  In this paper, we present and analyse optical photometry and spectra of the
extremely luminous and slowly evolving Type Ia supernova (SN Ia) 2009dc, and
offer evidence that it is a super-Chandrasekhar mass (SC) SN Ia and thus had a
SC white dwarf (WD) progenitor. Optical spectra of SN 2007if, a similar object,
are also shown. SN 2009dc had one of the most slowly evolving light curves ever
observed for a SN Ia, with a rise time of ~23 days and Delta m_15(B) = 0.72
mag. We calculate a lower limit to the peak bolometric luminosity of ~2.4e43
erg/s, though the actual value is likely almost 40% larger. Optical spectra of
SN 2009dc and SN 2007if obtained near maximum brightness exhibit strong C II
features (indicative of a significant amount of unburned material), and the
post-maximum spectra are dominated by iron-group elements. All of our spectra
of SN 2009dc and SN 2007if also show low expansion velocities. However, we see
no strong evidence in SN 2009dc for a velocity "plateau" near maximum light
like the one seen in SN 2007if (Scalzo et al. 2010). The high luminosity and
low expansion velocities of SN 2009dc lead us to derive a possible WD
progenitor mass of more than 2 M_Sun and a Ni-56 mass of about 1.4-1.7 M_Sun.
We propose that the host galaxy of SN 2009dc underwent a gravitational
interaction with a neighboring galaxy in the relatively recent past. This may
have led to a sudden burst of star formation which could have produced the SC
WD progenitor of SN 2009dc and likely turned the neighboring galaxy into a
"post-starburst galaxy." No published model seems to match the extreme values
observed in SN 2009dc, but simulations do show that such massive progenitors
can exist (likely as a result of the merger of two WDs) and can possibly
explode as SC SNe Ia.
