281 research outputs found
An ultrahigh-speed digitizer for the Harvard College Observatory astronomical plates
A machine capable of digitizing two 8 inch by 10 inch (203 mm by 254 mm)
glass astrophotographic plates or a single 14 inch by 17 inch (356 mm by 432
mm) plate at a resolution of 11 microns per pixel or 2309 dots per inch (dpi)
in 92 seconds is described. The purpose of the machine is to digitize the
\~500,000 plate collection of the Harvard College Observatory in a five year
time frame. The digitization must meet the requirements for scientific work in
astrometry, photometry, and archival preservation of the plates. This paper
describes the requirements for and the design of the subsystems of the machine
that was developed specifically for this task.Comment: 12 pages, 9 figures, 1 table; presented at SPIE (July, 2006) and
published in Proceeding
A new bright z = 6.82 quasar discovered with VISTA: VHS J0411-0907
We present the discovery of a new quasar discovered with the
near-IR VISTA Hemisphere Survey (VHS) which has been spectroscopically
confirmed by the ESO New Technology Telescope (NTT) and the Magellan telescope.
This quasar has been selected by spectral energy distribution (SED)
classification using near infrared data from VISTA, optical data from
Pan-STARRS, and mid-IR data from WISE. The SED classification algorithm is used
to statistically rank two classes; foreground Galactic low-mass stars and high
redshift quasars, prior to spectroscopic observation. Forced photometry on
Pan-STARRS pixels for VHS J0411-0907 allows to improve the SED classification
reduced- and photometric redshift. VHS J0411-0907 (, mag, mag) has the brightest J-band continuum magnitude of
the nine known quasars at and is currently the highest redshift
quasar detected in the Pan-STARRS survey. This quasar has one of the lowest
black hole mass ()
and the highest Eddington ratio () of the known quasars at
. The high Eddington ratio indicates that some very high- quasars are
undergoing super Eddington accretion. We also present coefficients of the best
polynomials fits for colours vs spectral type on the Pan-STARRS, VISTA and WISE
system for MLT dwarfs and present a forecast for the expected numbers of
quasars at
Discovery of three z>6.5 quasars in the VISTA Kilo-degree Infrared Galaxy (VIKING) survey
Studying quasars at the highest redshifts can constrain models of galaxy and
black hole formation, and it also probes the intergalactic medium in the early
universe. Optical surveys have to date discovered more than 60 quasars up to
z~6.4, a limit set by the use of the z-band and CCD detectors. Only one z>6.4
quasar has been discovered, namely the z=7.08 quasar ULAS J1120+0641, using
near-infrared imaging. Here we report the discovery of three new z>6.4 quasars
in 332 square degrees of the Visible and Infrared Survey Telescope for
Astronomy Kilo-degree Infrared Galaxy (VIKING) survey, thus extending the
number from 1 to 4. The newly discovered quasars have redshifts of z=6.60,
6.75, and 6.89. The absolute magnitudes are between -26.0 and -25.5, 0.6-1.1
mag fainter than ULAS J1120+0641. Near-infrared spectroscopy revealed the MgII
emission line in all three objects. The quasars are powered by black holes with
masses of ~(1-2)x10^9 M_sun. In our probed redshift range of 6.44<z<7.44 we can
set a lower limit on the space density of supermassive black holes of
\rho(M_BH>10^9 M_sun) > 1.1x10^(-9) Mpc^(-3). The discovery of three quasars in
our survey area is consistent with the z=6 quasar luminosity function when
extrapolated to z~7. We do not find evidence for a steeper decline in the space
density of quasars with increasing redshift from z=6 to z=7.Comment: 14 pages, 9 figures. Published in Ap
The Gaseous Environment of High-z Galaxies: Precision Measurements of Neutral Hydrogen in the Circumgalactic Medium of z ~ 2-3 Galaxies in the Keck Baryonic Structure Survey
We present results from the Keck Baryonic Structure Survey (KBSS), a unique
spectroscopic survey designed to explore the connection between galaxies and
intergalactic baryons. The KBSS is optimized for the redshift range z ~ 2-3,
combining S/N ~ 100 Keck/HIRES spectra of 15 hyperluminous QSOs with densely
sampled galaxy redshift surveys surrounding each QSO sightline. We perform
Voigt profile decomposition of all 6000 HI absorbers within the full Lya forest
in the QSO spectra. Here we present the distribution, column density,
kinematics, and absorber line widths of HI surrounding 886 star-forming
galaxies with 2.0 < z < 2.8 and within 3 Mpc of a QSO sightline. We find that
N_HI and the multiplicity of HI components increase rapidly near galaxies. The
strongest HI absorbers within ~ 100 physical kpc of galaxies have N_HI ~ 3 dex
higher than those near random locations in the IGM. The circumgalactic zone of
most enhanced HI absorption (CGM) is found within 300 kpc and 300 km/s of
galaxies. Nearly half of absorbers with log(N_HI) > 15.5 are found within the
CGM of galaxies meeting our photometric selection, while their CGM occupy only
1.5% of the cosmic volume. The spatial covering fraction, multiplicity of
absorption components, and characteristic N_HI remain elevated to transverse
distances of 2 physical Mpc. Absorbers with log(N_HI) > 14.5 are tightly
correlated with the positions of galaxies, while absorbers with lower N_HI are
correlated only on Mpc scales. Redshift anisotropies on Mpc scales indicate
coherent infall toward galaxies, while on scales of ~100 physical kpc peculiar
velocities of 260 km/s are indicated. The median Doppler widths of absorbers
within 1-3 virial radii of galaxies are ~50% larger than randomly chosen
absorbers of the same N_HI, suggesting higher gas temperatures and/or increased
turbulence likely caused by accretion shocks and/or galactic winds.Comment: Accepted to Ap
The Keck+Magellan Survey for Lyman Limit Absorption II: A Case Study on Metallicity Variations
We present an absorption line analysis of the Lyman limit system (LLS) at
z=3.55 in our Magellan/MIKE spectrum of PKS2000-330. Our analysis of the Lyman
limit and full HI Lyman series constrains the total HI column density of the
LLS (N_HI = 10^[18.0 +/- 0.25] cm^{-2} for b_HI >= 20 km/s) and also the N_HI
values of the velocity subsystems comprising the absorber. We measure ionic
column densities for metal-line transitions associated with the subsystems and
use these values to constrain the ionization state (>90% ionized) and relative
abundances of the gas. We find an order of magnitude dispersion in the
metallicities of the subsystems, marking the first detailed analysis of
metallicity variations in an optically thick absorber. The results indicate
that metals are not well mixed within the gas surrounding high galaxies.
Assuming a single-phase photoionization model, we also derive an N_H-weighted
metallicity, = -1.66 +/- 0.25, which matches the mean metallicity in
the neutral ISM in high z damped Lya systems (DLAs). Because the line density
of LLSs is ~10 times higher than the DLAs, we propose that the former dominate
the metal mass-density at z~3 and that these metals reside in the galaxy/IGM
interface. Considerations of a multi-phase model do not qualitatively change
these conclusions. Finally, we comment on an anomalously large O^0/Si^+ ratio
in the LLS that suggests an ionizing radiation field dominated by soft UV
sources (e.g. a starburst galaxy). Additional abundance analysis is performed
on the super-LLS systems at z=3.19.Comment: 20 pages, 7 figures (most in color). Accepted to Ap
Sizes, Shapes, and Correlations of Lyman Alpha Clouds and Their Evolution in the CDM Universe
This study analyzes the sizes, shapes and correlations of \lya clouds
produced by a hydrodynamic simulation of a spatially flat CDM universe with a
non-zero cosmological constant (, , ), over the redshift range . The \lya clouds range in
size from several kiloparsecs to about a hundred kiloparsecs in proper units,
and they range in shape from roundish, high column density regions with
\nhi\ge 10^{15} cm^{-2} to low column density sheet-like structures with
\nhi \le 10^{13} cm^{-2} at z=3. The most common shape found in the
simulation resembles that of a flattened cigar. The physical size of a typical
cloud grows with time roughly as while its shape hardly evolves
(except for the most dense regions ). Our result indicates that
any simple model with a population of spheres (or other shapes) of a uniform
size is oversimplified; if such a model agrees with observational evidence, it
is probably only by coincidence. We also illustrate why the use of double
quasar sightlines to set lower limits on cloud sizes is useful only when the
perpendicular sightline separation is small ( kpc).
Finally, we conjecture that high column density \lya clouds (\nhi\ge 10^{15}
cm^{-2}) may be the progenitors of the lower redshift faint blue galaxies.
This seems plausible because their correlation length, number density
(extrapolated to lower redshift) and their masses are in fair agreement with
those observed.Comment: ApJ, in press, 34 pages, 21 figures, figs (1a,b,c) can be at
http://astro.princeton.edu/~cen/LYASSC/lyassc.htm
SPIRITS 15c and SPIRITS 14buu: Two Obscured Supernovae in the Nearby Star-Forming Galaxy IC 2163
SPIRITS---SPitzer InfraRed Intensive Transients Survey---is an ongoing survey
of nearby galaxies searching for infrared (IR) transients with Spitzer/IRAC. We
present the discovery and follow-up observations of one of our most luminous
( mag, Vega) and red (
mag) transients, SPIRITS 15c. The transient was detected in a dusty spiral arm
of IC 2163 ( Mpc). Pre-discovery ground-based imaging revealed an
associated, shorter-duration transient in the optical and near-IR (NIR). NIR
spectroscopy showed a broad ( km s), double-peaked
emission line of He I at m, indicating an explosive origin. The NIR
spectrum of SPIRITS 15c is similar to that of the Type IIb SN 2011dh at a phase
of days. Assuming mag of extinction in SPIRITS 15c
provides a good match between their optical light curves. The IR light curves
and the extreme color cannot be explained using only a standard
extinction law. Another luminous ( mag) event, SPIRITS
14buu, was serendipitously discovered in the same galaxy. The source displays
an optical plateau lasting days, and we suggest a scenario similar
to the low-luminosity Type IIP SN 2005cs obscured by mag.
Other classes of IR-luminous transients can likely be ruled out in both cases.
If both events are indeed SNe, this may suggest of nearby
core-collapse SNe are missed by currently operating optical surveys.Comment: 19 pages, 7 Figures, 4 Table
Spectroscopic Discovery of the Broad-Lined Type Ic Supernova 2010bh Associated with the Low-Redshift GRB 100316D
We present the spectroscopic discovery of a broad-lined Type Ic supernova (SN
2010bh) associated with the nearby long-duration gamma-ray burst (GRB) 100316D.
At z = 0.0593, this is the third-nearest GRB-SN. Nightly optical spectra
obtained with the Magellan telescopes during the first week after explosion
reveal the gradual emergence of very broad spectral features superposed on a
blue continuum. The supernova features are typical of broad-lined SNe Ic and
are generally consistent with previous supernovae associated with low-redshift
GRBs. However, the inferred velocities of SN 2010bh at 21 days after explosion
are a factor of ~2 times larger than those of the prototypical SN 1998bw at
similar epochs, with v ~ 26,000 km/s, indicating a larger explosion energy or a
different ejecta structure. A near-infrared spectrum taken 13.8 days after
explosion shows no strong evidence for He I at 1.083 microns, implying that the
progenitor was largely stripped of its helium envelope. The host galaxy is of
low luminosity (M_R ~ -18.5 mag) and low metallicity (Z < 0.4 Z_solar), similar
to the hosts of other low-redshift GRB-SNe.Comment: 6 pages, 4 figures, 1 table, submitted to ApJ Letter
Extremely metal-poor gas at a redshift of 7
In typical astrophysical environments, the abundance of heavy elements ranges from 0.001 to 2 times the solar value. Lower abundances have been seen in selected stars in the Milky Way’s halo and in two quasar absorption systems at redshift z = 3 (ref. 4). These are widely interpreted as relics from the early Universe, when all gas possessed a primordial chemistry. Before now there have been no direct abundance measurements from the first billion years after the Big Bang, when the earliest stars began synthesizing elements. Here we report observations of hydrogen and heavy-element absorption in a spectrum of a quasar at z = 7.04, when the Universe was just 772 million years old (5.6 per cent of its present age). We detect a large column of neutral hydrogen but no corresponding metals (defined as elements heavier than helium), limiting the chemical abundance to less than 1/10,000 times the solar level if the gas is in a gravitationally bound proto-galaxy, or to less than 1/1,000 times the solar value if it is diffuse and unbound. If the absorption is truly intergalactic, it would imply that the Universe was neither ionized by starlight nor chemically enriched in this neighbourhood at z ≈ 7. If it is gravitationally bound, the inferred abundance is too low to promote efficient cooling, and the system would be a viable site to form the predicted but as yet unobserved massive population III stars
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