LOFAR Looks

1
LOFAR Looks at the Milky Way
John M. Dickey University of Minnesota
Sydney University January 30, 2003
2
Outline

• Mapping the emissivity of the
• Galactic non-thermal background

• Mapping the structure of the
• Galactic magnetic field

• Mapping the diffuse CII in the
• cool neutral interstellar medium

3
First, a quick look at the continuum sources in
the 4th quadrant of the Galactic Plane ...
Work-in-progress from the Southern Galactic
Plane Survey McClure-Griffiths, Dickey, Gaensler
and Green, 2003 in prep.
4
high contrast
low contrast
SGPS mosaic of the 21-cm continuum emission in
the fourth Galactic quadrant.
5
Test Region Continuum
6
ATCA only
Slices across the continuum maps
ATCA Parkes
at l21-cm
Parkes only
McClure-Griffiths, Dickey, Gaensler and Green,
2001
7
Milky Way centimeter-wave continuum
non-thermal (synchrotron)
thermal (bremstrahlung)
log( flux )
log( brightness Temp )
log ( n )
log ( n )
8
ATCA only
ATCA Parkes
at 100 MHz
Parkes only
9
ATCA only
HII regions are optically thick with TB
10,000 K.
ATCA Parkes
at 100 MHz
a distant HII region TB 10,000 plus foreground
synchrotron emission
Parkes only
a nearby HII region
10
Shadows of HII regions in front of the diffuse
synchrotron emission
Abramenko and Krymkin 1992, Astrophysics 35, 274.
11
21 cm emission spectrum, TB(v)
21 cm absorption spectrum, t(v)
(plotting e-t)
We can find the distances to HII regions by
their 21-cm absorption.
Dickey, McClure-Griffiths, Gaensler and Green,
2003 Ap. J. (March 20 issue).
12
sun
Galactic centre
EM 104 cm-6 pc
squares radio selected giant HII regions
Georgelin and Georgelin 1976, A. and A. 49, 57
from Kurtz, Churchwell, and Wood 1994, Ap. J.
Supp. 91, 659.
circles optically selected giant HII regions
13
VLA catalog of ultracompact HII regions (EMgt107
cm-6 pc).
Kurtz, Churchwell, and Wood 1994, Ap. J. Supp.
91, 659.
14
Telescopes in the Northern Hemisphere miss nearly
half of the inner Milky Way galaxy.
This is where the continuum sources are!
15
SGPS
The SGPS area covers 44 of the total disk mass
(or whatever stuff), assuming an
exponential surface density with scale length 3.2
kpc.
16
Given a declination limit of -370 (elev150 at
Green Bank)...
Missed
d lt -37o
we miss 34 of an exponential disk (scale
length 3.5 kpc).
longitude 3500
17
Given a declination limit of 00 ...
Missed
d lt 0o
we miss 84 of an exponential disk (scale
length 3.5 kpc).
18
Outline

• Mapping the emissivity of the
• Galactic non-thermal background

• Mapping the structure of the
• Galactic magnetic field

• Mapping the diffuse CII in the
• cool neutral interstellar medium

19
At low frequencies, diffuse linearly polarized
synchrotron emission will be everywhere.
What does it mean ?
20

• l21-cm total intensity (Stokes I)
• in a small area of the Galactic plane

Extragalactic sources
H II regions
SNRs
21

• Polarised Intensity PI ? (Q2 U2)
• in the same area

Point sources
Gaensler, Dickey, McClure-Griffiths, Green, 2001,
Ap. J. 549, 959.
22
Polarised Intensity PI ? (Q2 U2)
Diffuse emission
Gaensler, Dickey, McClure-Griffiths, Green, 2001,
Ap. J. 549, 959.
23
Polarised Intensity PI ? (Q2 U2)
Voids
Gaensler, Dickey, McClure-Griffiths, Green, 2001,
Ap. J. 549, 959.
24
Polarization position angle vs. frequency
indicates Faraday Rotation. We compute the
Rotation Measure RM ne2 dr
Gaensler, Dickey, McClure-Griffiths, Green, 2001,
Ap. J. 549, 959.
25
The diffuse polarized emission allows us to
measure the RM smoothly over a wide area on the
sky (but only in the foreground).
Gaensler, Dickey, McClure-Griffiths, Green, 2001,
Ap. J. 549, 959.
26
At lower frequencies, the Galactic diffuse
linearly polarized background gets brighter at
high latitudes, but more tangled at low
latitudes. Faraday depolarization causes
the distance to the emission to decrease. We
need the zero spacing, and good sensitivity on
short spacings, to make a clear map of the local
magnetic field and ionized gas distribution.
27
WSRT map of the polarized intensity at 327 MHz
Haverkorn, de Bruyn, Katgert, Schnitzeler, 2003
in preparation
28
Polarization position angle measured at 327 MHz
Haverkorn, de Bruyn, Katgert, Schnitzeler, 2003
in preparation
29
Milky Way disk magnetic field model by
Han, Manchester, Lyne, and Qiao, 2002, (Ap. J.
570, L17) based on analysis of pulsar
rotation measures.
30
The goal is to make a full three dimensional
model of the Galactic magnetic field and the free
electron distribution, based on all
possible tracers of the field. Surveys of the
diffuse linear polarization at a range of
frequencies will help make this possible.
31
Outline

• Mapping the emissivity of the
• Galactic non-thermal background

• Mapping the structure of the
• Galactic magnetic field

• Mapping the diffuse CII in the
• cool neutral interstellar medium

32
Recombination lines at low frequencies offer
a way to trace the diffuse ionized gas.
Carbon (only) recombination line regions
apparently trace the cool, atomic clouds.
33
Hydrogen recom lines at 327 MHz trace the WIM
(ELD-WIM ?)
Anantharamaiah, 1985, J. Ap. Ast. 6, 177. see
also Anish Roshi, Kantharia and
Anantharamaiah 2002, Astron Astroph 391, 1079.
34
At low frequencies the lines get brighter, as
long as the emission region is not
optically thick (low EM). In ordinary HII
regions the line width gets so large that the
lines all blend together.
Anish Roshi and Anantharamaiah 2001, Ap. J. 557,
226.
35
At still lower frequencies (lt100 MHz), we see no
H recom line emission, but we see carbon
lines in absorption !
Erickson, McConnell, and Anantharamaiah 1995, Ap.
J. 454, 125.
36
Edge-on spirals at 75 MHz show evidence for a
cool, ionized medium.
Israel and Mahoney 1990, Ap. J. 352, 30.
37
The low frequency recombination lines are telling
us about the diffuse ionization in the
interstellar medium. There may be more than one
phase (temperature) represented.
Carbon lines and hydrogen lines trace completely
different regions.
38
Conclusion Galactic astrophysics
applications of LOFAR are very diverse
and interesting, and LOFARs contribution is
unique, so we must build LOFAR in the Southern
Hemisphere.