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- ItemScaling relations of z ∼ 0.25–1.5 galaxies in various environments from the morpho-kinematics analysis of the MAGIC sample(Les Ulis : EDP Sciences, 2022) Mercier, W.; Epinat, B.; Contini, T.; Abril-Melgarejo, V.; Boogaard, L.; Brinchmann, J.; Finley, H.; Krajnović, D.; Michel-Dansac, L.; Ventou, E.; Bouché, N.; Dumoulin, J.; Pineda, J.C.B.Context. The evolution of galaxies is influenced by many physical processes, which may vary depending on their environment. Aims. We combine Hubble Space Telescope (HST) and Multi-Unit Spectroscopic Explorer (MUSE) data of galaxies at 0.25-1.5 to probe the impact of environment on the size-mass relation, the main sequence (MS) relation, and the Tully-Fisher relation (TFR). Methods. We perform a morpho-kinematics modelling of 593 [O-II] emitters in various environments in the COSMOS area from the MUSE-gAlaxy Groups In Cosmos survey. The HST F814W images are modelled with a bulge-disk decomposition to estimate their bulge-disk ratio, effective radius, and disk inclination. We use the [O-II]λλ3727, 3729 doublet to extract the galaxies ionised gas kinematics maps from the MUSE cubes, and we model those maps for a sample of 146 [O-II] emitters, including bulge and disk components constrained from morphology and a dark matter halo. Results. We find an offset of 0.03 dex (1 significant) on the size-mass relation zero point between the field and the large structure sub-samples, with a richness threshold of N=10 to separate between small and large structures, and of 0.06 dex (2) with N=20. Similarly, we find a 0.1 dex (2A) difference on the MS relation with N=10 and 0.15 dex (3) with N=20. These results suggest that galaxies in massive structures are smaller by 14% and have star formation rates reduced by a factor of 1.31.5 with respect to field galaxies at z 0.7. Finally, we do not find any impact of the environment on the TFR, except when using N=20 with an offset of 0.04 dex (1). We discard the effect of quenching for the largest structures, which would lead to an offset in the opposite direction. We find that, at z0.7, if quenching impacts the mass budget of galaxies in structures, these galaxies would have been affected quite recently and for roughly 0.7-+1.5 Gyr. This result holds when including the gas mass but vanishes once we include the asymmetric drift correction.
- ItemNew criteria for the selection of galaxy close pairs from cosmological simulations: Evolution of the major and minor merger fraction in MUSE deep fields(Les Ulis : EDP Sciences, 2019) Ventou, E.; Contini, T.; Bouché, N.; Epinat, B.; Brinchmann, J.; Inami, H.; Richard, J.; Schroetter, I.; Soucail, G.; Steinmetz, M.; Weilbacher, P.M.It remains a challenge to assess the merger fraction of galaxies at different cosmic epochs in order to probe the evolution of their mass assembly. Using the Illustris cosmological simulation project, we investigate the relation between the separation of galaxies in a pair, both in velocity and projected spatial separation space, and the probability that these interacting galaxies will merge in the future. From this analysis, we propose a new set of criteria to select close pairs of galaxies along with a new corrective term to be applied to the computation of the galaxy merger fraction. We then probe the evolution of the major and minor merger fraction using the latest Multi-Unit Spectroscopic Explorer (MUSE) deep observations over the Hubble Ultra Deep Field, Hubble Deep Field South, COSMOS-Gr30, and Abell 2744 regions. From a parent sample of 2483 galaxies with spectroscopic redshifts, we identify 366 close pairs spread over a large range of redshifts (0:2 < z < 6) and stellar masses (107-1011 M ). Using the stellar mass ratio between the secondary and primary galaxy as a proxy to split the sample into major, minor, and very minor mergers, we found a total of 183 major, 142 minor, and 47 very minor close pairs corresponding to a mass ratio range of 1:1-1:6, 1:6-1:100, and lower than 1:100, respectively. Due to completeness issues, we do not consider the very minor pairs in the analysis. Overall, the major merger fraction increases up to z ≈2-3 reaching 25% for pairs where the most massive galaxy has a stellar mass M· = 109:5 M . Beyond this redshift, the fraction decreases down to ∼5% at z≈6. The major merger fraction for lower-mass primary galaxies with M· = 109:5 M seems to follow a more constant evolutionary trend with redshift. Thanks to the addition of new MUSE fields and new selection criteria, the increased statistics of the pair samples allow us to significantly shorten the error bars compared to our previous analysis. The evolution of the minor merger fraction is roughly constant with cosmic time, with a fraction of 20% at z < 3 and a slow decrease to 8-13% in the redshift range 3 ≤ z ≤ 6.
- ItemAn atlas of MUSE observations towards twelve massive lensing clusters(Les Ulis : EDP Sciences, 2021) Richard, Johan; Claeyssens, Adélaïde; Lagattuta, David; Guaita, Lucia; Bauer, Franz Erik; Pello, Roser; Carton, David; Bacon, Roland; Soucail, Geneviève; Lyon, Gonzalo Prieto; Kneib, Jean-Paul; Mahler, Guillaume; Clément, Benjamin; Mercier, Wilfried; Variu, Andrei; Tamone, Amélie; Ebeling, Harald; Schmidt, Kasper B.; Nanayakkara, Themiya; Maseda, Michael; Weilbacher, Peter M.; Bouché, Nicolas; Bouwens, Rychard J.; Wisotzki, Lutz; de la Vieuville, Geoffroy; Martinez, Johany; Patrício, VeraContext. Spectroscopic surveys of massive galaxy clusters reveal the properties of faint background galaxies thanks to the magnification provided by strong gravitational lensing. Aims. We present a systematic analysis of integral-field-spectroscopy observations of 12 massive clusters, conducted with the Multi Unit Spectroscopic Explorer (MUSE). All data were taken under very good seeing conditions (~0".6) in effective exposure times between two and 15 h per pointing, for a total of 125 h. Our observations cover a total solid angle of ~23 arcmin2 in the direction of clusters, many of which were previously studied by the MAssive Clusters Survey, Frontier Fields (FFs), Grism Lens-Amplified Survey from Space and Cluster Lensing And Supernova survey with Hubble programmes. The achieved emission line detection limit at 5? for a point source varies between (0.77-1.5) × 10-18 erg s-1 cm-2 at 7000 Å. Methods. We present our developed strategy to reduce these observational data, detect continuum sources and line emitters in the datacubes, and determine their redshifts. We constructed robust mass models for each cluster to further confirm our redshift measurements using strong-lensing constraints, and identified a total of 312 strongly lensed sources producing 939 multiple images. Results. The final redshift catalogues contain more than 3300 robust redshifts, of which 40% are for cluster members and ∼30% are for lensed Lyman-α emitters. Fourteen percent of all sources are line emitters that are not seen in the available HST images, even at the depth of the FFs (∼29 AB). We find that the magnification distribution of the lensed sources in the high-magnification regime (μ = 2–25) follows the theoretical expectation of N(z) ∝ μ−2. The quality of this dataset, number of lensed sources, and number of strong-lensing constraints enables detailed studies of the physical properties of both the lensing cluster and the background galaxies. The full data products from this work, including the datacubes, catalogues, extracted spectra, ancillary images, and mass models, are made available to the community.
- ItemThe MUSE Extremely Deep Field: The cosmic web in emission at high redshift(Les Ulis : EDP Sciences, 2021) Bacon, Roland; Mary, David; Garel, Thibault; Blaizot, Jeremy; Maseda, Michael; Schaye, Joop; Wisotzki, Lutz; Conseil, Simon; Brinchmann, Jarle; Leclercq, Floriane; Abril-Melgarejo, Valentina; Boogaard, Leindert; Bouché, Nicolas; Contini, Thierry; Feltre, Anna; Guiderdoni, Bruno; Herenz, Christian; Kollatschny, Wolfram; Kusakabe, Haruka; Matthee, Jorryt; Michel-Dansac, Léo; Nanayakkara, Themiya; Richard, Johan; Roth, Martin; Schmidt, Kasper B.; Steinmetz, Matthias; Tresse, Laurence; Urrutia, Tanya; Verhamme, Anne; Weilbacher, Peter M.; Zabl, Johannes; Zoutendijk, Sebastiaan L.We report the discovery of diffuse extended Lyα emission from redshift 3.1 to 4.5, tracing cosmic web filaments on scales of 2.5-4 cMpc. These structures have been observed in overdensities of Lyα emitters in the MUSE Extremely Deep Field, a 140 h deep MUSE observation located in the Hubble Ultra-Deep Field. Among the 22 overdense regions identified, five are likely to harbor very extended Lyα emission at high significance with an average surface brightness of 5 × 10-20 erg s-1 cm-2 arcsec-2. Remarkably, 70% of the total Lyα luminosity from these filaments comes from beyond the circumgalactic medium of any identified Lyα emitter. Fluorescent Lyα emission powered by the cosmic UV background can only account for less than 34% of this emission at z ≈ 3 and for not more than 10% at higher redshift. We find that the bulk of this diffuse emission can be reproduced by the unresolved Lyα emission of a large population of ultra low-luminosity Lyα emitters (< 1040 erg s-1), provided that the faint end of the Lyα luminosity function is steep (α ⪅ -1.8), it extends down to luminosities lower than 1038 - 1037 erg s-1, and the clustering of these Lyα emitters is significant (filling factor < 1/6). If these Lyα emitters are powered by star formation, then this implies their luminosity function needs to extend down to star formation rates < 10-4M yr-1. These observations provide the first detection of the cosmic web in Lyα emission in typical filamentary environments and the first observational clue indicating the existence of a large population of ultra low-luminosity Lyα emitters at high redshift. © R. Bacon et al. 2021.
- ItemFaint end of the z ∼ 3-7 luminosity function of Lyman-alpha emitters behind lensing clusters observed with MUSE(Les Ulis : EDP Sciences, 2019) de La Vieuville, G.; Bina, D.; Pello, R.; Mahler, G.; Richard, J.; Drake, A.B.; Herenz, E. C.; Bauer, F.E.; Clément, B.; Lagattuta, D.; Laporte, N.; Martinez, J.; Patrício, V.; Wisotzki, L.; Zabl, J.; Bouwens, R.J.; Contini, T.; Garel, T.; Guiderdoni, B.; Marino, R.A.; Maseda, M.V.; Matthee, J.; Schaye, J.; Soucail, G.Contact. This paper presents the results obtained with the Multi-Unit Spectroscopic Explorer (MUSE) at the ESO Very Large Telescope on the faint end of the Lyman-alpha luminosity function (LF) based on deep observations of four lensing clusters. The goal of our project is to set strong constraints on the relative contribution of the Lyman-alpha emitter (LAE) population to cosmic reionization. Aims. The precise aim of the present study is to further constrain the abundance of LAEs by taking advantage of the magnification provided by lensing clusters to build a blindly selected sample of galaxies which is less biased than current blank field samples in redshift and luminosity. By construction, this sample of LAEs is complementary to those built from deep blank fields, whether observed by MUSE or by other facilities, and makes it possible to determine the shape of the LF at fainter levels, as well as its evolution with redshift. Methods. We selected a sample of 156 LAEs with redshifts between 2.9 ≤ z ≤ 6.7 and magnification-corrected luminosities in the range 39 ≳ log LLyα [erg s-1] ≲ 43. To properly take into account the individual differences in detection conditions between the LAEs when computing the LF, including lensing configurations, and spatial and spectral morphologies, the non-parametric 1/Vmax method was adopted. The price to pay to benefit from magnification is a reduction of the effective volume of the survey, together with a more complex analysis procedure to properly determine the effective volume Vmax for each galaxy. In this paper we present a complete procedure for the determination of the LF based on IFU detections in lensing clusters. This procedure, including some new methods for masking, effective volume integration and (individual) completeness determinations, has been fully automated when possible, and it can be easily generalized to the analysis of IFU observations in blank fields. Results. As a result of this analysis, the Lyman-alpha LF has been obtained in four different redshift bins: 2.9 < z < 6, 7, 2.9 < z < 4.0, 4.0 < z < 5.0; and 5.0 < z < 6.7 with constraints down to log LLyα = 40.5. From our data only, no significant evolution of LF mean slope can be found. When performing a Schechter analysis also including data from the literature to complete the present sample towards the brightest luminosities, a steep faint end slope was measured varying from α = -1.69+0.08-0.08 to α = -1.87+0.12-0.12 between the lowest and the highest redshift bins. Conclusions. The contribution of the LAE population to the star formation rate density at z z ∼ 6 is ≲50% depending on the luminosity limit considered, which is of the same order as the Lyman-break galaxy (LBG) contribution. The evolution of the LAE contribution with redshift depends on the assumed escape fraction of Lyman-alpha photons, and appears to slightly increase with increasing redshift when this fraction is conservatively set to one. Depending on the intersection between the LAE/LBG populations, the contribution of the observed galaxies to the ionizing flux may suffice to keep the universe ionized at z ∼ 6.
- ItemEvidence for ram-pressure stripping in a cluster of galaxies at z = 0.7(Les Ulis : EDP Sciences, 2019) Boselli, A.; Epinat, B.; Contini, T.; Abril-Melgarejo, V.; Boogaard, L. A.; Pointecouteau, E.; Ventou, E.; Brinchmann, J.; Carton, D.; Finley, H.; Michel-Dansac, L.; Soucail, G.; Weilbacher, P.M.Multi-Unit Spectroscopic Explorer (MUSE) observations of the cluster of galaxies CGr32 (M200≅ 2×1014 M⊙) at = 0.73 reveal the presence of two massive star-forming galaxies with extended tails of diffuse gas detected in the [O II]λλ3727-3729 Å emission-line doublet. The tails, which have a cometary shape with a typical surface brightness of a few 10-18 erg s-1 cm-2 arcsec-2, extend up to ≅ 100 kpc (projected distance) from the galaxy discs, and are not associated with any stellar component. All this observational evidence suggests that the gas was removed during a ram-pressure stripping event. This observation is thus the first evidence that dynamical interactions with the intracluster medium were active when the Universe was only half its present age. The density of the gas derived using the observed [O II]λ3729/[O II]λ3726 line ratio implies a very short recombination time, suggesting that a source of ionisation is necessary to keep the gas ionised within the tail.
- ItemThe tully-fisher relation in dense groups at z ∼ 0.7 in the MAGIC survey(Les Ulis : EDP Sciences, 2021) Abril-Melgarejo, Valentina; Epinat, Benoît; Mercier, Wilfried; Contini, Thierry; Boogaard, Leindert A.; Brinchmann, Jarle; Finley, Hayley; Michel-Dansac, Léo; Ventou, Emmy; Amram, Philipe; Krajnović, Davor; Mahler, Guillaume; Pineda, Juan C.B.; Richard, JohanContext. Galaxies in dense environments are subject to interactions and mechanisms that directly affect their evolution by lowering their gas fractions and consequently reducing their star-forming capacity earlier than their isolated counterparts. Aims. The aim of our project is to get new insights into the role of environment in the stellar and baryonic content of galaxies using a kinematic approach, through the study of the Tully-Fisher relation (TFR). Methods. We study a sample of galaxies in eight groups, over-dense by a factor larger than 25 with respect to the average projected density, spanning a redshift range of 0.5 < z < 0.8 and located in ten pointings of the MAGIC MUSE Guaranteed Time Observations program. We perform a morpho-kinematics analysis of this sample and set up a selection based on galaxy size, [O » II]λλ3727,3729 emission line doublet signal-to-noise ratio, bulge-to-disk ratio, and nuclear activity to construct a robust kinematic sample of 67 star-forming galaxies. Results. We show that this selection considerably reduces the number of outliers in the TFR, which are predominantly dispersion-dominated galaxies. Similar to other studies, we find that including the velocity dispersion in the velocity budget mainly affects galaxies with low rotation velocities, reduces the scatter in the relation, increases its slope, and decreases its zero-point. Including gas masses is more significant for low-mass galaxies due to a larger gas fraction, and thus decreases the slope and increases the zero-point of the relation. Our results suggest a significant offset of the TFR zero-point between galaxies in low- and high-density environments, regardless of the kinematics estimator used. This can be interpreted as a decrease in either stellar mass by ∼0.05 - 0.3 dex or an increase in rotation velocity by ∼0.02 - 0.06 dex for galaxies in groups, depending on the samples used for comparison. We also studied the stellar and baryon mass fractions within stellar disks and found they both increase with stellar mass, the trend being more pronounced for the stellar component alone. These fractions do not exceed 50%. We show that this evolution of the TFR is consistent either with a decrease in star formation or with a contraction of the mass distribution due to the environment. These two effects probably act together, with their relative contribution depending on the mass regime. © V. Abril-Melgarejo et al. 2021.
- ItemThe MUSE Hubble Ultra Deep Field surveys: Data release II(Les Ulis : EDP Sciences, 2023) Bacon, Roland; Brinchmann, Jarle; Conseil, Simon; Maseda, Michael; Nanayakkara, Themiya; Wendt, Martin; Bacher, Raphael; Mary, David; Weilbacher, Peter M.; Krajnović, Davor; Boogaard, Leindert; Bouché, Nicolas; Contini, Thierry; Epinat, Benoît; Feltre, Anna; Guo, Yucheng; Herenz, Christian; Kollatschny, Wolfram; Kusakabe, Haruka; Leclercq, Floriane; Michel-Dansac, Léo; Pello, Roser; Richard, Johan; Roth, Martin; Salvignol, Gregory; Schaye, Joop; Steinmetz, Matthias; Tresse, Laurence; Urrutia, Tanya; Verhamme, Anne; Vitte, Eloise; Wisotzki, Lutz; Zoutendijk, Sebastiaan L.We present the second data release of the MUSE Hubble Ultra-Deep Field surveys, which includes the deepest spectroscopic survey ever performed. The MUSE data, with their 3D content, amazing depth, wide spectral range, and excellent spatial and medium spectral resolution, are rich in information. Their location in the Hubble ultra-deep field area, which benefits from an exquisite collection of ancillary panchromatic information, is a major asset. This update of the first release incorporates a new 141-h adaptive-optics-assisted MUSE eXtremely Deep Field (MXDF; 1 arcmin diameter field of view) in addition to the reprocessed 10-h mosaic (3 × 3 arcmin2) and the single 31-h deep field (1 × 1 arcmin2). All three data sets were processed and analyzed homogeneously using advanced data reduction and analysis methods. The 3σ point-source flux limit of an unresolved emission line reaches 3.1 × 10-19 and 6.3 × 10-20 erg s-1 cm-2 at 10-and 141-h depths, respectively. We have securely identified and measured the redshift of 2221 sources, an increase of 41% compared to the first release. With the exception of eight stars, the collected sample consists of 25 nearby galaxies (z < 0.25), 677 [O II] emitters (z = 0.25-1.5), 201 galaxies in the MUSE redshift desert range (z = 1.5-2.8), and 1308 Lyα emitters (z = 2.8-6.7). This represents an order of magnitude more redshifts than the collection of all spectroscopic redshifts obtained before MUSE in the Hubble ultra-deep field area (i.e., 2221 versus 292). At high redshift (z > 3), the difference is even more striking, with a factor of 65 increase (1308 versus 20). We compared the measured redshifts against three published photometric redshift catalogs and find the photo-z accuracy to be lower than the constraints provided by photo-z fitting codes. Eighty percent of the galaxies in our final catalog have an HST counterpart. These galaxies are on average faint, with a median AB F775W magnitude of 25.7 and 28.7 for the [O II] and Lyα emitters, respectively. Fits of their spectral energy distribution show that these galaxies tend to be low-mass star-forming galaxies, with a median stellar mass of 6.2 × 108 M· and a median star-formation rate of 0.4 M· yr-1. We measured the completeness of our catalog with respect to HST and found that, in the deepest 141-h area, 50% completeness is achieved for an AB magnitude of 27.6 and 28.7 (F775W) at z = 0.8-1.6 and z = 3.2-4.5, respectively. Twenty percent of our catalog, or 424 galaxies, have no HST counterpart. The vast majority of these new sources are high equivalent-width z > 2.8 Lyα emitters that are detected by MUSE thanks to their bright and asymmetric broad Lyα line. We release advanced data products, specific software, and a web interface to select and download data sets.
- ItemFirst constraints on the AGN X-ray luminosity function at z 6 from an eROSITA-detected quasar(Les Ulis : EDP Sciences, 2021) Wolf, J.; Nandra, K.; Salvato, M.; Liu, T.; Buchner, J.; Brusa, M.; Hoang, D. N.; Moss, V.; Arcodia, R.; Brüggen, M.; Comparat, J.; de Gasperin, F.; Georgakakis, A.; Hotan, A.; Lamer, G.; Merloni, A.; Rau, A.; Rottgering, H. J. A.; Shimwell, T. W.; Urrutia, T.; Whiting, M.; Williams, W. L.Context. High-redshift quasars signpost the early accretion history of the Universe. The penetrating nature of X-rays enables a less absorption-biased census of the population of these luminous and persistent sources compared to optical/near-infrared colour selection. The ongoing SRG/eROSITA X-ray all-sky survey offers a unique opportunity to uncover the bright end of the high-z quasar population and probe new regions of colour parameter space. Aims. We searched for high-z quasars within the X-ray source population detected in the contiguous 140 deg2 field observed by eROSITA during the performance verification phase. With the purpose of demonstrating the unique survey science capabilities of eROSITA, this field was observed at the depth of the final all-sky survey. The blind X-ray selection of high-redshift sources in a large contiguous, near-uniform survey with a well-understood selection function can be directly translated into constraints on the X-ray luminosity function (XLF), which encodes the luminosity-dependent evolution of accretion through cosmic time. Methods. We collected the available spectroscopic information in the eFEDS field, including the sample of all currently known optically selected z > 5.5 quasars and cross-matched secure Legacy DR8 counterparts of eROSITA-detected X-ray point-like sources with this spectroscopic sample. Results. We report the X-ray detection of eFEDSU J083644.0+005459, an eROSITA source securely matched to the well-known quasar SDSS J083643.85+005453.3 (z = 5.81). The soft X-ray flux of the source derived from eROSITA is consistent with previous Chandra observations. The detection of SDSS J083643.85+005453.3 allows us to place the first constraints on the XLF at z > 5.5 based on a secure spectroscopic redshift. Compared to extrapolations from lower-redshift observations, this favours a relatively flat slope for the XLF at z 6 beyond L∗, the knee in the luminosity function. In addition, we report the detection of the quasar with LOFAR at 145 MHz and ASKAP at 888 MHz. The reported flux densities confirm a spectral flattening at lower frequencies in the emission of the radio core, indicating that SDSS J083643.85+005453.3 could be a (sub-) gigahertz peaked spectrum source. The inferred spectral shape and the parsec-scale radio morphology of SDSS J083643.85+005453.3 indicate that it is in an early stage of its evolution into a large-scale radio source or confined in a dense environment. We find no indications for a strong jet contribution to the X-ray emission of the quasar, which is therefore likely to be linked to accretion processes. Conclusions. Our results indicate that the population of X-ray luminous AGNs at high redshift may be larger than previously thought. From our XLF constraints, we make the conservative prediction that eROSITA will detect 90 X-ray luminous AGNs at redshifts 5.7 < z < 6.4 in the full-sky survey (De+RU). While subject to different jet physics, both high-redshift quasars detected by eROSITA so far are radio-loud; a hint at the great potential of combined X-ray and radio surveys for the search of luminous high-redshift quasars.
- ItemAstraeus I: The interplay between galaxy formation and reionization(Oxford : Oxford Univ. Press, 2021) Hutter, Anne; Dayal, Pratika; Yepes, Gustavo; Gottlöber, Stefan; Legrand, Laurent; Ucci, GrazianoWe introduce a new self-consistent model of galaxy evolution and reionization, ASTRAEUS (seminumerical rAdiative tranSfer coupling of galaxy formaTion and Reionization in N-body dArk mattEr simUlationS), which couples a state-of-the-art N-body simulation with the semi-analytical galaxy evolution DELPHI and the seminumerical reionization scheme CIFOG. ASTRAEUS includes all the key processes of galaxy formation and evolution (including accretion, mergers, supernova, and radiative feedback) and follows the time and spatial evolution of the ionized regions in the intergalactic medium (IGM). Importantly, it explores different radiative feedback models that cover the physically plausible parameter space, ranging from a weak and delayed to a strong and immediate reduction of gas mass available for star formation. From our simulation suite that covers the different radiative feedback prescriptions and ionization topologies, we find that radiative feedback continuously reduces star formation in galaxies with Mh ≲ 109.5 M☉ upon local reionization; larger mass haloes are unaffected even for the strongest and immediate radiative feedback cases during reionization. For this reason, the ionization topologies of different radiative feedback scenarios differ only on scales smaller than 1–2 comoving Mpc, and significant deviations are found only when physical parameters (e.g. the escape fraction of ionizing photons) are altered based on galactic properties. Finally, we find that observables (the ultraviolet luminosity function, stellar mass function, reionization histories and ionization topologies) are hardly affected by the choice of the used stellar population synthesis models that model either single stars or binaries.