The Arecibo Legacy Fast ALFA Survey: The alpha.40 HI Source Catalog, its Characteristics and their Impact on the Derivation of the HI Mass Function (1109.0027v1)

Abstract: 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 alpha.40 catalog. Covering 40% of the final survey area, the alpha.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 alpha.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 alpha.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.

• The paper demonstrates the derivation of the HI mass function using robust methods like the 1/V_max and 2DSWML techniques.
• It highlights enhanced catalog completeness with over 15,800 HI detections and precise matching of optical counterparts using SDSS data.
• The study addresses survey limitations, including RFI challenges, to improve our understanding of the distribution of neutral hydrogen in galaxies.

Overview of "The Arecibo Legacy Fast ALFA Survey: The α.40 HI Source Catalog, its Characteristics and their Impact on the Derivation of the HI Mass Function"

The paper "The Arecibo Legacy Fast ALFA Survey: The α.40 HI Source Catalog, its Characteristics and their Impact on the Derivation of the HI Mass Function" by Haynes et al. presents a comprehensive analysis of the partial (40%) release of the Arecibo Legacy Fast ALFA (ALFALFA) survey, focusing on its impact on deriving the HI mass function (HIMF). This survey provides a catalog of 21 cm HI line sources culled from the ALFALFA survey covering approximately 2800 square degrees of the sky. The paper highlights the advancements in cataloging extragalactic HI line sources and addresses the biases and completeness of the survey concerning the HI mass function.

Key Findings and Methodology

1. Catalog Characteristics and Completeness: The α.40 catalog contains 15,855 HI detections, with 15,041 extragalactic sources—an enhancement by a factor of 29 over previous surveys like HIPASS. The catalog reflects an average source density of 5.3 galaxies per square degree. Notably, the ALFALFA survey benefits from improved spatial resolution to accurately associate HI detections with optical counterparts.
2. Optical Counterparts and Multiwavelength Correlation: A thorough methodology correlates HI detections with optical counterparts primarily using SDSS and DSS2 imaging data. The paper elaborates on criteria used to assign probable optical counterparts to detect HI sources, emphasizing accuracy through cross-referencing with SDSS DR7 photometric and spectroscopic data. A comprehensive cross-reference to the SDSS DR7 provides insights into the stellar counterparts of the HI sources.
3. Survey Limitations and RFI Challenges: The paper acknowledges regions of the survey affected by radio frequency interference (RFI), particularly impacting the redshift range at ~15,000 km/s, mainly due to interference from the FAA radar located at the San Juan Airport. Consequently, cosmological applications such as HIMF are carefully considered, keeping such limitations in mind.
4. HI Mass Function Analysis: The impact of survey completeness and large-scale structure is evaluated extensively for deriving the HI mass function using both the 1/V_max method and the 2D stepwise maximum likelihood (2DSWML) method. The results are robust against biases introduced by systematic velocity flow models and assumptions of pure Hubble flow, with the latter found to underestimate low-mass slopes.
5. Volume Sensitivity and Implications: The paper underscores the need for thorough volume sensitivity analyses in HI line surveys. It highlights the survey’s ability to detect massive HI galaxies previously underestimated by HIPASS due to its limitations in volume sampling. The α.40 catalog, therefore, provides a more comprehensive representation of the high-mass end of the HIMF.

Theoretical and Practical Implications

• HI Mass Function and Galaxy Studies: The α.40 catalog and its thorough analysis offer significant contributions to our understanding of the HI mass function, particularly regarding the completeness of HI-bearing sources in the local universe. This survey offers a critical dataset for studying galaxy formation and evolution, informing models related to gas accretion and star formation processes.
• Survey Methodology: The methodology adopted in ALFALFA sets a precedent for future HI surveys. Its robust pipeline, combining automatic detection and interactive evaluation of data, ensures a high reliability of detections, setting a new benchmark in extragalactic HI surveys.
• Future Potential and Extensions: The completion of the ALFALFA survey promises further refinement in characterizing the extragalactic HI population at z~0, contributing to a more representative sampling of the cosmological volume and enhancing the statistical power of cosmological inferences obtainable from HI detected galaxy samples.

In conclusion, this paper presents a pivotal step in mapping the HI distribution in the nearby universe, resolving systematic biases from previous catalogs, and enriching the tools available for studying the HI content of galaxies. Future analyses fully incorporating the entire ALFALFA sample will likely further validate these findings and augment our understanding of the mass distribution of neutral hydrogen in galaxies.