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.
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.
We compare the dust attenuation properties of two samples of galaxies purely selected in the Galaxy Evolution Explorer (GALEX) near-ultraviolet band (NUV; 1750-2750 Å, λmean=2310 Å) and in the ...far-infrared (FIR) at 60 μm. These samples are built using the GALEX and IRAS sky surveys over ~600 deg2. The NUV-selected sample contains 95 galaxies detected down to NUV=16 mag (AB system). Eighty-three galaxies in this sample are spiral or irregular, and only two of them are not detected at 60 μm. The FIR-selected sample is built from the IRAS PSCz survey, which is complete down to 0.6 Jy. Among the 163 sources, we select 118 star-forming galaxies that are well measured by IRAS; all but one are detected in NUV, and 14 galaxies are not detected in the far-ultraviolet band (FUV; 1350-1750 Å, λmean=1530 Å). The dust-to-ultraviolet (NUV and FUV) flux ratio is calibrated to estimate the dust attenuation at both wavelengths. The median value of the attenuation in NUV is found to be ~1 mag for the NUV-selected sample, versus ~2 mag for the FIR-selected one. Within both samples, the dust attenuation is found to correlate with the luminosity of the galaxies. Almost all the NUV-selected galaxies and two-thirds of the FIR-selected sample exhibit a lower dust attenuation than expected from the tight relation found previously for starburst galaxies between dust attenuation and the slope of the ultraviolet continuum. The situation is reversed for the remaining third of the FIR-selected galaxies: their extinction is higher than that deduced from their FUV-NUV color and the relation for starbursts.
In a companion paper (Arnouts et al. 2004) we presented new measurements of the galaxy luminosity function at 1500 Angstroms out to z~1 using GALEX-VVDS observations (1039 galaxies with NUV<24.5 and ...z>0.2) and at higher z using existing data sets. In this paper we use the same sample to study evolution of the FUV luminosity density. We detect evolution consistent with a (1+z)^{2.5+/-0.7} rise to z~1 and (1+z)^{0.5+/-0.4} for z>1. The luminosity density from the most UV-luminous galaxies (UVLG) is undergoing dramatic evolution (x30) between 025%) of the total FUV luminosity density at z<1. We measure dust attenuation and star formation rates of our sample galaxies and determine the star formation rate density as a function of redshift, both uncorrected and corrected for dust. We find good agreement with other measures of the SFR density in the rest ultraviolet and Halpha given the still significant uncertainties in the attenuation correction.
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.
We test the empirical relation between ultraviolet color and attenuation as derived for starburst galaxies with a wide assortment of galaxy types detected by the Galaxy Evolution Explorer and find ...that it systematically overestimates the far-ultraviolet attenuation of our sample by ~0.5 mag. Our efforts to find an additional parameter that could improve the starburst reddening relation were unsuccessful. In particular, UV-Ks colors (in nonmatching apertures) show no correlation with the offset from the starburst reddening relation, suggesting either that UV-Ks colors are a poor tracer of present to past average star formation history (the ``b'' parameter) or that the intrinsic dust distribution/geometry may be responsible for moving galaxies off the correlation. It is possible to reduce the systematic overestimate of AFUV by using the linear correlation derived from our sample, which simply lowers the starburst predicted values of AFUV by 0.58 mag. The scatter, however, remains large at 0.89 mag.
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.
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