APOD: Galaxy Group Hickson 90 (2017 May 17)

[starsurfer]

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NGC 891.
Jean-Charles Cuillandre, Hawaiian Starlight, CFHT

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NGC 5866.
NASA, ESA and the Hubble Heritage Team.

douglas wrote:
I was referring to a more general observation that ellipticals appear so devoid of features. That's a signatory aspect of them.

Indeed, ellipticals seem devoid of features, and they seem devoid of dust. But for one thing that doesn't necessarily mean that they have no dust at all, and for another, it doesn't mean that all spiral galaxies are the same, or that they have the same amount of dust.

Take a look at edge-on galaxies NGC 891 and NGC 5866. As you can see, NGC 891 has a pretty massive dust lane that appears to stretch along the full length of the visible disk. NGC 5866, by contrast, has a thin, small dust disk, much smaller in extent that the visible stellar disk. I believe that the dust disk of NGC 5866 has shrunk. Not only that, but it seems certain that galaxies in general used to contain very much more gas in the past than they do now.

All the hydrogen that the universe is ever going to get was created in the Big Bang. In the very, very early universe there were no stars at all. But then stars started to form, and very many of them were low-mass stars. Today, according to Ken Croswell and his book Planet Quest, 80% of all stars in the Milky Way are small, faint red dwarfs, and only 4% are G-type stars like the Sun. But although the red dwarfs are small and faint, they are more massive than you'd think. The typical mass of a red dwarf may be 30-50% of the mass of the Sun, but because the red dwarfs are so numerous, their combined mass is much greater than the combined mass of stars like the Sun.

The thing about red dwarfs is that they evolve so terrifically slowly that they, in effect, remove hydrogen gas from the universe and lock it up inside themselves as they form, and they don't give back appreciable amounts of this hydrogen for perhaps trillions of years. Stars like the Sun, by contrast, will give back probably at least half of their mass during their drawn-out death processes, and they are not likely to live for more than 10-12 billion years at most. This means that since most stars that in the universe are low-mass stars, star formation effectively removes large quantities of "free gas" from the universe. And as the amount of gas is continually being depleted in the universe, how can we expect the dust lanes of all galaxies to remain as thick and massive as they once were?

NGC 5866 is fast on its way to using up its gas and dust. At the same time its stellar disk is puffing up, becoming more similar to an elliptical galaxy. As spiral galaxies use up their "free gas" they seem bound to slowly evolve into more puffed-up disk galaxies with shrinking dust lanes and little star formation.

Ann

Some elliptical galaxies do have dust and other features such as tidal shells or ionized outflows.

[douglas]

• Elliptical formation probably has been found at every stage along its formative continuum
  ...but like with humanoid fossils it is hard to know exactly how to catalogue them all.

Being devoid of structural detail, with a surface brightness that must rival their dark matter halos, and a bright central region appears to be consistent in their description.

Is stare time during imaging that important?

Ann's picture of NGC 5866 has a surface brightness that approaches ellipticals'. And yes, at its furthest extent and "extents-plural" upon multiple viewings. ( )
Surely the process that gave 5866 its appearance is related to ellipticals'. Black holes are not powering visible light galactic halos to that degree of uniformity of appearance. That is simply not possible. The distances are too vast and black hole powering would be too turbulent.

[douglas]

Some elliptical galaxies do have dust and other features such as tidal shells or ionized outflows.

"The discovery of kinematic shells and a peculiar core deep within the potential well of NGC 474 is important in this context. It is very unlikely that the mass transfer of material could form shells at very small radii, and which would exist for the timescales required. Kinematically-distinct core (KDC) ellipticals have central regions that rotate rapidly, and often in the opposite direction to the stars in the outer parts of the galaxy."
pg9
"A key result from the present study of NGC 474 and NGC 7600 is that the shell surface brightness is roughly constant with radius. This is a strong argument against the Weak Interaction Model (Thomson 1991), which predicts that the surface brightness profile of the shells should follow that of the galaxy."
https://arxiv.org/abs/astro-ph/9905041

Makes you wonder if the entire elliptical galaxy brightness over radius is related to shell brightness constancy over radius. (matter rendered "luminous" in "extents" /sarc )

Now if we could find how these shells evolve over time ..

[douglas]

Ellipticals do have features:
https://www.ifa.hawaii.edu/users/barnes ... /oeg1.html

"Traditionally, elliptical galaxies were seen as rather simple systems. Their overall luminosity profiles were fit by the de Vaucouleurs (1948) law, while the cores seen in surface photometry of their central regions were frequently fit using King (1966) models. In form, these galaxies were often assumed to be oblate spheroids, flattened by rotation. The stars within them were viewed as belonging to a single, ancient population analogous to the bulge and halo populations of our galaxy; gas and dust were thought to be absent. Finally, elliptical galaxies were considered to be dynamically unevolved.

At one level or another, all of the above are incorrect."

Gotta love it: "random orbital motions" in rotationally flattened oblate spheroids.

[Chris Peterson]

Being devoid of structural detail, with a surface brightness that must rival their dark matter halos, and a bright central region appears to be consistent in their description.

What the heck does that mean?

Is stare time during imaging that important?

Exposure time? The longer it is, the better the signal-to-noise ratio.

Bear in mind that virtually no image you see of a galaxy presents a realistic picture of the brightness profile. There is almost always a wide dynamic range, with the outer sections several orders of magnitude less bright than the core. So when processing, we create a (typically) S-shaped transfer function, bringing up the brightness of dim regions and decreasing the brightness of bright regions. This allows us to see structure and detail in the entire galaxy, at the expense of flattening the brightness profile.

Look at a raw, unprocessed image of a galaxy directly on a computer screen and it's very common to see nothing at all except the central region. Everything else is black.

[douglas]

Being devoid of structural detail, with a surface brightness that must rival their dark matter halos, and a bright central region appears to be consistent in their description.

What the heck does that mean?

The extent, Chris, the "extents" to which they are lit up, "luminous", in their visible halos.

Bear in mind that virtually no image you see of a galaxy presents a realistic picture of the brightness profile. There is almost always a wide dynamic range, with the outer sections several orders of magnitude less bright than the core. So when processing, we create a (typically) S-shaped transfer function, bringing up the brightness of dim regions and decreasing the brightness of bright regions. This allows us to see structure and detail in the entire galaxy, at the expense of flattening the brightness profile.

Look at a raw, unprocessed image of a galaxy directly on a computer screen and it's very common to see nothing at all except the central region. Everything else is black.

Sounds like its become a Wild West of image presentation. I must say, also, if the S-shaped transfer is being used to that extent it would imply the central region is incredibly bright and/or the outer regions incredibly dark in these ellipticals.

Reminds of how Saturn when viewed from terrestrial backyard scopes takes on a ghostly glow due to the water in the rings.
And your dark matter, is it really there .. or does it signify a lack of input/the unknown? /bait

[Ann]

douglas wrote:
Sounds like its become a Wild West of image presentation. I must say, also, if the S-shaped transfer is being used to that extent it would imply the central region is incredibly bright and/or the outer regions incredibly dark in these ellipticals.

Click to view full size image

Galaxy M100. Photo: Jeff Bryant.

Click to view full size image

Galaxy M100. Photo: Adam Block.

Here is how it works. In Jeff Bryant's quite realistic photo, little can be seen of galaxy M100 apart from its very bright center and some very faint spiral arms. In Adam Block's image, the brightness of the center has been decreased and the brightness of the outer parts of the galaxy has been increased.

You can certainly argue that Adam Block's image is "less realistic" than Jeff Bryant's, but it does reveal many more details than Jeff Bryant's picture.

I want to extend my thanks to both photographers, Jeff Bryant and Adam Block, for highlighting important facts and details of galaxy M100, both in their own way.

Ann

[Chris Peterson]

What the heck does that mean?

The extent, Chris, the "extents" to which they are lit up, "luminous", in their visible halos.

How do you make any inference at all about the extent of any dark matter halo? That is something that can only be determined by studying the movement of stars in a galaxy.

Look at a raw, unprocessed image of a galaxy directly on a computer screen and it's very common to see nothing at all except the central region. Everything else is black.

Sounds like its become a Wild West of image presentation. I must say, also, if the S-shaped transfer is being used to that extent it would imply the central region is incredibly bright and/or the outer regions incredibly dark in these ellipticals.

We do not make astronomical images to provide "realistic" images, in the sense of showing the same thing we'd see with our eyes. We make them with the intent of extracting information that is beyond our eyes. Images are processed to make the most information available to our eyes. The same data is used to extract non-visual information, as well. Photometric data is an example of this. A brightness profile can be generated from the raw data before any processing is applied, and that information used for purposes such as estimating the amount of luminous matter in a galaxy. But published images are usually processed with the intent of showing structural detail. Do not make any assumptions about absolute brightness from looking at an image!

[douglas]

... But published images are usually processed with the intent of showing structural detail. Do not make any assumptions about absolute brightness from looking at an image!

Published images are usually processed as if captured in much closer proximity.

Absolute brightness looks to have become a playground of sorts for demonstrating differences in visible halos, right? Versus the entirely different concept of dark matter halos as demonstrating problems with modeling.

This is where the "furthest extents" mentioned came into the discussion, as you'll recall. Visible halos.

The topic of some ellipticals' cores being sped up in rotation due to mergers is an important clue. Shells are only visible after images are highly processed, too, and occur only in ellipticals, never spirals. Newer stars in shells yet all the shells are nearly the same brightness at different radii.

[Chris Peterson]

... But published images are usually processed with the intent of showing structural detail. Do not make any assumptions about absolute brightness from looking at an image!

Published images are usually processed as if captured in much closer proximity.

Only in the sense of apparent size. A galaxy would not look any different to our eyes when close than it does from far away. It would not be brighter, it would not show greater contrast.

Absolute brightness looks to have become a playground of sorts for demonstrating differences in visible halos, right? Versus the entirely different concept of dark matter halos as demonstrating problems with modeling.

The technique of extracting information from accurate measurements of intensity is called photometry. It is one of the foundational methods of observational astronomy, and is used for countless purposes.

The topic of some ellipticals' cores being sped up in rotation due to mergers is an important clue. Shells are only visible after images are highly processed, too, and occur only in ellipticals, never spirals. Newer stars in shells yet all the shells are nearly the same brightness at different radii.

Elliptical galaxies don't have rotating cores. What distinguishes an elliptical from a disk is the fact that its constituent stars have random inclinations (as with globular clusters).

[douglas]

Here is how it works.

.. You can certainly argue that Adam Block's image is "less realistic" than Jeff Bryant's, but it does reveal many more details than Jeff Bryant's picture.

I want to extend my thanks to both photographers, Jeff Bryant and Adam Block, for highlighting important facts and details of galaxy M100, both in their own way.

Ann

Details are revealed, and for that reason is worthwhile, yes. It's also good to occasionally include, if only a thumbnail, the unprocessed appearance.
A commenter had made a seemingly offhand reference to my referring to "furthest extents" being illuminated, so that prompted this needed drawdown of 'working materials'.

I say ellipticals are as worthy of study as spirals as decades-old concepts about ellipticals are falling by the wayside. Concepts like "randomly orbiting stars" appear to be more related to triaxiality and disk remnants than any true lack of galactic structure.

I wonder if Taleb has comments on 'learning curves' in Gaussian image processing? self-updating in conceptual integration? /possibly inappropriate sarc

[starsurfer]

douglas wrote:
Sounds like its become a Wild West of image presentation. I must say, also, if the S-shaped transfer is being used to that extent it would imply the central region is incredibly bright and/or the outer regions incredibly dark in these ellipticals.

Click to view full size image

Galaxy M100. Photo: Jeff Bryant.

Click to view full size image

Galaxy M100. Photo: Adam Block.

Here is how it works. In Jeff Bryant's quite realistic photo, little can be seen of galaxy M100 apart from its very bright center and some very faint spiral arms. In Adam Block's image, the brightness of the center has been decreased and the brightness of the outer parts of the galaxy has been increased.

You can certainly argue that Adam Block's image is "less realistic" than Jeff Bryant's, but it does reveal many more details than Jeff Bryant's picture.

I want to extend my thanks to both photographers, Jeff Bryant and Adam Block, for highlighting important facts and details of galaxy M100, both in their own way.

Ann

A lot of deep sky objects have a high brightness dynamic range with very bright parts and then very faint parts. To show both the bright and faint parts simultaneously, the brightness dynamic range has to be compressed unfortunately.

[douglas]

ALMA should pry even more secrets from ellipticals.

http://www.astronomy.com/news/2017/05/n ... -astronomy