Transcript Aspen - NMSU Astronomy

Evidence For Cosmological
Evolution of the
Fine Structure Constant?
Chris Churchill
(Penn State)
a = e2/hc
Da = (az-a0)/a0
John Webb (UNSW)
- Analysis; Fearless Leader
Steve Curran (UNSW)
- QSO (mm and radio) obs.
Vladimir Dzuba (UNSW)
- Computing atomic parameters
Victor Flambaum (UNSW)
- Atomic theory
Michael Murphy (UNSW)
- Spectral analysis
John Barrow (Cambridge)
- Interpretations
Fredrik T Rantakyrö (ESO)
- QSO (mm) observations
Chris Churchill (Penn State)
- QSO (optical) observations
Jason Prochaska (Carnegie Obs.)
- QSO (optical) observations
Arthur Wolfe (UC San Diego)
- QSO optical observations
Wal Sargent (CalTech)
- QSO (optical) observations
Rob Simcoe (CalTech)
- QSO (optical) observations
Juliet Pickering (Imperial)
- FT spectroscopy
Anne Thorne (Imperial)
- FT spectroscopy
Ulf Greismann (NIST)
- FT spectroscopy
Rainer Kling (NIST)
- FT spectroscopy
Webb etal. 2001 (Phys Rev Lett 87, 091391)
QSO Spectra
Intrinisic QSO Emission/Absorption Lines
H I (Lyman-a) 1215.67
C IV 1548, 1550 & Mg II 2796, 2803
And, of course…The Beam Collector.
Keck Twins
10-meter Mirrors
The High Resolution Echelle Spectrograph (HIRES)
2-Dimensional Echelle Image of the Sun
Dark features are absorption lines
We require high resolution spectra…
Interpreting cloud-cloud velocity splittings….
Parameters describing ONE absorption line
3 Cloud parameters:
b, N, z
b (km/s)
N (atoms/cm2)
“Known” physics
parameters: lrest, f, G...
(1+z)lrest
Cloud parameters describing TWO (or more)
absorption lines from the same species…
(eg. MgII 2796 + MgII 2803 A)
N
b
b
3 cloud parameters
(no assumptions),
z
We decompose the complex profiles as multiple clouds, using
Voigt profile fitting
natural line broadening + Gaussian broadening
Gaussian is line of sight thermal broadening gives “b”
The “alkali doublet method”
Resonance absorption lines such as CIV, SiIV, MgII are commonly
seen at high redshift in intervening gas clouds. Bethe & Salpeter 1977
showed that the l1, l2 of alkali-like doublets, i.e transitions of the sort
l1 :
l2 :
2
S1 / 2 2 P3 / 2
2
S1 / 2  P1 / 2
2
are related to a by
l1  l2 Dl

 a 2 which leads to
l
l
l2
l1
 a  ( Dl )

a
2Dl
Note, measured relative to
same ground state
But there is more than just
The doublets… there are
other transitions too!
Cloud parameters describing TWO absorption lines
from different species (eg. MgII 2796 + FeII 2383 A)
b(FeII)
b(MgII)
N(FeII)
maximum of 6 cloud
parameters, without
assumptions
N(MgII)
z(FeII)
z(MgII)
We reduce the number of cloud parameters
describing TWO absorption lines from different
species:
b
Kb
N(FeII)
N(MgII)
4 cloud parameters,
with assumptions:
no spatial or velocity
segregation for different
species
z
The “Many-Multiplet method” - using different
multiplets and different species simultaneously Ei
Ec
In addition to alkali-like doublets,
many other more complex species
are seen in quasar spectra. Now we
measure relative to different ground
states
Low mass nucleus
Electron feels small
potential and moves
slowly: small relativistic
correction
Represents different
FeII multiplets
High mass nucleus
Electron feels large
potential and moves
quickly: large relativistic
correction
Procedure
1. Compare heavy (Z~30) and light (Z<10) atoms, OR
2. Compare s
p and d
p transitions in heavy atoms.
Shifts can be of opposite sign.
E

E
z
z 0
Illustrative formula:
 a
 q  z
 a 0


  1


2
Ez=0 is the laboratory frequency. 2nd term is non-zero only if a has
changed. q is derived from relativistic many-body calculations.
Relativistic shift of the
q  Q  K (L.S )
central line in the multiplet
Numerical examples:
K is the spin-orbit splitting
parameter.
Z=26 (s
p) FeII 2383A: w0 = 38458.987(2) + 1449x
Z=12 (s
p) MgII 2796A: w0 = 35669.298(2) + 120x
Z=24 (d
p) CrII 2066A: w0 = 48398.666(2) - 1267x
where x = (az/a0)2 - 1
MgII “anchor”
Low-z (0.5 – 1.8)
High-z (1.8 – 3.5)
ZnII
FeII
SiIV
FeII
Positive
Mediocre
Anchor
Mediocre
Negative
CrII
MgI, MgII
High-z
Da/a = -5×10-5
Low-z
Low-z vs. High-z constraints:
Current results:
Possible Systematic Errors
1. Laboratory wavelength errors
2. Heliocentric velocity variation
3. Differential isotopic saturation
4. Isotopic abundance variation (Mg and Si)
5. Hyperfine structure effects (Al II and Al III)
6. Magnetic fields
7. Kinematic Effects
8. Wavelength mis-calibration
9. Air-vacuum wavelength conversion (high-z sample)
10. Temperature changes during observations
11. Line blending
12. Atmospheric dispersion effects
13. Instrumental profile variations
2-Dimensional Echelle Image of the Sun
Dark features are absorption lines
Using the ThAr calibration spectrum to see if wavelength
calibration errors could mimic a change in a
ThAr lines
Quasar spectrum
Modify equations used on quasar data:
quasar line: w = w0(quasar) + q1x
ThAr line: w = w0(ThAr) + q1x
w0(ThAr) is known to
high precision (better than
0.002 cm-1)
ThAr calibration results:
Atmospheric dispersion effects:
Rotator
Isotopic ratio evolution:
Isotopic ratio evolution results:
Isotope
Correcting for both systematics:
Rotator + Isotope
Uncorrected: Quoted Results
Conclusions and the next step

~100 Keck nights; QSO optical results are “clean”, i.e. constrain a
directly, and give ~6s result. Undiscovered systematics? If
interpreted as due to a, a was smaller in the past.

3 independent samples from Keck telescope. Observations and data
reduction carried out by different people. Analysis based on a
RANGE of species which respond differently to a change in a:

Work for the immediate future:
(a) 21cm/mm/optical analyses.
(b) UVES/VLT, SUBARU data, to see if same effect is seen in
independent instruments;
(c) new experiments at Imperial College to verify/strengthen
laboratory wavelengths;
CMB Behavior and Constraints
Smaller a delays epoch of last scattering and results in first peak at
larger scales (smaller l) and suppressed second peak due to larger
baryon to photon density ratio.
Last scattering vs. z
CMB spectrum vs. l
Solid (a=0); Dashed (a=-0.05); dotted (a=+0.05)
(Battye etal 2000)
BBN Behavior and Constraints
D, 3He, 4He, 7Li abundances depend upon baryon fraction, Wb.
Changing a changes Wb by changing p-n mass difference and
Coulomb barrier.
Avelino etal claim no statistical
significance for a changed a from
neither the CMB nor BBN data.
They refute the “cosmic concordance”
results of Battye etal, who claim that
da=-0.05 is favored by CMB data.
(Avelino etal 2001)
49 Systems ; 0.5 < z < 3.5 ; 28 QSOs
Da/a = -0.72 +/- 0.18 x 10-5 (4.1s)
Numerical procedure:
 Use minimum no. of free parameters to fit the data
 Unconstrained optimisation (Gauss-Newton) nonlinear least-squares method (modified version of
VPFIT, Da/a explicitly included as a free parameter);
 Uses 1st and 2nd derivates of c2 with respect to each
free parameter ( natural weighting for estimating
Da/a);
 All parameter errors (including those for Da/a
derived from diagonal terms of covariance matrix
(assumes uncorrelated variables but Monte Carlo
verifies this works well)
However…
b
2
observed
b
2
thermal
b
2
bulk
2kT

 cons tan t
m
T is the cloud temperature, m is the atomic mass
So we understand the relation between (eg.)
b(MgII) and b(FeII). The extremes are:
A: totally thermal broadening, bulk motions
negligible, b(MgII)  m(Fe) b(FeII)  Kb(FeII)
(
m(Mg)
)
B: thermal broadening negligible compared to
bulk motions, b(MgII)  b(FeII)
How reasonable is the previous assumption?
Line of sight to Earth
Cloud rotation or outflow
or inflow clearly results in
a systematic bias for a
given cloud. However,
this is a random effect
over and ensemble of
clouds.
FeII
MgII
The reduction in the number of free parameters
introduces no bias in the results
We model the complex profiles as multiple clouds, using
Voigt profile fitting (Lorentzian + Gaussian convolved)
Free parameters are redshift, z, and Da/a
Lorentzian is natural line broadening
Gaussian is thermal line broadening (line of sight)
Dependence of atomic transition frequencies on a
1. Zero Approximation – calculate transition frequencies using
complete set of Hartree-Fock energies and wave functions;
2. Calculate all 2nd order corrections in the residual electronelectron interactions using many-body perturbation theory to
calculate effective Hamiltonian for valence electrons including
self-energy operator and screening; perturbation V = H-HHF.
This procedure reproduces the MgII energy levels to 0.2%
accuracy (Dzuba, Flambaum, Webb, Phys. Rev. Lett., 82, 888, 1999)
Important points:
(1) size of corrections are proportional to Z2, so effect is small in
light atoms;
(2) greatest precision will be achieved when considering all
relativistic effects (ie. including ground state)
Wavelength precision and q values
Line removal checks:
Pre-removal
Post-removal
Removing MgII2796:
Line Removal
Pre-removal
Post-removal
Removing MgII2796:
Line Removal
Number of
systems where
transition(s) can
be removed
Transition(s)
removed
Pre-removal
Post-removal
The position of a potential interloper “X”
Suppose some unidentified weak contaminant is present, mimicking a
change in alpha. Parameterise its position and effect by dl, Dl:
MgII line generated with
N = 1012 atoms/cm2
b = 3 km/s
Interloper strength can vary
Position of fitted profile is
measured
2-Dimensional Echelle Image
Dark features are absorption lines