Galaxy Zoo Talk

Merging Galaxies?

  • RichardHogg by RichardHogg

    Would these galaxies be considered merging?

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  • Budgieye by Budgieye moderator

    Welcome RichardHogg . Galaxies that seem deformed and pulling on each other would be merging. These look unaffected, so are probably all at different distances.

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  • mlpeck by mlpeck

    If you click through to NED the first line gives an erroneous photometric redshift of z=0.059. The central galaxy is CGCG 407-053NED02 with zsp=0.0402 from the CFA redshift survey. Its two nearest neighbors (CGCG 407-053NED01 and NED02) have redshifts compatible with being at ≈the same distance.

    It's also either the brightest or second brightest galaxy in the cluster Abell 2657 and was classified as a cD galaxy by Tovmassian and Anderbach (2012).

    I wonder about the wisdom of mods making declarative statements about objects in the classification pipeline, especially when something is a judgement call. Doesn't anyone worry about biasing future classifications? To me the spiral to the north looks disturbed, and I see faint hints of a shell and other peculiarities in the central galaxy -- enough to click yes on "is there anything odd?"

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  • Budgieye by Budgieye moderator

    Thank you for your analysis and suggestions for improvements, mlpeck. Since I don't have half an hour to answer every question in detail, I will happily pass them on to you, but please complete them quickly because newbies don't wait around.

    There is an example of a completed analysis below, and for all that work that I did,, I don't think they are merging. My judgement call wasn't too bad after all.


    started analysis18:34

    No easy way to check spectral charts, so I will assume NED has the correct value.

    The target galaxy

    enter image description here z= 0.059100 or z=0.040238 no spectral charts easily availble to check.

    http://skyserver.sdss.org/dr9/en/tools/explore/obj.asp?ra=356.23925258009285&dec=9.193153673765927

    the galaxy to the left

    enter image description here z=0.036455

    http://skyserver.sdss.org/dr9/en/tools/explore/obj.asp?id=1237678905707200518

    The galaxy above

    enter image description here z=0.041232

    http://www.wolframalpha.com/input/?i=redshift+z%3D0.040238&rawformassumption="FSelect"+->+{{"LookbackTimeFromRedshift"}}

    Distance from us (lookback time)

    Target galaxy 543 million light-years away.

    Galaxy to left 493 million light-years

    Galaxy above 556 million light-years

    I don't think galaxies within 5 million light years are interacting, certainly not merging.


    by mlpeck Remove
    If you click through to NED the first line gives an erroneous photometric redshift of z=0.059. The central galaxy is CGCG 407-053NED02 with zsp=0.0402 from the CFA redshift survey. Its two nearest neighbors (CGCG 407-053NED01 and NED02) have redshifts compatible with being at ≈the same distance.

    It's also either the brightest or second brightest galaxy in the cluster Abell 2657 and was classified as a cD galaxy by Tovmassian and Anderbach (2012).

    I wonder about the wisdom of mods making declarative statements about objects in the classification pipeline, especially when something is a judgement call. Doesn't anyone worry about biasing future classifications? To me the spiral to the north looks disturbed, and I see faint hints of a shell and other peculiarities in the central galaxy -- enough to click yes on "is there anything odd?"

    Posted

  • mlpeck by mlpeck in response to Budgieye's comment.

    Well, if you suspect that a set of galaxies might be dynamically linked you can't just plug their redshifts into a cosmological distance calculator and assume those are the actual distances. You also need to look at peculiar velocities relative to whatever you think is the correct cosmological redshift and see if they're reasonable. The formula for peculiar velocity for an object with redshift z relative to redshift z0 is vrel = c(z-z0)/(1+z0) (this is derived in Davis and Scrimgeour 2014 among other places).

    According to Struble & Rood (1999) Abell 2657 has a mean redshift of 0.0402 and velocity dispersion σ of 829 km/sec. An object with velocity within about 3σ would usually be accepted as a cluster member if it's also close on the sky.

    The three nearby galaxies in the classification image have between 2 and 5 redshift measurements listed in NED: for some reason NED just picks one rather than, say, try to combine them somehow. For CGCG 407-053NED02 the adopted redshift was 0.040238, which is within about 10km/sec. of the cluster redshift. The 4 distinct measurements are all reasonably close: I get a weighted average z=0.04037 using stated uncertainties to form weights. The value 0.059 is a photometric redshift from a catalog by Wen and Han (2015). It's obviously wrong so throw it out.

    NED01 has two redshift measurements, one of which is wrong so we take the NED choice of z=0.041232. That works out to vrel=+286 km/sec relative to the dominant galaxy NED02.

    NED03 has 3 redshift measurements, and the NED adopted one is the lowest by several hundred km/sec. Using the adopted redshift of 0.036455 the velocity relative to NED02 is -1090 km/sec, which seems like a lot but is only 1.3σ. I get 0.03837 as the weighted mean of the 3 measurements, or 0.04096 if I toss the outlier. The peculiar velocities then work out to -538 and +208 km/sec. So regardless of its exact redshift its peculiar velocity makes it a certain cluster member.

    Whether anything is merging is another matter. Clusters in the nearby universe apparently aren't favorable environments for mergers because velocity dispersions are too high. I'd still flag this galaxy as "odd" though.

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  • Budgieye by Budgieye moderator

    You are welcome to check on my posts for errors and omissions, and make Galaxy Zoo Talk a better scientific arena. Galaxy Zoo Talk should be a place where we can all do some of the work. I think there is not enough posting by other members, giving the impression that Talk is the moderator's own playground. If other people would help out, then moderators don't have to make judgement calls in 10 seconds on the images. I would like to mention for anyone unfamiliar with Talk, that several of the moderators are unpaid volunteers, just like yourselves.

    Whether anything is merging is another matter.

    I thought that was the question?

    The galaxies are apparently not merging based on my 30 minutes work which supported my 10 second judgement call. If a galaxy is "odd" I put this down to past interactions and mergers, not a current merger.

    Thank you for the information of the peculiar velocity of the galaxy group. I did not know how much variations in local redshift would affect the answer that I give. Perhaps you can elaborate on that.

    I also ignored the effect of proper motion, on the assumption that galaxies are widely separated in space, so "sideways" motion would be insignificant in comparison.

    Posted

  • JeanTate by JeanTate

    Galaxy clusters can be very complex environments. The high space density of galaxies, near the centers of rich and/or condensed clusters, can be very high, making chance alignments of two or more cluster galaxies almost inevitable. So two galaxies apparently close (on the sky) could be a long way apart, e.g. one well in the foreground of the other ... and both are still cluster members.

    If you get good spectroscopic redshifts of lots (up to thousands) of members of a galaxy cluster, you'll usually (but not always!) find that the mean (or median) redshift is close to that of the BCG (brightest cluster galaxy), which is often a cD. The variation in redshift can be estimated by the standard deviation of the distribution of redshifts of all cluster galaxies; this is nearly always ~a to several hundred km/s.

    But not all rich/condensed galaxy clusters are nice and symmetrical ... some are merging, some have 'lobes' or 'extensions', some ...

    The universe is a big place, and sometimes we come across the most (apparently) amazing things. For example, a galaxy whose redshift is well inside the redshift distribution of a cluster that's very close (on the sky), yet it may turn out to be completely unrelated (a "field" galaxy)! 😮

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  • Ghost_Sheep_SWR by Ghost_Sheep_SWR

    If I read it correctly, I'd say Budgieye's main point is not about the science.

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  • Budgieye by Budgieye moderator

    Correction to my previous post: All (not some) of the moderators are unpaid. Scientist have jobs, but I doubt that posting in Talk is part of their job description.

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  • Ghost_Sheep_SWR by Ghost_Sheep_SWR in response to Budgieye's comment.

    Was wondering about that sentence, good to clarify 😃 When I first read about mods being volunteers here I was flabbergasted, so much time and dedication (also on multiple projects, not just here!).

    That's why I hope there will be a scientist / scientists that do have that on their job description, either fully or partially. Not justto help in Talk but pick out interesting things for possible follow-up / publication. Probably a lot more science / papers can be extracted from GZ than is happening right now.

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  • GoldenRule by GoldenRule

    Thank you to all moderators for volunteering your time to help answer questions. No one(except me) is right 100% of the time. Question everything never follow blindly.

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  • JeanTate by JeanTate in response to Ghost_Sheep_SWR's comment.

    A bit OT, sorry ...

    That's why I hope there will be a scientist / scientists that do have that on their job description, either fully or partially. Not justto help in Talk but pick out interesting things for possible follow-up / publication. Probably a lot more science / papers can be extracted from GZ than is happening right now.

    Something I read recently (from memory, which could be faulty): one of the new radio surveys well on the way (EMU? SKA?) is expected to discovery/produce/whatever 70 million new sources. Current automated methods are expected be able to classify ~90% of these. So who's going to classify the other ~7 million?

    RGZ (Radio Galaxy Zoo) may be releasing its first DR soon. It wouldn't surprise me in the least to learn that FIRST sources with good associations (i.e. the host galaxy/object can be identified with high confidence) comprise only ~80-90% of those for which classification data (i.e. our clicks) is now in (i.e. in Zooniverse jargon, these Subjects have been Retired). And I suspect that the professional radio astronomers on the team may be feeling a little overwhelmed by the huge numbers of sources identified with difficult-to-classify morphologies (or just downright weird).

    So who'd going to do the research to discover all the strange new beasts that exist in the radio universe? Certainly there are far too few professionals to do the job!

    Bringing this back to GZ: I reckon for every new zooite-powered discovery (Voorwerps, GPs, ...), there are at least a hundred in the waiting room. Maybe we zooites could run a lot further with many of these, to the point of writing papers even?

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  • Ghost_Sheep_SWR by Ghost_Sheep_SWR in response to JeanTate's comment.

    I understand where you are coming from and respect your long-time dedication and achievements at Zooniverse / GZ so I hope you don't get me wrong, but I really feel you are turning the world upside-down.

    It's like we have a treasure chest full of rough gemstones in front of us but then say ''ah well, not enough gem cutters around, hmm maybe the locals can give it a try?'' Get more gem cutters ASAP would be my first reaction.

    I've also thought it a bit that strange that not more scientists are attracted until I realized that's not true; they all just start their own Zooniverse projects instead of doing GZ. And ofcourse a number of other reasons I don't know, probably budgets / grants etc.

    For me without a solid background I can only collect so much information about something interesting as is already available online at this moment, although it's probably not 100% impossible to actually write a correct and useful paper. We'll see what will happen in the future, but it is kind of a waste at the moment. If I'm correct it's happenend before that non-GZ papers are written about stuff that was already marked in the old forum.....

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  • Budgieye by Budgieye moderator

    The Forum (the first Talk) was set so volunteers could discuss the galaxies. I think it was a surprise that volunteers were making scientific discoveries.

    Now we need Talk Lite, where newbies are introduced, and we post poetry and

    Talk: Heavy?, where we can do rigorous science?

    http://www.galaxyzooforum.org/

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  • hallkr by hallkr

    ya'all have pretty mouth words.

    while the science is way over my head in respect to BFG and red spectroscopes and the like, I appreciate the debate.

    I was curious, too, if these were merging galaxies. and here's my take way from the sciency-speak (in case anybody else is as plebian as I):

    nope. not merging.
    they could have been interacting once upon a time, but they're probably not interacting now.
    and likely, they're not even near enough to be interacting with each other.
    just because they look close to each other, or one looks larger than the other, that's not necessarily the case.
    That's how they normal sized actors to look pint sized next to Will Ferrell in "Elf:" camera angles and perspective.

    Thank you, science community for allowing me to participate. ❤️

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  • Budgieye by Budgieye moderator

    Please do not use the comments by zoob1172 as an indicator of how the rest of us classify. His classification was his own system. I only found his NGC galaxy identification useful, and also his cross links.

    Sorry, I don't know what a "mouth word" is, and I am a native English speaker. I try to use plain language so the other people here who speak only a little English can understand me, or use computer translation with some success. I haven't seen the movie "Elf" but I can guess the analogy.

    Yes, this group of galaxies is probably not a cluster, looking at shape and colour.

    I agree that we look at the galaxies and use shape and colour to classify. That is why Galaxy Zoo is successful. People can do this while computers cannot. The really expensive studies can only be used on 10,000 galaxies, and we classify the next several million and hope we are correct.

    Talk was set up for our fun, socialization and learning. None of us are astrophysicists. We learn astrophysics a little at a time. If we need an expert opinion, we call in a scientist. Eventually, we learn enough to make discoveries of something new.

    Here is my chart of how the colour of galaxies change with distance. It took me about 150 hours to make, but I learned a lot by doing it. I share it so that we may all learn to understand the images that we see.

    Galaxy Redshift Chart https://talk.galaxyzoo.org/#/boards/BGZ0000007/discussions/DGZ0000ulp?page=2

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