337 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
Lensed Arcs and Inner Structure of Abell 697
We present new optical observations of the z=0.282 cluster Abell 697 from the
Keck II telescope. Images show an unusual disturbed structure in the cD halo
and a previously unknown faint gravitational lens arc. A spectrum of the arc
did not yield a redshift, but its spectrum and colors suggest it lies at z>1.3.
We construct models to reproduce the arc that show the potential is likely to
be highly elliptical. We suggest that this cluster may have undergone a recent
merger and is in the process of forming its cD galaxy. Analysis of X-ray data
from ROSAT and ASCA suggests that the merging process is sufficiently advanced
that the gas in the cluster has relaxed, and A697 lies near the L_x-T_x
relation for normal clusters.Comment: LaTeX; 12 pages, 3 figures, submitted to ApJ Letter
A Search for Distant Galactic Cepheids Toward l=60
We present results of a survey of a 6-square-degree region near l=60, b=0 to
search for distant Milky Way Cepheids. Few MW Cepheids are known at distances
>~ R_0, limiting large-scale MW disk models derived from Cepheid kinematics;
this work was designed to find a sample of distant Cepheids for use in such
models. The survey was conducted in the V and I bands over 8 epochs, to a
limiting I~=18, with a total of ~ 5 million photometric observations of ~ 1
million stars. We present a catalog of 578 high-amplitude variables discovered
in this field. Cepheid candidates were selected from this catalog on the basis
of variability and color change, and observed again the following season. We
confirm 10 of these candidates as Cepheids with periods from 4 to 8 days, most
at distances > 3 kpc. Many of the Cepheids are heavily reddened by intervening
dust, some with implied extinction A_V > 10 mag. With a future addition of
infrared photometry and radial velocities, these stars alone can provide a
constraint on R_0 to 8%, and in conjunction with other known Cepheids should
provide good estimates of the global disk potential ellipticity.Comment: 18 pages, 4 tables, 13 figures (LaTeX / AASTeX
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
HST and Palomar Imaging of GRB 990123: Implications for the Nature of Gamma-Ray Bursts and their Hosts
We report on HST and Palomar optical images of the field of GRB 990123,
obtained on 8 and 9 February 1999. We find that the optical transient (OT)
associated with GRB 990123 is located on an irregular galaxy, with magnitude
V=24.20 +/- 0.15. The strong metal absorption lines seen in the spectrum of the
OT, along with the low probability of a chance superposition, lead us to
conclude that this galaxy is the host of the GRB. The OT is projected within
the ~1'' visible stellar field of the host, nearer the edge than the center. We
cannot, on this basis, rule out the galactic nucleus as the site of the GRB,
since the unusual morphology of the host may be the result of an ongoing
galactic merger, but our demonstration that this host galaxy has extremely blue
optical to infrared colors more strongly supports an association between GRBs
and star formation. We find that the OT magnitude on 1999 Feb 9.05, V = 25.45
+/- 0.15, is about 1.5 mag fainter than expected from extrapolation of the
decay rate found in earlier observations. A detailed analysis of the OT light
curve suggests that its fading has gone through three distinct phases: an early
rapid decline (f_{nu} \propto t^{-1.6} for t < 0.1 days), a slower intermediate
decline power-law decay (f_{nu} \propto t^{-1.1} for 0.1 < t < 2 days), and
then a more rapid decay (at least as steep as (f_{\nu} \propto t^{-1.8} for t >
2 days). The break to steeper slope at late times may provide evidence that the
optical emission from this GRB was highly beamed.Comment: Accepted for publication in Astrophysical Journal (Letters). Fourteen
pages. Three encapsulated figure
The Shape and Scale of Galactic Rotation from Cepheid Kinematics
A catalog of Cepheid variables is used to probe the kinematics of the
Galactic disk. Radial velocities are measured for eight distant Cepheids toward
l = 300; these new Cepheids provide a particularly good constraint on the
distance to the Galactic center, R_0. We model the disk with both an
axisymmetric rotation curve and one with a weak elliptical component, and find
evidence for an ellipticity of 0.043 +/- 0.016 near the Sun. Using these
models, we derive R_0 = 7.66 +/- 0.32 kpc and v_circ = 237 +/- 12 km/s. The
distance to the Galactic center agrees well with recent determinations from the
distribution of RR Lyrae variables, and disfavors most models with large
ellipticities at the solar orbit.Comment: 36 pages, LaTeX, 10 figure
The Fading Optical Counterpart of GRB~970228, Six Months and One Year Later
We report on observations of the fading optical counterpart of the gamma-ray
burst GRB 970228, made with the Hubble Space Telescope STIS CCD approximately
six months after outburst and with the HST/NICMOS and Keck/NIRC approximately
one year after outburst. The unresolved counterpart is detected by STIS at
V=28.0 +/- 0.25, consistent with a continued power-law decline with exponent
-1.14 +/- 0.05. The counterpart is located within, but near the edge of, a
faint extended source with diameter ~0."8 and integrated magnitude V=25.8 +/-
0.25. A reanalysis of HST and NTT observations performed shortly after the
burst shows no evidence of proper motion of the point source or fading of the
extended emission. Only the extended source is visible in the NICMOS images
with a magnitude of H=23.3 +/- 0.1. The Keck observations find K = 22.8 +/-
0.3. Several distinct and independent means of deriving the foreground
extinction in the direction of GRB 970228 all agree with A_V = 0.75 +/- 0.2.
After adjusting for Galactic extinction, we find that the size of the observed
extended emission is consistent with that of galaxies of comparable magnitude
found in the Hubble Deep Field (HDF) and other deep HST images. Only 2% of the
sky is covered by galaxies of similar or greater surface brightness; therefore
the extended source is almost certainly the host galaxy. Additionally, we find
that the extinction-corrected V - H and V - K colors of the host are as blue as
any galaxy of comparable or brighter magnitude in the HDF. Taken in concert
with recent observations of GRB 970508, GRB 971214, and GRB 980703 our work
suggests that all four GRBs with spectroscopic identification or deep
multicolor broad-band imaging of the host lie in rapidly star-forming galaxies.Comment: 24 pages, Latex, 4 PostScript figures, to appear in the May 10 issue
of The Astrophysical Journal (Note: displayed abstract is abridged
Lensing in the Hercules Supercluster
We report Keck LRIS observations of an arc-like background galaxy near the
center of Abell 2152 (z=0.043), one of the three clusters comprising the
Hercules supercluster. The background object has a redshift z=0.1423 and is
situated 25 arcsec north of the primary component of the A2152 brightest
cluster galaxy (BCG). The object is about 15 arcsec in total length and has a
reddening-corrected R-band magnitude of . Its spectrum
shows numerous strong emission lines, as well as absorption features. The
strength of the H-alpha emission would imply a star formation rate \SFR
\approx 3h^{-2} \msun yr in the absence of any lensing. However, the
curved shaped of this object and its tangential orientation along the major
axis of the BCG suggest lensing. We model the A2152 core mass distribution
including the two BCG components and the cluster potential. We present velocity
and velocity dispersion profile measurements for the two BCG components and use
these to help constrain the potential. The lens modeling indicates a likely
magnification factor of for the lensed galaxy, making A2152 the
nearest cluster in which such significant lensing of a background source has
been observed. Finally, we see evidence for a concentration of early-type
galaxies at near the centroid of the X-ray emission previously
attributed to A2152. We suggest that emission from this background
concentration is the cause of the offset of the X-ray center from the A2152
BCG. The background concentration and the dispersed mass of the Hercules
supercluster could add further to the lensing strength of the A2152 cluster.Comment: Accepted for publication in AJ (January 2001). 9 pages; uses
emulateapj.sty. The all-important "Figure 1" is included here in GIF format;
for a version which includes Figure 1 as a high-resolution Postscript image,
see: http://adcam.pha.jhu.edu/~jpb/a2152.ps.g
Molecular Analysis Confirming the Introduction of Nile Crocodiles, Crocodylus niloticus Laurenti 1768 (Crocodylidae), in Southern Florida, with an Assessment of Potential for Establishment, Spread, and Impacts.
The state of Florida, USA, has more introduced herpetofauna than any other governmental region on Earth. Four species of nonnative crocodilians have been introduced to Florida (all since 1960), one of which is established. Between 2000â2014 we field-collected three nonnative crocodilians in Miami-Dade County, Florida, and one in Hendry County, Florida. We used DNA barcoding and molecular phylogenetics to determine species identification and native range origin. Also, we described diet, movement, and growth for one crocodile. Our molecular analyses illustrated that two of the crocodiles we collected are most closely related to Nile Crocodiles (Crocodylus niloticus) from South Africa, suggesting this region as a source population. We, thus, documented the first known introduction of C. niloticus in Florida. Two, and possibly three of the introduced crocodiles shared the same haplotype, suggesting they are likely from the same introduction pathway or source. One animal was captured, measured, marked, and released, then recaptured 2 y later allowing us to calculate growth rate (40.5 cm/y) and movement. The most likely route of travel by waterway (i.e., canal) illustrates that this animal traveled at least 29 km from its original capture site. One crocodile escaped from a facility in Hendry County, Florida, and survived in 1,012 ha of semi-wild habitat for three to four years, confirming that this species can survive in southern Florida
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