by Ann » Thu Mar 17, 2022 6:16 am
AVAO wrote: ↑Thu Mar 17, 2022 5:38 am
Can someone explain to me the conventions used to calculate the size of a galaxy? In purely geometric terms, this is clear to me if the distance and angle are known. But how do you define beginning and end? There where the last visible bands of dust can be seen? What about the halo, jets, IR tails, and radio bubbles? What factors determines the size of a galaxy and on which wavelength?
Very good question. You don't ask me anything that has anything to do with math, but I can offer a few thoughts. Consider the Andromeda Galaxy.
Take a look at the Andromeda Galaxy in infrared and ultraviolet light. You can sort of see the edge, can't you?
Yes, but that seeming visual edge is not the end of Andromeda, because our large sister galaxy has a huge halo:
The huge halo surrounding Andromeda. Image credit: NASA.
So where is the edge of Andromeda? You tell me!
Okay, edit:
Swinburne University wrote:
The sizes of galaxies are difficult to measure since they don’t possess clearly defined boundaries. Most galaxies simply get fainter and fainter in their outer regions, and the apparent size of the galaxy depends almost entirely on the sensitivity of the telescope used and the length of time for which the object is observed.
To overcome this ambiguity, astronomers define the ‘half-light’, or ‘effective’ radius (re) as the radius within which half of the galaxy’s luminosity is contained.
Of course, with the humongous halo of Andromeda, the "half-light radius" should perhaps be a giant sphere much farther out than the apparent visible disk of the galaxy?
Ann
[quote=AVAO post_id=321397 time=1647495520 user_id=144694]
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Spanning over 60,000 light-years, .....
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Can someone explain to me the conventions used to calculate the size of a galaxy? In purely geometric terms, this is clear to me if the distance and angle are known. But how do you define beginning and end? There where the last visible bands of dust can be seen? What about the halo, jets, IR tails, and radio bubbles? What factors determines the size of a galaxy and on which wavelength?
[img]https://youtu.be/bOjCrVQusYI[/img]
[/quote]
Very good question. You don't ask me anything that has anything to do with math, but I can offer a few thoughts. Consider the Andromeda Galaxy.
[float=left][img3="The Andromeda Galaxy in infrared light by Spitzer Space Telescope. Image credit: NASA/JPL-Caltech/UCLA"]https://upload.wikimedia.org/wikipedia/commons/thumb/0/0e/WISE-_Andromeda.jpg/1024px-WISE-_Andromeda.jpg[/img3][/float][float=right][img3="The Andromeda Galaxy in ultraviolet light by GALEX. Image credit: NASA/JPL-Caltech"]https://upload.wikimedia.org/wikipedia/commons/thumb/0/0d/Andromeda_galaxy_2.jpg/1280px-Andromeda_galaxy_2.jpg[/img3][/float]
[clear][/clear]
Take a look at the Andromeda Galaxy in infrared and ultraviolet light. You can sort of see the edge, can't you?
Yes, but that seeming visual edge is not the end of Andromeda, because our large sister galaxy has a huge halo:
[float=left][attachment=1]stsci-h-p2046b-m-1999x2000[1].jpg[/attachment][c][size=85][color=#0040FF]The huge halo surrounding Andromeda. Image credit: NASA.[/color][/size][/c][/float][float=right][img3="This illustration shows the location of the 43 quasars scientists used to probe Andromeda’s gaseous halo. These quasars—the very distant, brilliant cores of active galaxies powered by black holes—are scattered far behind the halo, allowing scientists to probe multiple regions. Looking through the immense halo at the quasars’ light, the team observed how this light is absorbed by the halo and how that absorption changes in different regions. By tracing the absorption of light coming from the background quasars, scientists are able to probe the halo’s material.
Credits: NASA, ESA, and E. Wheatley (STScI)"]https://www.nasa.gov/sites/default/files/styles/full_width/public/thumbnails/image/stsci-h-p2046a-f-3000x2400.jpg?itok=SYErXSGJ[/img3][/float]
[clear][/clear]
So where is the edge of Andromeda? You tell me!
Okay, edit:
[float=right][img3="The half-light (or ‘effective’) radius is the radius from within which half of the galaxy light is contained."]https://astronomy.swin.edu.au/cms/cpg15x/albums/userpics/half-lightradius1.jpg[/img3][/float][quote][url=https://astronomy.swin.edu.au/cosmos/h/Half-light+Radius]Swinburne University[/url] wrote:
The sizes of galaxies are difficult to measure since they don’t possess clearly defined boundaries. Most galaxies simply get fainter and fainter in their outer regions, and the apparent size of the galaxy depends almost entirely on the sensitivity of the telescope used and the length of time for which the object is observed.
To overcome this ambiguity, astronomers define the ‘half-light’, or ‘effective’ radius (re) as the radius within which half of the galaxy’s luminosity is contained.[/quote]
[attachment=0]Possible half light radius of Andromeda NASA.png[/attachment]
[clear][/clear]
Of course, with the humongous halo of Andromeda, the "half-light radius" should perhaps be a giant sphere much farther out than the apparent visible disk of the galaxy?
Ann