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Old 05-07-2011, 10:46 AM
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AGCS 805 Observed 02072011

Hi All,

It has been a little while since I posted here but most of the (relatively) few observations I've made in the last 6 months (due to cruddy weather) have been done for the purposes of publication elsewhere so I've had to withhold them.

Last Saturday night was a goodish night at the Southern Highlands site we use near Bargo. Not particularly dark but the seeing was pretty good -- I'd rate it 7/10 with a nice crisp image of Saturn earlier in the evening at x247. The SQM-L reading around midnight was a bit better than 21.0. I observed a whole heap of galaxies in Pavo and then Tucana later in the evening after some high cloud stopped us from serious observing until about 9pm. I won't bore you with all of them but I though I'd post here one observation of an object I know is a favourite of SAB (PGC) -- he made a post about it here a couple of years ago now -- AGCS 805. I enjoyed this cluster of galaxies for about 25 minutes and decided to write an entry about it for the Sydney Observatory blog that I hope will be published in the next few days.

Here is a copy of the blog note I have submitted that some might find interesting and below that are the observations:

Big picture part of an even bigger picture

Galaxies, like stars, have a tendency to live in clusters. I came upon this cluster of galaxies —Abell Galaxy Cluster South (AGCS) 805 while observing last weekend in the Southern Highlands. As I viewed it for almost a half-hour and identified nine of its galaxies, I was struck not only by their beauty, but also by how they fitted into the larger picture of the Universe.

We see AGCS 805 through a scattering of stars that make up the constellation of Pavo (The Peacock) in our southern sky. These stars (like the Sun) are in our Milky Way Galaxy and “only” 10s to 100s of light years away — one light-year is about 9.5 trillion kilometres. AGCS 805 includes at least eight major galaxies and probably several dozen dwarf galaxies. In this flat “2-D” photograph, all the galaxies seem to be equally distant from us. Most here are true members of AGCS 805 about 220 million light-years away. However a few are several tens of millions of light-years in the foreground while a few others are about twice as far away in the background. The brightest galaxy in the centre of the image is IC 4765 — a 12th magnitude supergiant elliptical galaxy that gravitationally dominates the lesser cluster members.

Adding an extra dimension to this big picture, because AGCS 805 is about 220 million light-years distant, we cannot see these galaxies in “real time”. We can only see them as they were 220 million years ago because it takes that long for their light to reach our telescopes. The light from these galaxies we see now left them about the same time the first true dinosaurs roamed the Earth in the mid-Triassic period.

The image is 1/3 of a degree across or about 2/3rds the apparent diameter of the Moon in our sky. It is also the same size as the view in a 9mm eyepiece in my 18” Newtonian at x247 magnification. At 220 million light-years distance, this image (and my eyepiece field) measures about 1.3 million light-years across.

AGCS 805 is a massive cluster of dozens of galaxies containing trillions of stars (so vast and remote as to be beyond meaningful human comprehension). However, it is just one component of a vastly bigger panorama — in the same way a masonry block is part of a wall or building. AGCS 805 is part of the gargantuan Pavo-Indus super-cluster of galaxies including more than a dozen other similar galaxy clusters — a cluster of galaxy clusters. The Pavo-Indus super-cluster contains well over a thousand galaxies. These super-clusters, chains and walls of galaxies that can stretch up to 500 million light-years are the largest structures in the visible Universe.

* * *

Here is a link to an image:

http://stdatu.stsci.edu/cgi-bin/dss_...e&fov=NONE&v3=


Below are the observations 12mm TII Nagler at x185 and 9mm Nagler at x247

IC 4765 Galaxy *
RA: 18h 47m 17.8s Dec: -63 19' 52"
Mag: 12.3 (P) S.B.: --- B-V: --- Size: 3.4'x1.8' Class: E+4
P.A.: 115 Inclination: --- R.V.: +4669 Source: RC3 *

ESO 104-7 PGC 62408 Galaxy *
RA: 18h 47m 18.3s Dec: -63 21' 32"
Mag: 12.9 (V) S.B.: --- B-V: +1.05 Size: 1.6'x0.8' Class: E?
P.A.: 97 Inclination: --- R.V.: +3899 Source: RC3 *

PGC 62391 Galaxy *
RA: 18h 46m 51.6s Dec: -63 18' 51"
Mag: --- S.B.: --- B-V: --- Size: 0.4'x0.3' Class:
P.A.: 39 Inclination: --- R.V.: +10795 Source: PGC *

ESO 104-2 PGC 62393 Galaxy *
RA: 18h 46m 53.9s Dec: -63 21' 39"
Mag: 14.1 (V) S.B.: --- B-V: +1.01 Size: 1.0'x0.3' Class: S0?
P.A.: 30 Inclination: --- R.V.: +4055 Source: RC3 *

ESO 104-8 PGC 62412 Galaxy *
RA: 18h 47m 23.1s Dec: -63 18' 35"
Mag: 15.6 S.B.: --- B-V: --- Size: 0.9'x0.4' Class: L
P.A.: 153 Inclination: --- R.V.: +4075 Source: PGC *

IC 4766 Galaxy *
RA: 18h 47m 35.6s Dec: -63 17' 30"
Mag: 13.8 (V) S.B.: --- B-V: +1.04 Size: 1.1'x0.4' Class: SA(r)0+
P.A.: 112 Inclination: --- R.V.: +4998 Source: RC3 *

IC 4770 Galaxy *
RA: 18h 48m 10.4s Dec: -63 23' 01"
Mag: 15.4 (P) S.B.: --- B-V: --- Size: 0.8'x0.5' Class: (R)SAB(rs)a:
P.A.: 65 Inclination: --- R.V.: +5040 Source: RC3 *

IC 4767 Galaxy *
RA: 18h 47m 41.8s Dec: -63 24' 20"
Mag: 14.3 (B) S.B.: 13.6 B-V: +0.93 Size: 1.4'x0.4' Class: S0+: pec sp
P.A.: 30 Inclination: --- R.V.: +3600 Source: RC3 *

IC 4764 Galaxy *
RA: 18h 47m 07.9s Dec: -63 29' 05"
Mag: 14.7 (P) S.B.: --- B-V: --- Size: 1.2'x0.3' Class: S?
P.A.: 128 Inclination: --- R.V.: +3900 Source: RC3 *

These members plus others are part of AGCS 805. The brightest eg and presumed to be the gravitationally dominant cluster member is IC 4765 at centre which is also somewhat the brightest. -65 is a moderately bright eg 1.5 to perhaps approaching 2' diameter, round with weak edges but growing weakly to centre and then within the central 1' broadly and slightly to centre without nucleus.

ESO 104-7 is about 2' S of -65 not quite at a 10th mag * that is directly further S. Small, perhaps 30" diameter of goodish SB and rising moderately to centre.

PGC 62391 is 3' WNW of -65 and is about 20-30" diameter spot of goodish SB, comparable to 104-7 above but perhaps not quite as good.

ESO 104-2 is again similar but perhaps the faintest of these inner central 4 eg. The 4 make a rough square about 3' a side.

ESO 104-8 is 2.5' N of IC 4765 as a small faint spot about 20" diameter brightening a little to centre and

IC 4766 is 3.5' NNE looks similar but a little bigger and brighter and oval in PA 45, 30 x 20".

IC 4767 is an elong or edge on eg of very low SB, very diffuse slash in about PA 45, 1.5' x 10" of consistent very LSB. Diffuse edged. It is 5’ SSE of IC 4765.

IC 4764 is 8' S of -65 and looks similar to -67 save that it is in PA 135 and a fraction smaller -- perhaps 1.25'long.

IC 4770 is 6' SE of -65 and presents as a 40" dia round patch of very weak gossamer with no central brightening. Very, very faint and of very LSB.


Best,

Les D

It's now on the blog:
http://www.sydneyobservatory.com.au/...Observatory%29

Last edited by ngcles; 05-07-2011 at 02:09 PM.
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Old 05-07-2011, 02:06 PM
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kustard (Simon)
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I was wondering what had you enthralled and now I know!

Got to get me a bigger bucket so I can enjoy that group
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Old 06-07-2011, 10:47 AM
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Great post Les. Plenty of interesting information, description and an image to boot. Have to put this on the list.
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Old 08-07-2011, 10:04 AM
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The Pavo-Indus-Telescopium supergalactic structure seems to be well characterized in a sky map found at:
//www.atlasoftheuniverse.com/superc/pavind.htm
The major concentrations of galaxies (clusters, groups) in this Supercluster and in the Centaurus Supercluster are well shown at this website.

I don't know who is responsible for this website, but on the whole it seems to be accurate.

An excellent book that characterizes the supergalactic structures (superclusters, voids, walls, shells, bubbles, etc), a book that can be understood by anyone with a really good background in astronomy, is:
"Large-Scale Structures in the Universe"
by Anthony Fairall (published by Wiley-Praxis)
This book is a model of clarity, and the maps and descriptions really give the reader a feel for how the largest scale structures in the universe actually look.

The author of this book, Anthony P. Fairall (1943-2008), usually known as Tony Fairall, was one of the great authorities on redshift surveys and on the largest scale structures in the southern hemisphere skies. He died not long ago in a tragic accident, which deprived the world not only of a first-rate scientist but also of one of the great gentlemen of science; he was always quick to assist me if I needed information, and (which is rare for a practicing researcher) he had a parallel
career as a planetarium director and talented popularizer of astronomy.

It was entirely characteristic of Fairall that he would create a website making the abstract findings of the 6dF Redshift Survey accessible to others....

Go to :
http://mensa.ast.uct.ac.za/6df-survey
and then click on "redshift shells" to download a .ppt presentation on the sky distribution of galaxies on the largest scales, grouped according to specific shells of redshift.
(choose the option "2MRS features enabled")

The maps in this powerpoint presentation show us what are the major features in the universe, with one sky map per each range of Galaxy Distance.

Last edited by madbadgalaxyman; 08-07-2011 at 01:44 PM. Reason: need to add more information
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Old 08-07-2011, 09:52 PM
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IC 4765 in the cluster Abell S805 (alias LGG 422 group)

The dwarf galaxy population of Abell S805 (alias the IC 4765 cluster)(alias the IC 4765 group) was studied in:
Carrasco et al., (2006), AJ, 132, 1796.

They note that this cluster is also known as Pavo II and LGG 422. Of these two alternative identifications, the LGG catalog of nearby galaxy groups and clusters is still very much in use;
indeed the LGG is, in fact, a sort of de facto standard for naming groups of galaxies.
(the LGG tidied up the "dog's breakfast" of competing catalogues of nearby groups of galaxies, moreover it is not one of those many existing catalogues of galaxy groups in which the mean group size was pared down to two or three galaxies just to make sure that the groups can be analyzed properly for statistical purposes)

Thus, the LGG designation for this cluster should have about the same priority as the Abell designation.

The central galaxy, IC 4765, was observed by Loubser et al., (2008), MNRAS, 391, 1009 .
They were primarily interested in the central stellar velocities and the central Velocity Dispersion of this galaxy.
This was one of the first times anybody has studied this prominent galaxy in any level of detail!

IC 4765 is normally classified as a cD galaxy (an elliptical galaxy with a very extended and extremely faint halo), and it has a blue absolute magnitude of about -22 which is very typical for these objects that usually sit near the centres of galaxy clusters.
However, I doubt that the morphology and radial surface brightness falloff of the outer regions of IC 4765 have ever been studied in any detail, so this classification is only approximate.

cheers,
mad galaxy man
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Old 09-07-2011, 03:46 PM
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Thanks !

Hi Robert & All,

Thank you for doing a bit of research there to find that small amount of info. I had a very quick poke through SIMBAD (have I mentioned before how much I hate the SIMBAD interface?) and found very little like you.

It was one of those interesting moments when I went to observe IC 4567 (not the central galaxy but a spiral near the southern edge), I picked up the name off my list of things to do, entered it in the Argo, hit the slew button and took a look in the eyepiece. I wasn't expecting to see a galaxy cluster! I hadn't looked at the map before-hand. As it happened the centre of the cluster was not far inside the field-stop and I saw 5 galaxies immediately and said to myself "holy-mackerel!" -- or words to that effect anyway. Took a look at the map and then found I had a whole cluster to see. It took 25-odd minutes to tease out and positively ID all nine eg.

The comment I made on the number of dwarfs persent is a reasonable assumption -- there is no proof but dwarves outnumber large eg more than 5/1 generally so that would mean about 40-odd dwarves that are too small or too faint to show even on the DSS -- maybe even double that number.

About half-way through observing, I just began to wonder how big (in light-years my eyepiece field was at the cluster distance? I didn't want to get too carried away calculating it but after a couple of minutes of mental maths I came up with a figure approx 1.4 million ly for my 9mm. It turns out to be 1.28million ly actually with a proper calculation (assuming the average recessional velocity ~ 4800km/sec) accurately reflects actual distance.

That's a biiiiiig picture!


Best,

Les D

Last edited by ngcles; 09-07-2011 at 04:11 PM.
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Old 09-07-2011, 08:26 PM
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Ic 4767

One galaxy in this cluster that has had some research done on it, over the years, is the edge-on large-bulged S0 galaxy IC 4767

This is one of the prototypes of a galaxy that has a very large Boxy and x-shaped bulge. The bulge component of this galaxy appears very rectangular when displayed at some scales:

Click image for larger version

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This logarithmic scale image (and the isophotes on the right) are from Whitmore and Bell's paper on this galaxy (1988, ApJ, 324, 741)

Large bulges that are boxy and which also show an X-shape are variously ascribed to the presence of a bar structure that is seen edge-on and/or to the accretion of a smaller companion galaxy.

Here are the NIR isophotes of IC 4767 from a very recent study by M.J. Williams et al., (see this preprint:
arxiv 1102.2438 .....which can be found at //arxiv.org )(accepted for publication in 2011 MNRAS)

Click image for larger version

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(the coloured lines have nothing at all to do with the galaxy; they mark slit positions)

These bulges are also referred to in the literature as "peanut-shaped" and "boxy/peanut shaped".


I have been doing a bit of an informal study of these edge-on S0 galaxies that have giant boxy bulges, and I find that they are very common in some galaxy cluster environments (accretion origin?!?). I also find that there is a connection between the presence of a very large boxy bulge and the presence of a strong warp or tilt (bending away from the plane) in the equatorial dust lane.

A somewhat larger example of this type of galaxy is NGC 6771.
Here is an image of NGC 6771:

Click image for larger version

Name:	N6771_U+B+V = blue+green+red_(w.VLT)_(PR Photo 12-04).jpg
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Old 11-07-2011, 12:02 PM
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Very interesting Rob. I've often been curious about why some galactic bulges are like this. And I love the word isophote. What a great sound!

Thanks for the posts
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Old 11-07-2011, 11:15 PM
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Bulges - classical, "pseudo", and boxy

You are right, Paddy, bulges that look rather like boxes or peanuts are hard to explain.

Professional astronomers often loosely talk of bulges and of elliptical galaxies as being "spheroids" due to fact that, technically, many of them have the geometrical shape of a spheroid.
But Even within the category "spheroid" there are various types of spheroid: e.g. an oblate spheroid (hamburger bun shape) and a prolate spheroid (hot dog bun shape) and a triaxial ellipsoid.
(as an example, some elliptical galaxies are oblate and some elliptical galaxies are prolate)

But, just to make things more complicated, many of the objects that we call bulges are not actually spheroids at all......

In the days of the old photographic galaxy atlases, such as the Hubble Atlas of Galaxies and the Carnegie Atlas of Galaxies, atlases which formed our basic conceptions (and prejudices) about galaxian morphology till about 1980, it was assumed that any discrete large-spatial-scale component, surrounding the centre of a galaxy, that shows a very strong radial brightening towards the centre .......is spheroidal. This is why - for historical reasons - we call all of these features bulges.

However, it has since become apparent that not all bulges are spheroidal!!

The disk (planar) component within a galaxy, in those cases where a galaxy does actually contain a disk component, has an observed surface brightness that tends to rise fairly gently towards the centre of a galaxy. Superposed on this disk component is excess light from a "bulge" component; which generally rises very rapidly towards the galaxian centre.

Anyhow, things have got ever more complicated in bulge studies, and what we observe as a bright central bulge component can have at least three different explanations:

(1) a "classical" bulge, which is genuinely much "fatter" than the typically very thin disk component of a galaxy, and which is spheroidal in shape. These structures tend to be quite massive and in general they contain a lot of very old red stars.
At one time, this was thought to be the only type of bulge that exists.

(2) A pseudo-bulge : when we look at the images of spiral galaxies and we observe a rapid central brightening due to what we usually call a bulge component, a brightening which looks either circular or oval to the eye or camera (it looks this way from our line-of-sight), it is not required that the object we are viewing in 2 dimensions is actually a spheroid in actual (real) three dimensional space.
In fact, a bright central structure which is flat, or fairly flat, can also give the impression of a central bulge.
These relatively flat central objects found in galaxies are therefore called pseudo-bulges (pseudobulges).
While a photograph may show something that looks like it could be spheroidal, the actual shape of the apparent bulge can be relatively flattened and disk-like. These pseudo-bulges are common in galaxies of type Sbc and later (in the Hubble sequence), and they often contain young stars due to ongoing bulge formation at the centre of a galaxy.

(3) Boxy and Peanut bulges:
Some bulges don't even look spheroidal to the eye ; they are noticeably boxy or peanut-shaped. The smaller examples of these boxy/peanut bulges are very likely to be bar structures that are seen edge-on. The bigger examples are not very well understood - they seem to have remarkably complex structures, in some cases.

I would like to include some actual examples of these various bulge types, but this contribution is already too long!

Some of the Really Big boxy bulges do look very odd, relative to the examples of bulges that astronomers used to use as "baseline truth", indeed these bulges have had little detailed study in the literature. I will eventually attach a few examples of giant boxy bulges, as they seem to me show evidence of galaxy interactions.
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Old 15-07-2011, 08:15 AM
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Well its about time I'm able to read a report from you Les without having to pay $$$ for it.

But I beat you to it, I've already read it on the Sydney Observatory Site. Clearly, I'm still stalking you well!

A very interesting read (as usual), thank you.
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Old 15-07-2011, 12:35 PM
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Bulges - some examples of the various types

Hi suzy,

How do bulges form and evolve?
Sometimes they don't form....as in the case of a "pure disk" galaxy that has only a planar component; such as NGC 5907 (see image later on).

I will try to give some pictorial examples of the various types of bulges, to aid in understanding my previous post about bulge types.

(I should be the expert on this topic, as I have a bulge forming right here.....caused by sitting slumped in front of the computer, day and night.
Ha Ha.)

An example of a galaxy which has a small apparent bulge that is not exactly spheroidal ("elliptical" in shape) is our own Milky Way galaxy.

Here is a COBE/DIRBE infrared map that shows the Milky Way in infrared light....which means that this image is minus most of the extinction of the background stellar light that we see in visual observations or in our usual CCD images.
Essentially, this image is minus most of the dust (dark nebulae and lanes) that we normally see:

Click image for larger version

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Note how the bulge of our own Galaxy is:
(1) Small (quite unlike the bulge of M31)
(2) Rather box shaped

In this respect the bulge of the Milky Way is similar to the bulge of the galaxy NGC 4565:

Click image for larger version

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But the Milky Way is dissimilar to the galaxy NGC 5907 ...... which has no bulge at all:

Click image for larger version

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It was originally thought that small boxy bulges like N4565 and the Milky Way are just small versions of the Very Large bulges found in galaxies like The Sombrero:

Click image for larger version

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However, in actual fact, Small Boxy Bulges have turned out to be similar to a small bar structure of the sort that can be seen in a face-on spiral galaxy, but which we now see in an edge-on orientation. Small bulges also have a very different internal structure, as compared to large bulges (for instance: the surface brightness of the Bulge falls off in a very different way)(the falloff of surface brightness with increasing radius)
Small Boxy Bulges also turn out to be younger objects than the massive spheroidal bulges like those of M104 and M31.......
indeed, small boxy bulges often continue to form stars to this day, and some of them form new stars at a high rate (this is called "ongoing bulge building").

The history of a small boxy bulge is probably very different from the history of a large spheroidal bulge......the big bulge is likely to have have formed most of its constituent stars Early in the history of the universe....... whereas the small boxy bulge may still be building itself up by means of ongoing star formation that is fuelled by gas making its way to the central regions of a galaxy.

Professor Kenneth C. Freeman of ANU, one of the most important extragalactic astronomers of our time, says that large spheroidal bulges are the products of the merger of galaxies; which contrasts with Small Boxy Bulges that form by means of the buckling of the planar disk component of a spiral or S0 galaxy. (Doctor Freeman is one of my gurus. I have abstracted this simplified exposition from his lecture notes!)

Another type of bulge, a so-called pseudobulge, is typified by NGC 4030, and by M94:

Click image for larger version

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In these pseudo-bulges, there is plenty of excess light near the centre, yet these bulges seem to be relatively planar structures rather than actually sticking out of the plane of a galaxy on both sides of its principal plane. Pseudobulges like that of M94 are essentially a continuation of the Flat Disk Component inwards, but at much higher surface brightness. Pseudobulges often have very active current star formation; with OB stars and dust lanes.

In addition to small boxy bulges, large spheroidal bulges, and pseudobulges, it is possible to identify one more type of bulge: a giant boxy bulge, often containing an X or Peanut shape. A good example of a giant boxy bulge is that of NGC 128:

Click image for larger version

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The origin and evolution of these giant boxy bulges is still a highly contentious matter!!
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Old 15-07-2011, 09:26 PM
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Boxy bulges & etc

Hi Robert & All,

Thanks for all the fascinating information Robert!

Another couple of galaxies that fit this description are NGC 3628 (the well-known third member of the Leo Triplet with M66 & M65).

Not a particularly strong example, you can make-out a wide "X" superimposed on the centre of NGC 3628 that is a strongly disrupted spiral with a warped disc that clearly shows up with the bifurcated dust lane in the plane of the disc.

http://apod.nasa.gov/apod/ap050408.html

For the last day I've been racking my brain to remember the designation of another stronger example that I could recall from seeing a Hubble image but do you think I could remember the catalogue number? Well tonight it came to me -- it is ESO 597-36 which is also Hickson 87A in Capricornus. It too has a bifurcated lane in the plane of the disc.

http://apod.nasa.gov/apod/ap100706.html

ESO 597-36 is the brightest in the group (that's why it has the "A" designation) at mag 15.1 -- it is on my hit-list for a fortnight's time! The eg in Hickson 87 lie at about the 390 million ly mark.


Best,

Les D

Last edited by ngcles; 15-07-2011 at 09:42 PM.
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Old 16-07-2011, 02:48 PM
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Hi Les,

You are absolutely right.....NGC 3628 is an excellent example of what I am talking about.

I am not sure whether the unusual morphology of NGC 3628 is due to an encounter with another galaxy or due to the accretion of another galaxy. I don't even think the models of these types of Boxy/X-shaped galaxies are good enough to be able to tell us what causes this type of appearance in a galaxy.

There is an HI (atomic hydrogen) bridge between N3628 and M66, and the galaxy M66 shows an Extremely Faint anomalous tidal arm or extension on one side of it;
so perhaps an interaction with M66 has something to do with the unusual appearance of N3628.

(also, in my view, one of the two principal spiral arms of M66 may be lifted out of the principal plane of this galaxy)

The accretion of a low mass companion is also a possible cause....it is a common event in the history of a giant galaxy.

The "NGC 3628 look" is actually common in some galaxy cluster and galaxy group environments.....it is not uncommon to see this type of bulge together with a warp or tilt in the dust lane.

cheers, mad galaxy man

P.S.
I have been working on an Atlas of warps, bends, twists, and bifurcations in edge-on galaxy disks, for some time!!
Non-planar phenomena in disks are the rule, rather than the exception......(do you believe me?)

Last edited by madbadgalaxyman; 16-07-2011 at 02:50 PM. Reason: type
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Old 22-07-2011, 10:18 AM
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In spite of appalling transparency I paid a visit to AGCS 805 last night. Easy star hopping to find it and through all the murk I could make out quite a few galaxies in one 28' FOV. I'll definitely be back for more detailed obs when the sky is clearer.
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Old 29-09-2013, 07:22 AM
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See the recent excellent image of this cluster of galaxies, by Strongmanmike, in the imaging forum.
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Old 02-10-2013, 12:30 PM
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Hi Robert

Robert's Post #12
Quote:
In addition to small boxy bulges, large spheroidal bulges, and pseudobulges, it is possible to identify one more type of bulge: a giant boxy bulge, often containing an X or Peanut shape. A good example of a giant boxy bulge is that of NGC 128:
And wow, we've just found out our galaxy is peanut shaped too .
Reading what you said about those pseudo bulges was fascinating to me. I hadn't realised they existed until I saw the latest news on our galaxy.
So, in simple terms (if possible) what causes the peanut shape, re the disc matter (if I got that right?) falling inwards- do you know?

I'm sure you've seen it, but I'll pop it up anyway, this is the new pic of our milky way galaxy showing that X/peanut shape.

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Old 02-10-2013, 12:49 PM
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Robert, I've just had a look at Mike's image and it's mind blowing!!!

I take it that the x/peanut shape one you're talking about is the one I've put an arrow next to it? Because I'm sure I see it there and wow that is really something!
I've never seen anything like that before in images.
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Old 02-10-2013, 09:28 PM
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Quote:
Originally Posted by Suzy View Post
Robert, I've just had a look at Mike's image and it's mind blowing!!!

I take it that the x/peanut shape one you're talking about is the one I've put an arrow next to it? Because I'm sure I see it there and wow that is really something!
I've never seen anything like that before in images.
Attachment 148815
Hi Suzy,

Yes, that is the galaxy with the boxy/peanut/x shaped bulge. These bulges have a remarkably complex shape.

I may try to answer your question about what causes the boxy and peanut shape of some bulges: I have had the relevant papers on my computer for a long time, but I have not seriously come to grips with how these bulges form and evolve! Box/peanut bulges are a phenomenon that evolves from the planar disk component of a spiral or S0 galaxy, rather than being spheroidal (elliptical) in shape;
so Box/peanut bulges are very different from big elliptical bulges like those in M104 and M31.

Another very good example of a giant boxy/peanut bulge is NGC 6771 in the famous galaxy triplet in Pavo:

Click image for larger version

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This bulge is quite bright and it might even be visible to deep sky observers, given a really dark sky and largish aperture (say 10-12 inches plus)

A closer example is NGC 3628; I suspect that the "boxy" nature of its bulge might be a possible for visual detection, though the X-shaped extensions of this bulge are exceedingly faint.

Here is a helpful overlay of the visual Milky Way with the infrared contours that show the shape of the small boxy bulge of the Milky Way Galaxy; our Milky Way bulge is thought to be an elongated bar structure that is seen edge on (Ken Freeman of ANU called it a "bar/bulge") :

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[[Incidentally, M31 is not actually "like the Milky Way"; M31 has a biggish spheroidal bulge! ]]

The Milky Way "bulge" that we see in the sky looks bigger than its actual bulge, so much of the "apparent bulge" that we see must be material relatively near to the sun, which only looks like it extends a long way above the plane of our galaxy mainly because it is nearby.

It wasn't so long ago that bulges were mostly thought to be spheroidal in shape , with a sort of generic elliptical shape.
But then we found that some bulges are elongated spheroids shaped like a rugby football and other bulges are symmetrical spheroids shaped like a hamburger bun.

Also, various bulges have many and diverse profiles of the falloff of surface brightness with increased radius. All in all, a very complex situation!#$$%%^^!

The small boxy-peanut bulges are probably caused by bending upwards of the material in the planar disk.

Sometimes, a so-called bulge is just an abrupt central brightening in a galaxy, and therefore this "bulge" is barely sticking above the plane of a galaxy;
these flat "disky" bulges are hardly more than an abrupt brightening of the central part of the planar component of a spiral galaxy (the planar part of a galaxy is usually called its disk component, to distinguish it from the spheroidal component(s) )

The really big boxy/peanut bulges are a puzzle, as they really do look very very different from a standard "elliptical" bulge, as per this comparison:

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Best regards,
Robert Lang

It has been a very long road for astronomers to actually figure out what our own Galaxy looks like; definitely a case of "not seeing the forest because of the trees in the way". The exact bar structure and the spiral arms structure are still controversial!

Here is another helpful graphic, comparing galaxies with various bulge shapes. Note the "disky" bulge, which is just an inner extension of the planar disk, plus the fact that there is a galaxy shown with no bulge:

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Attachment 148835
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Old 04-10-2013, 04:57 PM
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Thanks for more fantastic reading Robert - hope for some clear skies tomorrow night to see if I can pin down the bulge in NGC 6771!
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Old 05-10-2013, 01:30 PM
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Robert you're amazing, thank you so much for all that fabulous information.

I've just looked up NGC 6771 and drats it's got a surface brightness of mag. 12.5- that puts me out of observing it. From by light polluted backyard anyway.
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