Ways of knowing how distant from us is a radio source.
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sisifolibre
Hi. Provably there are a thread about this but I can't find it.
Looking to hourglass whose lobes seems disconnected and separated thousand (millions for giants) of parsecs, I think that the only way to know the origin of a far radio source with more certainty, is to know if the radio source is in a similar redshift that his possible origin or host. As far I know we can't to know the redshift with a only-radio source but, There are any way to know how distant from us is a radio source?.
To know how distant is a radio source by itself can be very useful for a lot of reasons, but one of the most interesting reason is to know with certainly his host. In any case, this can be a part of a other general question: how do you determine, with a high degree of confidence, what AGN is the ultimate source of any particular, extended, radio emission? Appart of observing to fainter levels of radio emission, and with higher resolution. I think that we would not be here if there is an easy method, but may there are a "difficult" method?
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ivywong
scientist, admin
in response to sisifolibre's comment.
Hi @sisifolibre,
We need to identify the host galaxy before we can infer the size of the radio jets. Of course, if you have SDSS distance measurements then you'll know the answer easily. However this will only apply for a subset of the radio AGN here. So the rough guide to distance is as follows.The nearest galaxies are observed in both SDSS and WISE. These nearest galaxies will appear bright and well-resolved in SDSS and are likely to be observed spectroscopically by SDSS. As we move further away in redshift to ~z>0.2, these galaxies will appear fairly unresolved and much fainter. If the host galaxy is observed in WISE but not SDSS, then you're more distant again. If you click on the WISE link in RadioTalk, you'll see that WISE has 4 bands. The nearer sources will be detected typically in WISE bands 3 and 4. However more distant sources will only be seen in WISE bands 1 and 2. Lastly, the most distant sources are the #IFRS sources where you do not see the host galaxy in WISE. These are the AGN that exists when the Universe was very young.
In terms of connecting the radio emission to the host galaxy, this is difficult without more sensitive observations and we generally rely on the symmetry of the hourglass and double lobes to determine the host. Sometimes the radio "colours" can help but we only have single band radio data here. Does this help?
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sisifolibre
Hmm I haven't explained well my question, or I can't understand well your answer.... I don't want to know the size of radio jets. I want to know if its possible to know how distant from us is a radio source (a jet- lobe) without identify the host galaxy. Even with little accuracy, for example to know if the radio lobe is at z>~0.4 or at z~0.1.
May be these radio "colours" (this is radio wavelength?) can help?
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ivywong
scientist, admin
Sorry @sisifolibre for misunderstanding your question. No, it is not possible to tell the distance from just the radio morphology. We can only infer that a large angular extent translates to a nearer set of emission than one with a smaller extent.
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sisifolibre
Thanks @ivywong. I asked myself if there are something in the radio data itself of the lobes that have any clue about his distance, but by your words, I guess that there aren't nothing.
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ivywong
scientist, admin
You are very welcome @sisifolibre. This is why cross-identifications across all wavelengths are so important in astronomy. We can never see the full picture if we only look in one small band of the EM spectrum.
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sisifolibre
I express here a feeling that I have lately, I hope you don't misunderstand me. The problem I see lately is that the classifications that we do here to identify the host galaxy are very subjective sometimes. For examle look at these cases below. In all these subjects we doubt about who is the host. Even in the last one (ARG000153g) we doubt about if it's a real radio source. I think that in these cases, if we change the "suspicious host" for an ETG, everybody would say that the ETG is the host, and these subjects would not call us attention. I have the impression that the misalignements that we consider important here, would be insignificant if the "suspicious host" were an usual ETG, and we search other explanations only because the most evident host is a rare object. Because this I asked about if there are something in the radio emission that can give us some objective or impartial clue about who is the host. But I guess that we are doing what we can with what we have, and it isn't very important dobts, at last. Or may be I'm absolutely wrong at all! XDXD
https://radiotalk.galaxyzoo.org/#/boards/BRG0000002/discussions/DRG0000e5j
https://radiotalk.galaxyzoo.org/#/boards/BRG0000006/discussions/DRG0000efq
https://radiotalk.galaxyzoo.org/#/boards/BRG0000002/discussions/DRG0000ev5
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ivywong
scientist, admin
Hi @sisifolibre, you are absolutely correct about the issues that you highlighted. There is a level of subjectivity that is inherent from these cross-matches. This is why we typically require 20 independent participants to classify each subject. We hope to use statistics to average out these "randomness". When we conducted an experiment of 100 subjects on the science team, as per paper #1, we found that there are 3 categories of classifications: 1) where everyone is in agreement; 2)where there is in general agreement; and 3) there aren't any obvious answers without further observations. The main gain in RGZ is in the second class of objects. It might not sound like much but the fact that we're making new discoveries with this very old dataset suggest that you are all doing a fantastic job!
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sisifolibre
Well, second group is a big group. Thank you for you patience and compresion with my questions.
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ivywong
scientist, admin
You are very welcome 😃
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