1,469 research outputs found
Star Formation, Supernovae Feedback and the Angular Momentum Problem in Numerical CDM Cosmogony: Half Way There?
We present a smoothed particle hydrodynamic (SPH) simulation that reproduces
a galaxy that is a moderate facsimile of those observed. The primary failing
point of previous simulations of disk formation, namely excessive transport of
angular momentum from gas to dark matter, is ameliorated by the inclusion of a
supernova feedback algorithm that allows energy to persist in the model ISM for
a period corresponding to the lifetime of stellar associations. The inclusion
of feedback leads to a disk at a redshift , with a specific angular
momentum content within 10% of the value required to fit observations. An
exponential fit to the disk baryon surface density gives a scale length within
17% of the theoretical value. Runs without feedback, with or without star
formation, exhibit the drastic angular momentum transport observed elsewhere.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter
Photometric evolution of dusty starburst mergers:On the nature of ultra-luminous infrared galaxies
By performing N-body simulations of chemodynamical evolution of galaxies with
dusty starbursts, we investigate photometric evolution of gas-rich major
mergers in order to explore the nature of ultraluminous infrared galaxies
(ULIRGs) with the total infrared luminosity ( for
m) of . Main results are the following three.
(1) Global colors and absolute magnitudes the during dusty starburst of a major
merger do not change with time significantly, because interstellar dust heavily
obscures young starburst populations that could cause rapid evolution of
photometric properties of the merger. (2) Dust extinction of stellar
populations in a galaxy merger with large infrared luminosity (
) is selective in the sense that younger stellar
populations are preferentially obscured by dust than old ones. This is because
younger populations are located in the central region where a larger amount of
dusty interstellar gas can be transferred from the outer gas-rich regions of
the merger. (3) Both and the ratio of to band
luminosity ) increases as the star formation rate increase during
the starburst of the present merger model, resulting in the positive
correlation between and .Comment: 32 pages 25 figures,2001,ApJ,in press. For all 25 PS figures
(including fig25.ps), see
http://newt.phys.unsw.edu.au/~bekki/res.dir/paper.dir/apj06.dir/fig.tar.g
Working Healthy Enrollees Report New and Persisting Challenges
Working Healthy participants are sent an annual satisfaction survey to evaluate their experiences with the program. Participants have consistently said that Working Healthy is a good program that allows them to work and maintain their health benefits, which reduces their stress and eliminates worry about whether or not they will be able to afford the health care and medication they need. Working Healthy not only benefits the state through premium collection and increased taxes paid, participants say it improves their mental health and quality of lif
Bostonia. Volume 13
Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs
On Compact Routing for the Internet
While there exist compact routing schemes designed for grids, trees, and
Internet-like topologies that offer routing tables of sizes that scale
logarithmically with the network size, we demonstrate in this paper that in
view of recent results in compact routing research, such logarithmic scaling on
Internet-like topologies is fundamentally impossible in the presence of
topology dynamics or topology-independent (flat) addressing. We use analytic
arguments to show that the number of routing control messages per topology
change cannot scale better than linearly on Internet-like topologies. We also
employ simulations to confirm that logarithmic routing table size scaling gets
broken by topology-independent addressing, a cornerstone of popular
locator-identifier split proposals aiming at improving routing scaling in the
presence of network topology dynamics or host mobility. These pessimistic
findings lead us to the conclusion that a fundamental re-examination of
assumptions behind routing models and abstractions is needed in order to find a
routing architecture that would be able to scale ``indefinitely.''Comment: This is a significantly revised, journal version of cs/050802
The Rest-Frame UV Luminosity Density of Star-Forming Galaxies at Redshifts z>3.5
We have measured the rest--frame lambda~1500 Ang comoving specific luminosity
density of star--forming galaxies at redshift 3.5<z<6.5 from deep images taken
with the Hubble Space Telescope (HST)and the Advanced Camera for Surveys (ACS),
obtained as part of the Great Observatories Origins Deep Survey (GOODS). We
used color selection criteria to construct samples of star--forming galaxies at
redshifts z~4, 5 and 6, identified by the signature of the 912 Ang Lyman
continuum discontinuity and Lyman-alpha forest blanketing in their rest--frame
UV colors (Lyman--break galaxies). The ACS samples cover ~0.09 square degree,
and are also relatively deep, reaching between 0.2 and 0.5 L_3^*, depending on
the redshift, where is the characteristic UV luminosity of Lyman--break
galaxies at z~3. The specific luminosity density of Lyman--break galaxies
appears to be nearly constant with redshift from z~3 to z~6, although the
measure at z~6 remains relatively uncertain, because it depends on the accurate
estimate of the faint counts of the z~6 sample. If Lyman--break galaxies are
fair tracers of the cosmic star formation activity, our results suggest that at
z~6 the universe was already producing stars as vigorously as it did near its
maximum several Gyr later, at 1<~z<~3. Thus, the onset of large--scale star
formation in the universe is to be sought at around z~6 or higher, namely at
less than ~7% of the current cosmic age.Comment: AAS LaTeX macros 4.0, 11 pages, 1 postscript figure. Accepted for
publication in The Astrophysical Journal, Letter. Minor changes to the figure
caption. The data and the GOODS-group papers can be found at
http://www.stsci.edu/science/goods
The HI Content of Local Late-Type Galaxies
We present a solid relationship between the neutral hydrogen (HI) disk mass
and the stellar disk mass of late-type galaxies in the local universe. This
relationship is derived by comparing the stellar disk mass function from the
Sloan Digital Sky Survey and the HI mass function from the HI Parkes All Sky
Survey (HIPASS). We find that the HI mass in late-type galaxies tightly
correlates with the stellar mass over three orders of magnitude in stellar disk
mass. We cross-check our result with that obtained from a sample of HIPASS
objects for which the stellar mass has been obtained by inner kinematics. In
addition, we derive the HI versus halo mass relationship and the dependence of
all the baryonic components in spirals on the host halo mass. These
relationships bear the imprint of the processes ruling galaxy formation, and
highlight the inefficiency of galaxies both in forming stars and in retaining
their pristine HI gas.Comment: 6 pages, 5 figures. Match to the published version. References
update
On the distribution of initial masses of stellar clusters inferred from synthesis models
The fundamental properties of stellar clusters, such as the age or the total
initial mass in stars, are often inferred from population synthesis models. The
predicted properties are then used to constrain the physical mechanisms
involved in the formation of such clusters in a variety of environments.
Population synthesis models cannot, however, be applied blindy to such systems.
We show that synthesis models cannot be used in the usual straightforward way
to small-mass clusters (say, M < few times 10**4 Mo). The reason is that the
basic hypothesis underlying population synthesis (a fixed proportionality
between the number of stars in the different evolutionary phases) is not
fulfilled in these clusters due to their small number of stars. This incomplete
sampling of the stellar mass function results in a non-gaussian distribution of
the mass-luminosity ratio for clusters that share the same evolutionary
conditions (age, metallicity and initial stellar mass distribution function).
We review some tests that can be carried out a priori to check whether a given
cluster can be analysed with the fully-sampled standard population synthesis
models, or, on the contrary, a probabilistic framework must be used. This leads
to a re-assessment in the estimation of the low-mass tail in the distribution
function of initial masses of stellar clusters.Comment: 5 pages, 1 figure, to appear in ``Young Massive Star Clusters -
Initial Conditions and Environments'', 2008, Astrophysics & Space Science,
eds. E. Perez, R. de Grijs, R. M. Gonzalez Delgad
Modeling the dynamical evolution of the M87 globular cluster system
We study the dynamical evolution of the M87 globular cluster system (GCS)
with a number of numerical simulations. We explore a range of different initial
conditions for the GCS mass function (GCMF), for the GCS spatial distribution
and for the GCS velocity distribution. We confirm that an initial power-law
GCMF like that observed in young cluster systems can be readily transformed
through dynamical processes into a bell-shaped GCMF. However,only models with
initial velocity distributions characterized by a strong radial anisotropy
increasing with the galactocentric distance are able to reproduce the observed
constancy of the GCMF at all radii.We show that such strongly radial orbital
distributions are inconsistent with the observed kinematics of the M87 GCS. The
evolution of models with a bell-shaped GCMF with a turnover similar to that
currently observed in old GCS is also investigated. We show that models with
this initial GCMF can satisfy all the observational constraints currently
available on the GCS spatial distribution,the GCS velocity distribution and on
the GCMF properties.In particular these models successfully reproduce both the
lack of a radial gradient of the GCS mean mass recently found in an analysis of
HST images of M87 at multiple locations, and the observed kinematics of the M87
GCS.Our simulations also show that evolutionary processes significantly affect
the initial GCS properties by leading to the disruption of many clusters and
changing the masses of those which survive.The preferential disruption of inner
clusters flattens the initial GCS number density profile and it can explain the
rising specific frequency with radius; we show that the inner flattening
observed in the M87 GCS spatial distribution can be the result of the effects
of dynamical evolution on an initially steep density profile. (abridged)Comment: 15 pages,14 figures;accepted for publication in The Astrophysical
Journa
The CORALS Survey I: New Estimates of the Number Density and Gas Content of Damped Lyman Alpha Systems Free from Dust Bias
(Abridged) We present the first results from the Complete Optical and Radio
Absorption Line System (CORALS) survey. We have compiled a homogeneous sample
of radio-selected QSOs from the Parkes Catalogue and searched for damped Lyman
alpha systems (DLAs) towards every target, irrespective of its optical
magnitude. This approach circumvents selection effects -- particularly from
intervening dust -- which have long been suspected to affect DLA surveys in
optically-selected, magnitude-limited QSO samples. The CORALS data set consists
of 66 z_em > 2.2 QSOs in which 22 DLAs with absorption redshifts 1.8 < z_abs <
z_em have been identified over a total redshift interval Delta z = 55.46. In
this first paper of the CORALS series we describe the sample, present
intermediate resolution spectroscopy and determine the population statistics of
DLAs. We deduce a value of the neutral gas mass density traced by DLAs
(expressed as a fraction of the closure density) log Omega_DLA h =
-2.59^{+0.17}_{-0.24}, and a number density of DLAs per unit redshift n(z) =
0.31^{+0.09}_{-0.08}, both at a mean redshift = 2.37. Taking into account
the errors, we conclude that dust-induced bias in previous surveys may have led
to an underestimate of these quantities by at most a factor of two and we have
not uncovered a previously unrecognised population of high column density DLAs
in front of faint QSOs.Comment: 25 pages, accepted for publication in A&
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