The fourth generation of the Sloan

The fourth generation of the Sloan

We present a current catalog of 21 cm HI line sources extracted from the Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) survey over ∼2800 deg 2 of sky: the α.40 catalog. Covering 40% of the final survey area, the α.40 catalog contains 15855 sources in the regionsOf those, 15041 are certainly extragalactic, yielding a source density of 5.3 galaxies per deg 2 , a factor of 29 improvement over the catalog extracted from the HI Parkes All Sky Survey. In addition to the source centroid positions, HI line flux densities, recessional velocities and line widths, the catalog includes the coordinates of the most probable optical counterpart of each HI line detection, and a separate compilation provides a crossmatch to identifications given in the photometric and spectroscopic catalogs associated with the Sloan Digital Sky Survey Data Release 7. Fewer than 2% of the extragalactic HI line sources cannot be identified with a feasible optical counterpart; some of those may be rare OH megamasers at 0.16 < z <0.25. A detailed analysis is presented of the completeness, width dependent sensitivity function and bias inherent of the α.40 catalog. The impact of survey selection, distance errors, current volume coverage and local large scale structure on the derivation of the HI mass function is assessed. While α.40 does not yet provide a completely representative sampling of cosmological volume, derivations of the HI mass function using future data releases from ALFALFA will further improve both statistical and systematic uncertainties.

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We present interferometric H i 21 cm line observations of the extended gas around the irregular galaxy NGC 4449 covering 67Ј on the sky at a resolution of ∼1Ј. The main star-forming body of NGC 4449 is relatively normal for a Magellanic irregular galaxy, but the galaxy is unusual in that it has two counterrotating gas systems and H i that extends to 6 times the Holmberg radius. Our new, detailed H i maps of this extended gas show that most of the extended H i is located in large, highly structured, extended clouds and very long streamers. We compare NGC 4449 with other systems in the context of possible models for the origin of these structures, the most likely of which involves an interaction with another galaxy. Thus, NGC 4449 no longer fits the standard picture of an irregular galaxy quietly evolving in isolation.