arXiv subjects

M. Arnaboldi

Publications and source records attributed to M. Arnaboldi.

At least 19 recordsLinked to original sources

arXiv2026

The Merger-Driven Origin of the Vast Extended Stellar Disc Around the Andromeda Galaxy

The closest giant spiral, the Andromeda galaxy (M31), shows compelling evidence for a recent, gas-rich major merger event. Pronounced substructures in its inner halo and a kinematically hot stellar disc, whose star formation history shows a widespread star formation episode 2.5 Gyr ago, are telltale evidence that may be directly linked to a major (mass ratio 1 to 4) merger event that took place 2-4 Gyr ago. Spectroscopy of resolved giant stars in the remote outskirts of M31's disc revealed a vast extended structure that rotates with a circular velocity close to the HI gas. In addition, the spatial distribution and significant prograde rotation of two distinct, compact groups of globular clusters (GCs) in the disc outskirts are unusual for typical inner halo GCs. We employ an available N-body hydrodynamical simulation of a major merger that reproduces the morphology of the inner halo substructures, the age-velocity dispersion relation, and the star formation history in the disc. We compare model particles with resolved tracers in the M31 disc. To examine the evolution of the progenitor M31 disc -- that appears to get stretched, distorted, and warped due to the gravitational perturbation inflicted by the major merger -- we investigate the properties of the pre- versus post-merger discs of the simulated analog. The merger transforms the disc of the progenitor galaxy, which becomes kinematically hot and asymmetric. In addition, the post-merger disc gets stretched by almost a factor of 2, and its extent spans distances greater than 40 kpc. The stellar warp in populations older than 2 Gyr is characterized by a monotonic decrease of inclination with radius, with the outer stellar distribution appearing less edge-on at larger galactic radii. These results provide a comprehensive picture of the evolution of the giant disc of M31, the closest merger-inflicted massive galaxy.

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arXiv2025

Tracing nitrogen enrichment across cosmic time with JWST

We present a comprehensive analysis of the nitrogen-to-oxygen (N/O) abundance ratio in a sample of ~ 660 star-forming galaxies at redshift z ~ 1-6, with a median redshift of z = 3.0, using deep JWST/NIRSpec spectroscopy. Leveraging detections of faint auroral emission lines in 92 galaxies at z>1 from both the MARTA survey and a large compilation of high-redshift literature objects, we derive direct electron temperature-based abundances for nitrogen and oxygen using rest-frame optical lines. We establish the first high-redshift calibrations of strong-line N/O diagnostics based on 'direct' abundance measurements, finding a mild evolution for the N2O2 = log([NII]6585/[OII]3727,3729) diagnostic and no clear evolution for the N2S2 = log([NII]6585/[SII]6717,6731) diagnostic compared to local realisations. We then investigate the N/O-O/H relation across cosmic time using both 'direct' abundances and strong-lines based measurements (additional 535 galaxies). We find evidence for mild but systematic nitrogen enhancement at high redshift: galaxies at z>1 exhibit N/O ratios elevated by ~0.18 dex (median offset) at fixed O/H compared to local trends, with a more pronounced enhancement at low metallicity (i.e. 12 + log(O/H) < 8.1), where the offset reaches up to ~0.4-0.5 dex. We consider several scenarios to explain the observed trends, including bursty star formation, differential metal loading, and inflows of pristine gas. Our results provide the most extensive confirmation of elevated N/O ratios at high-redshift to date based on rest-optical diagnostics and within a self-consistent frame.

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arXiv2025

MARTA: Temperature-temperature relationships and strong-line metallicity calibrations from multiple auroral-line detections at cosmic noon

We present the first results from MARTA (Measuring Abundances at high Redshift with the T$_e$ Approach), a programme leveraging ultra-deep, medium-resolution JWST/NIRSpec spectroscopy to probe the interstellar medium (ISM) of star-forming galaxies at $z \sim 2 - 3$. We report detections of one or more auroral lines, including [O III]$\lambda4363$, [O II]$λ\lambda7320,7330$, [S II] $\lambda4068$, and [S III] $\lambda6312$ for 16 galaxies in the sample, providing measurements of multiple ionic temperatures. We tested the validity of the T[O II]-T[O III] relation at high redshift considering a total sample of 21 objects including literature data, and obtained a shallower slope than in the low-$z$ literature. However, such a slope is consistent with low-redshift data when ultra-low metallicity objects are considered. We assessed the correlation of the T[O II]-T[O III] relationship and its scatter on different physical parameters, finding a mild correlation with the ionisation parameter and radiation field hardness, while no significant correlation with gas density. The location of high-redshift data is also consistent with the low-$z$ literature in the T[O II]-T[S II], and T[S III]-T[O III] relations, although this conclusion is limited with low-number statistics. Finally, we leveraged our sample together with a comprehensive compilation of galaxies with [O III]$\lambda4363$ detections from the literature to recalibrate classical strong-line diagnostics at high redshift. MARTA represents a key addition in this space because it provides direct metallicities at moderately high oxygen abundances (12+log(O/H) $\sim8.0-8.4$).

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arXiv2025

The survey of planetary nebulae in Andromeda (M31) VII. Predictions of a major merger simulation model compared with chemodynamical data of the disc and inner halo substructures

The nearest giant spiral, M31, exhibits a kinematically hot stellar disc, a global star formation episode ~2-4 Gyr ago, and conspicuous substructures in its stellar halo that are suggestive of a recent accretion event. Recent chemodynamical measurements in the M31 disc and inner halo can be used as additional constraints for N-body hydrodynamical simulations that successfully reproduce the disc age-velocity dispersion relation and star formation history as well as the morphology of the inner halo substructures. We combined a simulation of a major merger (mass ratio 1:4) with a well-motivated chemical model to predict abundance distributions and gradients in the merger remnant at z=0. We computed the projected phase space and the [M/H] distributions for the substructures in the M31 inner halo, namely, the Giant Stellar Stream (GSS) and the North-East (NE) and Western (W) shelves, and compared them with recent measurements for the M31 stars in the inner halo. This major merger model predicts (i) multiple distinct components within each of the substructures; (ii) a high mean metallicity and large spread in the GSS and NE and W Shelves which explain various photometric and spectroscopic metallicity measurements; (iii) simulated phase space diagrams that qualitatively reproduce various features identified in the projected phase space of the substructures in published data from the DESI; (iv) a large distance spread in the GSS, as suggested by previous tip of the RGB measurements; and (v) phase space ridges caused by several wraps of the secondary as well as up-scattered main M31 disc stars that also have plausible counterparts in the observed phase spaces. These results provide further strong and independent arguments for a major satellite merger in M31 ~3 Gyr ago and a coherent explanation for many of the observational results that make M31 appear so different from the Milky Way.

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arXiv2025

INSPIRE: INvestigating Stellar Populations In RElics. IX. KiDS J0842+0059: the first fully confirmed relic beyond the local Universe

Relics are massive, compact and quiescent galaxies that assembled the majority of their stars in the early Universe and lived untouched until today, completely missing any subsequent size-growth caused by mergers and interactions. They provide the unique opportunity to put constraints on the first phase of mass assembly in the Universe with the ease of being nearby. While only a few relics have been found in the local Universe, the {\tt INSPIRE} project has confirmed 38 relics at higher redshifts ($z \sim 0.2-0.4$), fully characterising their integrated kinematics and stellar populations. However, given the very small sizes of these objects and the limitations imposed by the atmosphere, structural parameters inferred from ground-based optical imaging are possibly affected by systematic effects that are difficult to quantify. In this paper, we present the first high-resolution image obtained with Adaptive Optics Ks-band observations on SOUL-LUCI@LBT of one of the most extreme {\tt INSPIRE} relics, KiDS~J0842+0059 at $z \sim 0.3$. We confirm the disky morphology of this galaxy (axis ratio of $0.24$) and its compact nature (circularized effective radius of $\sim 1$ kpc) by modelling its 2D surface brightness profile with a PSF-convolved S\'ersic model. We demonstrate that the surface mass density profile of KiDS~J0842+0059 closely resembles that of the most extreme local relic, NGC~1277, as well as of high-redshift red nuggets. We unambiguously conclude that this object is a remnant of a high-redshift compact and massive galaxy, which assembled all of its mass at $z>2$, and completely missed the merger phase of the galaxy evolution.

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arXiv2025

Looking into the faintEst WIth MUSE (LEWIS): Exploring the nature of ultra-diffuse galaxies in the Hydra I cluster III. Untangling UDG 32 from the stripped filaments of NGC 3314A with multi-wavelength data

UDG 32 is an ultra-diffuse galaxy (UDG) candidate in the Hydra I cluster that was discovered in the extended network of stellar filaments of the jellyfish galaxy NGC 3314A. This galaxy is affected by ram pressure stripping and it is hypothesised that UDG 32 may have formed from its stripped material. In this paper, we address whether UDG 32 can be associated with the stripped material of NGC 3314A and constrain its formation scenario in relation to its environment. We use new integral-field spectroscopic data from the MUSE large programme `LEWIS' in conjunction with deep multi-band photometry to constrain the kinematics and stellar populations of UDG 32. The new MUSE data allow us to reveal that the stripped material from NGC 3314A, traced by emission lines such as H$α$, extends much further from its parent galaxy than previously known, completely overlapping with UDG 32 in projection, and with ram pressure induced star formation. We determine the line-of-sight velocity of UDG 32 ($v_{\rm LOS} = 3080\pm120$ km/s) and confirm that UDG 32 is part of the same kinematic structure as NGC 3314A, the Hydra I cluster south-east subgroup. By fitting the UV and optical spectral energy distribution, we constrain the stellar population properties of UDG 32. We determine its mass-weighted age to be $7.7^{+2.9}_{-2.8}$ Gyr and its metallicity to be [M/H] = $0.07^{+0.19}_{-0.32}$ dex. We confirm the presence of two globular clusters (GCs) in the MUSE field of view, bound to the Hydra I cluster rather than to UDG 32, thus part of the Hydra I intracluster GC population. The metal-rich and intermediate-age nature of UDG 32 points towards its formation from pre-enriched material in the south-east group of the Hydra I cluster that was liberated from a more massive galaxy via tidal or ram-pressure stripping, but we cannot establish a direct link to the ram-pressure stripped material from NGC 3314A.

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arXiv2024

The VISTA Variables in the Vía Láctea eXtended (VVVX) ESO public survey: Completion of the observations and legacy

The ESO public survey VISTA Variables in the Vía Láctea (VVV) surveyed the inner Galactic bulge and the adjacent southern Galactic disk from $2009-2015$. Upon its conclusion, the complementary VVV eXtended (VVVX) survey has expanded both the temporal as well as spatial coverage of the original VVV area, widening it from $562$ to $1700$ sq. deg., as well as providing additional epochs in $JHK_{\rm s}$ filters from $2016-2023$. With the completion of VVVX observations during the first semester of 2023, we present here the observing strategy, a description of data quality and access, and the legacy of VVVX. VVVX took $\sim 2000$ hours, covering about 4% of the sky in the bulge and southern disk. VVVX covered most of the gaps left between the VVV and the VISTA Hemisphere Survey (VHS) areas and extended the VVV time baseline in the obscured regions affected by high extinction and hence hidden from optical observations. VVVX provides a deep $JHK_{\rm s}$ catalogue of $\gtrsim 1.5\times10^9$ point sources, as well as a $K_{\rm s}$ band catalogue of $\sim 10^7$ variable sources. Within the existing VVV area, we produced a $5D$ map of the surveyed region by combining positions, distances, and proper motions of well-understood distance indicators such as red clump stars, RR Lyrae, and Cepheid variables. In March 2023 we successfully finished the VVVX survey observations that started in 2016, an accomplishment for ESO Paranal Observatory upon 4200 hours of observations for VVV+VVVX. The VVV+VVVX catalogues complement those from the Gaia mission at low Galactic latitudes and provide spectroscopic targets for the forthcoming ESO high-multiplex spectrographs MOONS and 4MOST.

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arXiv2023

Planetary nebulae populations in the haloes of nearby massive early-type galaxies

Planetary nebulae (PNe) are excellent tracers of the metal-poor haloes of nearby early-type galaxies. They are commonly used to trace spatial distribution and kinematics of the halo and intracluster light at distances of up to 100 Mpcs. The results on the early-type galaxy M105 in the Leo I group represent a benchmark for the quantitative analysis of halo and intragroup light. Since the Leo I group lies at just a 10 Mpc distance, it is at the ideal location to compare results from resolved stellar populations with the homogeneous constraints over a much larger field of view from the PN populations. In M105, we have -- for the first time -- established a direct link between the presence of a metal-poor halo as traced by resolved red-giant branch stars and a PN population with a high specific frequency ($α$-parameter). This confirms our inferences that the high $α$-parameter PN population in the outer halo of M49 in the Virgo Cluster traces the metal-poor halo and intra-group light.

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arXiv2023

INSPIRE: INvestigating Stellar Population In RElics V. A catalogue of ultra-compact massive galaxies outside the local Universe and their degree of relicness

This paper presents the third data release of the INvestigating Stellar Population In RElics (INSPIRE) project, comprising 52 ultra-compact massive galaxies (UCMGs) observed with the X-Shooter spectrograph. We measure integrated stellar velocity dispersion, [Mg/Fe] abundances, ages, and metallicities for all the INSPIRE objects. We thus infer star formation histories and confirm the existence of a degree of relicness (DoR), defined in terms of the fraction of stellar mass formed by $z=2$, the time at which a galaxy has assembled 75\% of its mass, and the final assembly time. Objects with a high DoR assembled their stellar mass at early epochs, while low-DoR objects show a non-negligible fraction of later-formed populations and hence a spread in ages and metallicities. A higher DoR correlates with larger [Mg/Fe], super-solar metallicity, and larger velocity dispersion values. The 52 UMCGs span a large range of DoR from 0.83 to 0.06, with 38 of them having formed more than 75\% of their mass by $z=2$. Of these, nine are extreme relics (DoR$>0.7$), since they formed the totality ($>99\%$) of their stellar mass by redshift $z=2$. The remaining 14 UCMGs cannot be considered relics, as they are characterised by more extended star formation histories. With INSPIRE, we built the first sizeable sample of relics outside the local Universe, up to $z\sim0.4$, increasing the number of confirmed relics by a factor of $>10$, and opening up an important window to explain the mass assembly of massive galaxies in the high-z Universe.

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arXiv2023

The extended Planetary Nebula Spectrograph (ePN.S) early-type galaxy survey: The specific angular momentum of ETGs

Mass and angular momentum are key parameters of galaxies. Their co-evolution establishes an empirical relation between the specific stellar angular momentum j* and the stellar mass M* that depends on morphology. In this work, we measure j* in a sample of 32 early type galaxies (ETGs) from the ePN.S survey, using full 2D kinematic information out to a mean 6Re. We present lambda and j* profiles. We derive the distribution of these galaxies on the total j*-M* plane and determine the ratio between the stellar j* and the specific angular momentum of the host dark matter halo. The radially extended, 2D kinematic data show that the stellar halos of ETGs do not contain large stellar mass fractions of high j*. The j*-profiles of fast-rotator ETGs are largely converged within the range of the data. For slow rotators, j* is still rising and is estimated to increase beyond 6Re by up to 40%. More than 60% of their halo angular momentum is in misaligned rotation. We find that the ePN.S ETG sample displays the well-known correlation between j*, M*, and morphology: elliptical galaxies have systematically lower j* than similar mass S0 galaxies. However, fast and slow rotators lie on the same relation within errors with the slow rotators falling at the high M* end. A power-law fit to the j*-M* relation gives a slope of 0.55+-0.17 for the S0s and 0.76+-0.23 for the ellipticals, with normalisation about 4 and 9 times lower than spirals, respectively. The estimated retained fraction of angular momentum at 10^10-10^10.5 Msun is 25% for S0s and >10% for ellipticals, and decreases by 1.5 orders of magnitude at M*~10^12 Msun. Our results show that ETGs have substantially lower j* than spiral galaxies with similar M*. Their j* must be lost during their evolution, and/or retained in the hot gas component and the satellite galaxies that have not yet merged with the central galaxy. [abridged]

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arXiv2023

INSPIRE: INvestigating Stellar Population In RElics IV. The Initial Mass Function slope in relics

In the last decade, growing evidence has emerged supporting a non-universal stellar Initial Mass Function (IMF) in massive galaxies, with a larger number of dwarf stars with respect to the Milky-Way (bottom-heavy IMF). However, a consensus about the mechanisms that cause IMF variations is yet to be reached. Recently, it has been suggested that stars formed early-on in cosmic time, via a star formation burst, could be characterised by a bottom-heavy IMF. A promising way to confirm this is to use relics, ultra-compact massive galaxies, almost entirely composed by these "pristine" stars. The INSPIRE Project aims at assembling a large sample of confirmed relics, that can serve as laboratory to investigate on the conditions of star formation in the first 1-3 Gyr of the Universe. In this third INSPIRE paper, we build a high signal-to-noise spectrum from five relics and one from five galaxies with similar sizes, masses, and kinematical properties, but characterised by a more extended star formation history (non-relics). Our detailed stellar population analysis suggests a systematically bottom-heavier IMF slope for relics than for non-relics, adding new observational evidence for the non-universality of the IMF at various redshifts and further supporting the above proposed physical scenario.

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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

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

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

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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arXiv2021

The stellar halos of ETGs in the IllustrisTNG simulations: II. Accretion, merger history, and dark halo connection

Stellar halos in early-type galaxies (ETGs) are shaped by their accretion and merger histories. We use a sample of 1114 ETGs in the TNG100 simulation with stellar masses $10^{10.3}\leq M_{*}/M_\odot\leq 10^{12}$, selected at z=0 within the range of g-r colour and lambda-ellipticity diagram populated by observed ETGs. We study how the rotational support and intrinsic shapes of the stellar halos depend on the fraction of accreted stars, overall and separately by major, minor, and mini mergers. Accretion histories in TNG100 ETGs as well as the radial distributions of ex-situ stars $f_{ex}(R)$ strongly correlate with stellar mass. Low-mass ETGs have characteristic peaked rotation profiles and near-oblate shapes with rounder halos that are completely driven by the in-situ stars. At high $f_{ex}$ major mergers decrease the in-situ peak in rotation velocity, flatten the $V_{*}/σ_{*}(R)$ profiles, and increase the triaxiality of the stellar halos. Kinematic transition radii do not trace the transition between in-situ and ex-situ dominated regions, but for systems with $M_{*}>10^{10.6}M_\odot$ the local rotational support decreases with the local ex-situ fraction $f_{ex}(R)$ and triaxiality increases with $f_{ex}$. These correlations are followed by fast and slow rotators alike with a continuous and overlapping sequence of properties. Merger events dynamically couple stars and dark matter: in high mass ETGs and at large radii where $f_{ex}\gtrsim0.5$, both components tend to have similar intrinsic shapes and rotational support, and nearly aligned principal axes and spin directions. Based on these results we suggest that extended photometry and kinematics of massive ETGs ($M_{*}>10^{10.6}M_\odot$) can be used to estimate the local fraction of ex-situ stars and to approximate the intrinsic shapes and rotational support of the co-spatial dark matter component. [abridged]

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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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