Author Topic: Merger Checking Examples  (Read 38707 times)

waveney

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Merger Checking Examples
« on: July 10, 2008, 01:36:19 pm »
Following from the OOTD on Saturday 21st of July, I sent these objects to Daniel Darg at Oxford for his expert opinion. 
See also OOTD for 6th September 2008.

All that follows is from Daniel (formatted by me):

Firstly, there are some tricks people should know about if they want to make really informed decisions about these mergers. The first trick is this:

1) If you want to have information about an entity next to your galaxy that might or might not be a mergering galaxy, you can usually find it straight away by plugging the object id of your galaxy into the SDSS page and then changing the very last digit + or - a few. Normally this will put you right on top of the object.

E.g. 587722982279938052
... then try changing the last digit to,say, 1:
587722982279938051

... and voila! you are centred on your object in question. There are then two major questions to ask to help you decide whther this is a merging galaxy.

i) Does it have a redshift? If so, compare it with the GZoo object's redshift to see if they are similar. If theyre different by more than ~0.005 then its probably a projection (though this number depends on certain things so you need to be careful).

Often these extra objects will not have a redshift. The next question thats often useful to ask is whether SDSS reckons if this is a star or not. If you go from the above webpage and click the left hand link called 'PhotoTag' then you'll see a bunch of data relating to the photometry of this SDSS object. The one were most interested is an integer quantity called 'type.' SDSS photometry can tell whether or not an object is 'point-like' which is normally what a star looks like or 'extended.' On this basis it can decide if the object is a star or a galaxy. If it is a galaxy, itll return type=3 and if it thinks its a star then itll return type=6. When you do this with the example object id I used above, you get a type = 6 for the closeby object so its almost certainly a star! (Ive found this tag to be very reliable).

As for the list, here is what I would say by applying these tricks:



587722981747982511

Probably a merger. Appears to be distortion in the shape of the central object.



587722982279938052

Def. a star as revealed by the profile of surface brightness (very constant with radius) and phototag type=6



587722982281117913

Probably a minor merger. Looks like there's a small (second) core at the bottom and slight disruption to the top right



587722982811172894

These are defiantly interacting (you can see some ejected material in the wider view). Whther or not they will completely merge is impossible to say for sure though my guess is they would. It should be counted as merger in any case due to the tidal interaction.



587722982815694956

These are defiantly merging. Im surprised this one is in question. They are nicely blended together and there likes like theres some disturbed/ejected material to the top right. The bottom galaxy also has spectra (object 587722982815694957 which I found by adding/subtracting to the last digit) and its redshift is 0.071 which is very similar to the upper objects redshift 0.072. So theyre not a projection.



587724197469028506

This is again a star overlapping a non-merging galaxy. the stars object id is 587724197469028507 and when you look at the phototag, it has type=6. its a bit hard to tell, butId say the surface brightnes profile does look starlike as well.



587724197746901168

This system is clearly perturbed and, to be honest, Im in two minds whether or not to call this a merger. It could be that a galaxy whizzed past it ages ago and has dissapeared or that it was the result of a minor merger. In general, Id lump these sorts of things in.



587724198280036453

This is defiantly a merger. Its significantly perturned and there looks to be ejected material way up at top of the image (if you zoom out a little).



587724231568916583

This is most certainly a star. You shouldnt need to look at the phototag (which if you want to check is type=6 for 587724231568916582) to see that the surface brightness profile is way too constant to be the product of billions of red stars all packed together in a superdense and constant distribution.

:) Dan
« Last Edit: September 09, 2008, 02:01:57 pm by waveney »
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j_doe

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Re: Merger Checking Examples
« Reply #1 on: July 10, 2008, 02:24:19 pm »
The voting for this one from July 5th is what I do not understand.



588848900466016559
Merger: 10
Not a Merger: 8

If you zoom out you get a fairly strong interaction.



Am I missing something?  The tutorial stated that we should use “Anything that has a disturbed morphology and is
the product of two or more galaxies” as our definition.  What did I get wrong?

waveney

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Re: Merger Checking Examples
« Reply #2 on: July 10, 2008, 02:26:43 pm »
The voting for this one from July 5th is what I do not understand.

If you zoom out you get a fairly strong interaction.

What did I get wrong?

You did nothing wrong.  You looked...

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ElisabethB

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Re: Merger Checking Examples
« Reply #3 on: July 10, 2008, 02:27:32 pm »
maybe some people don't use the zoom ?

saiyette

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Re: Merger Checking Examples
« Reply #4 on: July 16, 2008, 02:42:42 am »
Hi Dan!

Thanks for the explanation on how to figure out if a set of objects are a merger or not.

I used the step you outlined above on these two objects:

Object 1: ref #587739304753758323; z value = .088; phototag type 3

Object 2: ref #587739304753758319; z value = .088; phototag type 3

I think these are an honest to goodness merger. Can you (or someone else!) take a look at these and tell me if I am reading the data correctly?  :-\

Thanks!!  :)

~Sai

elizabeth

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Re: Merger Checking Examples
« Reply #5 on: July 16, 2008, 03:00:12 am »
 ;D ;D Hi welcome  to the zoo.http://cas.sdss.org/astro/en/tools/explore/obj.asp?id=587739304753758323
by just the look of this I would say  merger. It has been posted in the mergers thread and also the overlap thread. Any one else? :)
« Last Edit: July 16, 2008, 03:02:46 am by elizabeth »

saiyette

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Re: Merger Checking Examples
« Reply #6 on: July 16, 2008, 03:22:46 am »
;D ;D Hi welcome  to the zoo.http://cas.sdss.org/astro/en/tools/explore/obj.asp?id=587739304753758323
by just the look of this I would say  merger. It has been posted in the mergers thread and also the overlap thread. Any one else? :)

Hiya!

Thanks for the re-welcome. I started on the temp forum and am finding more time to spend on GZoo now. So much great info here!!! When you say this has been posted in mergers and overlaps, do you mean that someone already asked about this in those forums? Sorry for the cluttering up but I only found this thread and this galaxy tonight so I wanted to use it as a handy example.

~Sai

j_doe

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Re: Merger Checking Examples
« Reply #7 on: July 16, 2008, 03:48:52 am »
After looking at the picture, I think it could be either.  ??? ??? ???  I would
need a sharper picture to tell.  (Maybe I will check the original images some day.)

elizabeth

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Re: Merger Checking Examples
« Reply #8 on: July 16, 2008, 04:26:01 am »
;D ;D Hi welcome  to the zoo.http://cas.sdss.org/astro/en/tools/explore/obj.asp?id=587739304753758323
by just the look of this I would say  merger. It has been posted in the mergers thread and also the overlap thread. Any one else? :)

Hiya!

Thanks for the re-welcome. I started on the temp forum and am finding more time to spend on GZoo now. So much great info here!!! When you say this has been posted in mergers and overlaps, do you mean that someone already asked about this in those forums? Sorry for the cluttering up but I only found this thread and this galaxy tonight so I wanted to use it as a handy example.

~Sai
  ;D yes to your question ( have you check out the search feature yet?) We sometimes don't agree on classification I think that what makes it intresting. ;D

slimeslayer

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Re: Merger Checking Examples
« Reply #9 on: August 06, 2008, 08:42:03 pm »


would this be considered a merger?

still waiting for Galaxy Zoo version 2...

waveney

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Re: Merger Checking Examples
« Reply #10 on: August 06, 2008, 08:45:59 pm »
would this be considered a merger?

Yes definitely.
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Nereid

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Re: Merger Checking Examples
« Reply #11 on: August 06, 2008, 09:04:48 pm »
Coming a bit late to this party ...

I have difficulty with 587722982815694956 (in the OP)*.

Agreed, the two object have similar zs, and their outer parts certainly overlap.

However, they seem more like a couple of (giant?) ellipticals often found near the centre of a relaxed, old, rich(-ish) cluster than definitely merging.  Further, the fluff at the top right is just not obviously associated with the pair (projections in rich clusters are anything but rare).

But the part I have most difficulty with is "nicely blended together" - by eye alone, I can't tell if the nice symmetrical outer parts show any sign of distortion, and the blending seems to me to be pretty much what you'd expect from projecting two ellipticals (one considerably more elliptical than the other).  Of course, if we could plot isophotes, and overlay with some simple, suitably aligned, symmetric profiles ....

* the others I agree with.
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Half65

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Re: Merger Checking Examples
« Reply #12 on: August 06, 2008, 09:06:29 pm »
http://cas.sdss.org/dr6/en/tools/explore/obj.asp?id=587722982815694956

overlap
overlap
overlap
overlap
overlap
overlap
overlap
overlap
overlap
overlap
overlap
overlap
overlap
overlap
overlap
overlap
overlap
overlap
overlap
overlap

Animation courtesy made by pluk.

vaevictus

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Re: Merger Checking Examples
« Reply #13 on: August 07, 2008, 02:10:06 am »
587742190440939799

NED returns that there are two objects here--and only one posts a redshift. What is this? Thanks.

waveney

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Re: Merger Checking Examples
« Reply #14 on: August 07, 2008, 05:28:07 am »
There is no merger here it is a faint (small) elliptical or dwarf galaxy.  There is no disruption, only one core seen - definitely not a merger.

Just because NED gives two objects, doesn't mean there are two, just it is listed twice under different reference numbers.  SDSS only thinks there is one object.
« Last Edit: August 07, 2008, 05:29:45 am by waveney »
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