2011MNRAS.415..431B

We investigate the nature of objects in a complete sample of 28 galaxies selected from the first sky area fully covered by the Arecibo Legacy Fast ALFA (ALFALFA) survey, being well detected and having H I profiles wider than 550 km/s. The selection does not use brightness, morphology or any other property derived from optical or other spectral bands. We investigate the degree of isolation, the morphology and other properties gathered or derived from open data bases and show that some objects have wide H I profiles probably because they are disturbed or are interacting, or might be confused in the ALFALFA beam. We identify a subsample of 14 galaxies lacking immediate interacting neighbours and showing regular, symmetric, two-horned H I profiles that we propose as candidate high-mass disc galaxies (CHMDs). We measure the net-Hα emission from the CHMDs and combine this with public multispectral data to model the global star formation (SF) properties of each galaxy. The Hα observations show SF rates not higher than a few M_☉/yr. Simple SF models indicate that the CHMDs formed most of their stars almost a Hubble time ago, but most also underwent an SF event in the last ∼10⁶–10⁷ yr; the young stars now produce 10–30 per cent of the visible light. The spatial distribution of the SF regions is compatible with recycled stellar ejecta. We calculate representative dynamical masses, M_dyn, for the CHMDs ranging from ∼10¹¹ to ∼7 {x} 10¹¹ M_☉. The M_dyn values are larger by factors of 2.5–7.5 than the baryonic masses of the luminous stars and gas but are affected by rather large errors. We test the Tully–Fisher relation for the CHMDs and show that these lie below the relation defined by lower mass galaxies, that is, their M_dyn is lower than expected when extrapolating the relation from lower mass galaxies to higher H I linewidths.