Author Topic: Wednesday, 16th May, 2012: Seeing Is Believing (Or Is It?)  (Read 17958 times)

JeanTate

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Strange sort of thing to start an OOTD with, don't you think? Is it, perhaps, some sort of color chart? Let's expand the picture to 600x600:



Hmm. Not a color chart; what if we zoom out a bit, five-fold in fact?



Ah, that's better; this is obviously a giant spiral galaxy, a rather beautiful one. Zooming out, by another factor of five:



Some of you probably now recognize it, it's M81, i.e. number 81 in Messier's list of fuzzies that he compiled in order to avoid mistaking them for comets. And one day there'll be a full entry on it in Messier catalogue LIST, a zooite forum project.

Here's another rather beautiful, classical spiral, as seen by the Hubble Space Telescope (from this OOTD):



And here it is, as seen by the SDSS telescope, at the same scale as the first image of M81 (the orientation is different from that of the Hubble image):



 :o :o Yep, that beautiful spiral is the small, fuzzy blob in the center of this SDSS image. The two galaxies look so different in large part because they are at very different distances from us: M81's is 3.6 Mpc (which is derived using M81's estimated distance modulus; 3.6 Mpc is 11.7 million light-years); SDSS J141822.30+524306.1's is 1.186 Gpc (derived using the Hubble redshift-distance relationship - the galaxy's redshift is 0.2388 - and Ned Wright's Cosmology Calculator with generally accepted values for the various cosmological parameters[1]; that's 3.87 billion light-years).

NGC3314 wrote an excellent OOTD on spirals, and how there's no way to tell, from their appearance alone, how big they actually are (or were): Friday 23rd October 2009: Spirals big and small. In that he explains that M81 is, in fact, pretty wimpy in the big spiral stakes; two of the biggest, in his OOTD (and in the SDSS footprint) are M104 (the Sombrero galaxy, z=0.0034) and NGC 309 (z=0.0178). Here are the SDSS images of these two - M104 first - at the same scale as the last of the M81 images:





Time for a history-of-astronomy trivia quiz question: you have all heard, I'm sure, of the fact that some people - even some astronomers - do not accept that the Hubble distance-redshift relationship holds for all galaxies (once you take account of some small, local variations); what role did M81 and NGC 309 play, as evidence against this relationship? For an extra bonus point, what is the name of the astronomer made the connection? For two bonus points, what's wrong with this evidence (that involving M81 and NGC 309)? I'll post answers later.  :D  :-*

Now for two rather boring-looking galaxies; the first I posted in last week's OOTD, the second has not been posted in this forum before (as far as I can tell):



Respectively, SDSS J235925.15-102608.8 (DR7 ObjId 587727225690063043, z=0.305), and SDSS J155752.37+273049.5 (DR7 ObjId 587736618787602957, z=0.389). These two images have the same scale as the first of the M81 images.

What if all the galaxies were shown with the same physical scale? That is, if, in each image, 1 pixel corresponded to, say, 3,000 light-years? Here they are, in the same order they appear in this post [2]




It is traditional to add a scale bar to images, to give the eye a chance to appreciate the size better. I'm going to go with a somewhat different tradition, I'm going to put a circle around the nucleus of each of these six galaxies, one that is 36 kpc in radius - this is just shy of 120 thousand light-years. How big is 120 thousand light-years? Well, the lovely Large Magellanic Cloud, a satellite galaxy of our own, is estimated to be just ~150 thousand light-years away, so, for a galaxy, it's pretty awesomely big ...




So, M81, despite appearances, isn't that big of a deal, so far as spiral galaxies go. And SDSS J141822.30+524306.1 is even wimpier, despite its appearance in the Hubble image. Further, I found two galaxies, at redshifts > 0.3, that are apparently larger than NGC 309 (and are, very likely, even larger than they appear, given that the fainter outer bits - visible in the SDSS image of NGC 309 - would be too faint to show)!  :o  8)

But are these the very biggest spiral galaxies, at z > 0.3, in the entire SDSS footprint? Stay tuned!  ;)

[1] H0 = 71 km/s/Mpc, ΩM = 0.27, and ΩΛ = 0.73; for the cognoscenti, the distances I quote are luminosity distances
[2] the scale is actually ~2930 light-years per pixel, or ~900 pc/pix

egalaxy

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Re: Wednesday, 16th May, 2012: Seeing Is Believing (Or Is It?)
« Reply #1 on: May 16, 2012, 12:16:54 am »
wonderful OOTD and beautiful objects!

mitch

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Re: Wednesday, 16th May, 2012: Seeing Is Believing (Or Is It?)
« Reply #2 on: May 16, 2012, 07:02:55 am »
Wow - boggles the mind :o Nice OOTD  8) 8)

elizabeth

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Re: Wednesday, 16th May, 2012: Seeing Is Believing (Or Is It?)
« Reply #3 on: May 16, 2012, 08:53:33 am »
 :D :D  8) 8) 8) Great OOTD!

AlexandredOr

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Re: Wednesday, 16th May, 2012: Seeing Is Believing (Or Is It?)
« Reply #4 on: May 16, 2012, 10:12:06 am »
Ah yes, great OotD !
Thx Jean  :D

Baby star opening its eyes on the Universe.

JeanTate

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Re: Wednesday, 16th May, 2012: Seeing Is Believing (Or Is It?)
« Reply #5 on: May 17, 2012, 12:11:45 pm »
For completeness, here, at the same scale as the third M81 image (and the first M104 and NGC 309 images), is the "Or yet larger, NGC 3646 in Leo" from NGC3314's OOTD (this is the largest spiral in the SDSS footprint*):



Taking its redshift as 0.01417, from NED, here's how it would appear at the same 900 pc = 1 pixel scale as the last six images (also with a "36 kpc" circle added); I've also included the corresponding pair for SDSS J155752.37+273049.5:




Oh, and I should also add that the M104 entry in the Messier catalogue LIST is blank, that there's also a corresponding - and more ambitious! - NGC catalogue LIST, but that it lacks even a placeholder for NGC 309 or NGC 3646 :( However, there is a post with NGC 3646 in the NGC Catalogue thread  :), but not one for NGC 309  :(

Is it possible to compare the sizes of spirals, in some quantitative fashion? Such a simple question, yet the answer is surprisingly complicated.  ??? If you want a one word Yes-or-No answer, I'd have to go for "No"  :o; at the short phrase level, "There is a way, but the answers come with caveats, and can be misleading" (OK, maybe not such a short phrase!  ::)).

For example: the SDSS photometric pipeline automatically, and without-fear-or-favor, calculates a robust estimate of the size of galaxies, which it calls "petroRad_r". This is the "Petrosian radius" in the r-band. Since we're looking at spirals, and since spiral galaxies have an "exponential profile" (well, mostly, sorta, with important exceptions, etc, etc, etc), another parameter in the photometric pipeline may be used, "expRad_r". This is the "effective radius" of a "fitted exponential profile", in the r-band. The first (petroRad_r) is helpful because "The aperture 2rP is large enough to contain nearly all of the flux for typical galaxy profiles, but small enough that the sky noise in FP is small"; the latter because the effective radius (re) of an "exponential profile" galaxy is a quantitative, objective measure of its size (you can say, for example, that its "edge" is at 4re). You can read more about this - and a hundred and one other things you never knew you needed to know - at the SDSS SkyServer Algorithm Description page, here8)

So, you're thinking, since SDSS is totally objective, and its treatment of all galaxies is uniform and identical (think of it as a totally impartial judge), why not just look up the values of petroRad_r and expRad_r for M81, M104, NGC 309, NGC 3646, SDSS J141822.30+524306.1, SDSS J235925.15-102608.8, and SDSS J155752.37+273049.5, convert the angular sizes (in arcsecs) to linear ones (in kpc) using the Ned Wright Cosmology Calculator (NWCC), and award gold/silver/bronze rank the seven galaxies accordingly?

To see why not, let's start with M81: clicking on PhotoObj, and scanning down the list, we find that petroRad_r is 52.459774", and that expRad_r is 20.663347"; all impressively precise**. We have to be careful with the NWCC, because M81 is so close that its redshift may not be an accurate measure of its distance; fortunately, we have redshift independent estimates of its distance, so we can work backwards (i.e. plug in different values of z until we get 3.6 Mpc as the distance). Doing that we get 0.017 arcsec per kpc kpc per arcsec as the scale, that almost all of M81's light ("nearly all of the flux") comes from a circle only 892 pc in radius (from the first measure), and that its edge is 1,405 pc from its nucleus (from the second).  :o

Clearly, that's complete nonsense!  >:( You only have to have a look at an SDSS image, with a scale bar, to see how ridiculous this is:



Obviously, a great deal of M81's light comes from beyond 2' from its nucleus, and 120 > 83 > 52.

The automated photometry pipeline doesn't work for such huge (on the sky) objects as M81!

Maybe it'll work for NGC 309, which has the highest redshift of the galaxies in NGC3314's OOTD? Let's see.

petroRad_r is 46.109917", and expRad_r is 21.010206"; at z=0.0178, 1 arcsec is 0.357 kpc, so "almost all" of NGC 309's light comes from within 16.46 kpc of its nucleus, and its edge is at 30 kpc (in the r-band). OK, that works, sorta, but we really should use just two significant digits!

How big, then, are the three distant galaxies, the ones whose names are just "SDSS" followed by strings of numbers? Here's how - radius of the edge (4re,r) - big:

17 kpc re,r = 1.15" (4.3 kpc); petroRad_r = 2.53" (9.5 kpc)
53 kpc re,r = 2.96" (13 kpc); petroRad_r =  7.90" (35 kpc)
81 kpc re,r = 3.87" (20 kpc); petroRad_r =  7.22" (38 kpc)

Oh, and to give you an idea of just how "sorta" these comparisons are, consider that the estimated value of re for the second galaxy, in the g-band, is 4.36" (makes sense, kinda; the arms look blue), which would put its edge at a whopping 78 kpc from the nucleus!  :o :o :o

Bottom line: these two z > 0.3 spirals are waaaay bigger than any in NGC3314's OOTD (those in the SDSS footprint anyway).

UPDATE: (server's down; I'll edit this post later, and put the numbers in) Server's back up, so I added the numbers for the more distant, huge spiral. I've also added the values for petroRad_r, and amended re to re,r, to show that it's the r-band estimate of re. Too, I put the physical size that each arcsec number corresponds to, at each galaxy's redshift (using the NWCC, and generally accepted values of the parameters). Oh, and I fixed a really silly mistake (how silly? well, the original is still in the post, so you can judge for yourself ...)

* or at least it was at the time NGC3314 wrote that OOTD, back in October, 2009; I do not know if there are any larger ones in DR8
** OK, I'm being unfair; in the list is also the estimated error, which means only the first two, or perhaps three, digits are meaningful
« Last Edit: May 17, 2012, 09:54:37 pm by JeanTate »

JeanTate

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Re: Wednesday, 16th May, 2012: Seeing Is Believing (Or Is It?)
« Reply #6 on: May 17, 2012, 04:29:56 pm »
egalaxy, mitch, elizabeth, AlexandredOr, many thanks for your feedback and kind words.

As you might have guessed, from reading my OOTD of last week if nothing else, I've been bitten by the 'giant spirals at z > 0.3' bug!   :D I intend to do more work have more fun on this topic, and would like to get your inputs on a number of things, if you would be kind enough to spare me your time and attention. Nothing heavy!  ;D For now, just looking at a few images and giving me your opinions; a kind of informal, very-small-scale Galaxy Zoo. For example, do you agree that these two galaxies look very much like (nearly) face-on spirals* (and not at all like edge-on spirals, ellipticals, or mergers)?





Everyone else reading this: of course I'd be delighted if you were to post an opinion too!  :-*

* even if you can't say, with any certainty, whether they're CW or ACW (well, the first is pretty obvious)

JeanTate

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Re: Wednesday, 16th May, 2012: Seeing Is Believing (Or Is It?)
« Reply #7 on: May 17, 2012, 05:59:43 pm »


That's UGC 2885, with a 36 kpc circle added*. The source is the SARA image which NGC3314 posted, in his 2009 OOTD Spirals big and small. Beside it is the SDSS image of SDSS J155752.37+273049.5, to the same physical scale (i.e. 1 pixel = 900 pc), as I've posted several times before in this thread.

As UGC 2885 is not in the SDSS footprint, I can't say what its estimated petroRad_r and re,r are.  :(

Also, like M81, M104, NGC 3646, and SDSS J141822.30+524306.1, UGC 2885 is not (nearly) face-on to us (unlike NGC 309, SDSS J235925.15-102608.8, and SDSS J155752.37+273049.5), so it's hard to say - just by looking at the image - whether UGC 2885 is bigger than SDSS J155752.37+273049.5 or not.  :'(

* I hope think; there are more sums involved in getting to this image, so more opportunities for me to have made a (silly) mistake or two. If anyone would like to check my sums, please send me a PM, and I'll gladly go over them with you ...  :-*

egalaxy

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Re: Wednesday, 16th May, 2012: Seeing Is Believing (Or Is It?)
« Reply #8 on: May 17, 2012, 10:17:09 pm »
i just checked, and someone today edited the NGC 309 place holder in the NGC LIST thread.

Budgieye

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Re: Wednesday, 16th May, 2012: Seeing Is Believing (Or Is It?)
« Reply #9 on: May 18, 2012, 01:38:28 pm »
egalaxy, mitch, elizabeth, AlexandredOr, many thanks for your feedback and kind words.

As you might have guessed, from reading my OOTD of last week if nothing else, I've been bitten by the 'giant spirals at z > 0.3' bug!   :D I intend to do more work have more fun on this topic, and would like to get your inputs on a number of things, if you would be kind enough to spare me your time and attention. Nothing heavy!  ;D For now, just looking at a few images and giving me your opinions; a kind of informal, very-small-scale Galaxy Zoo. For example, do you agree that these two galaxies look very much like (nearly) face-on spirals* (and not at all like edge-on spirals, ellipticals, or mergers)?





Everyone else reading this: of course I'd be delighted if you were to post an opinion too!  :-*

* even if you can't say, with any certainty, whether they're CW or ACW (well, the first is pretty obvious)

 

I think there is no doubt that they are both face-on spirals and both Anti Clock Wise

(Follow the arms from the outside-in
That's the way that galaxyies spin)

and here is another biggy,
I think is it very far away with a PhotoZ of about 0.4
so far away that its blue spiral arms are turning green


http://cas.sdss.org/dr6/en/tools/explore/obj.asp?id=587735348022477078
posted by elisabethB

Blackprojects

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Re: Wednesday, 16th May, 2012: Seeing Is Believing (Or Is It?)
« Reply #10 on: May 18, 2012, 02:21:22 pm »
Ermm this is a Face on Anti Clockwise Spiral but i am wondering if it could also be a Lens?

I May be seeing things others will say are not there!

http://casjobs.sdss.org/ImgCutoutDR6/getjpeg.aspx?ra=359.85480682&dec=-10.43578329&scale=0.09903&width=512&height=512&opt=SP&query=&PhotoObjs=on&SpecObjs=on




This Looks like a Dual Arm Anti Clockwise Spiral with a Possible BAR but with a Possible AGN and Face on But rite at the Telescopes Limmit

http://cas.sdss.org/dr6/en/tools/explore/obj.asp?id=587736618787602957





Wish there was a better Image?

« Last Edit: May 18, 2012, 02:24:18 pm by Blackprojects »

LynnSeguin

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Re: Wednesday, 16th May, 2012: Seeing Is Believing (Or Is It?)
« Reply #11 on: May 18, 2012, 02:32:06 pm »
Dear Jean,

Thank you for the wonderful and instructive OOTD.  My impression is that these are both ACW; the first, a face on spiral and the second face on with a slight tilt.

Cheers,
Lynn

Lovethetropics

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Re: Wednesday, 16th May, 2012: Seeing Is Believing (Or Is It?)
« Reply #12 on: May 18, 2012, 10:44:46 pm »


That's UGC 2885, with a 36 kpc circle added*. The source is the SARA image which NGC3314 posted, in his 2009 OOTD Spirals big and small. Beside it is the SDSS image of SDSS J155752.37+273049.5, to the same physical scale (i.e. 1 pixel = 900 pc), as I've posted several times before in this thread.

As UGC 2885 is not in the SDSS footprint, I can't say what its estimated petroRad_r and re,r are.  :(

Also, like M81, M104, NGC 3646, and SDSS J141822.30+524306.1, UGC 2885 is not (nearly) face-on to us (unlike NGC 309, SDSS J235925.15-102608.8, and SDSS J155752.37+273049.5), so it's hard to say - just by looking at the image - whether UGC 2885 is bigger than SDSS J155752.37+273049.5 or not.  :'(

* I hope think; there are more sums involved in getting to this image, so more opportunities for me to have made a (silly) mistake or two. If anyone would like to check my sums, please send me a PM, and I'll gladly go over them with you ...  :-*

Wow!!  The second one is also an ACW spiral!  And it's so beautiful  ;D

 *and find lots of asteroids  ;D

JeanTate

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Re: Wednesday, 16th May, 2012: Seeing Is Believing (Or Is It?)
« Reply #13 on: May 19, 2012, 06:46:48 am »
Thanks Budgieye, Blackprojects, LynnSeguin, and Lovethetropics!  And thanks all of you who sent me your opinions by PM!! ;D  :-*

Everyone thinks both distant galaxies are spirals.  8)

That has encouraged me to ask you to take a look at these four galaxies, and tell me what you think they are. The choices again (somewhat modified) are:

-> certainly some sort of spiral (if you'd like to say what sort - nearly face-on, barred, ring(ed), CW, ... - that'd be a bonus)
-> certainly an elliptical
-> certainly a merger or overlap
-> not really sure, but more likely to be spiral/elliptical/merger (pick one) than either of the other two choices
-> really, truly, cross-my-heart-and-hope-to-die can't tell! :'( (but for sure it's not a star or an artifact  :D )

If you think a galaxy looks like an edge-on spiral, then it's a spiral (either "certainly" or "more likely than not"); in this case, I'd appreciate it if you could explain why you feel "spiral" and not "cigar-shaped elliptical".

All the galaxies have the same physical scale, 1 pixel = 900 pc [1], and all images are the same size, 200 x 200 pixels.

14: 05:
14: 05:

41: 11:
41: 11:

Please post your opinions in this thread (though you can PM them to me if you'd prefer).

The labels/numbers are my way of keeping track (sure beats writing out long strings, like SDSS names or ObjIds). For reference, the DR7 ObjIds are, respectively (left to right, top to bottom): 588297865255387308, 587739380987265247, 587741490911772750, 588011219134054611.

[1] assuming generally accepted values for the standard cosmological model; the spectroscopic redshifts of the galaxies are all quite good

UPDATE: Following Lovethetropics' EXCELLENT suggestion (see Reply#15, on the next page), I've added inverted images ...

Thank you VERY MUCH!!!  ;D :-* :-*
« Last Edit: May 19, 2012, 10:58:28 am by JeanTate »

elizabeth

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Re: Wednesday, 16th May, 2012: Seeing Is Believing (Or Is It?)
« Reply #14 on: May 19, 2012, 10:10:46 am »
i just checked, and someone today edited the NGC 309 place holder in the NGC LIST thread.
;D Awesome was that because it might be NGC 0309 ?