Deep MERLIN 5 GHz radio imaging of supernova remnants in the M82 starburst

Abstract

The results of an extremely deep, 8‐d long observation of the central kpc of the nearby starburst galaxy M82 using Multi‐Element Radio Linked Interferometer Network (MERLIN) at 5 GHz are presented. The 17 μJy beam−1 rms noise level in the naturally weighted image makes it the most sensitive high‐resolution radio image of M82 made to date. Over 50 discrete sources are detected, the majority of which are supernova remnants, but with 13 identified as H ii regions. Sizes, flux densities and radio brightnesses are given for all of the detected sources, which are all well resolved with a majority showing shell or partial shell structures. Those sources within the sample which are supernova remnants have diameters ranging from 0.3 to 6.7 pc, with a mean size of 2.9 pc. From a comparison with previous MERLIN 5‐GHz observations made in 1992 July, which gives a 9.75‐yr timeline, it has been possible to measure the expansion velocities of 10 of the more compact sources, eight of which have not been measured before. These derived expansion velocities range between 2200 and 10 500 km s−1.

Evidence of nuclear disks in starburst galaxies from their radial distribution of supernovae

“…In the case of M 82, the closest of the three galaxies discussed here, we used the source list in McDonald et al (2002) and Fenech et al (2008), where the authors already discriminate SNe from HII regions based on their spectral shape, brightness temperature, and observed shell structure. In the cases of the more distant galaxies, Arp 299 and Arp 220, we used available spectral information from the literature, and especially, the brightness temperatures inferred from VLBI observations, which for all of the sources correspond to non-thermal radio emitters (e.g., Pérez-Torres et al 2009, for Arp 299A; Parra et al 2007, for Arp 220).…”

“…For our analysis, we combined sources catalogued as SNe (or SNR) detected by Wills et al (1997), Allen & Kronberg (1998), McDonald et al (2002), Fenech et al (2008), and Brunthaler et al (2009. These comprise observations at several frequencies with MERLIN, VLA, and VLBA, including a total of 39 sources.…”

Flux density variations of radio sources in M82

“…The data include eight epochs between May 2009 and December 2010, and were compared with data from the April 2002 monitoring session of Fenech et al (2008).…”

“…). Based on expansion velocity measurements from VLBI and MERLIN data, Fenech et al (2008) and Beswick et al (2006) determined that these sources are amongst the youngest in M82, with ages of 55 and 140 yrs (as of 2002). Consequently, their short term brightness variations could be explained by changes in the circumstellar and interstellar mediums in which the shocks travel, with these sources being at a different temporal stage in their evolution, compared with the older more stable sources.…”

“…At the distance of M82 (3.2 Mpc), this gives an estimated radius of 2.1 pc. Fenech et al (2008) measured expansion velocities of SNRs in M82 to be between 2200 and 10500 km/s, with a mean velocity of v exp = 5650 km/s. Using the latter value, we can estimate the age of 44.28+61.1 to be ∼360 yrs.…”