Radio monitoring of NGC 7469: late-time radio evolution of SN 2000ft and the circumnuclear starburst in NGC 7469

We present the results of an eight-year long monitoring of the radio emission from the luminous infrared galaxy (LIRG) NGC 7469, using 8.4 GHz Very Large Array (VLA) observations at 0.3 arcsec resolution. Our monitoring shows that the late-time evolution of the radio supernova (RSN) SN 2000ft follows a decline very similar to that displayed at earlier times of its optically thin phase. The late-time radio emission of SN 2000ft is, therefore, still being powered by its interaction with the pre-SN stellar wind, and not with the interstellar medium (ISM). Indeed, the ram pressure of the pre-SN wind is ρ w υ 2 w ≈ 7.6 x 10- 9 dyn cm ―2 , at a SN age of t ≈ 2127 d, which is significantly larger than the expected pressure of the ISM around SN 2000ft. At this age, the SN shock has reached a distance r sh ≈ 0.06 pc, and our observations are probing the interaction of the SN with dense material that was ejected by the pre-SN star about 5820 yr prior to its explosion. From our VLA monitoring, we estimate that the swept-up mass by the SN shock after about six years of expansion is M sw ≈ 0.29 M ⊙ , assuming an average expansion speed of the SN of 10 4 km s ―1 . We also searched for recently exploded core-collapse SNe in our VLA images. Apart from SN 2000ft (S v ≈ 1760 μJy at its peak, corresponding to 1.1 × 10 28 erg s ―1 Hz ―1 ), we found no evidence for any other RSN more luminous than ≈6.0 × 10 26 erg s ―1 Hz ―1 , which suggests that no other Type IIn SN has exploded since 2000 in the circumnuclear starburst of NGC 7469.