arXiv subjects

A. Vazdekis

Publications and source records attributed to A. Vazdekis.

At least 19 recordsLinked to original sources

arXiv2026

Probing IMF Variations in High-Redshift Early-Type Galaxies with SHARP

The stellar initial mass function (IMF), which describes the distribution of stellar masses at birth, is a fundamental ingredient in shaping galaxy evolution. Recent observations indicate that the IMF varies between galaxies, depending on their mass, morphology, and stellar content. In local early-type galaxies (ETGs), spectroscopy, dynamics, and lensing reveal bottom-heavy IMFs in dense central regions, with radial gradients toward a Milky Way-like distribution in the outskirts. Yet, the chemical enrichment of massive ETGs implies a dominant role of massive stars during their early formation phases. These findings can be reconciled if the IMF evolves over cosmic time -- initially more top-heavy to enable rapid enrichment, and later dominated by long-lived, low-mass stars. Directly measuring the IMF at z>1 is therefore essential to test such time-dependent IMF scenarios, including variations in the dwarf-to-giant and stellar mass-to-light ratios. To date, no direct observational confirmation of these IMF variations -- or of their physical origin -- has been obtained. The SHARP spectrograph on the E-ELT, with unprecedented spatial resolution and sensitivity compared to facilities such as JWST, and broader spectral coverage than other E-ELT instruments, will enable spatially resolved spectroscopy of IMF-sensitive features in high-redshift ETGs up to z~3, providing unique insights into the origin of the non-universal IMF in massive galaxies.

astro-ph.GA↗

arXiv2026

Chemical hints of Population III stars from silicon abundances in massive galaxies

The formation of massive galaxies remains a fundamental question in astrophysics, with recent JWST observations suggesting that intense star formation occurred earlier than previously expected. We used the prototypical massive relic galaxy NGC 1277 as a fossil record to probe the chemical signatures of early star formation. With a stellar mass of $1.2 \pm 0.4 \times 10^{11}M_\odot$ and a compact structure (half-light radius of 1.2 kpc), NGC 1277 is representative of massive relic galaxies-systems that formed rapidly at high redshifts ("red nuggets" at $z>2$) and have since undergone largely passive evolution, preserving the imprint of their earliest stellar populations. Using deep near-infrared spectroscopy, we identified unusually strong silicon (Si) absorption features in this galaxy. We measure $[\mathrm{Si}/\mathrm{Fe}]=1.05^{+0.45}_{-0.27}$ and $[\mathrm{Mg}/\mathrm{Fe}]=0.36^{+0.27}_{-0.24}$, corresponding to a significantly enhanced silicon-to-magnesium ratio ($[\mathrm{Si}/\mathrm{Mg}]=0.67^{+0.45}_{-0.27}$) which cannot be reproduced by current stellar population models or typical core-collapse supernova yields. This enhancement suggests a contribution from very massive, metal-poor progenitors during the earliest phases of star formation. Such conditions are consistent with nucleosynthetic yields from pair-instability supernovae or other enrichment channels associated with extremely massive stars, whose chemical signatures are expected to remain less diluted in systems like NGC 1277 due to their rapid formation timescales and minimal late-time accretion. This result provides rare chemical evidence of early massive-star enrichment preserved in a local relic galaxy and opens a new observational window into the chemical evolution of the Universe.

astro-ph.GA↗

arXiv2025

Why the Northern Hemisphere Needs a 30-40m Telescope and the Science at Stake: Shaping Galaxies and Their Stars with Stellar Population Gradients, IMF Variations and Environmental Drivers in Cluster Early-Type Galaxies

This white paper highlights how stellar population gradients, chemical abundance patterns, stellar initial mass function (IMF) variations, and structural signatures in early-type galaxies (ETGs), measured at faint and large galactocentric radii, out to $\sim4R_e$, provide powerful diagnostics of their formation and evolutionary histories. These observables encode the combined effects of early dissipative star formation, subsequent accretion and mergers, and internal feedback processes. Achieving such measurements requires high-signal-to-noise, spatially resolved U-band--optical--near-IR spectroscopy at large radii, with enough spatial resolution to study the variation of these properties on $\sim$kpc scales. These capabilities can only be delivered by a 30\,m-class telescope. Disentangling these internal processes from environmental influences further demands observations of galaxies across clusters spanning a wide range of evolutionary stages and local environments. The nearby Virgo, Perseus, and Coma clusters, without any comparable nearby counterparts in the Southern Hemisphere, offer ideal laboratories for this work. Such observations will place stringent constraints on the formation mechanism of ETGs, connecting local cluster ETGs to their high-redshift progenitors. This white paper outlines several key science cases enabled by such a facility: (1) mapping stellar population gradients across environments; (2) tracing IMF variations as a function of evolutionary stage and environment; (3) reconstructing the three-dimensional structure of galaxies through deep integral-field spectroscopy and imaging; and (4) identifying and studying compact and relic systems as progenitors of present-day ETGs.

astro-ph.IM↗

arXiv2025

Why the Northern Hemisphere Needs a 30-40 m Telescope and the Science at Stake: Resolved Stellar Populations Studies in M31 and its Satellites

A 30 m class optical/near-IR telescope in the Northern Hemisphere, equipped for diffraction-limited imaging and high-resolution, multi-object spectroscopy of faint stars, would enable a transformational investigation of the formation and evolution of M31 and its satellite system - on par with what Gaia, the HST, and other major photometric and spectroscopic facilities have achieved for the Milky Way (MW) and its satellites. The unprecedented detail obtained for our home system has reshaped our understanding of the assembly of the MW disk, halo, and bulge, and that of its satellites, which now serve as a benchmark for galaxy formation and evolution models. Extending this level of insight to the M31 system - that of the nearest massive spiral and the only one for which such a comprehensive, resolved stellar population study is feasible - will allow us to address a fundamental question: how representative is the MW and its satellite system within the broader context of galaxy evolution?

astro-ph.IM↗

arXiv2025

Elemental abundance ratios for the bulge of M31

We present radial trends of metallicity ([Fe/H]) and abundance ratios ([X/Fe]) for several chemical elements -- including C, N, Na, and the so-called alpha-elements (O, Mg, Si, Ca, and Ti) -- in the bulge of M31, out to ~0.6kpc from the center. We estimated abundances using full-spectrum fitting, full-index fitting, and line-strength analysis, in combination with different stellar population models. We first tested these techniques on mock spectra and SDSS stacked spectra of ETGs, and then applied them to high-quality long-slit spectroscopy of the M31 bulge obtained with the OSIRIS spectrograph at the GTC. We find that O, N, and Na are significantly enhanced relative to Fe across the bulge, with typical abundances >0.3~dex. In particular, N and Na show steep central enhancements, reaching ~0.5dex. C, Mg, and Si exhibit intermediate enhancements of [X/Fe]~0.2dex, with C and Mg decreasing toward the center to <~0.1dex; while Ca, and to a lesser extent Ti, closely follow Fe, with [X/Fe]<0.1dex. Applying the same analysis to SDSS stacked spectra of ETGs revealed that the abundance pattern of the M31 bulge closely resembles that of the most massive galaxies, except for N, which is significantly more enhanced (by ~0.1dex) in the bulge. For the bulk of the bulge, chemical evolution models assuming high star-formation efficiency and a short gas infall timescale reproduce the overall trends in [Fe/H] and [X/Fe]. In the central region (<~100pc), the high metallicity content of the bulge can be explained by either an IMF flatter than Salpeter at high mass, or a prolonged star formation. Additional processes, such as differential galactic winds, appear necessary to account for the observed decoupling among alpha elements and the strong central N enhancement. Our results support a scenario whereby the bulk of the M31 bulge formed during a fast and intense episode of star formation.

astro-ph.GA↗

arXiv2025

Puzzling radial gradients of K-band absorption features in the giant elliptical galaxy M87

We present new K-band spectroscopy for the giant elliptical galaxy M87 in the Virgo cluster, taken with the LUCI spectrograph at the Large Binocular Telescope (LBT). The new data are used to study line-strengths of K-band absorption features from different chemical species, namely Fe, Mg, Ca, Na, and CO, as a function of galactocentric distance, out to 40arcsec from the center (about half of the galaxy effective radius). The radial trends of spectral indices are compared to those for the bulge of M31, observed with the same instrument. For M87, most K-band indices exhibit flat radial profiles, with the exception of NaI2.21, that decreases outwards, with a negative radial gradient. Significant offsets are found between indices for M87 and those for the bulge of M31, the latter having weaker line-strengths for almost all features, but Fe and Ca, for which we find similar trends in both systems. We find that the behavior of CO features - most prominent in giant stars - is difficult to explain, consistent with previous results for the central regions of massive galaxies. In particular, the CO indices are stronger in M87 than M31, and do not exhibit significant radial gradients in M87, despite its IMF being bottom heavier than M31 especially in its central region. Predictions of state-of-the-art stellar population models, based on results from the optical spectral range, are able to match only the Na and Ca indices of M87, while a significant mismatch is found for all other indices. This shows that state-of-the-art stellar population models should be improved significantly in order to provide reliable constraints on the stellar population content of galaxies in the NIR spectral range.

astro-ph.GA↗

arXiv2025

Chemical abundance ratios for the bulge of M31

We present abundance ratio estimates of individual elements, namely C, N, Na, and the so-called alpha elements, Mg, O, Si, Ca, and Ti, for the bulge of M31. The analysis is based on long-slit, high-quality spectroscopy of the bulge, taken with the OSIRIS spectrograph at the Gran Telescopio CANARIAS (GTC). Abundance ratios, [X/Fe]s, are inferred by comparing radially binned spectra of M31 with different state-of-the-art stellar population models, averaging out results from various methods, namely full-spectral, full-index, and line-strength fitting, respectively. For the bulk of the bulge, we find that O, N, and Na are significantly enhanced compared to Fe, with abundances of about 0.3dex, followed by C, Mg, and Si, with [X/Fe] about 0.2dex, and lastly, Ti and Ca, mostly tracking Fe ([X/Fe]<0.1dex), within the error bars. Performing the same analysis on SDSS stacked spectra of early-type galaxies with different velocity dispersion, we find that the abundance pattern of the M31 bulge is very similar to that of most massive galaxies, supporting a scenario where most of the bulge formed in a fast and intense episode of star-formation.

astro-ph.GA↗

arXiv2024

Extragalactic globular cluster near-infrared spectroscopy: II. The stellar population synthesis model zero-point problem extends to the near-infrared

Many recent studies have pointed out significant discrepancies between observations and models of stellar populations in the near-infrared (NIR). With current and future observing facilities being focused in this wavelength range, properly assessing and solving these issues is of utmost importance. Here, we present the first application of the extragalactic globular cluster (GC) near-infrared spectroscopy survey, and present evidence that these GCs reveal an age zero-point problem of stellar population synthesis (SPS) models. This problem has already been identified in the optical range for the GCs of the Milky Way. Such an issue arises when derived GC spectroscopic ages appear older than the Universe itself. We extend this discussion for the first time to the NIR, specifically using the Pa$_{\rmβ}$ line at 1.28~microns. We focus on the GCs of the nearby Centaurus~A galaxy using their NIR spectra. This work broadens our understanding of the age zero-point problem and emphasises the necessity to revisit and refine SPS models, especially in the NIR domain.

astro-ph.GA↗

arXiv2024

Extragalactic globular cluster near-infrared spectroscopy: I. Integrated near-infrared spectra of Centaurus A/NGC 5128

One way to constrain the evolutionary histories of galaxies is to analyse their stellar populations. In the local Universe, our understanding of the stellar population properties of galaxies has traditionally relied on the study of optical absorption and emission-line features. In order to overcome limitations intrinsic to this wavelength range, such as the age-metallicity degeneracy and the high sensitivity to dust reddening, we must use wavelength ranges beyond the optical. The near-infrared (NIR) offers a possibility to extract information on spectral signatures that are not as obvious in traditional optical bands. Moreover, with the current and forthcoming generation of instrumentation focusing on the NIR, it is mandatory to explore possibilities within this wavelength range for nearby-Universe galaxies. However, although the NIR shows great potential, we are only beginning to understand it. Widely used techniques such as a full spectral fitting and line strength indices need to be tested on systems that are as close to simple stellar populations as possible, and the result from the techniques need to be compared to the yields from a traditional optical analysis. We present a NIR spectral survey of extragalactic globular clusters (GCs). The set was composed of 21 GCs from NGC5128 that were obtained with SOAR/TripleSpec4, which covered the ~1.0-2.4$μ$m range with a spectral resolution R=$λ/Δλ$ of 3500. These spectra cover H$β$ equivalent widths between 0.98$Å$ and 4.32$Å$, and [MgFe]' between 0.24$Å$ and 3.76$Å$. This set is ideal for performing absorption band measurements and a full spectral fitting, and it can be used for kinematic studies and age and abundance measurements. With this library, we expect to be able to probe the capabilities of NIR models, as well as to further improve stellar population estimates for the GCs around NGC5128.

astro-ph.GA↗

arXiv2024

Counting stars from the integrated spectra of galaxies

Over the last decades, evolutionary population synthesis models have powered an unmatched leap forward in our understanding of galaxies. From dating the age of the first galaxies in the Universe to detailed measurements of the chemical composition of nearby galaxies, the success of this approach built upon simple stellar population (SSP) spectro-photometric models is unquestionable. However, the internal constraints inherent to the construction of SSP models may hinder our ability to analyze the integrated spectra of galaxies in situations where the SSP assumption does not sufficiently hold. Thus, here we revisit the possibilities of fitting galaxy spectra as a linear combination of stellar templates without assuming any a priori knowledge on stellar evolution. We showcase the sensitivity of this alternative approach to changes in the stellar population properties, in particular the direct connection to variations in the stellar initial mass function, as well as its advantages when dealing with non-canonical integrated populations and semi-resolved observations. Furthermore, our analysis demonstrates that the absorption spectra of galaxies can be used to independently constrain tellar evolution theory beyond the limited conditions of the solar neighborhood.

astro-ph.GA↗

arXiv2024

Retrieval of the physical parameters of galaxies from WEAVE-StePS-like data using machine learning

The WHT Enhanced Area Velocity Explorer (WEAVE) is a new, massively multiplexing spectrograph. This new instrument will be exploited to obtain high S/N spectra of $\sim$25000 galaxies at intermediate redshifts for the WEAVE Stellar Population Survey (WEAVE-StePS). We test machine learning methods for retrieving the key physical parameters of galaxies from WEAVE-StePS-like spectra using both photometric and spectroscopic information at various S/Ns and redshifts. We simulated $\sim$105000 galaxy spectra assuming SFH with an exponentially declining star formation rate, covering a wide range of ages, stellar metallicities, sSFRs, and dust extinctions. We then evaluated the ability of the random forest and KNN algorithms to correctly predict such parameters assuming no measurement errors. We checked how much the predictive ability deteriorates for different S/Ns and redshifts, finding that both algorithms still accurately estimate the ages and metallicities with low bias. The dispersion varies from 0.08-0.16 dex for ages and 0.11-0.25 dex for metallicity, depending on the redshift and S/N. For dust attenuation, we find a similarly low bias and dispersion. For the sSFR, we find a very good constraining power for star-forming galaxies, log sSFR$\gtrsim$ -11, where the bias is $\sim$ 0.01 dex and the dispersion is $\sim$ 0.10 dex. For more quiescent galaxies, with log sSFR$\lesssim$ -11, we find a higher bias, 0.61-0.86 dex, and a higher dispersion, $\sim$ 0.4 dex, for different S/Ns and redshifts. Generally, we find that the RF outperforms the KNN. Finally, the retrieved sSFR was used to successfully classify galaxies as part of the blue cloud, green valley, or red sequence. We demonstrate that machine learning algorithms can accurately estimate the physical parameters of simulated galaxies even at relatively low S/N=10 per angstrom spectra with available ancillary photometric information.

astro-ph.GA↗

arXiv2024

CO radial gradients in the bulge of M31

We present new H- and K-band spectroscopy for the bulge of M31, taken with the LUCI spectrograph at the Large Binocular Telescope (LBT). We studied radial trends of CO absorption features (namely, CO1.58, CO1.60, CO1.64, CO1.66, CO1.68, CO2.30, CO2.32, CO2.35) in the bulge of M31, out to a galactocentric distance of 100'' (380pc). We find that most COs do not exhibit a strong radial gradient, despite the strong metallicity gradient inferred from the optical spectral range, except for CO1.64, showing a steep increase in the center. We compared the observed line strengths to predictions of different state-of-the-art stellar population models, including an updated version of EMILES models, which also uses the extended IRTF spectral library. The observed COs are close to models' predictions, but in some models they turn out to be underestimated. We find that the lack of radial gradients is due to the combination of increasing CO strength with metallicity and C abundance, and decreasing CO strength with IMF slope and O abundance. We speculate that the steep gradient of CO1.64 might be due to Na overabundance. Remarkably, we were able to fit, at the same time, optical indices and all the NIR COs except for CO1.68, leaving abundance ratios (i.e., [C/Fe], [O/Fe], and [Mg/Fe]) as free-fitting parameters, imposing age and metallicity constraints from the optical, with no significant contribution from intermediate-age populations. For the majority of the bulge, we find [Mg/Fe]~0.15dex, [O/Fe] larger than [Mg/Fe] (by ~0.1dex), and C abundance consistent with that of Mg. In the central (few arcsec) region, we still find an enhancement of O and Mg, but significantly lower [C/Fe]. We find that the COs' line strengths of the bulge are significantly lower than those of massive galaxies, possibly because of a difference in carbon abundance, as well as, to some extent, total metallicity.

astro-ph.GA↗

arXiv2024

Modelling of surface brightness fluctuation measurements: Methodology, uncertainty, and recommendations

The goal of this work is to scrutinise the surface brightness fluctuation (SBF) calculation methodology. We analysed the SBF derivation procedure, measured the accuracy of the fitted SBF under controlled conditions, retrieved the uncertainty associated with the variability of a system that is inherently stochastic, and studied the SBF reliability under a wide range of conditions. Additionally, we address the possibility of an SBF gradient detection. We also examine the problems related with biased measurements of the SBF and low luminosity sources. All of this information allows us to put forward guidelines to ensure a valid SBF retrieval. To perform all these experiments, we carried out Monte Carlo simulations of mock galaxies as an ideal laboratory. Knowing its underlying properties, we attempted to retrieve SBFs under different conditions. We show how the SBF uncertainty can be obtained and we present a collection of qualitative recommendations for a safe SBF retrieval: it is important to model the instrumental noise, rather than fitting it; the target galaxies must be observed under appropriate observational conditions; in a traditional SBF derivation, one should avoid pixels with fluxes lower than ten times the SBF estimate to prevent biased results. We offer our computational implementation in the form of a simple code designed to estimate the uncertainty of the SBF measurement. This code can be used to predict the quality of future observations or to evaluate the reliability of those already conducted.

astro-ph.GA↗

arXiv2023

Stellar metallicity from optical and UV spectral indices: Test case for WEAVE-StePS

The upcoming generation of optical spectrographs on four meter-class telescopes, with their huge multiplexing capabilities, excellent spectral resolution, and unprecedented wavelength coverage, will provide high-quality spectra for thousands of galaxies. These data will allow us to examine of the stellar population properties at intermediate redshift, an epoch that remains unexplored by large and deep surveys. We assess our capability to retrieve the mean stellar metallicity in galaxies at different redshifts and S/N, while simultaneously exploiting the UV and optical rest-frame wavelength coverage. The work is based on a comprehensive library of spectral templates of stellar populations, covering a wide range of age and metallicity values and built assuming various SFHs. We simulated realistic observations of a large sample of galaxies carried out with WEAVE at the WHT at different redshifts and S/N values. We measured all the reliable indices on the simulated spectra and on the comparison library. We then adopted a Bayesian approach to obtain the probability distribution of stellar metallicity. The analysis of the spectral indices has shown how some mid-UV indices can provide reliable constraints on stellar metallicity, along with optical indicators. The analysis of the mock observations has shown that even at S/N=10, the metallicity can be derived within 0.3 dex, in particular, for stellar populations older than 2 Gyr. Our results are in good agreement with other theoretical and observational works in the literature and show how the UV indicators can be advantageous in constraining metallicities. This is very promising for the upcoming surveys carried out with new, highly multiplexed, large-field spectrographs, such as StePS at the WEAVE and 4MOST, which will provide spectra of thousands of galaxies covering large spectral ranges at relatively high S/N.

astro-ph.GA↗

arXiv2023

WEAVE-StePS. A stellar population survey using WEAVE at WHT

The upcoming new generation of optical spectrographs on four-meter-class telescopes will provide valuable opportunities for forthcoming galaxy surveys through their huge multiplexing capabilities, excellent spectral resolution, and unprecedented wavelength coverage. WEAVE is a new wide-field spectroscopic facility mounted on the 4.2 m William Herschel Telescope in La Palma. WEAVE-StePS is one of the five extragalactic surveys that will use WEAVE during its first five years of operations. It will observe galaxies using WEAVE MOS (~950 fibres across a field of view of ~3 deg2 on the sky) in low-resolution mode (R~5000, spanning the wavelength range 3660-9590 AA). WEAVE-StePS will obtain high-quality spectra (S/N ~ 10 per AA at R~5000) for a magnitude-limited (I_AB = 20.5) sample of ~25,000 galaxies, the majority selected at z>=0.3. The survey goal is to provide precise spectral measurements in the crucial interval that bridges the gap between LEGA-C and SDSS data. The wide area coverage of ~25 deg2 will enable us to observe galaxies in a variety of environments. The ancillary data available in each observed field (including X-ray coverage, multi-narrow-band photometry and spectroscopic redshift information) will provide an environmental characterisation for each observed galaxy. This paper presents the science case of WEAVE-StePS, the fields to be observed, the parent catalogues used to define the target sample, and the observing strategy chosen after a forecast of the expected performance of the instrument for our typical targets. WEAVE-StePS will go back further in cosmic time than SDSS, extending its reach to encompass more than ~6 Gyr, nearly half of the age of the Universe. The spectral and redshift range covered by WEAVE-StePS will open a new observational window by continuously tracing the evolutionary path of galaxies in the largely unexplored intermediate-redshift range.

astro-ph.GA↗

arXiv2022

Modelling simple stellar populations in the near-ultraviolet to near-infrared with the X-shooter Spectral Library (XSL)

We present simple stellar population models based on the empirical X-shooter Spectral Library (XSL) from NUV to NIR wavelengths. The unmatched characteristics of relatively high resolution and extended wavelength coverage ($350-2480$ nm, $R\sim10\,000$) of the XSL population models bring us closer to bridging optical and NIR studies of intermediate and old stellar populations. It is now common to find good agreement between observed and predicted NUV and optical properties of stellar clusters due to our good understanding of the main-sequence and early giant phases of stars. However, NIR spectra of intermediate-age and old stellar populations are sensitive to cool K and M giants. The asymptotic giant branch, especially the thermally pulsing asymptotic giant branch, shapes the NIR spectra of $0.5-2$ Gyr old stellar populations; the tip of the red giant branch defines the NIR spectra of populations with ages larger than that. We construct sequences of the average spectra of static giants, variable-rich giants, and C-rich giants to include in the models separately. The models span the metallicity range $-2.2<[Fe/H]<+0.2$ and ages above 50 Myr, a broader range in the NIR than in other models based on empirical spectral libraries. Our models can reproduce the integrated optical colours of the Coma cluster galaxies at the same level as other semi-empirical models found in the literature. In the NIR, there are notable differences between the colours of the models and Coma cluster galaxies. The XSL models expand the range of predicted values of NIR indices compared to other models based on empirical libraries. Our models make it possible to perform in-depth studies of colours and spectral features consistently throughout the optical and the NIR range to clarify the role of evolved cool stars in stellar populations.

astro-ph.GA↗

arXiv2022

The X-shooter Spectral Library (XSL): Data Release 3

We present the third data release (DR3) of the X-shooter Spectral Library (XSL). This moderate-to-high resolution, near-ultraviolet-to-near-infrared ($350-2480$ nm, R $\sim$ 10 000) spectral library is composed of 830 stellar spectra of 683 stars. DR3 improves upon the previous data release by providing the combined de-reddened spectra of the three X-shooter segments over the full $350-2480$ nm wavelength range. It also includes additional 20 M-dwarf spectra from the ESO archive. We provide detailed comparisons between this library and Gaia EDR3, MILES, NGSL, CaT library, and (E-)IRTF. The normalised rms deviation is better than $D=0.05$ or 5$\%$ for the majority of spectra in common between MILES (144 spectra of 180), NGSL (112$/$116), and (E-)IRTF (55$/$77) libraries. Comparing synthetic colours of those spectra reveals only negligible offsets and small rms scatter, such as the median offset(rms) 0.001$\pm$0.040 mag in the (box1-box2) colour of the UVB arm,-0.004$\pm$0.028 mag in (box3-box4) of the VIS arm, and -0.001$\pm$0.045 mag in (box2-box3) colour between the UVB and VIS arms, when comparing stars in common with MILES. We also find an excellent agreement between the Gaia published (BP-RP) colours and those measured from the XSL DR3 spectra, with a zero median offset and an rms scatter of 0.037 mag for 449 non-variable stars. The unmatched characteristics of this library, which combine a relatively high resolution, a large number of stars, and an extended wavelength coverage, will help us to bridge the gap between the optical and the near-IR studies of intermediate and old stellar populations, and to probe low-mass stellar systems.

astro-ph.SR↗

arXiv2021

Mild radial variations of the stellar IMF in the bulge of M31

Using new, homogeneous, long-slit spectroscopy in the wavelength range from ~0.35 to ~1micron, we study radial gradients of optical and near-infrared (NIR) IMF-sensitive features along the major axis of the bulge of M31, out to a galacto-centric distance of ~200'' (~800pc). Based on state-of-the-art stellar population synthesis models with varying Na abundance ratio, we fit a number of spectral indices, from different chemical species (including TiO's, Ca, and Na indices), to constrain the low-mass (<0.5M_Sun) end slope (i.e. the fraction of low-mass stars) of the stellar IMF, as a function of galacto-centric distance. Outside a radial distance of ~10'', we infer an IMF similar to a Milky-Way-like distribution, while at small galacto-centric distances, an IMF radial gradient is detected, with a mildly bottom-heavy IMF in the few inner arcsec. We are able to fit Na features (both NaD and NaI8190), without requiring extremely high Na abundance ratios. [Na/Fe] is ~0.4dex for most of the bulge, rising up to ~0.6dex in the innermost radial bins. Our results imply an overall, luminosity-weighted, IMF and mass-to-light ratio for the M31 bulge, consistent with those for a Milky-Way-like distribution, in contrast to results obtained, in general, for most massive early-type galaxies.

astro-ph.GA↗