1,983 research outputs found
The role of thermal evaporation in galaxy formation
In colour-magnitude diagrams most galaxies fall in either the ``blue cloud''
or the ``red sequence'', with the red sequence extending to significantly
brighter magnitudes than the blue cloud. The bright-end of the red sequence
comprises elliptical galaxies (Es) with boxy isophotes and luminosity profiles
with shallow central cores, while fainter Es have disky isophotes and power-law
inner surface-brightness (SB) profiles. An analysis of published data reveals
that the centres of galaxies with power-law central SB profiles have younger
stellar populations than the centres of cored galaxies. We argue that thermal
evaporation of cold gas by virial-temperature gas plays an important role in
determining these phenomena. In less massive galaxies, thermal evaporation is
not very efficient, so significant amounts of cold gas can reach the galaxy
centre and fill a central core with newly formed stars, consistent with the
young stellar ages of the cusps of Es with power-law SB profiles. In more
massive galaxies, cold gas is evaporated within a dynamical time, so star
formation is inhibited, and a core in the stellar density profile produced by
dissipationless dynamics cannot be refilled. The different observed properties
of AGN in higher-mass and lower-mass ellipticals are also explained because in
the former the central black holes invariably accrete hot gas, while in the
latter they typically accrete cold gas. An important consequence of our results
is that at the present time there cannot be blue, star-forming galaxies in the
most massive galactic halos, consistent with the observed truncation of the
blue cloud at L*. [abridged]Comment: MNRAS, accepted. Added discussion and references, conclusions
unchanged. 14 pages, 6 figures (2 color
Probing Brownstein-Moffat Gravity via Numerical Simulations
In the standard scenario of the Newtonian gravity, a late-type galaxy (i.e.,
a spiral galaxy) is well described by a disk and a bulge embedded in a halo
mainly composed by dark matter. In Brownstein-Moffat gravity, there is a claim
that late-type galaxy systems would not need to have halos, avoiding as a
result the dark matter problem, i.e., a modified gravity (non-Newtonian) would
account for the galactic structure with no need of dark matter. In the present
paper, we probe this claim via numerical simulations. Instead of using a
"static galaxy," where the centrifugal equilibrium is usually adopted, we probe
the Brownstein-Moffat gravity dynamically via numerical -body simulations.Comment: 33 pages and 14 figures - To appear in The Astrophysical Journa
Gas flow in barred potentials
We use a Cartesian grid to simulate the flow of gas in a barred Galactic
potential and investigate the effects of varying the sound speed in the gas and
the resolution of the grid. For all sound speeds and resolutions, streamlines
closely follow closed orbits at large and small radii. At intermediate radii
shocks arise and the streamlines shift between two families of closed orbits.
The point at which the shocks appear and the streamlines shift between orbit
families depends strongly on sound speed and resolution. For sufficiently large
values of these two parameters, the transfer happens at the cusped orbit as
hypothesised by Binney et al. over two decades ago. For sufficiently high
resolutions the flow downstream of the shocks becomes unsteady. If this
unsteadiness is physical, as appears to be the case, it provides a promising
explanation for the asymmetry in the observed distribution of CO.Comment: Accepted for publication in MNRA
Gas flow in barred potentials II. Bar Driven Spiral Arms
Spiral arms that emerge from the ends of a galactic bar are important in
interpreting observations of our and external galaxies. It is therefore
important to understand the physical mechanism that causes them. We find that
these spiral arms can be understood as kinematic density waves generated by
librations around underlying ballistic closed orbits. This is even true in the
case of a strong bar, provided the librations are around the appropriate closed
orbits and not around the circular orbits that form the basis of the epicycle
approximation. An important consequence is that it is a potential's orbital
structure that determines whether a bar should be classified as weak or strong,
and not crude estimates of the potential's deviation from axisymmetry.Comment: Accepted for publication in MNRA
Intrinsic Shapes of Molecular Cloud Cores
We conduct an analysis of the shapes of molecular cloud cores using recently
compiled catalogs of observed axis ratios of individual cores mapped in ammonia
or through optical selection. We apply both analytical and statistical
techniques to deproject the observed axis ratios in order to determine the true
distribution of cloud core shapes. We find that neither pure oblate nor pure
prolate cores can account for the observed distribution of core shapes.
Intrinsically triaxial cores produce distributions which agree with
observations. The best-fit triaxial distribution contains cores which are more
nearly oblate than prolate.Comment: 20 pages, 10 figures. To appear in ApJ (2001 April 1). Color figures
available at http://www.astro.uwo.ca/~cjones/ or
http://www.astro.uwo.ca/~basu/pub.htm
The Planetary Nebula System and Dynamics in the Outer Halo of NGC 5128
The halos of elliptical galaxies are faint and difficult to explore, but they
contain vital clues to both structure and formation. We present the results of
an imaging and spectroscopic survey for planetary nebulae (PNe) in the nearby
elliptical NGC 5128. We extend the work of Hui et al.(1995) well into the halo
of the galaxy--out to distances of 100 and 50 kpc along the major and minor
axes. We now know of 1141 PNe in NGC 5128, 780 of which are confirmed. Of these
780 PNe, 349 are new from this survey, and 148 are at radii beyond 20 kpc. PNe
exist at distances up to 80 kpc (~15 r_e), showing that the stellar halo
extends to the limit of our data. This study represents by far the largest
kinematic study of an elliptical galaxy to date, both in the number of velocity
tracers and in radial extent. We confirm the large rotation of the PNe along
the major axis, and show that it extends in a disk-like feature into the halo.
The rotation curve of the stars flattens at ~100 km/s with V/sigma between 1
and 1.5, and with the velocity dispersion of the PNe falling gradually at
larger radii. The two-dimensional velocity field exhibits a zero-velocity
contour with a pronounced twist, showing that the galaxy potential is likely
triaxial in shape, tending toward prolate. The total dynamical mass of the
galaxy within 80 kpc is ~5 x 10^{11} M_sun, with M/L_B ~ 13. This mass-to-light
ratio is much lower than what is typically expected for elliptical galaxies.Comment: 21 pages, 13 figures (figures 3-8 best viewed in color), accepted for
publication in the Astrophysical Journa
Black Hole Motion as Catalyst of Orbital Resonances
The motion of a black hole about the centre of gravity of its host galaxy
induces a strong response from the surrounding stellar population. We treat the
case of a harmonic potential analytically and show that half of the stars on
circular orbits in that potential shift to an orbit of lower energy, while the
other half receive a positive boost and recede to a larger radius. The black
hole itself remains on an orbit of fixed amplitude and merely acts as a
catalyst for the evolution of the stellar energy distribution function f(E). We
show that this effect is operative out to a radius of approx 3 to 4 times the
hole's influence radius, R_bh. We use numerical integration to explore more
fully the response of a stellar distribution to black hole motion. We consider
orbits in a logarithmic potential and compare the response of stars on circular
orbits, to the situation of a `warm' and `hot' (isotropic) stellar velocity
field. While features seen in density maps are now wiped out, the kinematic
signature of black hole motion still imprints the stellar line-of-sight mean
velocity to a magnitude ~18% the local root mean-square velocity dispersion
sigma.Comment: revised version, typos fixed, added references, 20 pages MN styl
Heating, conduction and minimum temperatures in cooling flows
There is mounting observational evidence from Chandra for strong interaction
between keV gas and AGN in cooling flows. It is now widely accepted that the
temperatures of cluster cores are maintained at a level of 1 keV and that the
mass deposition rates are lower than earlier ROSAT/Einstein values. Recent
theoretical results suggest that thermal conduction can be very efficient even
in magnetized plasmas. Motivated by these discoveries, we consider a ``double
heating model'' which incorporates the effects of simultaneous heating by both
the central AGN and thermal conduction from the hot outer layers of clusters.
Using hydrodynamical simulations, we demonstrate that there exists a family of
solutions that does not suffer from the cooling catastrophe. In these cases,
clusters relax to a stable final state, which is characterized by minimum
temperatures of order 1 keV and density and temperature profiles consistent
with observations. Moreover, the accretion rates are much reduced, thereby
reducing the need for excessive mass deposition rates required by the standard
cooling flow models.Comment: 7 pages, 2 figures, minor changes, accepted for The Astrophysical
Journa
Quasi-stationary states and the range of pair interactions
"Quasi-stationary" states are approximately time-independent out of
equilibrium states which have been observed in a variety of systems of
particles interacting by long-range interactions. We investigate here the
conditions of their occurrence for a generic pair interaction V(r \rightarrow
\infty) \sim 1/r^a with a > 0, in d>1 dimensions. We generalize analytic
calculations known for gravity in d=3 to determine the scaling parametric
dependences of their relaxation rates due to two body collisions, and report
extensive numerical simulations testing their validity. Our results lead to the
conclusion that, for a < d-1, the existence of quasi-stationary states is
ensured by the large distance behavior of the interaction alone, while for a >
d-1 it is conditioned on the short distance properties of the interaction,
requiring the presence of a sufficiently large soft-core in the interaction
potential.Comment: 5 pages, 3 figures; final version to appear in Phys. Rev. Let
Accretion by the Galaxy
Cosmology requires at least half of the baryons in the Universe to be in the
intergalactic medium, much of which is believed to form hot coronae around
galaxies. Star-forming galaxies must be accreting from their coronae. HI
observations of external galaxies show that they have HI halos associated with
star formation. These halos are naturally modelled as ensembles of clouds
driven up by supernova bubbles. These models can fit the data successfully only
if clouds exchange mass and momentum with the corona. As a cloud orbits, it is
ablated and forms a turbulent wake where cold high-metallicity gas mixes with
hot coronal gas causing the prompt cooling of the latter. As a consequence the
total mass of HI increases. This model has recently been used to model the
Leiden-Argentina-Bonn survey of Galactic HI. The values of the model's
parameters that are required to model NGC 891, NGC 2403 and our Galaxy show a
remarkable degree of consistency, despite the very different natures of the two
external galaxies and the dramatic difference in the nature of the data for our
Galaxy and the external galaxies. The parameter values are also consistent with
hydrodynamical simulations of the ablation of individual clouds. The model
predicts that a galaxy that loses its cool-gas disc for instance through a
major merger cannot reform it from its corona; it can return to steady star
formation only if it can capture a large body of cool gas, for example by
accreting a gas-rich dwarf. Thus the model explains how major mergers can make
galaxies "red and dead."Comment: Invited review at "Assembling the Puzzle of the Milky Way", Grand
Bornand, April 2011; 6 page
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