Simple Stellar Population Research Articles

Counting stars from the integrated spectra of galaxies

Over the last few 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 providing 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 can 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 noncanonical integrated populations and semi-resolved observations. Furthermore, our analysis demonstrates that the absorption spectra of galaxies can be used to independently constrain stellar evolution theory beyond the limited conditions of the solar neighborhood.

Extragalactic globular cluster near-infrared spectroscopy

Context. 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. Aims. 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. Methods. We present a NIR spectral survey of extragalactic globular clusters (GCs). The set was composed of 21 GCs from the Centaurus A galaxy that were obtained with SOAR/TripleSpec4, which covered the ∼1.0–2.4 μm range with a spectral resolution (R = λ/Δλ) of 3500. These spectra cover Hβ equivalent widths between 0.98 Å and 4.32 Å, and [MgFe]′ between 0.24 Å and 3.76 Å. Results. This set was 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 the Centaurus A galaxy.

Validating full-spectrum fitting with a synthetic integral-field spectroscopic observation of the milky way

Abstract Ongoing deep IFS observations of disk galaxies provide opportunities for comparison with the Milky Way (MW) to understand galaxy evolution. However, such comparisons are marred by many challenges such as selection effects, differences in observations and methodology, and proper validation of full-spectrum fitting methods. In this study, we present a novel code GalCraft to address these challenges by generating mock IFS data cubes of the MW using simple stellar population models and a mock MW stellar catalog derived from E-Galaxia. We use the widely adopted full-spectrum fitting code pPXF to investigate the ability to recover kinematics and stellar populations for an edge-on mock MW IFS observation. We confirm that differences in kinematics, mean age, [M/H], and [α/Fe] between thin and thick disks can be distinguished. However, the age distribution is overestimated in the ranges between 2 − 4 and 12 − 14 Gyr compared to the expected values. This is likely due to the age spacing and degeneracy of SSP templates. We find systematic offsets in the recovered kinematics due to insufficient spectral resolution and the variation of line-of-sight velocity distribution with age and [M/H]. With future higher resolution and multi-[α/Fe] SSP templates, GalCraft will be useful to validate key signatures such as [α/Fe]-[M/H] distribution at different R and |z| and potentially infer radial migration and kinematic heating efficiency to study detailed chemodynamical evolution of MW-like galaxies.

Observational constraints on the stellar recycled gas in active galactic nuclei feeding

ABSTRACT Near-infrared long-slit spectroscopy has been used to study the stellar population (SP) of the low luminosity active galactic nuclei (AGNs) and matched analogues (LLAMA) sample. To perform the SP fits we have employed the X-shooter simple stellar population models together with the starlight code. Our main conclusions are: The star formation history of the AGNs is very complex, presenting many episodes of star formation during their lifetimes. In general, AGN hosts have higher fractions of intermediate-age SP (light-weighted mean ages, <t > L ≲ 4.5 Gyr) when compared with their analogues (<t > L ≲ 8.0 Gyr). AGNs are more affected by reddening and require significant fractions of featureless continuum and hot dust components. The ratio between the AGN radiated energy and the gravitational potential energy of the molecular gas (ERad/EPG) for the AGN is compared with the <t > L and a possible anticorrelation is observed. This suggests that the AGN is affecting the star formation in these galaxies, in the sense that more energetic AGN [log(ERad/EPG) ≳ 3] tend to host nuclear younger SP (<t > L ≲4 Gyr). We found that the recent (t <2 Gyr) returned (recycled) stellar mass is higher in AGN than in the controls. We also provide evidence that the mass-loss of stars would be enough to feed the AGN, thus providing observational constraints for models that predict that AGN feeding is partially due to the recycled gas from dying stars.

Measuring the Chemodynamics and Ages of the M32 and M110 Dwarf Galaxies with APOGEE

We present a full-spectrum-fitting analysis of the central kinematics and chemistry of the Andromeda dwarf satellite galaxies M32 and M110. We use an Markov Chain Monte Carlo routine to fit high-resolution, near-infrared, integrated-light spectra from APOGEE with empirical simple stellar population templates constructed from individual APOGEE stellar spectra. This yields the best-fitting mean radial velocity, velocity dispersion, metallicity, α abundance, and age for each spectrum. In general, our results are consistent with literature values where available, and we explore possible reasons where offsets are measured. This study was presented in a poster at the 243rd meeting of the American Astronomical Society in 2024 January.

SMARTY: The mileS Moderate resolution neAr-infRared sTellar librarY

Abstract Most of the observed galaxies cannot be resolved into individual stars and are studied through their integrated spectrum using simple stellar populations (SSPs) models, with stellar libraries being a key ingredient in building them. Spectroscopic observations are increasingly being directed towards the near-infrared (NIR), where much is yet to be explored. SSPs in the NIR are still limited, and there are inconsistencies between different sets of models. One of the ways to minimize this problem is to have reliable NIR stellar libraries. The main goal of this work is to present smarty (mileS Moderate resolution neAr-infRared sTellar librarY) a ∼0.9 − 2.4 μm stellar spectral library composed of 31 stars observed with the Gemini Near-IR Spectrograph (GNIRS) at the 8.1 m Gemini North telescope and make it available to the community. The stars were chosen from the miles library, for which the atmospheric parameters are reliable (and well tested), to populate different regions of the Hertzsprung-Russell (HR) diagram. Furthermore, five of these stars have NIR spectra available that we use to assess the quality of smarty. The remaining 26 stars are presented for the first time in the NIR. We compared the observed smarty spectra with synthetic and interpolated spectra, finding a mean difference of $\sim 20\%$ in the equivalent widths and ∼1% in the overall continuum shape in both sets of comparisons. We computed the spectrophotometric broadband magnitudes and colours and compared them with the 2MASS ones, resulting in mean differences up to 0.07 and 0.10 mag in magnitudes and colours, respectively. In general, a small difference was noted between the smarty spectra corrected using the continuum from the interpolated and the theoretical stars.

Supernova environments in J-PLUS

We investigated the local environmental properties of 418 supernovae (SNe) of all types using data from the Javalambre Photometric Local Universe Survey (J-PLUS), which includes five broad-band and seven narrow-band imaging filters. Our study involves two independent analyses: (1) the normalized cumulative-rank (NCR) method, which utilizes all 12 single bands along with five continuum-subtracted narrow-band emission and absorption bands, and (2) simple stellar population (SSP) synthesis, where we build spectral energy distributions (SED) of the surrounding 1 kpc2 SN environment using the 12 broad- and narrow-band filters. Improvements on previous works include: (i) the extension of the NCR technique to other filters (broad and narrow) and the use a set of homogeneous data (same telescope and instruments); (ii) a correction for extinction to all bands based on the relation between the g − i color and the color excess E(B − V); and (iii) a correction for the contamination of the [N II] λ6583 line that falls within the Hα filter. All NCR distributions in the broad-band filters, tracing the overall light distribution in each galaxy, are similar to each other. The main difference is that type Ia, II, and IIb SNe are preferably located in redder environments than the other SN types. The radial distribution of the SNe shows that type IIb SNe seem to have a preference for occurring in the inner regions of galaxies, whereas other types of SNe occur throughout the galaxies without a distinct preference for a specific location. For the Hα filter we recover the sequence from SNe Ic, which has the highest NCR, to SNe Ia, which has the lowest; this is interpreted as a sequence in progenitor mass and age. All core-collapse SN types are strongly correlated to the [O II] emission, which also traces star formation rate (SFR), following the same sequence as in Hα. The NCR distributions of the Ca II triplet show a clear division between II-IIb-Ia and Ib-Ic-IIn subtypes, which is interpreted as a difference in the environmental metallicity. Regarding the SSP synthesis, we found that including the seven J-PLUS narrow filters in the fitting process has a more significant effect on the core-collapse SN environmental parameters than for SNe Ia, shifting their values toward more extincted, younger, and more star-forming environments, due to the presence of strong emission lines and stellar absorptions in those narrow bands.

A Study of the Merging Dwarf Galaxy VCC322

Galaxy interactions and mergers can enhance or reduce star formation, but a complete understanding of the involved processes is still lacking. The effect of dwarf galaxy mergers is even less clear than their massive counterpart. We present a study on a dwarf merger remnant in the Virgo cluster, VCC322, which might form a triple system with VCC334 and VCC319. We identify a prominent long and straight tail-like substructure that has a size comparable to its host galaxy VCC322. By comparing the color–color (g − r versus r − H) distribution with simple stellar population models, we infer that the metallicity and stellar age of this tail are Z ⋆ ∼ 0.02 Z ⊙ and t ⋆ ∼ 10 Gyr, respectively. In VCC319, we find a sign of isophotal twisting. This suggests that VCC319 may be subject to tidal interaction. An analysis of the Sloan Digital Sky Survey optical spectra of VCC322 indicates mass- and light-weighted ages of about 109.8 yr and 107.5 yr, respectively, indicating an ongoing star formation activity. However, the star formation in VCC322 seems suppressed when compared to other star-forming dwarfs of comparable stellar masses. Our finding of shock excitation of optical emission lines indicates that interaction-induced shock may contribute to the heating of cold gas and suppression of star formation.

Near-infrared spectroscopic indices for unresolved stellar populations. III. Composite indices definition as age and metallicity tracers and model comparison

ABSTRACT Recent advances in the stellar population studies of unresolved galaxies in the NIR domain demonstrated that it contains several line-strength indices to be potentially used as diagnostics for stellar population properties. Reduction of the extinction and possibility to disentangle different stellar populations dominating different spectral ranges are obviously beneficial. To this aim, we have investigated the connections between 19 Lick/IDS indices and 39 NIR indices measured in the central regions of 32 galaxies observed with X-shooter. We adopted a systematic approach to deriving a correlation matrix using all the optical and NIR indices measured for the galaxies and building new NIR composite indices to maximize their correlations with the best age and metallicity optical tracers. We found that the new T1 and [AlFeCr] composite indices are promising age and metallicity diagnostics in NIR, respectively. We finally tested the T1 and [AlFeCr] indices with the predictions of simple stellar populations models, and we found that the models show a general agreement with the data. Some fine tuning and further comparison between models and data, which are now largely available, is necessary to reach the prediction level of the optical line-strength indices.

Revisit of 77 Clusters Candidates with Multiple Main Sequences using Gaia DR3

Gaia DR3 data provide the latest and full astrometric data for more than 1.5 billion sources, and these allow us to revisit the 77 special open cluster candidates collected from LISC star cluster catalog. According to their unclear color–magnitude-diagram (CMD) morphology, they are difficult to be ensured as real open clusters (OCs), and their cluster properties are still poorly known. We perform a cluster census by reidentification for these clusters using pyUPMASK based on the photometric and astrometric stellar information. We have redetected these 77 objects and they should be real OCs, of which five clusters are potential binary clusters. Their basic parameters are obtained by fitting isochrones to observed CMDs. Our results show that their CMDs display more clean main sequences (MSs) and less member stars lying on the right of lower MSs relative to most of clusters in our initial sample, but broad MSs have not changed significantly. We suggest that efficient clustering algorithm combined with precision data can only partly reduce some stars lying on the right of lower MSs, and a simple stellar population of binary and rotating stars can partly reproduce multiple MS phenomena, including split and broad MSs.

Modeling the High-energy Ionizing Output from Simple Stellar and X-Ray Binary Populations

We present a methodology for modeling the joint ionizing impact due to a “simple X-ray population” (SXP) and its corresponding simple stellar population (SSP), where “simple” refers to a single age and metallicity population. We construct composite spectral energy distributions (SEDs) including contributions from ultraluminous X-ray sources and stars, with physically meaningful and consistent consideration of the relative contributions of each component as a function of instantaneous burst age and stellar metallicity. These composite SEDs are used as input for photoionization modeling with Cloudy, from which we produce a grid for the time- and metallicity-dependent nebular emission from these composite populations. We make the results from the photoionization simulations publicly available. We find that the addition of the SXP prolongs the high-energy ionizing output from the population—and correspondingly increases the intensity of nebular lines such as He ii λ1640,4686, [Ne v] λ3426,14.3 μm, and [O iv] 25.9 μm by factors of at least two relative to models without an SXP spectral component. This effect is most pronounced for instantaneous bursts of star formation on timescales >10 Myr and at low metallicities (∼0.1 Z ⊙), due to the imposed time- and metallicity-dependent behavior of the SXP relative to the SSP. We propose nebular emission line diagnostics accessible with JWST suitable for inferring the presence of a composite SXP + SSP, and we discuss how the ionization signatures compare to models for sources such as intermediate-mass black holes.

Stellar Neutrino Emission across the Mass–Metallicity Plane

We explore neutrino emission from nonrotating, single-star models across six initial metallicities and 70 initial masses from the zero-age main sequence to the final fate. Overall, across the mass spectrum, we find metal-poor stellar models tend to have denser, hotter, and more massive cores with lower envelope opacities, larger surface luminosities, and larger effective temperatures than their metal-rich counterparts. Across the mass–metallicity plane we identify the sequence (initial CNO → 14N → 22Ne → 25Mg → 26Al → 26Mg → 30P → 30Si) as making primary contributions to the neutrino luminosity at different phases of evolution. For the low-mass models we find neutrino emission from the nitrogen flash and thermal pulse phases of evolution depend strongly on the initial metallicity. For the high-mass models, neutrino emission at He-core ignition and He-shell burning depends strongly on the initial metallicity. Antineutrino emission during C, Ne, and O burning shows a strong metallicity dependence with 22Ne(α, n)25Mg providing much of the neutron excess available for inverse-β decays. We integrate the stellar tracks over an initial mass function and time to investigate the neutrino emission from a simple stellar population. We find average neutrino emission from simple stellar populations to be 0.5–1.2 MeV electron neutrinos. Lower metallicity stellar populations produce slightly larger neutrino luminosities and average β decay energies. This study can provide targets for neutrino detectors from individual stars and stellar populations. We provide convenient fitting formulae and open access to the photon and neutrino tracks for more sophisticated population synthesis models.

The elemental abundance of quiescent galaxies in the LEGA-C survey: the (non-)evolution of [α/Fe] from z = 0.75 to z = 0

ABSTRACT We measure the [α/Fe] abundances for 183 quiescent galaxies at z = 0.60−0.75 with stellar masses ranging 10.4 ≤ log10(M*/M⊙) ≤ 11.6 selected from the Large Early Galaxy Astrophysics Census survey. We estimate [α/Fe] from the ratio of the spectral indices Mgb (λ ∼ 5177 Å) and Fe4383, compared to predictions of simple stellar population models. We find that 91 per cent of quiescents in our sample have supersolar [α/Fe], with an average value of [α/Fe] = +0.24 ± 0.01. We find no significant correlation between [α/Fe] and stellar metallicity, mass, velocity dispersion, and average formation time. Galaxies that formed the bulk of their stellar mass on time-scales shorter than 1 Gyr follow the same [α/Fe] distribution as those which formed on longer time-scales. In comparison to local early-type galaxies and to stacked spectra of quiescent galaxies at z = 0.38 and z = 0.07, we find that the average [α/Fe] has not changed between z = 0.75 and the present time. Our work shows that the vast majority of massive quiescent galaxies at z ∼ 0.7 are α-enhanced, and that no detectable evolution of the average [α/Fe] has taken place over the last ∼6.5 Gyr.

The creation of a massive UCD by tidal threshing from NGC 936

ABSTRACT We study a compact nucleus embedded in an early-type dwarf galaxy, MATLAS-167, which is in the process of disruption by the tidal force of the neighbouring giant S0 galaxy, NGC 936, in a group environment. Using the imaging data of the MATLAS survey, we analyse the stellar tidal tail of MATLAS-167 and its central compact nucleus, designated as NGC 936_UCD. We find that NGC 936_UCD has a luminosity of Mg = −11.43 ± 0.01 mag and a size of 66.5 ± 17 pc, sharing the global properties of ultra-compact dwarf galaxies (UCDs) but significantly larger and brighter compared to the typical UCD populations observed in the Virgo cluster. By integrating the total luminosity of both the tidal stream and MATLAS-167, we estimate that the disrupted dwarf progenitor possesses a luminosity of Mg = −15.92 ± 0.06 mag, a typical bright early-type dwarf galaxy luminosity. With the help of the optical spectrum observed by the SDSS survey, we derive the simple stellar population properties of NGC 936_UCD: a light-weighted age of 5.6 ± 0.7 Gyr and metallicity of [Z/H] = −0.83 ± 0.3 dex. Our findings suggest that tidal threshing is a possible formation mechanism of bright UCD populations in close proximity to giant galaxies.

Relation between photometric and parameter errors of star clusters

ABSTRACT Many observations supply only photometry data with large uncertainties. The study of star clusters based on such data is important for astrophysical studies, although photometric uncertainty affects the accuracy of results. In order to estimate the parameter uncertainties that are caused by photometric errors, this work quantifies the influence of photometric error on the uncertainties of some basic parameters when colour-magnitude diagram (CMD) fitting is used to determine these parameters. The observed data are derived from Hubble Space Telescope (HST) observations with the WFPC2 in four bands because of the large coverage of photometric error. The photometric errors that are estimated by Artificial Star Test (AST) are taken, and a binary star stellar population synthesis model and the Powerful CMD code are adopted to determine the parameters of star clusters. Two popular types of stellar population models, i.e. binary star simple stellar population (bsSSP) and single star simple stellar population (ssSSP), are used. The effects of photometric errors on the uncertainties of distance modulus (m − M), colour excess (E), metallicity (Z), and age (t), i.e. Δm − M, ΔE, ΔZ, and Δt, are studied via 19 globular clusters. The results show that Δm − M, ΔE, ΔZ, and Δt have positive correlations with photometric error. Some linear fitting formulae are given to make the results easy to use. The results can be used to estimate the errors of parameters that are caused by photometric error when determining the parameters by CMD fitting, for star clusters with relatively large photometric uncertainties (>∼0.01 mag).

The Chemodynamics of the Stellar Populations in M31 from APOGEE Integrated-light Spectroscopy

We present an analysis of nearly 1000 near-infrared, integrated-light spectra from APOGEE in the inner ∼7 kpc of M31. We utilize full-spectrum fitting with A-LIST simple stellar population spectral templates that represent a population of stars with the same age, [M/H], and [α/M]. With this, we determine the mean kinematics, metallicities, α abundances, and ages of the stellar populations of M31's bar, bulge, and inner disk (∼4–7 kpc). We find a nonaxisymmetric velocity field in M31 resulting from the presence of a bar. The bulge of M31 is less metal-rich (mean [M/H] = dex) than the disk, features minima in metallicity on either side of the bar ([M/H] ∼ −0.2), and is enhanced in α abundance (mean [α/M] = ). The disk of M31 within ∼7 kpc is enhanced in both metallicity ([M/H] = ) and α abundance ([α/M] = ). Both of these structural components are uniformly old at ≃12 Gyr. We find the mean metallicity increases with distance from the center of M31, with the steepest gradient along the disk major axis (0.043 ± 0.021 dex kpc−1). This gradient is the result of changing light contributions from the bulge and disk. The chemodynamics of stellar populations encodes information about a galaxy’s chemical enrichment, star formation history, and merger history, allowing us to discuss new constraints on M31's formation. Our results provide a stepping stone between our understanding of the Milky Way and other external galaxies.

The Absolute Age of M92

The absolute age of a simple stellar population is of fundamental interest for a wide range of applications but is difficult to measure in practice, as it requires an understanding of the uncertainties in a variety of stellar evolution processes as well as the uncertainty in the distance, reddening, and composition. As a result, most studies focus only on the relative age by assuming that stellar evolution calculations are accurate and using age determinations techniques that are relatively independent of distance and reddening. Here, we construct 20,000 sets of theoretical isochrones through Monte Carlo simulation using the Dartmouth Stellar Evolution Program to measure the absolute age of the globular cluster M92. For each model, we vary a range of input physics used in the stellar evolution models, including opacities, nuclear reaction rates, diffusion coefficients, atmospheric boundary conditions, helium abundance, and treatment of convection. We also explore variations in the distance and reddening as well as its overall metallicity and α enhancement. We generate simulated Hess diagrams around the main-sequence turn-off region from each set of isochrones and use a Voronoi binning method to fit the diagrams to Hubble Space Telescope Advanced Camera for Surveys data. We find the age of M92 to be 13.80 ± 0.75 Gyr. The 5.4% error in the absolute age is dominated by the uncertainty in the distance to M92 (∼80% of the error budget); of the remaining parameters, only the total metallicity, α element abundance, and treatment of helium diffusion contribute significantly to the total error.

Extreme broad He ii emission at high and low redshifts: the dominant role of VMS in NGC 3125-A1 and CDFS131717

ABSTRACT Super star cluster (SSC) A1 ($3.1\times 10^5\, \mathrm{M}_\odot$) in NGC 3125 has one of the strongest ($EW=4.6\pm 0.5\,$ Å) broad ($FWHM=1131\pm 40\,$ km s−1) He ii λ1640 emission lines in the nearby universe and constitutes an important template for interpreting observations of extreme He ii emitters out to redshifts of z ∼ 2 – 3. We use Cosmic Origins Spectrograph (COS) observations of A1 to show that there is no significant contamination of the He ii line with nebular emission and that the line is redshifted by 121 ± 17 km s−1 relative to ISM lines. We compare the COS G130M + G160M observations of A1 to recent binary BPASS and single-star Charlot & Bruzual (C&B) simple stellar population (SSP) models with very massive stars (VMS) of up to $300\, \mathrm{M}_\odot$. On the other hand, we suggest why BPASS models fail to reproduce A1’s He ii emission. On the other hand, a C&B model with Z = 0.008, age = 2.2 Myr, and VMS approaching the Eddington limit provides an excellent fit to the He ii emission and fits reasonably well C iii λ1175, N v λλ1238, 1241, and C iv λλ1548, 1551. We present O v λ1371 line-profile predictions showing that this line constitutes an important tracer of youth and VMS in galaxies. Finally, we discuss the presence of VMS in CDFS131717, a highly star-forming low-metallicity galaxy located at z = 3.071, which has a tentative detection of O v absorption and strong broad He ii emission. These features are rare and hint to the presence of short-lived VMS in the galaxy. Our results show the effect of the latest developments of stellar wind theory and the importance of accounting for VMS in models.

Search strategies for supermassive stars in young clusters and application to nearby galaxies

Context. Supermassive stars (SMSs) with masses M ≳ 103 − 104 M⊙ formed by runaway collisions in young, massive, and dense star clusters have been invoked as a possible solution to the problem of the presence of multiple stellar populations and peculiar abundance patterns observed in globular clusters (GCs). However, no such objects have been observed so far. Aims. We aim to develop observational strategies to search for SMSs hosted within young massive clusters (thought to be the precursors of GCs) using both photometric and spectroscopic observations. Such strategies could be applicable in a relatively general fashion. Methods. We used theoretical predictions of the spectra of SMSs and SMS-hosting clusters, together with predictions from standard simple stellar populations to examine their impact on color–color diagrams and on individual optical spectral lines (primarily hydrogen emission and absorption lines). As a first step, we applied our search strategies to a sample of about 3000 young star clusters (YSCs) from two nearby galaxies with multiband observations from the HST and optical integral-field spectroscopy obtained with MUSE on the Very Large Telescope. Results. We focus on models for SMSs with large radii (corresponding to Teff ≲ 7000 K), which predict strong Balmer breaks, and construct proper color–color diagrams to select the corresponding SMS-hosting cluster candidates. We show that the spectrophotometric properties of these latter are similar to those of normal clusters with ages of a few hundred million years. However, the cluster SEDs show signs of composite stellar populations due to the presence of nebular lines (Hα and others). Examining the photometry, overall SEDs, and the spectra of approximately 100 clusters with strong Balmer breaks, we find several objects with peculiar SEDs, the presence of emission lines, or other peculiar signatures. After careful inspection of the available data, we do not find good candidates of SMS-hosting clusters. In most cases, the composite spectra can be explained by multiple clusters or H II regions inside the aperture covered by the spectra, by contamination from a planetary nebula or diffuse gas, or by improper background subtraction. Furthermore, most of our candidate clusters are too faint to host SMSs. Conclusions. We demonstrate a strategy to search for SMSs by applying it to a sample of YSCs in two nearby galaxies. Our method can be applied to larger samples and also extended to higher redshifts with existing and upcoming telescopes, and therefore should provide an important test for GC-formation scenarios invoking such extreme stars.

Digging deeper into NGC 6868 I: Stellar population

ABSTRACT We use Gemini integral field unit observations to map the stellar population properties in the inner region (∼680 × 470 pc2) of the galaxy NGC 6868. In order to understand the physical and chemical properties of the stellar content of this galaxy, we performed stellar population synthesis using the starlight code with the MILES simple stellar population models. We measured the absorption line indices Fe4383, Mg2, Mgb, Fe5270, and Fe5335 for the whole FoV, and used them to derive Fe3 and [MgFe]’. These indices were used to derive [α/Fe]. This galaxy is dominated by old metal-rich populations (12.6 Gyr; 1.0 and 1.6 Z⊙) with a negative metallicity gradient. We also found a recent (∼63 Myr) metal-rich (1.6 Z⊙) residual star formation in the centre of the galaxy. A dust lane with a peak extinction in the V band of 0.65 mag is seen. No signs of ordered stellar motion are found and the stellar kinematics is dispersion dominated. All indices show a spatial profile varying significantly along the FoV. Mg2 shows a shallow gradient, compatible with the occurrence of mergers in the past. Mgb and Fe3 profiles suggest different enrichment processes for these elements. We observe three distinct regions: for R< 100 pc and R > 220 pc, Mg2, Mgb anticorrelate with respect to Fe3 and [MgFe]’, and for 100 pc <R< 220 pc, they correlate, hinting at different enrichment histories. The [α/Fe] profile is really complex and has a central value of ∼0.2 dex. We interpret this as the result of a past merger with another galaxy with a different [α/Fe] history, thus explaining the [α/Fe] maps.