We study the Kennicutt-Schmidt star formation law and efficiency in the gaseous disk of the isolated galaxy CIG 96 ( NGC 864 ; Tully 65 ) , with special emphasis on its unusually large atomic gas ( H i ) disk ( r _ { HI } /r _ { 25 } = 3.5 , r _ { 25 } = 1 \farcm 85 ) .
We present deep GALEX near and far ultraviolet observations , used as a recent star formation tracer , and we compare them with new , high resolution ( 16″ , or 1.6 kpc ) VLA H i observations .
The UV and H i maps show good spatial correlation outside the inner 1′ , where the H i phase dominates over H _ { 2 } .
Star-forming regions in the extended gaseous disk are mainly located along the enhanced H i emission within two ( relatively ) symmetric giant gaseous spiral arm-like features , which emulate a H i pseudo-ring at a r \simeq 3′ .
Inside such structure , two smaller gaseous spiral arms extend from the NE and SW of the optical disk and connect to the previously mentioned H i pseudo-ring .
Interestingly , we find that the ( atomic ) Kennicutt-Schmidt power law index systematically decreases with radius , from N \simeq 3.0 \pm 0.3 in the inner disk ( 0 \farcm 8 – 1 \farcm 7 ) to N = 1.6 \pm 0.5 in the outskirts of the gaseous disk ( 3 \farcm 3 – 4 \farcm 2 ) .
Although the star formation efficiency ( SFE ) , the star formation rate per unit of gas , decreases with radius where the H i component dominates as is common in galaxies , we find that there is a break of the correlation at r = 1.5 r _ { 25 } .
At radii 1.5 r _ { 25 }  <  r  <  3.5 r _ { 25 } , mostly within the H i pseudo-ring structure , there exist regions whose SFE remains nearly constant , SFE \simeq 10 ^ { -11 } yr ^ { -1 } .
We discuss about possible mechanisms that might be triggering the star formation in the outskirts of this galaxy , and we suggest that the constant SFE for such large radii r  > 2 r _ { 25 } and at such low surface densities might be a common characteristic in extended UV disk galaxies .