493 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
The stellar correlation function from SDSS - A statistical search for wide binary stars
We study the statistical properties of the wide binary population in the
Galaxy field with projected separations larger than 200 AU by constructing the
stellar angular two-point correlation function (2PCF) from a homogeneous sample
of nearly 670'000 main sequence stars. The selected stars lie within a
rectangular region around the Northern Galactic Pole and have apparent r-band
magnitudes between 15 and 20.5 mag and spectral classes later than G5 (g-r >
0.5 mag). The data were taken from the Sixth Data Release of the Sloan Digital
Sky Survey. We model the 2PCF by means of the Wasserman-Weinberg technique
including several assumptions on the distribution of the binaries' orbital
parameters, luminosity function, and density distribution in the Galaxy. In
particular, we assume that the semi-major axis distribution is described by a
single powerlaw. The free model parameters - the local wide binary number
density and the power-law index of the semi-major axis distribution - are
inferred simultaneously by least-square fitting. We find the separation
distribution to follow Oepik's law up to the Galactic tidal limit, without any
break and a local density of 5 wide binaries per 1'000 cubic parsec with both
components having spectral type later than G5. This implies that about 10% of
all stars in the solar neighbourhood are members of such a late-type wide
binary system. With a relative statistical (2 sigma) error of about 10%, our
findings are in general agreement with previous studies of wide binaries. The
data suggest that about 800 very wide pairs with projected separations larger
than 0.1 pc exist in our sample, whereas none are found beyond 0.8 pc.Comment: 18 pages, 14 figures, 7 tables; added reference for section
Structure and stellar content of dwarf galaxies. III: B and R photometry of dwarf galaxies in the M101 group and the nearby field
We have carried out CCD photometry in the Cousins B and R bands of 21 dwarf
galaxies in and around the M101 group. Eleven are members and suspected members
of the group and ten are field galaxies in the projected vicinity of the group.
We have derived total magnitudes, effective radii, effective surface
brightnesses, as well as galaxy diameters at various isophotal levels in both
colours. Best-fitting exponential parameters and colour gradients are also
given for these galaxies. Some of the galaxies show a pronounced luminosity
excess above the best-fitting exponential at large radii, or surface
brightnesses fainter than approx 26 mag/sq_arcsec in R. This feature, while
non-significant for a single case and technically difficult to interpret, might
be an indication of the existence of an extended old stellar halo in dwarf
irregulars. The photometric parameters of the galaxies presented here will be
combined with previously published data for nearby dwarf galaxies and
statistically analysed in a forthcoming paper.Comment: 15 pages, 4 tables and 13 figures. For a full resolution version see
http://www.astro.unibas.ch/galaxies/papers.html To appear in A&A
Structure and stellar content of dwarf galaxies IV. B and R photometry of dwarf galaxies in the CVnI cloud
We have carried out CCD photometry in the Cousins B and R bands of 15
galaxies in the Canes Venatici I cloud. Total magnitudes, effective radii,
effective surface brightnesses, as well as galaxy radii at various isophotal
levels in both colors were determined. Best-fitting exponential parameters and
color gradients are also given for these galaxies. The photometric parameters
presented here will analyzed in a forthcoming paper, together with previously
published data for nearby dwarf galaxies.Comment: 10 pages, submitted to A&AS. For a full resolution version see
ftp://merkur.astro.unibas.ch/pub/bremnes/canesv.ps.g
More evidence for hidden spiral and bar features in bright early-type dwarf galaxies
Following the discovery of spiral structure in IC3328 (Jerjen et al.~2000),
we present further evidence that a sizable fraction of bright early-type dwarfs
in the Virgo cluster are genuine disk galaxies, or are hosting a disk
component. Among a sample of 23 nucleated dwarf ellipticals and dS0s observed
with the Very Large Telescope in and , we found another four systems
exhibiting non-axisymmetric structures, such as a bar and/or spiral arms,
indicative of a disk (IC0783, IC3349, NGC4431, IC3468). Particularly remarkable
are the two-armed spiral pattern in IC0783 and the bar and trailing arms in
NGC4431. For both galaxies the disk nature has recently been confirmed by a
rotation velocity measurement (Simien & Prugniel 2002). Our photometric search
is based on a Fourier decomposition method and a specific version of unsharp
masking. Some ``early-type'' dwarfs in the Virgo cluster seem to be former
late-type galaxies which were transformed to early-type morphology, e.g. by
``harassment'', during their infall to the cluster, while maintaining part of
their disk structure.Comment: A&A accepte
The Formation of Low-Mass Cluster Galaxies and the Universal Initial Galaxy Mass Function
Clusters of galaxies have an observed over-density of low-luminosity systems
in comparison to the field, although it is not yet agreed whether this effect
is the result of initial galaxy mass functions that vary with environment or
galaxy evolutionary effects. In this letter we argue that this over-density is
the result of low-mass systems with red colors that are over-populating the
faint-end of the observed luminosity function in the nearby rich cluster Abell
0426. We show that the luminosity function of Abell 0426 becomes steeper, from
the field value alpha = -1.25+/-0.05 to alpha=-1.44+/-0.04, due to a recently
identified population of red low-mass cluster galaxies that are possibly the
remnants of dynamical stripped high-mass systems. We further demonstrate,
through simple models of stripping effects, how cluster luminosity functions
can become artificially steep over time from the production of these low-mass
cluster galaxies.Comment: Accepted to ApJ letter
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
Dissipative transformation of non-nucleated dwarf galaxies into nucleated systems
Recent photometric observations by the {\it Hubble Space Telescope (HST)}
have revealed the physical properties of stellar galactic nuclei in nucleated
dwarf galaxies in the Virgo cluster of galaxies. In order to elucidate the
formation processes of nucleated dwarfs, we numerically investigate gas
dynamics, star formation, and chemical evolution within the central 1 kpc of
gas disks embedded within the galactic stellar components of non-nucleated
dwarfs. We find that high density, compact stellar systems can be formed in the
central regions of dwarfs as a result of dissipative, repeated merging of
massive stellar and gaseous clumps developed from nuclear gaseous spiral arms
as a result of local gravitational instability. The central stellar components
are found to have stellar masses which are typically 5% of their host dwarfs
and show very flattened shapes, rotational kinematics, and central velocity
dispersions significantly smaller than those of their host dwarfs. We also find
that more massive dwarfs can develop more massive, more metal-rich, and higher
density stellar systems in their central regions, because star formation and
chemical enrichment proceed more efficiently owing to the less dramatic
suppression of star formation by supernovae feedback effects in more massive
dwarfs. Based on these results, we suggest that gas-rich, non-nucleated dwarfs
can be transformed into nucleated ones as a result of dissipative gas dynamics
in their central regions. We discuss the origin of the observed correlations
between physical properties of stellar galactic nuclei and those of their host
galaxies.Comment: 13 pages, 4 figures (1 color), ApJL in pres
Deep griz GMOS Imaging of the Dwarf Irregular Galaxy Kar 50
Images obtained with the Gemini Multi-Object Spectrograph (GMOS) are used to
investigate the stellar content and distance of the dwarf irregular galaxy Kar
50. The brightest object is an HII region, and the bright stellar content is
dominated by stars with g'-r' < 0. The tips of the main sequence and the red
giant branch are tentatively identified near r' = 24.9 and i' = 25.5,
respectively. The galaxy has a blue integrated color with no significant color
gradient, and we conclude that Kar 50 has experienced a recent galaxy-wide
episode of star formation. The distance estimated from the brightest blue stars
indicates that Kar 50 is behind the M81 group, and this is consistent with the
tentative RGB-tip brightness. Kar 50 has a remarkably flat central surface
brightness profile, even at wavelengths approaching 1um, although there is no
evidence of a bar. In the absence of another large star-forming episode, Kar 50
will evolve into a very low surface brightness galaxy.Comment: 17 pages of text and 8 postscript figures. Accepted for publication
in the PAS
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