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We present photometric and spectroscopic observations of SN 2007if, anoverluminous (M_V = -20.4), red (B-V = 0.16 at B-band maximum), slow-rising(t_rise = 24 days) type Ia supernova in a very faint (M_g = -14.10) hostgalaxy. A spectrum at 5 days past B-band maximum light is a direct match to thesuper-Chandrasekhar-mass candidate SN Ia 2003fg, showing Si II and C II at~9000 km/s. A high signal-to-noise co-addition of the SN spectral time seriesreveals no Na I D absorption, suggesting negligible reddening in the hostgalaxy, and the late-time color evolution has the same slope as the Lirarelation for normal SNe Ia. The ejecta appear to be well mixed, with no strongmaximum in I-band and a diversity of iron-peak lines appearing innear-maximum-light spectra. SN2007 if also displays a plateau in the Si IIvelocity extending as late as +10 days, which we interpret as evidence for anoverdense shell in the SN ejecta. We calculate the bolometric light curve ofthe SN and use it and the \ion{Si}{2} velocity evolution to constrain the massof the shell and the underlying SN ejecta, and demonstrate that SN2007 if isstrongly inconsistent with a Chandrasekhar-mass scenario. Within the context ofa "tamped detonation" model appropriate for double-degenerate mergers, andassuming no host extinction, we estimate the total mass of the system to be 2.4+/- 0.2 solar masses, with 1.6 +/- 0.1 solar masses of nickel-56 and with0.3-0.5 solar masses in the form of an envelope of unburned carbon/oxygen. Ourmodeling demonstrates that the kinematics of shell entrainment provide a moreefficient mechanism than incomplete nuclear burning for producing the lowvelocities typical of super-Chandrasekhar-mass SNeIa.

Nearby Supernova Factory Observations of SN 2007if: First Total Mass Measurement of a Super-Chandrasekhar-Mass Progenitor