The "Total" Mass of DDO 154

Combining data from the Dominion Radio Astrophysical Observatory (DRAO) interferometer array with previous Very Large Array (VLA) observations, it was possible to recover all the single-dish flux of DDO 154, of which ~⅓ was missing from the VLA data. The missing flux was found to be in an extended low surface brightness component in the outer parts of this dwarf irregular galaxy. In the new combined data, the H I disk can be traced out to ~6 optical radii (R_Ho) or 21 optical scale lengths (α^-1) at a level of ~1.0 × 10¹⁹ cm^-2. Even at that level, there is no sign of the abrupt truncation seen in most other galaxies, especially on the receding side. A total H I mass of 2.5 × 10⁸ M_☉ is derived, which gives an (M_{H I}/L_B) ≃ 8, making DDO 154 one of the most gas-rich galaxies known. The recovered H I flux makes it possible to extend the rotation curve by more than 33% in radii with the result that the rotation curve of DDO 154 is one of the most extensive rotation curves ever derived relative to the optical size of the galaxy. The new combined data confirm that the rotation curve is declining in the outer parts. Detailed mass models show that we may have reached the edge of the mass distribution for this galaxy, which would allow, for the first time, an estimate of the total mass and the total extent of a galaxy. For DDO 154, this gives an estimated total mass of M_tot ≃ 3.0 × 10⁹ M_☉ and an upper limit to the radial extent of R_tot ≤ 8 kpc.