We present two luminous UV/optical flares from the nuclei of apparently inactive early-type galaxies at image and 0.33 that have the radiative properties of a flare from the tidal disruption of a ...star. In this paper we report the second candidate tidal disruption event discovery in the UV by the GALEX Deep Imaging Survey and present simultaneous optical light curves from the CFHTLS Deep Imaging Survey for both UV flares. The first few months of the UV/optical light curves are well fitted with the canonical image power-law decay predicted for emission from the fallback of debris from a tidally disrupted star. Chandra ACIS X-ray observations during the flares detect soft X-ray sources with image or image and place limits on hard X- ray emission from an underlying AGN down to image. Blackbody fits to the UV/optical spectral energy distributions of the flares indicate peak flare luminosities of image10 super(44)-10 super(45) ergs s super(-1). The temperature, luminosity, and light curves of both flares are in excellent agreement with emission from a tidally disrupted main-sequence star onto a central black hole of several times 10 super(7) M sub(image). The observed detection rate of our search over image2.9 deg super( 2) of GALEX Deep Imaging Survey data spanning from 2003 to 2007 is consistent with tidal disruption rates calculated from dynamical models, and we use these models to make predictions for the detection rates of the next generation of optical synoptic surveys.
A supermassive black hole in the nucleus of a galaxy will be revealed when a star passes close enough to be torn apart by tidal forces and a flare of radiation is emitted by the stream of stellar ...debris that plunges into the black hole. Since common active galactic nuclei have accreting black holes that can also produce flares, a convincing demonstration that a stellar tidal disruption has occurred generally begins with a "normal" galaxy that has no evidence of prior nuclear activity. Here we report a luminous UV flare from an elliptical galaxy at z = 0.37 in the Groth field of the GALEX Deep Imaging Survey that has no evidence of a Seyfert nucleus from optical spectroscopy and X-ray imaging obtained during the flare. Multiwavelength data collected at the time of the event, and for 2 years following, allow us to constrain, for the first time, the spectral energy distribution of a candidate tidal disruption flare from optical through X-rays. The luminosity and temperature of the radiation and the decay curve of the flare are in excellent agreement with theoretical predictions for the tidal disruption of a star, and provide the strongest empirical evidence for a stellar disruption event to date.
This work presents the main ultraviolet (UV) and far-infrared (FIR) properties of two samples of nearby galaxies selected from the GALEX(l = 2315 Ae, hereafter NUV) and IRAS (l = 60 km) surveys, ...respectively. They are built in order to obtain detection at both wavelengths for most of the galaxies. Star formation rate (SFR) estimators based on the UV and FIR emissions are compared. Systematic differences are found between the SFR estimators for individual galaxies based on the NUV fluxes corrected for dust attenuation and on the total IR luminosity. A combined estimator based on NUV and IR luminosities seems to be the best proxy over the whole range of values of SFR. Although both samples present similar average values of the birthrate parameter b, their star-formation-related properties are substantially different: NUV-selected galaxies tend to show larger values of b for lower masses, SFRs, and dust attenuation, supporting previous scenarios of star formation history (SFH). Conversely, about 20% of the FIR-selected galaxies show high values of b, SFR, and NUV attenuation. These galaxies, most of them being LIRGs and ULIRGs, break down the downsizing picture of SFH; however, their relative contribution per unit volume is small in the local universe. Finally, the cosmic SFR density of the local universe is estimated in a consistent way from the NUV and IR luminosities.
Understanding hydrologic connectivity between wetlands and perennial streams is critical to understanding the reliance of stream flow on inputs from wetlands. We used the isotopic evaporation signal ...in water and remote sensing to examine wetland‐stream hydrologic connectivity within the Pipestem Creek watershed, North Dakota, a watershed dominated by prairie‐pothole wetlands. Pipestem Creek exhibited an evaporated‐water signal that had approximately half the isotopic‐enrichment signal found in most evaporatively enriched prairie‐pothole wetlands. Groundwater adjacent to Pipestem Creek had isotopic values that indicated recharge from winter precipitation and had no significant evaporative enrichment, indicating that enriched surface water did not contribute significantly to groundwater discharging into Pipestem Creek. The estimated surface water area necessary to generate the evaporation signal within Pipestem Creek was highly dynamic, varied primarily with the amount of discharge, and was typically greater than the immediate Pipestem Creek surface water area, indicating that surficial flow from wetlands contributed to stream flow throughout the summer. We propose a dynamic range of spilling thresholds for prairie‐pothole wetlands across the watershed allowing for wetland inputs even during low‐flow periods. Combining Landsat estimates with the isotopic approach allowed determination of potential (Landsat) and actual (isotope) contributing areas in wetland‐dominated systems. This combined approach can give insights into the changes in location and magnitude of surface water and groundwater pathways over time. This approach can be used in other areas where evaporation from wetlands results in a sufficient evaporative isotopic signal.
Plain Language Summary
Connectivity of wetlands within watersheds is critical to wetland function at local‐to‐landscape scales. However, actual measures of connectivity between wetlands and streams at the watershed scale are difficult to make, but critical for quantifying these functions, and for supporting policy and decision making. In this manuscript, we combine and compare direct field measurements of connectivity using stable isotopes of water, and remotely sensed measurements of connectivity using Landsat data. We found that prairie‐pothole wetlands contributed significant amounts of water to the perennial stream from high to low‐flow conditions. This wetland water flowed through surface water connections and not through groundwater flow paths. Hydrologic connectivity between prairie‐pothole wetlands and the stream was dynamic, fluctuating with streamflow. Combining the isotopic and Landsat approaches to estimating contributing area appears to be a promising technique for determining potential and actual contributing areas in these flat wetland‐dominated systems.
Key Points
The evaporation signal in water isotopes was used to assess connectivity between prairie‐pothole wetlands and streamflow
Prairie‐pothole wetlands contributed water to streamflow through surface water connections, but not through groundwater connections
Hydrologic connectivity between prairie‐pothole wetlands and the stream was dynamic, fluctuating with streamflow
Acute physical or psychological stress can elicit adaptive behaviors that allow an organism maintain homeostasis. However, intense and/or prolonged stressors often have the opposite effect, resulting ...in maladaptive behaviors and curbing goal-directed action; in the extreme, this may contribute to the development of psychiatric conditions like generalized anxiety disorder, major depressive disorder, or post-traumatic stress disorder. While treatment of these disorders generally focuses on reducing reactivity to potentially threatening stimuli, there are in fact impairments across multiple domains including valence, arousal, and cognition. Here, we use the genetically stress-susceptible 129S1 mouse strain to explore the effects of stress across multiple domains. We find that 129S1 mice exhibit a potentiated neuroendocrine response across many environments and paradigms, and that this is associated with reduced exploration, neophobia, decreased novelty- and reward-seeking, and spatial learning and memory impairments. Taken together, our results suggest that the 129S1 strain may provide a useful model for elucidating mechanisms underlying myriad aspects of stress-linked psychiatric disorders as well as potential treatments that may ameliorate symptoms.
We present the selection and classification of over a thousand ultraviolet (UV) variable sources discovered in ~40 degsup 2 of GALEX Time Domain Survey (TDS) NUV images observed with a cadence of 2 ...days and a baseline of observations of ~3 years. The GALEX TDS fields were designed to be in spatial and temporal coordination with the Pan-STARRS1 Medium Deep Survey, which provides deep optical imaging and simultaneous optical transient detections via image differencing. We characterize the GALEX photometric errors empirically as a function of mean magnitude, and select sources that vary at the 5sigma level in at least one epoch. We measure the statistical properties of the UV variability, including the structure function on timescales of days and years. We also calculate a surface density rate in the UV for transient sources, using the effective survey time at the cadence appropriate to each class, of ~15 and 52 degsup -2 yrsup -1 for M dwarfs and extragalactic transients, respectively.
UV Star Formation Rates in the Local Universe Salim, Samir; Rich, R. Michael; Charlot, Stéphane ...
The Astrophysical journal. Supplement series,
12/2007, Letnik:
173, Številka:
2
Journal Article
Recenzirano
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We measure star formation rates (SFRs) of approximately 50,000 optically selected galaxies in the local universe (z approximately 0.1)--from gas-rich dwarfs to massive ellipticals. We obtain ...dust-corrected SFRs by fitting the GALEX (ultraviolet) and SDSS photometry to a library of dust-attenuated population synthesis models. For star-forming galaxies, our UV-based SFRs compare remarkably well with those from SDSS-measured emission lines (H alpha ). Deviations from perfect agreement are shown to be due to differences in the dust attenuation estimates. In contrast to H alpha measurements, UV provides reliable SFRs for galaxies with weak H alpha , and where H alpha is contaminated with AGN emission (1/2 of the sample). Using full-SED SFRs, we calibrate a simple prescription that uses GALEX far- and near-UV magnitudes to produce dust-corrected SFRs for normal star-forming galaxies. The specific SFR is considered as a function of stellar mass for (1) star-forming galaxies with no AGNs, (2) those hosting an AGN, and (3) galaxies without Ha emission. We find that the three have distinct star formation histories, with AGNs lying Intermediate between the star-forming and the quiescent galaxies. Star-forming galaxies without an AGN lie on a relatively narrow linear sequence. Remarkably, galaxies hosting a strong AGN appear to represent the massive continuation of this sequence. On the other hand, weak AGNs, while also massive, have lower SFRs, sometimes extending to the realm of quiescent galaxies. We propose an evolutionary sequence for massive galaxies that smoothly connects normal star-forming galaxies to quiescent galaxies via strong and weak AGNs. We confirm that some galaxies with no H alpha show signs of star formation in the UV. We derive a cosmic star formation density at z = 0.1 with significantly smaller total error than previous measurements.
The Calibration and Data Products of GALEX Morrissey, Patrick; Conrow, Tim; Barlow, Tom A ...
The Astrophysical journal. Supplement series,
12/2007, Letnik:
173, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We describe the calibration status and data products pertaining to the GR2 and GR3 data releases of the Galaxy Evolution Explorer (GALEX). These releases have identical pipeline calibrations that are ...significantly improved over the GR1 data release. GALEX continues to survey the sky in the far-ultraviolet (FUV, similar to 154 nm) and near-ultraviolet (NUV, similar to 232 nm) bands, providing simultaneous imaging with a pair of photon-counting, microchannel plate, delay line readout detectors. These 1.25 degree field of view detectors are well suited to ultraviolet observations because of their excellent red rejection and negligible background. A dithered mode of observing and photon list output pose complex requirements on the data processing pipeline, entangling detector calibrations, and aspect reconstruction algorithms. Recent improvements have achieved photometric repeatability of 0.05 and 0.03 m sub(AB) in the FUV and NUV, respectively. We have detected a long-term drift of order 1% FUV and 6% NUV over the mission. Astrometric precision is of order 0.5" rms in both bands. In this paper we provide the GALEX user with a broad overview of the calibration issues likely to be confronted in the current release. Improvements are likely as the GALEX mission continues into an extended phase with a healthy instrument, no consumables, and increased opportunities for guest investigations.
We give an overview of the Galaxy Evolution Explorer (GALEX), a NASA Explorer Mission launched on April 28, 2003. GALEX is performing the first space UV sky-survey, including imaging and grism ...surveys in two bands (1350-1750 Angstroms and 1750-2750 Angstroms). The surveys include an all-sky imaging survey (mAB ~ 20.5), a medium imaging survey of 1000 square degrees (mAB ~ 23), a deep imaging survey of 100 square degrees (mAB ~ 25), and a nearby galaxy survey. Spectroscopic grism surveys (R=100-200) are underway with various depths and sky coverage. Many targets overlap existing or planned surveys. We will use the measured UV properties of local galaxies, along with corollary observations, to calibrate the UV-global star formation rate relationship in local galaxies. We will apply this calibration to distant galaxies discovered in the deep imaging and spectroscopic surveys to map the history of star formation in the universe over the redshift range 0 < z < 1.5, and probe the physical drivers of star formation in galaxies. The GALEX mission includes a Guest Investigator program supporting the wide variety of programs made possible by the first UV sky survey.
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