The velocity width function of galaxies from the 40% ALFALFA survey: shedding light on the cold dark matter overabundance problem

Understand

The ongoing Arecibo Legacy Fast ALFA (ALFALFA) survey is a wide-area, extragalactic HI-line survey conducted at the Arecibo Observatory.

• Sources have so far been extracted over ~3,000 sq.deg of sky (40% of its final area), resulting in the largest HI-selected sample to date.
• We measure the space density of HI-bearing galaxies as a function of their observed velocity width (uncorrected for inclination) down to w = 20 km/s, a factor of 2 lower than the previous generation HI Parkes All-Sky Survey.
• We confirm previous results that indicate a substantial discrepancy between the observational distribution and the theoretical one expected in a cold dark matter (CDM) universe, at low widths.