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Jets and radio lobes are generally associated with elliptical galaxies. Why not spirals?

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That's a really great question---and one that isn't fully understood. There are two directions to look at it from

1) Spiral Galaxies don't form jets 2) Jets tend to make galaxies more elliptical

Most likely, both of these factors contribute; but how much of each is, again, not well understood. To elaborate:

1) Spiral Galaxies don't form jets as readily

  • The SMBH's in the center of spirals seem to be less massive than those of ellipticals. Jet activity seems to correlate with SMBH mass.
  • Spiral galaxies, while richer in gas overall, might not be efficient at feeding that gas toward the SMBH to feed a jet

2) Jets tend to make galaxies more elliptical

  • Jets are associated with activity in the nucleus (obviously), in recent years studies have been finding that 'feedback' from the nucleus can have a surprisingly strong effect on the galaxy---specifically decreasing star formation, and blowing out gas. Both of these are characteristic of ellipticals, thus it is feasible to suggest that possessing a jet tends to make the galaxy more elliptical. Note: I don't think this has actually been seen in simulations.
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