Download Stellar population models in the Near-Infrared Meneses

University of Groningen
Stellar population models in the Near-Infrared
Meneses-Goytia, Sofia
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2015
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Meneses-Goytia, S. (2015). Stellar population models in the Near-Infrared [Groningen]: University of
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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
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0.90
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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
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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.