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FirstSight
Carpal Tunnel
Reged: 12/26/05
Posts: 2351
Loc: Raleigh, NC
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Once you move east (in rising ascension) past the densely packed heart of the Milky Way in Sagittarius, there is a vast region in the central and southern celestial latitudes that is relatively sparsely populated with bright or interesting deep space objects that extends around nearly to Orion (a span of from about 20h r.a. to about 5h r.a.). It is, of course, an exaggeration to call this region an "empty quarter", and there are islands and pockets of stuff, e.g. the Fornax galaxy cluster, and a scattering of galaxies and planetary nebulae here and there. Nevertheless for example, the descriptive note in the Collins Atlas of the Night Sky for the constellation Cetus quite aptly starts by saying: "While a very large constellation (the fourth largest), Cetus has surprisinly few objects of interest." Capricorn, Aquarius, Aries, and Eridanus share this trait (ok, so Capricorn has a couple of the smaller, dimmer Messier globulars, but but the dense globs in Sagittarius/Ophicus are a hard act to follow and Cap has little in the way of open clusters, nebulae, or galaxies).
That the density of intra-galactic objects should grow sparser east of Sagittarius is hardly surprising, given that the prominent western region of Sagittarius lies astride the central core of the Milky Way, and as one moves eastward in r.a. out of Sagittarius one is progressively moving away from the galactic plane. However, the central/northward region around Ursa Major and extending through Coma Berenices and Leo/Virgo (roughly 10h r.a. to 15h r.a.) also extend sharply away from the galactic plane, and are very richly populated with galaxies (the relative thinness of material from our own Milky Way in this direction helps greatly reduce interference with their visibility).
QUESTION: ARE THERE ANY PARTICULAR REASONS WHY THE CAPRICORN/CETUS et.al. REGIONS ARE RELATIVELY SPARSE OF GALAXIES (from our earth perspective) COMPARED TO THE URSA MAJOR/COMA/LEO/VIRGO REGION? Is this simply a product of where the denser vs sparse filaments of galactic density wound up falling through random processes, or does it also/instead have to do with a much greater amount of obstructing material in the direction of the southern "empty quarter" (if so, shouldn't there be more nebulae?) Or are there still other reasons having to do with differences in orientation of the two respective regions relative to the galactic plane and ourselves?
Maybe there is no better reason than "that's just how matter fell out in this region", or to put it another way, the Virgo region (grand galaxtic cluster closest to us) is centered in one direction from us, not the other, and other dense clusters in other directions are much farther away.
Curious minds want to know.
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Chris M., aka "First Sight"
Orion XT12i Dob with Moonlite CR-2 focuser
WO Megrez 90 refractor on UniStar Light mount
Nikon 10x50 Binoculars
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Tony Flanders
Carpal Tunnel
Reged: 05/18/06
Posts: 1792
Loc: Cambridge, MA, USA
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Quote:
ARE THERE ANY PARTICULAR REASONS WHY THE CAPRICORN/CETUS et.al. REGIONS ARE RELATIVELY SPARSE OF GALAXIES (from our earth perspective) COMPARED TO THE URSA MAJOR/COMA/LEO/VIRGO REGION?
Yes. A better way of asking the same question is why UMa, Com, Leo, and Vir are so rich in galaxies.
The answer is obvious in the case of Virgo and Coma Berenices. This is where the Virgo Cluster is centered, so the area is extraordinarily rich in medium-bright galaxies some 50-100 million light-years distant.
The Virgo Cluster is the center of the Virgo Supercluster, which includes our own Local Cluster, and also all of the brighter galaxies in nearby Leo and Ursa Major.
But the Virgo Supercluster, huge as it is, is just part of a much bigger "filament" of galaxies stretching hundreds of millions of light-years into space. One end of the filament stretches into Ursa Major, the other end into Centaurus -- where northern observers can't see it. The great majority of all galaxies visible in typical backyard scopes lie within this filament, which is why such galaxies are not distributed uniformly outside the plane of the Milky Way.
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Tony Flanders
eyeglasses
6x15 and 8x32 monoculars
8x25, 7x35, 10x30 IS, 10x50, and 15x70 binoculars
70mm and 100mm achromatic refractors
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bicparker
Pooh-Bah
Reged: 02/07/05
Posts: 1353
Loc: Plano, TX
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Chris,
I set up the Tully 3D database of galaxies in Starry Night and "traveled in the direction of capicorn, et al and picked out a random galaxy in that region (this was actually more in the direction of Eridanus) and took a quick look at what I could tell from the relative density of that area.
First, there are a lot of galaxies there, but what you said about this being a product of a less dense filament of galaxies appears to be correct. It actually looks to be between filaments, which, while certainly less dense, isn't devoid of galaxies, either.
The accuracy of such 3D models is only so-so, though they do give a good statistical measure of relative densities. One also has to take into account that our accuracy is reduced as objects become more distant (heck, it is even hard to get a really accurate measure of things close to us!).
Probably, the fact that groups like the Virgo Cluster are simply closer also accounts, to some degree, how other areas may seem less dense. There are likely galaxies in these less denser areas that we can't see and they may obscured due to intergalactic medium. Since most of our observations are in the very narrow band of visible and near visible objects, we have a bit of a narrower perspective on what is around us. The Spitzer Space Telescope as well as other "out of visible range" instruments such as Chandra have already found several galaxies and other objects that were otherwise not visible in our normal optical spectrum.
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Bic Parker
17.5" f/5 dob
10" f/10 SCT
5" f/8 refractor
80mm f/6 refractor
66mm f/6 refractor
Plus a few others out of the rotation
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astrokido
super member
Reged: 06/09/08
Posts: 158
Loc: Phoenix, AZ
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Some of those regions may seem empty only because the DSOs in them are too far away. That's why the Fornax region was chosen to obtain the deepest image of galaxies so far by the HST. The region is actually pretty crowded if you can see far enough away, as seen in the linked images. It's just that nearer objects easier to view are distributed over other regions of the sky. The spacial distribution isn't uniform like you say "that's just how matter fell out in this region". The DSO's there just fell further away from where we are in the Universe. We'd see a very different sky if our galaxy was among those distant galaxies.
Also, many areas near the galactic plane are obstructed from view by dense clouds of dust. That keeps many objects out of typical views, like the Milky Way center, which is only visible in some infrared wavelengths that can see through the dust.
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- Gill C. - All opinions subject to a speed limit of 299,792,458 m/s.
Nikon D40 - Bushnell 10x25 - Zhumell 20x80 - Celestron Cometron CO-100
skyatlas.rgbstore.com - cool sky charts, photo overlays, and data tables
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tatarjj
scholastic sledgehammer
Reged: 04/20/04
Posts: 890
Loc: Auburn, AL
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We're sitting in the middle of a cosmic web of matter. Long filaments and vast voids. These "empty" regions of the sky simply represent areas where we are looking out into one of the voids in the universe. With medium and large telescopes you can see galaxies on the other side of the void, and these relatively "sparse" regions fill in. In fact, as a large telescope user, I consider Bootes, Capricornus, and Cetus as rather fertile ground for distant galaxy observing. Many of these galaxies aren't very distant at all, only a few hundred million light years, if that. In fact, as long as there isn't the milky way in your way, you can see huge numbers of distant galaxies in every constellation with a large telescope.
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John T.
Auburn, AL
25" f/4.2 Dob
18" Obsession #701
4" Stellar Vue Achromat
8X56 Binos
Edited by tatarjj (07/28/08 02:03 PM)
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moron392
scholastic sledgehammer
Reged: 08/20/07
Posts: 799
Loc: Charlotte, NC
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the galaxies in the capriconus region are appearantly in a different filament than we are. it is like how the globular clusters are fainter looking in the andreomeda galaxy than the ones in our galaxy. it is because of distance.
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"If you've done something right, people won't be sure you've done anything at all."
http://www.freewebs.com/moron392/index.htm
60mm meade ngc60 refractor.
Meade 70AZ-TR (short one)
50mm homemade refractor (occasionally with a solar filter)
starblast4.5EQ
6mm,15mm expanse eyepieces
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nikon 7x35's
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FirstSight
Carpal Tunnel
Reged: 12/26/05
Posts: 2351
Loc: Raleigh, NC
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Quote:
We're sitting in the middle of a cosmic web of matter. Long filaments and vast voids. These "empty" regions of the sky simply represent areas where we are looking out into one of the voids in the universe. With medium and large telescopes you can see galaxies on the other side of the void, and these relatively "sparse" regions fill in. In fact, as a large telescope user, I consider Bootes, Capricornus, and Cetus as rather fertile ground for distant galaxy observing. Many of these galaxies aren't very distant at all, only a few hundred million light years, if that. In fact, as long as there isn't the milky way in your way, you can see huge numbers of distant galaxies in every constellation with a large telescope.
From the outset I understood that on the grand macro scale of things, the universe is similar in every direction, but as you reduce to the finer scale of individual galaxies and galaxy clusters, the local distributions become more filamenty and lumpy, with relatively sparse regions in-between. And so, yes if you proceed far enough in any given direction, there's stuff out there, just not much that's particularly near, bright, or accessible without larger telescopes and dark sky sites, unless you're looking in a direction either toward a nearby filament, or within your own galaxy.
My question was directed at making sure I understood the correct reasons why the region toward e.g. Capricorn and Cetus appeared to be relatively sparsely filled with more easily accessible objects, versus e.g. the UMa region, when both are in a direction away from the plane of our own galaxy. More from a trick of perspective and obstructed vision, or more from a true relative void in that direction for quite a distance?
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Chris M., aka "First Sight"
Orion XT12i Dob with Moonlite CR-2 focuser
WO Megrez 90 refractor on UniStar Light mount
Nikon 10x50 Binoculars
Edited by FirstSight (07/28/08 05:54 PM)
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Tony Flanders
Carpal Tunnel
Reged: 05/18/06
Posts: 1792
Loc: Cambridge, MA, USA
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Quote:
From the outset I understood that on the grand macro scale of things, the universe is similar in every direction ...
Well, that's an open question. Most cosmologists would agree with you, but the subject is certainly debatable.
However, the scale has to get really grand before homogeneity sets in. At the billion-light-year scale, the universe is still quite clumpy and highly organized. Galaxy filaments and sheets are genuinely gigantic structures! And since almost everything visible in backyard scopes lies within a billion l-y of Earth, the night sky as amateurs know it is quite the opposite of homogeneous.
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Tony Flanders
eyeglasses
6x15 and 8x32 monoculars
8x25, 7x35, 10x30 IS, 10x50, and 15x70 binoculars
70mm and 100mm achromatic refractors
4.5", 7", and 12.5" Dobs
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FirstSight
Carpal Tunnel
Reged: 12/26/05
Posts: 2351
Loc: Raleigh, NC
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Thanks Tony. On a more humble human-scale level, the effect of the apparent "empty quarter" east of Sagittarius is to add a bittersweet element to the richness of the southern summer sky with the Sagittarius/Scorpius/Ophicus region peaking about now at progressively earlier and earlier hours of the night: all too soon, that easily accessible richness to the south will give way to the much sparser "empty quarter, where it's much more work to extract smaller, fainter, and less showy celestial nuggets until Orion finally returns at a reasonable hour.
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Chris M., aka "First Sight"
Orion XT12i Dob with Moonlite CR-2 focuser
WO Megrez 90 refractor on UniStar Light mount
Nikon 10x50 Binoculars
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HfxObserver
professor emeritus
Reged: 11/12/04
Posts: 577
Loc: Waterloo ON, Canada
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There is a spot West of Cetus that Barnard charted as a potential Dark Neb. due to it;s lack of anything in the FOV!
-Chris
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Chris
7X50 Vixen,22X100 Antares
80mm William Optics Megrez II ED
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Starman1
Postmaster
Reged: 06/24/03
Posts: 10713
Loc: Los Angeles
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Might be a point to what you say.
My observing log has only 22 objects in Capricornus, and 407 in Cetus (a superb constellation, by the way, for great galaxies), vs 708 in Virgo and 528 in Ursa Major.
Capricornus would appear to be a great "dead spot".
I don't spend much time there, obviously.
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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Bill Weir
professor emeritus
Reged: 06/01/04
Posts: 706
Loc: Metchosin (Victoria), Canada
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Except for the globular clusters and of course galaxies, the majority of of what the average observer observes is actually not very far from us, within our galaxy. This freeware is very useful in giving a perspective of where in our galaxy, and beyond are the common objects we observe.
http://www.thinkastronomy.com/M13/
Bill
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6'' Orion SkyQuest
12.5'' f/5 Custom Truss Dob
William Optics 80mm ZenithStar II ED Doublet
f/5 25" newtonian on a giant GEM, any time I want
Observing sessions grand total for 2007, 171.
So far in 2008, 87
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bicparker
Pooh-Bah
Reged: 02/07/05
Posts: 1353
Loc: Plano, TX
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Even taking into account galaxies, the items we observe visually aren't very far away in the scheme of things. Most galaxies you can visually observe are less than 500 million LYs. Clusters like Virgo are less than 100 million LYs. That's not very far away at all, which means that what we are visually observing can hardly be considered representative.
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Bic Parker
17.5" f/5 dob
10" f/10 SCT
5" f/8 refractor
80mm f/6 refractor
66mm f/6 refractor
Plus a few others out of the rotation
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tatarjj
scholastic sledgehammer
Reged: 04/20/04
Posts: 890
Loc: Auburn, AL
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Quote:
Even taking into account galaxies, the items we observe visually aren't very far away in the scheme of things. Most galaxies you can visually observe are less than 500 million LYs. Clusters like Virgo are less than 100 million LYs. That's not very far away at all, which means that what we are visually observing can hardly be considered representative.
Yes, even the "extremely" distant galaxy clusters that lie 1-3 billion light years away and are visible in 18+ scopes are, cosmologically speaking, in our backyard.
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John T.
Auburn, AL
25" f/4.2 Dob
18" Obsession #701
4" Stellar Vue Achromat
8X56 Binos
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Tony Flanders
Carpal Tunnel
Reged: 05/18/06
Posts: 1792
Loc: Cambridge, MA, USA
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Quote:
Yes, even the "extremely" distant galaxy clusters that lie 1-3 billion light years away and are visible in 18+ scopes are, cosmologically speaking, in our backyard.
Well, I dunno. Three billion light-years is almost a quarter of the way to the edge of the observable universe. If I had to define our cosmic backyard, I'd pick the edge of the Virgo Supercluster as a natural boundary.
Here's one way of looking at that. Within the Virgo Supercluster, galaxy motions relative to the Milky Way are primarily due to local effects. Outside the Supercluster, they're primarily due to cosmic expansion.
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Tony Flanders
eyeglasses
6x15 and 8x32 monoculars
8x25, 7x35, 10x30 IS, 10x50, and 15x70 binoculars
70mm and 100mm achromatic refractors
4.5", 7", and 12.5" Dobs
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tatarjj
scholastic sledgehammer
Reged: 04/20/04
Posts: 890
Loc: Auburn, AL
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Quote:
Quote:
Yes, even the "extremely" distant galaxy clusters that lie 1-3 billion light years away and are visible in 18+ scopes are, cosmologically speaking, in our backyard.
Well, I dunno. Three billion light-years is almost a quarter of the way to the edge of the observable universe. If I had to define our cosmic backyard, I'd pick the edge of the Virgo Supercluster as a natural boundary.
Here's one way of looking at that. Within the Virgo Supercluster, galaxy motions relative to the Milky Way are primarily due to local effects. Outside the Supercluster, they're primarily due to cosmic expansion.
That is a way to look at it, and while I've read in some places we are falling towards the center of the Virgo Supercluster, a simple search on Simbad of redshift values for galaxies within that cluser does not support this assertion. I searched and found the redshifts of M61, M84, M87, M99, and M100 all were positive and consistant with a distance of 40-70MLY assuming a Ho of 70km/s/Mpc. The odd man out was M86, which had a redshift of z = -0.000901, or a closing speed of 270km/s. If we are indeed expanding away from the Virgo Cluster, then no doubt the combined gravity of it is lessening our velocity away from it, but the universal expansion would still be overwhelming gravity. I'll have to pull up more Virgo cluster members' redshifts and see for sure.
Three billion light years may be a quarter of the way back in time to the big bang, but the universe, by and large, looked and behaved pretty much like it does today- i.e., quasar density, metalicy, galaxy interactions and types, etc. Cosmologically speaking, it's our backyard.
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John T.
Auburn, AL
25" f/4.2 Dob
18" Obsession #701
4" Stellar Vue Achromat
8X56 Binos
Edited by tatarjj (07/30/08 01:14 PM)
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tatarjj
scholastic sledgehammer
Reged: 04/20/04
Posts: 890
Loc: Auburn, AL
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Quoting Wikipedia:
"The large mass of the cluster is indicated by the high peculiar velocities of many of its galaxies, sometimes as high as 1,600 km/s with respect to the cluster's center.
The Virgo cluster lies within the Local Supercluster, and its gravitational effects slow down the nearby galaxies. The large mass of the cluster has the effect of slowing down the recession of the Local Group from the cluster by approximately ten percent."
As I suspected, M86's rate closure towards our galaxy is probably due to its high velocity through the Virgo cluster. A typical value I got for z for a Virgo Cluster member was 0.0045, which is 1350km/s. 1600-1350 is 250km/s, which is very close to the closure rate that the z of M86 indicates.
So indeed, we are receeding from the Virgo Cluster.
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John T.
Auburn, AL
25" f/4.2 Dob
18" Obsession #701
4" Stellar Vue Achromat
8X56 Binos
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DeepSpaceTour
Carpal Tunnel
Reged: 10/25/05
Posts: 2942
Loc: In the dark and"WAY"out there!
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Well don't forget a very special large galaxy NGC-253,I was out last night and checked this gem of a galaxy out again,and it never fails to impress,it looked awesome through all my eyepieces but the 13E was particularly exquisite,showing a LOT of detail,in this tilted from edge on galaxy,you could really see the swirling arms and mottling to the core of this galaxy through the 17.5".
Clear skies.
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Bill
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