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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