4,214 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
The Semantic Automated Discovery and Integration (SADI) Web service Design-Pattern, API and Reference Implementation
Background. 
The complexity and inter-related nature of biological data poses a difficult challenge for data and tool integration. There has been a proliferation of interoperability standards and projects over the past decade, none of which has been widely adopted by the bioinformatics community. Recent attempts have focused on the use of semantics to assist integration, and Semantic Web technologies are being welcomed by this community.

Description. 
SADI – Semantic Automated Discovery and Integration – is a lightweight set of fully standards-compliant Semantic Web service design patterns that simplify the publication of services of the type commonly found in bioinformatics and other scientific domains. Using Semantic Web technologies at every level of the Web services “stack”, SADI services consume and produce instances of OWL Classes following a small number of very straightforward best-practices. In addition, we provide codebases that support these best-practices, and plug-in tools to popular developer and client software that dramatically simplify deployment of services by providers, and the discovery and utilization of those services by their consumers.

Conclusions.
SADI Services are fully compliant with, and utilize only foundational Web standards; are simple to create and maintain for service providers; and can be discovered and utilized in a very intuitive way by biologist end-users. In addition, the SADI design patterns significantly improve the ability of software to automatically discover appropriate services based on user-needs, and automatically chain these into complex analytical workflows. We show that, when resources are exposed through SADI, data compliant with a given ontological model can be automatically gathered, or generated, from these distributed, non-coordinating resources - a behavior we have not observed in any other Semantic system. Finally, we show that, using SADI, data dynamically generated from Web services can be explored in a manner very similar to data housed in static triple-stores, thus facilitating the intersection of Web services and Semantic Web technologies
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
AGN outflows trigger starbursts in gas-rich galaxies
Recent well resolved numerical simulations of AGN feedback have shown that
its effects on the host galaxy may be not only negative but also positive. In
the late gas poor phase, AGN feedback blows the gas away and terminates star
formation. However, in the gas-rich phase(s), AGN outflows trigger star
formation by over-compressing cold dense gas and thus provide positive feedback
on their hosts. In this paper we study this AGN-triggered starburst effect. We
show that star formation rate in the burst increases until the star formation
feedback counteracts locally the AGN outflow compression. Globally, this
predicts a strong nearly linear statistical correlation between the AGN and
starburst bolometric luminosities in disc galaxies, L_* \propto L_{AGN}^{5/6}.
The correlation is statistical only because AGN activity may fluctuate on short
time scales (as short as tens of years), and because AGN may turn off but its
effects on the host may continue to last until the AGN-driven outflow leaves
the host, which may be up to 10 times longer than the duration of the AGN
activity. The coefficient in front of this relation depends on the clumpiness
and morphology of the cold gas in the galaxy. A "maximum starburst" takes place
in am azimuthally uniform gas disc, for which we derive an upper limit of L_*
\sim 50 times larger than L_{AGN} for typical quasars. For more clumpy and/or
compact cold gas distributions, the starburst luminosity decreases. We also
suggest that similar AGN-triggerred starbursts are possible in hosts of all
geometries, including during galaxy mergers, provided the AGN is activated.
Finally, we note that due to the short duration of the AGN activity phase the
accelerating influence of AGN on starbursts may be much more common than
observations of simultaneous AGN and starbursts would suggest.Comment: 14 pages, 6 figures. Accepted for publication in MNRA
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