589 research outputs found
The Globular Cluster Luminosity Function as a Distance Indicator: Dynamical Effects
The dynamical evolution of the globular cluster systems in galaxies is
predicted, based on the standard dynamical theory normalized to the example of
the Milky Way. The major processes varying with the galactocentric distance are
the tidal shocks and dynamical friction. Our simple model explains, on a
quantitative basis, the observed differences of the inner and outer populations
of globular clusters. We can thus calculate corrections for dynamical evolution
for the luminosity function of globular clusters with the assumption that the
initial luminosity function is identical in all galaxies (and we can test this
assumption as well, in certain cases). Then we can compute the expected
distribution of absolute magnitudes and compare it with the observed
distribution of apparent magnitudes to estimate the distance moduli for M31 and
M87. Using this new method we find dm(M31)=24.05 +- 0.23, dm(M87)=30.83 +-
0.17, as compared to current best estimates using other methods of
dm(M31)=24.30 +- 0.20, dm(M87)=31.0 +- 0.1. As a check on the method we
compute, and compare with observations, the differences between the inner and
outer globular clusters in all three galaxies. This new method, coupled with
HST observations, promises to provide an independent method of estimating
distances to galaxies with recession velocities < 10,000 km/s, or D < 100 Mpc.Comment: 12 pages, 2 figures; submitted to ApJ Letter
The Expected Mass Function for Low Mass Galaxies in a CDM Cosmology: Is There a Problem?
It is well known that the mass function for_halos_ in CDM cosmology is a
relatively steep power law for low masses, possibly too steep to be consistent
with observations. But how steep is the_galaxy_ mass function? We have analyzed
the stellar and gas mass functions of the first massive luminous objects formed
in a \Lambda CDM universe, as calculated in the numerical simulation described
in Gnedin (2000ab). We found that while the dark matter mass function is steep,
the stellar and gas mass functions are flatter for low mass objects. The
stellar mass function is consistently flat at the low mass end. Moreover, while
the gas mass function follows the dark matter mass function until reionization
at z~7, between z=7 and z=4, the gas mass function also flattens considerably
at the low mass end. At z=4, the gas and stellar mass functions are fit by a
Schechter function with \alpha ~ -1.2 +/- 0.1, significantly shallower than the
dark matter halo mass function and consistent with some recent observations.
The baryonic mass functions are shallower because (a) the dark matter halo mass
function is consistent with the Press-Schechter formulation at low masses n(M)
M^-2 and (b) heating/cooling and ionization processes appear to cause baryons
to collect in halos with the relationship M_b M_d^4 at low masses. Combining
(a) and (b) gives n(M_b) M_b^-5/4, comparable to the simulation results. Thus,
the well known observational fact that low mass galaxies are underabundant as
compared to expectations from numerical dark matter simulations or
Press-Schechter modeling of CDM universes emerges naturally from these results,
implying that perhaps no ``new physics'' beyond the standard model is needed.Comment: Submitted to ApJ, 17 pages including 6 figure
Evidence for coupling between the Sagittarius dwarf galaxy and the Milky Way warp
Using recent determinations of the mass and orbit of Sagittarius, I calculate
its orbital angular momentum. From the latest observational data, I also
calculate the angular momentum of the Milky Way's warp. I find that both
angular momenta are directed toward l=270, b=0, and have magnitude 2-8x10^12
M_Sun kpc km s^-1, where the range in both cases reflects uncertainty in the
mass. The coincidence of the angular momenta is suggestive of a coupling
between these systems. Direct gravitational torque of Sgr on the disk is ruled
out as the coupling mechanism. Gravitational torque due to a wake in the halo
and the impulsive deposition of momentum by a passage of Sgr through the disk
are still both viable mechanisms pending better simulations to test their
predictions on the observed Sgr-MW system.Comment: 11 pages, to appear in the February 1 issue of ApJ
Turbulent Flow-Driven Molecular Cloud Formation: A Solution to the Post-T Tauri Problem?
We suggest that molecular clouds can be formed on short time scales by
compressions from large scale streams in the interstellar medium (ISM). In
particular, we argue that the Taurus-Auriga complex, with filaments of 10-20 pc
2-5 pc, most have been formed by H I flows in Myr,
explaining the absence of post-T Tauri stars in the region with ages Myr. Observations in the 21 cm line of the H I `halos' around the Taurus
molecular gas show many features (broad asymmetric profiles, velocity shifts of
H I relative to CO) predicted by our MHD numerical simulations, in which
large-scale H I streams collide to produce dense filamentary structures. This
rapid evolution is possible because the H I flows producing and disrupting the
cloud have much higher velocities (5-10 kms) than present in the molecular gas
resulting from the colliding flows. The simulations suggest that such flows can
occur from the global ISM turbulence without requiring a single triggering
event such as a SN explosion.Comment: 26 pages, 12 ps figures. Apj accepte
Analysis of Clumps in Molecular Cloud Models: Mass Spectrum, Shapes, Alignment and Rotation
Observations reveal concentrations of molecular line emission on the sky,
called ``clumps,'' in dense, star-forming molecular clouds. These clumps are
believed to be the eventual sites of star formation. We study the
three-dimensional analogs of clumps using a set of self-consistent,
time-dependent numerical models of molecular clouds. The models follow the
decay of initially supersonic turbulence in an isothermal, self-gravitating,
magnetized fluid. We find the following. (1) Clumps are intrinsically triaxial.
This explains the observed deficit of clumps with a projected axis ratio near
unity, and the apparent prolateness of clumps. (2) Simulated clump axes are not
strongly aligned with the mean magnetic field within clumps, nor with the
large-scale mean fields. This is in agreement with observations. (3) The clump
mass spectrum has a high-mass slope that is consistent with the Salpeter value.
There is a low-mass break in the slope at \sim 0.5 \msun, although this may
depend on model parameters including numerical resolution. (4) The typical
specific spin angular momentum of clumps is . This is larger than the median specific angular momentum of binary
stars. Scaling arguments suggest that higher resolution simulations may soon be
able to resolve the scales at which the angular momentum of binary stars is
determined.Comment: 14 pages, 13 figures, to appear in 2003 July 20 Ap
Cooling flows and quasars: II. Detailed models of feedback modulated accretion flows
Most elliptical galaxies contain central black holes (BHs), and most also
contain significant amounts of hot gas capable of accreting on to the central
BH due to cooling times short compared to the Hubble time. Why therefore do we
not see AGNs at the center of most elliptical galaxies rather than in only (at
most) a few per cent of them? We propose here the simple idea that feedback
from accretion events heats the ambient gas retarding subsequent infall. In
this context, we present a class of 1D hydrodynamical evolutionary sequences
for the gas flows in elliptical galaxies with a massive central BH. The
resulting evolution is characterized by strong oscillations, in which very fast
and energetic bursts of the BH are followed by longer periods in which the
X-ray galaxy emission comes from the coronal gas. We also allow for departures
from spherical symmetry by examining scenarios in which the central engine is
either an ADAF or a more conventional accretion disk that is optically thick
except for a polar region. In all cases the duty cycle (fraction of the time
that the system will be seen as an AGN) is quite small and in the range 10^{-4}
- 10^{-3}. Thus, for any reasonable value of the efficiency, the presence of a
massive BH at the center of a galaxy seems to be incompatible with the presence
of a long-lived cooling flow.Comment: 43 pages, 10 figures. Main additions concern observed Compton
temperatures and few extra numerical models. Conclusions unchanged. 1 new
table and 3 new figures. Accepted for publication on ApJ (main journal
Accretion of gas onto nearby spiral galaxies
We present evidence for cosmological gas accretion onto spiral galaxies in
the local universe. The accretion is seen through its effects on the dynamics
of the extra-planar neutral gas. The accretion rates that we estimate for two
nearby spiral galaxies are of the order of their star formation rates. Our
model shows that most of the extra-planar gas is produced by supernova feedback
(galactic fountain) and only 10-20 % comes from accretion. The accreting
material must have low specific angular momentum about the disc's spin axis,
although the magnitude of the specific angular-momentum vector can be higher.
We also explore the effects of a hot corona on the dynamics of the extra-planar
gas and find that it is unlikely to be responsible for the observed kinematical
pattern and the source of accreted gas. However, the interaction with the
fountain flow should profoundly affect the hydrodynamics of the corona.Comment: 11 pages, 6 figures, accepted for publication in MNRA
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