Extended optical and near-IR observations reveal that SN 2009dc shares a number of similarities with normal Type Ia supernovae ( SNe Ia ) , but is clearly overluminous , with a ( pseudo-bolometric ) peak luminosity of \log ( L ) = 43.47 [ erg s ^ { - } 1 ] .
Its light curves decline slowly over half a year after maximum light ( \Delta m _ { 15 } ( B ) _ { \mathrm { true } } = 0.71 ) , and the early-time near-IR light curves show secondary maxima , although the minima between the first and the second peaks are not very pronounced .
The bluer bands exhibit an enhanced fading after \sim 200 d , which might be caused by dust formation or an unexpectedly early IR catastrophe .
The spectra of SN 2009dc are dominated by intermediate-mass elements and unburned material at early times , and by iron-group elements at late phases .
Strong C ii lines are present until \sim 2 weeks past maximum , which is unprecedented in thermonuclear SNe .
The ejecta velocities are significantly lower than in normal and even subluminous SNe Ia .
No signatures of CSM interaction are found in the spectra .
Assuming that the light curves are powered by radioactive decay , analytic modelling suggests that SN 2009dc produced \sim 1.8 M _ { \odot } of ^ { 56 } Ni assuming the smallest possible rise time of 22 d. Together with a derived total ejecta mass of \sim 2.8 M _ { \odot } , this confirms that SN 2009dc is a member of the class of possible super-Chandrasekhar-mass SNe Ia similar to SNe 2003fg , 2006gz and 2007if .
A study of the hosts of SN 2009dc and other superluminous SNe Ia reveals a tendency of these SNe to explode in low-mass galaxies .
A low metallicity of the progenitor may therefore be an important pre-requisite for producing superluminous SNe Ia .
We discuss a number of possible explosion scenarios , ranging from super-Chandrasekhar-mass white-dwarf progenitors over dynamical white-dwarf mergers and Type I \frac { 1 } { 2 } SNe to a core-collapse origin of the explosion .
None of the models seem capable of explaining all properties of SN 2009dc , so that the true nature of this SN and its peers remains nebulous .