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Reflections on ULAS J1120+0641 and GN-z11



 
 
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Old January 7th 17, 10:31 AM posted to sci.astro.research
Steve Willner
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Default Reflections on ULAS J1120+0641 and GN-z11

In article ,
jacobnavia writes:
https://arxiv.org/pdf/1603.00461.pdf


Published in ApJ at
http://dx.doi.org/10.3847/0004-637X/819/2/129

There, we can read that this galaxy is very bright (across more than 13
billion years imagine). How they estimated the stellar population?

I went to Appendix B, "Estimating stellar population properties" and
there, I could read:


Answering your own question there.

quote
The allowed star-formation histories were based on standard
exponentially declining models with parameters tau = 108, 109, and
constant star-formation, with stellar metallicities of Z = 0.05 - 0.57
Zo and an initial Chabrier mass function
end quote

[Here Zo is a Z followed by a circle with a dot. I assume here that that
is the metallicity of the sun. Please correct me if I am wrong.]


That last is correct; circle with dot is the symbol for the Sun.

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?


The stellar evolution tracks are probably OK -- why wouldn't they be?
The initial mass function (IMF) is strictly an assumption, but one
has to assume something. For both local galaxies and distant ones,
the observed _light_ comes from the massive stars. In local stellar
populations, the _mass_ is mostly in the low-mass stars. The
"Chabrier" IMF assumption is typical, but there is no direct evidence
that any low-mass stars at all are present. In that sense, the
derived stellar mass is more or less an upper limit.

By definition this galaxy shouldn't contain much "metals"


Why not? A "generation" of massive stars takes only about 10 Myr to
produce supernovae, which will pollute the interstellar medium with
metals. The zero-metallicity IMF is likely to be top-heavy, so a
large fraction of the mass making up the first generation of stars is
likely to be recycled into metal-rich gas.

How can we have any metallicity or dust at all?


The dust extinction actually derived from the data was an upper
limit, i.e., consistent with zero, so you should be happy enough with
the result.

And besides that, the problem is that to make stars efficiently,
galaxies need "metals" as seeds. Making stars without any metals is a
MUCH SLOWER process...


Why do you think that? It's harder to make low-mass stars, but I
don't see why the process for high-mass stars should be slower.

Stars form in VERY cold environments in galaxies, protected from
radiation by dust and cool gas.


They do _now_. That doesn't make it impossible for them to have
formed in warmer environments in the past. The required mass would
have been higher, of course. Look up "Jeans mass."

Massive galaxies at large redshift were very rare (so far as we know
now), but their abundance was (apparently) not strictly zero. The
evidence should get much stronger when JWST starts working.

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Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA

 




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