A Stationary Core with a One‐sided Jet in the Center of M81

Abstract

The nucleus of the nearby spiral galaxy M81 was observed at 8.3 GHz with a global VLBI array at 20 epochs over four and a half years, with a linear resolution at the source of about 2000 AU or 0.01 pc. Phase-referenced mapping with respect to the geometric center of supernova 1993J in the same galaxy enabled us to find, with a standard error of about 600 AU, a stationary point in the source south-east of the brightness peak. We identify this point as the location of the core and the putative black hole at the gravitational center of the galaxy. The 2σ upper bound on the core's average velocity on the sky is ≤ 40 µas yr −1 or ≤ 730 km s −1 (relative to the center of SN1993J, in excess of galactic rotation). A short, one-sided jet extends towards the north-east from the core, in projection approximately in line with the rotation axis of the galaxy, and towards the well-known extended emission 1 kpc further out. The orientation of the jet varies smoothly, with timescales of about one year, and an rms of 6 • , around its mean position angle of 50 • . Occasionally the jet appears to bend to the east. The length of the jet is only about 1 mas (3,600 AU), and varies with an rms of about 20% from epoch to epoch. The inferred speeds are below 0.08c. The total flux density of the core-jet varies erratically, changing on occasion by a factor of two over a few weeks, without any significant changes in the source size and orientation. The inferred velocity of the plasma flow is > 0.25c. The results are consistent with a model in which plasma condensations with short lifetimes are ejected relativistically from the core on a timescale of less than a few weeks. The condensations travel along a tube whose pattern and geometry are also variable but only on a timescale of about one year. The central engine of M81 has qualitative similarities to those of powerful active galactic nuclei of radio galaxies and quasars, and may also represent in power and size a scaled-up version of the largely hidden nucleus in our own Galaxy.

Detection of jet precession in the active nucleus of M 81

“…The images of M 81* consist of a compact source, which dominates the emission, and a weak extension towards the northeast. These images agree with the results reported in Bietenholz et al (2000).…”

“…For the analysis of evolution in the M 81* structure, we characterized the model emission at each epoch in a similar way to what was modeled in Bietenholz et al (2000). We used an elliptical Gaussian to model the central dominant emission and a point source to model the weak jet extension.…”

“…These observations allow us to study the evolution in the structure and position of M 81*, in the frame of its host galaxy and at different frequencies and times. Results for some of the M 81* observations at 8.4 GHz between years 1993 and 1997 are reported in Bietenholz et al (2000), and results for the astrometry analysis of a subset of observations at all frequencies between years 1993 and 2002 were reported in Bietenholz et al (2004). These authors conclude that the proper motion of M 81*, relative to the shell center of SN 1993J, was consistent with zero up to the precision level of the observations (11.4 ± 9.3 μas yr −1 ).…”

SN 1993J VLBI. I. The Center of the Explosion and a Limit on Anisotropic Expansion

“…All our 8.4 GHz images were phase-referenced ; for this purpose, we alternated short scans of SN 1993J and M81, the latter being the compact source in the center of the galaxy M81 and about 170A away from SN 1993J (Bietenholz et al 1996 ; B00). We used a cycle time of D3 minutes, in which SN 1993J was observed for 120 s and M81 for 70 s, except during the Ðrst few months in 1993 when we used a longer cycle time.…”

“…This technique allowed us to obtain the most unbiased images and, in addition and at least equally important, relative position determinations with an accuracy of up to 40 kas. We recently found a quasi-stationary part in M81Ïs central radio source that is most likely the core and the gravitational center of the galaxy (Bietenholz, Bartel, & Rupen 2000 ;B00 hereafter). This Ðnding enables us to investigate the motion of the supernova in a more fundamental way than if we did not have such a Ðxed galactic reference frame.…”