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 \sim 23 days and \Delta m _ { 15 } ( B ) = 0.72 mag .
We calculate a lower limit to the peak bolometric luminosity of \sim 2.4 \times 10 ^ { 43 } erg s ^ { -1 } , 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 ( ) .
The high luminosity and low expansion velocities of SN 2009dc lead us to derive a possible WD progenitor mass of more than 2 M _ { \odot } and a ^ { 56 } Ni mass of about 1.4–1.7 M _ { \odot } .
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 .