High-resolution VLA Imaging of SDSS Stripe 82 at 1.4 GHz

We present a high-resolution radio survey of the Sloan Digital Sky Survey (SDSS) Southern Equatorial Stripe, a.k.a. Stripe 82. This 1.4 GHz survey was conducted with the Very Large Array (VLA) primarily in the A-configuration, with supplemental B-configuration data to increase sensitivity to extended structure. The survey has an angular resolution of 1.8 and achieves a median rms noise of 52 microJy/bm over 92 deg^2. This is the deepest 1.4 GHz survey to achieve this large of an area, filling a gap in the phase space between small, deep and large, shallow surveys. It also serves as a pilot project for a larger high-resolution survey with the Expanded Very Large Array (EVLA). We discuss the technical design of the survey and details of the observations, and we outline our method for data reduction. We present a catalog of 17,969 isolated radio components, for an overall source density of ~195 sources/deg^2. The astrometric accuracy of the data is excellent, with an internal check utilizing multiply-observed sources yielding an rms scatter of 0.19 in both right ascension and declination. A comparison to the SDSS DR7 Quasar Catalog further confirms that the astrometry is well tied to the optical reference frame, with mean offsets of 0.02 +/- 0.01 in right ascension, and 0.01 +/- 0.02 in declination. A check of our photometry reveals a small, negative CLEAN-like bias on the level of 35 microJy. We report on the catalog completeness, finding that 97% of FIRST-detected quasars are recovered in the new Stripe 82 radio catalog, while faint, extended sources are more likely to be resolved out by the resolution bias. We conclude with a discussion of the optical counterparts to the catalog sources, including 76 newly-detected radio quasars. The full catalog as well as a search page and cutout server are available online at http://third.ucllnl.org/cgi-bin/stripe82cutout.

Implications of Dramatic Broad Absorption Line Variability in the Quasar FBQS J1408+3054

We have observed a dramatic change in the spectrum of the formerly heavily absorbed `overlapping-trough iron low-ionization broad absorption line (FeLoBAL) quasar FBQS J1408+3054. Over a time span of between 0.6 to 5 rest-frame years, the Mg II trough outflowing at 12,000 km/s decreased in equivalent width by a factor of two and the Fe II troughs at the same velocity disappeared. The most likely explanation for the variability is that a structure in the BAL outflow moved out of our line of sight to the ultraviolet continuum emitting region of the quasars accretion disk. Given the size of that region, this structure must have a transverse velocity of between 2600 km/s and 22,000 km/s. In the context of a simple outflow model, we show that this BAL structure is located between approximately 5800 and 46,000 Schwarzschild radii from the black hole. That distance corresponds to 1.7 to 14 pc, 11 to 88 times farther from the black hole than the H-beta broad-line region. The high velocities and the parsec-scale distance for at least this one FeLoBAL outflow mean that not all FeLoBAL outflows can be associated with galaxy-scale outflows in ultraluminous infrared galaxies transitioning to unobscured quasars. The change of FBQS J1408+3054 from an FeLoBAL to a LoBAL quasar also means that if (some) FeLoBAL quasars have multiwavelength properties which distinguish them from HiBAL quasars, then some LoBAL quasars will share those properties. Finally, we extend previous work on how multiple-epoch spectroscopy of BAL and non-BAL quasars can be used to constrain the average lifetime of BAL episodes (currently >60 rest-frame years at 90% confidence).

Optimal Compression of Floating-point Astronomical Images Without Significant Loss of Information

We describe a compression method for floating-point astronomical images that gives compression ratios of 6 -- 10 while still preserving the scientifically important information in the image. The pixel values are first preprocessed by quantizing them into scaled integer intensity levels, which removes some of the uncompressible noise in the image. The integers are then losslessly compressed using the fast and efficient Rice algorithm and stored in a portable FITS format file. Quantizing an image more coarsely gives greater image compression, but it also increases the noise and degrades the precision of the photometric and astrometric measurements in the quantized image. Dithering the pixel values during the quantization process can greatly improve the precision of measurements in the images. This is especially important if the analysis algorithm relies on the mode or the median which would be similarly quantized if the pixel values are not dithered. We perform a series of experiments on both synthetic and real astronomical CCD images to quantitatively demonstrate that the magnitudes and positions of stars in the quantized images can be measured with the predicted amount of precision. In order to encourage wider use of these image compression methods, we have made available a pair of general-purpose image compression programs, called fpack and funpack, which can be used to compress any FITS format image.

Radio Detection of Radio-Quiet Galaxies

We investigate the radio emission of ~185,000 quiescent (optically unclassifiable) galaxies selected from the Sloan Digital Sky Survey (SDSS). By median-stacking FIRST cutouts centered on the optically-selected sources, we are able to reach flux densities down to the 10s of microJy. The quiescent galaxy sample is composed of two subgroups inhabiting vastly different regimes: those targeted for the SDSS MAIN Galaxy Sample (~55%), and those targeted for the Luminous Red Galaxy (LRG) sample (~45%). To investigate the star-formation rates (SFRs) of these quiescent galaxies, we calibrate a radio-SFR conversion using a third sample of star-forming galaxies. Comparing this SFR-indicator with indicators in the optical and UV, we derive conflicting SFR estimates for the MAIN sample quiescent galaxies. These radio-derived SFRs intersect those calculated using the 4000-Angstrom break (D4000) around an SFR of 1 Msun/yr and agree to within a factor of 3 over the range of SFRs. However, we find that the radio-derived SFRs are too high relative to the SFRs estimated for similar populations of galaxies using analysis of UV emission, implying either contamination of the radio by Active Galactic Nuclei (AGN) or incomplete dust modeling. If AGN activity is dominant in these galaxies, then a relation between AGN radio luminosity and galaxy mass is required to explain the observed trends. For the LRGs, on the other hand, we find the radio luminosity to be independent of SFR as derived from D4000, indicating an AGN component dominates their radio emission. AGN-based radio emission often implies the existence of radio jets, providing evidence of a mechanism for low-level feedback in these quiescent LRGs. (Abridged)