156 research outputs found
Tidal dwarf galaxies as a test of fundamental physics
Within the cold dark matter (CDM) framework tidal dwarf galaxies (TDGs)
cannot contain dark matter, so the recent results by Bournaud et al. (2007)
that 3 rotating TDGs do show significant evidence for being dark matter
dominated is inconsistent with the current concordance cosmological theory
unless yet another dark matter component is postulated. We confirm that the TDG
rotation curves are consistent with Newtonian dynamics only if either an
additional dark matter component is postulated, or if all 3 TDGs happen to be
viewed nearly edge-on, which is unlikely given the geometry of the tidal
debris. We also find that the observed rotation curves are very naturally
explained without any free parameters within the modified Newtonian dynamics
(MOND) framework if inclinations are adopted as derived by Bournaud et al. We
explore different inclination angles and two different assumptions about the
external field effect. The results do not change significantly, and we conclude
therefore that Newtonian dynamics has severe problems while MOND does
exceedingly well in explaining the observed rotation curves of the 3 TDGs
studied by Bournaud et al.Comment: Accepted for publication in A&A Letters, 5 pages, 3 figure
Reproducing properties of MW dSphs as descendants of DM-free TDGs
The Milky Way (MW) dwarf spheroidal (dSph) satellites are known to be the
most dark-matter (DM) dominated galaxies with estimates of dark to baryonic
matter reaching even above one hundred. It comes from the assumption that
dwarfs are dynamically supported by their observed velocity dispersions.
However their spatial distributions around the MW is not at random and this
could challenge their origin, previously assumed to be residues of primordial
galaxies accreted by the MW potential. Here we show that alternatively, dSphs
could be the residue of tidal dwarf galaxies (TDGs), which would have
interacted with the Galactic hot gaseous halo and disk. TDGs are gas-rich and
have been formed in a tidal tail produced during an ancient merger event at the
M31 location, and expelled towards the MW. Our simulations show that low-mass
TDGs are fragile to an interaction with the MW disk and halo hot gas. During
the interaction, their stellar content is progressively driven out of
equilibrium and strongly expands, leading to low surface brightness feature and
mimicking high dynamical M/L ratios. Our modeling can reproduce the properties,
including the kinematics, of classical MW dwarfs within the mass range of the
Magellanic Clouds to Draco. An ancient gas-rich merger at the M31 location
could then challenge the currently assumed high content of dark matter in dwarf
galaxies. We propose a simple observational test with the coming GAIA mission,
to follow their expected stellar expansion, which should not be observed within
the current theoretical framework.Comment: 17 pages, 11 figures, accepted by the Monthly Notices of the Royal
Astronomical Society (MNRAS
The frequency and properties of young tidal dwarf galaxies in nearby gas-rich groups
We present high-resolution Giant Metrewave Radio Telescope (GMRT) HI
observations and deep Canada-France-Hawaii Telescope (CFHT) optical imaging of
two galaxy groups: NGC 4725/47 and NGC 3166/9. These data are part of a
multi-wavelength unbiased survey of the gas-rich dwarf galaxy populations in
three nearby interacting galaxy groups. The NGC 4725/47 group hosts two tidal
knots and one dIrr. Both tidal knots are located within a prominent HI tidal
tail, appear to have sufficient mass (M_gas~10^8 M_sol) to evolve into
long-lived tidal dwarf galaxies (TDGs) and are fairly young in age. The NGC
3166/9 group contains a TDG candidate, AGC 208457, at least three dIrrs and
four HI knots. Deep CFHT imaging confirms that the optical component of AGC
208457 is bluer -- with a 0.28 mag g-r colour -- and a few Gyr younger than its
purported parent galaxies. Combining the results for these groups with those
from the NGC 871/6/7 group reported earlier, we find that the HI properties,
estimated stellar ages and baryonic content of the gas-rich dwarfs clearly
distinguish tidal features from their classical counterparts. We optimistically
identify four potentially long-lived tidal objects associated to three separate
pairs of interacting galaxies, implying that TDGs are not readily produced
during interaction events as suggested by some recent simulations. The tidal
objects examined in this survey also appear to have a wider variety of
properties than TDGs of similar mass formed in current simulations of
interacting galaxies, which could be the result of pre- or post-formation
environmental influences.Comment: 18 pages, 14 figures, accepted for publication in MNRA
The vast thin plane of M31 co-rotating dwarfs: an additional fossil signature of the M31 merger and of its considerable impact in the whole Local Group
The recent discovery by Ibata et al. (2013) of a vast thin disk of satellites
(VTDS) around M31 offers a new challenge for the understanding of the Local
Group properties. This comes in addition to the unexpected proximity of the
Magellanic Clouds (MCs) to the Milky Way (MW), and to another vast polar
structure (VPOS), which is almost perpendicular to our Galaxy disk. We find
that the VTDS plane is coinciding with several stellar, tidally-induced streams
in the outskirts of M31, and, that its velocity distribution is consistent with
that of the Giant Stream (GS). This is suggestive of a common physical
mechanism, likely linked to merger tidal interactions, knowing that a similar
argument may apply to the VPOS at the MW location. Furthermore, the VTDS is
pointing towards the MW, being almost perpendicular to the MW disk, as the VPOS
is.
We compare these properties to the modelling of M31 as an ancient, gas-rich
major merger, which has been successfully used to predict the M31 substructures
and the GS origin. We find that without fine tuning, the induced tidal tails
are lying in the VTDS plane, providing a single and common origin for many
stellar streams and for the vast stellar structures surrounding both the MW and
M31. The model also reproduces quite accurately positions and velocities of the
VTDS dSphs. Our conjecture leads to a novel interpretation of the Local Group
past history, as a gigantic tidal tail due to the M31 ancient merger is
expected to send material towards the MW, including the MCs. Such a link
between M31 and the MW is expected to be quite exceptional, though it may be in
qualitative agreement with the reported rareness of MW-MCs systems in nearby
galaxies.Comment: Accepted for publication in MNRAS, 8 pages, 3 figure
The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the Milky Way
It has been known for a long time that the satellite galaxies of the Milky
Way (MW) show a significant amount of phase-space correlation, they are
distributed in a highly inclined Disc of Satellites (DoS). We have extended the
previous studies on the DoS by analysing for the first time the orientations of
streams of stars and gas, and the distributions of globular clusters within the
halo of the MW. It is shown that the spatial distribution of MW globular
clusters classified as young halo clusters (YH GC) is very similar to the DoS,
while 7 of the 14 analysed streams align with the DoS. The probability to find
the observed clustering of streams is only 0.3 per cent when assuming isotropy.
The MW thus is surrounded by a vast polar structure (VPOS) of subsystems
(satellite galaxies, globular clusters and streams), spreading from
Galactocentric distances as small as 10 kpc out to 250 kpc. These findings
demonstrate that a near-isotropic infall of cosmological sub-structure
components onto the MW is essentially ruled out because a large number of
infalling objects would have had to be highly correlated, to a degree not
natural for dark matter sub-structures. The majority of satellites, streams and
YH GCs had to be formed as a correlated population. This is possible in tidal
tails consisting of material expelled from interacting galaxies. We discuss the
tidal scenario for the formation of the VPOS, including successes and possible
challenges. The potential consequences of the MW satellites being tidal dwarf
galaxies are severe. If all the satellite galaxies and YH GCs have been formed
in an encounter between the young MW and another gas-rich galaxy about 10-11
Gyr ago, then the MW does not have any luminous dark-matter substructures and
the missing satellites problem becomes a catastrophic failure of the standard
cosmological model.Comment: 21 pages, 8 figures, 2 tables. Accepted for publication in MNRAS. An
animation of Figure 5 can be found at http://youtu.be/nUwxv-WGfH
The orbital poles of Milky Way satellite galaxies: a rotationally supported disc-of-satellites
Available proper motion measurements of Milky Way (MW) satellite galaxies are
used to calculate their orbital poles and projected uncertainties. These are
compared to a set of recent cold dark-matter (CDM) simulations, tailored
specifically to solve the MW satellite problem. We show that the CDM satellite
orbital poles are fully consistent with being drawn from a random distribution,
while the MW satellite orbital poles indicate that the disc-of-satellites of
the Milky Way is rotationally supported. Furthermore, the bootstrapping
analysis of the spatial distribution of theoretical CDM satellites also shows
that they are consistent with being randomly drawn. The theoretical CDM
satellite population thus shows a significantly different orbital and spatial
distribution than the MW satellites, most probably indicating that the majority
of the latter are of tidal origin rather than being DM dominated
sub-structures. A statistic is presented that can be used to test a possible
correlation of satellite galaxy orbits with their spatial distribution.Comment: Accepted for publication in Ap
Candidate Tidal Dwarf Galaxies in the Compact Group CG J1720-67.8
This is the second part of a detailed study of the ultracompact group CG
J1720-67.8: in the first part we have focused the attention on the three main
galaxies of the group and we have identified a number of candidate tidal dwarf
galaxies (TDGs). Here we concentrate on these candidate TDGs. Absolute
photometry of these objects in BVRJHKs bands confirms their relatively blue
colors, as we already expected from the inspection of optical and near-infrared
color maps and from the presence of emission-lines in the optical spectra. The
physical conditions in such candidate TDGs are investigated through the
application of photoionization models, while the optical colors are compared
with grids of spectrophotometric evolutionary synthesis models from the
literature. Although from our data self-gravitation cannot be proved for these
objects, their general properties are consistent with those of other TDG
candidates. Additionally we present the photometry of a few ``knots'' detected
in the immediate surroundings of CG J1720-67.8 and consider the possibility
that these objects might belong to a dwarf population associated with the
compact group.Comment: Accepted for publication in the Astrophysical Journa
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