EXTRAGALACTIC RESEARCH IN OULU:
We use data from ground-based and space telescopes, including modern
instruments at European Southern Observatory (ESO). We do dynamical
modeling of galaxies using both N-body codes developed in our group
(Salo & Laurikainen 2000a,b), and with other codes in collaboration
with other researchers. We have developed both 2D-decomposition
structural decomposition methdods (see BDBAR, Laurikainen et
al. 2005), and methods for estimating the gravitational field of the
galaxies from near-IR images (NIRQB, Laurikainen and Salo 2002).
1. Structural decompositions:
The Astronomy Research unit in Oulu has been one of the pioneers in
developing methods of detailed multi-component photometric
decompositions, and applying them to large galaxy samples. Detailed
decompositions (Laurikainen et al. 2010) were obtained for the Near-IR
S0 Galaxy Survey ( NIRS0S , Laurikainen et al. 2011) of ~200
galaxies. The two-dimensional flux distributions of galaxies were
fitted with functions describing bulges, disks, bars, and lenses,
using BDBAR-code. We found that even in the "bulge
dominated" lenticular galaxies (S0s), in fact only 10-30 % of the light belongs
to relaxed central spheroids, i.e., classical bulges. In the same
study we found that some SOs have extremely low mass bulges, which led us to
suggest that S0s form a sequence parallel with that of Sa - Sc
spirals in the Hubble sequence, with systematically decreasing
bulge-to-total flux ratio. A similar conclusion has been made by
Kormendy & Bender (2012), and also by Cappellari et al. (2011), using
kinematic observations.
We have applied a similar decomposition approach to the Spitzer Survey
of Stellar Structure in Galaxies (S4G, Sheth et al. 2010) at 3.6
micron wavelength, for ~2300 nearby galaxies. These
S4G "pipeline 4"
decompositions (Salo et al. 2015) were carried out with the
GALFIT-code (Peng et al. 2002, 2010). The S4G decompositions form
one of the largest databases of detailed photometric multi-component
decompositions of galaxies, covering a large range of galaxy masses
(10^8 - 10^11 solar masses). We are currently applying a similar
decomposition method to other galaxy surveys.
2. Book on 'Galactic Bulges':
When theoretical models are compared with observations the observers
do not always mean the same thing when they talk about galactic
bulges. Also, due to the colorful background of the concept of the
bulge, it can refer to extra flux in the disk plane, to a spheroidal central
component in a galaxy, or to the vertically extended central part of the bar. Some of these
structures can be recurrent, while others are long lasting. In order
to bring the observers and people working with theoretical models
closer to each other, we initiated a book on "Galactic Bulges"
(eds. Laurikainen, Peletier, Gadotti) published by Springer in
2016. The book collects 16 reviews of different aspects of
extra-galactic bulges, as well as of the Milky Way bulge, written by
specialists in the field. The book contains also a historical and
philosophical background of the concept of the bulge. Also, a connection
of bulges with Super-massive Black Holes, and with Modified Newtonian
Dynamics (MOND) are discussed, and possible yet unresolved
problems in the field are summarized.
3. Barlenses, face-on counterparts of Boxy/Peanut bulges:
It was emphasized in the book on "Galactic Bulges" that most of the
flux generally associated to spheroids (i.e., classical bulges) can
actually form part of the bar. In the edge-on view such "bulges" are
known to appear as Boxy/Peanut or X-shaped structures, formed in
processes where stars are buckled to higher galactic latitudes. We
have further shown, based on observations (Laurikainen et al. 2014) and
on simulation models (Athanassoula et al. 2015), that also the
roundish structures in the central parts of bars in nearly face-on
view are manifestations of the same bar components (see Fig. 1). This is
important because these roundish components are generally interpreted
as spheroids, which in the simulation models typically form in
galaxy mergers. The models by Athanassoula et al. (2015) used
hydrodynamical simulations, in which models barlenses were formed in
conjunction with the bar formation.
Our recent simulation models (Salo & Laurikainen 2016; see also
Laurikainen & Salo 2016) have further demonstrated, that in order to
see a barlens in face-on view, the galaxies need to have massive
compact central regions, in a scale much smaller than the vertically
thick bar component. What are the classical bulges in galaxies remains
a puzzle that we plan to tackle by comparing bar/bulge photometry with
kinematic and stellar population analysis, in collaboration with the
specialists in these analysis methods. This study we are carrying out in
different galaxy environments.
4. Integral-Field observations with MUSE:
Integral-field-unit (IFU) spectroscopic observations are a powerful
tool to study the two dimensional distribution of stellar populations
and kinematics of galaxies. A new IFU instrument, MUSE, has been
recently attached to the 8m VLT telescope at the European Southern
Observatory (ESO). It already has the sensitivity and large enough
field of view to study the structure components of nearby galaxies. We
are particularly interested in thick disks (e.g., are thicker than the
thin disks of almost same size) in galaxies, structures which contain
stars that are almost as old as the Universe itself. Those stars are
also nearly as old as the stars in galactic bulges, including the
vertically thick inner bar components. An open question is, is this
just a coincidence, or are the formative processes of bulges and thick
disks coupled. MUSE kinematics for one galaxy with a
thick disk, ESO 243-49, is shown in Fig. 2 (from Comeron et al. 2016):
Fig 2: Top: image of ESO 243-49 made by collapsing the MUSE
data cube along its spectral direction. Middle: radial
velocity map. Bottom: velocity dispersion map.
The continuous line indicates the midplane, and the
dashed lines indicate heights of 0.8, 1.6, and 2.4 kpc above and
below the midplane. From Comeron et al. 2016.
We have adopted a multi-wavelength approach to solve the problem. We
first studied thick disc using mid-infrared imaging (Comer\'on et
al. 2014) and are now producing a large spectroscopic
survey of several tens of edge-on galaxies with MUSE, in order to
obtain the kinematics and the star formation history of thick disks.
5. Dwarf galaxies in clusters:
New instruments have been developed at ESO (MegaCam, OmegaCAM at ESO)
for making extremely deep images of galaxies, with a large
field-of-view. The OmegaCAM instrument has been developed by the
University of Groningen. Recent observations with these instruments
have shown that fairly isolated galaxies, previously classified as
featureless, can have huge, extended low surface brightness
structures (Duc et al. 2015; Iodice et al. 2016). Also, new type of
low mass, and low surface brightness galaxies have been discovered in
galaxy clusters, including the Virgo, Coma, and Fornax clusters.
We are involved in the imaging studies of the low surface brightness
galaxies in the Fornax cluster led by University of Groningen, via a
double-degree PhD project, and via a new SUNDIAL Marie
Sklodowska-Curie Innovative training network, starting 2017. The dwarf
galaxies provide an excellent test laboratory to study the
environmental effects. This is because they, besides being the most
numerous galaxies in the Universe, are also expected to be very
vulnerable to environmental processing because of thei low masses.
One Fornax field in r'-band is shown in Fig. 3 (Venhola et al. in
preparation).
Fig.3: A r'-band cut from the field 16 of the Fornax cluster. The
red ellipses point out the objects which were classified as Ultra
Diffuse Galaxies. The image is shown in the logarithmic scale.
(From Venhola et al. in preparation).
6. Interacting galaxies:
We have also experience of detailed dynamical modeling of interacting
galaxy pairs, of which an example is the model for M51 (Salo & Laurikainen
2000a,b), compared with S4G Spitzer image.
PARTICIPATION TO INTERNATIONAL NETWORKS:
During the last four years the Astronomy Research unit of Oulu has
been part of DAGAL (Detailed
Anatomy of GAlaxies) , which formed part of the European Union
(EU) programme 'Marie Curie Actions Initial Training Networks (ITN)'.
DAGAL (PI. Knapen, IAC Spain) consisted of six European institutes. As
the main database the network used the Spitzer Space telescope
observations of ~2300 galaxies, obtained by a large international team
of astronomers in Europa and in USA. In total 2 postdocs and 8 PhD
students were trained, of which two defended their thesis in the
University of Oulu, in September 2016.
A new SUNDIAL (SUrvey Network for Deep Imaging Analysis and Leaning)
innovative training network in the EU MSCA HORIZON 2020 project, will start in
summer 2017 (P.I. Peletier, Groningen, Netherlands). It consists of
nine European institutes, including the Astronomy Research unit of
Oulu. SUNDIAL is a combined effort of astronomers and computer
scientists to learn how to handle big databases, and how to apply the
new algorithms to solve problems of galaxy evolution, in particular in
dense galaxy environment. One of the PhD students will be trained in Oulu
References
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'Galactic Bulges', eds. Laurikainen E., Peletier R., Gadotti D. 2016, AStrophysics and Space Science Library, Vol. 418, Springer
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Salo & Laurikainen 2016, XXX
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