62 research outputs found
Dwarf Galaxies in Voids: Dark Matter Halos and Gas Cooling
Galaxy surveys have shown that luminous galaxies are mainly distributed in
large filaments and galaxy clusters. The remaining large volumes are virtually
devoid of luminous galaxies. This is in concordance with the formation of the
large-scale structure in Universe as derived from cosmological simulations.
However, the numerical results indicate that cosmological voids are abundantly
populated with dark matter haloes which may in principle host dwarf galaxies.
Observational efforts have in contrast revealed, that voids are apparently
devoid of dwarf galaxies. We investigate the formation of dwarf galaxies in
voids by hydrodynamical cosmological simulations. Due to the cosmic
ultra-violet background radiation low-mass haloes show generally are reduced
baryon fraction. We determine the characteristic mass below which dwarf
galaxies are baryon deficient. We show that the circular velocity below which
the accretion of baryons is suppressed is approximately 40 km/s. The suppressed
baryon accretion is caused by the photo-heating due to the UV-background. We
set up a spherical halo model and show that the effective equation of state of
the gas in the periphery of dwarf galaxies determines the characteristic mass.
This implies that any process which heats the gas around dwarf galaxies
increases the characteristic mass and thus reduces the number of observable
dwarf galaxies.Comment: 23 pages. To appear in Advances in Astronomy, Dwarf-Galaxy Cosmology
issu
Radio signature of cosmological structure formation shocks
In the course of the formation of cosmological structures, large shock waves
are generated in the intra-cluster medium. In analogy to processes in supernova
remnants, these shock waves may generate a significant population of
relativistic electrons which, in turn, produce observable synchrotron emission.
The extended radio relics found at the periphery of several clusters and
possibly also a fraction of radio halo emission may have this origin. Here we
derive an analytic expression for (i) the total radio power in the downstream
region of a cosmological shock wave and (ii) the width of the radio-emitting
region. These expressions predict a spectral slope close to -1 for strong
shocks. Moderate shocks, such as those produced in mergers between clusters of
galaxies, lead to a somewhat steeper spectrum. Moreover, we predict an upper
limit for the radio power of cosmological shocks. Comparing our results to the
radio relics in Abell115, 2256, and 3667, we conclude that the magnetic field
in these relics is typically at a level of 0.1 mu G. Magnetic fields in the
intra-cluster medium are presumably generated by the shocks themselves, this
allows us to calculate the radio emission as a function of the cluster
temperature. The resulting emissions agree very well with the radio
power-temperature relation found for cluster halos. Finally, we show that
cosmic accretion shocks generate less radio emission than merger shock waves.
The latter may, however, be detected with upcoming radio telescopes.Comment: 28 pages, 8 figures, MNRAS accepte
Too small to form a galaxy: How the UV background determines the baryon fraction
The cosmic ultraviolet background (UVB) heats the intergalactic medium (IGM),
as a result the gas in dark matter halos below a certain mass is too hot to
cool within a Hubble time. The UVB effectively suppresses the formation of
dwarf galaxies. Using high resolution cosmological hydrodynamical simulations
we show that photo heating leads to small baryon fractions in halos below ~
6x10^9 h^{-1}M_sun, independent of the cosmic environment. The simulations are
carried out assuming a homogeneous UVB with flux densities as given by Haardt &
Madau (1996). A halo may stop to condense gas significantly after the universe
is reionised, namely when its mass falls below the characteristic mass scale
set by the photo heating. Assuming a spherical halo model we derive this
characteristic mass analytically and identify the main mechanisms that prevent
the gas from cooling in small halos. The theoretically derived characteristic
mass is smaller than the one obtained from observations. Increasing the energy
per ionising photon by a factor between four and eight would be sufficient to
reconcile both. This is equivalent to an average temperature of the IGM of ~
10^4 K. In this sense the faint end of the luminosity function may serve as a
calorimeter for the IGM.Comment: To appear in Proceedings of IAU Symp #244, "Dark Galaxies and Lost
Baryons", June 2007, 5 pages including 3 figure
Reconciling radio relic observations and simulations: The NVSS sample
The diffusive shock acceleration scenario is usually invoked to explain radio
relics, although the detailed driving mechanism is still a matter of debate.
Our aim is to constrain models for the origin of radio relics by comparing
observed relic samples with simulated ones. Here we present a framework to
homogeneously extract the whole sample of known radio relics from NVSS so that
it can be used for comparison with cosmological simulations. In this way, we
can better handle intrinsic biases in the analysis of the radio relic
population. In addition, we show some properties of the resulting NVSS sample
relics such as the correlation between relic shape and orientation with respect
to the cluster. Also, we briefly discuss the typical relic surface brightness
and its relation to projected cluster distance and relic angular sizes.Comment: 4 pages, 2 figures. Proceedings of "The many facets of extragalactic
radio surveys: towards new scientific challenges" (EXTRA-RADSUR2015). 20-23
October 2015. Bologna, Ital
Dwarf galaxies in voids: Suppressing star formation with photo-heating
We study structure formation in cosmological void regions using
high-resolution hydrodynamical simulations. Despite being significantly
underdense, voids are populated abundantly with small dark matter halos which
should appear as dwarf galaxies if their star formation is not suppressed
significantly. We here investigate to which extent the cosmological
UV-background photo-evaporates baryons out of halos of dwarf galaxies, and
thereby limits their cooling and star formation rates. Assuming a Haardt &
Madau UV-background with reionisation at redshift z=6, our samples of simulated
galaxies show that halos with masses below a characteristic mass of M_c(z=0) =
6.5 x 10^9 h^{-1} M_sun are baryon-poor, but in general not completely empty,
because baryons that are in the condensed cold phase or are already locked up
in stars resist evaporation. In halos with mass M < M_c, we find that
photo-heating suppresses further cooling of gas. The redshift and UV-background
dependent characteristic mass M_c(z) can be understood from the equilibrium
temperature between heating and cooling at a characteristic overdensity of
\delta ~ 1000. If a halo is massive enough to compress gas to this density
despite the presence of the UV background, gas is free to `enter' the condensed
phase and cooling continues in the halo, otherwise it stalls. By analysing the
mass accretion histories of dwarf galaxies in voids, we show that they can
build up a significant amount of condensed mass at early times before the epoch
of reionisation. Later on, the amount of mass in this phase remains roughly
constant, but the masses of the dark matter halos continue to increase.
(abridged)Comment: revised version as accepted by MNRAS, 15 pages, 15 figures, new
simulation results and a significantly extended discussion have been include
Can cluster merger shocks reproduce the luminosity and shape distribution of radio relics?
Radio relics in galaxy clusters are believed to trace merger shock fronts. If cosmological structure formation determines the luminosity, size and shape distributions of radio relics, then merger shocks need to be lighted up in a homogeneous way. We investigate if a mock relic sample, obtained from zoomed galaxy cluster simulations, is able to match the properties of relics measured in the NRAO VLA Sky Survey (NVSS). We compile a list of all radio relics known to date and homogeneously measure their parameters in all NVSS images and apply the same procedure to relics in our simulations. Number counts in the mock relic sample increase more steeply towards lower relic flux densities, suggesting an incompleteness of NVSS in this regime. Overall, we find that NVSS and mock samples show similar properties. However, large simulated relics tend to be somewhat smaller and closer to the cluster centre than observed ones. Besides this, the mock sample reproduces very well-known correlations for radio relics, in particular those relating the radio luminosity with the largest linear size and the X-ray luminosity. We show that these correlations are largely governed by the sensitivity of the NVSS observations. Mock relics show a similar orientation with respect to the direction to the cluster centre as the NVSS sample. Moreover, we find that their maximum radio luminosity roughly correlates with cluster mass, although displaying a large scatter. The overall good agreement between NVSS and the mock sample suggests that properties of radio relics are indeed governed by merger shock fronts, emitting in a homogeneous fashion. Our study demonstrates that the combination of mock observations and data from upcoming radio surveys will allow us to shed light on both the origin of radio relics and the nature of the intracluster medium.Fil: Nuza, Sebastian Ernesto. Consejo Nacional de InvestigaciĂłnes CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de AstronomĂa y FĂsica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de AstronomĂa y FĂsica del Espacio; Argentina. Universidad de Buenos Aires; ArgentinaFil: Gelszinnis, Jakob. ThĂĽringer Landessternwarte; AlemaniaFil: Hoeft, Matthias. ThĂĽringer Landessternwarte; AlemaniaFil: Yepes, Gustavo. Universidad AutĂłnoma de Madrid; Españ
Radio relics in a cosmological cluster merger simulation
Motivated by the discovery of a number of radio relics we investigate the
fate of fossil radio plasma during a merger of clusters of galaxies using
cosmological smoothed-particle hydrodynamics simulations. Radio relics are
extended, steep-spectrum radio sources that do not seem to be associated with a
host galaxy. One proposed scenario whereby these relics form is through the
compression of fossil radio plasma during a merger between clusters. The
ensuing compression of the plasma can lead to a substantial increase in
synchrotron luminosity and this appears as a radio relic. Our simulations show
that relics are most likely to be found at the periphery of the cluster at the
positions of the outgoing merger shock waves. Relics are expected to be very
rare in the centre of the cluster where the life time of relativistic electrons
is short and shock waves are weaker than in the cooler, peripheral regions of
the cluster. These predictions can soon be tested with upcoming low-frequency
radio telescopes.Comment: 6 pages, 3 figures (high-resolution figures are available at
http://www.faculty.iu-bremen.de/~mbrueggen/), submitted to MNRA
Recommended from our members
Reconciling radio relic observations and simulations: The NVSS sample
The diffusive shock acceleration scenario is usually invoked to explain radio relics, although the
detailed driving mechanism is still a matter of debate. Our aim is to constrain models for the
origin of radio relics by comparing observed relic samples with simulated ones. Here we present
a framework to homogeneously extract the whole sample of known radio relics from NVSS so
that it can be used for comparison with cosmological simulations. In this way, we can better
handle intrinsic biases in the analysis of the radio relic population. In addition, we show some
properties of the resulting NVSS sample relics such as the correlation between relic shape and
orientation with respect to the cluster. Also, we briefly discuss the typical relic surface brightness
and its relation to projected cluster distance and relic angular sizes
Polarisation of Radio Relics in Galaxy Clusters
Radio emission in the form of giant radio relics is observed at the periphery
of galaxy clusters. This non-thermal emission is an important tracer for
cosmic-ray electrons and intracluster magnetic fields. One striking
observational feature of these objects is their high degree of polarisation
which provides information on the magnetic fields at the relics' positions. In
this contribution, we test if state-of-the-art high resolution cosmological
simulations are able to reproduce the polarisation features of radio relics.
Therefore, we present a new analysis of high-resolution cosmological
simulations to study the polarisation properties of radio relics in detail. In
order to compare our results with current and future radio observations, we
create mock radio observations of the diffuse polarised emission from a massive
galaxy clusters using six different projections, for different observing
frequencies and for different telescopes. Our simulations suggest that, due to
the effect of Faraday rotation, it is extremely difficult to relate the
morphology of the polarised emission for observing frequencies below $1.4 \
\mathrm{GHz}$ to the real magnetic field structure in relics. We can reproduce
the observed degree of polarisation and also several small-scale structures
observed in real radio relics, but further work would be needed to reproduce
some large-scale spectacular features as observed in real radio relics, such as
the "Sausage" and the "Toothbrush" relics.Comment: Accepted for publication in MNRAS, 20 pages, 13 figure
The Fossil Phase in the Life of a Galaxy Group
We investigate the origin and evolution of fossil groups in a concordance
LCDM cosmological simulation. We consider haloes with masses between
(1-5)\times10^{13} \hMsun and study the physical mechanisms that lead to the
formation of the large gap in magnitude between the brightest and the second
most bright group member, which is typical for these fossil systems. Fossil
groups are found to have high dark matter concentrations, which we can relate
to their early formation time. The large magnitude-gaps arise after the groups
have build up half of their final mass, due to merging of massive group
members. We show that the existence of fossil systems is primarily driven by
the relatively early infall of massive satellites, and that we do not find a
strong environmental dependence for these systems. In addition, we find
tentative evidence for fossil group satellites falling in on orbits with
typically lower angular momentum, which might lead to a more efficient merger
onto the host. We find a population of groups at higher redshifts that go
through a ``fossil phase'': a stage where they show a large magnitude-gap,
which is terminated by renewed infall from their environment.Comment: 9 pages and 8 figures, submitted to MNRA
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