Deep optical surveys using new POSS-II plate material (Schombert et al. 1997), automated plate scanners (Sprayberry et al. 1996), and clocked-CCD drift-scan surveys (Dalcanton et al. 1997) are succeeding in identifying faint LSB galaxies. Since many of these are gas-rich, they can be detected with sensitive radio observations (Schombert et al. 1997; Sprayberry et al. 1996; Impey et al. 1996; Zwaan et al. 1998).
One concern is how to appraise the mass density (both for optically luminous material and for neutral gas) contained in these newly identified populations in comparison to what has historically been included using the older catalogues that were based on higher surface brightness selection. Briggs (1997a) addressed this problem and noted that the new LSB catalogs contain galaxies at systematically greater distances than the older catalogues, and that estimates of luminosity functions, HI mass functions and integral mass densities are not substantially altered by the addition of these new catalogues. Either the new surveys are not sensitive to nearby, large angular diameter LSB galaxies, or the nearby LSB discs were already included in the historical catalogues because their inner regions are sufficiently bright that at small distances they surpass the angular diameter threshold to make it into catalogues.
The question still
remains whether a population of nearby ultra-low surface brightness
objects with HI to optical luminosity ratios 
/LB far in excess
of 1 could have escaped identification even to this day. For this
reason we have performed a systematic, sensitive (
after Hanning smoothing) blind HI-line survey in driftscan mode in
the nearby universe (
kms-1)
using the Nançay decimetric radio telescope. A blind HI-survey
will directly distinguish nearby gas-rich dwarfs at low redshifts
from intrinsically brighter background galaxies and allow us
to study the faint end of the HI mass function.
In order to directly compare the mass density detected in neutral hydrogen with the optically luminous mass density we have selected the Canes Venatici (CVn) region, which contains both loose galaxy groups and voids and which has previously been surveyed for low-luminosity dwarfs on deep optical plates (Binggeli et al. 1990, henceforth BTS).
The Nançay CVn group blind survey scanned this volume of space at a slightly deeper HI detection level than the pointed observations that Fisher & Tully (1981b) made of their large optically selected sample. Thus, these new observations allow at the same time a test of the completeness of late-type galaxies in this historical catalog.
In Sect. 2, the region observed with the Nançay telescope is described, followed by a description of the telescope, the driftscan observing mode as well as the reduction and data analysis procedures. In Sect. 3, the results from the driftscan survey are given including a detailed discussion on the sensitivy and completeness limit of the survey (3.1), the results from pointed follow-up observations of galaxy candidates without an unambigous optical counterpart on the one hand (3.2.2), and deep pointed observations of optically identified dwarfs not detected in the blind HI survey on the other hand (3.2.3). This is followed by a comparison of the driftscan data with pointed observations (3.3). In Sect. 4, the global properties of the HI-detected galaxies are discussed including the effects of Virgocentric flow on the derived parameters and a discussion of the discrepancy between the observed velocity and the independent distance determination of the galaxy UGC 7131 (4.1). This is followed by a discussion of the depth of this survey in comparison to the Fisher-Tully (1981b) Nearby Galaxy Catalog (4.2) and the HI mass function for the here performed blind HI driftscan survey (4.3). In Sect. 5, the conclusions are summarized.
Copyright The European Southern Observatory (ESO)