Thanks again, Mark. Fascinating topic, to be sure. Imaging the ICL, among the glare of the galaxies and foreground stars, is another wonderful example of human ingenuity.
I was thinking that it's reasonable to expect that some black holes can have highly elliptical orbits, similar to a comet in our solar system, having been slung during a galactic merger or during the AGN stage of formation. Would LIGO detect one of these, or would we have to use gravitational lensing techniques, or other means? Is the expectation reasonable, in 'light' of the 23% figure and the merger simulations?
- edit - For what period of time does a black hole 'ring'?
ChipperQ:
If you mean black holes which orbit the galactic centre of mass in a heighly elliptic orbit the answer is no. A black hole in such an orbit will send out GWs but they will be much to weak, and of too long wavelength, for LIGO and any other concieved GW detector.
However, lensing and e.g. gamma radiation from infalling matter would still be available for detection.
Is the 23% dark matter a general distribution, or is it concentrated in bands or rings? Have we cataloged any visible stars with highly elliptical orbits?
Is the 23% dark matter a general distribution, or is it concentrated in bands or rings?
According to what I have read the non-normal matter is expected to form an elliptical halo around isolated galaxies. This based on need to provide a mass distribution which expalins the observed velocity distribution of the component stars. I haven’t found any articles which describe the CDM distribution for galactic clusters or super clusters.
Quote:
Have we cataloged any visible stars with highly elliptical orbits?
Not that I have heard of , they probably do exist. In order to characterize an orbit you must know 3D velocity and position at a minimum of two points; assuming you know the gravity field. Normally you would want many such observation points. We know the 3D velocity/position history of comparatively few stars and no galaxies out side the local cluster. For all celestial objects we can get good values for the angular position on the celestial spherer. For very close stars we can get all six valuses. For less close stars we can get the radial velocity from their spectrum and we can sometimes estimate their radial position but know little about their non radial velocity. The same thing applies to nearby galaxies for more distant galaxies we can estimate their radial distance.
For the stars in distant galaxies we only know the median radial velocity distribution across the face of the galaxy and can estimate their radial distance. Knowing this distribution we can make statistical estimates of the 3D velocity and distance and work backwards to discribe the gravational field.
I recently learned that objects on the celestial sphere can even appear to move faster than light, being due to the object having a motion towards the observer as well as to the side. The travel time of the light must also be included in the calculations – Wikipedia has a good page on it: Superluminal motion
After following some of the links on that page, I just learned that the branch of Astronomy concerned with measuring positions and motions of celestial bodies is known as Astrometry. Unfortunately, it seems that only one probe was dedicated to the task, and thanks to the extra effort of the ESA after a booster rocket failure, data was collected from '89-'93, in an all-sky survey on a million stars to visual magnitude 11, and the data is now in “The Millennium Star Atlas”. The database can be queried online from the Hipparcos/Tycho catalogue home-page but the values that the user must enter to define the query are idiosyncratic, and the output is (if I'm not mistaken) tabular.
Frankly, after viewing the E@H Starsphere screensaver, tables seem a bit inadequate. It would be nice to be able to stop the rotation of the sphere (without halting the crunching, of course), and then roll the mouse-pointer over an object to see astrometrics (and history) of the object – the screensaver could provide the parameters and query the H/T catalogue automatically. Is anyone aware of anything like this already on the web?
Thanks, klasm. I see Gaia is scheduled for launch in '11. Additionally, NASA has the SIM PlanetQuest also scheduled to go up in '11. I didn't realize the magnitude of the astrometric 'task' I was asking about: SIM will need a reference frame whose precision must be commensurate with the precision of the instruments, and will thus construct an Atrometric Grid. Impressive work.
Since a star with more mass lives a shorter life, how does this carry over on a galactic scale? Perhaps the galaxies that are formed when the universe was less than 1 billion years old contain more massive stars, on average, and so can appear more mature?
Thanks again, Mark.
)
Thanks again, Mark. Fascinating topic, to be sure. Imaging the ICL, among the glare of the galaxies and foreground stars, is another wonderful example of human ingenuity.
I was thinking that it's reasonable to expect that some black holes can have highly elliptical orbits, similar to a comet in our solar system, having been slung during a galactic merger or during the AGN stage of formation. Would LIGO detect one of these, or would we have to use gravitational lensing techniques, or other means? Is the expectation reasonable, in 'light' of the 23% figure and the merger simulations?
- edit - For what period of time does a black hole 'ring'?
ChipperQ: If you mean black
)
ChipperQ:
If you mean black holes which orbit the galactic centre of mass in a heighly elliptic orbit the answer is no. A black hole in such an orbit will send out GWs but they will be much to weak, and of too long wavelength, for LIGO and any other concieved GW detector.
However, lensing and e.g. gamma radiation from infalling matter would still be available for detection.
[edit] Removed double posting
)
[edit]
Removed double posting of the previous post
Is the 23% dark matter a
)
Is the 23% dark matter a general distribution, or is it concentrated in bands or rings? Have we cataloged any visible stars with highly elliptical orbits?
RE: Is the 23% dark matter
)
According to what I have read the non-normal matter is expected to form an elliptical halo around isolated galaxies. This based on need to provide a mass distribution which expalins the observed velocity distribution of the component stars. I haven’t found any articles which describe the CDM distribution for galactic clusters or super clusters.
Not that I have heard of , they probably do exist. In order to characterize an orbit you must know 3D velocity and position at a minimum of two points; assuming you know the gravity field. Normally you would want many such observation points. We know the 3D velocity/position history of comparatively few stars and no galaxies out side the local cluster. For all celestial objects we can get good values for the angular position on the celestial spherer. For very close stars we can get all six valuses. For less close stars we can get the radial velocity from their spectrum and we can sometimes estimate their radial position but know little about their non radial velocity. The same thing applies to nearby galaxies for more distant galaxies we can estimate their radial distance.
For the stars in distant galaxies we only know the median radial velocity distribution across the face of the galaxy and can estimate their radial distance. Knowing this distribution we can make statistical estimates of the 3D velocity and distance and work backwards to discribe the gravational field.
I recently learned that
)
I recently learned that objects on the celestial sphere can even appear to move faster than light, being due to the object having a motion towards the observer as well as to the side. The travel time of the light must also be included in the calculations – Wikipedia has a good page on it: Superluminal motion
After following some of the links on that page, I just learned that the branch of Astronomy concerned with measuring positions and motions of celestial bodies is known as Astrometry. Unfortunately, it seems that only one probe was dedicated to the task, and thanks to the extra effort of the ESA after a booster rocket failure, data was collected from '89-'93, in an all-sky survey on a million stars to visual magnitude 11, and the data is now in “The Millennium Star Atlas”. The database can be queried online from the Hipparcos/Tycho catalogue home-page but the values that the user must enter to define the query are idiosyncratic, and the output is (if I'm not mistaken) tabular.
Frankly, after viewing the E@H Starsphere screensaver, tables seem a bit inadequate. It would be nice to be able to stop the rotation of the sphere (without halting the crunching, of course), and then roll the mouse-pointer over an object to see astrometrics (and history) of the object – the screensaver could provide the parameters and query the H/T catalogue automatically. Is anyone aware of anything like this already on the web?
ESA is preparing a follow-up
)
ESA is preparing a follow-up mission called Gaia which will proivde much more detailed data, and positions for many more stars.
Gaia
Thanks, klasm. I see Gaia is
)
Thanks, klasm. I see Gaia is scheduled for launch in '11. Additionally, NASA has the SIM PlanetQuest also scheduled to go up in '11. I didn't realize the magnitude of the astrometric 'task' I was asking about: SIM will need a reference frame whose precision must be commensurate with the precision of the instruments, and will thus construct an Atrometric Grid. Impressive work.
Spitzer & Hubble something
)
Spitzer & Hubble
something new about the early universe.
RE: Spitzer &
)
Since a star with more mass lives a shorter life, how does this carry over on a galactic scale? Perhaps the galaxies that are formed when the universe was less than 1 billion years old contain more massive stars, on average, and so can appear more mature?