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Accurate distance gauge out to z ~ 4 alleged (AGN)

  1. Sep 28, 2011 #1


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    Accurate distance gauge out to z ~ 4 alleged (AGN reverberation)

    Because AGN (active galactic nuclei) are so bright, it would be nice if we could tell their intrinsic luminosity or "wattage". Then it would be like having a "standard candle" to tell distance with, by seeing how less bright it looks at our remove from it.

    These people claim to have found a way to tell the inherent wattage of an AGN by measuring a time-lag which tells them the radius of a certain surrounding photo-ionized region. This surrounding region is called the BLR (broad line region). If their method is confirmed it would be very important for cosmology.

    One would then have a direct way to gauge the distance out beyond where we can judge it using Type IA supernovae as standard candles. Let's hope it is confirmed! Then we will come to understand the cosmos much better.

    A new cosmological distance measure using AGN
    D. Watson (1), K. D. Denney (1), M. Vestergaard (1), T. M. Davis (2) ((1) Dark Cosmology Centre, U. Copenhagen, (2) U. Queensland)
    (Submitted on 21 Sep 2011)
    Accurate distances to celestial objects are key to establishing the age and energy density of the Universe and the nature of dark energy. A distance measure using active galactic nuclei (AGN) has been sought for more than forty years, as they are extremely luminous and can be observed at very large distances. We report here the discovery of an accurate luminosity distance measure using AGN. We use the tight relationship between the luminosity of an AGN and the radius of its broad line region established via reverberation mapping to determine the luminosity distances to a sample of 38 AGN. All reliable distance measures up to now have been limited to moderate redshift -- AGN will, for the first time, allow distances to be estimated to z~4, where variations of dark energy and alternate gravity theories can be probed.
    Apj in press; 5 pages, 3 figures

    One of these authors is Tamara Davis. She was Charley Lineweaver's PhD student and wrote an excellent thesis in 2003 IIRC. She co-authored the SciAm article I have in my signature, which explains misconceptions people have about expansion cosmology (aka "big bang" although this is a misleading nickname). I'm definitely a Lineweaver-Davis fan and urge you to read the SciAm article---the "charley" link. Also since she is onboard with this AGN thing I tend to give it good odds of turning out right.

    Others: Marianne Vestergaard, Kelly Denney, Darach Watson at the Copenhagen center for dark cosmology http://www.dark-cosmology.dk/~darach/About%20Me.html [Broken]
    Darach must be an Irish name, new to me.

    This team has quite a bit of credibility as I see it. Also see their figure. The correlation of this measure with known distances is good. What I want to understand better is this delay time tau.
    AGN are black holes with stuff spiraling in. And their output fluctuates depending on how much hot gas is on the way in. The light emitted is thermal, as the gas heats up as it spirals in, and glows---with a continuum spectrum. This is the main source of luminosity and it is what we want to gauge.

    Now this hot central glow falls off as the square of the radius outwards into the surrounding clouds and how luminous it is determines how far out it can excite secondary radiation. If we knew the radius we could tell the luminosity. But we can tell the radius by the delay time of a flare-up.
    The central glow flares up first, and then the secondary radiation happens a little while later because the light takes time to get out to the surrounding BLR gas clouds. I think that is roughly the idea. You can read the explanation on page 1. It is called the "reverberation" method.

    The BLR radiation is a kind of "echo" of the initial flareup. So you have to observe the AGN and record when things happen and this way you learn the absolute luminosity. Then you see how much dimmer it looks to us, and that tells the distance.
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  3. Oct 1, 2011 #2


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    Very interesting paper, marcus. The advantage they have is, as they state, the ability to repeat measurements on a single AGN to reduce error. The disadvantage, as I see it, is the time delay for reverberation that one uses to compute DL is going to be a function of the cosmology model used. Do you agree?
  4. Oct 1, 2011 #3


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    Well you are the pro (Elizabethtown prof IIRC) so I would tend to defer on a point like that. But you would need to explain how the model dependence works in a bit more detail.

    At the moment I don't agree because I don't understand your point.

    As I see it, they have a well defined measure and they just need to calibrate it using nearer AGN where the distance is known by well-established Cepheid means. And then they have to see how much scatter there is.

    If their measure can be calibrated to match (without much scatter) the Cepheid distances in the cases where these are known, then I don't see needing to worry about cosmo model! In that case it is just a good measure----the next rung in the ladder.

    Of course the delay times are affected by the same z+1 factor as the wavelengths of the light---the delay times we measure have been stretched out by the same factor as the light---but we can measure that directly and allow for it. We don't have to use a cosmo model for that.

    So the question comes down AFAIK to calibration and scatter. Here's what they say on page 2:
    ...We therefore calibrated τ/√ F to the luminosity distance to the galaxy NGC 3227 based on a distance modulus of m − M = 31.86 ± 0.24 .... Due to the occurrence of a supernova, 2002bo, in another member of the Leo III group to which NGC 3227 belongs, the SBF distance quoted above has been examined in detail and seems likely to be correct within the quoted uncertainty (Krisciunas et al. 2004). However, the uncertainty in this calibration is relatively large, and we use it here only to determine an initial estimate of the luminosity distances. NGC 4051, NGC 4151, and NGC 3227 are certainly close enough that it should be possible to obtain more reliable Cepheid-derived distances with HST to these galaxies for a better absolute calibration. In practice, we expect that Cepheid distances can in fact be determined to multiple nearby AGN, allowing at least a dozen AGN to be distance-calibrated in this way.

    I think they are saying that they need HST time to get Cepheid distances for a dozen or so AGN in order to calibrate their "next rung in the distance ladder" more precisely.

    Right now their calibration has the uncertainty illustrated by the quoted figure of 31.86 ± 0.24 (which already looks pretty nice to me!) but they want to do better. And also if they get a dozen or so directly measured Cepheid distances to compare with their new AGN measure they will be able to redo Figure 1 and show how much scatter there is or isn't. Ultimately that is what will (or will not) persuade people to trust this proposed next rung.

    That's how I see it. I'm interested if you see something different I've missed. Right now I'm excited, but in suspense. Will this pan out? And for no good reason I'm glad that Tamara Davis is involved. Here's a picture and audio of a 2009 interview:

    And Marianne Vestergaard to, for that matter. I tend to like astronomers as a group. Here's a Vestergaard thing:
    She just got tenure last month at the Niels Bohr Institute's Dark Cosmology Center

    Here's another Tamara Davis thing:
    Last edited: Oct 1, 2011
  5. Oct 2, 2011 #4


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    Here's what I mean. When we measure redshift, we assume we know the frequency of emission, then we measure the frequency at reception. No matter what happens between emission and reception, if we're right to assume the emission frequency, we have our redshift, by definition. In this case, what we want to measure is a time delay at the galaxy. That means anything that happens to the signals in route to effect that difference (which is a function of cosmology model) will have to be accounted for in order to determine the time delay at emission. You note for example the relationship between scale factor at emission and redshift, ae = 1/(1+z), potentially coming into play. But that relationship is model dependent, which is exactly my point.

    I think they'll be in the clear if they calibrate out to the max supernova z (~1.5 now). That should allow them to figure out if there is something model dependent about their time delays. Having read the paper, I'm optimistic. That would be great to have reliable distance moduli versus redshifts out to z = 4.
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