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Reflections on ULAS J1120+0641 and GN-z11
[[Mod. note -- I have rewrapped overly-long lines. -- jt]]
On Wednesday, December 28, 2016 at 5:14:16 PM UTC-5, jacobnavia wrote: Most of the original snipped ... I worked a few hours for this post, and I think it contain physics arguments that I calculated by hand. I think my calculations are right, and I posted them to you. Steve Willner has already asked you about several of these calculations of = yours; I too am interested in reading more about why you think they are "ri= ght". Well, ULAS J1120_0641 is the most dstant quasar yet discovered. GN-z11 is the most distant galaxy ever seen... until tomorrow, of course. First, thanks for this post. It gave me an excuse to read up on an active p= art of contemporary astrophysics that I don't follow. I haven't yet had a l= ook at papers on the history of formation of SMBH; maybe later. And we have also GN-z11, a one billion solar masses galaxy at just... 400 Million years after that bang. I couldn't understand how people could explain how that fits into our cosmological models and went to the= original science article. {snip} But wait a minute here. We are just a few hundred million years after a big "bang"... Can we apply any models based on data MUCH later in the history of the universe? {snip} That is completely impossible. And we aren't done yet. I am sure in the next time astronomers will discover EVEN FURTHER OUT galaxies since the Hubble team says 90% of the galaxies are further out and are invisible to us. Just wondering... There are quite a few papers which report the results of simulations of galaxy formation (etc) in the early universe (z~5). One in particular directly addresses GN-z11 (preprint: https://arxiv.org/abs/1605.08054). I recommend that you use ADS to find papers this references, and others which cite it (and the references) to dig into this topic in some detail. Figure 1 is, I think, particularly impressive. Myself, I was struck by just how conservative the models are! For example, there's nothing about what effects CDM self-interaction might have. A suggestion or two: redshifts are more-or-less directly observed; "Myr" (e.g. "million years after a big "bang"") is not. And so actual values will depend on the (values of the) parameters used in the models, such as the Hubble constant. In comparing "Myr" values, don't you think it would be sensible to check what models were used to estimate them, if only to see if they are compatible? One more, out of order: Stars form in VERY cold environments in galaxies, protected from radiation by dust and cool gas. A great many galaxies have intense star-formation in or near their nuclei, and even our own MW has some very impressive star clusters within a kpc or so of SgrA*. How do you explain the formation of the stars in such clusters/environments? |
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Reflections on ULAS J1120+0641 and GN-z11
In article ,
writes: A suggestion or two: redshifts are more-or-less directly observed; "Myr" (e.g. "million years after a big "bang"") is not. And so actual values will depend on the (values of the) parameters used in the models, such as the Hubble constant. In comparing "Myr" values, don't you think it would be sensible to check what models were used to estimate them, if only to see if they are compatible? Good point. While the values of the standard model appear to have really converged and many people use them, older papers might use different values not just for the Hubble constant, but for Omega and lambda as well. While the Hubble constant scales inversely with the age of the universe, the relationship of observable quantities on lambda and Omega is more complicated. Of course, time is more relevant than redshift for star formation. Also, while lookback time increases relatively rapidly with redshift at low redshift, at higher redshifts this is much less the case. After all, the big bang was a finite time ago but at infinite redshift. So while the difference between redshifts of, say, 1 and to corresponds to a relatively big difference in lookback time, there isn't much difference between redshifts of, say, 10 and 11. So even if galaxies are discovered at higher and higher redshifts, the higher the redshift, the less the relative increase in lookback time (and decrease in time since the big bang). |
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Reflections on ULAS J1120+0641 and GN-z11
Le 08/01/2017 =E0 19:23, Phillip Helbig (undress to reply) a =E9crit :
In article , writes: A suggestion or two: redshifts are more-or-less directly observed; "Myr" (e.g. "million years after a big "bang"") is not. And so actual values will depend on the (values of the) parameters used in the models, such as the Hubble constant. In comparing "Myr" values, don't you think it would be sensible to check what models were used to estimate them, if only to see if they are compatible? Good point. While the values of the standard model appear to have really converged and many people use them, older papers might use different values not just for the Hubble constant, but for Omega and lambda as well. I used Ned's calculator with the default options. I think that is enough. While the Hubble constant scales inversely with the age of the universe, the relationship of observable quantities on lambda and Omega is more complicated. I suppose Ned uses the latest value. Of course, time is more relevant than redshift for star formation. YES. I am trying to calculate the threshold mass for star formation, that depends on the temperature power 3/2, divided by the square root of the density. Problem is, for a density we need mass and volume, and we have the mass (1e9 Solar masses), just the volume in cubic meters is missing. The units are difficult to follow for an uninitiated: Half light radius... It is the radius of something, so probably a volume, but I have to look it up. Once I figure out that, we can then see how things come out. [[Mod. note -- It's explained by the figure caption in the Wikipedia article https://en.wikipedia.org/wiki/Effective_radius which is the very first result of a google search galaxy "half light radius" -- jt]] |
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Reflections on ULAS J1120+0641 and GN-z11
On Tuesday, January 10, 2017 at 7:39:30 PM UTC-5, jacobnavia wrote:
{snip} I used Ned's calculator with the default options. I think that is enough. For future reference, I think it would help your readers if you state things like this explicitly, and copy the actual "default options" (from my own experience, these change, as the authors decide to update them). While the Hubble constant scales inversely with the age of the universe, the relationship of observable quantities on lambda an= d Omega is more complicated. I suppose Ned uses the latest value. I suspect there's a lot more involved than merely reading "the latest value" ... there's the model (LCDM, but that's not set in stone) as well as the actual values (three, not just H0). Also, I suspect they're conservative, in the sense that the model and values do not get changed unless they feel there's strong consensus, based on published results. Of course, time is more relevant than redshift for star formation. YES. I am trying to calculate the threshold mass for star formation, that depends on the temperature power 3/2, divided by the square root of the density. Problem is, for a density we need mass and volume, and we have the mass (1e9 Solar masses), just the volume in cubic meters is missing. {snip} I would strongly recommend that you also include some estimate of uncertainty, particularly where a variable depends sensitively on one or more values of the input parameters, and/or the model used in the derivation. Too, be very clear in your definitions; example: "mass", as in "the threshold mass for star formation" ... do you mean total mass (including dark matter)? total baryonic mass (i.e. ignoring DM)? "cold gas" mass? ... |
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