Jumei Yao

Dick Manchester

2015/03/11

A new model for the Galactic free electron density

Outline

• Background and significance

• Deficiencies of existing models

• Advantages of new model

• Components and model-fitting

• Results

Background and significance

Electron density model :distribution of electron density of our galaxy

1. determine the distances to pulsars

2. investigate the large scale structure and variations of Galactic magnetic field

3. assess scattering in the ionized interstellar medium

Model based on pulsars with known DM and independent distances

Deficiencies of existing models TC93 (Taylor et al.1993)

Components: Thin disk, Thick disk ,Spiral arms( four arm), Gum

Asymmetry of DM with respect to Gl caused by spiral structure

Distances to some local pulsars and to pulsars near Carina-Sagittarius arm are overestimated, even with spiral arm component.

TC93 (Overestimated & underestimated)

For 146 pulsars, Only half of pulsar distances can be estimated accurate to 20% .

red star: distance overestimated

blue square:distance underestimated

NE2001 (Cordes et al. 2003)

Components: Thin disk, Thick disk, Spiral arms( four arm+ local arm), Galactic center, Clumps, Voids, Local ISM

a) Distances to High-latitude pulsars (|b|≥400)are underestimated

S. Chatterjee et al. 2009

new

b) Distances to some local pulsars are overestimated

rred star: model distance overestimated(need to increase ne)

blue square:model distance underestimated(need to decrease ne)

cyan star:distance within 20%

Advantages of new model

PX Kinematic Globular Cluster

Association (GC,HII,WD,SNR)

Total

63 62 13 8 146

ne

c) Using spiral arm model based on HII regions

NE2001 :n1=0.034 H1=950pc

New Model: n1=0.011 cm-3 H1=2382 pc

a) More pulsars with independent distance

b) New model much simpler: no "fine tuning" for particular pulsars

d) Different thick disk based on new globular cluster associations

Hou & Han 2014

Compared with spiral arm model from Georgelin et al. 1976, new spiral structure obtained by using 1800 HII regions with more accurate distances is more complete and accurate.

Averaged electron density for spiral arm is 0.09 cm¯³

except for local arm. The electron density of local

arm is 0.02 cm¯³

e) New Hα image of Gum nebula

Finkbeiner et al. 2003

Purcell et al. 2015

Purcell use a shell structure to model structure of Gum

D=450pc, dr=20 pc, ne=1.4 cm¯3

But new model use ellipsoidal shell structure:

f) New structure of Local Bubble

Welsh et al. 2009

Chimney like structure

Tilted 20 degrees to the Galactic plane

ne=0.005 cm¯³ (Snowden et al. 2014)

Dense clouds around Local Bubble

Welsh et al. 2010

Components and model-fitting a) Components of new model: Thick disk, Thin disk, Spiral arms( four arm+ local arm), Molecular Ring, Galactic Center, Gum nebula, local bubble, local enhancements (There are 52 free parameters in our model).

b) Model fitting

Definition of goodness-of-fit F:

Plot of Fi (Dl=2kpc, D0=3kpc, Du=5kpc)

We obtain algorithms from NLOPT (http://ab_initio.mit.edu/wiki/index.php/NLopt) ESCH (evolutionary algorithm) is used to do global optimization and MEAD (Nelder Mead Simplex algorithm) is used to do local optimization.

Algorithms:

Definition of distance error:

1) Model distance to local pulsars

Results

red star: distance overestimated

blue square:distance underestimated

cyan star:distance within 20%

2) Model distance to all pulsars

3) Distance to high latitude pulsars (|b|≥400)

4) Distances to all 146 pulsars

Thanks