Radio Galaxy Zoo Talk

morphology of distant galaxies

  • WizardHowl by WizardHowl

    In this case a slightly #overedge #doublelobe has an optical match for the bright IR source between the lobes as SDSS J140023.50+143833.9 Z_ph~0.61-0.77 which appears as an elongated red blob. I am wondering what type of galaxy this and similar objects are?

    Out of curiosity, I've been looking at SDSS images of a lot of the galaxies where I'm confident of the identity of the host and it's not unusual for objects with redshifts of ~0.6-0.7 to appear as a fuzzy red blob. They are normally too faint for a spectrum (so the redshifts are photometric and more uncertain) and seem near the detection limit of SDSS, although they can be quite bright in WISE (as here). The small number I have seen spectra for (sorry I can't give examples, I find it difficult to trace through my past classifications for this kind of detail) mostly seemed closer to ellipticals than any other type, being fairly featureless with weak absorption and little or no emission but sometimes stronger emission lines are present - but not the very strong, wide emission lines of quasars. Does that mean these red blobs are what ordinary ellipticals look like at greater redshift? If I come across examples with a spectrum I'll add them to the discussion, but for optical/IR this is a typical case.

    I should point out that I have no experience of analysing spectra and it is only through looking at examples in SDSS after classifying that I have come to distinguish three main types: quasars (strong, wide emission lines); ellipticals (absorption lines but little or no emission, absorption lines often seem wide at their base whilst emission lines, if present, are narrow) and disks (little or no absorption but strong, narrow emission lines, often very many of them).

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  • DocR by DocR scientist

    I'm not confident of the classification. Could be two independent sources. But your questions about appearance in SDSS (spectra and image) are right on the mark. I'm not the best to respond, but i'll make sure other members of science team see the above thoughts.

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  • WizardHowl by WizardHowl in response to DocR's comment.

    Thanks for responding. I've found two such objects with spectra this afternoon:

    http://radiotalk.galaxyzoo.org/#/subjects/ARG0003bpd - SDSS J120613.12+065652.1 Z_sp=0.655 red colour, fairly featureless spectrum with weak emission and absorption, radio emission compact but slightly extended in shape

    http://radiotalk.galaxyzoo.org/#/subjects/ARG000256b - SDSS J082547.29+232238.1 Z_sp=0.632 red colour, both emission and absorption present, hourglass radio emission

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  • ivywong by ivywong scientist, admin

    Thanks for checking these objects out. The spectra for these objects are all fairly low signal-to-noise and apart from telling us that they are fairly red and evolved galaxies. I consulted my more optically-oriented colleagues and they too were not able to discern much from these spectra and suggested that the only way to get more information is to do follow-up observations with a larger telescope.

    In general we do find that most radio jets come from spheroidal/ early-type/ elliiptical galaxies.

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  • klmasters by klmasters scientist, admin

    Hi, I'm Karen Masters - I'm the Project Scientist for the optical Galaxy Zoo, and I've been working with SDSS data for some time. In SDSS (or any ground based imaging) beyond a certain redshift pretty much everything looks like a fuzzy blob. This is why in Galaxy Zoo we recommend serious mistrust of any classification of a galaxy beyond z=0.08. This is not because the citizen scientists can't classify those, just that the imaging itself isn't good enough to really tell.

    These objects are classic examples of the types of galaxies targeted in the BOSS survey part of SDSS. That was looking to measure redshifts for massive galaxies at around these redshifts to better map large scale structure in the Universe. So I bet any examples with redshifts have BOSS redshifts.

    I wrote a paper about the morphology of BOSS galaxies a while ago - in SDSS they are all fuzzy red blobs, but in a tiny fraction which have HST imaging you can see all sorts of details. That's described in more detail here: http://blog.sdss3.org/2011/08/11/what-types-of-galaxies-are-in-boss/

    Thanks again to everyone participating in both Galaxy Zoo and Galaxy Zoo: Radio. We're really very glad you're all here. ๐Ÿ˜ƒ

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  • WizardHowl by WizardHowl

    Many thanks for your interesting and educational replies!

    I suppose you've already checked the 240 BOSS galaxies imaged by HST for radio emission (those that are in FIRST)?

    Some interesting examples in the paper you mention (arXiv:1106.3331) of mergers - and there are still more galaxies in the background of some HST images. As many of the BOSS galaxies were selected to be massive ellipticals it's perhaps no surprise that there are other galaxies around and these massive galaxies are also often radio sources.

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  • KWillett by KWillett scientist, admin, translator

    Similar thoughts from another science team member, Graham Cotter:

    "I think what we are seeing here is how the population mix changes with radio flux. Crudely speaking the quasars are beamed in the radio, so they are a larger fraction of the population at bright flux level than at faint. And so the the optical counterparts tend to be preferentially red E-gals, and indeed these donโ€™t necessarily always have bright emission lines."

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  • garretcotter by garretcotter

    Thanks for posting that Kyle while I was getting my account set up, and Hi everyone. I'm Garret Cotter, I'm a lecturer in the astrophysics group at Oxford University in the UK and I'm interested in many aspects of active galaxies, especially the jets.

    To expand a bit on this -

    The material in the radio jets is travelling very close to the speed of light. The theory of relativity tells us that there will be a "searchlight" effect, where the radio emission from the jet is concentrated in the direction of motion. When we look around the sky, if we come across a source where the jet is pointing towards us, we see it is being extremely bright, sometimes tens of thousands of times brighter than if we were viewing it sideways-on. This means that if we make a radio survey of the sky, containing objects with a range of observed fluxes down to some limit, we'll find that the sources where the jets are pointing towards us tend to be over-represented at the bright fluxes, and as we go down to the fainter objects, we tend to see more and more of the objects that don't get boosted by the relativistic effect.

    But that's not the whole story - why do the objects then look different in their optical images and spectra? We think that in many of these objects, the material swirling around that eventually spirals into the black hole initially collects in a big, dusty, doughnut shape, with the jets pointing outwards from the hole in the doughnut. So if we're looking at an object close to the direction of the jets, we can see right into the core, near the black hole - and in the optical, this is where we see the very hot emission and bright, broad lines from clouds of material swirling round the black hole at very high speed. But if we're looking more sideways on, the dusty material blocks out our view of the bright core. And without the blindingly bright core swamping our image ... we can see that these objects actually live in big, red elliptical galaxies. Overall, as we get to fainter radio fluxes, we tend to see more sources identified with galaxies.

    Of course, this is a very broad-brush picture based on our knowledge from existing surveys. The devil is in the detail and finding new and interesting objects with the new surveys is what we need to make our physical models more complete and more confident. Finding these interesting objects in Radio Galaxy Zoo is exactly what we need to do this!

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  • JeanTate by JeanTate

    Quite by accident, I stumbled across this old thread, started January 19 2014! ๐Ÿ˜ฎ

    It's very cool, and I wonder why no one has posted to it, for many months ... ๐Ÿ˜ƒ

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