We present an analysis of HI , H \alpha , and oxygen abundance data for NGC 3741 .
This galaxy has a very extended gas disk ( \sim 8.8 times the Holmberg radius ) , and a dark to luminous ( i.e .
stellar ) mass ratio of \sim 149 , which makes it one of the “ darkest ” dwarf irregular galaxies known .
However its ratio of baryons ( i.e .
gas + stellar ) mass to dark mass is typical of that in galaxies .
Our new high-resolution HI images of the galaxy show evidence for a large scale ( purely gaseous ) spiral arm and central bar .
From our HI data , a rotation curve can be derived out to \sim 37 - 44 disk scale lengths in the J and B band respectively .
This is just slightly short of the radius at which one would expect an NFW type rotation curve to start falling .
The galaxy has an integrated star formation rate of \sim 0.0034 M _ { \odot } yr ^ { -1 } , while the average star formation rate within the optical disk is \sim 0.0049 M _ { \odot } yr ^ { -1 } kpc ^ { -2 } .
Despite the gaseous spiral feature and the on-going star formation , we find that the global gas density in NGC 3741 is significantly lower than the Toomre instability criterion .
This is consistent with the behaviour seen in other dwarf galaxies .
We also find that the star formation rate is consistent with that expected from the observed correlations between HI mass and SFR and the global Kennicutt-Schmidt law respectively .
We measure the oxygen abundance to be 12+ log ( O/H ) =7.66 \pm 0.10 , which is consistent with that expected from the metallicity-luminosity relation , despite its extreme gas mass ratio .
We also examine the issue of chemical evolution of NGC 3741 in the context of closed-box model of chemical evolution .
The effective oxygen yield of NGC 3741 is consistent with recent model estimates of closed-box yields , provided one assumes that the gas has been efficiently mixed all the way to edge of the HI disk ( i.e . > 8 times the optical radius ) .
This seems a priori unlikely .
On the other hand , using a sample of galaxies with both interferometric HI maps and chemical abundance measurements , we find that the effective yield is anti-correlated with the total dynamical mass , as expected in leaky box models .