30 research outputs found
On the compared accuracy and reliability of spectroscopic and photometric redshift measurements
We present a comparison between the spectroscopic catalog of the HDF recently
published by Cohen et al and the redshifts measured by our group for those
objects using photometric techniques. We perform this comparison in order to
characterize the errors associated to the photometric redshift technique. The
spectroscopic sample includes over 140 objects in the HDF proper, representing
the deepest, cleanest, most complete spectroscopic catalog ever compiled. We
study each object for which our redshift and the one measured by Cohen et al
disagree. In most cases the photometric evidence is strong enough to call for a
careful review of the spectroscopic values, as they seem to be in error. It is
possible to characterize the systematic errors associated to our technique,
which when combined with the photometric errors allow us to obtain complete
information on the redshift of each galaxy and its associated confidence
interval, regardless of apparent magnitude. One of the main conclusions of this
study is that, to date, all the redshifts from our published catalog that have
been checked have been shown to be correct (within the stated confidence
limits). This implies that our galaxy template set is a fair representation of
the galaxy population at all redshifts (0<z<6) and magnitudes (R<24) explored
to date. On the other hand, spectroscopy of faint sources is subject to unknown
and uncharacterized systematic errors, that will in turn be transmitted to any
photometric redshift technique which uses spectroscopic samples as calibration.
Our analysis proves that photometric redshift techniques can and must be used
to extend the range of applicability of the spectroscopic redshift
measurements. (Abridged)Comment: Accepted for publication in ApJSS. Minor changes in presentation and
discussion. No changes to main results. ApJ preprint format, 27 pages, 16
embedded figure
The Ultraviolet Luminosity Density of the Universe from Photometric Redshifts of Galaxies in the Hubble Deep Field
Studies of the Hubble Deep Field (HDF) and other deep surveys have revealed
an apparent peak in the ultraviolet (UV) luminosity density, and therefore the
star-formation rate density, of the Universe at redshifts 1<z<2. We use
photometric redshifts of galaxies in the HDF to determine the comoving UV
luminosity density and find that, when errors (in particular, sampling error)
are properly accounted for, a flat distribution is statistically
indistinguishable from a distribution peaked at z~1.5. Furthermore, we examine
the effects of cosmological surface brightness (SB) dimming on these
measurements by applying a uniform SB cut to all galaxy fluxes after correcting
them to redshift z=5. We find that, comparing all galaxies at the same
intrinsic surface brightness sensitivity, the UV luminosity density contributed
by high intrinsic SB regions increases by almost two orders of magnitude from
z~0 to z~5. This suggests that there exists a population of objects with very
high star formation rates at high redshifts that apparently do not exist at low
redshifts. The peak of star formation, then, may occur somewhere beyond a
redshift z~>5.Comment: 4 pages total, including 3 embedded figures, to appear in the
proceedings of the Xth Rencontres de Blois, "The Birth of Galaxies." LaTeX
style file include
The Star Formation Rate Intensity Distribution Function--Implications for the Cosmic Star Formation Rate History of the Universe
We address the effects of cosmological surface brightness dimming on
observations of faint galaxies by examining the distribution of "unobscured"
star formation rate intensities versus redshift. We use the star formation rate
intensity distribution function to assess the ultraviolet luminosity density
versus redshift, based on our photometry and photometric redshift measurements
of faint galaxies in the HDF and the HDF--S WFPC2 and NICMOS fields. We find
that (1) previous measurements have missed a dominant fraction of the
ultraviolet luminosity density of the universe at high redshifts by neglecting
cosmological surface brightness dimming effects, which are important at
redshifts larger than z = 2, (2) the incidence of the highest intensity star
forming regions increases monotonically with redshift, and (3) the ultraviolet
luminosity density plausibly increases monotonically with redshift through the
highest redshifts observed. By measuring the spectrum of the luminosity density
versus redshift, we also find that (4) previous measurements of the ultraviolet
luminosity density at redshifts z < 2 must be reduced by a factor 2 to allow
for the spectrum of the luminosity density between rest-frame wavelengths 1500
and 2800 A. And by comparing with observations of high-redshift damped
Lyman-alpha absorption systems detected toward background QSOs, we further find
that (5) the distribution of star formation rate intensities matches the
distribution of neutral hydrogen column densities at redshifts z = 2 through 5,
which establishes a quantitative connection between high-redshift galaxies and
high column density gas and suggests that high-redshift damped Lyman-alpha
absorption systems trace lower star formation rate intensity regions of the
same galaxies detected in star light in the HDF and HDF--S.Comment: 28 pages, 9 figures; accepted for publication in the Astrophysical
Journa
AN ACOUSTIC / RADAR SYSTEM FOR AUTOMATED DETECTION, LOCALIZATION, AND CLASSIFICATION OF BIRDS IN THE VICINITY OF AIRFIELDS
Bird-aircraft collisions present a significant threat to military and commercial aircraft, and as bird populations and air traffic continue to grow, and airport/airbase operations continue to expand, the problem will steadily get worse. To help mitigate bird strike hazards, we propose a multi-sensor system consisting of ground radar and acoustic sensors that can directly monitor bird activity and provide an alert when a threat condition occurs. Radar offers a large detection range and the ability to detect in all weather conditions, while acoustic sensors allow the ability to detect targets in the midst of clutter and add the capability to classify. A multi-sensor approach ensures that the system can provide bird strike monitoring capability in any situation with a low false alarm rate. As the Phase II effort of an Air Force STTR project, we have constructed and tested a microphone array adapted from state-of-the-art undersea warfare sensor technology that measures accurate angles to any acoustic source (broadband or narrowband) and a parabolic dish microphone which provides high-gain data on targets of interest. A test was conducted near Panama City / Bay County International Airport in conjunction with the Merlin Bird Detection Radar designed by DeTect, Inc. Results of this test will be presented and show that the acoustic array is capable of detecting, localizing in angle, and tracking multiple targets simultaneously, including birds, bats, aircraft, automobiles, people, and boats. The parabolic dish microphone was able to provide very high-gain acoustic data on several of these targets. The radar data was used as truth data for acoustic sensor performance evaluation and to determine situations in which the acoustic data can benefit the radar. Altogether, almost three days of continuous acoustic and radar data were collected, and analysis of these data show that the hybrid radar-acoustic system can provide bird strike avoidance capability