APOD: Centaurus A (2022 Mar 17)

[APOD Robot]

Centaurus A

Explanation: A mere 11 million light-years away, Centaurus A is the closest active galaxy to planet Earth. Spanning over 60,000 light-years, the peculiar elliptical galaxy also known as NGC 5128, is featured in this sharp telescopic view. Centaurus A is apparently the result of a collision of two otherwise normal galaxies resulting in a fantastic jumble of star clusters and imposing dark dust lanes. Near the galaxy's center, leftover cosmic debris is steadily being consumed by a central black hole with a billion times the mass of the Sun. As in other active galaxies, that process likely generates the enormous radio, X-ray, and gamma-ray energy radiated by Centaurus A.

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[AVAO]

Centaurus A
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?

https://www.nasa.gov/topics/universe/fe ... -jets.html
"At its heart lies a black hole with a mass of 55 million suns."

This all makes me a bit confused...

[Ann]

Multi-wavelength image of Centaurus A.
X-ray: NASA/CXC/SAO; optical: Rolf Olsen; infrared: NASA/JPL-Caltech;
radio: NRAO/AUI/NSF/Univ.Hertfordshire/M.Hardcastle

Click to view full size image

What can you say about such a titanic collision but Blam, Poof, Owtch?

Ann

[shaileshs]

I like the APOD from 10th Jan 2008 (https://apod.nasa.gov/apod/ap080110.html) because it shows all 3 individual and composite. Seems Ann has given same image above. I guess back then we thought the black hole at center was only 10 million times Sun mass, now we feel it's billion times, who knows in 10 more years, we may have discovered something even different .. No one can predict/know how many new things we'll discover.. (some of which maybe completely contradictory to original thinking..)..

[Ann]

Centaurus A
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?

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.

The Andromeda Galaxy in infrared light by Spitzer Space Telescope. Image credit: NASA/JPL-Caltech/UCLA

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The Andromeda Galaxy in ultraviolet light by GALEX. Image credit: NASA/JPL-Caltech

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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.

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)

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So where is the edge of Andromeda? You tell me!

Okay, edit:

The half-light (or ‘effective’) radius is the radius from within which half of the galaxy light is contained.

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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

[AVAO]

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.

If the size depends on the luminosity, does that mean the galaxy gets bigger when the quasar at its core shines right in my face? (And does the galaxy start to wobble when its luminosity changes periodically due to the rotation of the quasar at its core ;-? Seems a bit difficult to define...

[Ann]

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.

If the size depends on the luminosity, does that mean the galaxy gets bigger when the quasar at its core shines right in my face? (And does the galaxy start to wobble when its luminosity changes periodically due to the rotation of the quasar at its core ;-? Seems a bit difficult to define...

I'd say it is the luminosity that depends on the size. Well, mostly.

Check out the (unfortunately not all that informative) Wikipedia page about giant low-luminosity galaxy Malin 1. Its humongous set of spiral arms is very faint, but because it is so ginormous, its total luminosity is really quite high - except that
this light is so spread out that it is extremely hard to detect.

Ann

[Tszabeau]

Who’s blob is that anyway? Is there any way to tell if the dark blob below Centaurus A and to the right of the larger blue spikey star which I assume is in the Milky Way… is also in the Milky Way or does it belong to Cent. A? And… if it is part of the Milky Way… is it perhaps a piece of sweet sweet nugget attached by a an invisible strand of buttery rich caramel?

[orin stepanek]

Dirty old Galaxy; hasn't been dusted in years!


from Wikipedia;

https://en.wikipedia.org/wiki/Andromeda_Galaxy

https://en.wikipedia.org/wiki/Andromeda_Galaxy
Isn't Andromeda an active Galaxy? And 2.5 M light years is a lot
closer than 11 M light years? How is active galaxy defined? Does it
depend on how much stars it eats?
I've heard that the Milky Way and Andromeda are already interacting !
can that be the case?
https://www.youtube.com/watch?v=OO0pkPPocvw

[Chris Peterson]

Centaurus A
Spanning over 60,000 light-years, .....

<< Previous APOD

This Day in APOD

Next APOD >>

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?

There is no convention. Or perhaps it would be better to say there are different ones, depending on context. At the simplest (for example, the size reported in a lot of catalogs) it's just an optical metric, like where the intensity has dropped to some fraction of the total. In some cases, it makes more sense to consider the dark matter halo. Or the region where you still have significant material that is gravitationally bound. If you're looking across a broad range of wavelengths, it might be where you still see significant signal.

In short, it's best not to worry too much about any absolute definition, and just consider what we're seeing in context.

[AVAO]

In short, it's best not to worry too much about any absolute definition, and just consider what we're seeing in context.

I think you're absolutely right Chris. thanks for your explanations, and also to Ann.

Jac

[MoreInput]

* The black hole does not have a block hole with a billion times the mass of the sun, it is just 55 million sun masses. (https://arxiv.org/abs/0812.1000).

If you want an introduction to the structure of Centaurus A I suggest looking an my video:
** https://de.wikipedia.org/wiki/Datei:Centaurus_A_EN.webm
** https://de.wikipedia.org/wiki/Datei:Centaurus_A_DE.webm

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