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  General
Stellar Populations in Nearby Galaxies
SAURON - Integral Field Spectroscopy of Galaxies
The Formation and Evolution of Dwarf Galaxies |
| General |
My main research focus is on nearby galaxies. I
work on kinematics, morphology but mainly on stellar populations of
galaxies that are so far away that one cannot resolve them into
individual stars. This matches well with the field of other researchers
at the Kapteyn Institute. Although traditionally the nearby galaxy
group has always been strong, the institute has seen a strong
rejuvenation since 2002 with the appointment of 5 new staff
members in this field, including myself. With van der Kruit,
Peletier, Sancisi (part time) and Verheijen, Groningen has a
strong presence in the field of galaxy morphology and dynamics.
Particularly impressive, however, is the stellar population group,
which includes both people studying resolved (Helmi and Tolstoy) and
unresolved (Peletier, Trager, van der Kruit
and van der Hulst)
stellar populations in galaxies, with the aim of studying galaxy
evolution. There is a good mix of theory and observations, and of
expertise in the various wavelength regions. For plans concerning the PhD project advertised by the Faculty of Mathematics and Natural Sciences, click here.
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Stellar Populations in Nearby Galaxies
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A significant part of the information
about the evolution of galaxies is contained in their stellar
populations. So, to know the age of faraway galaxies, one can study
the spectra of their integrated light. The way one does this is to
make stellar population models based on nearby star clusters and
galaxies, and apply them to spectra of faraway galaxies. These models make
use of a library of observed stars, and use stellar evolution models
to predict what kinds of stars a galaxy of a certain age and
metallicity (mass fraction of elements other than H and He)
contains. The stellar evolution theory is calibrated using globular
clusters in our Galaxy. I specialise in making these models and
applying them to nearby elliptical and spiral galaxies. For my Ph.D.
Thesis in 1989 I
developed a stellar population model. With my ex-Ph.D. student
Alexandre Vazdekis we have refined and extended this model,
so that it can be used to determine ages and abundance distributions of
galaxies. Although I am interested in applying these models to
galaxies, my main aim is to continuously improve them. At the moment, I do this
in collaboration with Scott Trager, a stellar population
specialist at the Kapteyn Astronomical Institute. A stellar population
models contains two main parts: a state-of-the-art stellar evolution
code, and a radiative transfer code modelling the atmospheres of all
possible kinds of stars. Since for many, especially cool, stars such
model atmospheres are not very accurate yet, they are often replaced by
observed spectra of standard stars. Our main projects in this area are
at present:
- We have been developing a new stellar library
of 1000 stars at a resolution of 2.3Å between 3500 and 7500Å, named MILES.
With this new library of well-calibrated spectra it is possible to make
much more accurate stellar population models than before, and study
many more element abundances. This library has been adopted by all
major stellar population models in the world, e.g. the new models by
Charlot & Bruzual (2008, in preparation). We have used it for
detailed fits of nearby elliptical galaxies (Yamada et al. 2006). MILES can be downloaded freely.
- Up to now almost all stellar population results
have been obtained by studying spectra in the optical. The
near-infrared region between 1 and 2.5 microns has almost not been
studied, since spectra there are rather difficult to obtain due to
the large atmospheric OH emission and H2O
absorption bands, small detectors etc. Using telescopes at Hawaii, La
Palma and La Silla (Chile) we now have obtained a library of 150 stars
in the infrared H and K-band at a resolution of 2000, that will allow
us to extend the stellar population models to this spectral region. The
models will be particularly sensitive to AGB stars, with ages between
0.5 and 2 Gyr. Applying the models to nearby elliptical galaxies we
find that galaxies in clusters are generally old (8 Gyr or older),
while ellipticals in the field contain a significant fraction of AGB
stars. Since it is clear that the availability of such libraries is
crucial for the interpretation of galaxy spectra, we plan to observe a
new, much better quality, library using the new X-Shooter instrument at
the VLT in Chile, available from the end of 2009. This partly
Dutch-built instrument will allow us to obtain spectra all the way from
the blue to 2.5 micron at once.
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When
using the stellar population models to obtain abundance ratios of
galaxies, it is important that the metal composition of the stellar
evolution models is identical to the composition of the model spectra,
or of the standard stars. Since up to now this rarely is the case, we
have started making a new set of self-consistent stellar population
models. To do this, we collaborate with an American group making
stellar interior models, and use novel ways to determine the abundance
distribution of stars on the standard spectra. A new PhD student has
started last year to do this work.
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| SAURON - Integral Field Spectroscopy of Galaxies |
SAURON
is an integral field spectrograph, built by a consortium consisting of
the Observatoire de Lyon, the University of Durham and NOVA, the
association of astronomy institutes in the Netherlands. It has been
operating since 1999 on the WHT at La Palma. I have been a member of
this consortium since this time. Contrary to ordinary spectrographs,
who provide spectra along a one-dimensional slit, SAURON takes spectra
of all the light in an area of 40x30" on the sky sampled with lenslets
of 0.95". The SAURON spectra provide information about the kinematics
of gas and stars, and about age and metallicity of the stars. The
instrument SAURON is strongly linked with a scientific project, the
main goal of which is to understand the formation and evolution of
elliptical
and lenticular galaxies and of spiral bulges from 2D-spectroscopy. To
do this, the consortium has made a survey of a representative sample of
72 nearby E, S0, and Sa galaxies drawn from both cluster and field
environments. The survey is aimed at determining the intrinsic shape of
the galaxies, their orbital structure, the mass-to-light ratio as a
function of radius, the age and metallicity of their stellar
populations, and the frequency of kinematically decoupled cores and
nuclear black holes. Integral-field mapping uniquely connects
measurements of the kinematics and stellar populations with the galaxy
morphology.
Work on the survey is in its final stages. Several large papers have
been publlished about the various topics, i.e. stellar kinematics,
absorption line strength maps, gas kinematics, etc.. The survey is
having a major impact in the field. The 37 SAURON papers that have
appeared in refereed journals have already received 1080 citations. Not
only have new results appeared, also several new analysis techniques
have been developed by the collaboration. In Groningen we have added an
additional sample of 18 late-type spirals, for which we have analyzed
the same parameters. Although in the coming years I plan to continue
analysing some SAURON data, I will direct my attention more towards
applying the same techniques to data obtained using new integral field
spectrographs, which provide similar data, but in different wavelength
regions, and at a much higher spatial resolution, or covering a much
larger field on the sky.
- One
of the unique capabilities of SAURON is that it can obtain absorption
line indices in a large, two-dimensional area on the sky. The SAURON
observations of the sample of early-type spirals (Sa) indicate that the
stellar populations in such galaxies show a variety of behaviours. Some
objects are currently forming stars in rings at a very fast rate, while
others only show old stellar populations. About half the galaxies
show so-called sigma-drops, central minima in the velocity dispersion.
These sigma-drops are probably caused by central fast-rotating disks,
which remain kinematically cold for up to 5 Gyr (see Figure).
- We
have obtained Spitzer Space Telescope mid-infrared imaging data (PI is my
PhD student Guido van der Wolk) for the full sample of 72 galaxies. The
Spitzer data are excellent to study quantitatively the recent star
formation in these objects. At the moment we are doing this for the 24
Sa galaxies, trying to understand why stars form, and how the locus of
star formation correlates with the mass distribution, the gas
distribution and its kinematics, the position of the dust lanes and the
old stars.
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 | Figure: Diagnostic
observations of NGC 3623, an early-type spiral. I show in the top left:
unsharp-masked F555W HST image, showing the places of non-negligible
extinction (from Falcón-Barroso et al. 2006). Top right: major axis surface brightness profile, from the same HST data.
A bulge-disk decomposition is also shown, with an exponential disk and
a Sérsic bulge. Bottom row (from left to right): H beta absorption line
map (Peletier et al. 2007), stellar velocity and velocity dispersion map (Falcón-Barroso et al. 2006). Overlayed on all maps is the reconstructed SAURON intensity.
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| The Formation and Evolution of Dwarf Galaxies |
Although
dwarf galaxies constitute the dominant galaxy population, they have
received much less attention in the literature. Defining dwarf galaxies
as those fainter than MV = -18 (I use this definition
rather than the much more uncertain mass), one finds, for example, that
in the Local Group, dwarf ellipticals alone outnumber high luminosity
galaxies by a factor of 6, and more than 50% of the
galaxies in the Virgo cluster are dwarf ellipticals. In hierarchical structure formation scenarios dwarf
galaxies, or small dark matter halos, are important building blocks for
more massive galaxies and should have been even more numerous in the
early Universe. Accordingly, the dwarfs that we observe today are
considered survivors of an initially much richer population.
The
dwarfs that we know in the nearby Universe can be divided into 2
groups: the star forming dwarfs or dIrr, and the quiescent dwarfs or
dwarf ellipticals (dE) or spheroidals (dSph). Quiescent dwarfs are only
found in clusters, while star forming dwarfs are mainly found in the
field. To really understand the origin and evolution of dwarf
galaxies, and to be able to classify e.g. objects such as UCD's, one
needs to have a database containing structural parameters such as size,
surface brightness and velocity dispersion, colour and colour gradient
of galaxies in different environments. I am building up this database
as part of two different international collaborations:
- The
Coma-ACS Survey (PI: David Carter) is an HST Treasury programme awarded 164 orbits in
Cycle 15 to provide a comprehensive dataset on samples of thousands of
galaxies to MV=-9. The Survey provides ultra-deep
observations in the B and I-band at HST resolution, and provides
therefore the best available structural parameters and radial colour
profiles for Coma galaxies. In addition to the ACS dataset, the project
contains a variety of studies of the cluster that have been done with ground-based and
space-based observatories, and at wavelengths from the X-ray to the
radio. For understanding dwarf galaxies in a rich environment, the
Coma-ACS Survey clearly provides the best dataset curently available.
- To
obtain a comparable dataset in a less dense environment, we have been
allocated 60 nights on the 4 different telescopes at La Palma as part
of the International Time Programme 2005-2006 (with an extension
2006-2007). In this ITP, in which I was the Principle
Investigator, different kinds of data, including optical imaging and
spectroscopy, have been obtained for a sample of dwarfs (both quiescent
and star forming) in the Virgo cluster and in the field. I am
currently analyzing these data with a team from the MAGPOP EU
Research/Training Network, to obtain structural parameters, colours,
colour gradients and line strengths. The main aim of both surveys is to
understand the formation and evolution of dwarf galaxies, and what the
role of the environment is in this process.
At
present we are
analyzing the Coma-ACS data in Groningen using the Astro-Wise system
developed by Edwin Valentijn. That system allows us to automatically
analyze
thousands of galaxies, and to get better much better statistics than
has been possible in the past. |
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Gert Sikkema
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Ph.D. student (MSc Groningen)
Started 2003
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Elif Kutdemir
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Ph.D. student supervisor jointly with Bodo
Ziegler.
Started in 2005.
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Guido van der Wolk
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Ph.D. student of Barthel and Peletier. (MSc Groningen)
Started 2006
in Groningen
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Mark den Brok
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Ph.D. student working on the Coma ACS Survey project.
Started in 2008.
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| Previous Group Members: |
| Dr. Kambiz Fathi |
Now at the IAC, Tenerife |
| Dr. Peter Kamphuis | Now at the Kapteyn Astronomical Institute, Groningen |
| Dr. Michael Pohlen |
Now at the University of Cardiff |
| Dr. Katia Ganda |
Now living in Amsterdam |
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- Unresolved Stellar Populations in Galaxies
- Resolved Stellar Populations in the Local Group
- Stellar Streams in the Milky Way
- Galaxies in Clusters and in the Field
- Edge-on Galaxies
- Neutral Hydrogen in Galaxies
- Active Galactic Nuclei
- Planetary Nebulae
- High Velocity Clouds
- Galaxy Evolution with redshift
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SAURON
SAURON is an integral field spectrograph,
built by a consortium consisting of the Observatoire de Lyon, the Sterrewacht
Leiden, University of Durham and NOVA.
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Coma-ACS Collaboration
VISIR
is an infrared imager/spectrThe Coma core is the densest galaxy
environment in the local Universe. As such, it provides a key local,
high density benchmark for comparison to surveys of less dense and
relaxed environments (Virgo, Fornax, Perseus), high-redshift HST
cluster surveys, and field surveys such as HUDF, GOODS, and GEMS. The
Coma-ACS survey has obtained deep data in B and I of parts of the Coma
cluster to study galaxy evolution. |  |
VISIR
VISIR is an infrared imager/spectrograph built by Saclay (Paris) and ASTRON,
with strong involvement from the Kapteyn Institute. The instrument has been
put on one of the VLT telescopes (Melipal) in 2004.
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OmegaCAM
OmegaCAM is a 1 square degree wide field, optical,
16k x 16k CCD camera for the VLT Survey Telescope (VST) at Paranal observatory
in Chile, which is expected to become operational at Paranal during the
year 2006 or 2007. OmegaCAM is built in collaboration with institutes in Italy and
Germany. In Groningen the OmegaCEN group is developing software to handle
the enormous dataflow that will come from OmegaCAM once it is operational. |
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All Refereed Publications
All Non-Refereed Publications
| Recent preprints: |
| The nature of late-type spiral galaxies: structural parameters, optical and near-infrared colour profiles, and dust extinction, Katia Ganda; Reynier Peletier; Marc Balcells; Jesus Falcon-Barroso, 2009, accepted by MNRAS | | The present-day galaxy population in spiral galaxies, Reynier Peletier, Invited Review, to appear in Proceedings of 'Probing Stellar Populations out to the Distant Universe', Cefalu, Italy, Sep 7-19, 2008, AIP Conf. Proc. Series | | Exploring the Star Formation History of Elliptical Galaxies: Beyond
Simple Stellar Populations with a New Estimator of Line Strengths, Ben Rogers; Ignacio Ferreras; Reynier F. Peletier; Joseph Silk, 2009, submitted to MNRAS |
New: My PhD Thesis has been scanned (190MB) and is now available in electronic form:
Reynier Peletier, Elliptical Galaxies: Structure and Stellar Content, 1989, Ph.D. Thesis, University of Groningen. |
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A link to my New Astronomy article with images of galactic bulges:
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| The MILES stellar library. | | Database of the study by Katia Ganda, Reynier Peletier et al. of late-type spiral galaxies with SAURON. |
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My old homepage
from the University of Nottingham |
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| Last major update: 1 March 2009 |
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