Page 1 of 6 123 ... LastLast
Results 1 to 30 of 179

Thread: Bias effects in galaxy detection

  1. #1
    Join Date
    Mar 2004
    Posts
    2,689

    Bias effects in galaxy detection

    Mike Disney has worried about selection effects in detection of galaxies (and their results on what we know about the galaxy population) for about 35 years. Here's his latest, no doubt deliberately provocative but a set of conclusions to be seriously wrestled with:

    The galaxy ancestor problem, Mike Disney and Huw Lang:

    HST finds galaxies whose Tolman dimming should exceed 10 mag. Could evolution alone explain these as our ancestor galaxies? Or could they be representatives of quite a different dynasty whose descendants are no longer prominent today? We explore this latter hypothesis and argue that Surface Brightness Selection Effects naturally bring into focus quite different dynasties from different redshifts. Thus the HST z=7 galaxies could be examples of galaxies whose descendants are both too small and too choked with dust to be recognizable in our neighborhood easily today. Conversely the ancestors of the Milky Way and its obvious neighbors will have completely sunk below the sky at z>1.2 although their diffuse light could account for the missing Reionization flux. This Succeeding Prominent Dynasties Hypothesis (SPDH) fits the existing observations both naturally and well,including the bizarre distributions of galaxy surface brightnesses found in deep fields, the angular size ~ inverse (1+z) law,'Downsizing' which turns out to be an 'illusion' in the sense that it does not imply evolution, 'Infant Mortality', i.e. the discrepancy between stars born and stars seen, and finally the recently discovered and unexpected excess of QSOAL DLAs at high redshift. If the SPDH is true then a large proportion of galaxies remain sunk from sight, probably at all redshifts. We show that fishing them out of the sky by their optical emissions alone will be practically impossible, even when they are nearby. More ingenious methods will be needed to detect them. It follows that disentangling galaxy evolution through studying ever higher redshift galaxies may be a forlorn hope because one will be comparing young apples with old oranges, not descendants with their own ancestors.
    There are a lot of wrinkles I plan to ping him about - gains from the very blue colors of young stellar populations, detection of galaxies from their UV-bright compact regions at substantial redshift, dusty-galaxy detections in the FIR and submm when we have accurate enough coordinates - but these visibility limits are important issues. In the most extreme cases they consider, much of what we think we know about galaxy evolution is instead a reflection of selection effects operating on a galaxy population much broader than we can easily detect at any single redshift.

  2. #2
    Join Date
    Apr 2005
    Posts
    1,553
    Thanks for posting this, looks very interesting. Not read it all yet, and I won't understand it all when I have, but look at this quote:

    .....high redshift galaxies are truly bizarre.
    They are one or two orders of magnitude smaller in physical size, while their intrinsic surface
    brightnesss must be 9 mag or 4000 times higher. Moreover, and this is even more extraordi-
    nary, they must have systematically adjusted their sizes and their SBs [surface brightness] over cosmic time...

  3. #3
    Join Date
    Mar 2004
    Location
    Ocean Shores, Wa
    Posts
    5,228
    Quote Originally Posted by kzb View Post
    Thanks for posting this, looks very interesting. Not read it all yet, and I won't understand it all when I have, but look at this quote:

    .....high redshift galaxies are truly bizarre.
    They are one or two orders of magnitude smaller in physical size, while their intrinsic surface
    brightnesss must be 9 mag or 4000 times higher. Moreover, and this is even more extraordi-
    nary, they must have systematically adjusted their sizes and their SBs [surface brightness] over cosmic time...
    Truly bizarre, or once again, as Disney suggests, are selection effects playing havoc with our perceptions. Are there galaxies that suffer from lens pinching? Could there be a condensation of the light path so that the size is artificially small and the magnitude artificially bright?

  4. #4
    Join Date
    Nov 2009
    Posts
    267
    The linked paper was fairly dense (at least to me).

    If anyone can provide more of a layman's explanation to the concept of galaxies such as the MW or Andromeda, "falling off the horizon" due to low redshift scores, I would appreciate it. On the surface it made it sound like there could be a multitude of Local Cluster type galaxies much closer than other outlies within our observable horizon that are simply no longer visible. So where exactly are they? Simply out there but luminouse enough to be detected with our current tecnhology?

  5. #5
    Join Date
    Mar 2004
    Posts
    2,689
    There are several factors at play in the detectability of galaxies. One is observed surface brightness - energy received per square arcsecond, or example. As this drops below the interfering level of sky brightness (airglow, light pollution, starlight scattered from interstellar dust) the required exposure times for detection grow rapidly for fainter levels. This sets a fairly hard limit on what galaxies we can detect in a particular regime - better from space, and better at wavelengths with the lowest levels of interfering diffuse light.

    Another major effect is the Tolman dimming. Cosmological pictures with expanding spacetime (or any coordinate equivalent) make a robust prediction that the surface brightness of otherwise identical objects, measured in the same way, drops off with redshift z according to (1+z)-4. This is what would make Milky Way analogs "drop out" at higher redshifts, although the details depend on how we observe them - higher resolution lets us see the bright cores and star-forming regions, while redshifting ultraviolet into the visible band would exacerbate light loss due to dust while presenting us brighter and more compact star-forming regions when they are not heavily dust-reddened. One clear result of the Tolman dimming is that our optcally-chosen galaxy samples at high redshifts cannot fail to be biased in favor of galaxies or pieces of galaxies which are blue (UV-bright) of unusually high surface brightness.

    Their visibility selection function for galaxies includes a third factor - sufficiently small galaxies, which have the highest surface brightness, will not be selected by typical surveys and will be identified only spectroscopically or when their colors are uifficiently different from stars.

    None of the foregoing is at all controversial, this is standard stuff. Where these authors set off fireworks is in the claim that these effects limit our view of the galaxy population so strongly that most of what is usually interpreted as evolution in the galaxy population instead results from redshift-dependent selection of galaxies from a wider range of properties than normally assumed, so strongly that our empirical constrains on galaxy evolution from multi-redshift comparison weakens dramatically. There are a lot of details to pry into to see how strong an effect this is. For example, H I wide-field surveys turn up very few "dark" galaxies where we see the gas but not starlight - but how true this is depends on how precisely we can associate the 21-cm objects with detected galaxies. And as above, we could still pick up star-forming regions in a galaxy which has otherwise faded past our detection limits, and very dusty galaxies below our optical threshold should show up as far-infrared sources (as indeed some do).

  6. #6
    Join Date
    Mar 2004
    Posts
    13,441
    Surely wide surveys like WISE and UKIDSS, and narrow ones like the various multi-wavelength/band observation campaigns of Chandra-S etc, provide a very good handle on this sort of thing?

    In any case, doesn't it come down, in the end, to the extent to which various hypotheses are consistent with all the relevant observational data?

    I must be missing something ...

  7. #7
    Join Date
    Apr 2005
    Posts
    1,553
    Another point from the paper is that before Hubble was launched, they assumed that redshift z>2 galaxies could not be observed, precisely because of this Tolman dimming. But "that got forgotten about" and they are observing galaxies now at redshift 7 and beyond.

    I've a question though: current galaxies could have formed from mergers of smaller galaxies. So could the current crop of large but low surface brightness galaxies not have formed from mergers of the small but high SB galaxies?

  8. #8
    Join Date
    Mar 2004
    Posts
    2,689
    Their claim (which I'm still looking at in the context of details listed above) is that the redshift/surface brightness selection is so strong that it may dominate over actual evolutionary effects. For example, Lyman-break galaxies at z~3 will have surface brightness dimmed by 44=256 times, so in order to be as bright as we see, they must have prodigious (UV) surface brightness which has virtually no local analogs among entire galaxies. Elliptical galaxies fare a bit better observationally because of their light concentration. The authors like to point out (in Disney's case, not for the first time) that the range of average surface brightness of catalogued galaxies (by various standard measures - within the half-light radius, Petrosian radius, isophotal radius which changes with z-dimming) is much narrower than the amount of Tolman dimming expected to significant redshifts, so it would be a remarkable coincidence of the mean star-forming rate just managed to cancel it out. This looks like much more of a problem in a global view than when looking at one particular class of galaxy (although they further claim that we could be getting it wrong when trying to trace a single galaxy type across wide spans on redshift).

    On the other hand, the maximum surface brightness of starburst regions in galaxies is pretty constant with z when taking Tolman dimming into account, although the linear scale and thus luminosity changes a lot. And it is clear that there is no significantly numerous local population of galaxies so compact that they masquerade as stars in, or example, the SDSS (all those words are significant) - they claim that these would be the descendants of galaxies easy to see in HST images at high z, unless they are now so dusty as to be mostly far-infrared sources..

  9. #9
    Join Date
    Apr 2005
    Posts
    1,553
    The other alternative of course is there is something wrong about universal expansion/redshift theories.

  10. #10
    Join Date
    Mar 2004
    Posts
    13,441
    Quote Originally Posted by kzb View Post
    The other alternative of course is there is something wrong about universal expansion/redshift theories.
    Never say never, of course, but no, that's not really a viable alternative.

    Galaxies are really complicated things, and how they evolve (change over time) doubly so. For example, not that long ago it was widely accepted that they mostly just evolved in isolation; now I think it's fair to say that very few galaxies we see today, locally, got to be the way they are/seem without all sorts of influences from their environment (including 'major mergers').

    The alternative you mention would involve 'unexplaining' (and so re-explaining) a staggering number of observations, of many different kinds.

  11. #11
    Join Date
    Sep 2003
    Posts
    10,397
    Quote Originally Posted by ngc3314 View Post
    Another major effect is the Tolman dimming. Cosmological pictures with expanding spacetime (or any coordinate equivalent) make a robust prediction that the surface brightness of otherwise identical objects, measured in the same way, drops off with redshift z according to (1+z)-4.
    Wasn't this tweaked from a 4th power to something between 2.6 to 3.4? Lobin & Sandage, 2001

    [I'm just drilling a hole in the wall here so I can watch.
    We know time flies, we just can't see its wings.

  12. #12
    Join Date
    Mar 2004
    Posts
    13,441
    I've just started to read the preprint*, and I figure it'd take me a solid month of work to really understand it. And it would, indeed, take me that much effort, how many of BAUT's regulars would be able to 'get it' significantly faster? ngc3314, obviously, and perhaps a dozen others?

    Anyway, here are some immediate thoughts (mostly questions):

    -> "objects with an exponentially declining light distribution (virtually all galaxies bar Giant Ellipticals; see later)": we've now 'seen' galaxies in wavebands from GeV gammas to MHz radio; in what slices of this enormous range are virtually all (local) galaxies exponential?

    -> are all quasars (BL Lac objects, etc) the nuclei of galaxies?

    -> what happens to the Visibility window when a (distant) galaxy is (strongly, gravitationally) lensed?

    -> for the Hubble, the 'sky' is very dark, and 'size visibility' sharp. How does Hubble's P and FWHM compare with those of a small, ground-based telescope (say, 0.4m) in a heavily light-polluted location (around full Moon perhaps)?

    -> some galaxies emit a great deal of their visual waveband light in the form of emission lines, e.g. H-alpha, [OIII]. Are such galaxies exponential when observed in narrow bands around these lines?

    * it certainly is a preprint! There are a number of things - all minor? - that need to be fixed before it's ready for publication

  13. #13
    Join Date
    Mar 2004
    Posts
    13,441
    OK, maybe not a month, but at least a week!

    Having now read the whole paper, some immediate impressions:

    * it certainly is a preprint! In the Acknowledgements section: "Nino Disney, Richard Elliott, and Joe Romano (U Texas, Brownsville) for indispensable help with word processing": well, there are plenty of typos (etc) that need fixing

    * of the points in my last post, the preprint deals explicitly (and at some length) with the 4th ("for the Hubble, the 'sky' is very dark, ...")

    * some of the predictions of the SPD hypothesis (as presented in this preprint) would seem to be fairly easily tested, doing nothing more than some analyses of data from readily available astronomical datasets

    * general question: what's the current state-of-art re spectroscopic redshift estimations in the mid-IR and far-IR?

  14. #14
    Join Date
    Mar 2004
    Posts
    13,441
    I've now got quite a few ideas of some research into the things Disney and Lang cover in their preprint. Unfortunately, I can do little, if any, of this research entirely on my own. So, is anyone reading this interested in collaborating?

    (you can send me a PM if you don't want to write a post)

  15. #15
    Join Date
    Mar 2004
    Location
    Ocean Shores, Wa
    Posts
    5,228
    Quote Originally Posted by Nereid View Post
    Never say never, of course, but no, that's not really a viable alternative.

    Galaxies are really complicated things, and how they evolve (change over time) doubly so. For example, not that long ago it was widely accepted that they mostly just evolved in isolation; now I think it's fair to say that very few galaxies we see today, locally, got to be the way they are/seem without all sorts of influences from their environment (including 'major mergers').

    The alternative you mention would involve 'unexplaining' (and so re-explaining) a staggering number of observations, of many different kinds.
    Ya, a lot more than a small group of researchers could sort out.

    The same argument, by the by, has been made concerning the 'staggering number' of other observations that must be wrong if the 'dark energy' solution is not the correct answer to the observed brightness of distant supernovae.

    Galaxy brightness and supernove magnitude determinations should more-or-less trump a staggering number of lesser approximations based upon weaker assumptions.

  16. #16
    Join Date
    Sep 2006
    Posts
    1,424
    Quote Originally Posted by Nereid View Post
    -> "objects with an exponentially declining light distribution (virtually all galaxies bar Giant Ellipticals; see later)": we've now 'seen' galaxies in wavebands from GeV gammas to MHz radio; in what slices of this enormous range are virtually all (local) galaxies exponential?
    The statement refers to visible light, and to the near-IR to a lesser extent. At high energies, emission from galaxies is dominated by a relatively few discrete sources, not the billions of stars which produce the visible and near-IR.

    -> are all quasars (BL Lac objects, etc) the nuclei of galaxies?
    Yes. Some people are searching for cases in which a supermassive black hole has left its home galaxy, due to the gravitational influence of another supermassive black hole, but they haven't found many yet.

    -> what happens to the Visibility window when a (distant) galaxy is (strongly, gravitationally) lensed?
    That galaxy becomes visible at higher redshifts than it would be otherwise. This isn't very common, by the way, so it won't have any major effect on the statistics of large samples.

    -> for the Hubble, the 'sky' is very dark, and 'size visibility' sharp. How does Hubble's P and FWHM compare with those of a small, ground-based telescope (say, 0.4m) in a heavily light-polluted location (around full Moon perhaps)?
    HST's FWHM can be approximated by the simple diffraction limit: wavelength / diameter of telescope. So, in the optical, 500 nm / 2.4 m = 2.1 x 10^(-7) radians = 0.04 arcsec. Small ground-based telescopes without active optics will have a FWHM of order 1 arcsec, due to the atmosphere.

    The sky brightness from the optical depends on location and wavelength. Choose B-band, for an example. The B-band sky brightness at the best mountain-top sites is of order 22 magnitudes per square arcsecond. The B-band sky brightness at poor sites is much higher -- perhaps mag 17 mag per square arcsecond, and worse if the full moon is present. The difference is much larger if one moves to the near-IR, where HST's sky background is much, much, MUCH lower than the sky background from any ground-based site.

    Are you trying to compute some sort of detection limit between these two cases?


    -> some galaxies emit a great deal of their visual waveband light in the form of emission lines, e.g. H-alpha, [OIII]. Are such galaxies exponential when observed in narrow bands around these lines?
    No, in the narrow bands, one will see chunky bits of emission scattered around the extent of the galaxy. After all, the emission comes from HII regions, which occur only in the neighborhood of hot young stars (yes, and a tiny bit from planetary nebulae, but they are discrete little sources, too).

  17. #17
    Join Date
    Mar 2004
    Posts
    13,441
    Quote Originally Posted by Jerry View Post
    Ya, a lot more than a small group of researchers could sort out.

    The same argument, by the by, has been made concerning the 'staggering number' of other observations that must be wrong if the 'dark energy' solution is not the correct answer to the observed brightness of distant supernovae.
    Can we stick with the topic, please?

    Galaxy brightness and supernove magnitude determinations should more-or-less trump a staggering number of lesser approximations based upon weaker assumptions.
    I guess, from this comment, it would be fair to say that you didn't actually read the Disney and Lang preprint? Or, that you read it, but didn't understand it?

  18. #18
    Join Date
    Mar 2004
    Posts
    13,441
    Thanks StupendousMan!
    Quote Originally Posted by StupendousMan View Post
    The statement refers to visible light, and to the near-IR to a lesser extent. At high energies, emission from galaxies is dominated by a relatively few discrete sources, not the billions of stars which produce the visible and near-IR.
    If so, then there's a major piece missing in the Disney and Lang presentation*: are galaxies overwhelmingly exponential in the UV, from the Lyman limit to ~300 nm? If not, then most of the case they make is irrelevant (or at least needs caveats/further work).

    Yes. Some people are searching for cases in which a supermassive black hole has left its home galaxy, due to the gravitational influence of another supermassive black hole, but they haven't found many yet.
    There are a lot of papers on the selection effects at work wrt quasars. However, if there's good reason to be confident we have a handle on their co-moving space density, as a function of redshift, then we already know something about the space density of at least some galaxies (i.e. the hosts of the quasars). At least in principle, that allows the opportunity to test some aspects of the Disney&Lang hypothesis (or hypotheses).

    That galaxy becomes visible at higher redshifts than it would be otherwise. This isn't very common, by the way, so it won't have any major effect on the statistics of large samples.
    Sure.

    Assuming that the object (galaxy, cluster) doing the lensing is not associated in any way with the object lensed (other than by serving as the lens!), then the distribution of (leased) galaxy properties (SB profile, size, etc) should provide a test of the Disney&Lang hypothesis (or hypotheses).

    HST's FWHM can be approximated by the simple diffraction limit: wavelength / diameter of telescope. So, in the optical, 500 nm / 2.4 m = 2.1 x 10^(-7) radians = 0.04 arcsec. Small ground-based telescopes without active optics will have a FWHM of order 1 arcsec, due to the atmosphere.

    The sky brightness from the optical depends on location and wavelength. Choose B-band, for an example. The B-band sky brightness at the best mountain-top sites is of order 22 magnitudes per square arcsecond. The B-band sky brightness at poor sites is much higher -- perhaps mag 17 mag per square arcsecond, and worse if the full moon is present. The difference is much larger if one moves to the near-IR, where HST's sky background is much, much, MUCH lower than the sky background from any ground-based site.

    Are you trying to compute some sort of detection limit between these two cases?
    The question was ill-posed and premature; the Disney&Lang preprint covers what I am interested in (I just hadn't read far enough).

    No, in the narrow bands, one will see chunky bits of emission scattered around the extent of the galaxy. After all, the emission comes from HII regions, which occur only in the neighborhood of hot young stars (yes, and a tiny bit from planetary nebulae, but they are discrete little sources, too).
    And that too suggests several possible tests of the Disney&Lang hypothesis (or hypotheses). In fact, they actually mention this indirectly, when they talk about HI surveys.

    * It may, perhaps, be there ... and I simply didn't read (or absorb) it

  19. #19
    Join Date
    Mar 2004
    Location
    Ocean Shores, Wa
    Posts
    5,228
    Quote Originally Posted by Nereid View Post
    Can we stick with the topic, please?
    which topic? Disney is proposing a convoluted solution to an evolution-induced problem: We can't find the progenitors of todays galaxies, and we can't find the children of yesterday's galaxies. Isolating a single, or even a pair of cosmic uncertainties while declaring that the bulk of the evidence as sound is poor justification for introducing new, sliding parameters to quench the probem. If, for example, increasingly dusty environments are quenching the surface brightness of AGN galaxies, we should see an increase in the dust-induced redness of this population that follows roughly the same inverse power rule as the AGN magnitude creep. I don't think that the evidence in hand suggests this.

    I guess, from this comment, it would be fair to say that you didn't actually read the Disney and Lang preprint? Or, that you read it, but didn't understand it?
    Perused, and jumped to the usual conclusion: The Tolman law, the theory that leads to the conclusion that surface brightness has evolved, should be suspected. Rather than resolving every one of these reoccurring problems by pounding every observation that is at odds with current theoretical expectations into submission we should look harder for other explanations for the discrepancy. The irony here is that I agree with Disney: Selection effects are masking our ability to trace galactic history. I would simply add that our obsession with mechanics developed in the early twentieth century hamstring theoretical development, too. If there is no obvious evolutionary trail from the past universe to the present; it may be the evolutionary theory that needs to be discarded rather than salting the theory with dark stuff to taste.

    A LOT of the probems Disney is dealing with evaporate if the Tolman z^-4 dimming rule is discarded.
    Last edited by Jerry; 2011-Sep-19 at 03:14 PM.

  20. #20
    Join Date
    Mar 2004
    Posts
    13,441
    Quote Originally Posted by Jerry View Post
    which topic? Disney is proposing a convoluted solution to an evolution-induced problem: We can't find the progenitors of todays galaxies, and we can't find the children of yesterday's galaxies. Isolating a single, or even a pair of cosmic uncertainties while declaring that the bulk of the evidence as sound is poor justification for introducing new, sliding parameters to quench the probem.
    The usual Jerry interpretation, eh?

    It'd be nice to once, just once, engage in a meaningful discussion with you Jerry, on topics like this.

    If, for example, increasingly dusty environments are quenching the surface brightness of AGN galaxies, we should see an increase in the dust-induced redness of this population that follows roughly the same inverse power rule as the AGN magnitude creep. I don't think that the evidence in hand suggests this.
    Perhaps it is, perhaps it isn't.

    However, as usual, it would seem that you and I have read completely different documents!

    For example, it would seem that you found nothing the least bit unusual, suspicious, questionable, assumptions-too-far, etc, etc, etc in the Disney&Lang preprint; rather, you bought the most speculative parts of their discussion section at face value.

    Doesn't it ever bother you that you approach this sort of material in such a blatantly biased way?

    Perused, and jumped to the usual conclusion: The Tolman law, the theory that leads to the conclusion that surface brightness has evolved, should be suspected. Rather than resolving every one of these reoccurring problems by pounding every observation that is at odds with current theoretical expectations into submission we should look harder for other explanations for the discrepancy.
    Thanks, you just made my point, in spades.

    Straight question Jerry: did you find anything, anything at all, in the Disney&Lang preprint that might give rise to even a niggle of doubt, in the mind of a 'cosmology skeptic' like you?

    The irony here is that I agree with Disney: Selection effects are masking our ability to trace galactic history.
    Don't you think it just the teensiest bit ironical that, based on what you've posted so far in this thread, it seems you haven't actually read the Disney&Lang preprint?

    I would simply add that our obsession with mechanics developed in the early twentieth century hamstring theoretical development, too. If there is no obvious evolutionary trail from the past universe to the present; it may be the evolutionary theory that needs to be discarded rather than salting the theory with dark stuff to taste.
    It's easy to spin an enjoyable yarn about this sort of stuff, isn't it?

    Is it too much to ask this: when do you expect to roll up your sleeves and do the hard yakka of putting your nice words to the rack of quantitative analysis?

  21. #21
    Join Date
    Nov 2002
    Posts
    6,238
    Quote Originally Posted by Jerry View Post
    A LOT of the probems Disney is dealing with evaporate if the Tolman z^-4 dimming rule is discarded.
    Can you show which problems, and by how much those problems go away if the Tolman dimming rule is discarded? Or,as is your history, is this just another throwaway claim that you have no intention of providing any kind of quantitative support for?

  22. #22
    Join Date
    Mar 2004
    Posts
    2,689
    I think this issue has come up before in the SN Ia context - an unresolved source displays two factors of (1+z) of the Tolman dimming (we don't see the other two unless we spatially resolve it since they come from angular size). To the range where SN Ia are detected (z~1.3), that part of the effect is about an order of magnitude greater than the deviations interpreted as due to acceleration. So getting rid of the expansion-attributed effects makes the SN data much, much worse instead of getting rid of a problem. One has to look carefully at a particular study's way of doing the K-correction to see where this is done, BTW - these are heuristically the photon-energy and arrival-time-dilation terms; bandwidth narrowing and band shifting are different beasts best done from a sample spectrum.

  23. #23
    Join Date
    Jul 2003
    Posts
    4,217
    This kind of reminds me of Radio Source
    counts in the sixties and conclusions
    being drawn.

    Poor Sir Fred...

  24. #24
    Join Date
    Mar 2004
    Posts
    13,441
    Quote Originally Posted by Nereid View Post
    I've now got quite a few ideas of some research into the things Disney and Lang cover in their preprint. Unfortunately, I can do little, if any, of this research entirely on my own. So, is anyone reading this interested in collaborating?

    (you can send me a PM if you don't want to write a post)
    No one eh?

  25. #25
    Join Date
    Sep 2006
    Posts
    1,424
    Quote Originally Posted by Nereid View Post
    No one eh?
    Fire away, Nereid. An open discussion on this thread might be interesting and educational for many readers.

  26. #26
    Join Date
    Sep 2003
    Posts
    10,397
    Quote Originally Posted by StupendousMan View Post
    Fire away, Nereid. An open discussion on this thread might be interesting and educational for many readers.
    I'm more the lurker, but this topic looks like we might get some nice juice for the squeeze. Just learning all the differenct effects and pricinciples involved will be worth it for me.
    We know time flies, we just can't see its wings.

  27. #27
    Join Date
    Mar 2004
    Posts
    13,441
    Quote Originally Posted by StupendousMan View Post
    Fire away, Nereid. An open discussion on this thread might be interesting and educational for many readers.
    Excellent point!

    OK, let's start with that fantastic, free, public resource, SDSS. Would it be better to use DR7, or DR8? Why? Myself, I think it depends on what we want to do.

    We could start by thinking up tests of the Disney&Lang assumption, that galaxies have exponential surface brightness (SB) profiles (other than giant ellipticals, or BCGs).

    For example*:
    -> bin galaxies by redshift, Petrosian radius, and dominance of spectrum by emission lines
    -> for bins with sufficiently many members, stack
    -> extract the stacked radial SB distribution, by band (u, g, r, i, z)

    I recall this has already been done, for z ~ 0.2 BCGs; I'll see if I can dig up the paper.

    * There's plenty of data; from memory, there are >600,000 objects identified as galaxies in DR7.

  28. #28
    Join Date
    Mar 2004
    Posts
    13,441
    Tal and van Dokkum (2011): "The faint stellar halos of massive red galaxies from stacks of more than 42000 SDSS LRG images".

  29. #29
    Join Date
    Aug 2003
    Posts
    1,110
    Quote Originally Posted by Nereid
    Would it be better to use DR7, or DR8? Why?
    DR8 has photometry over a larger area, with significantly better sky-subtraction than DR7. If you want to investigate surface brightness issues, you definitely would prefer DR8.

    Also, there are just under a million unique galaxies with spectra in DR7/8 (no new extra-galactic spectroscopy in DR8), with redshifts up to ~0.6.

  30. #30
    Join Date
    Mar 2004
    Posts
    13,441
    Thanks parejkoj.

    GALEX DR6 may be a good place to start to investigate the nature of galaxy surface brightness in two UV wavebands, as a function of redshift.

Similar Threads

  1. Arp 291 A "wind effects" galaxy
    By RickJ in forum Astrophotography
    Replies: 1
    Last Post: 2010-Nov-28, 03:23 AM
  2. observational bias
    By toothdust in forum Against the Mainstream
    Replies: 23
    Last Post: 2008-Jul-18, 10:29 PM
  3. Replies: 2
    Last Post: 2007-May-09, 06:47 PM
  4. Bias in Papers
    By RussT in forum Against the Mainstream
    Replies: 13
    Last Post: 2006-Oct-24, 08:31 AM
  5. The artful bias of the BBC
    By Glom in forum Off-Topic Babbling
    Replies: 33
    Last Post: 2006-Feb-07, 11:08 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
here
The forum is sponsored in-part by: