85 research outputs found
Ecosystem engineering creates a direct nutritional link between 600-m deep cold-water coral mounds and surface productivity
Cold-water corals (CWCs) form large mounds on the seafloor that are hotspots of biodiversity in the deep sea, but it remains enigmatic how CWCs can thrive in this food-limited environment. Here, we infer from model simulations that the interaction between tidal currents and CWC-formed mounds induces downwelling events of surface water that brings organic matter to 600-m deep CWCs. This positive feedback between CWC growth on carbonate mounds and enhanced food supply is essential for their sustenance in the deep sea and represents an example of ecosystem engineering of unparalleled magnitude. This âtopographically-enhanced carbon pumpâ leaks organic matter that settles at greater depths. The ubiquitous presence of biogenic and geological topographies along ocean margins suggests that carbon sequestration through this pump is of global importance. These results indicate that enhanced stratification and lower surface productivity, both expected consequences of climate change, may negatively impact the energy balance of CWCs
A Synoptic, Multiwavelength Analysis of a Large Quasar Sample
We present variability and multi-wavelength photometric information for the
933 known quasars in the QUEST Variability Survey. These quasars are grouped
into variable and non-variable populations based on measured variability
confidence levels. In a time-limited synoptic survey, we detect an
anti-correlation between redshift and the likelihood of variability. Our
comparison of variability likelihood to radio, IR, and X-ray data is consistent
with earlier quasar studies. Using already-known quasars as a template, we
introduce a light curve morphology algorithm that provides an efficient method
for discriminating variable quasars from periodic variable objects in the
absence of spectroscopic information. The establishment of statistically robust
trends and efficient, non-spectroscopic selection algorithms will aid in quasar
identification and categorization in upcoming massive synoptic surveys.
Finally, we report on three interesting variable quasars, including variability
confirmation of the BL Lac candidate PKS 1222+037.Comment: AJ, accepted for publication 15 Dec 200
Variability-selected quasars behind the Small Magellanic Cloud
We present followup spectroscopic observations of quasar candidates in the
Small Magellanic Cloud selected by Eyer from the OGLE database. Of twelve
observed objects identified as "QSO Candidate", five are confirmed quasars,
with the emission redshifts ranging from 0.28 to 2.16. Two of those quasars
were also recently identified independently in the MACHO database by Geha et
al. We discuss the prospects of using variability-based selection technique for
quasar searches behind other dense stellar fields. An additional criterion
utilizing the color-color diagram should reduce the number of stars in the
candidate lists.Comment: Revised version, AASTeX, 11 pages, 3 EPS figures, one table, accepted
14 Nov 2002 for publication in the Astronomical Journal, March 2003 issu
Time Domain Explorations With Digital Sky Surveys
One of the new frontiers of astronomical research is the exploration of time
variability on the sky at different wavelengths and flux levels. We have
carried out a pilot project using DPOSS data to study strong variables and
transients, and are now extending it to the new Palomar-QUEST synoptic sky
survey. We report on our early findings and outline the methodology to be
implemented in preparation for a real-time transient detection pipeline. In
addition to large numbers of known types of highly variable sources (e.g., SNe,
CVs, OVV QSOs, etc.), we expect to find numerous transients whose nature may be
established by a rapid follow-up. Whereas we will make all detected variables
publicly available through the web, we anticipate that email alerts would be
issued in the real time for a subset of events deemed to be the most
interesting. This real-time process entails many challenges, in an effort to
maintain a high completeness while keeping the contamination low. We will
utilize distributed Grid services developed by the GRIST project, and implement
a variety of advanced statistical and machine learning techniques.Comment: 5 pages, 2 postscript figures, uses adassconf.sty. To be published
in: "ADASS XIV (2004)", Eds. Patrick Shopbell, Matthew Britton and Rick
Ebert, ASP Conference Serie
The Low-Redshift Quasar-Quasar Correlation Function from an Extragalactic Halpha Emission-Line Survey to z=0.4
We study the large-scale spatial distribution of low-redshift quasars and
Seyfert~1 galaxies using a sample of 106 luminous emission-line objects
() selected by their H emission lines in a
far-red objective prism survey (). Of the 106 objects, 25 were
previously known AGN (Veron-Cetty and Veron 2000), and follow-up spectroscopy
for an additional 53 objects (including all object pairs with separation r <
20 \hmpc) confirmed 48 AGN and 5 narrow emission-line galaxies (NELGs). The
calculated amplitude of the spatial two-point correlation function for the
emission-line sample is A = 0.4 \cdot \bar{\xi}(r < 20 \hmpc) \cdot 20^{1.8} =
142 \pm 53. Eliminating the confirmed NELGs from the sample we obtain the AGN
clustering amplitude . Using Monte Carlo simulations we reject
the hypothesis that the observed pair counts were drawn from a random
distribution at the 99.97% and 98.6% confidence levels for the entire sample
and the AGN subset respectively. We measure a decrease in the quasar clustering
amplitude by a factor of between and ,
and present the coordinates, redshifts, and follow-up spectroscopy for the 15
previously unknown AGN and 4 luminous NELGs that contribute to the clustering
signal.Comment: ApJ, in press, Vol 548, added follow-up spectroscop
Discovery of the Optical Transient of GRB 990308
The optical transient of the faint gamma-ray burst GRB 990308 was detected by the QUEST camera on the Venezuelan 1 m Schmidt telescope starting 3.28 hr after the burst. Our photometry gives V= 18.32 ± 0.07, R = 18.14 ± 0. 06, B =18.65, and R=18.22 ± 0.05 for times ranging from 3.28 to 3.47 hr after the burst. The colors correspond to a spectral slope of close to f â v^1/3 . Within the standard synchrotron ïŹreball model, this rquires that the external medium be less dense than 10^4 cm^-3, the electrons contain more than 20% of the shock energy, and the magnetic ïŹeld energy be less than 24% of the energy in the electrons for normal interstellar or circumstellar densities. We also report upper limits of V \u3e 12.0 at 132 s (with LOTIS), V \u3e 13.4 from 132 to 1029 s (with LOTIS), V \u3e 15.3 at 28.2 minutes (with Super-LOTIS), and a 8.5 GHz ïŹux of less than 114 ÎŒJy at 110 days (with the Very Large Array). Wisconsin-Indiana-Yale-NOAO 3.5 m and Keck 10 m telescopes reveal this location to be empty of any host galaxy to R \u3e 25.7and K \u3e 123.3. The lack of a host galaxy likely implies that it is either substantially subluminous or more distant than a redshift of âŒ1.2
Discovery of four X-ray quasars behind the Large Magellanic Cloud
We present the discovery of four X-ray quasars (z_em = 0.26, 0.53, 0.61,
1.63) located behind the Large Magellanic Cloud; three of them are located
behind the bar of the LMC. The quasars were identified via spectroscopy of
optical counterparts to X-ray sources found serendipitously by the Chandra
X-ray Observatory satellite. All four quasars have archival VI photometry from
the OGLE-II project; one of them was found by OGLE to be variable. We present
the properties of the quasars and discuss their possible applications.Comment: AASTeX, 8 pages, 1 ps figure, accepted for publication in
Ap.J.Letter
The QUEST RR Lyrae Survey: Confirmation of the Clump at 50 kpc and Other Over-Densities in the Outer Halo
We have measured the periods and light curves of 148 RR Lyrae variables from
V=13.5 to 19.7 from the first 100 sq. degrees of the QUEST RR Lyrae survey.
Approximately 55% of these stars belong to the clump of stars detected earlier
by the Sloan Digital Sky Survey. According to our measurements, this feature
has ~10 times the background density of halo stars, spans at least 37.5 deg by
3.5 deg in right ascension and declination (>=30 by >=3 kpc), lies ~50 kpc from
the Sun, and has a depth along the line of sight of ~5 kpc (1 sigma). These
properties are consistent with the recent models that suggest it is a tidal
stream from the Sgr dSph galaxy. The mean period of the type ab variables, 0.58
d, is also consistent. In addition, we have found two smaller over-densities in
the halo, one of which may be related to the globular cluster Pal 5.Comment: 12 pages (including 4 figures). Accepted for publication in the ApJ
Letter
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