UV-to-optical regime. The entire sample resembles stellar disks as inferred from lensing-reconstructed images. Through stellar population synthesis analysis we infer that the targets are young (median age = 0.1-1.2 Gyr) and formed 80% of their stellar masses within 0.07-0.47 Gyr. Mg II $λλ2796,2803$ absorption is detected across the sample. Blue-shifted absorption and/or redshifted emission of Mg II is found in the two youngest sources, indicative of a galactic-scale outflow of warm ($T\sim10^{4}$ K) gas. The [O III] $\lambda5007$ luminosity is higher for the two young sources (median age less than 0.4 Gyr) than the two older ones, perhaps suggesting a decline in nuclear activity as quenching proceeds. Despite high-velocity ($v\approx1500$ km s$^{-1}$) galactic-scale outflows seen in the most recently quenched galaxies, warm gas is still present to some extent long after quenching. Altogether our results indicate that star formation quenching at high redshift must have been a rapid process (< 1 Gyr) that does not synchronize with bulge formation or complete gas removal. Substantial bulge growth is required if they are to evolve into the metal-rich cores of present-day slow-rotators.

astro-ph.GA↗

arXiv2021

Galaxy evolution across environments as probed by the ages, stellar metallicities and [alpha/Fe] of central and satellite galaxies

We explore how the star formation and metal enrichment histories of present-day galaxies have been affected by environment combining stellar population parameter estimates and group environment characterization for SDSS DR7. We compare stellar ages, stellar metallicities and element abundance ratios [alpha/Fe] of satellite and central galaxies, as a function of their stellar and host group halo mass, controlling for the current star formation rate and for the infall epoch. We confirm that below log(Mstar/Msun)=10.5 satellites are older and metal-richer than equally-massive central galaxies. On the contrary, we do not detect any difference in their [alpha/Fe]: this depends primarily on stellar mass and not on group hierarchy nor host halo mass. We also find that the differences in the median age and metallicity of satellites and centrals at stellar mass below 10^{10.5}Msun are largely due to the higher fraction of passive galaxies among satellites and as a function of halo mass. We argue that the observed trends at low masses reveal the action of satellite-specific environmental effects in a `delayed-then-rapid' fashion. When accounting for the varying quiescent fraction, small residual excess in age, metallicity and [alpha/Fe] emerge for satellites dominated by old stellar populations and residing in halos more massive than 10^{14}Msun, compared to equally-massive central galaxies. This excess in age, metallicity and [alpha/Fe] pertain to ancient infallers, i.e. satellites that have accreted onto the current halo more than 5 Gyr ago. This result points to the action of environment in the early phases of star formation in galaxies located close to cosmic density peaks.

astro-ph.GA↗

arXiv2020

The Fundamental Plane of Massive Quiescent Galaxies at z~2

We examine the Fundamental Plane (FP) and mass-to-light ratio ($M/L$) scaling relations using the largest sample of massive quiescent galaxies at $1.5 11.26$, subset of 8 quiescent galaxies at $z>2$, from Stockmann et al. (2020), we show that they cannot passively evolve to the local Coma cluster relation alone and must undergo significant structural evolution to mimic the sizes of local massive galaxies. The evolution of the FP and $M/L$ scaling relations, from $z=2$ to present-day, for this subset are consistent with passive aging of the stellar population and minor merger structural evolution into the most massive galaxies in the Coma cluster and other massive elliptical galaxies from the MASSIVE Survey. Modeling the luminosity evolution from minor merger added stellar populations favors a history of merging with "dry" quiescent galaxies.

astro-ph.GA↗

arXiv2020

Phase A Science Case for MAVIS -- The Multi-conjugate Adaptive-optics Visible Imager-Spectrograph for the VLT Adaptive Optics Facility

We present the Phase A Science Case for the Multi-conjugate Adaptive-optics Visible Imager-Spectrograph (MAVIS), planned for the Adaptive Optics Facility (AOF) of the Very Large Telescope (VLT). MAVIS is a general-purpose instrument for exploiting the highest possible angular resolution of any single optical telescope available in the next decade, either on Earth or in space, and with sensitivity comparable to (or better than) larger aperture facilities. MAVIS uses two deformable mirrors in addition to the deformable secondary mirror of the AOF, providing a mean V-band Strehl ratio of >10% (goal >15%) across a relatively large (30 arc second) science field. This equates to a resolution of <20mas at 550nm - comparable to the K-band diffraction limit of the next generation of extremely large telescopes, making MAVIS a genuine optical counterpart to future IR-optimised facilities like JWST and the ELT. Moreover, MAVIS will have unprecedented sky coverage for a high-order AO system, accessing at least 50% of the sky at the Galactic Pole, making MAVIS a truly general purpose facility instrument. As such, MAVIS will have both a Nyquist-sampled imager (30x30 arcsec field), and a powerful integral field spectrograph with multiple spatial and spectral modes spanning 370-1000nm. This science case presents a distilled set of thematically linked science cases drawn from the MAVIS White Papers (www.mavis-ao.org/whitepapers), selected to illustrate the driving requirements of the instrument resulting from the recent MAVIS Phase A study.

astro-ph.IM↗

arXiv2020

Inverse stellar population age gradients of post-starburst galaxies at z=0.8 with LEGA-C

We use deep, spatially resolved spectroscopy from the LEGA-C Survey to study radial variations in the stellar population of 17 spectroscopically-selected post-starburst (PSB) galaxies. We use spectral fitting to measure two Lick indices, $Hδ_A$ and $Fe4383$, and find that, on average, PSB galaxies have radially decreasing $Hδ_A$ and increasing $Fe4383$ profiles. In contrast, a control sample of quiescent, non-PSB galaxies in the same mass range shows outwardly increasing $Hδ_A$ and decreasing $Fe4383$. The observed gradients are weak ($\approx-0.2$ Å/$R_e$), mainly due to seeing convolution. A two-SSP model suggests intrinsic gradients are as strong as observed in local PSB galaxies ($\approx -0.8$ Å$/R_e$). We interpret these results in terms of inside-out growth (for the bulk of the quiescent population) vs star formation occurring last in the centre (for PSB galaxies). At $z\approx0.8$, central starbursts are often the result of gas-rich mergers, as evidenced by the high fraction of PSB galaxies with disturbed morphologies and tidal features (40%). Our results provide additional evidence for multiple paths to quiescence: a standard path, associated with inside-out disc formation and with gradually decreasing star-formation activity, without fundamental structural transformation, and a fast path, associated with centrally-concentrated starbursts, leaving an inverse age gradient and smaller half-light radius.

astro-ph.GA↗

arXiv2019

X-shooter Spectroscopy and HST Imaging of 15 Ultra Massive Quiescent Galaxies at $z\gtrsim2$

We present a detailed analysis of a large sample of spectroscopically confirmed ultra-massive quiescent galaxies (${\rm{log}}(M_{\ast}/M_{\odot})\sim11.5$) at $z\gtrsim2$. This sample comprises 15 galaxies selected in the COSMOS and UDS fields by their bright K-band magnitudes and followed up with VLT/X-shooter spectroscopy and HST/WFC3 $H_{F160W}$ imaging. These observations allow us to unambiguously confirm their redshifts ascertain their quiescent nature and stellar ages, and to reliably assess their internal kinematics and effective radii. We find that these galaxies are compact, consistent with the high mass end of the mass-size relation for quiescent galaxies at $z=2$. Moreover, the distribution of the measured stellar velocity dispersions of the sample is consistent with the most massive local early-type galaxies from the MASSIVE Survey showing that evolution in these galaxies, is dominated by changes in size. The HST images reveal, as surprisingly high, that $40\ \%$ of the sample have tidal features suggestive of mergers and companions in close proximity, including three galaxies experiencing ongoing major mergers. The absence of velocity dispersion evolution from $z=2$ to $0$, coupled with a doubling of the stellar mass, with a factor of four size increase and the observed disturbed stellar morphologies support dry minor mergers as the primary drivers of the evolution of the massive quiescent galaxies over the last 10 billion years.

astro-ph.GA↗

arXiv2019

Insights into formation scenarios of massive Early-Type galaxies from spatially resolved stellar population analysis in CALIFA

We perform spatially resolved stellar population analysis for a sample of 69 early-type galaxies (ETGs) from the CALIFA integral field spectroscopic survey, including 48 ellipticals and 21 S0's. We generate and quantitatively characterize profiles of light-weighted mean stellar age and metallicity within $\lesssim 2R_e$, as a function of radius and stellar-mass surface density $μ_*$. We study in detail the dependence of profiles on galaxies' global properties, including velocity dispersion $σ_e$, stellar mass, morphology. ETGs are universally characterized by strong, negative metallicity gradients ($\sim -0.3\,\text{dex}$ per $R_e$) within $1\,R_e$, which flatten out moving towards larger radii. A quasi-universal local $μ_*$-metallicity relation emerges, which displays a residual systematic dependence on $σ_e$, whereby higher $σ_e$ implies higher metallicity at fixed $μ_*$. Age profiles are typically U-shaped, with minimum around $0.4\,R_e$, asymptotic increase to maximum ages beyond $\sim 1.5\,R_e$, and an increase towards the centre. The depth of the minimum and the central increase anti-correlate with $σ_e$. A possible qualitative interpretation of these observations is a two-phase scenario. In the first phase, dissipative collapse occurs in the inner $1\,R_e$, establishing a negative metallicity gradient. The competition between the outside-in quenching due to feedback-driven winds and some form of inside-out quenching, possibly caused by central AGN feedback or dynamical heating, determines the U-shaped age profiles. In the second phase, the accretion of ex-situ stars from quenched and low-metallicity satellites shapes the flatter stellar population profiles in the outer regions.

astro-ph.GA↗