Thank you kindly for providing me with such thorough and concise answers to my questions. In fact, all of a sudden, I’m beginning to feel like a genuine astrophysicist.;)

Best,
Cynthia

The particular nature of the correlation is that the mass, M_BH, appears to be proportional to the dispersion velocity of stars in a galaxy’s central bulge, sigma.

That should be “appears to be proportional to a power of the dispersion velocity of stars …”. Big difference! The power is something like sigma^4 or sigma^5; there’s a bit of back and forth on this in the literature, with much of the sparring going on between the two groups who originally announced this correlation (Ferrarese & Merritt on the one hand [cf. ApJ 2000, v539, L9] and Gebhardt et al on the other [cf. ApJ 2000, v539, L13]).

All this is theory. No one has gotten up close and personal to one of these objects to experience one in practice. The difference between theory and practice is that, in theory, they’re the same. And yet, no one doubts Gravity. And the Theory of Gravity (called General Relativity) is on solid ground, so to speak.

Question#1: Is there a particular period in the Universe’s evolution where dormant varieties are more prevalent over active varieties, including vice versa?

Dormant black holes appear to be the norm now. It had been thought for years that quiet galaxies may be “quasar fossils”, and so would contain dormant black holes at their cores. The problem is that resolving the region over which the black hole has a strong influence is extremely difficult. Solid evidence only really began to pile up over the last decade, thanks largely to the advent of improved ground-based observations (e.g., Ghez’s group at UCLA, which takes advantage of adaptive optics technology at the Keck telescopes) and to observations with the Hubble Space Telescope. The rule of thumb appears to be that there is a dormant black hole at the core of every galaxy that has a central bulge.

Active galaxies and quasars are rarer, but become less rare as we look back in cosmic time. This actually leads us nicely to your second question …

Question#2: Are there certain conditions, either intrinsically or extrinsically, that act to transform galactic cores into either active ones or dormant ones?

One hypothesis is that quasars are activated by mergers. Let me back up: Simulations and observations now strongly suggest that structure in the universe is built hierarchically, from the repeated merger of smaller substructures. (Risa Wechsler over at cosmicvariance has done a lot of work on this topic.) Such a merger can bring a great deal of gas into a galaxy. If at least one of the galaxies hosts a black hole, then the gas (among other things) is likely to accrete onto the black hole, fueling both black hole growth and quasar emission. (If both galaxies host black holes, then you may form a binary black hole system, something near and dear to my heart …)

Mergers were a lot more common when the universe was younger and denser; this may be a partial explanation of why quasars were more common at higher redshift. The transition to dormant state appears to occur when the system runs out of fuel — all the gas has been fed to the black hole, and it no longer accretes at the high rates needed to power quasar emission.

Question#3; Do certain types of galaxies (for instance, spiral vs. elliptical vs. irregular) tend to have active cores while others types tend to have dormant cores?

That’s a good question, and I’m not so sure of the answer. Let me blather a little bit on a related issue, which may shed a bit of light; hopefully someone wiser can help out.

One thing that observations have established in recent years is that, at least in the nearby universe, the mass of black holes is correlated quite tightly with the properties of the galaxy in which it is embedded. This may not be terribly surprising, but the accuracy of the correlation suggests that in fact something deep is probably at work.

The particular nature of the correlation is that the mass, M_BH, appears to be proportional to the dispersion velocity of stars in a galaxy’s central bulge, sigma. The velocity dispersion tells us about the distribution of kinetic energy for stars in the bulge; since the stars are gravitationally bound, it therefore also tells us a lot about the bulge’s gravitational potential. What’s particularly compelling about this correlation to me is that the velocity dispersion is measured in regions where the central black hole’s influence is negligible. In other words, when we measure this velocity dispersion, we have no a priori reason to expect it to “know” anything about the black hole — the black hole is thousands of parsecs away, all the other matter between it and the stars there is far more important. And yet we find that it is this quantity that correlates to black hole masses!

So, what’s going on? This relationship seems to be telling us that the growth of the central bulge of galaxies and of the black hole at its center are very closely related to one another. I don’t think anyone really understands in detail how they are related, but the fact that there is a relationship does not seem to be in doubt. One compelling hypothesis is that a merger fuels a quasar or other black hole activity, and also sparks an episode of star formation. But the radiation from the quasar tends to “feedback” on the bulge, disrupting star formation close by, and preventing too many stars from forming. In this way galactic activity regulates bulge growth.

The way that this connects to your question is that elliptical galaxies appear to grow from smaller galaxies through mergers — each merger essentially makes the spheroidal component of disk galaxies larger and larger until eventually just the spheroid remains. Disk galaxies with small bulges have the least massive black holes (e.g., Milky Way, M_BH ~ a few million Msun); disk galaxies with larger bulges have more massive black holes (e.g., NGC4258, M_BH ~ a few tens of million Msun); massive ellipticals (essentially a giant bulge!) have the most massive black holes (e.g., M87, M_BH ~ a few billion Msun).

The way I’ve failed to answer your question is that I’m not up on the observational evidence well enough to concretely address what you asked. I know a lot of active galaxies are in disk galaxies; I’m not sure about the statistics, though.

Question#4: Are there any stars at risk for being consumed by SagittariusA*? Or is SagittariusA* (the Milky Way’s center black hole) too tame to partake in a feeding frenzy?

Hard to say. If you look at the link to Ghez’s group, you’ll see that there are a lot of stars down pretty close to the black hole. However, none of these stars are close enough that they are likely to be captured (unless there’s some dynamical interaction that seriously affects one of their orbits). It’s an open question whether there are even more stars down closer, though, which might be captured and disrupted in the future. It’s hard to tell because of the difficulty resolving things at that scale.

But it would really suck to be a lifeform on a planet that was part of a solar system being pulled, stretched, twisted, torn, ripped, and essentially “disappeared” by events such as these. Would we see it coming if we were proximal to the one in the Milky Way???

The regions near the supermassive black holes we see in galactic centers are rather dense — lots of stars, lots of gas, lots of highly energetic processes going on. Such black holes tend to sink to the bottom of the galactic potential well, so more or less by definition they are right in the guts of the beast. (More accurately, the galaxy and the black hole tend to coevolve with each other, so the picture of “big heavy black hole in the densest region” develops as galaxies and black holes grow.) For example, the stellar density in the region of the Milky Way center is orders of magnitude higher than it is in our neck of the woods. Andrea Ghez at UCLA has an excellent webpage describing her group’s work on this neighborhood:

http://www.astro.ucla.edu/~ghezgroup/gc/

My best guess (and I emphasize that this is a guess!) is that any life in a solar system attached to such a star would be pretty thoroughly cooked by all the activity going on in the vicinity of the black hole. Given all the stuff going on there, it’s just not a very hospitible region for life as we know it. So for events like this one observed by Gevari et al, chances are that we would in fact see it coming, and would be thoroughly toasted well before the tides ripped us up.

While dark matter seems to be the glue that holds galaxies together, supermassive black holes appear to serve as engines to fuel galaxies: engines where, strangely enough, life and death intertwine, figuratively speaking of course.;)

Several questions come to mind regarding this issue of active versus dormant black holes at the center of galaxies…

Question#1: Is there a particular period in the Universe’s evolution where dormant varieties are more prevalent over active varieties, including vice versa?

Question#2: Are there certain conditions, either intrinsically or extrinsically, that act to transform galactic cores into either active ones or dormant ones?

Question#3; Do certain types of galaxies (for instance, spiral vs. elliptical vs. irregular) tend to have active cores while others types tend to have dormant cores?

Question#4: Are there any stars at risk for being consumed by SagittariusA*? Or is SagittariusA* (the Milky Way’s center black hole) too tame to partake in a feeding frenzy?

-cvj

As for what it would look like, it depends on how it’s being illuminated. You don’t really see the hole itself, but radiation from things happening nearby. In this case, that radiation undergoes a characteristic evolution as the captured star is squashed and stripped; it is the measurement of this evolution that makes Gevari et al’s study so compelling.

If you don’t have something bright and dynamic lighting up the hole, you may be able to detect the shadow cast by the horizon on background radiation. A non-rotating black hole will make a circular dark spot with a radius 3/2 Sqrt(3) times the horizon radius; the numerical factor is just related to the impact parameter at which photons tend to be captured (as opposed to just scattered) by the hole. The same idea applies to rotating holes (expected to be the usual case — all astrophysical objects have angular momentum), but their shadows have a more complicated, lopsided shape. There are some plans to try to image the galactic center black hole in the next decade or so using very long baseline interferometry. Heino Falcke at the Max Planck for Radio Astronomy has a nice online slideshow about the galactic center black hole that talks about this idea:

http://www.mpifr-bonn.mpg.de/staff/hfalcke/bh/sld1.html

The stuff on imaging the horizon’s shadow starts around slide 10.