356 research outputs found
Triple Bars and Complex Central Structures in Disk Galaxies
We present an analysis of ground-based and HST images of three early-type
barred galaxies. The first, NGC 2681, may be the clearest example yet of a
galaxy with three concentric bars. The two other galaxies were previously
suggested as triple-barred. Our analysis shows that while NGC 3945 is probably
double-barred, NGC 4371 has only one bar; but both have intriguing central
structures. NGC 3945 has a large, extremely bright disk inside its primary bar,
with patchy dust lanes, a faint nuclear ring or pseudo-ring within the disk,
and an apparent secondary bar crossing the ring. NGC 4371 has a bright nuclear
ring only marginally bluer than the surrounding bulge and bar. There is no
evidence for significant dust or star formation in either of these nuclear
rings. The presence of stellar nuclear rings suggests that the centers of these
galaxies are dynamically cool and disklike.Comment: LaTeX: 6 pages, 3 figures, uses emulateapj.sty. Accepted by
Astrophysical Journal Letters. Version with full-resolution figures available
at: http://www.astro.wisc.edu/~erwin/research
Effective Radii and Color Gradients in Radio Galaxies
We present de Vaucouleurs' effective radii in B and R bands for a sample of
Molonglo Reference Catalogue radio galaxies and a control sample of normal
galaxies. We use the ratio of the scale lengths in the two bands as an
indicator to show that the radio galaxies tend to have excess of blue color in
their inner region much more frequently than the control galaxies. We show that
the scale length ratio is a useful indicator of radial color variation even
when the conventional color gradient is too noisy to serve the purpose.Comment: 11 pages, 4 figures, (LaTeX: aaspp4, epsfig), to appear in ApJL 199
Initial correlations in nonequilibrium Falicov-Kimball model
The Keldysh boundary problem in a nonequilibrium Falicov-Kimball model in
infinite dimensions is studied within the truncated and self-consistent
perturbation theories, and the dynamical mean-field theory. Within the model
the system is started in equilibrium, and later a uniform electric field is
turned on. The Kadanoff-Baym-Wagner equations for the nonequilibrium Green
functions are derived, and numerically solved. The contributions of initial
correlations are studied by monitoring the system evolution. It is found that
the initial correlations are essential for establishing full electron
correlations of the system and independent on the starting time of preparing
the system in equilibrium. By examining the contributions of the initial
correlations to the electric current and the double occupation, we find that
the contributions are small in relation to the total value of those physical
quantities when the interaction is weak, and significantly increase when the
interaction is strong. The neglect of initial correlations may cause artifacts
in the nonequilibrium properties of the system, especially in the strong
interaction case
A Normal Stellar Disk in the Galaxy Malin 1
Since its discovery, Malin 1 has been considered the prototype and most
extreme example of the class of giant low surface brightness disk galaxies.
Examination of an archival Hubble Space Telescope I-band image reveals that
Malin 1 contains a normal stellar disk that was not previously recognized,
having a central I-band surface brightness of mu_0 = 20.1 mag arcsec^-2 and a
scale length of 4.8 kpc. Out to a radius of ~10 kpc, the structure of Malin 1
is that of a typical SB0/a galaxy. The remarkably extended, faint outer
structure detected out to r~100 kpc appears to be a photometrically distinct
component and not a simple extension of the inner disk. In terms of its disk
scale length and central surface brightness, Malin 1 was originally found to be
a very remote outlier relative to all other known disk galaxies. The presence
of a disk of normal size and surface brightness in Malin 1 suggests that such
extreme outliers in disk properties probably do not exist, but underscores the
importance of the extended outer disk regions for a full understanding of the
structure and formation of spiral galaxies.Comment: 13 pages, 7 figures. To appear in AJ. Typographical error correcte
Surface Brightness Fluctuations from archival ACS images: a stellar population and distance study
We derive Surface Brightness Fluctuations (SBF) and integrated magnitudes in
the V- and I-bands using Advanced Camera for Surveys (ACS) archival data. The
sample includes 14 galaxies covering a wide range of physical properties:
morphology, total absolute magnitude, integrated color. We take advantage of
the latter characteristic of the sample to check existing empirical
calibrations of absolute SBF magnitudes both in the I- and V-passbands.
Additionally, by comparing our SBF and color data with the Teramo-SPoT simple
stellar population models, and other recent sets of population synthesis
models, we discuss the feasibility of stellar population studies based on
fluctuation magnitudes analysis. The main result of this study is that
multiband optical SBF data and integrated colors can be used to significantly
constrain the chemical composition of the dominant stellar system in the
galaxy, but not the age in the case of systems older than 3 Gyr.
SBF color gradients are also detected and analyzed. These SBF gradient data,
together with other available data, point to the existence of mass dependent
metallicity gradients in galaxies, with the more massive objects showing a
non--negligible SBF versus color gradient. The comparison with models suggests
that such gradients imply more metal rich stellar populations in the galaxies'
inner regions with respect to the outer ones.Comment: ApJ Accepte
Photometric observations of Southern Abell Cluster Redshifts Survey Clusters: Structure of galaxies in the inner region of clusters of galaxies
We analyze photometric properties of 1384 cluster galaxies as a function of
the normalized distance to cluster center. These galaxies were selected in the
central region ( 0.8) of 14 southern Abell clusters chosen from
the Southern Abell Cluster Redshifts Survey (SARS). For 507 of these galaxies
we also obtained their luminosity profiles. We have studied the
morphology-clustercentric distance relation on the basis of the shape parameter
of the S\'ersic's law. We also have analyzed the presence of a possible
segregation in magnitude for both, the galaxy total luminosity and that of
their components (i.e. the bulge and the disk).
Results show a marginal ( level) decrease of the total luminosity as
a function of normalized radius. However, when bulges are analyzed separately,
a significant luminosity segregation is found ( and for
galaxies in projection and member galaxies respectively). The fraction of
bulges brighter than is three times larger in the core of
clusters than in the outer region. Our analysis of the disk component suggests
that disks are, on average, less luminous in the cluster core than at
. In addition, we found that the magnitude-size relation as
a function of indicates (at level) that disks are smaller
and centrally brighter in the core of clusters. However, the Kormendy relation
(the bulge magnitude-size relation) appears to be independent of environment.Comment: To appear in the A
Stellar Kinematics of the Double Nucleus of M31
We report observations of the double nucleus of M31 with the f/48 long-slit
spectrograph of the HST Faint Object Camera. We obtain a total exposure of
19,000 sec. over 7 orbits, with the 0.063-arcsec-wide slit along the line
between the two brightness peaks (PA 42). A spectrum of Jupiter is used as a
spectral template. The rotation curve is resolved, and reaches a maximum
amplitude of ~250 km/s roughly 0.3 arcsec either side of a rotation center
lying between P1 and P2, 0.16 +/- 0.05 arcsec from the optically fainter P2. We
find the velocity dispersion to be < 250 km/s everywhere except for a narrow
``dispersion spike'', centered 0.06 +/- 0.03 arcsec on the anti-P1 side of P2,
in which sigma peaks at 440 +/- 70 km/s. At much lower confidence, we see local
disturbances to the rotation curve at P1 and P2, and an elevation in sigma at
P1. At very low significance we detect a weak asymmetry in the line-of-sight
velocity distribution opposite to the sense usually encountered. Convolving our
V and sigma profiles to CFHT resolution, we find good agreement with the
results of Kormendy & Bender (1998, preprint), though there is a 20%
discrepancy in the dispersion that cannot be attributed to the dispersion
spike. Our results are not consistent with the location of the maximum
dispersion as found by Bacon et al. We find that the sinking star cluster model
of Emsellem & Combes (1997) does not reproduce either the rotation curve or the
dispersion profile. The eccentric disk model of Tremaine (1995) fares better,
and can be improved somewhat by adjusting the original parameters. However,
detailed modeling will require dynamical models of significantly greater
realism.Comment: 29 pages, Latex, AASTeX v4.0, with 7 eps figures. To appear in The
Astronomical Journal, February 199
Calibrating Type Ia Supernovae using the Planetary Nebula Luminosity Function I. Initial Results
We report the results of an [O III] lambda 5007 survey for planetary nebulae
(PN) in five galaxies that were hosts of well-observed Type Ia supernovae: NGC
524, NGC 1316, NGC 1380, NGC 1448 and NGC 4526. The goals of this survey are to
better quantify the zero-point of the maximum magnitude versus decline rate
relation for supernovae Type Ia and to validate the insensitivity of Type Ia
luminosity to parent stellar population using the host galaxy Hubble type as a
surrogate. We detected a total of 45 planetary nebulae candidates in NGC 1316,
44 candidates in NGC 1380, and 94 candidates in NGC 4526. From these data, and
the empirical planetary nebula luminosity function (PNLF), we derive distances
of 17.9 +0.8/-0.9 Mpc, 16.1 +0.8/-1.1 Mpc, and 13.6 +1.3/-1.2 Mpc respectively.
Our derived distance to NGC 4526 has a lower precision due to the likely
presence of Virgo intracluster planetary nebulae in the foreground of this
galaxy. In NGC 524 and NGC 1448 we detected no planetary nebulae candidates
down to the limiting magnitudes of our observations. We present a formalism for
setting realistic distance limits in these two cases, and derive robust lower
limits of 20.9 Mpc and 15.8 Mpc, respectively.
After combining these results with other distances from the PNLF, Cepheid,
and Surface Brightness Fluctuations distance indicators, we calibrate the
optical and near-infrared relations for supernovae Type Ia and we find that the
Hubble constants derived from each of the three methods are broadly consistent,
implying that the properties of supernovae Type Ia do not vary drastically as a
function of stellar population. We determine a preliminary Hubble constant of
H_0 = 77 +/- 3 (random) +/- 5 (systematic) km/s/Mpc for the PNLF, though more
nearby galaxies with high-quality observations are clearly needed.Comment: 25 pages, 12 figures. Accepted for publication by the Astrophysical
Journal. Figures degraded to comply with limit. Full paper is available at:
http://www.as.ysu.edu/~jjfeldme/pnlf_Ia.pd
Hubble Space Telescope faint object camera instrument handbook (Post-COSTAR), version 5.0
The faint object camera (FOC) is a long-focal-ratio, photon-counting device capable of taking high-resolution two-dimensional images of the sky up to 14 by 14 arc seconds squared in size with pixel dimensions as small as 0.014 by 0.014 arc seconds squared in the 1150 to 6500 A wavelength range. Its performance approaches that of an ideal imaging system at low light levels. The FOC is the only instrument on board the Hubble Space Telescope (HST) to fully use the spatial resolution capabilities of the optical telescope assembly (OTA) and is one of the European Space Agency's contributions to the HST program
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