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#31
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An old galaxy at z=7.1
In article ,
Eric Flesch writes: I wonder what you'd think of this paper: "The Temperature Of The z=8.4 Intergalactic Medium" -- http://arxiv.org/abs/1503.00045 . They measure the "spin temperature" of the IGM at z=8.4 It's early days for this sort of observation, and I'd really like to have an expert -- which I am not -- explain what the expectations are. What's being measured, as you say, is the spin temperature T_S of the neutral hydrogen, or rather the _difference_ between T_S and the CMB temperature T_CMB. The measurement is complicated by the unknown neutral fraction of the intergalactic gas and (to a lesser extent) other physical parameters. What isn't clear to me is what T_S "should" be, though I gather it should equal the kinetic temperature T_g of the neutral hydrogen gas. Apparently in a model with no heating by stars, the gas decouples from the CMB at about z = 200 and then cools adiabatically faster than the CMB. According to the preprint cited, T_g should reach 1.8 K at z = 8.4, where T_CMB = ~25 K, but I don't understand why T_g isn't coupled to T_CMB. One caution is that the preprint is apparently an unrefereed draft. For all I know, there could be something horribly wrong that I can't spot. and are hard-pressed to reach as high as 10K. I think that's backwards: the _weakness_ of observed fluctuations requires either T_g is near T_CMB or the neutral fraction is either very large or very small. (That last seems quite unlikely to me, but the former might be the case as far as I can tell.) If the neutral fraction is near 50%, T_g 10 K (and presumably 90 K). The _high_ T_g, if you believe it, requires that there be some source of gas heating, presumably X-rays from SNe, and that requires fairly early star formation. However, I don't see how near-zero star formation at z 8.4 with neutral fraction = 100% is ruled out by these results. It may be ruled out by direct detection of z 8 galaxies, but that would take a quantitative argument. As I say, early days, but the approach is extremely promising. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#32
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An old galaxy at z=7.1
On 3/7/15 2:47 AM, Steve Willner wrote:
In article , jacob navia writes: http://www.eso.org/public/archives/r...8/eso1508a.pdf This is an interesting paper, but because it's in _Nature_, not all the information is given. In particular, it looks to me as though the uncertainties on the physical quantities are underestimated, and I don't see how the authors derive the expected equivalent width for the C III] line. (It isn't in the reference cited.) A dust temperature as low as 35 K also strikes me as unlikely; the CMBR temperature is 23 K, after all. None of this changes the basic and valuable result that there must be _some_ dust in the galaxy, and in fact more of the galaxy's luminosity comes out in the rest-frame FIR than in the UV. Why is CMBR presently at 2.7 K and (1+7.1)*2.7 K = 22 K considered dogma when the temperatures thermodynamically approaching zero are available not in thermal equilibrium with CMBR 2.7 K and (1+7.1)*2.7 K = 22 K particularly in the context that dust thermal emissivity may be an indicator of these low temperatures although related extremely long wavelengths are not measurable at this time as calculated by established black body spectrum peak wave length theory. wave length (cm) = h*c/(4.96536456*Boltzmann*T) Emission T K Emission wave length (cm) 2.2E+01 1.3E-02 2.7E+00 1.1E-01 1.0E+00 2.9E-01 1.0E-01 2.9E+00 1.0E-02 2.9E+01 1.0E-03 2.9E+02 1.0E-04 2.9E+03 1.0E-05 2.9E+04 1.0E-06 2.9E+05 1.0E-07 2.9E+06 1.0E-08 2.9E+07 1.0E-09 2.9E+08 1.0E-10 2.9E+09 1.0E-11 2.9E+10 1.0E-12 2.9E+11 1.0E-13 2.9E+12 1.0E-14 2.9E+13 1.0E-15 2.9E+14 1.0E-16 2.9E+15 1.0E-17 2.9E+16 Paraphrasing and amplifying your statement: more of the galaxy's luminosity comes out in the higher wave lengths with the possibility that non currently measurable long wave length luminosity represents large portions of galactic and extra-galactic dust. This concept is further amplified by introducing experimentally available dimensionless material emissivity factors(F) into black body spectrum peak wave length theory: wave length (cm) = h*c/(F^(1/4)*4.96536456*Boltzmann*T) Carbon and other dust candidates such as silicates or metal types have emissivity factors(F) on the order of .1 - .9 . I have looked for experimental emissivity factors(F) for gaseous types such as hydrogen agglomerate particles and have not found them but anticipate their emissivity factors(F) .1 making their thermal emissivity detection further problematic. Such anticipated increased dust would contribute to a more rapid star formation within the context of BB theory. Richard D Saam |
#33
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An old galaxy at z=7.1
Robert L. Oldershaw wrote:
Here is a direct, straightforward question that I would like to have answered. What quantitative or unique qualitative empirical result would lead us to think that there is a problem with our theoretical model of the early period of expansion? I presume that there are some limits to what the existing model could account for. So what are these "lines in the sand" that do offer clear and definitive tests of the model? The standard big bang model requires that the mass distribution in the universe should become more and more anisotropic over time. We have good evidence from CMBR observations that the universe was isotropic to within a few parts per million at the CMBR-last-scattering time (redshift somewhere around 1100 or so if I recall correctly). So.... Hypothetical case #1: If we were to observe some mass tracer to be highly *anisotropic* at times *earlier* (i.e., redshifts higher) than the CMBR-last-scattering time, that would be very hard to explain in (i.e., it would be a big problem for) the big bang model. Hypothetical case #2: If we were to observe some mass tracer to be *more* isotropic than the CMBR at a time *later* than (i.e., at a redshift lower than) the CMBR-last-scattering time, that would be also be very hard to explain in (i.e., it would be a big problem for) the big bang model. Hypothetical case #3: If we were to observe the distribution of some mass tracer (say, galaxies, as measured by their 2-point correlation function) to become *more* isotropic with increasing time over some redshift range, that would also be hard to explain, i.e., it would be a big problem for, the big bang model. [Technically, #2 and #3 might be better characterized as big problems for our theories of the evolution of anisotropies in an expanding universe, rather than for the big bang model itself.] [I suppose there might actually be some theoretical wiggle room around each of my cases if the "mass tracers" turn out not to accurately trace mass. One way to reduce that wiggle-room might be to use (galaxy-cluster) gravitational lensing measurements for #2 and #3, since these directly sample the mass distribution (at least in theory; in practice these are very delicate observations to make, and they require elaborate theoretical modelling of a type of which I don't think Robert Oldershaw approves).] ciao, -- -- "Jonathan Thornburg [remove -animal to reply]" Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA "There was of course no way of knowing whether you were being watched at any given moment. How often, or on what system, the Thought Police plugged in on any individual wire was guesswork. It was even conceivable that they watched everybody all the time." -- George Orwell, "1984" |
#34
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An old galaxy at z=7.1
On Monday, March 16, 2015 at 7:40:24 AM UTC-4, Jonathan Thornburg [remove -animal to reply] wrote:
The standard big bang model requires that the mass distribution in the universe should become more and more anisotropic over time. We have good evidence from CMBR observations that the universe was isotropic to within a few parts per million at the CMBR-last-scattering time (redshift somewhere around 1100 or so if I recall correctly). If I remember correctly, the Planck results at some point generated talk of an inherent "directionality" to the observable portion of the cosmos. If deeper and more sensitive observations turned up a dipole anisotropy that was not due to our relative motion, but rather due to the distribution of matter, would that require a qualitatively different model for the global expansion? [Mod. note: reformatted -- mjh] |
#35
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An old galaxy at z=7.1
In article , "Robert L.
Oldershaw" writes: The standard big bang model requires that the mass distribution in the universe should become more and more anisotropic over time. We have good evidence from CMBR observations that the universe was isotropic to within a few parts per million at the CMBR-last-scattering time (redshift somewhere around 1100 or so if I recall correctly). If I remember correctly, the Planck results at some point generated talk of an inherent "directionality" to the observable portion of the cosmos. If deeper and more sensitive observations turned up a dipole anisotropy that was not due to our relative motion, but rather due to the distribution of matter, would that require a qualitatively different model for the global expansion? You are confusing two different things. The directionality refers to the fact that the "direction" (not obvious what is meant for multipoles higher than 2) of some multipoles appear to be correlated. While this effect appears to be real, it is a very-few-sigma result. It is also not clear if it is primordial. However, it has nothing to do with the dipole direction. The primordial dipole is essentially unobservable, since there is a dipole due to our own motion. Only if one knew the latter precisely could one correct for it and hence see the primordial dipole. One DOES expect a primordial dipole. However, most people don't worry about it since it is essentially unobservable, which is why most plots of the power spectrum start at the quadrupole. |
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