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University of Groningen Stellar population models in the Near-Infrared Meneses-Goytia, Sofia IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2015 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Meneses-Goytia, S. (2015). Stellar population models in the Near-Infrared [Groningen]: University of Groningen Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 17-06-2017 Summary Most of the galaxies in the Universe are found so far away from us that it is no longer possible to resolve individual stars within them. However, it is still possible to obtain valuable information about the different types of stars within these systems, by analysing their electromagnetic spectrum in detail. This thesis is focussed on the near-infrared (NIR) wavelength range, which is especially important for studying the properties of old galaxies. This region of the spectrum is strongly influenced by cool stars of the so-called late-type, which do not contribute as strongly in the optical wavelength range. Among the cool, late-type stars, red giant branch (RGB) stars are present in every stellar population, but contribute strongly mainly for old (> 2 Gyr) systems and for redder wavelengths. Another type of late-type stars are the asymptotic giant branch (AGB) stars, which can be further subdivided in regular and thermally pulsating AGB stars. Regular AGB stars contributing to the spectrum of a galaxy for a wide range in ages, while thermally pulsating asymptotic giant branch stars, contribute most to the integrated light of a stellar population between 1 and 3 Gyr. In order to create stellar population models for galaxies it is necessary to obtain accurate spectra or models of these stars. The simplest, and at this moment best way of doing this is by obtaining observations of these and other types of stars and using those to create a library of stars which can be used to create stellar population models. 0.5 160 0.0 0 1.0 -0.5 3.0 4.0 120 -1 -1.5 [Z/Z⊙] 2.0 140 100 N log g -1.0 80 60 -2 -2.5 5.0 40 20 -3 0 -2.7 8000 7000 6000 5000 4000 3000 2000 1000 Teff (K) -2.3 -1.9 -1.5 -1.1 [Z/Z⊙] -0.7 -0.3 0.1 0.5 Figure 1 – The stellar atmospheric parameters for the stars of the IRTF spectral library. The left panel shows the parameter coverage of this library for stellar population models. The right panel shows the metallicity distribution function for the stars. In Chapter II, we present a detailed study of the stars of the empirical IRTF spectral library to understand its full extent and reliability for use with stellar population 137 Summary 138 modelling. This library consists of 210 individual stars, with a total of 292 spectra covering the wavelength range of 0.94 to 2.41 µm (mainly covering the NIR J , H and K bands) at a resolution of R = λ/∆λ ≈ 2000. Here the ∆λ value is the minimum distance in wavelength for which information is present in a spectrum. Resolution therefore determines how close together in wavelength two values can be while they can still be separated from each other. For every star in the library, we infer its atmospheric parameters, which are the effective temperature (Teff ), gravity at the surface of the star (log g ) and metallicity ([Z /Z¯ ]) (Astronomers designate all element heavier than Helium as "metals"). These parameters are calculated in different ways, such as by making use of relations between the temperature of a star and its NIR colour (Figure 1). Furthermore, we also employ an advanced method which compares the observed spectrum in a section of the K -band (2.19 to 2.34 µm) with spectra from an empirical stellar library (for which we know the exact temperature, gravity and metallicity of each star). For each star in the IRTF library we find a spectrum which best fits the spectrum for the observed star and adopt those atmospheric parameters. 14 FWHM (Å) 12 Figure 2 – Behaviour of the spectral resolution (FWHM) of the G stars of the IRTF spectral library (grey lines) as a function of wavelength. The black points represent the mean values for those effective wavelengths and the blue line mark the mean dispersion. The red line corresponds to a liner relation of the mean FWHM for each wavelength. 10 8 6 4 2 0 10000 12500 15000 17500 20000 22500 25000 Wavelength (Å) With accurate atmospheric parameters and NIR colours in place, we can investigate the spectral library in more detail. First we investigate if the atmospheric parameters of the library agree with those of other studies from the literature. The NIR colours are also compared to those from a known reference library (the Pickles NIR library) by looking at the distribution of stars in colour-colour diagrams, from which we can conclude that the IRTF stars agree well with the reference sources. This is especially important for the construction of stellar population models. Besides these tests, we also measure the spectral resolution R as a function of wavelength. We find that the resolution increases as a function of lambda from about 6 Å in J to 10 Å in the red part of the K -band (Figure 2). With these tests we have established that the IRTF spectral library, the largest currently available general library of stars at intermediate resolution (R between 1500 and 3500) in the NIR, is an excellent candidate to be used in stellar population models. In Chapter III, we introduce the unresolved synthesis models for single stellar populations (SSP models, comprised of a single age and metallicity) in the near-infrared range. The extension to the NIR is very important for the study of early-type galaxies, since these galaxies are predominantly old and therefore emit most of their light 1.0 Si I Br γ Na I Fe I 2.0 Ca I Mg I CO Br δ C I C I Ni II Si I Fe I Al I Ca I Fe I Mg I Ca I Al I K I I C Ti I Mg I Fe I Ni I Si I C I Ca I Ni I Pa β Al I Si I Fe I K I Si I I Na I Si I C 3.0 Pa γ F/F1.65 µm+constant Ca I Fe I P I 139 MarS GirS BaSS 1.04 Ratios 1.02 1.00 0.98 GirS / MarS BaSS / MarS BaSS / GirS 0.96 1.0 1.2 1.4 1.6 1.8 2.0 Wavelength (µm) 2.2 2.4 Figure 3 – SED of our three SSP models at solar metallicity and 10 Gyr (upper panel) and ratios when comparing to each other (lower panel). Interesting spectral features (from Rayner et al., 2009) are marked in the upper panel. in this wavelength range. The models are created using the now well-calibrated IRTF spectral library of empirical stellar spectra. To construct a model, we determine the spectrum for each point in a so-called stellar isochrone, which is the distribution of stars with different masses in a single age and metallicity population. Subsequently, we integrate these spectra with a weight according to the number of stars formed for each mass, governed by the initial mass function (IMF). In this way, we have produced model spectra of single age-metallicity stellar populations at a resolution R ∼ 2000 (Figure 3. The models we have constructed as part of this thesis can be used to fit observed spectra of globular clusters and galaxies, to derive their age distribution, chemical abundances and IMF properties. The reliability of the models has been tested by comparing them to observed colours of elliptical galaxies and clusters in the Magellanic Clouds. Furthermore, predicted absorption line indices have been compared to published indices of other elliptical galaxies. The comparisons show that our models are well suited for studying stellar populations in unresolved galaxies, which are located sufficiently far away from us that it is not possible to distinguish individual stars. They are particularly useful for studying the old and intermediate-age stellar populations in galaxies, which are relatively free of contamination by young stars and extinction from dust. The models we have derived will also be invaluable for the study of data from future IR based facilities, such as the JWST and extremely large telescopes, such as the E-ELT. In Chapter IV, we derive the stellar population properties (such as the age, metallicity and star formation history) of a sample of galaxies inside and outside of a cluster environment (a large collection of galaxies held together by gravity in a group, influ- Summary 140 50 100 150 200 250 300 field Fornax − 0.7 dex − 0.4 dex + 0.0 dex + 0.2 dex 2 Gyr 7 Gyr 14 Gyr 350 −1 σ (km s ) 4.30 3.90 Na I 3.50 3.10 2.70 2.30 1.90 0.80 0.70 0.50 0.40 0.30 0.20 0.10 3.00 2.80 Ca I 2.60 2.40 2.20 2.00 1.80 Fe I 1.30 1.10 0.90 0.70 1.23 1.22 DCO Figure 4 – Indexindex diagrams for the combined sample from MármolQueraltó et al. (2009) and Silva et al. (2008), and the MarS models. The MIUSCAT models were used to complement the optical index predictions. The optical line-strength indices were collected from Kuntschner (2000) and SánchezBlázquez et al. (2003). Mg I 0.60 1.21 1.20 1.19 1.18 1.17 1.00 1.40 1.80 2.20 2.60 3.00 3.40 2.50 Hβ 4.50 6.50 C24668 8.50 1.50 2.50 3.50 Mg b 4.50 5.50 encing the evolution of individual systems). The galaxies are studied by comparing different stellar evolution tracers, such as line-strength indices (the relative strength of a spectral absorption line, determined by comparing the spectral flux on both sides of the feature with the flux inside the absorption line), integrated colours and Spectral Energy Distributions (SEDs). By combining optical and NIR models and data, we find evidence that the contribution of the AGB stellar phase behaves differently for elliptical galaxies in the field and cluster. This implies that the environment plays an important role in driving the evolutionary histories of the galaxies (Figure 4). We also determine that the NIR line-strength index DCO is an efficient indicator of the presence of AGB stars. Chapter IV shows that the contribution of AGB stars to the galaxy spectrum is clearly larger in the field (outside a cluster) than it is in the Fornax cluster. From this index and the redder (J −K ) values of field galaxies, we infer that the field galaxies must contain younger populations (Figure 5). To analyse the AGB contribution of the studied sample in more detail, we also applied a new method of spectral analysis, in which we fit the observed galaxy spectrum 141 field Fornax 2 Gyr 7 Gyr 14 Gyr 50 100 150 200 250 300 350 Ca I (Å) Fe I (Å) Na I (Å) σ (km s−1) − 0.7 dex − 0.4 dex + 0.0 dex + 0.2 dex 4.20 3.80 3.40 3.00 2.60 2.20 1.80 1.50 1.30 1.10 0.90 0.70 3.00 2.80 2.60 2.40 2.20 2.00 1.80 Mg I (Å) 0.70 0.50 0.30 DCO (mag) 0.10 1.23 1.22 1.21 1.20 1.19 1.18 1.17 0.85 0.90 0.95 1.00 (J−Ks) 0.57 0.62 0.67 0.72 0.77 0.14 (J−H) 0.19 0.24 (H−Ks) 0.29 Figure 5 – Colourindex diagrams for the combined sample from MármolQueraltó et al. (2009) and Silva et al. (2008) and our models. with models constructed according to a slightly different prescription than the classical stellar population synthesis. In this approach we only partially populate the points on each stellar isochrone, leaving out the points corresponding to the AGB phase. During the spectral fiting, we instead allow a free fraction of AGB stars from the IRTF spectral library to be included on top of the partially populated SED. In this way, we can derive the fraction of AGB stars in field and Fornax galaxies directly from the spectrum. We find strong evidence for the need of extra AGB stars, and that the contribution of AGB stars is once again stronger in field galaxies than in those of Fornax (Figure 6). The results in the NIR suggest that a more flexible and non-parametric SED fitting approach is needed to fully reproduce the behaviour of the galaxies. The analysis of the DCO index also suggests that a more detailed treatment of the AGB phase, including thermally pulsating AGB stars, is required to fully understand these galaxies. Finally, it is clear that environment plays a role in the NIR stellar populations, as Fornax galaxies require less additional AGB contribution than their field counterparts. In Chapter V, we summarise and discuss the conclusions drawn from this thesis. Furthermore, we also look to further studies which can be done in the future, and show Summary 142 Figure 6 – Comparison between fullspectrum fitting results of non-classical SSPs and added AGB stars for galaxies in bins of velocity dispersion, in both field and cluster environments. 4.10 Na I 3.70 3.30 2.90 2.50 2.10 0.70 350 300 Mg I 250 0.50 0.30 0.10 3.00 200 150 σ (km s−1) 1.70 100 50 2.80 Ca I 2.60 2.40 2.20 2.00 1.80 1.50 Fe I 1.30 Figure 7 – Comparison of four line-strength indices (Na I, Mg I, Ca I, and Fe I) as a function of DCO , at solar metallicity, for different initial mass functions. 1.10 0.90 field Fornax MarS MarBHa MarBHb MarCH 2 Gyr 7 Gyr 14 Gyr 0.70 1.161.171.181.191.201.211.221.231.24 DCO what happens to the models when we change the properties of the IMF. We present models in which the relative fraction of low-mass stars is greater (bottom-heavy mod- 143 els, with slopes of −3.0 and −3.5) and a model that follows the IMF recipe of Chabrier with parameter χ = −1.3. The different initial mass function tests presented there give us insights into the complex star formation scenario that these early-type galaxies present. In future work, we will need to consider other approaches as well, such as for example an initial mass function with an even stronger presence of low-mass stars (Figure 7). Also, we show the results of a multiple stellar populations approach, which indicates that in general, the star formation histories of early-type galaxies are better explained when more than one stellar component is present. There are several ways to improve our current models in the future, such as using a stellar spectral library, empirical or theoretical, with a better parameter coverage that includes α-enhancements (by either complementing the IRTF spectral library or using an entirely different one). Furthermore, a more accurate prescriptions for all AGB phases in the isochrones (by either modifying current isochrones or using new ones), multiple flexible populations, chemical evolution models, and using results from cosmological simulations could also improve the models and therefore our understanding of early-type galaxies in the NIR. In order to allow new insights and enrichments from the scientific community to the work presented in this thesis, we have created a website where all the information about the ingredients, the features obtain from our models and the models themselves are available to explore and test, http://smg.astro-reseach.net.