Star formation in the extended gaseous disk of the isolated galaxy CIG
  96
  We study the Kennicutt-Schmidt star formation law and efficiency in the
gaseous disk of the isolated galaxy CIG 96 (NGC 864), with special emphasis on
its unusually large atomic gas (HI) disk (r_HI/r_25 = 3.5, r_25 = 1.'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 HI observations. The UV and HI maps show good spatial correlation
outside the inner 1', where the HI phase dominates over H_2. Star-forming
regions in the extended gaseous disk are mainly located along the enhanced HI
emission within two (relatively) symmetric giant gaseous spiral arm-like
features, which emulate a HI 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 HI pseudo-ring.
Interestingly, we find that the (atomic) Kennicutt-Schmidt power law index
systematically decreases with radius, from N \simeq 3.0 +- 0.3 in the inner
disk (0.'8 - 1.'7) to N = 1.6 +- 0.5 in the outskirts of the gaseous disk (3.'3
- 4.'2). Although the star formation efficiency (SFE), the star formation rate
per unit of gas, decreases with radius where the HI 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 HI 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.
