NGC 5128 - disks & multiple mergers - Ongoing galaxy formation!

[madbadgalaxyman]

This is my recent post copied from a thread in the IIS Imaging Forum, as I am starting a science thread about the structure and evolution of NGC 5128, which is more affectionately known as Centaurus A. (what? emotion in a science thread?? )

(the imaging thread from which this is copied is Centaurus A in UV by sjastro)
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The three-dimensional morphology and evolution of the approximately disk-shaped "Extreme Population I" component of NGC 5128 (young stars + molecular hydrogen gas + dust) that manifests in our images as the dust lane, is a problem that has had a few different solutions in the professional literature. The most recent attempt was probably Quillen et al., 2010, PASA, Vol.27, p.396 (in Proceedings of the Astronomical Soc. of Australia)
Here is the preprint of this paper......the preprint reference is arxiv: 0912.0632 (you have to pay for the final version!)

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They have analysed a lot of the multi-wavelength imaging data about this galaxy and they fit a simplified model (or visualization) of the three-dimensional structure of the disk to these observations;
But they give their model in exclusively numerical terms, without a 3-D visualization of the structure that they have come up with. I may try to make a three-dimensional image of their model of the dusty disk galaxy that bisects NGC 5128, but I would have to find some tools to do so; typically, these 'tilted ring' models are usually made and displayed with the tools of radio astronomers.

The following near-infrared image from 2MASS (a composite of J and H and K bands) may be helpful, as it simplifies the structure of the dust lane by only showing the parts with the highest extinction:

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Here is the inner disk of NGC 5128, in the central region of this galaxy, as imaged at 8 microns by the Spitzer Space Telescope: (8 microns picks up primarily dust emission, whereas 3 microns detects primarily stellar photospheres)

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The disk is not planar, with a complex and warped/bendy structure.....this is a very complicated galaxy!

In the following image :
(1) The left panel shows a map of the two-dimensional distribution of the J-H colour (an infrared colour index) of this galaxy over the central parts of NGC 5128. The parts of the dust lane with the heaviest dust extinction are displayed as the lightest regions on this map. (This infrared colour map greatly resembles a negative of the dust lane. )
(2) The right panel shows a near-infrared (J band) image of the dust lane, displayed as a positive image from the 2MASS survey. (being infrared, it shows only the heaviest dust extinction). On this image are overlaid the contours (isophotes) of the previously displayed 8 micron image from the Spitzer Telescope.

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The above two images are from: 2006, ApJ, 645, 1092 (which is freely accessible on the internet)

( I am going to try to overlay this infrared disk with an image of the dust lane in the optical/UV regime, so you can get a better idea of where the infrared disk is located in this galaxy; I think there is a Herschel Space Observatory image that 'ties it all together' )

At present, the disk component that slices through the short axis of this galaxy is an unpopular subject for research; a search of the literature for recent papers about this galaxy using the SAO/NASA Astrophysics Data System reveals that since 2007, as regards NGC5128, professionals have written only one or two papers about the disk component (dust lane) but about 40 (!!!) papers about the orbital structures and kinematics and ages and metallicities of the objects in the spheroidal component, mainly studies of the very many globular clusters and planetary nebulae that belong to this galaxy ( these papers have the still unrealised goal of characterizing the time-evolution of the spheroidal component.....the data archives contain massive data about galaxies, but nobody in the professional community has been smart enough to use it to produce a generally accepted theory of how galaxies evolve!! ).

Here is a section of a still useful (but somewhat out of date) comprehensive overview about NGC 5128, which came out in 1998 (F.P. Israel , Astronomy & Astrophysics Review, 1998, Vol.8, p.237)

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(this is in MS Word 2000 .doc format)

The rest of this paper can be found at;
http://ned.ipac.caltech.edu/level5/March01/Israel/Israel_contents.html
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In the overall context of humankind's attempts to understand how galaxies evolve, this sort of observation and study of an individual case of galaxy merger and accretion can shed a light on how galaxies evolve and build up mass; NGC 5128 has previously undergone at least one major merger (between two large galaxies) in addition to its recent "eating" of the smaller galaxy that produced the dust lane. There is also evidence that the big elliptical has swallowed a few other small galaxies!
In the 'hierarchical model' of galaxy formation, galaxy formation is ongoing in today's universe, as smaller galaxies continue to merge with larger galaxies, while in another recently popular model of galaxy formation, galaxies continue to build up their masses (especially the outermost part of the disk component) by accretion of gas from the intergalactic medium;
we could be seeing both the "merger" model of galaxy evolution and the "gas accretion" model of galaxy evolution happening in NGC 5128, because the outermost portions of this galaxy (the 'ansae" at the two extreme ends of the long axis of this galaxy, as seen in Mr Sidonio's ultra-deep image) show evidence of disky kinematics (orbits in the same plane, rather than in many different planes);
there seems to be a disk forming in the very outer regions, which means that NGC5128 might even evolve into a spiral galaxy!

Here is a map showing the noticeably-elongated Disky distribution of (spectroscopically identified using OIII) planetary nebulae in the outer parts of NGC 5128, overlaid on a 2 by 2 degree DSS image. This image is taken from Peng, Ford, and Freeman (2004, ApJ, 602, 685 )

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Is this a giant disk structure that is currently in the process of formation in the outer regions of NGC5128?
(it would require infall of gaseous material for a 'traditional' gas+stars+spiral Disk Structure to form, but many giant elliptical galaxies already have a kinematically cold disk-like component in their outermost regions)
The planetary nebulae have substantial orbital velocities, with line-of-sight velocities that can depart from that of NGC 5128 itself by over 100 km/s.(the rotational component is significant in comparison to the random velocity component)

The current "mild two-component S0 morphology" of NGC 5128 may be evolving towards a true Disk+bulge galaxy (becoming, eventually, a spiral galaxy or an S0 galaxy);
Peng & Ford & Freeman characterise the velocities in the outermost regions as "fast disk-like rotation along the major axis" of this galaxy, together with significant rotation along the minor axis as well. The strong rotation of the outer parts of this galaxy continues out to a radius of at least 50 kpc! ( 163,000 light years)

cheers
bad galaxy man

[madbadgalaxyman]

Low contrast structures in the halo of NGC 5128, using an adaptive histogram equalization filter applied to a B+V+R image, as described in Peng et al., 2002, Astronomical Journal, 124, 3144 ( http://iopscience.iop.org/1538-3881/124/6/3144/fulltext )

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Note the shell structures (typical of ellipticals which have merged with a smaller galaxy), and also note some regions with blue knots (recent star formation) which are believed to be young stars formed from the interaction of the plasma jet with the interstellar gas in this galaxy.

The blue knots of stars formed by jet/ISM interactions have been seen in some IIS members' images of this galaxy.

Centaurus A is a very complex and intriguing critter.......
The recent literature about this galaxy is so copious that reading it is the mental equivalent of "drinking from a firehose".

[Weltevreden SA]

Really great stuff, Robert. Solid research, interesting facts, lucid & concise writing. I learned a lot off this thread. I sure wish you could ghost-write some of professional papers. Thanks . . . =Dana

[madbadgalaxyman]

The recent Imaging Forum thread by sjastro (= Steven), about his UV observations of this galaxy, contains far-ultraviolet and H-alpha images of this galaxy; these images are superb tracers of the two-dimensional distribution of OB stars in our images of NGC 5128.

As mentioned there, the Herschel Space Observatory far-infrared images are also uniquely informative about the true thee-dimensional structure of the dust lane; the dust lane does not absorb light at FIR wavelengths; it emits brightly!!

Thanks for the good review! A technical writer can never be 100 percent sure if the knowledge in his mind is clearly conveyed to his intended readership, as there is a subjective element in writing, in addition to its logical structure.

Actually, you do a very good job, yourself, in explaining things, Dana! (e.g. your Sextans A post)

Now I must get back to studying some of my soil samples!! Oddly, the 'galaxyman' also looks downwards and studies dirt!!
(just for fun, I work with a soil scientist on invertebrates and fungi)

Best regards,
Robert Lang

I agree with your comment about scientific papers....most of them are an absolute mess, as regards the clarity of the written communication.

"A good technical writer makes difficult things easy to understand. A poor technical writer makes simple ideas seem very difficult" - R.L.

[xelasnave]

Thank you I never miss your posts.alex

[madbadgalaxyman]

(( The above post was transferred from the IIS imaging forum, as we were discussing the effects of the dust lane in Centaurus A, as seen at various different wavelengths ))

Hello Steven,

This is a fairly basic discussion, but I do try to bear in mind that our IIS readership is composed of people with various amounts of astronomical knowledge.

The terminology regarding the dimming and reddening (due to the wavelength-dependent amount of extinction) of electromagnetic radiation as it passes through the interstellar medium is not always accurately or consistently used in the astronomical literature. In general, there is direct absorption of light by dust grains, as well as scattering of the light by the dust grains.

A word in common use for the total effect of absorption and scattering, which expresses the fact that the interstellar dust dims the light of an object, is extinction.

[[[
But Alessandro Boselli, in his recent (2012) textbook "A Panchromatic View of Galaxies", uses the non-standard word attenuation instead of the word extinction. This is a very accurate descriptor, though it will be a hard one to use for "vocabulary-poor people who can't spell"(an ever-increasing fraction of the population!)
]]]

Bruce Draine, in his book I mention below, presents some detailed physics (several chapters!) for the interstellar dust grains; there is nothing simple about trying to model the nature (size ; shape ; composition ; evolution) of the grains themselves, though their effects on electromagnetic radiation are well known.

Here are three quotations (edited for clarity) about interstellar dust, taken from some of my favourite reference handbooks about galaxies and astrophysics:

(1) "Normal spiral galaxies have dust masses in the order of 1 million to 100 million solar masses, with ~3 percent in PAHs, this fraction being slightly smaller in low-metallicity galaxies. The dust mass represents ~0.5 percent of the gaseous phase. Normal galaxies have a dust temperature of ~15-25 Kelvin in the diffuse Interstellar Medium.
(this quotation is from "A Panchromatic view of Galaxies"(2012) by Alessandro Boselli)

(the dust is warmer near to HII regions)
(Other authors give 1 percent of the ISM mass of a non-dwarf galaxy as being in the form of dust. But small galaxies without enough gravity to hold on to the products of mass ejection from stars can contain very little dust.)
(cold as it is, the interstellar dust emits very brightly at Far-infrared to sub-millimetre wavelengths)

(2) Interstellar grain material appears to be quite varied. Large organic molecules, and graphite and silicate grains are prominent constituents. The graphite grains appear to originate in the dense atmospheres of carbon stars. Water and other volatiles freeze out on these grains in the cold interior of dense clouds, where the grains are shielded from heating by starlight.
[ this quotation is from "Astrophysical Concepts (4th edition)"(2006) by Martin Harwit ]

(3) Based on observations of ultraviolet extinction, scattering of visible light, and polarization of starlight, it is clear that the interstellar grain population must have a broad size distribution, extending from radii as small as about 0.01 micrometers (or less) to radii of about 0.2 micrometers.
(this quotation is from "Physics of the Interstellar and Intergalactic Medium"(2011) by Bruce T. Draine)

[[ extinction of visible light is due to dust grains which are about 0.1 micron in radius ]]


Characteristically opinionated commentary.......

Comment No. 1
Draine says that some of the dust grains might be as small as 50 atoms! Maybe Fred Hoyle wasn't as 'cracked' as other astronomers thought he was when he posited the existence of interstellar viruses?

Reading Draine's book (he is a well-known modeller of the dust) gives me the impression that there is no 'typical' or 'characteristic' size for the interstellar dust particles, but much of the total mass of the dust in a galaxy will be in the larger grains (0.1 microns in diameter, and greater).
According to one popular model, the grains become progressively more numerous with progressively decreasing grain radius.

Comment no. 2:

Figuring out the nature of the interstellar dust grains is very much a 'black box' problem;
the 'box' of matter we are trying to look into is at a vast distance (x light years) from the observer and contains tiny particles no bigger than 1/3000 millimeter. So astronomers will never actually see these interstellar particles in a microscope or telescope or any other type of scope! (unless we can build a telescope that is much bigger than the solar system.....)
Therefore, humans can only observe the effects of this dust on the light that reaches it or the light that originates from it. This implies a significant probability that current ideas about the nature of the interstellar dust could be substantially wrong! So astronomers who talk about the composition and size-distribution of the Interstellar Dust very prudently talk about models rather than actual confirmed facts.

Comment No. 3 :

My "lady friend" (call her Virginia, for argument's sake) was very dismissive and sarcastic when I just told her I was writing about dust in outer space......

"Studying dust in outer space? Why not study the dust in the carpet?"
- Virginia's response on hearing that I am studying
the dust between the stars

[sjastro]

We have absorption, absorption/emission of photons and scattering. The higher the photon energy the larger the contribution from the absorption/emission mechanism. Conversely low energy photons (IR and lower) do not participate in absorption/emission.
In the UV range what is the primary mechanism for extinction?
My understanding is that scattering is the primary mechanism.

[[[


Very interesting Robert.




The important point here is whether these theories are falsifiable or in layman's terms can either be supported or proven wrong by experimental data or observation.
Some aspects of cosmology such as multiverse theory fall in the not falsifiable category and hover between science and philosophy.



A subtle reference to the dereliction of your household duties? ;)

You are a great source of information.

Regards

Steven

[Weltevreden SA]

Hey, Robert, I'm a soils scientist, too! Every time my wife sends me out to weed the garden.

Luckily, the nights are mine.

[Weltevreden SA]

Hi again, Robert. Your post on dust grains raised a question and a thought.

The question: Re 'According to one popular model, the grains become progressively more numerous with progressively decreasing grain radius,' I wonder if that would be a power-law relationship vis-a-vis linear or log. Power law relationships describe some pretty fundamental properties, e.g. initial mass function, luminosity function, etc. Thoughts?

Another question: Re your comment, 'astronomers who talk about the composition and size-distribution of the Interstellar Dust very prudently talk about models rather than actual confirmed facts.' I read so many papers based on modeling that I wonder where the boundary between 'prudent' and 'over-reaching' lies. I notice that reliance on models rather than data crunching seems to be a feature of younger teams (or at least lead authors representing their consensus), who have come into the field more familiar with data mining and algorithms than observational assessment. This is only my hunch, based on the names I do NOT see on many of the model-heavy papers—van den Bergh, Tolstoy, Elmgreen, your own Ken Freeman, Dan Bates, Krumholz. I really begin to wonder when I see authors citing mainly their own earlier papers.

Where do you draw the credibility line between solid and stretching as you wade through papers?

[madbadgalaxyman]

When it comes to the source of the information I supply in these fora,
one important source is my large personal library of books (paper-based, not electronic) on topics relevant to galaxies (observational techniques, star formation, ISM, galactic dynamics, galaxy morphology, stellar evolution, stellar populations, etc), and I make a habit of collecting astrononomy books which are likely to be useful for reference;
this costly paper-based 'intranet' contains a lot more astronomy books than I strictly need, but it gives me a "quick and dirty" answer to most every question about astronomy......

In many circumstances, this can be a more efficient and detailed method of information gathering than the internet.
[ Most 'explanations' on the internet seem very shallow, with the exception of the primary literature (papers) ]

I am still cogitating in respect of UV scattering.

I have a nice quotation from Draine about the observables that constrain theories about dust grains, which I shall shortly submit in answer to Dana's query about how theoretical is the field of dust-grain modelling.

cheers,
Robert

In general, I mainly tend to have a very practical need for quick 'rules of thumb' to estimate the values of extinction along various sight-lines 'drawn' in our own Galaxy and in other galaxies; but your comments about extinction are having the effect of making me look deeper at the issues!

[Weltevreden SA]

Hi Robert . . . I fear we are getting off-topic on NGC 5128 as the discussion deepens re. extinction. I'd like to start a new thread on it. Would you mind re-posting your material on the 5128 thread to the new one? I have a feeling this is a topic more IIS'ers would like to read about and add to. Cheers, =Dana

[Ric]

Hi guys, I'd just like to say thanks for this thread and others as well.

Even though it a bit out of my league to add to the comments, I am enjoying the read and definitely learning from it.

Cheers :thumbsup: