galaxy at redshift 8.56

In article , Juergen Barsuhn
writes:

One question arising there was, over which distance a photon
from this galaxy had actually to travel until it reached our
instruments. The answer 13 billion light years was thought
to be wrong, as the photon encounters the steadyly expanding
space on its journey. Based on a paper by Harald Lang from
the Swedish Royal Technical University KTH at Stockholm:
http://www.math.kth.se/~lang/distance.htm
a distance of 40 billion light years was "accepted". This
sounds incredibly large to me. What do you think?

It's not a matter of what one thinks; this is standard stuff. I had a
BRIEF look at the link above; nothing obviously wrong. I haven't
checked the figures myself, but it would probably be quicker to fire up
Ned Wright's cosmology calculator (google it!).

On has to specify the cosmological parameters H, Omega and lambda to
calculate the distance as a function of redshift. Also, one has to
decide WHICH distance one is interested in. The stuff above sounds like
the "proper distance at the present time" is the distance in question.

On Fri, 19 Nov 10, Juergen Barsuhn wrote:
One question arising there was, over which distance a photon
from this galaxy had actually to travel until it reached our
instruments.

I don't think you can sensibly add universal expansion to that
calculation. The photon tells its own full story by its redshift --
it's a pure relic of its source era. Its distance travelled and time
travelled are interchangably the same concept.


Eric Flesch

Le 19/11/10 10:01, Juergen Barsuhn a écrit :
Jonathan Thornburg [remove -animal to reply] schrieb:
Catching up on some of my reading, I see an impressive observational
paper:

Matthew D. Lehnert et al
Title: Spectroscopic confirmation of a galaxy at redshift z=8.6
Nature vol 467 (21 Oct 2010), pages 940-942
doi: 10.1038/nature09462
preprint: http://arxiv.org/abs/1010.4312

......
A few aspects of these findings were also discussed in the
German newsgroup de.sci.astronomie .

One question arising there was, over which distance a photon
from this galaxy had actually to travel until it reached our
instruments. The answer 13 billion light years was thought
to be wrong, as the photon encounters the steadyly expanding
space on its journey. Based on a paper by Harald Lang from
the Swedish Royal Technical University KTH at Stockholm:
http://www.math.kth.se/~lang/distance.htm
a distance of 40 billion light years was "accepted". This
sounds incredibly large to me. What do you think?
Regards Jurgen

I have some trouble following you:

What is a "light year"?

The distance light travels in a year. OK. It is a measure
of distance.

But does this "light year" include the distance added by the
space expansion in a year or not?

Because during that year, the universe has expanded, so the
light has to travel a bit "farther"...


If you say that light has "actually" travelled 40 billion light
years you are inventing an "absolute" light year, i.e. a NON EXPANDING
light year that is completely unobservable since we live in an
"expanding" universe...

If space is expanding, either the measurements units expand ALSO
(then that 40 billion years is bogus) or we invent a NON EXPANDING
unit of measure that is unobservable and unverifiable and depends
on the value we figure out for the Hubble constant...

Conclusion:
The universe is not expanding because if it would, my head would
explode...

:-)

Thanks in advance for a clarification of this.

[Mod. note: we can define non-expanding units of measure on Earth, or,
say, within the solar system, because the Earth, solar system,
Brooklyn etc are not expanding.
http://math.ucr.edu/home/baez/physic..._universe.html
When we talk about light years, we are not considering the expansion
of the universe -- mjh.]

Le 19/11/10 13:34, jacob navia a écrit :

Because during that year, the universe has expanded, so the
light has to travel a bit "farther"...


If you use:

https://www.cfa.harvard.edu/~huchra/hubble/

the value is 160 KM/sec /million light years/second.

For a single light year this is:

160 000 meters / 1 million -- 16 cm

In one second 16 cm, in a year 5045.76 KM.

Then EITHER:

(1) A light year is

c * (24hours *3600 sec/hour *365 days) meters

ÒR


(2) A light year is

c * (24hours *3600 sec/hour *365 days) meters + 5 045 760 meters

Note that (1) is UNOBSERVABLE since we can't stop the univers's
expansion to make the measurement.

In article , Eric Flesch
writes:

On Fri, 19 Nov 10, Juergen Barsuhn wrote:
One question arising there was, over which distance a photon
from this galaxy had actually to travel until it reached our
instruments.

I don't think you can sensibly add universal expansion to that
calculation. The photon tells its own full story by its redshift --
it's a pure relic of its source era.

True.

Its distance travelled and time
travelled are interchangably the same concept.

Not really. Both depend on the cosmological model, but in a different
way. Knowing one doesn't tell you the other, unless you know the
cosmological model, but even then you would have to calculate it from
the redshift; it's not a simple relationship. Depending on the
cosmological model and redshift, the light-travel--time distance and
another distance might be the same, or perhaps the latter is larger, or
the former.

In article , jacob navia
writes:

What is a "light year"?

The distance light travels in a year. OK. It is a measure
of distance.

OK. But one doesn't have to take it literally always. One can convert
this distance into millimetres or whatever.

But does this "light year" include the distance added by the
space expansion in a year or not?

Because during that year, the universe has expanded, so the
light has to travel a bit "farther"...

Right.

If you say that light has "actually" travelled 40 billion light
years you are inventing an "absolute" light year, i.e. a NON EXPANDING
light year that is completely unobservable since we live in an
"expanding" universe...

Define "observable". Yes, the distance being discussed is "less
observable" than some others, but still well defined.

If space is expanding, either the measurements units expand ALSO
(then that 40 billion years is bogus) or we invent a NON EXPANDING
unit of measure that is unobservable and unverifiable and depends
on the value we figure out for the Hubble constant...

Right, the latter. It also depends on the other cosmological
parameters.

In article ,
Thomas Smid writes:
The point I am making is that neutral hydrogen can not really absorb
the Ly-alpha line but only scatter.

Scattered Ly-alpha would come out of the scattering roughly
isotropically. Only a tiny fraction would make it to us on Earth,
and we would never see the galaxy in question as a distinct object.
Compare with the "Lyman-alpha forest" in quasar spectra.

Ned's cosmologogy calculator, for the standard cosmological
parameters, gives a light travel time of 13.07 Gyr. Comoving radial
distance, by contrast, is just over 30 Glyr. That last is the proper
distance of the galaxy "now," not the distance it had when the light
was emitted.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA

In article , Steve Willner
writes:

Ned's cosmologogy calculator, for the standard cosmological
parameters, gives a light travel time of 13.07 Gyr. Comoving radial
distance, by contrast, is just over 30 Glyr. That last is the proper
distance of the galaxy "now," not the distance it had when the light
was emitted.

The last two distances are the same if the universe is flat. Since
current data suggests it is at least pretty close to flat, the
distinction doesn't matter. In a spatially closed universe, the
comoving radial distance increases more slowly than the proper distance,
then starts decreasing and then even reaches zero. Think of it as
proportional to a parallel of latitude. As one goes away from the north
pole, it increases, until the equator, then it starts decreasing, then
is zero at the south pole. The proper distance (which one would travel
along the Earth's surface) keeps increasing though. This comoving
radial distance is important since "observable" distances such as the
luminosity and angular-size distance are closely related to it (just
multiply by the appropriate power of (1+z)).

On Nov 19, 8:57*am, Juergen Barsuhn wrote:
Thomas Smid schrieb:

On Nov 10, 9:42 pm, "Jonathan Thornburg [remove -animal to reply]"
wrote: [Mod. note: entire quoted article deleted -- mjh]

Am I missing something here? How should neutral hydrogen (in its
ground state) possibly absorb the Lyman alpha line at 1216 A when the
ionization threshold for hydrogen is 912 A? The only thing that
hydrogen could do is resonantly scatter the line, which would however
not affect the total Ly alpha flux escaping from the volume.

Reconsidering, this appears convincing to me. - But how can
I then understand the broad gap around the Lyman-Alpha
wavelength in the interstellar radiation field of our Milky
way: Should not the gap be filled by resonantly reemitted
Lyman-Alpha photons? So I am left a little bit worried.


It all depends on the geometry of the situation:
obviously, if you would just have a plane wall off scattering material
between the source and observer, then most of the radiation would be
scattered back and disappear into the half-space in the opposite
direction. So in this case the emission line would be much reduced in
its intensity in the forward direction (and a continuous spectrum
would show an absorption line).
However, if you have a (more or less) spherical shell of scattering
material surrounding the source, a photon will bounce back and forth
inside until it eventually manages to escape. In the process, the
photon density inside will build up to such a degree that it exactly
compensates for the high reflectivity of the shell, i.e outside you
will observe exactly the same flux as without the reflecting shell.

Another question arises to me. The finally redshifted
Lyman-Alpha line cannot be absorbed by neutral hydrogen.
However, on the way to us the photons will traverse regions
of slightly lower redshift that would still allow the
photons to be absorbed and then re-emitted at this lower
redshift. and so on. Should this effect change the profile
of the observed Lyman-Alpha line?

The Lyman alpha wavelength (1216 A) is the longest wavelength of
photons that can be affected by hydrogen in its ground state (be it
scattering (bound-bound transitions) or absorption (bound-free
transitions)). So a red-shifted Lyman alpha line can not really be
affected at all by ground-state hydrogen The latter can only affect
non-redshifted Lyman alpha lines (i.e. in close proximity to the
source) by resonant scattering. And as indicated above, it all depends
on the geometry of the situation by what amount the flux will appear
to be reduced through scattering (the line profile would be affected
in any case, but this is not discussed in the paper as the line was
not sufficiently resolved). So the data presented in the paper do not
at all warrant the conclusions drawn by the authors. In fact, they
rather seem to contradict them unless some unreasonable (or at least
completely unsubstantiated) assumptions regarding the geometry of the
situation are being made.

Thomas

On 11/17/10 1:46 AM, Thomas Smid wrote:

The point I am making is that neutral hydrogen can not really absorb
the Ly-alpha line but only scatter. In the end, every photon that is
emitted from the galaxy will escape, and the total escape flux will be
the same as without a surrounding hydrogen cloud (for a spherically
symmetric situation it will even be the same in each specific
direction). So their argument does not really apply.

Photons absorbed on the way
to the observer have only a very low chance to be scattered
into their previous direction by (multiple) re-emission. I
think your arguing only applies to a dense globule with
infinitely often re-emission. Look for an illustration at
our galactic interstellar radiation field: Ther you will
find e.g. the surrounding of Lyman-Alpha void of photons.

The scattering optical depth of the cloud does not really matter. It
only affects the photon density distribution within the cloud, but not
the escape flux. In a steady state, the total Ly-alpha flux leaving
the cloud must be equal to the original flux emitted by the central
galaxy. It could only be reduced if there is a significant amount of
hydrogen in excited states (so that it can be ionized by Ly-alpha), or
if there are other atoms or molecules having a ionization potential
low enough so that they can be ionized by Ly-alpha. But I don't see
anything in that paper in the way of a discussion of these
possibilities.

Thomas

Putting some numbers to your point:

The 1S-2S characteristic BEC hydrogen transition
(generally true for neutral hydrogen in all states)
is at 243 nm (1.23E+15 Hz) with a band width of ~1E6 Hz
Ref: Killian 1S-2S Spectrum of a Hydrogen Bose-Einstein Condensate
Physical Review A 61, 33611 (2000)

What are potential sources of 1.23E+15 Hz +/- 1E6 Hz for reionization?

Richard D. Saam