2 Goals and target selection

2.1 A pilot FGC survey

In May 1996, we began using the Nançay Radio Telescope to undertake a pilot survey aimed at measuring redshifts and H I parameters for a modest sample of galaxies selected from the FGC. Our preliminary sample consisted of 57 galaxies lying in the declination range accessible at Nançay ( $\delta\ge -38^{\circ}$) and without published redshifts and/or H I flux and linewidth measurements.

Figure 2: H I spectra of the galaxies detected in the present survey. Axes are flux density, in millijanskys, versus radial velocity, in kilometers per second, using the optical convention. Boxcar smoothing has been applied to the spectra for display purposes only

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A primary goal of our pilot survey was to find new examples of underevolved "extreme late-type'' spirals in the local universe ($V_{\rm h}$ <$\sim$ 5000 km s^-1). As shown by Matthews (1998), galaxies at the end of the spiral sequence possessing moderate-to-low surface brightness, optically-organized disks with little or no bulge component are often extremely gas-rich systems ( $\frac{{\cal M}_{\rm HI}}{L_{\rm V}}$$\ge$1). The faintest of these galaxies are often found to rotate faster than predicted by the standard Tully-Fisher relation (see also Matthews et al. 1998b), and appear to have evolved minimally in both a dynamical and in a star-formation sense over a Hubble time (see also Matthews & Gallagher 1997). The relative simplicity of such objects compared with giant spirals makes them suitable for investigations of the factors that govern various aspects of disk structure and evolution in all spirals. Extreme late-type spirals with superthin morphologies are of particular interest due to their minimal dynamical heating and their requirement of massive dark halos for stability (e.g., Zasov et al. 1991; Matthews 1998). This makes the FGC an excellent source for new examples of nearby extreme late-type spirals.

For our pilot survey we preferentially selected targets with large angular sizes (D₂₅>1'), very late-type Hubble classifications (Scd or later), and luminosity classes of III or IV (i.e. low optical surface brightness systems). Our selection criteria were imposed in order to maximize the likelihood that our targets would be relatively small, gas-rich, low-luminosity spirals (i.e., extreme late-type spirals) within in the local universe, rather than more distant, more luminous objects (see also Gallagher et al. 1995; Matthews et al. 1995; Matthews & Gallagher 1996).

2.2 An extended survey of pure disk spirals

Because the detection rate during our pilot survey was so high, in August 1996 we began a second phase of our survey program by significantly expanding our target list and extending our velocity search range for most of the undetected objects from the pilot survey. Our new goals included not only disk evolution studies and a continued search for interesting individual objects, but also to produce a more extensive database of H I properties suitable for: (1) exploring the diversity of galaxy properties within the FGC; (2) providing global parameters for a statistical sample of pure disk galaxies; (3) undertaking Tully-Fisher analyses of the lowest-luminosity pure disks spirals (cf. Matthews et al. 1998b); (4) better assessing the contribution of late-type disks to the local H I luminosity function; (5) exploring the 3-D spatial distribution of pure disk galaxies.

For the second phase of our survey we culled from the FGC a list of 380 additional targets for observation from Nançay. We included most of the remaining galaxies in the FGC meeting our initial selection criteria, but this time we also selected a number of galaxies with $D_{25}\le 1'$ or luminosity class II, as well as a few large angular size objects with earlier Hubble classifications, and a handful of galaxies with D₂₅>1' from the addendum to the FGC (the FGCA). Added to this target list were $\sim20$ galaxies culled from the Lyon-Meudon Extragalactic Database (LEDA) with $a/b\ge$ 7, $D_{25}\ge$ $1\hbox{$.\mkern-4mu^\prime$ }0$, Hubble type Scd or later, and no catalogued redshift, but which had not been included in the FGC. Finally, we compiled from the southern extension of the FGC (the FGCE) a list of high-priority targets in the declination range $-44.5^{\circ}\le\delta\le-38^{\circ}$ (i.e., south of the Nançay declination limit) for observation with the Green Bank 140-ft telescope (see below).