12,733 research outputs found
The Keck Lyman Continuum Spectroscopic Survey (KLCS): The Emergent Ionizing Spectrum of Galaxies at
We present results of a deep spectroscopic survey designed to quantify the
statistics of the escape of ionizing photons from star-forming galaxies at z~3.
We measure the ratio of ionizing to non-ionizing UV flux density
_obs, where f900 is the mean flux density evaluated over the range
[880,910] A. We quantify the emergent ratio of ionizing to non-ionizing UV flux
density by analyzing high-S/N composite spectra formed from sub-samples with
common observed properties and numbers sufficient to reduce the statistical
uncertainty in the modeled IGM+CGM correction to obtain precise values of
_out, including a full-sample average
_out=. We further show that _out
increases monotonically with Ly rest equivalent width, inducing an
inverse correlation with UV luminosity as a by-product. We fit the composite
spectra using stellar spectral synthesis together with models of the ISM in
which a fraction f_c of the stellar continuum is covered by gas with column
density N(HI). We show that the composite spectra simultaneously constrain the
intrinsic properties of the stars (L900/L1500)_int along with f_c, N(HI),
E(B-V), and , the absolute escape fraction of ionizing photons. We
find a sample-averaged , and that subsamples fall
along a linear relation . We use the FUV luminosity function, the distribution function
, and the relationship between and
_out to estimate the total ionizing emissivity of
star-forming galaxies with Muv < -19.5:
ergs/s/Hz/Mpc, exceeding the contribution of QSOs by a factor of ,
and accounting for % of the total at
estimated using indirect methods.Comment: 45 pages, 31 figures, ApJ, in pres
RELBET 4.0 user's guide
This manual describes the operation and use of RELBET 4.0 implemented on the Hewlett Packard model 9000. The RELBET System is an integrated collection of computer programs which support the analysis and post-flight reconstruction of vehicle to vehicle relative trajectories of two on-orbit free-flying vehicles: the Space Shuttle Orbiter and some other free-flyer. The manual serves both as a reference and as a training guide. Appendices provide experienced users with details and full explanations of program usage. The body of the manual introduces new users to the system by leading them through a step by step example of a typical production. This should equip the new user both to execute a typical production process and to understand the most significant variables in that process
GRB 030329: 3 years of radio afterglow monitoring
Radio observations of gamma-ray burst (GRB) afterglows are essential for our
understanding of the physics of relativistic blast waves, as they enable us to
follow the evolution of GRB explosions much longer than the afterglows in any
other wave band. We have performed a three-year monitoring campaign of GRB
030329 with the Westerbork Synthesis Radio Telescopes (WSRT) and the Giant
Metrewave Radio Telescope (GMRT). Our observations, combined with observations
at other wavelengths, have allowed us to determine the GRB blast wave physical
parameters, such as the total burst energy and the ambient medium density, as
well as investigate the jet nature of the relativistic outflow. Further, by
modeling the late-time radio light curve of GRB 030329, we predict that the
Low-Frequency Array (LOFAR, 30-240 MHz) will be able to observe afterglows of
similar GRBs, and constrain the physics of the blast wave during its
non-relativistic phase.Comment: 5 pages, 2 figures, Phil. Trans. R. Soc. A, vol.365, p.1241,
proceedings of the Royal Society Scientific Discussion Meeting, London,
September 200
Q1549-C25: A Clean Source of Lyman-Continuum Emission at
We present observations of Q1549-C25, an ~L* star-forming galaxy at z=3.15
for which Lyman-continuum (LyC) radiation is significantly detected in deep
Keck/LRIS spectroscopy. We find no evidence for contamination from a
lower-redshift interloper close to the line of sight in the high
signal-to-noise spectrum of Q1549-C25. Furthermore, the morphology of Q1549-C25
in V_606, J_125, and H_160 Hubble Space Telescope (HST) imaging reveals that
the object consists of a single, isolated component within 1". In combination,
these data indicate Q1549-C25 as a clean spectroscopic detection of LyC
radiation, only the second such object discovered to date at z~3. We model the
spectral energy distribution (SED) of Q1549-C25, finding evidence for
negligible dust extinction, an age (assuming continuous star formation) of ~1
Gyr, and a stellar mass of M_*=7.9x10^9 M_sun. Although it is not possible to
derive strong constraints on the absolute escape fraction of LyC emission,
f_esc(LyC), from a single object, we use simulations of intergalactic and
circumgalactic absorption to infer f_esc(LyC)>=0.51 at 95% confidence. The
combination of deep Keck/LRIS spectroscopy and HST imaging is required to
assemble a larger sample of objects like Q1549-C25, and obtain robust
constraints on the average f_esc(LyC) at z~3 and beyond.Comment: 6 pages, 5 figures, accepted to ApJ Letter
Superfaults and Pseudotachylytes: Evidence of Catastrophic Earth Movements
Superfaults often exhibit an unusual rock called pseudotachylyte, and together these two features are critical to our understanding of catastrophic tectonics. Superfaults are rapid-moving, single-slip displacement surfaces involving very large offsets with the moving-block side of the fault being unconfined or unpinned during rapid gravity offset. Pseudotachylyte is the dark-colored, metamorphic silicate glass formed by frictional melting upon the superfault surface at temperature exceeding 1000 oC, and often displaying distinctive isotropic or cryptocrystalline optical properties. Pseudotachylyte is understood to be evidence of high-speed rock movement during superquakes, where displacements occurred much faster than during modern magnitude 9 earthquakes. Superfaults, pseudotachylyte and superquakes are interpreted as support for global catastrophic tectonic activity. Hand specimens and field data were collected from Homestake Shear Zone in central Colorado, and from the Pasagshak Thrust, Kodiak Island, Alaska. Thin-section analysis shows the presence of glass melt and/or aphanitic black rocks containing pseudotachylyte, and the development of numerous pressure solution surfaces within cataclasite rocks. Cross-cutting relationships support repeated episodes of fault movement and subsequent melt development at both sites. Data gathered from the Homestake Shear Zone supports rapid catastrophic Earth movements during the formation of the North American continent on Day 3 of the Creation Week. Evidence from the Pasagshak Thrust supports the subduction model of catastrophic plate tectonics, with the fault being active during and after Noah’s Flood
A High Fraction of Ly-alpha-Emitters Among Galaxies with Extreme Emission Line Ratios at z ~ 2
Star-forming galaxies form a sequence in the [OIII]/H-beta vs. [NII]/H-alpha
diagnostic diagram, with low metallicity, highly ionized galaxies falling in
the upper left corner. Drawing from a large sample of UV-selected star-forming
galaxies at z~2 with rest-frame optical nebular emission line measurements from
Keck-MOSFIRE, we select the extreme ~5% of the galaxies lying in this upper
left corner, requiring log([NII]/H-alpha) =
0.75. These cuts identify galaxies with 12 + log(O/H) <~ 8.0, when oxygen
abundances are measured via the O3N2 diagnostic. We study the Ly-alpha
properties of the resulting sample of 14 galaxies. The mean (median) rest-frame
Ly-alpha equivalent width is 39 (36) A, and 11 of the 14 objects (79%) are
Ly-alpha-emitters (LAEs) with W_Lya > 20 A. We compare the equivalent width
distribution of a sample of 522 UV-selected galaxies at 2.0<z<2.6 identified
without regard to their optical line ratios; this sample has mean (median)
Ly-alpha equivalent width -1 (-4) A, and only 9% of these galaxies qualify as
LAEs. The extreme galaxies typically have lower attenuation at Ly-alpha than
those in the comparison sample, and have ~50% lower median oxygen abundances.
Both factors are likely to facilitate the escape of Ly-alpha: in less dusty
galaxies Ly-alpha photons are less likely to be absorbed during multiple
scatterings, while the harder ionizing spectrum and higher ionization parameter
associated with strong, low metallicity star formation may reduce the covering
fraction or column density of neutral hydrogen, further easing Ly-alpha escape.
The use of nebular emission line ratios may prove useful in the identification
of galaxies with low opacity to Ly-alpha photons across a range of redshifts.Comment: 12 pages, 5 figures, 2 tables. Accepted for publication in Ap
No Fossil Disk in the T Tauri Multiple System V773 Tau
We present new multi-epoch near-infrared and optical high-angular images of
the V773 Tau pre-main sequence triple system, a weak-line T Tauri (WTTS) system
in which the presence of an evolved, ``fossil'' protoplanetary disk has been
inferred on the basis of a significant infrared excess. Our images reveal a
fourth object bound to the system, V773 Tau D. While it is much fainter than
all other components at 2 micron, it is the brightest source in the system at
4.7 micron. We also present medium-resolution K band adaptive optics
spectroscopy of this object, which is featureless with the exception of a weak
Br gamma emission line. Based on this spectrum and on the spectral energy
distribution of the system, we show that V773 Tau D is another member of the
small class of ``infrared companions'' (IRCs) to T Tauri stars. It is the least
luminous, and probably the least massive, component of the system, as opposed
to most other IRCs, which suggests that numerous low-luminosity IRCs such as
V773 Tau D may still remain to be discovered. Furthermore, it is the source of
the strong IR excess in the system. We therefore reject the interpretation of
this excess as the signature of a fossil (or ``passive'') disk and further
suggest that these systems may be much less frequent than previously thought.
We further show that V773 Tau C is a variable classical T Tauri star (CTTS)
and that its motion provides a well constrained orbital model. We show that
V773 Tau D can be dynamically stable within this quadruple system if its orbit
is highly inclined. Finally, V773 Tau is the first multiple system to display
such a variety of evolutionary states (WTTS, CTTS, IRC), which may be the
consequence of the strong star-star interactions in this compact quadruple
system.Comment: Accepted for publication in Astrophysical Journal, 29 pages, 2
tables, 5 figure
- …