627 research outputs found
SBF Distances to Dwarf Elliptical Galaxies in the Sculptor Group
As part of an ongoing search for dwarf elliptical galaxies (dE) in the
vicinity of the Local Group, we acquired deep B and R-band images for five dE
candidates identified in the Sculptor (Scl) group region. We carried out a
surface brightness fluctuation (SBF) analysis on the R-band images to measure
the apparent fluctuation magnitude \bar{m}_R for each dE. Using predictions
from stellar population synthesis models the galaxy distances were determined.
All of these dE candidates turned out to be satellites of Scl group major
members. A redshift measurement of the dE candidate ESO294-010 yielded an
independent confirmation of its group membership: the [OIII] and H
emission lines from a small HII region gave a heliocentric velocity of 117(\pm
5) km s-1, in close agreement with the velocity of its parent galaxy NGC 55
(v_\odot=125 km s-1). The precision of the SBF distances (5 to 10%) contributes
to delineating the cigar-like distribution of the Scl group members, which
extend over distances from 1.7 to 4.4 Mpc and are concentrated in three,
possibly four subclumps. The Hubble diagram for nine Scl galaxies, including
two of our dEs, exhibits a tight linear velocity--distance relation with a
steep slope of 119 km s-1 Mpc-1. The results indicate that gravitational
interaction among the Scl group members plays only a minor role in the dynamics
of the group. However, the Hubble flow of the entire system appears strongly
disturbed by the large masses of our Galaxy and M31 leading to the observed
shearing motion. From the distances and velocities of 49 galaxies located in
the Local Group and towards the Scl group, we illustrate the continuity of the
galaxy distribution which strongly supports the view that the two groups form a
single supergalactic structure.Comment: To appear in The Astronomical Journal, December 1998; 28 pages with
22 figure
Virgo Early-Type Dwarfs in ALFALFA
Early-type dwarf galaxies dominate cluster populations, but their formation
and evolutionary histories are poorly understood. The ALFALFA (Arecibo Legacy
Fast ALFA) survey has completed observations of the Virgo Cluster in the
declination range of 6 - 16 degrees. Less than 2% of the early-type dwarf
population is detected, a significantly lower fraction than reported in
previous papers based on more limited samples. In contrast ~30 of the
irregular/BCD dwarf population is detected. The detected early-type galaxies
tend to be located in the outer regions of the cluster, with a concentration in
the direction of the M Cloud. Many show evidence for ongoing/recent star
formation. Galaxies such as these may be undergoing morphological transition
due to cluster environmental effects.Comment: 2 pages. To be published in proceedings of IAU Symposium 244: 'Dark
Galaxies and Lost Baryons', J. I. Davies & M. D. Disney. eds., Cambridge
University Pres
Consequences of gravitational radiation recoil
Coalescing binary black holes experience an impulsive kick due to anisotropic
emission of gravitational waves. We discuss the dynamical consequences of the
recoil accompanying massive black hole mergers. Recoil velocities are
sufficient to eject most coalescing black holes from dwarf galaxies and
globular clusters, which may explain the apparent absence of massive black
holes in these systems. Ejection from giant elliptical galaxies would be rare,
but coalescing black holes are displaced from the center and fall back on a
time scale of order the half-mass crossing time. Displacement of the black
holes transfers energy to the stars in the nucleus and can convert a steep
density cusp into a core. Radiation recoil calls into question models that grow
supermassive black holes from hierarchical mergers of stellar-mass precursors.Comment: 5 pages, 4 figures, emulateapj style; minor changes made; accepted to
ApJ Letter
Mn L edge resonant x-ray scattering in manganites: Influence of the magnetic state
We present an analysis of the dependence of the resonant orbital order and
magnetic scattering spectra on the spin configuration. We consider an arbitrary
spin direction with respect to the local crystal field axis, thus lowering
significantly the local symmetry. To evaluate the atomic scattering in this
case, we generalized the Hannon-Trammel formula and implemented it inside the
framework of atomic multiplet calculations in a crystal field. For an
illustration, we calculate the magnetic and orbital scattering in the CE phase
of \lsmo in the cases when the spins are aligned with the crystal lattice
vector (or equivalently ) and when they are rotated in the
-plane by 45 with respect to this axis. Magnetic spectra differ
for the two cases. For the orbital scattering, we show that for the former
configuration there is a non negligible ()
scattering component, which vanishes in the 45 case, while the () components are similar in the two cases. From the
consideration of two 90 spin canted structures, we conclude there is a
significant dependence of the orbital scattering spectra on the spin
arrangement. Recent experiments detected a sudden decrease of the orbital
scattering intensity upon increasing the temperature above the N\' eel
temperature in \lsmo. We discuss this behavior considering the effect of
different types of misorientations of the spins on the orbital scattering
spectrum.Comment: 8 figures. In the revised version, we added a note, a reference, and
a few minor changes in Figure 1 and the text. Accepted in Physical Review
Virgo cluster early-type dwarf galaxies with the Sloan Digital Sky Survey. II. Early-type dwarfs with central star formation
Despite the common picture of an early-type dwarf (dE) as a quiescent galaxy
with no star formation and little gas, we identify 23 dEs that have blue
central colors caused by recent or ongoing star formation in our sample of 476
Virgo cluster dEs. In addition, 14 objects that were mostly classified as
(candidate) BCDs have similar properties. Among the certain cluster members,
the dEs with blue centers reach a fraction of more than 15% of the dE
population at brighter (B<=16) magnitudes. A spectral analysis of the centers
of 16 galaxies reveals in all cases an underlying old population that dominates
the mass, with M(old)>=90% for all but one object. Therefore the majority of
these galaxies will appear like ordinary dEs within ~one Gigayear or less after
the last episode of star formation. Their overall gas content is less than that
of dwarf irregular galaxies, but higher than that of ordinary dEs. Their
flattening distribution suggests the shape of a thick disk, similar to what has
been found for dEs with disk features in Paper I of this series. Their
projected spatial distribution shows no central clustering, and their
distribution with projected local density follows that of irregular galaxies,
indicative of an unrelaxed population. This is corroborated by their velocity
distribution, which displays two side peaks characteristic of recent infall. We
discuss possible formation mechanisms (ram-pressure stripping, tidally induced
star formation, harassment) that might be able to explain both the disk shape
and the central star formation of the dEs with blue centers.Comment: 16 pages + 15 figures. Accepted for publication in AJ. We recommend
downloading the full resolution version from
http://www.virgo-cluster.com/lisker2006b.ps.g
On the unification of dwarf and giant elliptical galaxies
The near orthogonal distributions of dwarf elliptical (dE) and giant
elliptical (E) galaxies in the mu_e-Mag and mu_e-log(R_e) diagrams have been
interpreted as evidence for two distinct galaxy formation processes. However,
continuous, linear relationships across the alleged dE/E boundary at M_B = -18
mag - such as those between central surface brightness (mu_0) and (i) galaxy
magnitude and (ii) light-profile shape (n) - suggest a similar, governing
formation mechanism. Here we explain how these latter two linear trends
necessitate a different behavior for dE and E galaxies, exactly as observed, in
diagrams involving mu_e (and also _e). A natural consequence is that the
distribution of dEs and Es in Fundamental Plane type analyses that use the
associated intensity I_e, or _e, are expected to appear different. Together
with other linear trends across the alleged dE/E boundary, such as those
between luminosity and color, metallicity, and velocity dispersion, it appears
that the dEs form a continuous extension to the E galaxies. The presence of
partially depleted cores in luminous (M_B < -20.5 mag) Es does however signify
the action of a different physical process at the centers (< ~300 pc) of these
galaxies.Comment: 5 pages from the proceedings of the 2004 conference "Penetrating bars
through masks of cosmic dust: the Hubble tuning fork strikes a new note".
Edited by D. L. Block, I. Puerari, K. C. Freeman, R. Groess, and E. K. Bloc
Temperature dependence of surface stress across an order-disorder transition: p(1x2)O/W(110)
Strain relaxations of a p(1x2) ordered oxygen layer on W(110) are measured as
a function of temperature across the disordering transition using low-energy
electron diffraction. The measured strains approach values of 0.027 in the
[1-10] and -0.053 in the [001] direction. On the basis of the measured strain
relaxations, we give quantitative information on temperature-dependent surface
stress using the results of ab initio calculations. From the surface formation
energy for different strains, determined by first-principles calculations, we
estimate that surface stress changes from -1.1 for the ordered phase to -0.2N/m
for the disordered one along [1-10], and from 5.1 to 3.4 N/m along [001].
Moreover, our observation that the strains scale inversely with domain size
confirms that the strain relaxation takes place at the domain boundaries.Comment: 8 pages, 5 figure
Harassment Origin for Kinematic Substructures in Dwarf Elliptical Galaxies?
We have run high resolution N-body models simulating the encounter of a dwarf
galaxy with a bright elliptical galaxy. The dwarf absorbs orbital angular
momentum and shows counter-rotating features in the external regions of the
galaxy. To explain the core-envelope kinematic decoupling observed in some
dwarf galaxies in high-density environments requires nearly head-on collisions
and very little dark matter bound to the dwarf. These kinematic structures
appear under rather restrictive conditions. As a consequence, in a cluster like
Virgo ~1% of dwarf galaxies may present counter-rotation formed by harassment.Comment: 10 pages, 7 figures; Accepted for publication in Astronomy and
Astrophysic
The counter-streaming instability in dwarf ellipticals with off-center nuclei
n many nucleated dwarf elliptical galaxies (dE,N's), the nucleus is offset by
a significant fraction of the scale radius with respect to the center of the
outer isophotes. Using a high-resolution N-body simulation, we demonstrate that
the nucleus can be driven off-center by the m=1 counterstreaming instability,
which is strong in flattened stellar systems with zero rotation. The model
develops a nuclear offset on the order of 30% of the exponential scale length.
We compare our numerical results with the photometry and kinematics of FCC 046,
a Fornax Cluster dE,N with a nucleus offset by 1.2" we find good agreement
between the model and FCC 046. We also discuss mechanisms that may cause
counterrotation in dE,N's and conclude that the destruction of box orbits in an
initially triaxial galaxy is the most promising.Comment: 5 pages, 4 figure
The structure of elliptical galaxies in the Virgo cluster. Results from the INT Wide Field Survey
We report on a complete CCD imaging survey of 226 elliptical galaxies in the
North-East quadrant of the Virgo cluster, representative of the properties of
giant and dwarf elliptical galaxies in this cluster. We fit their radial light
profiles with the Sersic r^(1/n) model of light distribution. We confirm the
result of Graham & Guzman (2003) that the apparent dichotomy between E and dE
galaxies in the luminosity-_e plane no longer appears when other structural
parameters are considered and can be entirely attributed to the onset of "core"
galaxies at B_T sim -20.5 mag. When "core" galaxies are not considered, E and
dE form a unique family with n linearly increasing with the luminosity. For 90
galaxies we analyze the B-I color indices, both in the nuclear and in the outer
regions. Both indices are bluer toward fainter luminosities. We find also that
the outer color gradients do not show any significant correlation with the
luminosity. The scatter in all color indicators increases significantly toward
lower luminosities, e.g. galaxies fainter than B_T sim -15 have a B-I spread
0.5 mag.Comment: 21 pages, 14 figures, 2 tables. Accepted for publication on A&A. High
resolution figures at
http://goldmine.mib.infn.it/papers/WFS_04/WFS_04-frame.htm
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