A Method for Measuring the
Photoionization of Neutral Gas
by Soft X-Rays
Edward B. Jenkins
Princeton University Observatory
Motivation
 Relevance to the study of DLAs
 Monitor the strength and character of the soft X-ray
environment that irradiates the H I gas from
 Internal sources
 Radiation from embedded hot gas
 Population of very hot stars emitting EUV radiation
 The effects of an external penetrating radiation
 Metagalactic flux
 AGN radiation for proximate DLAs (PDLA).
 Radiation from the DLA’s own (otherwise invisible) AGN
 Obtain an understanding of how measurements of
element abundances could be influenced by this type
of radiation that can penetrate H I regions.
Requirements for a Reliable
Interpretation
 Considerations for the choice of species to
compare – should be immune to uncertainties
arising from the following:
Overall metallicity level
The effects of element depletions caused by dust
formation
3. [α/Fe]
4. [secondary elem./primary elem.]
5. Contributions from H II regions
1.
2.
 There must be a workable selection of
absorption lines.
An Ideal Combination:
Neutral Argon and Oxygen
 Basic premises:
1. Ar and O are both α-process elements.
2. They are not subject to strong depletions.
3. The ionization of O is locked to that of H through
very strong charge exchange reactions in both
directions (but beware of a temperature
dependence ), so its ionization level can serve as
a surrogate for that of H.
4. The ionization potentials of Ar and H are about
the same, but their photoionization cross
sections are vastly different from each other.
Ionization cross sections
(Recombination coefficients with free
electrons as a function of temperature αe(T)
for these two elements are virtually identical.)
Ionization cross sections
(Recombination coefficients with free
electrons as a function of temperature αe(T)
for these two elements are virtually identical.)
PAr
(Recombination coefficients with free
electrons as a function of temperature αe(T)
for these two elements are virtually identical.)
Photoionization Equilibrium Lite
• Relationships that are easy to understand:
(Ar ) (H)
PAr 
(H) (Ar )
• Where     ( E )(1   ) F ( E )dE and α = recomb.
coefficient with free electrons. (dependent on T)
It follows that
 1  xe 
 , where xe = n(e)/n(H)
[Ar I/O I]  log 
 1  PAr xe 
We can measure this with absorption lines.

Upgrade to
Photoionization Equilibrium Pro
In reality, we must include the effects of charge exchange with
hydrogen and helium to obtain more correct ion fractions,
Three lowest
fractional
ionizations
And include the extra electrons produced by the partial
ionization of helium,
This is a bit
tricky: must
solve iteratively
And add extra terms that account for recombination of ions on
the surfaces of dust grains …, etc.
Could Collisional Ionization Mimic
the Effects of Photoionization?
Log neutral fraction
Ionization by Collisions
H
Equilibrium (CIE)
Ar
Log neutral fraction
Ionization by Collisions
H
Ar
Radiative Isobaric Cooling
Log neutral fraction
Ionization by Collisions
H
Ar
Radiative Isochoric Cooling
Principles of Observing
Consider not Voigt
profiles and column
densities, but instead
compare equivalent
widths.
Available Lines
Log (Ar/O) = -2.29
[Ar I/O I] = 0.0
Ar I
2.29 dex
OI
Available Lines
Log (Ar/O) = -2.29
Ar I
[Ar I/O I] = 0.0
2.29 dex
OI
Available Lines
Log (Ar/O) = -2.29
[Ar I/O I] = -0.5
Ar I
0.5 dex
2.29 dex
OI
O I lines
Ar I
line
Equivalent widths in the spectrum of
J1014+4300
Least-squares
linear fit to the
lowest 4 points
Ar I
line
O I lines
[Ar I/O I] = -1.09 (+0.19,-0.29)
Conclusions from the low [Ar I/O I]
determination for J1014+4300
• Important constraint: Log lc = -27.25 [Wolfe et al.
(2008) ApJ, 681, 881]
• It is difficult to understand our finding of [Ar I/O I]
= -1.09 with either the Haardt & Madau description
for extragalactic irradiation or an addition of some
moderate-energy X-ray illumination without either
– (1) having in incredibly low n(H) which would increase
the ionizations (and their differences) but also would lead
to an unbelievably large length scale,
– or (2) creating an X-ray heating rate well in excess of
the observed value of Log lc shown above.
PAr
Plausible solution: the gas is
illuminated by an AGN at the
center of the DLA galaxy, but the
X-rays are obscured by a column
density of hydrogen with Log N(H)
≈ 22.5, thus creating an ionization
(Recombination coefficients with free
condition with a large value of PAr
electrons as a function of temperature αe(T)
for these two elements are virtually identical.)
Trend of [Ar/α] with z reported by Vladilo et al.
(2003) A&A 402, 487
Log lc = -26.66
Log lc = -27.48
Log lc = -27.09
“3/7 of the zabs < 3 absorbers in the
Vladilo et al. (2003) sample are
PDLAs. A further two have velocities
within 10 000 km s−1, leaving only two
which might be considered truly
intervening absorbers. At zabs > 3 all
three absorbers have v > 15 000 km
s−1. Given the evidence presented in
this paper that the effects of a hard
ionizing spectrum are seen out to at
least 3000 km s−1 (and previous results
that indicate narrow associated
absorbers may contribute out to 10
000 km s−1; e.g.Wild et al. 2008; Tytler
et al. 2009), the apparent redshift
evolution may actually arise from the
inclusion of proximate systems.”
(Ellison et al. 2010, MNRAS, 406,
1435)