We present the catalog of HI sources extracted from the ongoing Arecibo Legacy Fast ALFA ( ALFALFA ) extragalactic HI line survey , found within the sky region bounded by 9 ^ { h } 36 ^ { m } < \alpha < 11 ^ { h } 36 ^ { m } and +08 ^ { \circ } < \delta < +12 ^ { \circ } .
The HI catalog presented here for this 118-deg ^ { 2 } region is combined with ones derived from surrounding regions also covered by the ALFALFA survey to examine the large scale structure in the complex Leo region .
Because of the combination of wide sky coverage and superior sensitivity , spatial and spectral resolution , the ALFALFA HI catalog of the Leo region improves significantly on the numbers of low HI mass sources as compared with those found in previous HI surveys .
The HI mass function of the Leo I group presented here is dominated by low-mass objects : 45 of the 65 Leo I members have M _ { HI } < 10 ^ { 8 } M _ { \odot } , yielding tight constraints on the low-mass slope of the Leo I HI mass function .
The best-fit slope is \alpha \simeq - 1.41 + 0.2 - 0.1 .
A direct comparison between the ALFALFA HI line detections and an optical search of the Leo I region proves the advantage of the ALFALFA strategy in finding low mass , gas-rich dwarfs .
These results suggest the existence of a significant population of low surface brightness , gas-rich , yet still very low HI mass galaxies , and may reflect the same type of morphological segregation as is seen in the Local Group .
While the low mass end slope of the Leo I HI mass function is steeper than that determined for luminosity functions of the group , the slope still falls short of the values predicted by simulations of structure formation in the \Lambda CDM paradigm .