We present the results of multi-epoch observations of the southern starburst galaxy, NGC~253, with the Australian Long Baseline Array (LBA) at 2.3~GHz. Our observations, conducted in 2006, 2007 and 2008 are similar to previous 2.3~GHz LBA observations by Lenc and Tingay (2006) in angular resolution (15~mas), field of view and sensitivity. Eight sources were detected, corresponding to higher frequency VLA detection. Six sources correspond to detections in the previous 2.3 GHz LBA observations, while two are new detections. One supernova remnant, 5.48-43.3, was imaged with the highest resolution in all three epochs, revealing a morphology similar to that of SN~1987A and an upper limit of the expansion speed consistent with, 10,000~km/s. A free-free absorption model was fitted to the spectra of 20 compact sources in NGC~253,incorporating the flux densities from the 2008 epoch. Small differences were found in the spectra and fitted parameters of spectral index and free-free opacity from Lenc and Tingay (2006). These variations are possibly due to the surrounding medium. A value for the upper limit of the supernova rate, 0.29 per yr, was derived for the inner 300~pc of NGC~253 and is consistent with estimates determined from near infrared (NIR) observations of the [FeII] line. The model used to derive this limit was based on the principles Ulvestad & Antonucci (1991) and Lenc and Tingay (2006), and included data from 10 observations over a period of a 20.6 years. Improvements over previous models included better knowledge of the Type~II supernovae peak luminosities at 6~cm, an improved distance estimate and incorporation of time-dependent spectral indices from Type~II supernovae light curves. From the supernova rate we estimate a new upper limit to the star formation rate of SFR(M>5M_solar)<7.1 M_solar/yr , which is consistent with estimates at other wavelengths.
- Lenc, E. and Tingay, S. J. (2006). AJ, 132:1333–1345
- Ulvestad, J. S., & Antonucci, R. R. J. 1991, AJ, 102, 875