Elliptical vs Edge-On, Lens? either way it look interesting - AGZ0001v99
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Capella05
moderator
@Zutopian was the first to comment on this galaxy earlier today, tagging it as a lens.
It is difficult to debate interesting objects when you are limited to 150 characters at a time, so I thought it would be better to bring it here.
The SDSS images are not optimised to feature possible lenses, and yes, they can be visible, they are usually of low resolution.
I am sure a lot of you have classified images on Spacewarps CFHTLS and Spacewarps VICS82 (Star Gazing live) , and you must of thought it was quite easy to spot a lens ( 😉 ) but it is important to note that those images were specifically processed to make them more apparent.
Although both myself and Els do consider this is a possibility (and I have passed it on), but there are several red flags. Spiral galaxies do not tend to have the mass to cause strong gravitational lensing - there are a few but they are quite rare. There also seem to be a few artifacts in this particular image, and looking at SkyServer the arc seems less apparent.
Time will be needed to analyse the image 😃
I did not want this conversation to continue in its original format, as I felt we were going round in circles.
(Be nice, Be respectful, and do not attack the person posting, also a smilie would not go amiss 😃 )
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JeanTate
in response to Capella05's comment.
Thanks for doing this Capella05, and for the excellent summary. 😃
Spiral galaxies do not tend to have the mass to cause strong gravitational lensing - there are a few but they are quite rare.
This is a very important point! In some general sense - qualitative words, not quantitative plots or distributions - disk galaxies (to use the more general term; spirals are just the most obvious kind of disk galaxy) are less massive than the giant ellipticals (acting as lenses) found in Spacewarps. And the size of a lensed image (in arcsecs) depends strongly* on the mass of lens, so while a lower mass galaxy - disk or elliptical - may act as a lens, the image(s) may be invisible at the angular resolution of SDSS. However, there are very massive disk galaxies! 😮 But they are rare, so when you add the rarity of the alignment necessary to produce lensed images to the rarity of massive disk galaxies (actually, multiply would be a better word), you get ... rarerare 😉
This thread may also be a good place to start a discussion on the how of following up on lens candidates. For example, if there were a spectrum of this galaxy (there isn't, or at least not an SDSS one), at least several zooites could have a go at analyzing it, to see if there's any evidence of a lensed galaxy (or other features, such as a polar ring, perhaps).
Again, great post, good thread! 😄
*but not solely
Posted
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zutopian
Thanks for starting this discussion and for forwarding this lens candidate for further inspection !
In Skyserver the arc isn't recognizable, but the GZ images were created by using a different image processing technique in order to make details visible. Well, it wasn't actually done in order to make lenses recognizable, but it might be, that these images are also better for spotting lens candidates. There are a few other cases, where images were tagged as lens in GZ Talk, but the arcs aren't visible in Skyserver.:
e.g. AGZ000427d and AGZ0005n6d
The title of this discussion is "Elliptical vs Spiral-".: You might want to change the title into "Elliptical vs Edge-on".
I would like to mention, that I said in the other discussion, that I think, that it isn't an spiral/disc edgeon, but an elliptical= smooth galaxy. If so, it is more likely, that it might be indeed a lens.PS: I clicked the "follow" button at this discussion. 😃
Posted
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zutopian
in response to JeanTate's comment.
This thread may also be a good place to start a discussion on the how of following up on lens candidates. For example, if there were a spectrum of this galaxy (there isn't, or at least not an SDSS one), at least several zooites could have a go at analyzing it, to see if there's any evidence of a lensed galaxy (or other features, such as a polar ring, perhaps).
Maybe it might be useful to check the images by using the multiwavelength viewer.
Posted
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Capella05
moderator
Also, modelling the candidate would be useful - if a reasonable model can not be created it will be easy to discount.
It may not be possible as the lens modeller we used on SW will not be linked to this dataset. I will send a few emails to see if it is possible.
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Capella05
moderator
The title of this discussion is "Elliptical vs Spiral-".: You might want to change the title into "Elliptical vs Edge-on".
I was just using the terminology used by the original posters in the comments section, as we might be having some guests dropping by, I will change it. 😃
Posted
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Capella05
moderator
Just heard back from @psaha from SW 😃 He has had a look at AGZ0001v99 and says "there is nothing to discount it or to recommend it - but further investigation is always good" .That is not a bad comment from a scientist 😃
Also, he told me that the lens modelling tool (from SW) is down at the moment (after a recent server upgrade), but he will make sure it is up and running with the image I can model.
Now my dinner is cold, and I need some off time. Later 😃
Posted
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Aprajita
Hi @capella05 & @zutopian
This looks like a plausible lens candidate, but we really need better (more sensitive, better resolution) imaging to learn more about it as the potential arc is so faint in the SDSS images. The bulges of disk galaxies can be massive enough to cause lensing, so it's not impossible. I'm going to have a go with the examine tool and think what imaging possibilities might be available (or if there's archival imaging data) as the next observing deadline is end of Sept. and then it will be another 6-8 months before we get data IF the observing proposal is accepted!
Thanks
Aprajita.Posted
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Capella05
moderator
Just to add some clarification, although she does not have a tag - @Aprajita is a co-PI of Spacewarps 😃
We are very fortunate to have her popping by, and having a look. She does have a has a toddler (and a job!) to contend with 😄
Posted
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Aprajita
I forgot to add, that the faint blue arc may be the consequence of an interaction with a lower mass & star forming neighbouring galaxy where the star forming disk gets disrupted and can make blue tidal tails like this so we can't exclude that possibility. We equally can't exclude that this is a chance alignment along the line of sight.
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zutopian
Hi Aprajita,
welcome to GZ Talk ! 😃
Thanks for your comments !
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zutopian
I would like to mention following paper.:
A search for edge-on galaxy lenses in the CFHT Legacy Survey
Jean-Francois Sygnet (IAP), Hong Tu (IAP), Bernard Fort (IAP), Raphael Gavazzi (IAP)
(Submitted on 25 Jan 2010 (v1), last revised 31 May 2010 (this version, v2))
http://arxiv.org/abs/1001.4336Posted
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JeanTate
in response to zutopian's comment.
I would like to mention, that I said in the other discussion, that I think, that it isn't an spiral/disc edgeon, but an elliptical= smooth galaxy. If so, it is more likely, that it might be indeed a lens.
This is one aspect which any ordinary zooite can check up on, on their own! 😄 Here's how* ...
Click through to "View on SkyServer", then click the PhotoObj link in the left-hand panel.
A really long list unfolds; scroll down until you find "deVAB_r"; that stands for "deVaucouleurs b/a r-band", or the estimated ratio of the minor to the major axis, in the r-band, using a deVaucouleurs model. In plainer English, how 'oval' is the galaxy? This particular number is 0.389806.
Next, calculate the galaxy's "E" value: an integer, which is ten times (one minus the b/a ratio), or INT(10*(1-b/a)) for the Open Office spreadsheet I usually use; I get 6.
So if this galaxy is an elliptical, it's an E6 elliptical.
Now E6 ellipticals - if there are, in fact, any - are wimps, in terms of mass (see this post, over in the GZ forum; the whole thread may be of interest too). So, for this galaxy to be massive enough to be a likely lens, maybe it's better that we hope it's an edge-on, or highly inclined, disk galaxy! 😮
And that's what I reckon it is ... a quick rule of thumb I use is "does this have 'pointy ends'?" (if so, it's an edge-on disk galaxy). This one doesn't, quite, but it's close (I think).
Hmm, so how about the galaxy's likely mass, is there any way we - ordinary zooites - can find out?
Yes, there is, in general.
Not long after SDSS DR7 came out, a number of 'value-added' catalogs were published, and more followed later; here is a list of them. One such is the MPA-JHU (full names, anyone?) Total masses from fits to photometry catalog. Is our galaxy in this? If so, how can we find its estimated total mass? Are there other - freely available - catalogs which might give an estimate of the total mass (or even total stellar mass) of this galaxy? How could we find out?
Isn't astronomy fun! 😄
*Note to any astronomer reading this: yes, I know I'm simplifying (and I can expand my 'how to' considerably, so that it gives a more robust answer); however, it's important - I believe - to start simple ... (you are, of course, more than welcome to jump in and fix up any errors I have made; in fact, please do, that's how I learn)
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zutopian
The same galaxy is also available in DR7 (not a photometric object, but has an ID: 587729753282576710), but there is a filter problem in that field.:
http://cas.sdss.org/dr7/en/tools/explore/obj.asp?ra=253.686139&dec=36.715147
http://cas.sdss.org/dr7/en/tools/explore/obj.asp?id=587729753282576710Posted