For the reader who likes to browse original papers, this page contains
the articles in which quasar absorption lines were proposed (1965,1966
with Ed Salpter, before the absorption line were observed), in which
Monte Carlo simulations were first used (1968) to establish the
validity of the line identifications for multiple absorption redshifts
in a single QSO spectrum, in which the hypothesis of Galactic Halos as
the origin of metallic absorption line systems was suggested (1969,
with Lyman Spitzer), in which quasar absorption lines were first used
to constrain the temperature of the CMB at larger redshifts (1973,
with Paul Joss and Roger Lynds), in which the hypothesis was advanced
(1975) that absorption line systems can be resolved into sub-clouds
with structure on a scale of order 100 km/s, and in which the use of
the HST for quasar and galactic absorption line studies was initially
described (1980). The collection also includes the suggestion that 21
cm absorption lines may be detectable in the spectra of distant
On The Interaction of Radiation from Distant Sources with the
- Authors:John N. Bahcall and E. E. Salpeter
- Journal: The Astrophysical Journal, 142,
No. 4, 1677--1681 (November 1965).
We discuss several ways that a distant radiation source (with a large
due to the cosmological expansion) can provide information over a wide
range of distances about the intervening medium. As we shall show [cf. Gunn and
neutral hydrogen (or other atoms) at various distances between the source
will give rise to an ``absorption trough'' in the continuous spectrum of a
If the neutral hydrogen is instead concentrated in clusters of galaxies,
this trough is
replaced by a number of sharp absorption lines. Besides discussing (i)
and (ii) absorption lines from clusters, we also consider (iii) photon
scattering by free
electrons in the intervening medium and (iv) spreading of a radio beam due
to inhomogeneities in the ionized gas that is traversed. Present observations
stringent upper limits, and we suggest other feasible cosmological tests.
Absorption Lines in the Spectra of Distant Sources
- Authors:John N. Bahcall and E. E. Salpeter
- Journal: The Astrophysical Journal, 144,
No. 2, 847--852 (May 1966).
We discuss the properties of absorption lines that might appear in the
spectra of distant quasi-stellar sources (henceforth abbreviated ``QSS'') and
what can be learned from a search for such lines. We consider first
absorption by intergalactic gas in clusters of galaxies
that are in the line of sight. We
extend our previous
discussion (Bahcall and Salpeter 1965)1
of absorption lines caused by
to resonant absorption lines in other elements and use a variety of
estimate some of the uncertainties. If the intergalactic gas in clusters
abundances comparable to those inside our Galaxy, densities
above 10-5 cm-3, and
temperatures below 5 × 105° K, a
number of different ion species could
cause pronounced absorption lines; we present a table of the spectral lines expected
to be prominent in absorption for various temperatures. The detection of more than one
feature in the same spectrum corresponding to the same redshift for the
help to eliminate the possibility that an apparent absorption feature was
really a valley
between two emission peaks. In particularly favorable cases identification
of the absorber with an optically detectable cluster might be possible. A number of
known with sufficiently large redshifts so that an interposed cluster might
several absorption features redshifted into the visible. The
characteristics (such as wavelengths, depth, and width) of absorption lines formed in clusters of
galaxies could provide
otherwise unobtainable information about distant clusters (such as the
chemical composition, and velocity dispersion of the absorbing gas).
We show in what follows that the identification of absorption lines with a
redshift appreciably different from that of the emitting QSS's would
provide strong evidence for the hypothesis that the redshifts of QSS's are of cosmological
origin. We also
discuss absorption features which could be formed by gas clouds in the
a QSS and which might lead (under special circumstances) to apparent
variations in the
observed wavelengths of some QSS emission lines.
A program is under way at the California Institute of Technology (J. N.
B. A. Peterson, and M. Schmidt) to search for absorption lines in spectra
Schmidt) of QSS's.
1Eq. (6) of
our paper should contain the factor (/100 Mc/s)-4 instead of
(/100 Mc/s)4. Observations of the ultraviolet
spectra of 3C 9 (down to 3100 Å by Wampler )
and of 0106+01 (Burbidge
1966) enable one to conclude that the cosmological number density of
molecular hydrogen is less than
10-10 cm-3 if it is uniformly distributed.
A Systematic Method for Identifying Absorption Lines as
Applied to PKS 0237-23
- Authors:John N. Bahcall
- Journal: The Astrophysical Journal, 153,
679--688 (August 1968).
A systematic method for identifying the absorption lines in
quasi-stellar sources with rich absorption
spectra is described, and the lines that are expected on theoretical
grounds to have the highest probability
of appearing in absorption in quasi-stellar sources are listed.
The computational scheme that was developed for recognizing and
analyzing candidate redshifts is then described, this computational scheme is
supplemented by a set of rules used to determine if a candidate
redshift is acceptable. The computational
scheme and formal rules were applied to a set of nonsense spectra,
i.e., random-number spectra, with the
same main characteristics as the absorption spectrum of PKS 0237-23
determined by Greenstein and
Sargent. The average number of acceptable redshifts per nonsense
spectrum was 0.7. Five acceptable
redshifts were found in the real spectrum:
z = 2.2015, 1.671, 1.656, 1.513, and 1.364. No acceptable
redshift was found at z = 1.95. A list of additional lines
that should be detectable if the identifications
proposed here are correct is given for slightly longer and
shorter wavelengths than covered in the observations of Greenstein
and Sargent. Some characteristics of the identifications are also described.
Absorption Lines Produced by Galactic Halos
- Authors:John N. Bahcall and Lyman Spitzer, Jr.
- Journal: The Astrophysical Journal, 156,
L63-L65 (May 1969).
We propose that most of the absorption lines observed in quasi-stellar
sources with multiple absorption redshifts are caused by gas in
extended halos of normal galaxies.
On the Temperature of the Microwave Background Radiation at a
- Authors:John N. Bahcall, Paul C. Joss, and Roger Lynds
- Journal: The Astrophysical Journal, 182,
L95 (June 1973).
It is shown that the temperature of the microwave background radiation
at a redshift of the order of 2.5 is certainly less than 200° K
and probably less than 45° K.
Further detailed studies of the absorption spectra of large-redshift
quasars can improve these limits.
Splittings in Quasar Absorption Lines
- Authors:John N. Bahcall
- Journal: The Astrophysical Journal, 200,
L1-L3 (August 15, 1975).
It is shown that the apparent constancy of the splitting
between quasar absorption lines observed
by Boksenberg and Sargent could be understood if the typical
velocity in the absorbing material were somewhat less than the
observing resolution (estimated to be 100 km s-1).
If this interpretation were correct, then there should be many
more redshifts that are split with velocity separations that are
less than 100 km s-1 than have been so far observed
at velocity separations of the order of 150 km s-1, The
above conclusions are independent of whether or not the absorption
lines originate close to, or very far away from, the quasar.
Some results are presented for a specific model in which the
splittings are supposed due to absorption in halos or clouds
surrounding galaxies or small groups of galaxies and some tests
of this model are suggested. The model parameters are consistent
with recent radio observations of H I clouds or halos that
extend far from their parent galaxies.
Absorption Lines in the Spectra of Distant
- Authors:John N. Bahcall
- Journal: Scientific Research with the Space
Telescope, IAU Colloquium Number 54, ed. M. S. Longair and
J. W. Warner (1980), pp. 215-240.
This article discusses critical observations of quasar absorption
lines that are made possible by the Hubble Space Telescope. The
article reviews the state of quasar absorption line research circa
1980, including a prediction of the absorption lines that are most
likely to be observed with HST and two tests for the origins of the
absorption systems. According to the interpretation given here, many
or most of the absorption lines are caused by intervening material,
although references are given to contemporary claims that the
absorption lines are intrinsic to the quasars. An iconoclastic
proposal is made, namely, that most of the Ly-alpha lines are caused
by gas in the outer reaches of galaxies (halos 200 kpc) or of clusters
of galaxies (halos of 30 Mpc). Several important observing projects are
On the Possibility of Detecting Redshifted 21-cm Absorption
Lines in the Spectra of Quasi-Stellar Sources
- Authors:John N. Bahcall and Ronald D. Ekers
- Journal: ApJ, 157, 1055 (September 1969).
The expected strengths of redshifted 21-cm absorption lines are
estimated by using the properties of the observed optical absorption
lines in quasi-stellar sources. It is shown that the absence of OI
and NI absorption lines in the observed optical absorption spectra of
quasi-stellar radio sources indicates that 21-cm absorption lines
wider than 100 kHz are likely to be weak unless the heavy-element
abundance in the absorbing material is low. Lines narrower than 100
kHz may be strong; their optical counterparts would have escaped
detection. A general expression is given for the spin temperature of
neutral hydrogen when (following Field) Ly α excitation and
de-excitation, 21-cm absorption and emission, and particle collisions
are all included. The results are expressed simply in terms of the
strength and distance of the radio source and the separation between
absorber and emitter. It is shown that the spin temperature of
neutral hydrogen is large near a quasi-stellar radio source and is
determined either by the ambient 21-cm flux or the ambient Ly α
flux for separations between absorber and emitter of less than about
105 lt-yr. The expected absorption (and emission)
strengths of redshifted 21-cm lines are compared with observational
capabilities at radio observatories. Some interesting candidates for
study are listed.
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