HIghMass—HIGH H I MASS, H I-RICH GALAXIES AT z ∼ 0 HIGH-RESOLUTION VLA IMAGING OF UGC 9037 AND UGC 12506

We present resolved HI observations of two galaxies, UGC 9037 and UGC 12506, members of a rare subset of galaxies detected by the ALFALFA extragalactic HI survey characterized by high HI mass and high gas fraction for their stellar masses. Both of these galaxies have M$_*>10^{10}$ M$_\odot$ and M$_\text{HI}>$ M$_*$, as well as typical star formation rates for their stellar masses. How can such galaxies have avoided consuming their massive gas reservoirs? From gas kinematics, stability, star formation, and dark matter distributions of the two galaxies, we infer two radically different histories. UGC 9037 has high central HI surface density ($>10$ M$_\odot$ pc$^{-2}$). Its gas at most radii appears to be marginally unstable with non-circular flows across the disk. These properties are consistent with UGC 9037 having recently acquired its gas and that it will soon undergo major star formation. UGC 12506 has low surface densities of HI, and its gas is stable over most of the disk. We predict its gas to be HI-dominated at all except the smallest radii. We claim a very high dark matter halo spin parameter for UGC 12506 ($\lambda=0.15$), suggesting that its gas is older, and has never undergone a period of star formation significant enough to consume the bulk of its gas.