VLBA Observations of AU-scale HI Structures

VLBA Observations of AU-scale 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

Cold HI scale height (2z) ~200 pc

Local bubble ~100 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)

Changing HI spectrum with hour angle indicates structure

Comparison of averages of “early” and “late” hour angles in two epochs

3C147

Evidence for structures with size 3 x 10-4 pc (70 AU)

Under Pressure….

What is measured: N(HI)/Ts (HI column density/spin temperature) and the size scale L

Typical Tiny HI: - N(HI)/Ts ~ 1 x 1019 cm-2 * K

- size scale of tiny HI ~ 50 AU

- maximum deviation ~ 0.5

- spin temperature Ts ~ 50 K

Density of tiny n(HI) ~ 3 x 105 cm-3

Pressure of tiny HI P/k ~ 107 cm-3 K

Typical ISM - n ~ 50 cm-3

- P/k ~ 10(3-4) cm-3 K

• What is the nature of the TSAS?

• How long does it live?

• How common is it?

• Is it really a “structure” or just a statistical phenomenon?

Solutions:

- Skinny (Heiles 1997)

- Cold (Heiles 1997)

- Temporary (Jenkins 2004)

- Statistical (Deshpande 2000)

Quest for Better S/NLovell, 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 HIMERLIN + 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)

3C138 (20 mas)0404+786 (10 mas)

Faison et al. (1998, 2001)

Multi-epoch VLBA Study Toward 3c138

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

Average 2002 HI optical depth

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

5σ =7% 5σ =5%

5σ =11% 5σ =10%

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%

Could Optical Depth Changes be Due to a Change in Temperature?

Arecibo (Heiles & Troland 2003) effective resolution ~800 mas

For a temperature drop from 50 K to 15 K the line width should decrease by ~0.7 km/s!

Difference 2002 -1999

Is the Line of Sight to 3C 138 Special?

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

VLBA HI Absorption Measurements toward Quasars

Pulsar Observation Simulation

from 120 simulated pulsar observations with epochs separated by 50 mas = typical size of HI variations

Probability of landing on variation with a 1-D sampling method is very low

x x

What about Those Magnetic Fields?Zeeman effect Blos 3σ upper limits:

Blos < 45 G/pixel

Blos < 20 G for average

Consistent with Heiles & Troland Arecibo detection of 5.6 ± 1.0 G

What does it mean?

Assume magnetic and turbulent pressures equal:

B = 0.4 vNTn0.5 where vNT is the non-thermal linewidth

For n=50 cm-3 and vNT =2.1 km/s; B = 6 G

For n=105 cm-3 and vNT =2.1 km/s; B = 266 G

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?

MHD waves likely mediate magnetic field/turbulent pressure balance

MHD waves with frequencies larger than the ion-neutral collision frequency cannot propagate:

For a parent cloud with

size ~ 3.5 pc

ionization fraction 10-4

Alfven speed = 2.1 km/s

the cutoff wavelength is on the order of 10AU

Simulations are needed….

Preliminary Results for 3C 147

Special thanks to E. Fomalont, V. Dhawan, C. Walker

D= 100 – 1000 pc

10mas ~ 5 AU at 500 pcG161.7+10.3

S/N of Deviation

Summary of VLBA Tiny HI Results

Distinct structures with typical sizes of ~25AU are observed

• significant changes on few year timescales are observed

•The line widths of these features rule out the significantly cooler gas scenario

•The covering fraction of these features is only about 10%

• The filling factor is very tiny < 0.1%

•The magnetic fields of these features are not significantly enhanced

• 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

• What is the nature of the TSAS?

• How long does it live?

• How common is it?

• Is it really a “structure” or just a statistical phenomenon?