Galaxies wrap their black holes in loving arms

By Phil Plait | June 2, 2008 9:07 am

The thing about black holes is, they’re black. That makes them hard to find, of course, but once you find one it’s also hard to get any information about it. The only way we can figure out anything about them is by looking at how they affect things around them: how stars orbit them, how material falls in and gives off light, and so on.

After observing many galaxies over many decades, it was found that every large galaxy has a supermassive black hole at its core, where supermassive means thousands, millions, or even billions of times the mass of the Sun. The way to weigh a black hole is to carefully measure the velocity of stars in orbit around them; the faster they move, the more massive the black hole. Thanks to Kepler, we can use those measurements to get a decent estimate of the central black hole mass.

But this can be hard to do, especially for distant galaxies. It takes long exposure times, intensive analysis, and generally quite a bit of work. But now astronomers have announced a very interesting discovery: spiral galaxies with more massive central black holes tend to have their arms more tightly wrapped. Galaxies with lower mass black holes seem to have more loosely wound spiral arms.

Why this would be is something of a mystery, and to be honest the discovery is not on absolutely firm ground. What they have uncovered is a correlation, not a rock solid cause-and-effect, but their data so far look pretty good. This idea pans out, that means that getting the mass of the central black hole in a spiral galaxy may be as easy as simply taking the galaxy’s snapshot and looking at the spiral arms. Incredibly, this means the central black hole’s mass can be determined for galaxies that are eight billion light years away!

The Andromeda (left) and Triangulum (right) galaxies. Andromeda has tight arms and a massive black hole, while Triangulum has loose arms and a lightweight black hole. Images courtesy T. Rector and B. Wolpa, NOAO/AURA/NSF, and T. Rector and M. Hanna/NRAO/AUI/NSF/NOAO/AURA, repsectively.

So, for example, the Andromeda galaxy, which has a very massive black hole in its heart — nearly 200 million times the mass of the Sun, or about 50 times the mass of the black hole in the center of our Milky Way — has its arms relatively tightly wound up. But the Triangulum galaxy, which has loose arms, has a low-mass black hole in its core, just a few thousand times the Sun’s mass.

The mass of the central black hole turns out to be pretty important in the life of the galaxy… though probably not why you’d think. Even the most massive black hole is only a tiny fraction of the total mass of the parent galaxy — far less than even 1%! But it turns out that the mass of the black hole seems to play an important role in how the galaxy itself forms. The black hole forms at roughly the same time as the galaxy itself. As the black hole gobbles down matter, it can get what is basically indigestion, eating material too quickly. This sets up a wind of matter that blows out from the black hole, and that in turn disturbs the gas in the galaxy. That gas is what forms stars, so the star formation history of the galaxy can be affected by its central black hole. This in turn can affect how mass gets distributed in the disk of the galaxy, and that’s — maybe — why the arm structure is affected by the black hole.

However, galaxy history is fraught with danger. Galaxies collide, or slide past each other and mess each other up. This also affects how the disk and arms behave, so obviously this situation gets complicated quickly. Worse, dark matter may play a role as well, but it’s not clear how that might work either. But if the result that black hole mass somehow correlates with the spiral arm shape is correct, that will give astronomers yet another handle on how galaxies interact with the monsters at their hearts.

CATEGORIZED UNDER: Astronomy, Pretty pictures, Science

Comments (26)

  1. Gokuson123

    It’s like studying the atom again, except on a galactic level.

  2. Dave

    The monster at their heart just might be the monster that is their heart. I like the sound of that.

  3. Xerxes

    Cite: http://arxiv.org/abs/0804.0773 ; Seigar, Kennefick, Kennefick, Lacy – Discovery of a relationship between spiral arm morphology and supermassive black hole mass in disk galaxies

  4. SonOfSLJ

    Great article, BA.

    But more importantly, please tell me that was a Red Dwarf reference in the first sentence.

  5. SonofSLJ, sorry, but I never watched Red Dwarf, so that’s all mine.

  6. Matthew Reynolds

    Interesting observation, to say the least. Although, couldn’t the size and mass of a galaxy play a role in the shape of its arms? E.g, Andromeda is quite larger than most other known galaxies, so would the sheer mass of the trillion or so stars within it be a factor?

  7. aiabx

    How much blacker could black holes be? None. None more black.

  8. Sarcastro

    You ought to check out Red Dwarf BA. They have great fun playing with big physics ideas.

    “Well, the thing about a Black Hole, its main distinguishing feature, is it’s black. And the thing about space, your basic space colour is black. So how are you supposed to see them?”

    Later…

    “They weren’t Black Holes.”
    “What were they?”
    “Grit. Five specks of grit on the scanner-scope. See, the thing about grit is, it’s black, and the thing about scanner-scopes…”
    “Oh shut up!”

  9. Benudhar From- India

    Black hole is very strange to understand.It is imposible to know about his birth.

  10. Helioprogenus

    That’s an interesting correlation. Does that mean that older galaxies would be more likely to have tighter bound arms if their black holes have consumed enough gas through their evolution? Or is this dependent on the formative years, with little input in later years?

    Also, is it possible black holes are related to the bar structure in some spiral galaxies? Ours has a bar structure and last artist representation I remember, seems to have an intermediate bound arms.

  11. MoreInput

    Is this correlation not yet know? They found out some years ago that theres a correlation between the size of bulge and the size of the black hole.
    http://en.wikipedia.org/wiki/Supermassive_black_hole
    Big bulge means also tight arms, and small bulge usually means open arms.

  12. Tom Marking

    The following is a very good URL which talks about galaxy evolution:

    http://www.astro.caltech.edu/~george/ay20/eaa-galevol.pdf

    “Bulge and disk do not only differ for the composition of their stellar populations and interstellar medium, but also for their dynamics. Galaxies are large concentrations of mass, and they would collapse under the effect of their own self-gravity if some opposing force were not there to keep them in dynamical equilibrium. Disks are kept in equilibrium by rotation, which provides the centrifugal force to oppose gravity. Bulges have modest or no rotation, and are sustained against gravity by the ‘velocity dispersion’ of their stars, which is similar to the
    motions of molecules in a hot gas. These motions provide the pressure that contrasts gravity and keeps the structure in dynamical equilibrium.

    ELLIPTICAL GALAXIES consist only of the bulge component and have no disk. LENTICULAR GALAXIES are essentially ellipticals with a very thin, often almost invisible disk, which gives them the shape of a lentil. SPIRALGALAXIES have both a bulge and a disk with spiral arms. If a bar is present, this contains the bulge at its center and has the spiral arms departing from its extremities. IRREGULAR GALAXIES tend to
    resemble spiral ones, except that the bulge, the disk and the spiral arms are not regularly defined. There are also amorphous galaxies, which have morphology that cannot be easily classified into any of the main types.

    Initially, it was believed that the Hubble sequence was the manifestation of an evolutionary process such that galaxies originated as ellipticals, which were therefore called the ‘early types’, and evolved to become spirals and irregulars, which were called the ‘late types’. Although it has been shown that such ideas were incorrect, nonetheless the nomenclature has remained, and today it is common,
    for example, to refer to ellipticals and lenticulars as ‘earlytype galaxies’.”

    .
    .
    .

  13. David Blair

    What came first the black hole (the chicken) or the galaxy (the egg)?

  14. BA stated:
    I never watched Red Dwarf,

    Then smeg you….
    ;)

    I need to take some time and watch it on the NetFlix Instant View….

    J/P=?

  15. I’d say this might sound like a silly question, but it’s being asked and therefore not silly.

    As I understand relativity with respect to the distortion of space and time around massive objects, the massive object creates a curvature in space, which is observed as gravity, and a dilation of time, which is observed as time going slower for objects deeper withing the gravity well. I’ve also heard, read, and viewed that rotating massive objects may actually drag space around them in addition to the curvature that the mass produces. We know that neutron stars and white dwarf stars rotate at extremely fast rates, sometimes on the order of milliseconds per revolution.

    Do you think it possible that the supermassive black holes at the center of galaxies are rotating in this manner and that the twisting and dragging of space on such a large scale is a possible factor in the formation of the spiral arm structures?

    The stars in the galaxy seem to be traveling too fast to stay bound to the galaxy, and dark matter is being given credit for the bindng, but what if the rotation of the SMBH is dragging the space along so that they are not actually traveling as fast as they appear? Like a warp drive would allow a ship to travel at sublight speeds, but actually get somewhere faster than a photon traveling through normal space by warping the space surrounding the ship. I know that 50,000 LY is a very large radius for such influence, but the SMBH is a very large mass.

    Any ideas or references?

  16. Colin

    “LENTICULAR GALAXIES are essentially ellipticals with a very thin, often almost invisible disk, which gives them the shape of a lentil”

    …um, doesn’t lenticular mean that they have the shape of a LENS?

  17. Matthew Reynolds

    Yep. Lenticular galaxies are in between ellipticals and spirals as far as shape goes, with a more prolific bulge because of the apparent lack of interstellar mass. Hence the lens shape.

  18. Tom Marking

    “Do you think it possible that the supermassive black holes at the center of galaxies are rotating in this manner and that the twisting and dragging of space on such a large scale is a possible factor in the formation of the spiral arm structures?”

    The Schwarzchild radius (i.e., radius of the event horizon) for a 1 solar mass black hole is 2.96 km. If we assume that the black hole at the center of the Milky Way galaxy has a mass of 3.7 million solar masses then its Schwarzchild radius is

    http://en.wikipedia.org/wiki/Schwarzchild_radius is 11 million km or 16 times the radius of our sun.

    For a rotating black hole there is a region outside the event horizon in which space-time is dragged around faster than the speed of light. It is called the ergosphere and is perhaps the idea you mentioned.

    http://en.wikipedia.org/wiki/Ergosphere

    The extent of the ergosphere depends on how rapidly the black hole is rotating. In all the diagrams I’ve seen it is at most a few times the size of the event horizon diameter so I find it improbable that the ergosphere could be involved in what’s happening light-years away from the black hole itself.

  19. David Blair

    “The optical lens is named after the lentil (Latin: lens), whose shape it resembles.”

    The lentil came first.

  20. Irishman

    Matthew, Colin is commenting on the quote from Tom Marking that says lenticular galaxies are shaped like a “lentil”.

    A quick online dictionary search clears this up. While lenticular does mean “shaped like a bi-convex lens” (i.e. fat in the middle and tapered at both ends), the word origin is Latin lenticularis for, get this, “shaped like a lentil”. In fact, the word lens comes from the word lentil.

    http://www.merriam-webster.com/dictionary/lens

  21. Colin

    So a lens is a great glass lentil? You learn something new…

  22. The the Milky Way has only two major spiral arms , and how tightly the arms of a spiral galaxy are wrapped around their center seems to be determined by the mass of their central black hole. The larger the black hole the tighter are the arms of the spirals. The mass of the black holes can be determined approximately by math.

  23. The the Milky Way has only two major spiral arms , and how tightly the arms of a spiral galaxy are wrapped around their center seems to be determined by the mass of their central black hole. The larger the black hole the tighter are the arms of the spirals. The mass of the black holes can be determined approximately by math.

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