658 research outputs found
Chandra Observations and the Nature of the Anomalous Arms of NGC 4258 (M 106)
This paper presents high resolution X-ray observations with Chandra of NGC
4258 and infers the nature of the so called ``anomalous arms'' in this galaxy.
The anomalous arms dominate the X-ray image; diffuse X-ray emission from the
``plateaux'' regions, seen in radio and H imaging, is also found. X-ray
spectra have been obtained at various locations along the anomalous arms and
are well described by thermal (mekal) models with kT in the range 0.37 - 0.6
keV. The previously known kpc-scale radio jets are surrounded by cocoons of hot
X-ray emitting gas for the first 350 pc of their length. The radio jets, seen
in previous VLBA and VLA observations, propagate perpendicular to the compact
nuclear gas disk (imaged in water vapor maser emission). The angle between the
jets and the rotation axis of the galactic disk is 60. The jets shock
the normal interstellar gas along the first 350 pc of their length, causing the
hot, X-ray emitting cocoons noted above. At a height of z = 175 pc from the
disk plane, the jets exit the normal gas disk and then propagate though the low
density halo until they reach ``hot spots'' (at 870 pc and 1.7 kpc from the
nucleus), which are seen in radio, optical line and X-ray emission. These jets
must drive mass motions into the low density halo gas. This high velocity halo
gas impacts on the dense galactic gas disk and shock heats it along and around
a ``line of damage'', which is the projection of the jets onto the galactic gas
disk as viewed down the galaxy disk rotation axis. However, because NGC 4258 is
highly inclined ( = 64), the ``line of damage'' projects on the
sky in a different direction to the jets themselves. We calculate the expected
p.a. of the ``line of damage'' on the sky and find that it coincides with the
anomalous arms to within 2. (Abstract truncated).Comment: 12 pages plus 9 figures, to be published in the Astrophysical
Journal, v560, nr 1, pt 1 (Oct 10, 2001 issue
The Sloan-Lens ACS Survey II: stellar populations and internal structure of early-type lens galaxies
We derive Fundamental Plane parameters of 15 early-type lens galaxies
identified by the Sloan Lens ACS (SLACS) Survey. The size of the sample allows
us to investigate for the first time the distribution of lens galaxies in the
FP space. After correcting for evolution, we find that lens galaxies occupy a
subset of the local FP. The edge-on projection (approximately M vs M/L) is
indistinguishable from that of normal early-type galaxies. However -- within
the fundamental plane -- the lens galaxies appear to concentrate at the edge of
the region populated by normal early-type galaxies. We show that this is a
result of our selection procedure (approximately velocity dispersion
sigma>240km/s). We conclude that SLACS lenses are a fair sample of high
velocity dispersion early-type galaxies. By comparing the central stellar
velocity dispersion that of the best fit lens model, we find
== =1.01+-0.02 with 0.065 rms scatter. We conclude that
within the Einstein radii the SLACS lenses are very well approximated by
isothermal ellipsoids, requiring a fine tuning of the stellar and dark matter
distribution (bulge-halo ``conspiracy''). Interpreting the offset from the
local FP in terms of evolution of the stellar mass-to-light ratio, we find for
the SLACS lenses d log M/L_B/dz=-0.69+-0.08 (rms 0.11) consistent with the rate
found for field early-type galaxies and with a scenario where most of the stars
were formed at high redshift (>2) with secondary episodes of star formation
providing less than ~10% of the stellar mass below z=1. We discuss star
formation history and structural homogeneity in the context of formation
mechanisms such as collisionless (``dry'') mergers. [Abridged]Comment: 2006, ApJ, 604, 622; 13 pages, 7 figures, 2 tables. Replaced Table 2,
since the previous version was incorrectly sorted. Updated references. No
changes in plots or content. More info available at SLACS website
www.slacs.or
Cosmological Origin of the Stellar Velocity Dispersions in Massive Early-Type Galaxies
We show that the observed upper bound on the line-of-sight velocity
dispersion of the stars in an early-type galaxy, sigma<400km/s, may have a
simple dynamical origin within the LCDM cosmological model, under two main
hypotheses. The first is that most of the stars now in the luminous parts of a
giant elliptical formed at redshift z>6. Subsequently, the stars behaved
dynamically just as an additional component of the dark matter. The second
hypothesis is that the mass distribution characteristic of a newly formed dark
matter halo forgets such details of the initial conditions as the stellar
"collisionless matter" that was added to the dense parts of earlier generations
of halos. We also assume that the stellar velocity dispersion does not evolve
much at z<6, because a massive host halo grows mainly by the addition of
material at large radii well away from the stellar core of the galaxy. These
assumptions lead to a predicted number density of ellipticals as a function of
stellar velocity dispersion that is in promising agreement with the Sloan
Digital Sky Survey data.Comment: ApJ, in press (2003); matches published versio
Constraining global properties of the Draco dwarf spheroidal galaxy
By fitting a flexible stellar anisotropy model to the observed surface
brightness and line-of-sight velocity dispersion profiles of Draco we derive a
sequence of cosmologically plausible two-component (stars + dark matter) models
for this galaxy. The models are consistent with all the available observations
and can have either cuspy Navarro-Frenk-White or flat-cored dark matter density
profiles. The dark matter halos either formed relatively recently (at z~2...7)
and are massive (up to ~5x10^9 M_Sun), or formed before the end of the
reionization of the universe (z~7...11) and are less massive (down to ~7x10^7
M_Sun). Our results thus support either of the two popular solutions of the
"missing satellites" problem of Lambda cold dark matter cosmology - that dwarf
spheroidals are either very massive, or very old. We carry out high-resolution
simulations of the tidal evolution of our two-component Draco models in the
potential of the Milky Way. The results of our simulations suggest that the
observable properties of Draco have not been appreciably affected by the
Galactic tides after 10 Gyr of evolution. We rule out Draco being a "tidal
dwarf" - a tidally disrupted dwarf galaxy. Almost radial Draco orbits (with the
pericentric distance <15 kpc) are also ruled out by our analysis. The case of a
harmonic dark matter core can be consistent with observations only for a very
limited choice of Draco orbits (with the apocentric-to-pericentric distances
ratio of <2.5).Comment: 18 pages, 14 figures; accepted by Ap
The scale-free character of the cluster mass function and the universality of the stellar IMF
Our recent determination of a Salpeter slope for the IMF in the field of 30
Doradus (Selman and Melnick 2005) appears to be in conflict with simple
probabilistic counting arguments advanced in the past to support observational
claims of a steeper IMF in the LMC field. In this paper we re-examine these
arguments and show by explicit construction that, contrary to these claims, the
field IMF is expected to be exactly the same as the stellar IMF of the clusters
out of which the field was presumably formed. We show that the current data on
the mass distribution of clusters themselves is in excellent agreement with our
model, and is consistent with a single spectrum {\it by number of stars} of the
type with beta between -1.8 and -2.2 down to the smallest clusters
without any preferred mass scale for cluster formation. We also use the random
sampling model to estimate the statistics of the maximal mass star in clusters,
and confirm the discrepancy with observations found by Weidner and Kroupa
(2006). We argue that rather than signaling the violation of the random
sampling model these observations reflect the gravitationally unstable nature
of systems with one very large mass star. We stress the importance of the
random sampling model as a \emph{null hypothesis} whose violation would signal
the presence of interesting physics.Comment: 9 pages emulateap
Two dimensional bulge disk decomposition
We propose a two dimensional galaxy fitting algorithm to extract parameters
of the bulge, disk, and a central point source from broad band images of
galaxies. We use a set of realistic galaxy parameters to construct a large
number of model galaxy images which we then use as input to our galaxy fitting
program to test it. We find that our approach recovers all structural
parameters to a fair degree of accuracy. We elucidate our procedures by
extracting parameters for 3 real galaxies -- NGC 661, NGC 1381, and NGC 1427.Comment: 23 pages, LaTeX, AASTEX macros used, 7 Postscript figures, submitted
to Ap
Self-consistent nonspherical isothermal halos embedding zero-thickness disks
Disk-halo decompositions of galaxy rotation curves are generally performed in
a parametric way. We construct self-consistent models of nonspherical
isothermal halos embedding a zero-thickness disk, by assuming that the halo
distribution function is a Maxwellian. The method developed here can be used to
study other physically-based choices for the halo distribution function and the
case of a disk accompanied by a bulge. In a preliminary investigation we note
the existence of a fine tuning between the scalelengths R_{\Omega} and h,
respectively characterizing the rise of the rotation curve and the luminosity
profile of the disk, which surprisingly applies to both high surface brightness
and low surface brightness galaxies. This empirical correlation identifies a
much stronger conspiracy than the one required by the smoothness and flatness
of the rotation curve (disk-halo conspiracy). The self-consistent models are
characterized by smooth and flat rotation curves for very different
disk-to-halo mass ratios, hence suggesting that conspiracy is not as dramatic
as often imagined. For a typical rotation curve, with asymptotically flat
rotation curve at V_{\infty} (the precise value of which can also be treated as
a free parameter), and a typical density profile of the disk, self-consistent
models are characterized by two dimensionless parameters, which correspond to
the dimensional scales (the disk mass-to-light ratio M/L and the halo central
density) of standard disk-halo decompositions. We show that if the rotation
curve is decomposed by means of our self-consistent models, the disk-halo
degeneracy is removed and typical rotation curves are fitted by models that are
below the maximum-disk prescription. Similar results are obtained from a study
of NGC 3198. Finally, we quantify the flattening of the spheroidal halo, which
is significant, especially on the scale of the visible disk.Comment: accepted for publication in A&
Distortion Isomerism of Cu(II) Chloride Adducts with Bis(2-benzimidazolyl)ethane. Synthesis, Characterization, X-ray Structures and Spectroscopy of Four Different Isomers
Metals in Catalysis, Biomimetics & Inorganic Material
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