6,230 research outputs found
A Lensed Arc in the Low Redshift Cluster Abell 2124
We report the discovery of an arc-like object 27" from the center of the cD
galaxy in the redshift cluster A2124. Observations with the Keck II
telescope reveal that the object is a background galaxy at ,
apparently lensed into an arc of length \sim 8 \farcs5 and total R magnitude
. The width of the arc is resolved; we estimate it to be
0\farcs6 after correcting for seeing. A lens model of the A2124 core mass
distribution consistent with the cluster galaxy velocity dispersion reproduces
the observed arc geometry and indicates a magnification factor \gta 9. With
this magnification, the strength of the [OII] \lambda 3727 line implies a
star-formation rate of SFR \sim 0.4 h^{-2}\msun yr^{-1}$. A2124 thus appears to
be the lowest redshift cluster known to exhibit strong lensing of a distant
background galaxy.Comment: 6 pages using emulateapj.sty; 4 Postscript figures; Figure 4 uses
color. Accepted for publication, but ApJ Letters' new policy of counting data
images makes the manuscript too long; will appear in main journal. This final
version has minor correction
The Ballistic Phonon Signal in Low Temperature Scanning Electron Microscopy
By scanning the surface of a specimen cooled to liquid-helium temperature with the electron beam, ballistic phonons are generated which can be used for acoustic imaging. The anisotropy of the ballistic phonon propagation caused by the phonon focusing effect has been observed in recent experiments. The simultaneous operation of two or more small-area phonon detectors during the scanning process appears promising for three-dimensional acoustic imaging of structural inhomogeneities even far from the specimen surface. The principles of this scheme for acoustic imaging are discussed and the spatial resolution limits are estimated
An ultraviolet excess in the superluminous supernova Gaia16apd reveals a powerful central engine
Since the discovery of superluminous supernovae (SLSNe) in the last decade,
it has been known that these events exhibit bluer spectral energy distributions
than other supernova subtypes, with significant output in the ultraviolet.
However, the event Gaia16apd seems to outshine even the other SLSNe at
rest-frame wavelengths below \AA. Yan et al (2016) have recently
presented HST UV spectra and attributed the UV flux to low metallicity and
hence reduced line blanketing. Here we present UV and optical light curves over
a longer baseline in time, revealing a rapid decline at UV wavelengths despite
a typical optical evolution. Combining the published UV spectra with our own
optical data, we demonstrate that Gaia16apd has a much hotter continuum than
virtually any SLSN at maximum light, but it cools rapidly thereafter and is
indistinguishable from the others by -15 days after peak. Comparing
the equivalent widths of UV absorption lines with those of other events, we
show that the excess UV continuum is a result of a more powerful central power
source, rather than a lack of UV absorption relative to other SLSNe or an
additional component from interaction with the surrounding medium. These
findings strongly support the central-engine hypothesis for hydrogen-poor
SLSNe. An explosion ejecting M, where
is the opacity in cmg, and forming a magnetar with spin
period ms, and G (lower than other SLSNe with
comparable rise-times) can consistently explain the light curve evolution and
high temperature at peak. The host metallicity, Z, is
comparable to other SLSNe.Comment: Updated to match accepted version (ApJL
An X-ray measurement of Titan's atmospheric extent from its transit of the Crab Nebula
Saturn's largest satellite, Titan, transited the Crab Nebula on 5 January
2003. We observed this astronomical event with the {\it Chandra} X-ray
Observatory. An ``occultation shadow'' has clearly been detected and is found
to be larger than the diameter of Titan's solid surface. The difference gives a
thickness for Titan's atmosphere of 880 60 km. This is the first
measurement of Titan's atmospheric extent at X-ray wavelengths. The value
measured is consistent with or slightly larger than those estimated from
earlier Voyager observations at other wavelengths. We discuss the possibility
of temporal variations in the thickness of Titan's atmosphere.Comment: 14 pages, 5 figures, AASTeX preprint. Accepted for publication in the
Astrophysical Journa
The SBF Survey of Galaxy Distances. I. Sample Selection, Photometric Calibration, and the Hubble Constant
We describe a program of surface brightness fluctuation (SBF) measurements
for determining galaxy distances. This paper presents the photometric
calibration of our sample and of SBF in general. Basing our zero point on
observations of Cepheid variable stars, we find that the absolute SBF magnitude
in the Kron-Cousins I band correlates well with the mean (V-I)o color of a
galaxy according to
M_Ibar = (-1.74 +/- 0.07) + (4.5 +/- 0.25) [ (V-I)o - 1.15 ]
for 1.0 < (V-I) < 1.3. This agrees well with theoretical estimates from
stellar population models. Comparisons between SBF distances and a variety of
other estimators, including Cepheid variable stars, the Planetary Nebula
Luminosity Function (PNLF), Tully-Fisher (TF), Dn-sigma, SNII, and SNIa,
demonstrate that the calibration of SBF is universally valid and that SBF error
estimates are accurate. The zero point given by Cepheids, PNLF, TF (both
calibrated using Cepheids), and SNII is in units of Mpc; the zero point given
by TF (referenced to a distant frame), Dn-sigma and SNIa is in terms of a
Hubble expansion velocity expressed in km/s. Tying together these two zero
points yields a Hubble constant of H_0 = 81 +/- 6 km/s/Mpc. As part of this
analysis, we present SBF distances to 12 nearby groups of galaxies where
Cepheids, SNII, and SNIa have been observed.Comment: 29 pages plus 8 figures; LaTeX (AASTeX) uses aaspp4.sty (included);
To appear in The Astrophysical Journal, 1997 February 1 issue; Compressed
PostScript available from ftp://mars.tuc.noao.edu/sbf
Leonardo's rule, self-similarity and wind-induced stresses in trees
Examining botanical trees, Leonardo da Vinci noted that the total
cross-section of branches is conserved across branching nodes. In this Letter,
it is proposed that this rule is a consequence of the tree skeleton having a
self-similar structure and the branch diameters being adjusted to resist
wind-induced loads
Nickel-Rich Outflows Produced by the Accretion-Induced Collapse of White Dwarfs: Lightcurves and Spectra
The accretion-induced collapse (AIC) of a white dwarf to form a neutron star
can leave behind a rotationally supported disk with mass of up to ~ 0.1 M_sun.
The disk is initially composed of free nucleons but as it accretes and spreads
to larger radii, the free nucleons recombine to form helium, releasing
sufficient energy to unbind the remaining disk. Most of the ejected mass fuses
to form Ni56 and other iron group elements. We present spherically symmetric
radiative transfer calculations of the transient powered by the radioactive
heating of this ejecta. For an ejecta mass of 1e-2 M_sun (3e-3 M_sun), the
lightcurve peaks after <~ 1 day with a peak bolometric luminosity ~ 2e41 erg/s
(~ 5e40 erg/s), i.e., a "kilonova"; the decay time is ~ 4 (2) days. Overall,
the spectra redden with time reaching U-V ~ 4 after ~ 1 day; the optical colors
(B-V) are, however, somewhat blue. Near the peak in the lightcurve, the spectra
are dominated by Doppler broadened Nickel features, with no distinct spectral
lines present. At ~ 3-5 days, strong Calcium lines are present in the infrared,
although the Calcium mass fraction is only ~ 1e-4.5. If rotationally supported
disks are a common byproduct of AIC, current and upcoming transient surveys
such as the Palomar Transient Factory should detect a few AIC per year for an
AIC rate of ~ 1e-2 of the Type Ia rate. We discuss ways of distinguishing AIC
from other rapid, faint transients, including .Ia's and the ejecta from binary
neutron star mergers.Comment: 9 pages, 4 figures, 1 table, now accepted to MNRA
The SBF Survey of Galaxy Distances. IV. SBF Magnitudes, Colors, and Distances
We report data for band Surface Brightness Fluctuation (SBF) magnitudes,
V-I colors, and distance moduli for 300 galaxies. The Survey contains E, S0 and
early-type spiral galaxies in the proportions of 49:42:9, and is essentially
complete for E galaxies to Hubble velocities of 2000 km/s, with a substantial
sampling of E galaxies out to 4000 km/s. The median error in distance modulus
is 0.22 mag.
We also present two new results from the Survey. (1) We compare the mean
peculiar flow velocity (bulk flow) implied by our distances with predictions of
typical cold dark matter transfer functions as a function of scale, and find
very good agreement with cold, dark matter cosmologies if the transfer function
scale parameter , and the power spectrum normalization are
related by . Derived directly from
velocities, this result is independent of the distribution of galaxies or
models for biasing. The modest bulk flow contradicts reports of large-scale,
large-amplitude flows in the Mpc diameter volume surrounding our
Survey volume. (2) We present a distance-independent measure of absolute galaxy
luminosity, \Nbar, and show how it correlates with galaxy properties such as
color and velocity dispersion, demonstrating its utility for measuring galaxy
distances through large and unknown extinction.Comment: Accepted for publication in ApJ (10 January 2001); 23 page
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