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Michael Noonan
2008-Oct-14, 11:06 AM
What would the rate of change of the size of the super massive black hole at the center of our galaxy need to be for astronomers to measure it?

I am assuming thousands of solar masses would be insignificant but would hundreds of thousands of solar masses rate of change show up after a few years?

Disclaimer ... it is a proof of concept question but I do not want to discuss the concept, thank you.

antoniseb
2008-Oct-14, 11:41 AM
What would the rate of change of the size of the super massive black hole at the center of our galaxy need to be for astronomers to measure it?...

Our most accurate information about the mass of Sgr A* comes from observing the orbits of stars around the object. If the mass were changing even one percent a year (30,000 solar masses), we'd have observed it by now.

StupendousMan
2008-Oct-14, 01:04 PM
Our most accurate information about the mass of Sgr A* comes from observing the orbits of stars around the object. If the mass were changing even one percent a year (30,000 solar masses), we'd have observed it by now.

Hmmm. I'm not so sure about that. Yesterday in journal club, we discussed a paper which appeared on astro-ph: "Kinematics of the old stellar population at the Galactic Center", by Trippe et al.; see

http://xxx.lanl.gov/abs/0810.1040

The paper uses the kinematic properties of over 5000 stars, most with only proper motions, but a subset of about 650 with full 3-D motions, to estimate the mass of the black hole at the center of the Milky Way. The authors find a distance to the center of the Milky Way which agrees nicely with other determinations, 8.07 +/- 0.32 (stat) +/- 0.13 (sys) kpc, but they explain that it is difficult to extract masses from their data. There is a complex interplay between the properties of a central population of stars, the mass of a compact object at the center of the cluster, and the spatial resolution of the measurements. They conclude that their measurements may be consistent with other estimates of the mass of the black hole at the center of the Milky Way (about 4 million solar masses), but suggest that systematic uncertainties of a factor of 2 or so remain.

It's not clear to me that we could detect a one percent change in the mass of the black hole at Sgr A* .... Could you explain what sort of measurements we could make to do so well?

antoniseb
2008-Oct-14, 01:24 PM
It's not clear to me that we could detect a one percent change in the mass of the black hole at Sgr A* .... Could you explain what sort of measurements we could make to do so well?

No I can't, and that is one of the dangers of posting gut impressions. I was going on the basis of the motion of the closest observed star, which I think we've observed for most of one orbit now. That one star's motion is smooth and elliptical enough that clearly the mass hasn't changed by the factor of two you cite above. While the actual mass may have considerable uncertainty, it isn't changing sizeably, based on that one orbit... As to percentage detectability, that was a wild-guess based on impressions, not my own calculations.

Hornblower
2008-Oct-14, 04:15 PM
What does any of this have to do with stability?

Theophage
2008-Oct-14, 05:11 PM
I'm assuming by "stable" the OP was asking if it's mass was stable, in other words, not growing by devouring our galaxy. From what I understand, the central black hole of a galaxy helps the galaxy to form, but when it clears out the area directly around it of matter, it no longer feeds and thus stays a stable size.

tonybaloney
2008-Oct-14, 05:58 PM
Sorry for my ignorance, but in thinking about the stable black hole topic, I was wondering - in time will all matter in the universe eventually be in the form of black holes as it is swallowed up (stars, planets, etc)? Or, on the contrary, will all black holes eventually evaporate due to Hawking Radiation? Or will it be somewhere in between where they are constantly being fed and evaporated?

Thanks in advance.

Hornblower
2008-Oct-14, 08:32 PM
If the expansion continues unabated, the cosmos eventually would become cool and rarified enough for Hawking radiation to evaporate even the biggest black holes. That would be a very long time, one in which a billion years would be like the blink of an eye by comparison.

My source was Sky and Telescope back in the late '70s or early '80s, not long after Hawking published his theoretical findings on this topic.

Hornblower
2008-Oct-14, 08:34 PM
I'm assuming by "stable" the OP was asking if it's mass was stable, in other words, not growing by devouring our galaxy. From what I understand, the central black hole of a galaxy helps the galaxy to form, but when it clears out the area directly around it of matter, it no longer feeds and thus stays a stable size.
OK, he used "stable" where I would have said "constant". I understand the question now.

phunk
2008-Oct-15, 01:11 AM
Our most accurate information about the mass of Sgr A* comes from observing the orbits of stars around the object. If the mass were changing even one percent a year (30,000 solar masses), we'd have observed it by now.

If it were ever growing 0.1% per year, it would be an incredibly bright quasar.

Michael Noonan
2008-Oct-15, 05:25 AM
Thank you for your replies.

I had taken a couple of weeks break from the forums and was growing a beard. Trouble was the beard looked untidy and so the boss told me to shave it which meant writing to you good scientific folk again. I appreciate the attention the question received.

Knowing that some stars have a variable output brought to mind what would it take for a black hole to exhibit variable mass in relation to perhaps dark energy and how would it be recognized. I was not sure if this question had been asked before and the responses have made interesting reading.

antoniseb
2008-Oct-15, 01:16 PM
If it were ever growing 0.1% per year, it would be an incredibly bright quasar.

Yes, though I had read MN's original question as more isolated. i.e. do we know that the mass of SMBHs don't just change for no obvious reason? I'm a strong believer in the quasar model you reference, but I didn't think the original post was about that.

parejkoj
2008-Oct-15, 03:05 PM
For more on the growth-rate of Sgr A*, see the links in my post here (http://www.bautforum.com/questions-answers/79305-milky-way-quasar.html#post1337200).

As far as how well we can measure the mass of Sgr A*, even the latest measurements by Ghez et al. (http://adsabs.harvard.edu/abs/2008IAUS..248...52G) (that one's quite new!) and Genzel et al. (http://www.mpe.mpg.de/ir/GC/index.php) (sadly, they haven't kept their website up-to-date, and I didn't find their latest paper on ADS - sorry!) of the nearest few stars have errors of ~10%.

astromark
2008-Oct-15, 06:22 PM
Some how the growth of your beard, seems a little bit more information than I would want... and I too have been absent from these pages for a week as my computer had failed to proceed. Fortunately your shaving habits and my computer are all at a state of normality again... whats this got to do with this... I have no idea. Is it the fact that information is sparse and vaige...
By observation we have calculated orbital velocities, mass and distances. This information is imprecise and only with advancements and time will come a more accurate picture of the reality at the hart of Sag.,A.
From the information so far I would judge the core of this Galaxy as stable.
:)The human race will be a distant memory and the hart of Sag.A will look unchanged. Forever is a very long time.

Michael Noonan
2008-Oct-18, 01:54 PM
Fair enough. One reason among others I wont go into is that I find the black hole at the center of this galaxy to be quite unremarkable. Sure some two or three million solar masses is big but not in relation to the size of the Milky Way Galaxy which comes in at some 400 billion solar masses.

It seems a bit trivial and the beard appealed to my sense of humor, cheers.