Simple Stellar Population Models Research Articles

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 (AGN) 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). AGN 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 anti-correlation 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 ≲4Gyr). 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.

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.

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.

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.

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.

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.

The large-scale structure of globular clusters in the NGC 1052 group

ABSTRACT Prompted by the many controversial claims involving the NGC 1052 group, including that it hosts two dark matter-free galaxies with overluminous and monochromatic globular cluster (GC) systems, here, we map out the large-scale structure (LSS) of GCs over the entire group. To recover the LSS, we use archival optical CFHT imaging data. We recover two GC density maps, one based on universal photometric properties of GCs from simple stellar population models, and one based on the properties of spectroscopically confirmed GCs in DF2 and DF4 (the two dwarf galaxies with overluminous GC populations). Both selection methods reveal overdensities around the massive galaxies in the group, as well as around NGC 1052 itself, that are coincident with the positions of previously identified stellar streams and tidal features. No intragroup GCs are found connecting these structures to any of the dwarf galaxies. We find, however, two other dwarfs in the group hosting GC systems. These include RCP32 with two GCs with ages equivalent to the GCs around NGC 1052 and DF9 with three GCs with ages similar to the GCs around DF2 and DF4. We conclude that the GC distribution in the group does not strongly support any formation scenario in particular. It favours, none the less, scenarios relying on galaxy–galaxy interactions and on the coeval formation of GCs around the DM-free dwarf galaxies. These may include the recently proposed bullet-dwarf formation, as well as high-redshift tidal dwarf galaxy models.

Investigation of the connection between X-ray binaries and compact star clusters in NGC 628

ABSTRACTWe investigated for a possible connection between the types of X-ray binaries (XRBs) and the properties of compact star clusters in the nearby galaxy NGC 628. Using Chandra archival data covering the years 2001–2018, 75 X-ray sources were detected within the field of view of observations. A total of 69 XRBs, one of which is an ultraluminous X-ray source (ULX), were found to be in the luminosity range of 3 × 1036 ≤ LX ≤ 2 × 1039 erg s−1. We identified the optical counterpart(s) of 15 of the 42 XRBs that coincide with the HST field of view via improved astrometry. We classified 15 of them as HMXBs based on the presence of the optical counterparts. The remaining sources with no optical counterparts were classified as LMXBs. We also search compact star clusters in this galaxy using the multiband optical images drawn from HST archives. 864 compact star clusters were identified and their ages and masses were determined by applying the best-fit SSP (Simple Stellar Population) model to their colour–colour diagram. We found that in NGC 628, HMXBs are associated with younger star clusters and LMXBs with older ones. Our findings support a connection between different types of XRBs and cluster ages, already known to exist for other galaxies.

What Are Those Tiny Things? A First Study of Compact Star Clusters in the SMACS0723 Field with JWST

We use the unprecedented resolution and depth of the JWST NIRCam Early Release Observations at 1–5 μm to study the stellar mass, age, and metallicity of compact star clusters in the neighborhood of the host galaxies in the SMACS J0723.3–7327 galaxy cluster field at z = 0.39. The measured colors of these star clusters show a similar distribution as quiescent galaxies at the same redshift, but are >3 mag fainter than the current depths of wide-field galaxy survey. The star clusters are unresolved in the NIRCam/F150W data suggesting sizes smaller than 50 pc. This is significantly smaller than star-forming clumps or dwarf galaxies in local galaxies. From fitting their photometry with simple stellar population (SSP) models, we find stellar metallicities consistent with 0.2–0.3 Z ⊙ and ages of . We rule out metallicities <0.2 Z ⊙ and solar/supersolar at 4σ significance. Assuming mass-to-light ratios obtained from the best-fit SSP, we estimate stellar masses of . These are between average masses of local globular clusters and dwarf galaxies. Our analysis suggests middle-aged globular cluster with relatively recent formation times at z = 0.5–0.7, which could have been subsequently stripped away from their host galaxies due to interactions in the cluster environment or formed in cold flows onto the cluster core. However, we cannot rule out these objects being compact cores of stripped dwarf galaxies.

Stellar population analysis of MaNGA early-type galaxies: IMF dependence and systematic effects

ABSTRACT We study systematics associated with estimating simple stellar population (SSP) parameters – age, metallicity [M/H], α-enhancement [α/Fe], and initial mass function (IMF) shape – and associated M*/L gradients, of elliptical slow rotators (E-SRs), fast rotators (E-FRs), and S0s from stacked spectra of galaxies in the MaNGA survey. These systematics arise from (i) how one normalizes the spectra when stacking; (ii) having to subtract emission before estimating absorption line strengths; (iii) the decision to fit the whole spectrum or just a few absorption lines; (iv) SSP model differences (e.g. isochrones, enrichment, IMF). The MILES+Padova SSP models, fit to the Hβ, 〈Fe〉, TiO2SDSS, and [MgFe] Lick indices in the stacks, indicate that out to the half-light radius Re: (a) ages are younger and [α/Fe] values are lower in the central regions but the opposite is true of [M/H]; (b) the IMF is more bottom-heavy in the center, but is close to Kroupa beyond about Re/2; (c) this makes M*/L about 2 × larger in the central regions than beyond Re/2. While the models of Conroy et al. return similar [M/H] and [α/Fe] profiles, the age and (hence) M*/L profiles can differ significantly even for solar abundances and a Kroupa IMF; different responses to non-solar abundances and IMF parametrization further compound these differences. There are clear (model independent) differences between E-SRs, E-FRs, and S0s: younger ages and less enhanced [α/Fe] values suggest that E-FRs and S0s are not SSPs, but relaxing this assumption is unlikely to change their inferred M*/L gradients significantly.

The Low-Redshift Lyman Continuum Survey

Aims. Combining 66 ultraviolet (UV) spectra and ancillary data from the recent Low-Redshift Lyman Continuum Survey (LzLCS) and 23 LyC observations by earlier studies, we form a statistical sample of star-forming galaxies at z ∼ 0.2 − 0.4 with which we study the role of cold interstellar medium (ISM) gas in the leakage of ionizing radiation. We also aim to establish empirical relations between the H I neutral and low-ionization state (LIS) absorption lines with different galaxy properties. Methods. We first constrain the massive star content (stellar ages and metallicities) and UV attenuation by fitting the stellar continuum with a combination of simple stellar population models. The models, together with accurate LyC flux measurements, allow us to determine the absolute LyC photon escape fraction for each galaxy (fescabs). We then measure the equivalent widths and residual fluxes of multiple H I and LIS lines, and the geometrical covering fraction of the UV emission, adopting the picket-fence model. Results. The LyC escape fraction spans a wide range, with a median fescabs (0.16, 0.84 quantiles) of 0.04 (0.02, 0.20), and 50 out of the 89 galaxies detected in the LyC (1σ upper limits of fescabs ≲ 0.01 for non-detections, typically). The H I and LIS line equivalent widths scale with the UV luminosity and attenuation, and inversely with the residual flux of these lines. Additionally, Lyα equivalent widths scale with both the H I and LIS residual fluxes, but anti-correlate with the corresponding H I or LIS equivalent widths. The H I and LIS residual fluxes are correlated, indicating that the neutral gas is spatially traced by the low-ionization transitions. We find that the observed trends of the absorption lines and the UV attenuation are primarily driven by the geometric covering fraction of the gas. The observed nonuniform gas coverage also demonstrates that LyC photons escape through low-column-density channels in the ISM. The equivalent widths and residual fluxes of both the H I and LIS lines strongly correlate with fescabs: strong LyC leakers (highest fescabs) show weak absorption lines, low UV attenuation, and large Lyα equivalent widths. We provide several empirical calibrations to estimate fescabs from UV absorption lines. Finally, we show that simultaneous UV absorption line and dust attenuation measurements can, in general, predict the escape fraction of galaxies. We apply our method to available measurements of UV LIS lines of 15 star-forming galaxies at z ∼ 4 − 6 (plus 3 high-z galaxy composites), finding that these high-redshift, UV-bright galaxies (MUV ≲ −21) may have low escape fractions, fescabs ≲ 0.1. Conclusions. UV absorption lines trace the cold ISM gas of galaxies, which governs the physics of the LyC escape. We show that, with some assumptions, the absolute LyC escape can be statistically predicted using UV absorption lines, and the method can be applied to study galaxies across a wide redshift range, including in the epoch of cosmic reionization.

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 near-ultraviolet (NUV) to near-infrared (NIR) wavelengths. The unmatched characteristics of the relatively high resolution and extended wavelength coverage (350–2480 nm, R ∼ 10 000) of the XSL population models bring us closer to bridging optical and NIR studies of intermediate-age 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 older populations. We therefore construct sequences of the average spectra of static giants, variable O-rich giants, and C-rich giants to be included in the models separately. The models span the metallicity range −2.2 &lt; [Fe/H] &lt; +0.2 and ages above 50 Myr, a broader range in the NIR than in other models based on empirical spectral libraries. We focus on the behaviour of colours and absorption-line indices as a function of age and metallicity. 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. Furthermore, 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.

Inferring the helium abundance of extragalactic globular clusters using integrated spectra

ABSTRACT The leading method for the determination of relevant stellar population parameters of unresolved extragalactic Globular Clusters is through the study of their integrated spectroscopy, where Balmer line-strength indices are considered to be age sensitive. Previously, a splitting in the highly optimized spectral line-strength index H βo was observed in a sample of Galactic globular clusters at all metallicities resulting in an apparent ‘upper branch’ and ‘lower branch’ of globular clusters in the H βo–[MgFe] diagram. This was suggested to be caused by the presence of hot Blue straggler stars (BSSs), resulting in an underestimation of ‘spectroscopic’ ages in the upper branch. Over a decade on, we look to re-evaluate these findings. We make use of new, large Galactic Globular Cluster integrated spectroscopy data sets. To produce a large, homogeneously combined sample we have considered a number of factors including the radial dependence of Balmer and metal lines. Using this new sample, in disagreement with previous work, we find the splitting in H βo only occurs at intermediate to high metallicities ([M/H] &amp;gt; −1), and is not the result of an increased fraction of BSSs, but rather is due to an increased helium abundance. We explore the possible impact of varying helium on simple stellar population models to provide a theoretical basis for our hypothesis and then use the relationship between upper branch candidacy and enhanced helium to predict the helium content of three M31 clusters. We discuss what this can tell us about their mass and fraction of first generation stars.

Calibration of Surface Brightness Fluctuations for Dwarf Galaxies in the Hyper Suprime-Cam gi Filter System

Abstract Surface brightness fluctuation (SBF) magnitudes are a powerful standard candle to measure distances to semiresolved galaxies in the local universe, a majority of which are dwarf galaxies that often have bluer colors than bright early-type galaxies. We present an empirical i-band SBF calibration in a blue regime, 0.2 ≲ (g − i)0 ≲ 0.8 in the Hyper Suprime-Cam (HSC) magnitude system. We measure SBF magnitudes for 12 nearby dwarf galaxies of various morphological types with archival HSC imaging data, and use their tip of the red giant branch distances to derive fluctuation–color relations. In order to subtract contributions of fluctuations due to young stellar populations, we use five different g-band magnitude masking thresholds, M g,thres = −3.5, −4.0, −4.5, −5.0, and −5.5 mag. We find that the rms scatter of the linear fit to the relation is the smallest (rms = 0.16 mag) in the case of M g,thres = −4.0 mag, M ¯ i = (−2.65 ± 0.13) + (1.28 ± 0.24) × (g − i)0. This scatter is much smaller than those in the previous studies (rms = 0.26 mag), and is closer to the value for bright red galaxies (rms = 0.12 mag). This calibration is consistent with predictions from metal-poor simple stellar population models.

Nonlinear Color–Metallicity Relations of Globular Clusters. X. Subaru/FOCAS Multiobject Spectroscopy of M87 Globular Clusters

Abstract We obtained spectra of some 140 globular clusters (GCs) associated with the Virgo central cD galaxy M87 with the Subaru/FOCAS MOS mode. The fundamental properties of GCs such as age, metallicity, and α-element abundance are investigated by using simple stellar population models. It is confirmed that the majority of M87 GCs are as old as, more metal-rich than, and more enhanced in α-elements than the Milky Way GCs. Our high-quality, homogeneous data set enables us to test the theoretical prediction of inflected color–metallicity relations (CMRs). The nonlinear-CMR hypothesis entails an alternative explanation for the widely observed GC color bimodality, in which even a unimodal metallicity spread yields a bimodal color distribution by virtue of nonlinear metallicity-to-color conversion. The newly derived CMRs of old, high-signal-to-noise-ratio GCs in M87 (the V − I CMR of 83 GCs and the M−T2 CMR of 78 GCs) corroborate the presence of the significant inflection. Furthermore, from a combined catalog with the previous study on M87 GC spectroscopy, we find that a total of 185 old GCs exhibit a broad, unimodal metallicity distribution. The results corroborate the nonlinear-CMR interpretation of the GC color bimodality, shedding further light on theories of galaxy formation.

Cloud Computing Bayesian Approach to Study Effects of Binary Stars on Ages of Star Clusters

We apply bayesian algorithms to studying the effects of binary stellar population models on the age determination of 46 new star clusters in Gaia DR2. The ages of star clusters are obtained via color-magnitude diagram (CMDs). The data are selected from two works. Our analysis focus on exploring the potential of bayesian algorithm to estimate the effects of binary stars on the parameter determination of star clusters. Our work is based on simple stellar population models of binary stars and single stars. As a result, the bayesian analysis shows that binary stars have significant effects on the determination of ages of star clusters. The models with binary stars report older ages for most star clusters. The age difference between binary-star models and single-star models follows a Gaussian distribution with mean of 1.2 Gyr.

Dynamical masses and mass-to-light ratios of resolved massive star clusters – II. Results for 26 star clusters in the Magellanic Clouds

ABSTRACT We present spectroscopy of individual stars in 26 Magellanic Cloud (MC) star clusters with the aim of estimating dynamical masses and V-band mass-to-light (M/LV) ratios over a wide range in age and metallicity. We obtained 3137 high-resolution stellar spectra with M2FS on the Magellan/Clay Telescope. Combined with 239 published spectroscopic results of comparable quality, we produced a final sample of 2787 stars with good quality spectra for kinematic analysis in the target clusters. Line-of-sight velocities measured from these spectra and stellar positions within each cluster were used in a customized expectation-maximization (EM) technique to estimate cluster membership probabilities. Using appropriate cluster structural parameters and corresponding single-mass dynamical models, this technique ultimately provides self-consistent total mass and M/LV estimates for each cluster. Mean metallicities for the clusters were also obtained and tied to a scale based on calcium IR triplet metallicities. We present trends of the cluster M/LV values with cluster age, mass, and metallicity, and find that our results run about 40 per cent on average lower than the predictions of a set of simple stellar population (SSP) models. Modified SSP models that account for internal and external dynamical effects greatly improve agreement with our results, as can models that adopt a strongly bottom-light IMF. To the extent that dynamical evolution must occur, a modified IMF is not required to match data and models. In contrast, a bottom-heavy IMF is ruled out for our cluster sample as this would lead to higher predicted M/LV values, significantly increasing the discrepancy with our observations.

On the precision of full-spectrum fitting of simple stellar populations – II. The dependence on star cluster mass in the wavelength range 0.3–5.0 µm

ABSTRACT In this second paper of a series on the accuracy and precision of the determination of age and metallicity of simple stellar populations (SSPs) by means of the full-spectrum fitting technique, we study the influence of star cluster mass through stochastic fluctuations of the number of stars near the top of the stellar mass function, which dominate the flux in certain wavelength regimes depending on the age. We consider SSP models based on the Padova isochrones, spanning the age range $7.0 \le \mbox{log(age/yr}) \le 10.1$. Simulated spectra of star clusters in the mass range 104 ≤ M/M⊙ &amp;lt; 106 are compared with SSP model spectra to determine best-fitting ages and metallicities using a full-spectrum fitting routine in four wavelength regimes: the blue optical (0.35–0.70 µm), the red optical (0.6–1.0 µm), the near-infrared (near-IR; 1.0–2.5 µm), and the mid-IR (2.5–5.0 µm). We compare the power of each wavelength regime in terms of both the overall precision of age and metallicity determination and its dependence on cluster mass. We also study the relevance of spectral resolution in this context by utilizing two different spectral libraries (BaSeL and BT-Settl). We highlight the power of the mid-IR regime in terms of identifying young massive clusters in dusty star-forming regions in distant galaxies. The spectra of the simulated star clusters and SSPs are made available online to enable follow-up studies by the community.

MEGARA-IFU detection of extended He ii λ4686 nebular emission in the central region of NGC 1569 and its ionization budget

ABSTRACTWe here report the detection of extended He ii λ4686 nebular emission in the central region of NGC 1569 using the integral field spectrograph MEGARA at the 10.4 m Gran Telescopio Canarias. The observations cover a field of view (FoV) of 12.5 arcsec × 11.3 arcsec at a seeing-limited spatial resolution of ∼15 pc and at a spectral resolution of R = 6000 in the wavelength range 4330–5200 Å. The emission extends over a semicircular arc of ∼40 pc width and ∼150 pc diameter around the superstar cluster A (SSC-A). The AV derived using Balmer decrement varies from the Galactic value of 1.6 mag to a maximum of ∼4.5 mag, with a mean value of 2.65 ± 0.60 mag. We infer 124 ± 11 Wolf–Rayet (WR) stars in SSC-A using the He ii λ4686 broad feature and AV = 2.3 mag. The He+ ionizing photon rate from these WR stars is sufficient to explain the luminosity of the He ii nebula. The observationally determined total He+ and H0 ionizing photon rates, their ratio, and the observed number of WR stars in SSC-A are all consistent with the predictions of simple stellar population models at an age of 4.0 ± 0.5 Myr and a mass of (5.5 ± 0.5) × 105 M⊙. Our observations reinforce the absence of WR stars in SSC-B, the second most massive cluster in the FoV. None of the other locations in our FoV where He ii λ4686 emission has been reported from narrow-band imaging observations contain WR stars.