The Arecibo Galaxy Environment Survey - VII. A dense filament with extremely long H I streams

We present completed observations of the NGC 7448 galaxy group and background volume as part of the blind neutral hydrogen Arecibo Galaxy Environment Survey. Our observations cover a region spanning 5°× 4°, over a redshift range of approximately -2000 <cz < 20 000 km s^-1. A total of 334 objects are detected, mostly in three overdensities at cz ∼ 7500, cz ∼ 9600 and cz ∼ 11 400 km s^-1. The galaxy density is extremely high (15 deg^-2) and many (∼24 per cent) show signs of extended H I emission, including some features as much as 800 kpc in projected length. We describe the overall characteristics of this environment: kinematics, typical galaxy colours and mass-to-light ratios, and substructure. To aid in the cataloguing of this data set, we present a new FITS viewer (FRELLED: FITS Realtime Explorer of Low Latency in Every Dimension). This incorporates interactive source cataloguing tools which increase our source extraction speed by approximately a factor of 50.