VLBA Observations of AUscale HI Structures
Crystal Brogan (NRAO/NAASC)
W. M. Goss (NRAO), T. J. W. Lazio (NRL)
SINS Meeting, Socorro, NM, May 21, 2006
Probing Galactic HI through Absorption
Local bubble
~100 pc
Cold HI scale
height (2z)
~200 pc
~500 pc
G187.4-11.3
First Evidence for Small HI Structures
Hat Creek – Owens Valley interferometer with a fringe spacing of 0.09”
Dieter, Welch, & Romney (1976)
3C147
Comparison of averages of “early”
and “late” hour angles in two epochs
Changing HI spectrum with
hour angle indicates structure
 Evidence for structures with size
3 x 10-4 pc (70 AU)
Under Pressure….
What is measured:
columnof
density/spin
temperature)
• WhatN(HI)/Ts
is the(HI
nature
the TSAS?
and the  size scale L
• How long does it live?
Typical Tiny HI: - N(HI)/Ts ~ 1 x 1019 cm-2 * K
• How common is it?
-  size scale of tiny HI ~ 50 AU
deviation
Dt ~ 0.5
• Is- maximum
it really
a “structure”
or just a
- spin temperature
Ts ~ 50 K
statistical
phenomenon?
 Density of tiny n(HI) ~ 3 x
 Pressure of tiny HI P/k ~
Typical ISM
- n ~ 50 cm-3
- P/k ~ 10(3-4) cm-3 K
105
107
cm-3
cm-3
K
Solutions:
- Skinny (Heiles 1997)
- Cold (Heiles 1997)
- Temporary (Jenkins 2004)
- Statistical (Deshpande 2000)
Quest for Better S/N
Lovell, Effelsberg, & Westerbork VLBI with a resolution of 0.05”
Diamond et al. (1989)
 Evidence for structures with size ~25 AU and density 105 cm-3
First Imaging of Small Scale HI
MERLIN + EVN with a resolution of 0.1”
Davis, Diamond & Goss (1996)
 Evidence for structures with size
~70 AU and density 104 cm-3
Imaging with the VLBA (i.e. Epoch I)
Faison et al. (1998, 2001)
0404+786 (10 mas)
3C138 (20 mas)
Multi-epoch VLBA Study Toward 3c138
Average 2002
HI optical depth
Epochs: 1995, 1999, 2002
Resolution: 20 mas = 10AU at 500 pc
Superior dynamic range to any
previous study
 First attempt to study variabilty
Difference
2002 -1999
Position-Velocity Diagram
Cross-Cuts
Shaded lines are ± 1σ
Apparent sizescale
of features is
50mas or ~ 25 AU
Epoch to Epoch Opacity Changes
Percentage of “Significantly” Deviant Pixels
Dt  5σ =5%
Dt  5σ =7%
At typical ISM densities of ~50 cm-3 all of the observed HI column
would fit into a cloud only 3.5 pc in diameter
The filling factor of the CNM itself is low (<1% in this direction)
 Thus the filling factor of TSAS must be < 0.1%
Dt  5σ =11%
Dt  5σ =10%
Could Optical Depth Changes be Due to
a Change in Temperature?
Difference
Arecibo (Heiles & Troland 2003)
2002 -1999
effective resolution ~800 mas
For a temperature drop from 50 K to
15 K the line width should decrease
by ~0.7 km/s!
Is the Line of Sight to 3C 138 Special?
VLBA HI Absorption Measurements toward Quasars
 If the typical scale size of tiny scale HI is ~50mas, and the
“covering fraction” is low, then the best chance of seeing it is
toward a large source
Pulsar Observation Simulation
x x
Probability of
landing on variation
with a 1-D sampling
method is very low
Dt from 120 simulated
pulsar observations with
epochs separated by 50
mas = typical size of HI
variations
What about Those Magnetic Fields?
 MHD waves likely mediate magnetic field/turbulent pressure
Zeeman effect Blos 3σ upper limits:
balance
Blos < 45 mG/pixel
 MHD waves with frequencies larger than the ion-neutral collision
Bloscannot
< 20 mGpropagate:
for average
frequency
Consistent
with
Heiles &
Troland
For
a parent
cloud
withArecibo detection of 5.6 ± 1.0 mG
What does it mean?
size ~ 3.5 pc
Assume magneticionization
and turbulent
pressures
fraction
10-4 equal:
0.5
B = 0.4 Dv
where
is the non-thermal linewidth
NTn
Alfven
speed
= Dv
2.1NTkm/s
For n=50 cm-3 and DvNT =2.1 km/s; B = 6 mG
the cutoff wavelength is on the order of 10AU
For n=105 cm-3 and DvNT =2.1 km/s; B = 266 mG
Simulations are needed….
 Magnetic and turbulent pressures do not appear to be in equilibrium
 Given flux freezing, how can density change by 103 and not produce
appreciable increase in B?
Preliminary Results for 3C 147
D= 100 – 1000 pc
G161.7+10.3
10mas ~ 5 AU at 500 pc
S/N of
Deviation
Special thanks to E. Fomalont, V. Dhawan, C. Walker
Summary of VLBA Tiny HI Results
Distinct structures with typical sizes of ~25AU are observed
• •What
is
the
nature
of
the
TSAS?
significant changes on few year timescales are observed
• How long does it live?
•The line widths of these features rule out the significantly
cooler gas scenario
•The covering fraction of these features is only about 10%
• How
common
is
it?
• The filling factor is very tiny < 0.1%
magnetic fields of these features are not significantly
• •The
Is
it
really a “structure” or just a
enhanced
statistical
phenomenon?
• Perhaps MHD
waves can’t propagate?
These features are sufficiently rare that the probability of
observing them is low unless the search region is large
• Could explain the low rate of return from pulsar
observations and small quasars