146 research outputs found
Assembly Bias of Dwarf-sized Dark Matter Haloes
Previous studies indicate that assembly bias effects are stronger for lower
mass dark matter haloes. Here we make use of high resolution re-simulations of
rich clusters and their surroundings from the Phoenix Project and a large
volume cosmological simulation, the Millennium-II run, to quantify assembly
bias effects on dwarf-sized dark matter haloes. We find that, in the regions
around massive clusters, dwarf-sized haloes ([10^9,10^{11}]\ms) form earlier
( in redshift) and possess larger ()
than the field galaxies. We find that this environmental dependence is largely
caused by tidal interactions between the ejected haloes and their former hosts,
while other large scale effects are less important. Finally we assess the
effects of assembly bias on dwarf galaxy formation with a sophisticated
semi-analytical galaxy formation model. We find that the dwarf galaxies near
massive clusters tend to be redder () and have three times
as much stellar mass compared to the field galaxies with the same halo mass.
These features should be seen with observational data.Comment: 8 pages, 8 figures, accepted by MNRA
The Evolution of Sizes and Specific Angular Momenta in Hierarchical Models of Galaxy Formation and Evolution
We extend our previous work focused at , studying the redshift
evolution of galaxy dynamical properties using the state-of-the-art
semi-analytic model GAEA: we show that the predicted size-mass relation for
disky/star forming and quiescent galaxies is in good agreement with
observational estimates, up to . Bulge dominated galaxies have sizes
that are offset low with respect to observational estimates, mainly due to our
implementation of disk instability at high redshift. At large masses, both
quiescent and bulge dominated galaxies have sizes smaller than observed. We
interpret this as a consequence of our most massive galaxies having larger gas
masses than observed, and therefore being more affected by dissipation. We
argue that a proper treatment of quasar driven winds is needed to alleviate
this problem. Our model compact galaxies have number densities in agreement
with observational estimates and they form most of their stars in small and low
angular momentum high- halos. GAEA predicts that a significant fraction of
compact galaxies forming at high- is bound to merge with larger structures
at lower redshifts: therefore they are not the progenitors of normal-size
passive galaxies at . Our model also predicts a stellar-halo size relation
that is in good agreement with observational estimates. The ratio between
stellar size and halo size is proportional to the halo spin and does not depend
on stellar mass but for the most massive galaxies, where AGN feedback leads to
a significant decrease of the retention factor (from about 80 per cent to 20
per cent).Comment: Accepted for publication in MNRAS, 17 pages, 11 figure
Structural and Dynamical Properties of Galaxies in a Hierarchical Universe: Sizes and Specific Angular Momenta
We use a state-of-the-art semi-analytic model to study the size and the
specific angular momentum of galaxies. Our model includes a specific treatment
for the angular momentum exchange between different galactic components. Disk
scale radii are estimated from the angular momentum of the gaseous/stellar
disk, while bulge sizes are estimated assuming energy conservation. The
predicted size--mass and angular momentum--mass relations are in fair agreement
with observational measurements in the local Universe, provided a treatment for
gas dissipation during major mergers is included. Our treatment for disk
instability leads to unrealistically small radii of bulges formed through this
channel, and predicts an offset between the size--mass relations of central and
satellite early-type galaxies, that is not observed. The model reproduces the
observed dependence of the size--mass relation on morphology, and predicts a
strong correlation between specific angular momentum and cold gas content. This
correlation is a natural consequence of galaxy evolution: gas-rich galaxies
reside in smaller halos, and form stars gradually until present day, while
gas-poor ones reside in massive halos, that formed most of their stars at early
epochs, when the angular momentum of their parent halos is low. The dynamical
and structural properties of galaxies can be strongly affected by a different
treatment for stellar feedback, as this would modify their star formation
history. A higher angular momentum for gas accreted through rapid mode does not
affect significantly the properties of massive galaxies today, but has a more
important effect on low-mass galaxies at higher redshift.Comment: 26 pages, 14 figures, 4 appendices. Accepted for publication in MNRA
BSG alignment of SDSS galaxy groups
We study the alignment signal between the distribution of brightest satellite
galaxies (BSGs) and the major axis of their host groups using SDSS group
catalog constructed by Yang et al. (2007). After correcting for the effect of
group ellipticity, a statistically significant (~ 5\sigma) major-axis alignment
is detected and the alignment angle is found to be 43.0 \pm 0.4 degrees. More
massive and richer groups show stronger BSG alignment. The BSG alignment around
blue BCGs is slightly stronger than that around red BCGs. And red BSGs have
much stronger major-axis alignment than blue BSGs. Unlike BSGs, other
satellites do not show very significant alignment with group major axis. We
further explore the BSG alignment in semi-analytic model (SAM) constructed by
Guo et al. (2011). We found general good agreement with observations: BSGs in
SAM show strong major-axis alignment which depends on group mass and richness
in the same way as observations; and none of other satellites exhibit prominent
alignment. However, discrepancy also exists in that the SAM shows opposite BSG
color dependence, which is most probably induced by the missing of large scale
environment ingredient in SAM. The combination of two popular scenarios can
explain the detected BSG alignment. The first one: satellites merged into the
group preferentially along the surrounding filaments, which is strongly aligned
with the major axis of the group. The second one: BSGs enter their host group
more recently than other satellites, then will preserve more information about
the assembling history and so the major-axis alignment. In SAM, we found
positive evidence for the second scenario by the fact that BSGs merged into
groups statistically more recently than other satellites. On the other hand,
although is opposite in SAM, the BSG color dependence in observation might
indicate the first scenario as well.Comment: 8 pages, 11 figures, ApJ accepte
On the influence of environment on star forming galaxies
We use our state-of-the-art semi analytic model for GAlaxy Evolution and
Assembly (GAEA), and observational measurements of nearby galaxies to study the
influence of the environment on the gas content and gaseous/stellar disc sizes
of star-forming galaxies. We analyse the origin of differences between physical
properties of satellites and those of their central counterparts, identified by
matching the Vmax of their host haloes at the accretion time of the satellites.
Our model reproduces nicely the differences between centrals and satellites
measured for the HI mass, size of the star-forming region, and stellar radii.
In contrast, our model predicts larger differences with respect to data for the
molecular gas mass and star formation rate. By analysing the progenitors of
central and satellite model galaxies, we find that differences in the gas
content arise after accretion, and can be entirely ascribed to the
instantaneous stripping of the hot gas reservoir. The suppression of cold gas
replenishment via cooling and star formation leads to a reduction of the cold
gas and of its density. Therefore, more molecular gas is lost than lower
density HI gas, and model satellites have less molecular gas and lower star
formation rates than observed satellites. We argue that these disagreements
could be largely resolved with the inclusion of a proper treatment for
ram-pressure stripping of cold gas and a more gradual stripping of the hot gas
reservoir. A more sophisticated treatment of angular momentum exchanges,
accounting for the multi-phase nature of the gaseous disc is also required.Comment: 15 pages, 9 figures, accepted for publication in MNRA
Damped Lyman- absorbers and atomic hydrogen in galaxies: the view of the GAEA model
Using the GAEA semi-analytic model, we analyse the connection between Damped
Lyman- systems (DLAs) and HI in galaxies. Our state-of-the-art
semi-analytic model is tuned to reproduce the local galaxy HI mass function,
and that also reproduces other important galaxy properties, including the
galaxy mass - gas metallicity relation. To produce catalogs of simulated DLAs
we throw random lines of sight in a composite simulated volume: dark
matter haloes with log are extracted
from the Millennium Simulation, while for we use the Millennium II, and for a halo occupation distribution model. At
, where observational data are more accurate, our fiducial model
predicts the correct shape of the column density distribution function, but its
normalization falls short of the observations, with the discrepancy increasing
at higher redshift. The agreement with observations is significantly improved
increasing both the HI masses and the disk radii of model galaxies by a factor
2, as implemented 'a posteriori' in our model. In the redshift range of
interest, haloes with give the major
contribution to , and the typical DLA host halo mass is . The simulated DLA metallicity distribution is in
relatively good agreement with observations, but our model predicts an excess
of DLAs at low metallicities. Our results suggest possible improvements for the
adopted modelling of the filtering mass and metal ejection in low-mass haloes.Comment: 21 pages, 16 figures. Accepted for publication in MNRA
H2-based star formation laws in hierarchical models of galaxy formation
5We update our recently published model for GAlaxy Evolution and Assembly (GAEA), to include a self-consistent treatment of the partition of cold gas in atomic and molecular hydrogen. Our model provides significant improvements with respect to previous ones used for similar studies. In particular, GAEA (i) includes a sophisticated chemical enrichment scheme accounting for non-instantaneous recycling of gas, metals and energy; (ii) reproduces the measured evolution of the galaxy stellar mass function; (iii) reasonably reproduces the observed correlation between galaxy stellar mass and gas metallicity at different redshifts. These are important prerequisites for models considering a metallicity-dependent efficiency of molecular gas formation. We also update our model for disc sizes and show that model predictions are in nice agreement with observational estimates for the gas, stellar and star-forming discs at different cosmic epochs. We analyse the influence of different star formation laws including empirical relations based on the hydrostatic pressure of the disc, analytic models and prescriptions derived from detailed hydrodynamical simulations. We find that modifying the star formation law does not affect significantly the global properties of model galaxies, neither their distributions. The only quantity showing significant deviations in different models is the cosmic molecular-to-atomic hydrogen ratio, particularly at high redshift. Unfortunately, however, this quantity also depends strongly on the modelling adopted for additional physical processes. Useful constraints on the physical processes regulating star formation can be obtained focusing on low-mass galaxies and/or at higher redshift. In this case, self-regulation has not yet washed out differences imprinted at early time.openopenXie, Lizhi; De Lucia, Gabriella; Hirschmann, Michaela; Fontanot, Fabio; Zoldan, AnnaXie, Lizhi; De Lucia, Gabriella; Hirschmann, Michaela; Fontanot, Fabio; Zoldan, Ann
The atomic hydrogen content of the post-reionization Universe
We present a comprehensive analysis of atomic hydrogen (H\u2009i) properties using a semi-analytical model of galaxy formation and N-body simulations covering a large cosmological volume at high resolution. We examine the H\u2009i mass function and the H\u2009i density, characterizing both their redshift evolution and their dependence on hosting halo mass. We analyse the H\u2009i content of dark matter haloes in the local Universe and up to redshift z = 5, discussing the contribution of different galaxy properties. We find that different assembly history plays a crucial role in the scatter of this relation. We propose new fitting functions useful for constructing mock H\u2009i maps with halo occupation distribution techniques. We investigate the H\u2009i clustering properties relevant for future 21 cm intensity mapping (IM) experiments, including the H\u2009i bias and the shot-noise level. The H\u2009i bias increases with redshift and it is roughly flat on the largest scales probed. The scale dependence is found at progressively larger scales with increasing redshift, apart from a dip feature at z = 0. The shot-noise values are consistent with the ones inferred by independent studies, confirming that shot noise will not be a limiting factor for IM experiments. We detail the contribution from various galaxy properties on the H\u2009i power spectrum and their relation to the halo bias. We find that H\u2009i poor satellite galaxies play an important role at the scales of the one-halo term. Finally, we present the 21 cm signal in redshift space, a fundamental prediction to be tested against data from future radio telescopes such as Square Kilometre Array
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