From: njzy@vax5.cit.cornell.edu (T. Joseph Lazio, Cornell University)
Newsgroups: sci.astro
Subject: Re: Standard model of QSOs
Date: 27 Jul 92 08:38:22 EDT
Distribution: sci
Organization: Cornell University

In article <Bs1878.5Hw@well.sf.ca.us>,
metares@well.sf.ca.us (Tom Van Flandern) writes: 
> 
> njzy@vax5.cit.cornell.edu (T. Joseph Lazio, Cornell University) writes:
> 
>> The Gunn-Peterson test tells us that there is very little intergalactic HI.
>> The Gunn-Peterson test is, briefly, the following:  Examine the spectra of
>> QSOs which are redshifted to the point that the Lyman alpha emission line
>> can be seen in the optical (typically, the Lyman alpha emission line is at
>> 1215 Angstroms, too far into the UV to see from the ground).  Is there
>> emission in these spectra blueward of the Lyman alpha line?  If so, there
>> cannot be much intergalactic HI; the HI cross-section for a Lyman alpha
>> photon is huge.  If there was a reasonable amount of intergalactic HI,
>> there would be a sharp cut-off on the blue side of the Lyman alpha line.
>> Since sharp cut-offs are not observed, the conclusion is that there is
>> either very little hydrogren present in intergalactic space or that it is
>> largely ionized.
> 
>      That conclusion is in conflict with the observation of great numbers of
> intergalactic hydrogen clouds between us and several nearby quasars, such as
> 3C273.  One easy way out of that conflict is if high redshift does *not* mean
> great distance.  Then everything will be just as observed.  

 I disagree.  If one can correctly identify the Lyman alpha emission 
 line, then the Gunn-Peterson test can be applied.  The hydrogen 
 cross-section near Lyman alpha is huge.  If one sees emission blueward 
 of the Lyman alpha line, then there is little (i.e. essentially no) HI 
 between the source and the observer, regardless of distance between 
 source and observer.  As I noted in an earlier post, the column density
 through these intergalactic HI clouds is very small; although there
 may be many of them, the combined optical depth must be small. (I would 
 assume that somebody has done a more sophisticated analysis attempting 
 to figure out the actual optical depth in front of some QSOs although
 off-hand I don't know of any such references; anybody else?)

> I don't recall how you proposed to explain the lack of a monotonic 
> relationship between numbers of clouds seen in the Lyman alpha forest 
> and redshift.

 I didn't; however, I believe the standard interpretation is that 
 the QSO environment is partially to blame.  QSOs would have large 
 ionizing fluxes which result in a deficit of clouds near the source; 
 I vaguely remember a reference which showed just that.  Should I 
 dig for it?

-- 

 T. Joseph Lazio                | Why relativity? and Why  
 514 Space Sciences             | turbulence?  I really believe   
 Ithaca, NY  14853              | [God] will have an answer for the
 (607) 255-6420                 | first [question].
 lazio@astrosun.tn.cornell.edu  |   -- W. Heisenberg, on his death bed
ICBM:                           |
 42 deg. 20' 08" N              |              STOP RAPE
 76 deg. 28' 48" W              |