The Arecibo Legacy Fast ALFA Survey: The ALFALFA Extragalactic HI Source Catalog (1805.11499v1)

Abstract: We present the catalog of ~31500 extragalactic HI line sources detected by the completed ALFALFA survey out to z < 0.06 including both high signal-to-noise ratio (> 6.5) detections and ones of lower quality which coincide in both position and recessional velocity with galaxies of known redshift. We review the observing technique, data reduction pipeline, and catalog construction process, focusing on details of particular relevance to understanding the catalog's compiled parameters. We further describe and make available the digital HI line spectra associated with the catalogued sources. In addition to the extragalactic HI line detections, we report nine confirmed OH megamasers and ten OH megamaser candidates at 0.16 < z < 0.22 whose OH line signals are redshifted into the ALFALFA frequency band. Because of complexities in data collection and processing associated with the use of a feed-horn array on a complex single-dish antenna in the terrestrial radio frequency interference environment, we also present a list of suggestions and caveats for consideration by users of the ALFALFA extragalactic catalog for future scientific investigations.

• The paper details the creation of a 31,500-source HI catalog from a blind survey, significantly advancing our understanding of the HI mass function.
• It employs drift scan observations with dual polarization and rigorous RFI flagging to ensure accurate and reliable data quality.
• The integrated visualization and data handling tools, like GridView and GalFlux, enhance the study of galaxy formation and evolution.

The Arecibo Legacy Fast ALFA Survey: The ALFALFA Extragalactic HI Source Catalog

The paper "The Arecibo Legacy Fast ALFA Survey: The ALFALFA Extragalactic HI Source Catalog" presents a comprehensive catalog of extragalactic HI line sources detected via the ALFALFA survey. This extensive survey leverages the Arecibo L-band Feed Array (ALFA) to map over 7000 square degrees of the sky, focusing on regions accessible to the Arecibo radio telescope. The outcome is a catalog of approximately 31,500 extragalactic HI line sources detected out to a redshift $z < 0.06$.

The primary goal of ALFALFA is to determine the faint-end slope of the HI mass function (HIMF) and explore the abundance of low-mass gas-rich galaxies. This survey is performed as a “blind” survey, wherein a broad frequency range (1335-1435 MHz) is systematically examined for line emissions. The focus on HI emissions is strategic due to its role as a cornerstone in assessing the volume of atomic hydrogen in galaxies, which directly correlates with galactic star formation potential.

Methodological Framework

The survey adopts drift scan observations, optimized for the ALFA instrument's feed horn array on the Arecibo telescope. Notably, the calibration process was engineered to address the challenges posed by the telescope's complex optics and the terrestrial RF environment, particularly the ever-present RF interference. By employing dual polarization processing and extensive RFI flagging, the team ensures the resultant data is both reliable and high-quality.

The ALFALFA survey also specializes in cataloging OH megamasers, with nine confirmed and ten candidate OH megamasers noted, expanding knowledge of such phenomena to redshifts between 0.16 and 0.22.

Data Products

Critical to this work is the catalogue of HI sources, which embodies a plethora of relevant data, including heliocentric velocities, velocity widths, and integrated line flux densities. This catalogue supersedes previous data releases, benefiting from overlapping drift scans and more robust optical imaging counterparts, facilitating substantial improvements in data integrity and comprehensiveness.

The catalogue extends beyond numerical data; integrated tools for visualization and data handling, such as the GridView and GalFlux software, allow meticulous inspection, enabling researchers to make informed assignments of optical counterparts. The ALFALFA dataset eschews completeness for depth, making it a unique resource for studies in galaxy formation and evolution.

Implications and Future Prospects

The ALFALFA catalog is foundational in probing the distribution and behavior of HI clouds in various environments, setting a benchmark for future extragalactic HI surveys. The paper clearly illustrates the survey’s capacity to refine the current understanding of HI distribution across the universe, paving the way for future, more nuanced explorations in cosmology and extragalactic astronomy.

As a legacy survey, ALFALFA officers a vital repository of HI line data that could be pivotal for addressing open questions about galactic mass distribution and the role of environment in shaping galactic form and function. This survey also prepares ground for new methodologies in galactic archaeology using HI as a tracer for both dark and baryonic matter.

Continued advancements, perhaps leveraging upcoming radio astronomical facilities such as the Square Kilometre Array, promise to complement ALFALFA’s legacy by expanding its scope to deeper cosmic volumes and more intricate structural analyses. As data collection and processing methodologies evolve, the fundamental challenges and solutions delineated in this survey will serve as instructional bedrocks for subsequent generations of spectral line surveys.