3,715 research outputs found
Anisotropic Distribution of SDSS Satellite Galaxies: Planar (not Polar) Alignment
The distribution of satellite galaxies relative to isolated host galaxies in
the Sloan Digital Sky Survey (SDSS) is investigated. Host-satellite systems are
selected using three different methods, yielding samples of ~3300, ~1600, and
\~950 satellites. In the plane of the sky, the distributions of all three
samples show highly significant deviations from circular symmetry (> 99.99%, >
99.99%, and 99.79% confidence levels, respectively), and the degree of
anisotropy is a strong function of the projected radius, r_p, at which the
satellites are found. For r_p < 100 kpc, the SDSS satellites are aligned
preferentially with the major axes of the hosts. This is in stark contrast to
the Holmberg effect, in which satellites are aligned with the minor axes of
host galaxies. The degree of anisotropy in the distribution of the SDSS
satellites decreases with r_p and is consistent with an isotropic distribution
at of order the 1-sigma level for 250 kpc < r_p < 500 kpc.Comment: ApJ Letters (in press); Discussion section substantially revised,
SDSS DR3 included in the analysis, no significant changes to the result
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An analysis of the impact of floodplain regulations on residential land values: the Oak Grove, Oregon case study
Flood damage mitigation by means of land use regulations,
as mandated by the National Flood Insurance Act of 1968, has been
controversial and slow in implementation, in part due to the conjecture
that residential land values will be adversely affected
in the regulated areas. There has been relatively little research
to test this idea. This study tests the hypothesis that floodplain
regulations significantly lower the appreciation rate of residential
lots within the regulated area, when compared to adjacent similar
unregulated lots. The study was done in the old residential
neighborhood of Oak Grove, Clackamas County, beside the Willamette
River. Assessed land values from 1958 to 1981 were examined. The
study period includes a disastrous flood in 1964, and the adoption
of floodplain regulations in 1971. Examination of the data indicated
that the regulated lots need to be subdivided into three
groups, based on water frontage, and each of these was tested
against the unregulated control lots. Statistical testing shows
that after implementation of flood plain regulations there was
no significant reduction in appreciation rates of the regulated
lots when compared to the control lots. Contrary to the hypothesis
in several time intervals, lakefront and riverfront lots appreciated
at faster rates than the control lots did. The results suggest
that there are other factors that balance, and sometimes outweigh,
any negative economic effects due to floodplain regulations
The distortion of distant galaxy images by large-scale structure
Inhomogeneity in the distribution of mass in the universe on scales ≲ 100 Mpc can generate a coherent shear distortion or polarization of the images of background galaxies. This distortion may be measurable over patches of the sky up to a few square degrees in size. If this distortion is measured, or conversely, if its magnitude is limited, it should help us understand the degree to which luminosity traces the underlying mass over cosmological scales. A prescription is given for quantifying the galaxy distortion and a propagation equation for its evolution in an inhomogeneous universe is derived. The creation of shear by inhomogeneity is illustrated using model kinematic universes comprising random distributions of point masses, spheres and circular discs designed to simulate the superclusters, voids and ‘walls’ reported in galaxy velocity surveys. Using these simulations, we estimate that an rms induced ellipticity of |p|_(rms) ˜ 0.2Ω_(LSS) (where Ω_(LSS) is the fraction of the mass of the universe clustered on the large scale) will be produced. The angular correlation length is ˜ 1.6°.
In an alternative prescription, the universe is modelled using a power spectrum of density fluctuations and the mean correlation function is computed both analytically and numerically. In these simulations we find that |p|_(rms) ˜0.02 for biased cold dark matter models of an Einslein–De Sitter universe, and the effective correlation length is θ_(1/2) ˜ 0.5^ΰ. For a hot dark matter dominated universe the correlation length is θ_(1/2) ˜ 0.7^ΰ.
The faint, blue galaxies discovered by Tyson and collaborators have a surface density of ˜ 3 × 10^5 deg^(−2) and should provide an ideal set of sources for measuring this effect
The Extinction Towards the GRB970228 Field
We determine the local galactic extinction towards the field of gamma-ray
burst GRB970228 using a variety of methods. We develop a maximum likelihood
method for measuring the extinction by comparing galaxy counts in the field of
interest to those in a field of known extinction, and apply this method to the
GRB970228 field. We also measure the extinction by comparing the observed
stellar spectral energy distributions of stars in the GRB970228 field to the
spectral energy distribution of library spectra of the same spectral type.
Finally we estimate the extinction using the Balmer emission line ratios of a
galaxy in the GRB970228 field, and the neutral hydrogen column density and
amount of infrared dust emission toward this field. Combining the results of
these methods, we find a best-fit galactic extinction in the optical of
, which implies a a substantial dimming and change of
the spectral slope of the intrinsic GRB970228 afterglow.Comment: 22 pages, including 7 figures. Submitted to Ap
The Opacity of Nearby Galaxies from Counts of Background Galaxies: II. Limits of the Synthetic Field Method
Recently, we have developed and calibrated the Synthetic Field Method (SFM)
to derive the total extinction through disk galaxies. The method is based on
the number counts and colors of distant background field galaxies that can be
seen through the foreground object, and has been successfully applied to NGC
4536 and NGC 3664, two late-type galaxies located, respectively, at 16 and 11
Mpc. Here, we study the applicability of the SFM to HST images of galaxies in
the Local Group, and show that background galaxies cannot be easily identified
through these nearby objects, even with the best resolution available today. In
the case of M 31, each pixel in the HST images contains 50 to 100 stars, and
the background galaxies cannot be seen because of the intrinsic granularity due
to strong surface brightness fluctuations. In the LMC, on the other hand, there
is only about one star every six linear pixels, and the lack of detectable
background galaxies results from a ``secondary'' granularity, introduced by
structure in the wings of the point spread function. The success of the SFM in
NGC 4536 and NGC 3664 is a natural consequence of the reduction of the
intensity of surface brightness fluctuations with distance. When the dominant
confusion factor is structure in the PSF wings, as is the case of HST images of
the LMC, and would happen in M 31 images obtained with a 10-m diffraction-
limited optical telescope, it becomes in principle possible to improve the
detectability of background galaxies by subtracting the stars in the foreground
object. However, a much better characterization of optical PSFs than is
currently available would be required for an adequate subtraction of the wings.
Given the importance of determining the dust content of Local Group galaxies,
efforts should be made in that direction.Comment: 45 pages, 10 Postscript figure
Galactic Extinction from Colors and Counts of Field Galaxies in WFPC2 Frames: An Application to GRB 970228
We develop the ``simulated extinction method'' to measure average foreground
Galactic extinction from field galaxy number-counts and colors. The method
comprises simulating extinction in suitable reference fields by changing the
isophotal detection limit. This procedure takes into account selection effects,
in particular, the change in isophotal detection limit (and hence in isophotal
magnitude completeness limit) with extinction, and the galaxy color--magnitude
relation.
We present a first application of the method to the HST WFPC2 images of the
gamma-ray burster GRB 970228. Four different WFPC2 high-latitude fields,
including the HDF, are used as reference to measure the average extinction
towards the GRB in the F606W passband. From the counts, we derive an average
extinction of A_V = 0.5 mag, but the dispersion of 0.4 mag between the
estimates from the different reference fields is significantly larger than can
be accounted by Poisson plus clustering uncertainties. Although the counts
differ, the average colors of the field galaxies agree well. The extinction
implied by the average color difference between the GRB field and the reference
galaxies is A_V = 0.6 mag, with a dispersion in the estimated extinction from
the four reference fields of only 0.1 mag. All our estimates are in good
agreement with the value of 0.81\pm0.27 mag obtained by Burstein & Heiles, and
with the extinction of 0.78\pm0.12 measured by Schlegel et al. from maps of
dust IR emission. However, the discrepancy between the widely varying counts
and the very stable colors in these high-latitude fields is worth
investigating.Comment: 14 pages, 2 figures; submitted to the Astrophysical Journa
Faint K Selected Galaxy Correlations and Clustering Evolution
Angular and spatial correlations are measured for K-band--selected galaxies,
248 having redshifts, 54 with z>1, in two patches of combined area 27 arcmin^2.
The angular correlation for K<=21.5 mag is (theta/1.4+/-0.19 arcsec
e^{+/-0.1})^{-0.8}. From the redshift sample we find that the real-space
correlation, calculated with q_0=0.1, of M_K<=-23.5 mag galaxies (k-corrected)
is \xi(r) = (r/2.9e^{+/-0.12}1/h Mpc)^{-1.8} at a mean z= 0.34,
(r/2.0e^{+/-0.15}1/h Mpc)^{-1.8} at z= 0.62, (r/1.4e^{+/-0.15}1/h Mpc)^{-1.8}
at z= 0.97, and (r/1.0e^{+/-0.2}1/h Mpc)^{-1.8} at z= 1.39, the last being a
formal upper limit for a blue-biased sample. In general, these are more
correlated than optically selected samples in the same redshift ranges. Over
the interval 0.32 AB
mag, have \xi(r)=(r/2.4e^{+/-0.14}1/h Mpc)^{-1.8} whereas bluer galaxies, which
have a mean B of 23.7 mag and mean [OII] equivalent width W_{eq} = 41=\AA, are
very weakly correlated, with \xi(r)=(r/0.9e^{+/-0.22}1/h Mpc)^{-1.8}. For our
measured growth rate of clustering, this blue population, if non-merging, can
grow only into a low-redshift population less luminous than 0.4L_\ast. The
cross-correlation of low- and high-luminosity galaxies at z=0.6 appears to have
an excess in the correlation amplitude within 100/h kpc. The slow redshift
evolution is consistent with these galaxies tracing the mass clustering in low
density, Omega= 0.2, relatively unbiased, sigma_8=0.8, universe, but cannot yet
exclude other possibilities.Comment: to be published in the Aug 1 ApJ, 20 pages as a uuencoded postscript
file Postscript with all figures is available at
http://manaslu.astro.utoronto.ca/~carlberg/paper
Targeted disruption of ATM leads to growth retardation, chromosomal fragmentation during meiosis, immune defects, and thymic lymphoma
ATM, the gene mutated in the inherited human disease ataxia-telangiectasia, is a member of a family of kinases involved in DNA metabolism and cell-cycle checkpoint control. To help clarify the physiological roles of the ATM protein, we disrupted the ATM gene in mice through homologous recombination. Initial evaluation of the ATM knockout animals indicates that inactivation of the mouse ATM gene recreates much of the phenotype of ataxia-telangiectasia. The homozygous mutant (ATM-/-) mice are viable, growth-retarded, and infertile. The infertility of ATM-/- mice results from meiotic failure. Meiosis is arrested at the zygotene/pachytene stage of prophase I as a result of abnormal chromosomal synapsis and subsequent chromosome fragmentation. Immune defects also are evident in ATM-/- mice, including reduced numbers of B220+CD43- pre-B cells, thymocytes, and peripheral T cells, as well as functional impairment of T-cell-dependent immune responses. The cerebella of ATM-/- mice appear normal by histologic examination at 3 to 4 months and the mice have no gross behavioral abnormalities. The majority of mutant mice rapidly develop thymic lymphomas and die before 4 months of age. These findings indicate that the ATM gene product plays an essential role in a diverse group of cellular processes, including meiosis, the normal growth of somatic tissues, immune development, and tumor suppression
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