549 research outputs found
A link between feedback outflows and satellite galaxy suppression
We suggest a direct link between the two "missing" baryon problems of
contemporary galaxy formation theory: (1) that large galaxies are known to
contain too little gas and stars and (2) that too few dwarf satellite galaxies
are observed around large galaxies compared with cosmological simulations. The
former can be explained by invoking some energetic process -- most likely AGN
or star formation feedback -- which expels to infinity a significant fraction
of the gas initially present in the proto-galaxy, while the latter problem is
usually explained by star formation feedback inside the dwarf or tidal and ram
pressure stripping of the gas from the satellite galaxy by its parent. Here we
point out that the host galaxy "missing" baryons, if indeed ejected at
velocities of hundreds to a thousand km s, must also affect smaller
satellite galaxies by stripping or shocking the gas there. We estimate the
fraction of gas removed from the satellites as a function of the satellite
galaxy's properties. Applying these results to a Milky Way like dark matter
halo, we find that this singular shock ram pressure stripping event may be
quite efficient in removing the gas from the satellites provided that they are
close enough. We also use the orbital and mass modelling data for eight
Galactic dwarf spheroidal (dSph) satellites, and find that it is likely that
many of them have been affected by the Galactic outflow. We point out that
galactic outflows of the host may also trigger a starburst in the satellite
galaxies by over-pressuring their gas discs. This process may be responsible
for the formation of the globular clusters observed in some of the Milky Way's
dSphs (e.g. the Fornax and Sagittarius dSphs).Comment: appearing in MNRAS; 9 page
Dark Matter In Disk Galaxies II: Density Profiles as Constraints on Feedback Scenarios
The disparity between the density profiles of galactic dark matter haloes
predicted by dark matter only cosmological simulations and those inferred from
rotation curve decomposition, the so-called cusp-core problem, suggests that
baryonic physics has an impact on dark matter density in the central regions of
galaxies. Feedback from black holes, supernovae and massive stars may each play
a role by removing matter from the centre of the galaxy on shorter timescales
than the dynamical time of the dark matter halo. Our goal in this paper is to
determine constraints on such feedback scenarios based on the observed
properties of a set of nearby galaxies.
Using a Markov Chain Monte Carlo (MCMC) analysis of galactic rotation curves,
via a method developed in a previous paper, we constrain density profiles and
an estimated minimum radius for baryon influence, , which we couple with a
feedback model to give an estimate of the fraction of matter within that radius
that must be expelled to produce the presently observed halo profile. We show
that in the case of the gas rich dwarf irregular galaxy DDO 154, an outflow
from a central source (e.g. a black hole or star forming region) could produce
sufficient feedback on the halo without removing the disk gas.
We examine the rotation curves of 8 galaxies taken from the THINGS data set
and determine constraints on the radial density profiles of their dark matter
haloes. For some of the galaxies, both cored haloes and cosmological cusps are excluded. These intermediate central slopes require
baryonic feedback to be finely tuned. We also find for galaxies which exhibit
extended cores in their haloes (e.g. NGC 925), the use of a split power-law
halo profile yields models without the unphysical, sharp features seen in
models based on the Einasto profile.Comment: 17 pages, 19 figures Submitted to MNRA
Competitive feedback in galaxy formation
It is now well established that many galaxies have nuclear star clusters
(NCs) whose total masses correlate with the velocity dispersion (sigma) of the
galaxy spheroid in a very similar way to the well--known supermassive black
hole (SMBH) M - sigma relation. Previous theoretical work suggested that both
correlations can be explained by a momentum feedback argument. Observations
further show that most known NCs have masses < 10^8 Msun, while SMBHs
frequently have masses > 10^8 Msun, which remained unexplained in previous
work. We suggest here that this changeover reflects a competition between the
SMBH and nuclear clusters in the feedback they produce. When one of the massive
objects reaches its limiting M-sigma value, it drives the gas away and hence
cuts off its own mass and also the mass of the ``competitor''. The latter is
then underweight with respect to the expected M-sigma mass (abridged).Comment: To appear in MNRAS Letter
A low pre-infall mass for the Carina dwarf galaxy from disequilibrium modelling
Dark matter only simulations of galaxy formation predict many more subhalos
around a Milky Way like galaxy than the number of observed satellites. Proposed
solutions require the satellites to inhabit dark matter halos with masses
between one to ten billion solar masses at the time they fell into the Milky
Way. Here we use a modelling approach, independent of cosmological simulations,
to obtain a preinfall mass of 360 (+380,-230) million solar masses for one of
the Milky Way's satellites: Carina. This determination of a low halo mass for
Carina can be accommodated within the standard model only if galaxy formation
becomes stochastic in halos below ten billion solar masses. Otherwise Carina,
the eighth most luminous Milky Way dwarf, would be expected to inhabit a
significantly more massive halo. The implication of this is that a population
of "dark dwarfs" should orbit the Milky Way: halos devoid of stars and yet more
massive than many of their visible counterparts.Comment: 5 pages, 3 figures, 1 table, and supplementary material availabl
The mass distribution of the Fornax dSph: constraints from its globular cluster distribution
Uniquely among the dwarf spheroidal (dSph) satellite galaxies of the Milky
Way, Fornax hosts globular clusters. It remains a puzzle as to why dynamical
friction has not yet dragged any of Fornax's five globular clusters to the
centre, and also why there is no evidence that any similar star cluster has
been in the past (for Fornax or any other dSph). We set up a suite of 2800
N-body simulations that sample the full range of globular-cluster orbits and
mass models consistent with all existing observational constraints for Fornax.
In agreement with previous work, we find that if Fornax has a large dark-matter
core then its globular clusters remain close to their currently observed
locations for long times. Furthermore, we find previously unreported behaviour
for clusters that start inside the core region. These are pushed out of the
core and gain orbital energy, a process we call 'dynamical buoyancy'. Thus a
cored mass distribution in Fornax will naturally lead to a shell-like globular
cluster distribution near the core radius, independent of the initial
conditions. By contrast, CDM-type cusped mass distributions lead to the rapid
infall of at least one cluster within \Delta t = 1-2Gyr, except when picking
unlikely initial conditions for the cluster orbits (\sim 2% probability), and
almost all clusters within \Delta t = 10Gyr. Alternatively, if Fornax has only
a weakly cusped mass distribution, dynamical friction is much reduced. While
over \Delta t = 10Gyr this still leads to the infall of 1-4 clusters from their
present orbits, the infall of any cluster within \Delta t = 1-2Gyr is much less
likely (with probability 0-70%, depending on \Delta t and the strength of the
cusp). Such a solution to the timing problem requires that in the past the
globular clusters were somewhat further from Fornax than today; they most
likely did not form within Fornax, but were accreted.Comment: 12 pages, 8 figures, 3 tables, submitted to MNRA
A spectroscopic binary in the Hercules dwarf spheroidal galaxy
We present the radial velocity curve of a single-lined spectroscopic binary
in the faint Hercules dwarf spheroidal (dSph) galaxy, based on 34 individual
spectra covering more than two years of observations. This is the first time
that orbital elements could be derived for a binary in a dSph. The system
consists of a metal-poor red giant and a low-mass companion, possibly a white
dwarf, with a 135-days period in a moderately eccentric () orbit. Its
period and eccentricity are fully consistent with metal-poor binaries in the
Galactic halo, while the projected semimajor axis is small, at sin =
38 R. In fact, a very close orbit could inhibit the production of
heavier elements through -process nucleosynthesis, leading to the very low
abundances of neutron-capture elements that are found in this star. We discuss
the further implications for the chemical enrichment history of the Hercules
dSph, but find no compelling binary scenario that could reasonably explain the
full, peculiar abundance pattern of the Hercules dSph galaxy.Comment: 7 pages, 3 figures, accepted for publication in the Astrophysical
Journa
An inefficient dwarf: Chemical abundances and the evolution of the Ursa Minor dwarf spheroidal galaxy
We present detailed chemical element abundance ratios of 17 elements in three
metal poor stars in the Ursa Minor dwarf spheroidal galaxy, which we combine
with extant data from the literature to assess the predictions of a novel suite
of galaxy chemical evolution models. The spectroscopic data were obtained with
the Keck/HIRES instrument and revealed low metallicities of [Fe/H]=-2.12, -2.13
and -2.67 dex. While the most metal poor star in our sample shows an
overabundance of [Mn/Fe] and other Fe-peak elements, our overall findings are
in agreement with previous studies of this galaxy: elevated values of the
[alpha/Fe] ratios that are similar to, or only slightly lower than, the halo
values but with SN Ia enrichment at very low metallicity, as well as an
enhancement of the ratio of first to second peak neutron capture elements
[Y/Ba] with decreasing metallicity. The chemical evolution models which were
tailored to reproduce the metallicity distribution function of the dSph,
indicate that UMi had an extended star formation which lasted nearly 5 Gyr with
low efficiency and are able to explain the [Y/Ba] enhancement at low
metallicity for the first time. In particular, we show that the present day
lack of gas is probably due to continuous loss of gas from the system, which we
model as winds.Comment: 10 pages, 7 figures, table
Dark matter in disk galaxies I: a Markov Chain Monte Carlo method and application to DDO 154
We present a new method to constrain the dark matter halo density profiles of
disk galaxies. Our algorithm employs a Markov Chain Monte Carlo (MCMC) approach
to explore the parameter space of a general family of dark matter profiles. We
improve upon previous analyses by considering a wider range of halo profiles
and by explicitly identifying cases in which the data are insufficient to break
the degeneracies between the model parameters. We demonstrate the robustness of
our algorithm using artificial data sets and show that reliable estimates of
the halo density profile can be obtained from data of comparable quality to
those currently available for low surface brightness (LSB) galaxies. We present
our results in terms of physical quantities which are constrained by the data,
and find that the logarithmic slope of the halo density profile at the radius
of the innermost data point of a measured rotation curve can be strongly
constrained in LSB ([Vstar/Vobs]max ~ 0.16) galaxies. High surface brightness
galaxies ([Vstar/Vobs]max ~ 0.79) require additional information on the
mass-to-light ratio of the stellar population - our approach naturally
identifies those galaxies for which this is necessary. We apply our method to
observed data for the dwarf irregular galaxy DDO 154 and recover a logarithmic
halo slope of -0.39 +- 0.11 at a radius of 0.14 kpc. Our analysis validates
earlier estimates which were based on the fitting of a limited set of
individual halo models, but constitutes a more robust constraint than was
possible using other techniques since it marginalises over a wide range of halo
profiles.Comment: Accepted by MNRAS 20/05/1
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