227 research outputs found
Astronomical seeing and ground-layer turbulence in the Canadian High Arctic
We report results of a two-year campaign of measurements, during arctic
winter darkness, of optical turbulence in the atmospheric boundary-layer above
the Polar Environment Atmospheric Laboratory in northern Ellesmere Island
(latitude +80 deg N). The data reveal that the ground-layer turbulence in the
Arctic is often quite weak, even at the comparatively-low 610 m altitude of
this site. The median and 25th percentile ground-layer seeing, at a height of
20 m, are found to be 0.57 and 0.25 arcsec, respectively. When combined with a
free-atmosphere component of 0.30 arcsec, the median and 25th percentile total
seeing for this height is 0.68 and 0.42 arcsec respectively. The median total
seeing from a height of 7 m is estimated to be 0.81 arcsec. These values are
comparable to those found at the best high-altitude astronomical sites
QSO hosts and environments at z=0.9 to 4.2: JHK images with adaptive optics
We have observed nine QSOs with redshifts 0.85 to 4.16 at near-IR wavelengths
with the adaptive optics bonnette of the Canada-France-Hawaii telescope.
Exposure times ranged from 1500 to 24000s (mostly near 7000s) in J, H, or K
bands, with pixels 0.035 arcsec on the sky. The FWHM of the co-added images at
the location of the quasars are typically 0.16 arcsec. Including another QSO
published previously, we find associated QSO structure in at least eight of ten
objects, including the QSO at z = 4.16. The structures seen in all cases
include long faint features which appear to be tidal tails. In four cases we
have also resolved the QSO host galaxy, but find them to be smooth and
symmetrical: future PSF removal may expand this result. Including one object
previously reported, of the nine objects with more extended structure, five are
radio-loud, and all but one of these appear to be in a dense small group of
compact galaxy companions. The radio-quiet objects do not occupy the same dense
environments, as seen in the NIR. In this small sample we do not find any
apparent trends of these properties with redshift, over the range 0.8 < z <
2.4. The colors of the host galaxies and companions are consistent with young
stellar populations at the QSO redshift. Our observations suggest that adaptive
optic observations in the visible region will exhibit luminous signatures of
the substantial star-formation activity that must be occurring.Comment: 22 pages including 10 tables, plus 11 figures. To appear in A
Astronomical Sky Quality Near Eureka, in the Canadian High Arctic
Nighttime visible-light sky brightness and transparency are reported for the
Polar Environment Research Laboratory (PEARL), located on a 610-m high ridge
near the Eureka research station, on Ellesmere Island, Canada. Photometry of
Polaris obtained in V band with the PEARL All Sky Imager (PASI) over two
winters is supported by standard meteorological measurements and visual
estimates of sky conditions from sea level. These data show that during the
period of the study, October through March of 2008/09 and 2009/10, the sky near
zenith had a mean surface brightness of 19.7 mag/square-arcsec when the sun was
more than 12 deg below the horizon, reaching 20.7 mag/square-arcsec during
astronomical darkness with no moon. Skies were without thick cloud and
potentially usable for astronomy 86% of the time (extinction <2 mag). Up to 68%
of the time was spectroscopic (<0.5 mag), attenuated by ice crystals, or clear
with stable atmospheric transparency. Those conditions can persist for over 100
hours at a time. Further analysis suggests the sky was entirely free of ice
crystals (truly photometric) 48+/-3% of the time at PEARL in winter, and that a
higher elevation location nearby may be better.Comment: 14 pages, 1 table, 11 figures; accepted for publication in PAS
Characterizing the Adaptive Optics Off-Axis Point-Spread Function - I: A Semi-Empirical Method for Use in Natural-Guide-Star Observations
Even though the technology of adaptive optics (AO) is rapidly maturing,
calibration of the resulting images remains a major challenge. The AO
point-spread function (PSF) changes quickly both in time and position on the
sky. In a typical observation the star used for guiding will be separated from
the scientific target by 10" to 30". This is sufficient separation to render
images of the guide star by themselves nearly useless in characterizing the PSF
at the off-axis target position. A semi-empirical technique is described that
improves the determination of the AO off-axis PSF. The method uses calibration
images of dense star fields to determine the change in PSF with field position.
It then uses this information to correct contemporaneous images of the guide
star to produce a PSF that is more accurate for both the target position and
the time of a scientific observation. We report on tests of the method using
natural-guide-star AO systems on the Canada-France-Hawaii Telescope and Lick
Observatory Shane Telescope, augmented by simple atmospheric computer
simulations. At 25" off-axis, predicting the PSF full width at half maximum
using only information about the guide star results in an error of 60%. Using
an image of a dense star field lowers this error to 33%, and our method, which
also folds in information about the on-axis PSF, further decreases the error to
19%.Comment: 29 pages, 9 figures, accepted for publication in the PAS
CATS: CfAO Treasury Survey of distant galaxies, supernovae, and AGN's
The NSF Science and Technology Center for Adaptive Optics (CfAO) is
supporting a major scientific legacy project called the CfAO Treasury Survey
(CATS). CATS is obtaining near-infrared AO data in deep HST survey fields, such
as GEMS, GOODS-N, & EGS. Besides summarizing the main objectives of CATS, we
highlight some recent imaging work on the study of distant field galaxies,
AGNs, and a redshift z = 1.32 supernova. CATS plans the first data release to
the community in early 2007 (check
http://www.astro.ucla.edu/~irlab/cats/index.shtml for more details on CATS and
latest updates).Comment: 2 pages. Proceedings of the IAU Symposium 235, "Galaxy Evolution
across the Hubble Time", F. Combes & J. Palous (eds.
Triggered or Self-Regulated Star Formation within Intermediate Redshift Luminous Infrared Galaxies (I). Morphologies and Spatially Resolved Spectral Energy Distributions
We imaged a set of 15 intermediate redshift (z~0.8) luminous infrared
galaxies (LIRGs) with the Keck Laser Guide Star (LGS) AO facility. These
galaxies were selected from the GOODS-S field, allowing us to combine the high
spatial resolution HST optical (B, V, i, and z-bands) images with our
near-infrared (K'-band) images to study the LIRG morphologies and spatially
resolved spectral energy distributions (SEDs). Two thirds of the LIRGs are disk
galaxies, with only one third showing some evidence for interactions, minor, or
major mergers. In contrast with local LIRG disks (which are primarily barred
systems), only 10% of the LIRG disks in our sample contain a prominent bar.
While the optical bands tend to show significant point-like substructure,
indicating distributed star formation, the AO K-band images tend to be smooth.
The SEDs of the LIRGs are consistent with distributed dusty star formation, as
exhibited by optical to IR colors redder than allowed by old stellar
populations alone. This effect is most pronounced in the galaxy cores, possibly
indicating central star formation. We also observed a set of 11 intermediate
redshift comparison galaxies, selected to be non-ellipticals with apparent
K-band magnitudes comparable to the LIRGs. The "normal" (non-LIRG) systems
tended to have lower optical luminosity, lower stellar mass, and more irregular
morphology than the LIRGs. Half of the "normal" galaxies have SEDs consistent
with intermediate aged stellar populations and minimal dust. The other half
show evidence for some dusty star formation, usually concentrated in their
cores. Our work suggests that the LIRG disk galaxies are similar to large disk
systems today, undergoing self regulated star formation, only at 10 - 20 times
higher rates. (Abridged)Comment: Accepted for Publication in AJ. 27 pages, 21 figures, 3 table
Rest-Frame R-band Lightcurve of a z~1.3 Supernova Obtained with Keck Laser Adaptive Optics
We present Keck diffraction limited H-band photometry of a z~1.3 Type Ia
supernova (SN) candidate, first identified in a Hubble Space Telescope (HST)
search for SNe in massive high redshift galaxy clusters. The adaptive optics
(AO) data were obtained with the Laser Guide Star facility during four
observing runs from September to November 2005. In the analysis of data from
the observing run nearest to maximum SN brightness, the SN was found to have a
magnitude H=23.9 +/- 0.14 (Vega). We present the H-band (approximately
rest-frame R) light curve and provide a detailed analysis of the AO photometric
uncertainties. By constraining the aperture correction with a nearby (4"
separation) star we achieve 0.14 magnitude photometric precision, despite the
spatially varying AO PSF.Comment: 11 pages, 8 figures, Accepted for Publication in AJ Updated the
citations, fixed typo
- …