2004A&A...417..421B

We have used the Westerbork array to carry out an unbiased wide-field survey of HI emission features, achieving an rms sensitivity of about 18 mJy/Beam at a velocity resolution of 17km/s over 1800deg² and between -1000<V_Hel<+6500km/s. The primary data consists of auto-correlation spectra with an effective angular resolution of 49' FWHM. The survey region is centered approximately on the position of Messier 31 and is Nyquist-sampled over 60x30° in RAxDec. In this paper we present our HI detections at negative velocities which could be distinguished from the Galactic foreground. Fully 29% of the entire survey area has high velocity HI emission at a column density exceeding our 3σ limit of about 1.5x10¹⁷/cm² over 30km/s. We tabulate the properties of 95 discrete HVC components in the field, corresponding to more than an order of magnitude increase in number over that known previously. The distribution of discrete object number with flux density has an inflection point near 12 Jy-km/s which may correspond to a characteristic mass and distance for this population. A faint population of discrete HVCs is detected in the immediate vicinity of M 31 which spans a large fraction of the M 31 rotation velocity. This class of features is confined to about 12° (160kpc) projected radius of M 31 and appears to be physically associated. Independent confirmation of the features within 3.5° (47kpc) of M 31 has been obtained in our Green Bank Telescope survey (Thilker et al., 2004ApJ...601L..39T). We detect a diffuse northern extension of the Magellanic Stream (MS) from at least δ=+20->40°, which subsequently loops back toward the south. Recent numerical simulations had predicted just such an MS extension corresponding to the apo-galacticon portion of the LMC/SMC orbit at a distance of 125kpc. A faint bridge of HI emission appears to join the systemic velocities of M 31 with that of M 33 and continues beyond M 31 to the north-west. This may be the first detection of HI associated with the warm-hot intergalactic medium (WHIM). The distribution of peculiar velocity HI associated with M 31 can be described by a projected exponential of 25kpc scale-length and 5x10¹⁷cm^–2 peak column density. We present the distribution function of N_HI in the extended M 31 environment, which agrees well with the low red-shift QSO absorption line data over the range log(N_HI)=17.2 to log(N_HI)=21.9. Our data extend this comparison about two orders of magnitude lower than previously possible and provide the first image of the Lyman limit absorption system associated with an L_* Galaxy.