76,959 research outputs found

### How do galactic winds affect the Lyalpha forest?

We investigate the effect of galactic winds on the Lyalpha forest in
cosmological simulations of structure and galaxy formation. We combine high
resolution N-body simulations of the evolution of the dark matter with a
semi-analytic model for the formation and evolution of galaxies which includes
detailed prescriptions for the long-term evolution of galactic winds. This
model is the first to describe the evolution of outflows as a two-phase process
(an adiabatic bubble followed by a momentum--driven shell) and to include
metal--dependent cooling of the outflowing material. We find that the main
statistical properties of the Lyalpha forest, namely the flux power spectrum
P(k) and the flux probability distribution function (PDF), are not
significantly affected by winds and so do not significantly constrain wind
models. Winds around galaxies do, however, produce detectable signatures in the
forest, in particular, increased flux transmissivity inside hot bubbles, and
narrow, saturated absorption lines caused by dense cooled shells. We find that
the Lyalpha flux transmissivity is highly enhanced near strongly wind-blowing
galaxies, almost half of all high-redshift galaxies in our sample, in agreement
with the results of Adelberger et al. (2005). Finally, we propose a new method
to identify absorption lines potentially due to wind shells in the Lyalpha
forest: we calculate the abundance of saturated regions in spectra as a
function of region width and we find that the number with widths smaller than
about 1 Angstrom at z=3 and 0.6 Angstrom at z=2 may be more than doubled. This
should be detectable in real spectra.Comment: 14 pages, 11 figures. Minor changes in the text. Accepted for
publication in MNRA

### Effect of the W-term for a t-U-W Hubbard ladder

Antiferromagnetic and d_{x2-y2}-pairing correlations appear delicately
balanced in the 2D Hubbard model. Whether doping can tip the balance to pairing
is unclear and models with additional interaction terms have been studied. In
one of these, the square of a local hopping kinetic energy H_W was found to
favor pairing. However, such a term can be separated into a number of simpler
processes and one would like to know which of these terms are responsible for
enhancing the pairing. Here we analyze these processes for a 2-leg Hubbard
ladder

### Energetics of Domain Walls in the 2D t-J model

Using the density matrix renormalization group, we calculate the energy of a
domain wall in the 2D t-J model as a function of the linear hole density
\rho_\ell, as well as the interaction energy between walls, for J/t=0.35. Based
on these results, we conclude that the ground state always has domain walls for
dopings 0 < x < 0.3. For x < 0.125, the system has (1,0) domain walls with
\rho_\ell ~ 0.5, while for 0.125 < x < 0.17, the system has a possibly
phase-separated mixture of walls with \rho_\ell ~ 0.5 and \rho_\ell =1. For x >
0.17, there are only walls with \rho_\ell =1. For \rho_\ell = 1, diagonal (1,1)
domain walls have very nearly the same energy as (1,0) domain walls.Comment: Several minor changes. Four pages, four encapsulated figure

### Bulgeless Galaxies and their Angular Momentum Problem

The specific angular momentum of Cold Dark Matter (CDM) halos in a
$\Lambda$CDM universe is investigated. Their dimensionless specific angular
momentum $\lambda'=\frac{j}{\sqrt{2}V_{vir} R{vir}}$ with $V_{vir}$ and
$R_{vir}$ the virial velocity and virial radius, respectively depends strongly
on their merging histories. We investigate a set of $\Lambda$CDM simulations
and explore the specific angular momentum content of halos formed through
various merging histories. Halos with a quiet merging history, dominated by
minor mergers and accretion until the present epoch, acquire by tidal torques
on average only 2% to 3% of the angular momentum required for their rotational
support ($\lambda'=0.02$). This is in conflict with observational data for a
sample of late-type bulgeless galaxies which indicates that those galaxies
reside in dark halos with exceptionally high values of $\lambda' \approx
0.06-0.07$. Minor mergers and accretion preserve or slowly increase the
specific angular momentum of dark halos with time. This mechanism is however
not efficient enough in order to explain the observed spin values for late-type
dwarf galaxies. Energetic feedback processes have been invoked to solve the
problem that gas loses a large fraction of its specific angular momentum during
infall. Under the assumption that dark halos hosting bulgeless galaxies acquire
their mass via quiescent accretion, our results indicate yet another serious
problem: the specific angular momentum gained during the formation of these
objects is not large enough to explain their observed rotational
properties,even if no angular momentum would be lost during gas infall.Comment: 4 pages, 3 figures. To appear in September 1, 2004, issue of ApJ
Letter

### Testing Cosmological Models With A \lya Forest Statistic: The High End Of The Optical Depth Distribution

We pay particular attention to the high end of the \lya optical depth
distribution of a quasar spectrum. Based on the flux distribution
(Miralda-Escud\'e et al 1996), a simple yet seemingly cosmological model
-differentiating statistic, $\Delta_{\tau_0}$ -- the cumulative probability of
a quasar spectrum with \lya optical depth greater than a high value $\tau_0$
-- is emphasized. It is shown that two different models -- the cold dark matter
model with a cosmological constant and the mixed hot and cold dark matter
model, both normalized to COBE and local galaxy cluster abundance -- yield
quite different values of $\Delta_{\tau_0}$: 0.13 of the former versus 0.058 of
the latter for $\tau_0=3.0$ at $z=3$. Moreover, it is argued that
$\Delta_{\tau_0}$ may be fairly robust to compute theoretically because it does
not seem to depend sensitively on small variations of simulations parameters
such as radiation field, cooling, feedback process, radiative transfer,
resolution and simulation volume within the plausible ranges of the concerned
quantities. Furthermore, it is illustrated that $\Delta_{\tau_0}$ can be
obtained sufficiently accurately from currently available observed quasar
spectra for $\tau_0\sim 3.0-4.0$, when observational noise is properly taken
into account. We anticipate that analyses of observations of quasar \lya
absorption spectra over a range of redshift may be able to constrain the
redshift evolution of the amplitude of the density fluctuations on
small-to-intermediate scales, therefore providing an independent constraint on
$\Omega_0$, $\Omega_{0,HDM}$ and $\Lambda_0$.Comment: ApJ Letters, in press, substantial changes have been made from the
last versio

### The Structure and Clustering of Lyman Break Galaxies

The number density and clustering properties of Lyman-break galaxies (LBGs)
are consistent with them being the central galaxies of the most massive dark
halos present at z~3. This conclusion holds in all currently popular
hierarchical models for structure formation, and is almost independent of the
global cosmological parameters. We examine whether the sizes, luminosities,
kinematics and star-formation rates of LBGs are also consistent with this
identification. Simple formation models tuned to give good fits to low redshift
galaxies can predict the distribution of these quantities in the LBG
population. The LBGs should be small (with typical half-light radii of 0.6-2
kpc/h), should inhabit haloes of moderately high circular velocity (180-290
km/s) but have low stellar velocity dispersions (70-120 km/s) and should have
substantial star formation rates (15-100 Msun/yr). The numbers here refer to
the predicted median values in the LBG sample of Adelberger et al. (1998); the
first assumes an Omega=1 universe and the second a flat universe with
Omega=0.3. For either cosmology these predictions are consistent with the
current (rather limited) observational data. Following the work of Kennicutt
(1998) we assume stars to form more rapidly in gas of higher surface density.
This predicts that LBG samples should preferentially contain objects with low
angular momentum, and so small size, for their mass. In contrast, samples of
damped Lyman alpha systems (DLSs), should be biased towards objects with large
angular momentum. Bright LBGs and DLSs may therefore form distinct populations,
with very different sizes and star formation rates, LBGs being smaller and more
metal-rich than DLSs of similar mass and redshift.Comment: 27 pages, 9 figures, MNRAS submitte

### A Theoretical Model for the $M_{\rm bh}-\sigma$ Relation for Supermassive Black Holes in Galaxies

We construct a model for the formation of black holes within galactic bulges.
The initial state is a slowly rotating isothermal sphere, characterized by
effective transport speed \aeff and rotation rate $\Omega$. The black hole
mass is determined when the centrifugal radius of the collapse flow exceeds the
capture radius of the central black hole. This model reproduces the observed
correlation between black hole masses and galactic velocity dispersions, \mbh
\approx 10^8 M_\odot (\sigma/200 \kms)^4, where \sigma = \sqrt{2} \aeff.
This model also predicts the ratio \mrat of black hole mass to host mass:
\mrat $\approx$ 0.004 (\sigma/200 \kms).Comment: 9 pages, 2 figures, submitted to Astrophysical Journal Letter

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