What in the dickens is a quasar?

parejkoj · #1 (**permalink**) 15-September-2007, 08:31 AM

Because I obviously have plenty of time to spare from my research... I don't really need to graduate any time soon, do I?

Often in discussions of observations, new papers and even ATM vs. FTM (For the Mainstream?) conflicts, these strange objects called quasars come up. But one rarely sees a solid definition of what a quasar is, from any of the involved participants. Without a good definition, how does one distinguish a quasar from a QSO, Blazar, LIRG, ULIRG, OVV, BAL, loBAL, BL LAC, radio galaxy, LINER, Type 1, 1.5, 1.8, 1.9 or 2 Seyfert, active galaxy or active galactic nucleus (AGN)? (did I miss any?) Heck, how does one distinguish between those with a good definition?!?! I'll eventually get into the physics that powers these sources, since the currently accepted unification model brings them all together.

But first, what do they look like, and how do you pick one out of a crowd? And should you even try?

Since it is the dataset I work with daily, and also since the more I use it, the more enamored I become (baring occasional annoyance that its resolution is not as good as the VLT or HST...

), most of my examples will come from the Sloan Digital Sky Survey (SDSS). This has the wonderful advantage that it contains an immense number of photometrically calibrated spectra for galaxies, quasars, stars and wierd blobby things. Thus, it is a perfect place to pick examples from. If I'm ignoring your own personal favorite survey or telescope, it isn't out of spite, just simplicity. Though I will have to bring Keck, HST, Chandra, XMM-Newon, Suzaku, Spitzer, The VLA and the VLT into the mix by the end...

QSO: Quasi-Stellar Object.
To start with, the "common" observational definition of a quasar is an optical point source (star-like) that has a very strange spectrum (not star-like). Hence, Quasi-Stellar Object. That's enough to get us started, but vague enough to get us into trouble right away; different stars can have quite different spectra!

What does the spectrum of a star look like?
A good place to start learning about stellar spectra would be the Stellar Spectral Types Project from the SDSS Advanced Projects page (the one from Basic Projects is a simpler subset of that one, but the Advanced one has a lot more description). Remember how I said I'd be refering to SDSS a lot? That's another reason: they've put together some very nice student projects that anyone can learn a bit from.

I'll give folks some time to go through one of those two projects (the teacher's guide says 3 hours: you should all be able to finish by the time I wake up), while I get some much needed shut-eye. They might have apple cider at the farmer's market tomorrow! I don't want to miss that, even if it is raining. So, I suppose that means extra time for those projects...

For the record, this will evolve out of a discussion beginning with my post in this ATM thread, the basic thrust of which I will steal from liberally. This will be a slowly building thread, similar, I suppose to tusenfem's Plasma Physics for Dummies and Nereid's What is the observational basis for (cold, non-baryonic) dark matter? threads. I can't promise to provide a lot of math, as I'm more of an observtional and statistical astronomer myself, but if it is requested, I can certainly try. And if the mods feel that Q&A is a better location, they can feel free to move it there. I'm planning it as more of an A&Q, I think... And if nobody cares, feel free to point that out, as well!

jami cat · #2 (**permalink**) 15-September-2007, 04:42 PM

Quote:

Originally Posted by parejkoj

What in the dicken's is a quasar?

The Old Curiosity, David Copperfield spoke of, in his Shop, as Our Mutual Friend, The Italian Prisoner Chimes in from The Frozen Deep, A Tale of Two Cities and The Mystery of Edwin Drood. Nicholas Nickleby and Oliver Twist had looked over the Pickwick Papers with Great Expectations. The Haunted Men on Hard Times debating over Dinner at Poplar Walk.
Barnaby Rudge in the midst of The Battle of Life pondered A Christmas Carol, as Martin Chuzzlewit returned to the Bleak House with Dombey and Son searching for their belongings among the The Crickets and the Hearth.

Nope Dickens said nothing of any Quasi stellar Objects...

Ari Jokimaki · #3 (**permalink**) 15-September-2007, 05:30 PM

Ok, I went through the Stellar Spectral Types Project (although I must admit I didn't do all the exercises

), what's next?

KaiYeves · #4 (**permalink**) 16-September-2007, 02:42 AM

Responding to title:
A star that spins at set intervals of time and sends out radio waves when this happens, I think. My impression of one, which cracked up my English teacher was to stare blankly into space and go "neep" every two seconds. Why? Long story.

Ken G · #5 (**permalink**) 16-September-2007, 03:59 AM

I actually think that's a pulsar you're thinking of.

William · #6 (**permalink**) 16-September-2007, 04:21 AM

The following are some basic observations concerning Quasars from the big orange book “Introduction to Astrophysics”, Carrolle & Ostlie, 2nd Edition, pages 1099-1100.

Quote:

The Sloan Digital Sky Survey has catalogued 46,420 quasars. … The brightest entry in the catalog is … SDS 171000.62 at redshift of z = 2.736 having Mi = -30.2. The most distant quasar in the catalog is SDSS 023137 at a redshift of z = 5.416. … In fact there are 520 quasars in the SDSS catalog with redshifts greater than z = 4.

Evidence for Quasar Evolution

…Statistical studies indicate that there are more than 1000 times as many quasars per Mpc^3 (comoving space density) brighter than Mb = -25.9 at z = 2 than there are today at z = 0. However, there is strong evidence that the total number of quasars has not changed significantly from the present (z=0) back to roughly z = 2.

… Apparently, a picture in which a constant number of quasars grow dimmer as the universe expands is consistent with observations for z < 2.

The situation becomes more complicated between z =2 and z=3. … Statistical surveys at both optical and X-ray wavelengths show that the comoving space density of AGNs peak at redshift approximately z = 2.5 and then drops off for z > 3. These studies indicate that the comoving space density declines by roughly a factor of 10 from its peak value at approx z = 4.

This high-z deficit in the number of quasars could reflect a growth phase of supermassive black holes that power the nascent AGN. ….

The following from the “Extragalactic Astronomy and Cosmology an Introduction, by Schneider. Page 183, is a terminology note that radio-quiet quasars, have been found to be radio less loud rather than radio quiet. Hence the term QSO is used for both radio-loud and the more common radio less loud quasars.

Quote:

Quasi-Stellar Objects

… Therefore they were called radio-quiet quasars, quasi-stellar objects, QSOs. Today this terminology is no longer very common because the clear separation between sources with and without radio emission is not considered valid any more. Radio-quiet quasars also show radio emission if they are observed with sufficient high sensitivity. In modern terminology, the expression QSO encompasses both the quasars and the radio-quiet QSOs. .... About 10 times more radio-quiet QSOs than quasars are thought to exist.

William · #7 (**permalink**) 16-September-2007, 04:44 AM

The following is a link to Bill Keel’s web site that provides an overview of QSOs and AGN. An abridged and simplified version of Bill's review paper, was published in an article in Astronomy. The following is a quote from the paper concerning some of the key observational data concerning QSOs and AGN.

http://www.astr.ua.edu/keel/agn/quasar40.html

Quote:

Just where is the accretion disk?

The "standard" model of the central powerhouse in an active nucleus features a very massive black hole surrounded by an accretion disk, and it is in fact that accretion disk to which we might attribute most of the radiation we can actually see. But, to this point, the accretion disk has proven very shy, and observational tests for direct signatures of the disk have come up negative or ambiguous. This has held true with features in the optical spectrum, the overall shape of the ultraviolet spectrum, and lines from very hot matter seen in the X-ray spectral region. Searches for some feature that comes directly from the accretion disk rely either on the characteristic pattern of motions in a thin, orbiting disk, or on that fact that it will be heated by internal collisions and will thus be hot - like a large, hot, and very strangely shaped star. …

…The profiles of emission line in the spectra of most AGN are centrally peaked, telling us that only minor fractions of this light come from material at very high velocities. This sounds more like an amorphous cloud, or maybe a spherical distribution of clouds, than a thin, organized disk. Thus there was excitement when a class of quasars and radio galaxies turned up showing "twin peaks" - double emission-line profiles, with most of the radiation either redshifted or blueshifted compared to the center. Generically, this is just what we expect from a disk, and it is also just what we see from the hot disks around white dwarfs or neutron stars that are accreting gas from a companion. The first to be examined in detail was Arp 102B, part of an interacting galaxy pair that first seemed interesting based on an X-ray detection. A dedicated survey by Michael Eracleous and Jules Halpern turned up more of these objects, suggesting that they comprise nearly 10% of radio-loud objects if you look carefully enough. The double-peaked profiles did seem to show some of the right features to represent light coming directly from the accretion disk. In particular, the blue peak was often a little sharper and brighter than the red peak, effects predicted from relativity as the light from the approaching side is boosted in our frame of reference. Life was not to remain so simple, though - it turned out that the red and blue peaks in some of these spectra can vary independently, whereas the two halves of an accretion disk should remain coupled to each other within hours as material follows its independent orbits….

…Instead of emission lines, the accretion disk might well emit only continuum radiation in the optical range. If so, its signature might be such a broad spectral pattern that it would show up only when comparing data ranging from the ultraviolet into the infrared. Matt Malkan and Wallace Sargent carried out a widely-quoted study of this kind, showing that the broad emission pattern known as the Big Blue Bump does look like the kind of distorted stellar atmosphere that an accretion disk could give. It also showed some of the kinds of changes from one object to another that would result from effects of relativity as the central mass and viewing angle changed, according to calculations by Wei-Hsin Sun. So far so good. But the idea didn't pass other tests that these results suggested. In particular, the light output of such an object (including the surface of a supergiant star, which we understand rather better) should change dramatically at the Lyman limit in the ultraviolet, where radiation on the blue side of the limit can ionize hydrogen atoms. This change would be either a step in intensity or a change in polarization, unless the disk were carefully constructed to cancel these signals. Robert Antonucci and several sets of collaborators have tested this ideas, and find that they do not correspond with what we actually see in quasars and radio galaxies. As they suggest in a recent paper, if such quasars do exist, they are the ones we can't observe in the ultraviolet because the outer parts of the disk are so broad that they block our view. This is not only a suspiciously neat coincidence, but it violated the very small number of objects as powerful as quasars but showing only narrow emission lines (as if we don't see the core region at all). …

parejkoj · #8 (**permalink**) 16-September-2007, 05:01 AM

Ari: you've got a head start on this, and you'll recognize some of this, but a review might be good.

Before I go any further: could a mod please correct my grievous mistake in the title by removing the errant apostrophe? Thanks to Jami Cat for pointing that out: I blame the late (early?) hour at which it was written...

KaiYeves -- You are thinking of pulsars (a type of neutron star). Though I do rather like your artistic impression of them, from which I conclude that you are probably nowhere near dense enough to be a real neutron star.

William -- no spoiling the surprise! First we need to get through what they look like observationally: when two people are arguing about a bunch of quasars, they should agree on what do and don't fit the definition. Also, that's a big quote you've got there...

Moving on...
Now that you're all familiar with what stars look like (in short: blackbodies with perhaps some absorption), let's make an initial unambiguous comparison.

Stars vs. Quasars: the eternal showdown!
I've centered the SDSS finding chart on two separate SDSS objects in the links below. One is a star, one is a quasar. Can you guess which is which from the finding charts alone? If you aren't certain which object is in the center of the frame, check the "grid" box in the left hand panel under Drawing options.
Object 1 (finding chart)
Object 2 (finding chart)

Made your guesses?

It's ok, I'll wait...

Don't cheat and look ahead: make a note of it, and let's see how everybody did. Any reasons for guessing one way or the other?

Ok, now let's get a better idea, by looking at their spectra, as well as some other properties.
Object 1 (explore page)
Object 2 (explore page)

Number 1 is a star, and number 2 is a quasar. How did you do?

First off, a few things to note. They are both modrately bright: around 17-18th magnitude (ugriz filters). Some of their colors (difference in magnitudes) are fairly close: g-r ~= 0.4. SDSS selects targets for follow-up spectroscopy based on color and brightness limits, among others. (I'll eventually get into some of the details of the targeting algorithm, but we need more background first.) But the quasar has a whole slew of PrimTarget flags, which means SDSS is aware of a bunch of other things about it...

They are both labeled "Star" in the upper left of the pane. That's the photometric classification, which merely determines whether something is noise, a point source (star) or an extended source (galaxy). But they both also have TARGET_QSO_SKIRT in the PrimTarget field next to the spectrum (middle of the page). That means that the SDSS targeting algorithms thought they both could be quasars.

But how do we know that the second one is the quasar? Well, before we get to the spectrum, a very useful hint is that the quasar has substantial fluxes in both the FIRST radio catalog and the ROSAT x-ray catalog (see the bottom of the Explore page). Strong radio and X-ray fluxes are usually indicative of quasars (though they are not absolutely necessary). That is represented in the other PrimTarget flags I mentioned: the SDSS targetting algorithm recognized that the close ROSAT and FIRST matches meant it almost certainly had a quasar on its hands, besides the hints from the color.

Some spectroscopy
Here's where we start to see the real differences between stars and quasars. On the explore pages for these two objects, click the white box with the squiggles in it in the middle of the page. This is the SDSS quick-look spectrum. The spectroscopic pipeline automatically fits absorption and emission lines, black body and power law continua and galaxy and quasar templates to all its spectra. You can see the lines it was able to identify marked with black vertical dashed lines. The residual spectrum (what was left after the best-fit spectrum is subtracted) is in green at the bottom, and the locations of potentially contaminating sky-lines marked in magenta.

I'll leave you to ponder the difference between those two spectra. It is always fun to re-live the adventures of Schmidt (1963) and/or Hazard, Mackey, & Shimmins (1963). As they noted (in the spectrum of 3c273), a spectrum like our Object 2 would be a very, very weird star!

While we're on the topic of weirdness, what about this beast? No cheating and using NED, try and guess what it is on your own. I'll talk more about it next time...
http://cas.sdss.org/dr6/en/tools/exp...37826748596350

For those keeping track at home, they didn't actually have apple cider at the market, but it was the last batch of peaches for the year. I tried to stock up, but I'm sure we'll finish them before the end of the week.

Ari Jokimaki · #9 (**permalink**) 16-September-2007, 06:44 AM

Quote:

Originally Posted by parejkoj

But how do we know that the second one is the quasar? Well, before we get to the spectrum, a very useful hint is that the quasar has substantial fluxes in both the FIRST radio catalog and the ROSAT x-ray catalog (see the bottom of the Explore page).

All I see is a bunch of numbers for object 2, but I have no idea if they are substantial or not, I guess I should first understand what the numbers are there... But perhaps that's already an indication, that object 1 doesn't have an entry in these catalogs?

By the way, I succesfully determined (or guessed) that object 2 is a quasar (under assumption that one of these objects is a quasar), or rather that it's an extragalactic object, based on how it fades towards the edges. Object 1 has more of an appearance of a point source (it has "sharper" edges) than object 2, and that I think is a sign that object 1 is more probable to be a star of our own galaxy, and object 2 which fades at the edges in a similar manner as galaxies do, is more probable to be extragalactic object. But does this reasoning hold water? Also, I know that not all quasars look like this, some of them look even sharper at the edges than object 1, but is the galaxy-like fading at the edges good sign of an extragalactic object?

Quote:

Originally Posted by parejkoj

The residual spectrum (what was left after the best-fit spectrum is subtracted) is in green at the bottom, and the locations of potentially contaminating sky-lines marked in magenta.

Care to elaborate? I was wondering after that stellar spectrum thing what is the vertical line just below 5600 Å? At least one object had quite substantial absorption line there. What is doing the contamination there?

Quote:

Originally Posted by parejkoj

While we're on the topic of weirdness, what about this beast? No cheating and using NED, try and guess what it is on your own.

Well, I guess the SDSS spectrum section's mention of "QSO" in spectral class and the redshift of 0.377 are not indicative that this is a quasar then? It's hard to say. The spectrum looks featureless (perhaps only thermal?), but I don't know what to make of it. I checked the things you showed me from the earlier thread, but there wasn't anything like this. Well, because of featureless spectrum I'll just take a wild guess that it is a BL LAC.

Ken G · #10 (**permalink**) 16-September-2007, 05:33 PM

Quote:

Originally Posted by William

The following is a link to Bill Keel’s web site that provides an overview of QSOs and AGN. An abridged and simplified version of Bill's review paper, was published in an article in Astronomy. The following is a quote from the paper concerning some of the key observational data concerning QSOs and AGN.

http://www.astr.ua.edu/keel/agn/quasar40.html

Some interesting stuff there, but I find the "problems" with a disk interpretation may be a bit overblown. For example:

Quote:

Life was not to remain so simple, though - it turned out that the red and blue peaks in some of these spectra can vary independently, whereas the two halves of an accretion disk should remain coupled to each other within hours as material follows its independent orbits….

This is fairly easily resolved in terms of features in the disk that don't move with the gas itself, like the spiral arms of a galaxy. Features that move more slowly are seen in stellar disks, though they are believed to be global modes so are not independent. Causal disconnects in quasar disks might preclude such global behavior, I don't know.

and...

Quote:

In particular, the light output of such an object (including the surface of a supergiant star, which we understand rather better) should change dramatically at the Lyman limit in the ultraviolet, where radiation on the blue side of the limit can ionize hydrogen atoms.

As this objection is raised in analogy to stellar atmospheres that have very different radiation environments at higher energies than the Lyman limit than do quasars, is it not important to consider how the relative abundance of EUV and X-ray radiation in a quasar might alter the expectation for a Lyman jump?

KaiYeves · #11 (**permalink**) 17-September-2007, 01:46 AM

Yup, that's pulsars. SNIG- Situation Normal 'Till I Goofed. I was showing my teacher some scientific impressions to make her laugh on a field trip. My "two piranahs fighting over food" and "flounder going along with eyes on only one side of it's head" made her crack up, too.

William · #12 (**permalink**) 17-September-2007, 04:51 AM

Quote:

In reply to Ken G.'s comment:
This is fairly easily resolved in terms of features in the disk that don't move with the gas itself, like the spiral arms of a galaxy. Features that move more slowly are seen in stellar disks, though they are believed to be global modes so are not independent. Causal disconnects in quasar disks might preclude such global behavior, I don't know.

Ken, I do not necessary agree or disagree, with your comment. I need more facts before considering any hypothesis. I offered a link to Bill Keel's comments/article, with the hope that it would make people curious. I think we should not assume that Quasars/AGN have been explained. Keel seemed to be a knowledgeable, reasonable professional. I have read through Morley Bell's 16 papers on quasars/AGN and thought the same, about Bell. I was surprised at the number of interesting and new papers in this area.

What are some of the basic facts concerning Quasars that all can agree on?

The following are some more excerpts from Scheider's "Extragalactic Astronomy and Cosmology" which I believe is current, Section 5 "Active Galactic Nuclei"

Quote:

Some classes of AGNs, in particular quasars, belong to the most luminous sources in the Universe, ... they have been observed out to the highest measured redshifts (z approx. 6). The luminousity of a quasar can exceed the luminosity of normal galaxies by a factor of a thousand. This lumosity originates from a very small region of in space, r < 1 pc. The optical/UV are dominated by numerous strong and very broad emission lines, some of them emitted by highly ionized atoms ..... The processes in AGN are among the most energic in astrophysics. The enormous bandwidth of AGN spectra suggests that the radiation is nonthermal.

I will paraphrase to see if I understand what Scheider has written. This is not black body radiation, there is some process that is generating a significant amount of radiation in a small area, for a long period of time.

In addition (see my above comment concerning quasar statistics from the big orange book "Introduction to Astrophysics") the quasar hypothesis must explain why the luminosity of quasars has evolved with redshift (ie. Quasars are more luminous with greater redshift, in the past than they are in the local universe.). In addition, when we look at Bell's paper the strength of the radio component, for radio active quasars has also evolved with redshift. (i.e. The radio component of radio active quasars was stronger in the past than it is in the current, local universe.).

Ken is my paraphrase concerning the basic facts about quasars correct?

P.S. I would recommend both Schneider's book "Extragalactic Astronomy and Cosmology" and the big orange book "Introduction to Astrophysics".

parejkoj · #13 (**permalink**) 17-September-2007, 05:55 AM

We have a winner! Though there was a 50% chance just by guessing...

Quote:

Originally Posted by Ari Jokimaki

All I see is a bunch of numbers for object 2 ...But perhaps that's already an indication, that object 1 doesn't have an entry in these catalogs?

Absolutely. If there is no entry for a catalog, it means there was no match found with that catalog. FIRST has a resolution fairly close to that of SDSS (~4"), while ROSAT does not (~40"). So, if an SDSS object has FIRST data listed on its Explore page, there is a high likelihood that it is the source of those radio waves. On the other hand, there are generally a large number of SDSS sources within the ROSAT error circle, so you have to be very careful. (There's a paper to be published in the January AJ about that exact problem...

)

Making sense of FIRST and ROSAT
The numbers to pay attention to in the matched catalogs are delta (the separation between the SDSS and other catalog source position, in arcseconds), peak (peak radio emission in FIRST, in milli-Janskys) and cps (counts/second in ROSAT). The FIRST detection limit is ~1 mJy and the ROSAT limit is, very roughly, 0.01 cps. Looking at our object 2, the FIRST and ROSAT sources are well above the detection limits and both have relatively small separation distances (12" for a ROSAT source is pretty darn good).

Quote:

Originally Posted by Ari Jokimaki

By the way, I succesfully determined (or guessed) that object 2 is a quasar (under assumption that one of these objects is a quasar), or rather that it's an extragalactic object, based on how it fades towards the edges. ... But does this reasoning hold water? Also, I know that not all quasars look like this, some of them look even sharper at the edges than object 1, but is the galaxy-like fading at the edges good sign of an extragalactic object?

Hmm... Good eye. I initially attributed that to the way the SDSS finding charts are JPEG compressed, but object 2 may actually have some slight extent to the upper-right. I'm looking at a stack of the FITS images, and it is very hard to tell whether that is just noise, or actual structure. You may well be right! In general though, that method won't work---as you point out, many quasars are perfect point sources. Apparently I wasn't careful enough in picking my examples!

But you have to be very careful what you mean by "galaxy-like fading at the edges": object 2 has a nearly-perfect stellar light profile, except for that possible small extent at the top. Determining light profiles by eye is very unreliable, especially from JPEG compressed images. You'd need to get the FITS files and actually compare the light profile with a gaussian before you could really say anything, except for very obvious cases. But in general, yes, objects that do not have a stellar light profile are extra-galactic (excepting planetary nebula, supernova remnants, etc.).

A complete digression for something cool...
On completely separate note, while looking at the FIRST cutout for the area around object 2, I found the weird thing shown in the attachment (FIRST on the left, SDSS log-scaled gri photometry on the right; same physical scale and the coordinates of the center of the circle are given). None of those three radio sources have SDSS spectroscopy. The bottom one is probably a very high redshift quasar. The middle one---with all the wierd structure, and possible sidelobes---doesn't seem to have a matching SDSS photometric source, in any color band!

ds9.jpeg

Anyone have a 5-10 meter telescope that I could borrow for a couple of nights? Preferably with a good infrared spectrograph...

The thing is, this happens every time I compare FIRST and SDSS data. With many millions of SDSS sources, and nearly a million FIRST sources, there are going to be plenty of strange things. So much universe, so little time...

Atmospheric absorption

Quote:

Originally Posted by Ari Jokimaki

Quote:

Originally Posted by parejkoj

The residual spectrum (what was left after the best-fit spectrum is subtracted) is in green at the bottom, and the locations of potentially contaminating sky-lines marked in magenta.

Care to elaborate? I was wondering after that stellar spectrum thing what is the vertical line just below 5600 Å? At least one object had quite substantial absorption line there. What is doing the contamination there?

To be completely honest: I don't know, exactly. It is probably aerosol absorption from our atmosphere that wasn't correctly accounted for. You'll notice that the quasar has a small mark there as well. Since both that and the mark at ~7230 show up in the exact same place in both, they must be redshift independent and thus due to atmospheric absorption. I'll elaborate more on the fitting and residuals in a later post, if you don't mind. This one is already getting too long!

Quote:

Originally Posted by Ari Jokimaki

Quote:

Originally Posted by parejkoj

While we're on the topic of weirdness, what about this beast?

Well, I guess the SDSS spectrum section's mention of "QSO" in spectral class and the redshift of 0.377 are not indicative that this is a quasar then? It's hard to say. The spectrum looks featureless (perhaps only thermal?), but I don't know what to make of it. I checked the things you showed me from the earlier thread, but there wasn't anything like this. Well, because of featureless spectrum I'll just take a wild guess that it is a BL LAC.

You're doing good, Ari! Two for two, tonight. The big hint is the abso-friggin-ginormous radio source: 0.29 Jy is a lot of radio flux!

Yes, the "weird source" is a BL Lac. But all that means is that it is quasar-like source with a blackbody-like spectrum. And now we've come full circle! From things that look like stars but have strange spectra, to things which otherwise look like quasars, but have spectra that aren't like a "typical" quasar. I'll actually get to the spectral details tomorrow, I promise...

Ari Jokimaki · #14 (**permalink**) 17-September-2007, 07:51 AM

Thanks for the info. For now, I just comment on this:

Quote:

Originally Posted by parejkoj

But you have to be very careful what you mean by "galaxy-like fading at the edges": object 2 has a nearly-perfect stellar light profile, except for that possible small extent at the top. Determining light profiles by eye is very unreliable, especially from JPEG compressed images.

If you compare objects 1 and 2, there's a difference in light profile in the sense that where the "bright central light" stops and "haze" starts. And I mean this not just in one specific section of the object 2, I mean it all around (except N side, see below) Object 1 has very little "haze", roughly only one third of it's radius. Object 2 has more "haze", half or even two thirds of it's radius. In that sense object 2 resembles more a galaxy, with relatively small nucleus region and larger disk region, than object 1 does. But your comment about determining these things by eye being unreliable is a good one.

Now that I looked object 2 even more, I started wondering what the odd reddish coloring is at the North section of the object. SDSS images are not usually oddly coloured without reason, as far as I know, and I think I've only seen similar odd colorings in images of stars. Could there be two objects overlapping? Looking at spectrum, there's a small bulge around 6000 Å (roughly between 5000 and 7000 Å). Could that bulge come from a orange/red star having peak wavelength at 6000 Å? And other part of the spectrum would then be mainly from the quasar. Well, perhaps a bit far fetched...