A dwarf galaxy with a giant HI disk

Abstract

We present Giant Meterwave Radio Telescope (GMRT) HI 21 cm images of a nearby dwarf irregular galaxy NGC 3741 ( M_B ~ −13.13) which show it to have a gas disk that extends to ˜8.3 times its Holmberg radius. This makes it probably the most extended gas disk known. Our observations allow us to derive the rotation curve (which is flat in the outer regions) out to ˜38 optical scale lengths. NGC 3741 has a dynamical mass to light ratio of ~107 and is one of the "darkest" irregular galaxies known. However, the bulk of the baryonic mass in NGC 3741 is in the form of gas and the ratio of the dynamic mass to the baryonic mass (˜8), falls within the range that is typical for galaxies. Thus the dark matter halo of NGC 3741 has acquired its fair share of baryons, but for some reason, these baryons have been unable to collapse to form stars. A comparison of NGC 3741's dark halo properties with those of a sample of galaxies with well measured rotation curves suggests that if one has to reconcile the observations with the expectation that low mass galaxies suffer fractionally greater baryon loss then baryon loss from halos occurs in such a way that, in the net, the remaining baryons occupy a fractionally smaller volume of the total halo.