APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

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APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by APOD Robot » Wed May 03, 2023 4:06 am

Image Centaurus A: A Peculiar Island of Stars

Explanation: Galaxies are fascinating. In galaxies, gravity alone holds together massive collections of stars, dust, interstellar gas, stellar remnants and dark matter. Pictured is NGC 5128, better known as Centaurus A. Cen A is the fifth brightest galaxy on the sky and is located at a distance of about 12 million light years from Earth. The warped shape of Cen A is the result of a merger between an elliptical and a spiral galaxy. Its active galactic nucleus harbors a supermassive black hole that is about 55 million times more massive than our Sun. This central black hole ejects a fast jet visible in both radio and X-ray light. Filaments of the jet are visible in red in the upper left. New observations by the Event Horizon Telescope have revealed a brightening of the jet only towards its edges -- but for reasons that are currently unknown and an active topic of research.

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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by shaileshs » Wed May 03, 2023 4:20 am

Multiple things (questions) top of my head - 1) Why jet stream is visible only on 1 side ? 2) Why jet stream is only red in color ? 3) What's the while "halo" filled all around the center ? 4) With such distorted (mixed eliptical/circular) shape, how do we even know/how can we define edge of galaxy and size of galaxy ? 5) We can see stars with spikes obviously in our own milky way and we can clearly see some various size/shaped galaxies in background but apart from those obvious ones, there's 1000s of light sources seen - bright/dim, yellow/white/blue.. how do we know (or how we can make out) how many and which ones are stars v/s galaxies ? Ufffff... I need to read a bit more on these topics.. Thanks in advance for all comments/answers.

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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by Sa Ji Tario » Wed May 03, 2023 4:42 am

shaileshs wrote: Wed May 03, 2023 4:20 am Multiple things (questions) top of my head - 1) Why jet stream is visible only on 1 side ? 2) Why jet stream is only red in color ? 3) What's the while "halo" filled all around the center ? 4) With such distorted (mixed eliptical/circular) shape, how do we even know/how can we define edge of galaxy and size of galaxy ? 5) We can see stars with spikes obviously in our own milky way and we can clearly see some various size/shaped galaxies in background but apart from those obvious ones, there's 1000s of light sources seen - bright/dim, yellow/white/blue.. how do we know (or how we can make out) how many and which ones are stars v/s galaxies ? Ufffff... I need to read a bit more on these topics.. Thanks in advance for all comments/answers.
What is detailed is from other studies and images in past decades, I personally counted 46 somewhat more distant galaxies, although there may be more

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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by AVAO » Wed May 03, 2023 5:01 am

APOD Robot wrote: Wed May 03, 2023 4:06 am Image Centaurus A: A Peculiar Island of Stars

This central black hole ejects a fast jet visible in both radio and X-ray light. Filaments of the jet are visible in red in the upper left. New observations by the Event Horizon Telescope have revealed a brightening of the jet only towards its edges -- but for reasons that are currently unknown and an active topic of research.

Today's image is a worthy APOD of very high quality and detail. I've seen many images of Cen A, but never before have I been so conscious of the shell structure of the galaxy, or perhaps better the elliptical part of the galaxy, as in this image, especially when viewed at full size. Congratulations!


Image

"This composite image shows the galaxy and the surrounding intergalactic space at several different wavelengths. The radio plasma is displayed in blue and appears to be interacting with hot X-ray emitting gas (orange) and cold neutral hydrogen (purple). Clouds emitting Halpha (red) are also shown above the main optical part of the galaxy which lies in between the two brightest radio blobs. The ‘background’ is at optical wavelengths, showing stars in our own Milky Way that are actually in the foreground. Credit: Connor Matherne, Louisiana State University (Optical/Halpha), Kraft et al. (X-ray), Struve et al. (HI), Ben McKinley, ICRAR/Curtin. (Radio)."
https://www.icrar.org/centaurus/

... I don't think the sentence "Filaments of the jet are visible in red in the upper left." is correct ...



Cen A - a galaxy with a thousand different faces.
Image
https://www.space.fm/astronomy/planetar ... #gallery-2

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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by Ann » Wed May 03, 2023 6:21 am

shaileshs wrote: Wed May 03, 2023 4:20 am Multiple things (questions) top of my head - 1) Why jet stream is visible only on 1 side ? 2) Why jet stream is only red in color ? 3) What's the while "halo" filled all around the center ? 4) With such distorted (mixed eliptical/circular) shape, how do we even know/how can we define edge of galaxy and size of galaxy ? 5) We can see stars with spikes obviously in our own milky way and we can clearly see some various size/shaped galaxies in background but apart from those obvious ones, there's 1000s of light sources seen - bright/dim, yellow/white/blue.. how do we know (or how we can make out) how many and which ones are stars v/s galaxies ? Ufffff... I need to read a bit more on these topics.. Thanks in advance for all comments/answers.

1) Why is the jet stream visible only on 1 side? I don't know, but please note that the famous giant elliptical galaxy M87 also has a jet that is visible only on one side. But a combined X-ray and radar image of M87 shows activity on both sides, although somewhat asymmetrically distributed:


2) Why is jet stream only red in color? The red color is a false or mapped color to bring out the jet and make it easier to see. You can tell that this is false color because the red hue is so saturated, so "unvarying in intensity" and so extremely sharply outlined. To be so "visually red", the only optically visible wavelengths that the jet could emit would be between 650 and 750 nm. But the jet does not behave like that. In reality, the jet is mostly visible in X-rays:


3) What's the while "halo" filled all around the center? Not exactly sure what you mean by that, but the visible whitish halo surrounding Cen A is made up of old stars that were flung out of Cen A during its ongoing merger with the small spiral galaxy that is seen as a broad dark dust band across the face of Cen A. Take a look at this black and white image of elliptical galaxy NGC 474 and its amazing shells of old stars:


4) With such distorted (mixed elliptical/circular) shape, how do we even know/how can we define edge of galaxy and size of galaxy? That's a tricky question, but there is such a thing as the "half-light radius":

COSMOS - The SAO Encyclopedia of Astronomy wrote:

Half-light Radius

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.
But when astronomers measured the full size of the halo of Andromeda, they found that it reached halfway to the Milky Way.

stsci-h-p2046b-m-1999x2000[1].jpg
Halo of Andromeda (in purple). The visible galaxy is the small white object in the center.
Credits: NASA, ESA, J. DePasquale and E. Wheatley (STScI), and Z. Levay

5) We can see stars with spikes obviously in our own Milky Way and we can clearly see some various size/shaped galaxies in background but apart from those obvious ones, there's 1000s of light sources seen - bright/dim, yellow/white/blue.. how do we know (or how we can make out) how many and which ones are stars v/s galaxies ?

All the light points with spikes are stars in our own galaxy.
Sci News wrote:

Also known as NGC 5128, the galaxy is too far away to allow astronomers to see individual stars, but star clusters can be identified as such and used as fossil evidence of the galaxy’s tumultuous evolution.
I don't agree that it is impossible for astronomers to see individual stars in Cen A. Individual young blue stars are clearly seen in the dust lane of Cen A in this Hubble photo:


And the old red and yellow stars of the elliptical component of Cen A should be visible to James Webb. But we are not going to see them as individual stars in a picture like the APOD. We might - possibly - see some globular clusters, although I wasn't able to spot any obvious globular clusters in Cen A when I was scanning the APOD.

Omega Centauri and Cen A Marco Lorenzi.png
Milky Way globular cluster Omega Centauri (bottom)
and galaxy Cen A (top). Credit: Marco Lorenzi.

There are many background stars in the field of Cen A, so some of the faint points of light that seem to lack diffraction spikes may still be Milky Way stars. But all the objects that are fuzzy and extended are likely to be background galaxies.

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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by Ann » Wed May 03, 2023 6:39 am

AVAO wrote: Wed May 03, 2023 5:01 am
APOD Robot wrote: Wed May 03, 2023 4:06 am Image Centaurus A: A Peculiar Island of Stars

This central black hole ejects a fast jet visible in both radio and X-ray light. Filaments of the jet are visible in red in the upper left. New observations by the Event Horizon Telescope have revealed a brightening of the jet only towards its edges -- but for reasons that are currently unknown and an active topic of research.

Today's image is a worthy APOD of very high quality and detail. I've seen many images of Cen A, but never before have I been so conscious of the shell structure of the galaxy, or perhaps better the elliptical part of the galaxy, as in this image, especially when viewed at full size. Congratulations!


Image

"This composite image shows the galaxy and the surrounding intergalactic space at several different wavelengths. The radio plasma is displayed in blue and appears to be interacting with hot X-ray emitting gas (orange) and cold neutral hydrogen (purple). Clouds emitting Halpha (red) are also shown above the main optical part of the galaxy which lies in between the two brightest radio blobs. The ‘background’ is at optical wavelengths, showing stars in our own Milky Way that are actually in the foreground. Credit: Connor Matherne, Louisiana State University (Optical/Halpha), Kraft et al. (X-ray), Struve et al. (HI), Ben McKinley, ICRAR/Curtin. (Radio)."
https://www.icrar.org/centaurus/

Fascinating, AVAO! Those two main radio lobes of Cen A (in cyan, on both sides of the thick dust lane) are very reminiscent of the gamma ray Fermi bubbles of the Milky Way!


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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by Ann » Wed May 03, 2023 7:18 am

shaileshs wrote: Wed May 03, 2023 4:20 am Multiple things (questions) top of my head - 1) Why jet stream is visible only on 1 side ? 2) Why jet stream is only red in color ? 3) What's the while "halo" filled all around the center ? 4) With such distorted (mixed eliptical/circular) shape, how do we even know/how can we define edge of galaxy and size of galaxy ? 5) We can see stars with spikes obviously in our own milky way and we can clearly see some various size/shaped galaxies in background but apart from those obvious ones, there's 1000s of light sources seen - bright/dim, yellow/white/blue.. how do we know (or how we can make out) how many and which ones are stars v/s galaxies ? Ufffff... I need to read a bit more on these topics.. Thanks in advance for all comments/answers.
Rather than editing my first post, I'll make a new one to post an M87 image that I didn't see before, showing two jets from M87. (Or rather, the image shows two shock waves from the two jets of M87:


And yesterday bystander posted an X-ray picture of Cen A, where it is easy to imagine that the blue lobe to the lower right of the dust lane of Cen A has been caused by a jet that is still hidden from our view.


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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by VictorBorun » Wed May 03, 2023 1:05 pm

Ann wrote: Wed May 03, 2023 7:18 am
shaileshs wrote: Wed May 03, 2023 4:20 am Multiple things (questions) top of my head - 1) Why jet stream is visible only on 1 side ? 2) Why jet stream is only red in color ? 3) What's the while "halo" filled all around the center ? 4) With such distorted (mixed eliptical/circular) shape, how do we even know/how can we define edge of galaxy and size of galaxy ? 5) We can see stars with spikes obviously in our own milky way and we can clearly see some various size/shaped galaxies in background but apart from those obvious ones, there's 1000s of light sources seen - bright/dim, yellow/white/blue.. how do we know (or how we can make out) how many and which ones are stars v/s galaxies ? Ufffff... I need to read a bit more on these topics.. Thanks in advance for all comments/answers.
Rather than editing my first post, I'll make a new one to post an M87 image that I didn't see before, showing two jets from M87. (Or rather, the image shows two shock waves from the two jets of M87: Ann
On one-tail galaxies (I only mean tails starting right at the centre of the core) we now know one explanation:
a runaway central black hole.

Consider:
1) the current central BH of Cen A weighs only 55 million Suns and thus must be an impostor after the original billionaire BH ran away
2) there was in fact a merger, maybe less than a billion years ago
3) the long red tail at 11 o'clock, however dissipated, does seem to originate rather from the centre than from an arm or dust ring

The author of this APOD image writes:
The rarely imaged long filamentary jet, extending from the core in the upper left of the picture, is the visible result of the nucleo activity and is created by the outflow of gas accumulated in the accretion disk surrounding the supermassive black hole.

But the jets must go in pairs; even when the starting parts of the jets are relativistically fast and the one that speeds toward us is brightened for us and the other one is dimmed, still the warm clouds they create in the media are not relativistically fast and look more symmetrical to us.

To make a 300 kly tail without a counter part you have to merge two billionaire BHs with co-directional spins: such merger pinches the space-time fabric, fires a one-directional gravitation wave packet of some 100 million Suns mass-energy and kicks the merged BH out of the galaxy at some 1500 km/s.

Here is a more fresh and evident (less dissipated) one-tail case:
runaway super massive central black hole.png
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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by VictorBorun » Wed May 03, 2023 2:17 pm

Centaurus A.jpg
Centaurus A-.jpg
...
Why the one red 11 o'clock tail is visually co-directional with the X-ray-bright relativistically-fast jet going toward us?
After the original billionaire BH ran away it left its accretion disk behind, and the spin of the latter must have been inherited by young 55 million Suns BH. Maybe the accretion disk's spin is co-directional with the spin of infalling matter from the core of the galaxy, be it stellar population or some dark matter flows, that gave 55 million Suns to feed the baby BH and another 55 million Suns to feed the pair of jets.
The fact is that the tail of the runaway BH is co-axial with the current pair of jets.

Therefore we know that the red tail is going toward us in 3d. It is rather 3 Mly than 300 kly. At the speed of c/200 the runaway BH must have been travelling for 600 million years (or more if we allow for some invisible tip of the tail. The runaway billionaire BH may be already halfway to the Local cluster, having covered 6 Mly of 12 Mly. The media may be thinner there and the tail of disturbed media dimmer. If it's going to curve into Milky Way, we have another billion years to prepare)
Click to view full size image 1 or image 2
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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by shaileshs » Wed May 03, 2023 2:43 pm

Thank you so much for your detailed response Ann.

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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by orin stepanek » Wed May 03, 2023 6:26 pm

NGC5128_Lorenzi_960.jpg
If a Black hole lets no mater escape' what are
the jets? Is mater escaping with the jets?
maxresdefault.jpg
Kitty looks like he's been in a fight or too; I'm looking at his broken
tooth!!!!
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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by Cousin Ricky » Wed May 03, 2023 7:42 pm

APOD Robot wrote: Wed May 03, 2023 4:06 am Cen A is the fifth brightest galaxy on the sky ...
Let’s see...
  1. Milky Way
  2. Large Magellanic Cloud
  3. Small Magellanic Cloud
  4. M31
  5. M33
  6. Centaurus A
  7. ...
Obviously, the Milky Way is not included in the description’s ranking.

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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by Cousin Ricky » Wed May 03, 2023 7:49 pm

orin stepanek wrote: Wed May 03, 2023 6:26 pm If a Black hole lets no mater escape' what are
the jets? Is mater escaping with the jets?
Aside from Hawking radiation (which is negligible), no matter or light is escaping from the black hole itself. The jets are escaping from the accretion disc surrounding the black hole.

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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by Chris Peterson » Wed May 03, 2023 8:20 pm

Cousin Ricky wrote: Wed May 03, 2023 7:49 pm
orin stepanek wrote: Wed May 03, 2023 6:26 pm If a Black hole lets no mater escape' what are
the jets? Is mater escaping with the jets?
Aside from Hawking radiation (which is negligible), no matter or light is escaping from the black hole itself. The jets are escaping from the accretion disc surrounding the black hole.
I'd phrase that a little differently. I'd say the jets are being fed by matter from the accretion disc. The disc is equatorial, and the jets are polar. They are connected, and obviously both exist outside the BH's event horizon.
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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by VictorBorun » Wed May 03, 2023 8:36 pm

Chris Peterson wrote: Wed May 03, 2023 8:20 pm
Cousin Ricky wrote: Wed May 03, 2023 7:49 pm
orin stepanek wrote: Wed May 03, 2023 6:26 pm If a Black hole lets no mater escape' what are
the jets? Is mater escaping with the jets?
Aside from Hawking radiation (which is negligible), no matter or light is escaping from the black hole itself. The jets are escaping from the accretion disc surrounding the black hole.
I'd phrase that a little differently. I'd say the jets are being fed by matter from the accretion disc. The disc is equatorial, and the jets are polar. They are connected, and obviously both exist outside the BH's event horizon.
To add still more details, the accretion disk struggles to shed its kinetic and potential energy and its spin to let the matter spiral down to the BH.
One exit channel is to warm matter by the friction between a ring and the next ring (the smaller radius the smaller orbital period, so there is in fact some friction) and then to radiate the heat in thermal photons from the surface of the accretion disk.
Another exit channel is to develop orbital electric currents and generate doughnut-shaped magnetic field with a hole around the axis of the rotation.
Yet another way is to send the quickest ions and electrons away along the magnetic lines. That would make a pair of high-speed jets along the axis of the rotation.
Note: the thermal radiation channel does nothing to shed the spin. What saves the day are the ions and electrons that spirals along the magnetic lines, trying to force them into pigtails, and thus exchange their spin with the accretion disk. In the end those ions and electrons move away from the scene and somewhat from the rotation axis, taking spin.
Last edited by VictorBorun on Thu May 04, 2023 7:59 am, edited 1 time in total.

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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by MarkE » Thu May 04, 2023 3:06 am

Is that evidence of gravitational lensing around the 6 o'Clock position near the outer edge of the white spiral?

There's a neat little row of stars, then a similar more distant looking row close by, with other nearby objects seeming to mirror each other too?

*Best seen on the full resolution image!

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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by Chris Peterson » Thu May 04, 2023 4:38 am

MarkE wrote: Thu May 04, 2023 3:06 am Is that evidence of gravitational lensing around the 6 o'Clock position near the outer edge of the white spiral?

There's a neat little row of stars, then a similar more distant looking row close by, with other nearby objects seeming to mirror each other too?

*Best seen on the full resolution image!
I don't see anything that looks like gravitational lensing (something that produces neither lines of stars nor mirrored objects).
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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by VictorBorun » Thu May 04, 2023 9:02 am

Chris Peterson wrote: Thu May 04, 2023 4:38 am
MarkE wrote: Thu May 04, 2023 3:06 am Is that evidence of gravitational lensing around the 6 o'Clock position near the outer edge of the white spiral?

There's a neat little row of stars, then a similar more distant looking row close by, with other nearby objects seeming to mirror each other too?

*Best seen on the full resolution image!
I don't see anything that looks like gravitational lensing (something that produces neither lines of stars nor mirrored objects).
MarkE may have been meaning the objects a b c d and a' b' c' d'
Centaurus A lensing.jpg
Centaurus A-.jpg
...
Click to view full size image 1 or image 2
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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by MarkE » Thu May 04, 2023 3:24 pm

There are many examples of gravitational lensing producing what can be described as 'mirrored' effects, and the 'row of stars' is probably more a line of sight effect(something i expected the reader to figure out for themselves!?) that has another almost identical row nearby, which led to my question.

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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by Chris Peterson » Thu May 04, 2023 3:30 pm

VictorBorun wrote: Thu May 04, 2023 9:02 am
Chris Peterson wrote: Thu May 04, 2023 4:38 am
MarkE wrote: Thu May 04, 2023 3:06 am Is that evidence of gravitational lensing around the 6 o'Clock position near the outer edge of the white spiral?

There's a neat little row of stars, then a similar more distant looking row close by, with other nearby objects seeming to mirror each other too?

*Best seen on the full resolution image!
I don't see anything that looks like gravitational lensing (something that produces neither lines of stars nor mirrored objects).
MarkE may have been meaning the objects a b c d and a' b' c' d'
Centaurus A lensing.jpgCentaurus A-.jpg
...
Click to view full size image 1 or image 2
Maybe. Nothing there that looks at all like gravitational lensing to me.
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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by Chris Peterson » Thu May 04, 2023 3:31 pm

MarkE wrote: Thu May 04, 2023 3:24 pm There are many examples of gravitational lensing producing what can be described as 'mirrored' effects, and the 'row of stars' is probably more a line of sight effect(something i expected the reader to figure out for themselves!?) that has another almost identical row nearby, which led to my question.
But not mirrored star patterns, nor symmetric mirrored galaxies.
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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by VictorBorun » Thu May 04, 2023 3:38 pm

Chris Peterson wrote: Thu May 04, 2023 3:31 pm
MarkE wrote: Thu May 04, 2023 3:24 pm There are many examples of gravitational lensing producing what can be described as 'mirrored' effects, and the 'row of stars' is probably more a line of sight effect(something i expected the reader to figure out for themselves!?) that has another almost identical row nearby, which led to my question.
But not mirrored star patterns, nor symmetric mirrored galaxies.
not star pattern, rather background light dots pattern (if what we are hunting is lensing by some hidden mass concentration in Cen A)
But yes, it's hard to imagine a lensing that makes these

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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by Chris Peterson » Thu May 04, 2023 3:43 pm

VictorBorun wrote: Thu May 04, 2023 3:38 pm
Chris Peterson wrote: Thu May 04, 2023 3:31 pm
MarkE wrote: Thu May 04, 2023 3:24 pm There are many examples of gravitational lensing producing what can be described as 'mirrored' effects, and the 'row of stars' is probably more a line of sight effect(something i expected the reader to figure out for themselves!?) that has another almost identical row nearby, which led to my question.
But not mirrored star patterns, nor symmetric mirrored galaxies.
not star pattern, rather background light dots pattern (if what we are hunting is lensing by some hidden mass concentration in Cen A)
But yes, it's hard to imagine a lensing that makes these
The dots look to me like stars, and the smear(s) like background spiral galaxies.
Chris

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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by VictorBorun » Thu May 04, 2023 4:26 pm

Chris Peterson wrote: Thu May 04, 2023 3:43 pm
VictorBorun wrote: Thu May 04, 2023 3:38 pm
Chris Peterson wrote: Thu May 04, 2023 3:31 pm

But not mirrored star patterns, nor symmetric mirrored galaxies.
not star pattern, rather background light dots pattern (if what we are hunting is lensing by some hidden mass concentration in Cen A)
But yes, it's hard to imagine a lensing that makes these
The dots look to me like stars, and the smear(s) like background spiral galaxies.
but background dwarf galaxies with bright cores can look like dull distant stars of Milky Way, can not they

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Re: APOD: Centaurus A: A Peculiar Island of Stars (2023 May 03)

Post by Chris Peterson » Thu May 04, 2023 4:33 pm

VictorBorun wrote: Thu May 04, 2023 4:26 pm
Chris Peterson wrote: Thu May 04, 2023 3:43 pm
VictorBorun wrote: Thu May 04, 2023 3:38 pm

not star pattern, rather background light dots pattern (if what we are hunting is lensing by some hidden mass concentration in Cen A)
But yes, it's hard to imagine a lensing that makes these
The dots look to me like stars, and the smear(s) like background spiral galaxies.
but background dwarf galaxies with bright cores can look like dull distant stars of Milky Way, can not they
Visually, maybe. They are easy to distinguish if we're really interested, however. All you need to do is run a star extractor app on the image (better if it's not a JPEG, but even that usually works). Stars all have similar PSFs, quite different from galaxies or any other extended objects. So you get out a list of stars and a list of nonstellar objects. If you're looking for lensed structures, you'd typically be looking for nonstellar objects clustered around a fairly nearby galaxy cluster. Then you'd want spectroscopic data to determine if those objects all had the same redshift.
Chris

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Chris L Peterson
Cloudbait Observatory
https://www.cloudbait.com