First off, thanks for the reply. I'll try and provide a brief (or maybe not so brief... sorry) lesson in eyeball spectroscopy as part of my reply. To really understand these sources, you have to do a deeper analysis of the spectrum, fitting the various lines and the continuum. But you can make some obvious comparisons between sources much more simply.

Also, a technical question about the board itself: when I click the "quote" button, it only quotes your reply, not my comments that you are replying to. Is there a way to get it to quote the entire conversation, so I don't have to go back and fill it in?

Hmm... Unfortunately, the STSci archive is down, so I can't access the archival HST data for that region, and it's out of the SDSS coverage area. But glancing at their radio map, the radio source that they are assigning to the galaxy *could* just be a jet from the quasar. I can't say much without knowing more about the objects. According to the data from Lehnert et al. 1999, the redshift of 3c343 is actually 0.998, and they call it a Seyfert 2 galaxy. Though, that's a fine splitting of hairs, if you ask me.

If you know the point-spread function of your imaging system (telescope+camera), you can do PSF subtraction and masking to recover things that would be otherwise hidden due to the brightness of the point source. Here's an example with 3c273 (the brightest quasar, visible by eye in some amateur telescopes!) from Hubble's now dead ACS:

http://www.spacetelescope.org/images/html/opo0303b.html

By luminosity profiling, do you mean determining the light profile of the source? If so, then yes: the light profile tells you whether a source is a point source, or an extended source. But I don't think the DSS PSF is well enough determined to do PSF subtraction

That's ok, we needed a common place to start from. It's apparent that you've put some thought into this, but there's a *lot* of data out there. Your definition is pretty close to that of Arp et al., from what I can tell.

Absolutely! You can't determine the redshift to something without a spectrum (well, photometric redshifts are getting pretty accurate these days, but they are based off of an understanding of the spectral properties of the sources), and the spectrum of a source provides an immense amount of information, besides just redshift. The spectrum of a star and the spectrum of a quasar look drastically different, because they are produced by wildly different systems. I'll give examples below.

The problem that SDSS has had with "stars masquerading as quasars" is that at certain redshifts, quasars have very similar colors to certain stars. What exact redshifts this happens at depends on what the exact filters are that are used in the initial photometric measurement. In the case of SDSS, "quasars" (point sources with certain colors) are selected for follow up spectroscopy based on their u,g,r,i,z colors, and a variety of interesting and rare types of stars were actually discovered this way! For example, spectra that were similar to white dwarf stars, but different from any white dwarfs previously known.

Just making sure. The SDSS spectral classification algorithms are quite good, but there are always the edge cases that cause trouble. Heck, the photometric classification is hard itself, which is why galaxyzoo.org was created. But remember, if the spectral classification is wrong, the redshift could be as well!

I'm going to reorder things a bit in my reply.

Well, we agree on one thing, anyway! Take a good look at the spectrum (click the spectrum image below SpecObjId=... to get a better view of the spectrum), and keep it open for comparison. Broad emission lines, very blue continuum which does not appear to be black-body emission. This is a pretty classic quasar, by any definition. I'll refer to it as (1) below.

Good guess; this confused the heck out of me when I first saw it. It is an example of a class of very rare objects, called BL Lacs. The choice of name is unfortunate (as with so much of astronomical nomenclature), because the first one discovered, BL Lacertae, was initially classified as a variable star, so now they're all named like after it . Like most BL Lacs, this particular source is a whoppingly-powerful X-ray and radio source---notice the FIRST and ROSAT cross-ids at the bottom of the explore page. The optical spectrum is nearly featureless, but highly variable on short time scales. Because there are almost no spectral lines (absorption *or* emission), the redshift is somewhat questionable.

BL Lacs are currently thought to be systems where we are looking directly into the "mouth of the beast," if you will. Direct line of sight into the central black hole with the relativistic jet pointed at us. There are less than a thousand known BL Lacs: a hundred or so with SDSS spectroscopy. So, according to the standard view, this is very similar to a quasar, but viewed at a particular angle. It has many of the same properties of other quasars: bright in X-ray and radio, high variability, and a bright point source in the core of a galaxy.

Oh, good... I was just chiding myself for not including one of the quasar-like stars on my list, but you found one for me! In the Finding Chart and Navigation pages (I prefer the Navigation, as you can go directly from it to the Explore page for a selected object), you can click the "objects with spectra" box on the left to get red boxes around sources with SDSS spectroscopy. One of your three quasar candidates is actually a blue star. Possibly a white dwarf, though I'm no star expert (if any are reading this, please clarify!):

http://cas.sdss.org/dr6/en/tools/exp...98663046938710

Blue, pure-blackbody continuum, some absorption, no emission? Star. Compare it with (1).

I'll deal with these two together, as they are interacting companions (check the finding chart if you don't believe me; the redshifts are the same). The first is a galaxy hosting a quasar (notice the broad emission lines and spectrum that gets stronger towards blue?), while the second is a star-forming galaxy (notice the strong narrow emission in H-alpha and [SII], and black-body continuum that increases towards ~450 nm, and then falls off?). Compare the two spectra with (1): notice how similar the first is, while the second looks very different.

Also, the first is more than an order of magnitude brighter than the second, in this pair. Something is definitely going on in the nucleus of the first one (which is also a radio and X-ray source).

Keep these spectra in mind as we move on. I'm going to flip the order of the next two...

There is a spiral galaxy there, but compare the spectrum with the two above; which does it look more like, the star-forming galaxy, or the quasar? Also, compare it with (1). If you use Firefox, Opera, Safari, or another browser with tabs, open each spectrum in a separate tab and flip between them quickly: (1) looks like a redshifted version of this one.

I'd just call it pretty.

It is a spiral galaxy, but the nuclear spectrum is quite odd: notice how broad the H-alpha emission line is? That's a line-broadening of several thousand kilometers per second! The continuum is kinda funny as well. Definitely a disturbed system, and the galaxy that probably caused the mess is visible just north-west in the finding chart image. There aren't many ways to get an emission line that broad; the standard view is the accretion disk of the central black hole.

This one is neat, one of my favorite objects in SDSS, and I don't even study stars! It is a double star system, but the resolution of SDSS is not quite good enough to separate them. The spectrum looks odd, but that's because we are seeing the spectra of both stars (one cool and red, the other hot and blue) overlaid on top of each other. What might appear to be emission lines are actually broad absorption in the red star. I think it is a red giant and a white dwarf, though I don't remember what the experts' discussion of it decided.

Good call! I'd actually say this is more likely to be a planetary nebula in the blue, star-forming galaxy, rather like the owl nebula (M27) in our own galaxy. Compare it with this spectrum that was taken of a knot in the owl. Strong [OII], very weak H-alpha and H-beta, generally flat continuum. Also compare it to the not-quasar star-forming galaxy in interacting pair above.

The redshift is high: greater than 2, and it has a strange, very blue spectrum. None of the lines of the other objects above would even be in the SDSS spectral band anymore, due to the high redshift! The strange spectral shape may be due to iron emission (not fit by the SDSS spectroscopic pipeline), or very hot gas (at that redshift, the "peak" around 500 nm corresponds to ~150 nm, which is far-UV!). I might call it a quasar, but that's because the spectrum doesn't look anything like a typical star or galaxy.

Don't assume that anyone else can identify a quasar just by looking at it! Some of them are pretty odd, as we saw above. And whether something is a Seyfert 1, Seyfert 1.5, Seyfert 1.8, Seyfert 2, quasar, blazar, BL Lac, LoBAL or LINER depends on exactly where you make the dividing lines for a given set of definitions. The standard model has them all as more-or-less different views of similar objects, but that doesn't mean that astronomers won't happily make a dozen observational classifcations.

Personally? I don't really know if Arp has an actual working definition of the term quasar, beyond what you said above about looking like a star but having high redshift. From what I've seen of Arp's work, his quasars are simply those objects that he has selected from other catalogs (SDSS, 2df, or even NED) for his own analysis, thus they are whatever the given catalog classified as a quasar.

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Just to add one tiny thing to this excellent post! IIRC, you can, in fact, determine the PSF quite well ... on an individual plate (or region of a plate), but there is considerable variation between plates (certainly epochs!), and (possibly, I just don't remember) poorly constrained variation between different parts of the same plate.

The digitisation of the original plates was done quite well (the plate machines are - were? I don't know if they're still in use - good examples of precision engineering), but the project's objectives did not include producing a product from which consistent, accurate, well-defined PSFs could be extracted*.

If anyone's interested, since the DSS data is in the public domain, you could have a go at determining the PSFs for yourself ... there are certainly plenty of 'unresolved, point sources' (i.e. stars) all over most plates ... which you can find from one or more of the many online databases.

*At least I don't think the objectives included this; I could well be wrong though.

Thanks for the lesson. I ensure you that it's appreciated.

I don't think there's a way around that, other than constructing the quotes yourself.

Sorry, I was little sloppy there. I looked at NED and noticed they give same redshift you are citing, but they refer to a paper from 1985. So then I got to thinking that perhaps there's a mistake in the abstract of the paper I cited, because that's where I copied the numbers. I opened the abstract page of that paper again and then it hit me, they are talking about 3C 343.1 not 3C 343. A small but significant difference, sorry about that. (NED only gives one redshift, the 0.75, for 3C 343.1.)

Yes.

Just so that it is clear: without PSF subtraction, the luminosity profile of the point-like source is useless?

From what I've seen in SkyServer, they are still quite far from truth. I think currently I wouldn't trust studies using photometric redshifts to be more than preliminary results, I think they should be confirmed with real redshifts. (Incidentally, the Scranton et al. study mentioned in this thread uses photometric redshifts...)

This feels little strange to me. If there's many spectral lines in object's spectrum, isn't the redshift determination then quite solid (excluding some anomalous cases)? I mean it would be quite a coincidence to have all lines showing the same wrong redshift, wouldn't it?

You're not the only one, NED has two entries for it, one as a galaxy, and one as a QSO (with BLLAC as description). (BL LACs are something Arp considers to be a later stage to quasars in quasar to galaxy evolution cycle.)

So the redshift of BL LACs don't tell the distance to the objects, because there is quite substantial blueshift due to the velocity of the material in the jet towards us? (I tried to peek at the redshift distribution of BL LACs but apparently you can't query just for BL LACs in NED.)

And in "PrimTarget" it says "TARGET_QSO_CAP"...

As I'm willing to give Arp a benefit of doubt, I don't even need the redshifts to be the same to believe that two objects are interacting. However, due to that same reason, I also don't believe that two objects are interacting just because their redshift are the same.

Well, so far all objects seem to have spectrum that gets stronger towards blue, especially the one you called possibly a white dwarf. So, I guess that feature doesn't distinguish between quasars and stars then?

That probably requires some experience, because besides the fall off I don't notice much difference in lines, sure there's some difference like the couple very narrow lines in galaxy's spectrum, but the galaxy has also a quite broad line at 7000 Å. I don't think I could distinguish these two from each other (in the sense that are they a quasar or a galaxy) without the fall off.

Yes, but this one doesn't have stronger spectrum towards blue.

I can see that. Is this kind of spectrum happening only in double objects, or are there objects that produce similar spectrum by themselves?

Curiously, it says "z = 0.0052 +/- 0.0012 (0.40), QSO" in the image of the spectrum. I wonder if that's an automatically created text, or did someone write it there?

I'm not sure I understand why. This object has couple of quite narrow looking emission lines, and a fall off toward blue.

I don't, I just meant that Arp probably can do it as well as anyone else, I didn't mean I think he (or anyone else) can do it perfectly.

I don't remember seeing Arp address specifically his definition of quasar, but I think that browse through some of his papers would reveal at least some details more. I might check for some later if I have time.

For other parts of your post I didn't respond: thanks for the info!

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Well, it seems that Arp was prepared to this discussion, in his latest paper he suggests a new definition for quasar:

So there.

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Bumping this question ... it's now a week old.

rtomes, do you require clarification of the question? If so, please just ask.

If the answer is "I don't know" (or similar), please say so.

If you need more time to answer the question, please say so, and give an indication of when you expect to be answering it.

First things first: I just noticed that you are in Finland. That would explain why you are always posting so "late at night."

Just trying to be useful. It helps my understanding to try and explain it.

But before we go any further, I should ask how much you know about spectroscopy to begin with? Do you know what a thermal (blackbody) spectrum is, vs., say, a synchrotron (power law) spectrum? What causes absorption vs. emission lines? Things that can broaden emission lines?

Understanding those is necessary for really understanding what is going on with the spectra that I listed, and why they are different.

No, I think I wasn't clear. The light-profile of a source can tell you whether it is strictly a point source (that is, smaller than the resolution of the camera), and determining the light profile of a known point-source (say, a star) is the first step toward subtracting the PSF; the PSF of the camera is what an ideal point source with infinite signal to noise should look like.

If you have a well determined PSF, you can use the light profile to determine the size of things that are just beyond the PSF, as Mike Brown et al. did to determine the size of Eris (middle of the How Big is it? section, about HST). They got a very good constraint on Hubble's PSF, and used that knowledge to determine the angular size of Eris. They didn't subtract the PSF, just determined what it was for comparison.

They are actually probably better than you think: one of the two photo-z methods (the PhotoZ2 table in Sky Server) is described at this website. The absolute error plots (done on the validation set, which have spectra) are given for a few different brightness bins. Basically, for galaxies with r-band magnitude brighter than 20, the photo-z estimator has quite small errors.

I don't think Sky Server has photo-z's for quasar candidates listed yet (at least, computed based on knowledge of quasar spectra), but they are also quite accurate. I can't find a good paper about it online (the only ones are based on the SDSS early data release, which was years ago, and the methods are much better now), but there are some Bayesian statistical techniques that work quite well, with small scatter.

That depends. There are certain combinations of spectral lines that could potentially confuse an automatic classification system. Again, we're talking about the corner cases here, most of the time everything is fine. As you say, if there are many spectral lines, then there's generally nothing to worry about. But an invalid redshift can result from having few and/or weak lines. If they are weak (low signal-to-noise), or if the spectrum is otherwise odd, the spectral classifier can get it wrong (mis-interpreting lines, thinking that noise features are lines), and thus the redshift can be wrong.

Well, as you can see, there is definitely a galaxy there. There is also a very odd nuclear source. The spectrum of the nucleus in this case might include some of the galaxy's spectrum shining through, but I'm not really sure. Remember, NED just contains a list of objects that were described in a paper somewhere, it is not a telescopic survey itself. This object was probably first discovered as a galaxy (in DSS plates, I'd bet), and, from the NED reference list, the BL Lac was discovered in 1989 by Remillard et al. after follow-up from X-ray observations.

I'm rather curious what Arp et al. have to say about the immense X-ray and radio emission from these sources. We're talking outrageous fluxes here (note I said fluxes, not luminosities!). The standard model, where we are looking down the jet from the black hole, explains the features of these systems quite well. Some of the details are still a bit tricky...

That depends. Jet-induced blueshift could be one reason why the redshift might be wrong. Another reason is that there are very few lines in the spectrum, so it is hard to get a redshift in the first place (see my discussion above). In think the redshift of this particular system is ok, because there are enough absorption lines (possibly from the host galaxy) to get a good redshift determination. But for many BL Lacs, this is not the case, because the nuclear source is so much brighter than the host galaxy itself, and the optical spectrum of a BL Lac is nearly featureless.

Yup! That means it was targeted for spectroscopy because the pipeline thought it might be a quasar (though you may have already figured that out). The spectroscopy targeting flags are listed in this table and details on the spectroscopic target selection algorithm are available, if you'd like to peruse them. They aren't easy to digest, though.

Ok, here's a few questions for you (being the present potential supporter of Arp and company), though I'm sure they've all been asked before: how do you propose determining whether two objects are at the same distance, if you don't think their redshifts are relevant quantities for distance determination? I understand that Arp et al. claim that the redshifts of quasars are actually due to some, unknown, undetermined and undescribed physical process, but then how do you determine the distances to things? Remember, there are over a million objects in SDSS DR6 with spectroscopy, including more than a hundred thousand quasars, ~800,000 galaxies and ~300,000 stars... And if the redshifts to galaxies are trustworthy, why not quasars?

And what about galaxies where some other measure is used to determine the distance, and it agrees with the redshift (to a fairly good degree, if not exactly)? What about other measures of distance, say estimates based on normalized galaxy sizes, or rough luminosity distances? Those aren't precise, but they qualitatively match the theory that redshift is a measure of distance: further things are smaller and dimmer.

It does distinguish, but you have to understand the actual spectral shape. That's why I asked about Blackbody vs. power-law spectra above. Stars are generally nearly perfect blackbodies, with some absorption lines due to the photosphere. Quasars generally have a power-law spectrum (flux proportional to frequency^alpha), which you can see in the rate at which the flux increases toward blue wavelengths.

A good place to start about stellar spectra might be the Stellar Spectral Types Project from the Advanced Projects page (the one from Basic Projects is a simpler subset of that one, but the Advanced one has a lot more description).

And now we've run into the problem with using the quick-look spectra in SDSS! They are not designed to do science from, just for a quick glance.

In the star-forming galaxy's spectrum, the "quite broad line at ~7000Å" is, infact, the Hα, [NII] blended doublet. You can kind of make out the doublet in the image, but it is much more obvious in the complete FITS spectrum. The quasar spectrum, on the other hand, has little sign of the [NII] line, and very broad wings (look at the base of that line, as well as Hβ, in particular). This particular quasar has relatively narrow lines, compared to some!

If you want to examine the FITS spectra in detail, you can use specview a free Java application from the Space Telescope Science Institute.

Perhaps, but the broad lines are a give-away. Again, just how broad the lines are compared with the star-forming galaxy above is more apparent in the full FITS spectrum. Also, the continuum does not fall off towards short wavelengths, as happens with galaxy spectra. Again, flip back and forth between it and the one we both agree is a quasar (1): the similarities are striking.

I'm not sure I understand the question, but I'll try and answer what I think you were asking. A spectrum like this is the sum (linear combination, to be exact) of two very different blackbody spectra. It is hard to have two wildly different blackbodies together on the same object: they will thermodynamically equilibrate rather quickly (what's the temperature of things on the surface of the Earth? All roughly the same, and Earth isn't even a blackbody!)

However, what is a galaxy? Lots and lots of stars (and gas and dust, as well). So a galaxy's spectrum (ignoring the gas and dust, for now) should be the sum of a lot of blackbodies of different temperatures. The exact shape of a galaxy's spectrum depends on the stars in it: if they are old stars, it will be redder (cooler), if young stars, bluer (hotter). The gas and dust complicate things because dust preferentially absorbs blue light, and gas can produce absorption or emission lines, depending on how dense it is. So a starforming spiral galaxy might have a blue spectrum like the star earlier, with emission lines from hot gas and some dust absorption, while an old elliptical galaxy in a cluster might show just old, red stars and no signs of gas or dust, because it has all either turned into stars or been kicked out due to interactions.

You'll find that note on all the SDSS quick-look spectra (including stars). Almost everything data-related in SDSS is automatically generated. There are just too many sources for any kind of manual intervention, in general. The few cases where a person was in the loop are usually mentioned (e.g. the MANUAL_MAPPED specZWarning flag).

In this particular case, the spectroscopic pipeline found that the spectrum was best fit by a quasar template-