HI Cosmology in the Local Universe
The Arecibo Legacy Fast ALFA (ALFALFA) survey is an on-going second generation blind extragalactic HI survey exploiting Arecibo's superior sensitivity, angular resolution and digital technology to conduct a census of the local HI universe over a cosmologically significant volume. When complete in 2012, ALFALFA will have detected more than 30,000 extragalactic HI line sources out to z~0.06, and its catalog will be especially useful in synergy with wide area surveys conducted at other wavelengths. ALFALFA is detecting HI masses as low as 106 solar masses and as high as 1010.8 solar masses with positional accuracies typically better than 20 arcsec, allowing immediate identification of the most probable optical counterpart to each HI detection. ALFALFA results already suggest, in agreement with the suggestions of previous, more limited studies, that there does not appear to be a cosmologically significant population of optically dark but HI rich galaxies; fewer than 2% of the extragalactic HI line sources detected by ALFALFA cannot be identified with a feasible optical counterpart. In the region of the Virgo cluster of galaxies, a number of optically dark HI sources have been found. These all lie in the outskirts of the cluster and could be tidal or "harassment" debris, the result of high speed gravitational encounters. Additionally, ALFALFA has identified a intriguing population of massive galaxies with unusually high fractions of their baryons in the form of cool HI gas: how have these galaxies managed to maintain such massive HI disks without converting their gas into stars? At the low mass end, ALFALFA has already detected several hundred galaxies with HI masses less than 108 solar masses and we have identified a class of ultracompact high velocity clouds which match the predictions for gas-bearing Local Group dark matter mini-halos. The 2011 ALFALFA sample, dubbed α.40 because it covers 40% of the full survey area, approaches homogeneity but is not yet a perfectly representative sample of the nearby volume; the full ALFALFA survey, when completed, will be. ALFALFA promises a wealthy dataset for the exploration of many issues in near-field cosmology and galaxy evolution studies, setting the stage for their extension to higher redshifts with the Square Kilometer Array.
Haynes et al. 2011, AJ 142, 170 (link)
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 square degrees of sky: the α.40 catalog. Covering 40% of the final survey area, the α.40 catalog contains 15855 sources in the regions 07h30m < R.A. < 16h30m, +04 deg < Dec. < +16 deg and +24 deg < Dec. < +28 deg and 22h < R.A. < 03h, +14 deg < Dec. < +16 deg and +24 deg < Dec. < +32 deg. Of those, 15041 are certainly extragalactic, yielding a source density of 5.3 galaxies per square degree, 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 in the current 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.
Tables 1, 2 and 3 from Haynes et al. (2011) are available as ASCII and CSV files:
Note: This release supercedes the previous releases.
Older datasets: (superceded by above)
Kent et al. 2008, AJ 136, 713 (link)
We present the third installment of HI sources extracted from the Arecibo Legacy Fast ALFA extragalactic survey. This dataset continues the work of the Virgo ALFALFA catalog. The catalogs and spectra published here consist of data obtained during the 2005 and 2006 observing sessions of the survey. The catalog consists of 578 HI detections within the range 11h 36m < R.A.(J2000) < 13h 52m and +08 deg < Dec.(J2000) < +12 deg, and cz_sun < 18000 km/s. The catalog entries are identified with optical counterparts where possible through the examination of digitized optical images. The catalog detections can be classified into three categories: (a) detections of high reliability with S/N > 6.5; (b) high velocity clouds in the Milky Way or its periphery; and (c) signals of lower S/N which coincide spatially with an optical object and known redshift. 75% of the sources are newly published HI detections. Of particular note is a complex of HI clouds projected between M87 and M49 that do not coincide with any optical counterparts. Candidate objects without optical counterparts are few. The median redshift for this sample is 6500 km/s and the cz distribution exhibits the local large scale structure consisting of Virgo and the background void and the A1367-Coma supercluster regime at cz_sun ~7000 km/s. Position corrections for telescope pointing errors are applied to the dataset by comparing ALFALFA continuum centroid with those cataloged in the NRAO VLA Sky Survey. The uncorrected positional accuracy averages 27 arcsec ~(21 arcsec ~median) for all sources with S/N > 6.5 and is of order ~21 arcsec ~(16 arcsec ~median) for signals with S/N > 12. Uncertainties in distances toward the Virgo cluster can affect the calculated HI mass distribution.
The catalog contents can be accesed by using the Search tools.
Saintonge et al. 2008, AJ 135, 588 (ADS)
We present a second catalog of HI sources detected in the Arecibo Legacy Fast ALFA Survey. We report 488 detections over 135 deg^2, within the region of the sky having 22h < RA< 3h and +26d < Dec < +28d. We present here the detections that have either (a) S=N > 6.5, where the reliability of the catalog is better than 95% or (b) 5.0 < S=N < 6.5 and a previously measured redshift that corroborates our detection. Of the 488 objects presented here, 49 are High Velocity Clouds or clumps thereof with negative heliocentric recession velocities. These clouds are mostly very compact and isolated, while some of them are associated with large features such as Wright's Cloud or the northern extension of the Magellanic Stream. The remaining 439 candidate detections are identified as extragalactic objects and have all been matched with optical counterparts. Five of the six galaxies detected with MHI < 10^7.5Msun are satellites of either the NGC672/IC1727 nearby galaxy pair or their neighboring dwarf irregular galaxy NGC784. The data of this catalog release include a slice through the Pisces- Perseus foreground void, a large nearby underdensity of galaxies. We report no detections within the void, where our catalog is complete for systems with HI masses of 108 Msun. Gas-rich, optically-dark galaxies do not seem to constitute an important void population, and therefore do not suffce at producing a viable solution to the void phenomenon.
The catalog contents can be accesed by using the Search tools.
Giovanelli et al. 2007, AJ, 133, 2569 (ADS)
We present the first installment of HI sources extracted from the Arecibo Legacy Fast ALFA (ALFALFA) extragalactic survey, initiated in 2005. Sources have been extracted from 3-D spectral data cubes and then examined interactively to yield global HI parameters. A total of 730 HI detections are catalogued within the solid angle 11h44m < R.A.(J2000) < 14h00m and +12deg< Dec.(J2000) < +16deg, and redshift range -1600 \kms < cz < 18000 \kms. In comparison, the HI Parkes All-Sky Survey (HIPASS) detected 40 HI signals in the same region. Optical counterparts are assigned via examination of digital optical imaging databases. ALFALFA HI detections are reported for three distinct classes of signals: (a) detections, typically with S/N > 6.5; (b) high velocity clouds in the Milky Way or its periphery; and (c) signals of lower S/N (to ~ 4.5) which coincide spatially with an optical object of known similar redshift. Although this region of the sky has been heavily surveyed by previous targeted observations based on optical flux-- or size-- limited samples, 69% of the extracted sources are newly reported HI detections. The resultant positional accuracy of HI sources is 20" (median). The median redshift of the sample is ~7000 \kms and its distribution reflects the known local large scale structure including the Virgo cluster. Several extended HI features are found in the vicinity of the Virgo cluster. A small percentage (6%) of HI detections have no identifiable optical counterpart, more than half of which are high velocity clouds in the Milky Way vicinity; the remaining 17 objects do not appear connected to or associated with any known galaxy.