According to the current cosmological paradigm, ``Lambda Cold Dark
Matter'' (LambdaCDM), only ~20% of the gravitating matter in
the universe is made up of ordinary (i.e. baryonic) matter, while the
rest consists of invisible dark matter (DM) particles, which existence
can be inferred from their gravitational influence on baryonic matter
and light. Despite the large success of the LambdaCDM model in
explaining the large scale structure of the Universe and the
conditions of the early Universe, there has been debate on whether this
model can fully explain the observations of low mass (dwarf)
galaxies. The Local Group (LG), which hosts most of the known dwarf
galaxies, is a unique laboratory to test the predictions of the
LambdaCDM model on small scales.
I analyze the kinematics of LG members, including the
Milky~Way-Andromeda (MW-M31) pair and dwarf galaxies, in order to
constrain the mass of the LG. I construct samples of LG analogs from
large cosmological N-body simulations, according to the following
kinematics constraints: (a) the separation and relative velocity of
the MW-M31 pair; (b) the receding velocity of dwarf galaxies in the
outskirts of the LG. I find that these constraints yield a median
total mass of 2*10^{12} solar masses for the MW and M31, but with a
large uncertainty. Based on the mass and the kinematics constraints, I
select twelve LG candidates for the APOSTLE simulations project. The
APOSTLE project consists of high-resolution cosmological
hydrodynamical simulations of the LG candidates, using the EAGLE
galaxy formation model. I show that dwarf satellites of MW and M31
analogs in APOSTLE are in good agreement with observations, in terms
of number, luminosity and kinematics.
There have been tensions between the observed masses of LG dwarf
spheroidals and the predictions of N-body simulations within the
LambdaCDM framework; simulations tend to over-predict the mass of
dwarfs. This problem is known as the ``too-big-to-fail'' problem. I
find that the enclosed mass within the half-light radii of Galactic
classical dwarf spheroidals, is in excellent agreement with the
simulated satellites in APOSTLE, and that there is no too-big-to-fail
problem in APOSTLE simulations. A few factors contribute in solving
the problem: (a) the mass of haloes in hydrodynamical simulations are
lower compared to their N-body counterparts; (b) stellar mass-halo
mass relation in APOSTLE is different than the ones used to argue for
the too-big-to-fail problem; (c) number of massive satellites
correlates with the virial mass of the host, i.e. MW analogs with
virial masses above ~ 3*10^{12} solar masses would have faced
too-big-to-fail problems; (d) uncertainties in observations were
underestimated in previous works.
Stellar mass-halo mass relation in APOSTLE predicts that all isolated
dwarf galaxies should live in haloes with maximum circular velocity
(V_max) above 20 km/s. Satellite galaxies, however, can inhabit
lower mass haloes due to tidal stripping which removes mass from the
inner regions of satellites as they orbit their hosts. I examine all
satellites of the MW and M31, and find that many of them live in
haloes less massive than V_max=20 km/s. I additionally show that the
low mass population is following a different trend in stellar
mass-size relation compared to the rest of the satellites or field
dwarfs. I use stellar mass-halo mass relation of APOSTLE field
galaxies, along with tidal stripping trajectories derived in Penarrubia
et al., in order to predict the properties of the progenitors of the LG
satellites. According to this prediction, some satellites have
lost a significant amount of dark matter as well as stellar
mass. Cra~II, And~XIX, XXI, and XXV have lost 99 per-cent of their
stellar mass in the past.
I show that the mass discrepancy-acceleration relation of dwarf
galaxies in the LG is at odds with MOdified Newtonian Dynamics (MOND)
predictions, whereas tidal stripping can explain the observations very
well. I compare observed velocity dispersion of LG satellites with the
predicted values by MOND. The observations and MOND predictions are
inconsistent, in particular in the regime of ultra faint dwarf
galaxies. / Graduate
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/8576 |
| Date | 18 September 2017 |
| Creators | Fattahi, Azadeh |
| Contributors | Navarro, Julio |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Available to the World Wide Web |
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