1,947 research outputs found
Natural Resources Inventory of Wissahickon Valley Watershed Association Preserves and the Green Ribbon Trail
The purpose of this study is to provide The Wissahickon Valley Watershed Association with baseline information needed to help guide appropriate resource management and education decisions.
The area under consideration in this study includes the named preserves of the Wissahickon Valley Watershed Association (WVWA) (approximately 331 acres), and the Green Ribbon Trail and associated WVWA-owned lands from the trailhead at the Upper Gwynedd Township Park in North Wales to the boundary of Fort Washington State Park near Skippack Pike, a distance of about 13.1 kilometers (8.14 miles) not counting the trail interruption at the Cedarbrook Country Club
Morphology and evolution of emission line galaxies in the Hubble Ultra Deep Field
We investigate the properties and evolution of a sample of galaxies selected
to have prominent emission lines in low-resolution grism spectra of the Hubble
Ultra Deep Field (HUDF). These objects, eGRAPES, are late type blue galaxies,
characterized by small proper sizes (R_50 < 2 kpc) in the 4350A rest-frame, low
masses (5x10^9 M_sun), and a wide range of luminosities and surface
brightnesses. The masses, sizes and volume densities of these objects appear to
change very little up to a redshift of z=1.5. On the other hand, their surface
brightness decreases significantly from z=1.5 to z=0 while their mass-to-light
ratio increases two-folds. This could be a sign that most of low redshift
eGRAPES have an older stellar population than high redshift eGRAPES and hence
that most eGRAPES formed at higher redshifts. The average volume density of
eGRAPES is (1.8 \pm 0.3)x10^{-3} Mpc^{-3} between 0.3 < z < 1.5. Many eGRAPES
would formally have been classified as Luminous Compact Blue Galaxies (LCBGs)
if these had been selected based on small physical size, blue intrinsic color,
and high surface brightness, while the remainder of the sample discussed in
this paper forms an extension of LCBGs towards fainter luminosities.Comment: Accepted, to appear in Ap
Stars in the Hubble Ultra Deep Field
We identified 46 unresolved source candidates in the Hubble Ultra Deep Field,
down to i775 = 29.5. Unresolved objects were identified using a parameter S,
which measures the deviation from the curve-of-growth of a point source.
Extensive testing of this parameter was carried out, including the effects of
decreasing signal-to-noise and of the apparent motions of stars, which
demonstrated that stars brighter than i775 = 27.0 could be robustly identified.
Low resolution grism spectra of the 28 objects brighter than i775 = 27.0
identify 18 M and later stellar type dwarfs, 2 candidate L-dwarfs, 2 QSOs, and
4 white dwarfs. Using the observed population of dwarfs with spectral type M4
or later, we derive a Galactic disk scale height of 400 \pm 100 pc for M and L
stars. The local white dwarf density is computed to be as high as (1.1 \pm 0.3)
x10^(-2) stars/pc^3. Based on observations taken 73 days apart, we determined
that no object in the field has a proper motion larger than 0.027"/year (3
sigma detection limit). No high velocity white dwarfs were identified in the
HUDF, and all four candidates appear more likely to be part of the Galactic
thick disk. The lack of detected halo white dwarfs implies that, if the dark
matter halo is 12 Gyr old, white dwarfs account for less than 10% of the dark
matter halo mass.Comment: 35 pages, 11 figures, accepted by Ap
Chandra X-ray Sources in the LALA Cetus Field
The 174 ks Chandra Advanced CCD Imaging Spectrometer exposure of the Large
Area Lyman Alpha Survey (LALA) Cetus field is the second of the two deep
Chandra images on LALA fields. In this paper we present the Chandra X-ray
sources detected in the Cetus field, along with an analysis of X-ray source
counts, stacked X-ray spectrum, and optical identifications. A total of 188
X-ray sources were detected: 174 in the 0.5-7.0 keV band, 154 in the 0.5-2.0
keV band, and 113 in the 2.0-7.0 keV band. The X-ray source counts were derived
and compared with LALA Bootes field (172 ks exposure). Interestingly, we find
consistent hard band X-ray source density, but 36+-12% higher soft band X-ray
source density in Cetus field. The weighted stacked spectrum of the detected
X-ray sources can be fitted by a powerlaw with photon index Gamma = 1.55. Based
on the weighted stacked spectrum, we find that the resolved fraction of the
X-ray background drops from 72+-1% at 0.5-1.0 keV to 63+-4% at 6.0-8.0 keV. The
unresolved spectrum can be fitted by a powerlaw over the range 0.5-7 keV, with
a photon index Gamma = 1.22. We also present optical counterparts for 154 of
the X-ray sources, down to a limiting magnitude of r' = 25.9 (Vega), using a
deep r' band image obtained with the MMT.Comment: 21 pages, including 6 figures, 1 table, ApJ accepte
Growth and predation activity at deep-sea hydrothermal vents along the Galápagos Rift
Growth rates of unclassified mussels collected from hydrothermal vents on the Pacific Ocean (2500 m) are among the highest recorded for deep-sea species. Mature mussels have mean growth rates of about 1 cm yr-1 which are comparable to growth rates of shallow water mussels...
Microlensing of Globular Clusters as a Probe of Galactic Structure
The spatial distribution of compact dark matter in our Galaxy can be
determined in a few years of monitoring Galactic globular clusters for
microlensing. Globular clusters are the only dense fields of stars distributed
throughout the three-dimensional halo and hence are uniquely suited to probe
its structure. The microlensing optical depths towards different clusters have
varying contributions from the thin disk, thick disk, bulge, and halo of the
Galaxy. Although measuring individual optical depths to all the clusters is a
daunting task, we show that interesting Galactic structure information can be
extracted with as few as -- events in total for the entire globular
cluster system (observable with 2--5 years of monitoring). The globular cluster
microlensing is particularly sensitive to the core radius of the halo mass
distribution and to the scale length, surface mass density, and radial scale
height variations of the thin disk.Comment: 14 pages, 1 figure. Submitted to ApJ Letters. Uses aastex macro
Lyman-alpha emission galaxies at a redshift of z = 5.7 in the FORS Deep Field
We present the results of a search for Lyman-alpha emission galaxies at z~
5.7 in the FORS Deep Field. The objective of this study is to improve the faint
end of the luminosity function of high-redshift Lyman-alpha emitting galaxies
and to derive properties of intrinsically faint Lyman-alpha emission galaxies
in the young universe. Using FORS2 at the ESO VLT and a set of special
interference filters, we identified candidates for high-redshift Lyman-alpha
galaxies. We then used FORS2 in spectroscopic mode to verify the
identifications and to study their spectral properties. The narrow-band
photometry resulted in the detection of 15 likely Lyman-alpha emission
galaxies. Spectra with an adequate exposure time could be obtained for eight
galaxies. In all these cases the presence of Lyman-alpha emission at z = 5.7
was confirmed spectroscopically. The line fluxes of the 15 candidates range
between 3 and 16 * 10^-21 Wm^-2, which corresponds to star-formation rates not
corrected for dust between 1 and 5 Msun/yr. The luminosity function derived for
our photometrically identified objects extends the published luminosity
functions of intrinsically brighter Lyman-alpha galaxies. With this technique
the study of high-redshift Lyman-alpha emission galaxies can be extended to low
intrinsic luminosities.Comment: 9 pages, 17 figures. Accepted by A&A. PDF version with higher
resolution figures here:
http://www.lsw.uni-heidelberg.de/users/jheidt/fdf/pubs/fdflae5_7_110406.pd
Emission-Line Galaxies from the HST PEARS Grism Survey I: The South Fields
We present results of a search for emission-line galaxies in the Southern
Fields of the Hubble Space Telescope PEARS (Probing Evolution And Reionization
Spectroscopically) grism survey. The PEARS South Fields consist of five ACS
pointings (including the Hubble Ultra Deep Field) with the G800L grism for a
total of 120 orbits, revealing thousands of faint object spectra in the
GOODS-South region of the sky. Emission-line galaxies (ELGs) are one subset of
objects that are prevalent among the grism spectra. Using a 2-dimensional
detection and extraction procedure, we find 320 emission lines orginating from
226 galaxy "knots'' within 192 individual galaxies. Line identification results
in 118 new grism-spectroscopic redshifts for galaxies in the GOODS-South Field.
We measure emission line fluxes using standard Gaussian fitting techniques. At
the resolution of the grism data, the H-beta and [OIII] doublet are blended.
However, by fitting two Gaussian components to the H-beta and [OIII] features,
we find that many of the PEARS ELGs have high [OIII]/H-beta ratios compared to
other galaxy samples of comparable luminosities. The star-formation rates
(SFRs) of the ELGs are presented, as well as a sample of distinct giant
star-forming regions at z~0.1-0.5 across individual galaxies. We find that the
radial distances of these HII regions in general reside near the galaxies'
optical continuum half-light radii, similar to those of giant HII regions in
local galaxies.Comment: 15 pages, 13 figures; Accepted for publication in A
Dwarf Galaxy Formation Was Suppressed By Cosmic Reionization
A large number of faint galaxies, born less than a billion years after the
big bang, have recently been discovered. The fluctuations in the distribution
of these galaxies contributed to a scatter in the ionization fraction of cosmic
hydrogen on scales of tens of Mpc, as observed along the lines of sight to the
earliest known quasars. Theoretical simulations predict that the formation of
dwarf galaxies should have been suppressed after cosmic hydrogen was reionized,
leading to a drop in the cosmic star formation rate. Here we present evidence
for this suppression. We show that the post-reionization galaxies which
produced most of the ionizing radiation at a redshift z~5.5, must have had a
mass in excess of ~10^{10.6+/-0.4} solar masses or else the aforementioned
scatter would have been smaller than observed. This limiting mass is two orders
of magnitude larger than the galaxy mass that is thought to have dominated the
reionization of cosmic hydrogen (~10^8 solar masses). We predict that future
surveys with space-based infrared telescopes will detect a population of
smaller galaxies that reionized the Universe at an earlier time, prior to the
epoch of dwarf galaxy suppression.Comment: 19 pages, 3 figures. Accepted for publication in Nature; press
embargo until publishe
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