15 years of very long baseline interferometry observations of two compact radio sources in Messier 82

Abstract

We present the results of a second epoch of 18‐cm global very long baseline interferometry (VLBI) observations, taken on 2001 February 23, of the central kiloparsec of the nearby starburst galaxy Messier 82. These observations further investigate the structural and flux evolution of the most compact radio sources in the central region of M82. The two most compact radio objects in M82 have been investigated (41.95+575 and 43.31+592). Using this recent epoch of data in comparison with our previous global VLBI observations and two earlier epochs of European VLBI network observations, we measure expansion velocities in the range of 1500–2000 km s−1 for 41.95+575 and 9000–11 000 km s−1 for 43.31+592 using various independent methods. In each case, the measured remnant expansion velocities are significantly larger than the canonical expansion velocity (500 km s−1) of supernova remnants (SNRs) within M82 predicted from theoretical models. In this paper, we discuss the implications of these measured expansion velocities with respect to the high‐density environment that the SNRs are expected to reside in within the centre of the M82 starburst.

Radio supernovae

“…Radio supernovae R. J. Beswick 11000 km s −1 [41,42,44]. This value is close agreement with other radio expansion measurements of this source made using MERLIN by Muxlow et al [46].…”

“…Over the ∼20 years, high-resolution VLBI and MERLIN observations have been used to resolve and monitor the structural evolution of these sources in great detail [38,39,3,40,41,42,43,44,45] (see Fig. 5).…”

“…The two most compact sources in M82 (41.95+475 and 43.31+592) have been resolved in detail using a series of global VLBI observations over the last 20 years [41,42,44,45]. Three 18-cm global VLBI images of the most compact source in M82 are shown in Fig.…”

“…Whilst it is clear that 41.95+475 is evolving both in its radio flux density and milliarcsecond radio structure, it is still unclear as to the true nature of this source. It has recently been speculated that this source is not a true radio supernova event but something more exotic, such as the remnant of an off-axis GRB (see [43,44,45]). …”

VLBI observations of SN 2008iz

“…For instance, the mean expansion velocity of SN 1979C during the first year after explosion has been estimated to be ∼10 000-11 000 km s −1 Marcaide et al 2009b); for SN 1986J, a velocity of ∼14 700 km s 3 month after the explosion; Staveley- Smith et al (1993) report a mean expansion speed of ∼35 000 km s −1 during the first years for SN 1987A before it slowed down to ∼4800 km s −1 ; for SN 2004et, the expansion velocity was >15 700 km s −1 (Martí-Vidal et al 2007); and for SN 2008ax, an expansion velocity as large as 52 000 km s −1 was obtained (Martí-Vidal et al 2009). Estimates of the expansion velocities of other supernova remnants in M 82 (the host galaxy of SN 2008iz) have also been reported to range between ∼1500 and 11 000 km s −1 (Beswick et al 2006). The larger velocities above are much higher (by a factor of 3−22) than the predicted velocities from the model of Chevalier & Fransson (2001), based on the high pressure expected in the interstellar medium (ISM) of M 82.…”