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Old June 26th 17, 11:29 PM posted to sci.astro.research
jacobnavia
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Posts: 105
Default A new impossible galaxy

Le 23/06/2017 =C3=A0 05:06, wlandsman a =C3=A9crit :
he published paper
(https://arxiv.org/ftp/arxiv/papers/1706/1706.07030.pdf ) gives an
observed spectroscopic age of the galaxy of log[Age(yrs)] = 8.97
(+0.26,-0.25) or between 500 million and 1.7 billion years. This
is comfortably within its cosmological age at a redshift of z=2.15
of 3 billion years.


The paper says:

"The best fitting spectrum reveals a massive, old, post-starburst galaxy
consistent with negligible ongoing star formation, and at most solar
metallicity (that is log(M*/Mo) = 11.15 (+/- 0.23), log[Age(yr)]=8.97
(+/-0.26 0.25), Ai=0.6 (+/- 0.9 0.6)."

*Old* galaxy. I understood that. Now what the log[Age(yr)] = 8.97 that
is a mystery for me. exp(8.9) gives 7331.973. 7331 years ???

I must be doing something very wrong obviously. How do you come from
log(age) = 8.9 to 500 - 1700 million years is also a mystery (for me)

In any case I would like to underscore that we are measuring total light
emission from that galaxy, not looking at any individual star. So it
would have been useful if we would figure out the SPREADING of that
measure. Yes, MOST of the stars would have around 1Gy but there is
certain probability of having stars having 4, or even 5 Gy isn't it?

[[Mod. note -- The authors are likely using base-10 logarithms.
log10[Age(yr) = 8.97 (+0.26 -0.25) gives a range of roughly
500 to 1700 Myr.

It would indeed be useful to know the age distribution of the stars
in that galaxy. But it would surely take a lot more light to
measure that than to measure the integrated properties, i.e.,
compared to the current observations the age-distribution measurement
would need a much larger telescope and/or a much longer exposure time.
-- jt]]