Shooting movies of nature’s great particle accelerators with the Hubble Space Telescope
By Eileen Meyer, Postdoctoral Fellow at STScI
We know that the relativistic jets spewing out of the centers of disks around supermassive black holes start out very fast. VLBI studies in the radio of hundreds of jets1 in active galaxies have shown super-luminal apparent velocities of up to 50c, implying Lorentz factors of at least 50 (real speeds over 99.9% c) in the fastest jets. However, these studies are limited to measuring jet speeds on scales close to the black hole (typically < 1 pc), while the extent of the jet can range from a few kpc up to a Mpc, in some cases greatly beyond the scale of the host galaxy.
The long lifetime of Hubble has given us an opportunity to use over 13 years of archival images of one of nature’s most photogenic jets, M87, to map the complete velocity structure of a relativistic jet on kpc scales2. Using state-of-the art astrometry (thanks to Jay Anderson and others of the HST Proper Motions3 Team), I was able to register over 400 images of M87 (d: 16.4 Mpc, or 78pc/”) taken by 3 cameras on Hubble: WFPC2, ACS/HRC, and ACS/WFC (all F814W filter) using the positions of over 1000 globular clusters spread throughout the host galaxy. The resulting systematic astrometric error was only 0.17 mas, corresponding to an unprecendented 0.003c over the 13 year span of the study. Our goal was to get high-precision speeds of individual knots in the jet in order to measure not only the speeds along the jet as it extends out into the host galaxy, but also look for subtle accelerations and transverse motions which might give us insights into the jet structure and how it evolves on human timescales.
The most striking presentation of the data is in the form of movies (available at my personal webpage) , in which component speeds of up to 4.5c yield motions easily distinguished by eye. We found an impressive variety of behaviors in the jet, with some knots apparently stationary, others rapidly decelerating, and some with superluminal transverse speeds, challenging the previous picture of a jet that “smoothly decelerates” (see Figure 1 where our measurements are compared to previous studies in the radio and optical). We also found evidence for the first time of helical motions in the outermost part of the jet, where speeds were still superluminal nearly 2 kpc (projected) from the core. By overlaying the velocity vectors onto an image of the M87 jet, the apparent alignment of the vectors gives the impression of side-to-side motion (i.e., a flattened helix) as shown in Figure 2. Helical, ordered magnetic fields have long been suggested as a possible feature of relativistic jets in AGN4.
Figure 1: Velocities along the jet (upper panel) and transverse to the jet (lower panel) as a function of distance from the core. Previous measurements shown for comparison taken from Biretta et al. (1995), Biretta et al. (1999), Cheung et al. (2007), and Kovalev et al. (2007). Figure from Meyer et al., 2013
Figure 2: Various knots in the outer part of the M87 jet are shown, in 4 different epochs (1995, 1998, 2001, 2008), with vertical lines to guide the eye. Bottom panel: a depiction of velocities as vectors from their positions along the jet.
This work is ongoing, and the next publication will discuss the theoretical implications of our study and release the full dataset of multiwavelength spectra and positions as a function of time for almost 2 dozen individual components in the jet. In addition, we were recently awarded time in Cycle 21 for deep imaging of 3 more nearby jets in order to measure their kpc-scale proper motions. The most distant, 3C 273, is over 500 Mpc from Earth, making it the most distant optical proper motions target ever studied.
Further information on this study is available in Meyer et al. (2013), as well as the NASA/ESA press release.
- Lister et al 2009 (AJ, 138, 1874)
- Meyer et al 2013 (ApJ, 774, 21)
- Blandford & Znajek 1977 (MNRAS, 179, 433)