stars or globular clusters?
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jstoke
What is going on in the halo around this galaxy? Are those some globular clusters, or small companions, or just foreground stars? (I'm looking at the five or so fuzzy objects above the central elliptical galaxy.)
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JeanTate
Without going to the SDSS data and finding out what redshift(s) this/these have, I'd say companion galaxies ... the bigger ETG (elliptical) looks like a cD, the giant elliptical that is often found at (or near) the center of a rich cluster of galaxies. However, it may be that this is much closer, and so the fainter blobs are indeed globular clusters (GCs). If they're fuzzy, they're not stars (but checking with SDSS Navigate should tell for sure).
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jstoke
in response to JeanTate's comment.
Thank you for inspiring and directing me. This was the first time for me to click on Navigate and derive some information from that feature. I just learned a lot about the SDSS with some other links and help menus as well, and can now formulate my question. The z for this object is around 15.35 and the fuzzy objects all have a z of around 19-21, so I'd say they are moving the same relative to us, but not moving the same as the primary object. Here's my question. What sort of difference in the z value indicates "not associated by the same relative velocity" ... is it in the hundredths, or is 19 and 21 functionally very similar. I guess I have no frame of reference or previous experience looking at these z numbers, so am not sure how similar they have to be before I conclude with some certainty that the objects are likely associated. I'll start looking at that on the next items of interest, and learning what sort of range exists ... . Thank you for the time you took in commenting on my first question.
I think I'm still a Newbie ... I just looked at the z for a bunch of starforming regions in one galaxy, and the numbers varied widely ... I think I'm confusing the z band data (far infrared in u g r i z ... the 5 bands) with PhotoZ (the redshift).
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JeanTate
in response to jstoke's comment.
You're very welcome! 😃
You know, the way astronomers name things can sometimes seem as if they're deliberately trying to make it hard for newbies! (They're not; the way things get their names in astronomy is no more perverse or convoluted than the way crazy stuff happens in language in general; one of my faves is "what is a 'ghoti'?" 'gh' as in 'enough'; 'o' as in 'women'; 'ti' as in 'nation')
'z' has two meanings in SDSS-speak; it is the redshift, which can be either derived from spectroscopy or estimated from the photometry. This link will take you to an SDSS DR8 Explore page of a galaxy which has a spectroscopically derived redshift (0.174, in this case). You can see there's a whole panel devoted to the 'SpecObj'; that panel is missing for AGZ00061u7 (and indeed all nearby objects).
You probably know that the SDSS camera has (had; it's now mothballed) five filters, and that the images we get to classify come from just three. The five filters are named u', g', r', i', and z'. It is standard, for almost all the SDSS stuff we'll ever encounter, for the 'prime' symbol to be dropped, giving u, g, r, i, and ... wait for it ... z!
Except for some quasars, nothing we'll ever come across in SDSS images will have a redshift greater than ~0.4 to 0.5.
Photoz is an estimate of the redshift, based on the colors; it's often quite good (provided you take into account the stated uncertainty), but sometimes it's wildly off. For SDSS J153457.20+285423.0 - the galaxy in my link - Photoz is 0.156 +/- 0.019, which is pretty close to 0.174. For AGZ00061u7, the Photoz is 0.251 +/-0.039, which seems about right, given the apparent color (to my eye) of the SDSS image. That makes this galaxy a giant elliptical, and the fuzzy blobs around it smaller elliptical galaxies.
One thing that messes things up in this image is that it's near the edge of an SDSS field, and the bright star nearby makes the background bright (and red) in half the SDSS image (it's weaker in the GZ image, because of the superior processing, but still visible, and annoying).
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jstoke
in response to JeanTate's comment.
Thank you very much! So, for the redshift (not just the data from one of the five filters), I need an object with a spectrum ... and there it is on the sample you gave, z of 0.174. Great! Now I know where to look. I'll start looking more closely at objects with spectra and try to make sense of it all.
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