Observations of Black Holes
Black Holes Today
- Black Holes are called black because they emit no light. Since space
is curved in on itself, not even light can escape. Therefore, the only
way we can detect black holes is through their effect that they have
on nearby objects.
- An important consequence of this is that we can never see a black hole,
and we must ask ourselves if they really exist at all. There are observations
coming up that might allow us to witness the effect of an event horizon. To see
these we must look into the x-rays.
- The easiest way to measure the mass of a black hole is in a binary system.
This is identical to what we do to measure planets---we measure the
velocity wobble of an orbiting star. However, as opposed to planet detection,
we do not see the black hole, but only the companion (for planets we could
only the star and not the planet). This has been done for about 20 systems
for solar-mass black holes. We have been looking for these since the
early 1980s, so it's not a very common situation since they would be
easy to find.
- Detection of supermassive black holes is trickier, since they are believed
to be in the centers of galaxies and so are further away. Also, since we
can't see as close to the black hole, we do not see any direct motion. We aren't
able to watch something orbit the black hole.
- Thus, with a binary system we get a DIRECT mass measurement. But with
supermassive black holes we must INFER the mass from some model.
- Presently, there are two exceptions for which we have a
direct measurement: the supermassive black hole in the center of our galaxy and the
one in NGC4258.
- Our black hole has been found using proper motions
of stars in the center.
The two groups are led by Andrea Ghez (UCLA) and Genzel (Germany).
This happened in the mid 1990s and it was a very difficult observation.
New observations make the case very compelling.
- We are able to see a star orbit the central black hole in our
- How do we know that this is a black hole? We measure the mass and we know
how small an area it has to fit into to. At some point the density becomes
so high that we don't know of anything else possible; i.e., we eliminate
everything else that we know about.
- The black hole in our galaxy was an incredible success and excited the
media/public. This likely created interest in wanting to know more
about black holes in other galaxies.
- The black hole searches in other galaxies began around 1980. The first
"detection" was in 1986 by two groups looking at M32, a companion to Andromeda.
John Kormendy is one of
pioneers for the search for supermassive black holes.
- The main observation that suggested a black hole was measuring the rotation
of stars near the center relative to outside.
- Not many people in the community believed it though since there were
uncertainties about the data and the analysis. These indirect measurements
had to make simplistic assumptions about the galaxy. It was possible
that there was no black hole and that the galaxy model was inadequate.
- The Hubble Space Telescope changed all this in the mid 1990s, since it
could see close into the center. Since the effect is stronger there,
the black hole model was more appealing. Very slowly, scientists began
to accept these new data and models.
- The two most common ways to measure a black hole is to use a ROTATION CURVE
for gas and a VELOCITY DISPERSION PROFILE for stars. A
rotation curve represents how fast stars
are moving in a galaxy as a function of distance from the center.
- The velocity dispersion measures the distribution of speeds of stars in a galaxy.
- For both of these techniques, we try to detect the increase in either the
dispersion or in the rotation curve, since the closer one gets to a black hole
then the greater effect it will have on the velocities.
- For some current information see
my BH webpage .
- We can't see a black hole but we can see the distribution of stars and gas
near the black hole. Here are a few images of galaxies that have black holes:
M87 - One of the first black hole detection with HST.
This measurement confirmed the value that was first suggested in 1980 (see
press release ).
Other gas disks include NGC4261 and
press release ).
NGC4374 shows the definite signature of a massive object
near the center of the galaxy (see
press release ).
Stellar determination of a black hole with HST began
in the mid 1990s. Although, there were about 4-5 black hole detection based on
previous ground-based work (see
press release 1
and press release 2 ).
Centaurus A provides one of the most spectacular displays
of activity in a galaxy center (see
press release ).
- Every galaxy with a bulge that has been looked at with HST has required
a black hole. The Messier Catalogue contains a bunch of
galaxies, most of which have black holes.
- An interesting diversion: Messier was a comet hunter in the late 1700s who compiled
a list of objects that were fuzzy but did not move, in order to distinguish
them from comets. These were intended to be the objects to avoid looking at.
Today these objects are some of the most studied since they are nearby structures
that allow astronomers detailed observations.
- We have now shifted from just trying to detect black holes to
trying to uncover their relationship with the galaxy. The way to do this
is to correlate the black hole mass with everything that we can
think of. This plot of everything versus everything
is an example of how we do this.
- I will concentrate on two panels of that previous plot and show that
here . In this plot there are multiple colors. Each
colors refer to a different type of measurement. There are six different
measurements and they all appear to follow the same trend.
- Here is the latest correlation plot.