What We Are Learning from the

1
What We Are Learning from the Southern Galactic
Plane Survey
John Dickey
University of Minnesota
SGPS team Naomi McClure-Griffiths, ATNF
Bryan Gaensler, Harvard Uni.
Anne Green, Sydney Uni.
RSAA - MSSSO 13 March 2003
2
The SGPS combines data from both
The Australia Telescope
Compact Array
The Parkes Telescope
3
Combining data from the single dish and the
interferometer
Vela supernova remnant with nearby HII region
4
Outline
Or as far as I get in 45 minutes.
5
21-cm line formulae The emission
coefficient
density n
erg
-33
j 1.6 10 n f( )
n
sec Hz sterrad
n
The brightness temperature
T ( ) j ds
n
n
B
-2
cm
18
-1
K km s
if the line is optically thin all along the line
of sight !
6

• We map the 21-cm line and continuum
• radiation in the longitude range 2550 - 3570,
• latitude -10 to 10 with resolution 2 arcmin
• latitude -100 to 100 with resolution 15 arcmin.

7
The Longitude-Velocity Diagram
8
Locus of Tangent Points
The simplest thing on the l-v diagram is the ...
At the tangent point
d
Rg
Nominal spiral arms
Model electron density Cordes Lazio (2001)
9
A deeper version of the l-v diagram ...
The terminal velocity edge is quite sharp.
10
Consecutive l-v diagrams averaged over 0.1 degree
steps in latitude from b 0.5o to b -0.5o.
11
threshold in TB
l325.5 deg
12
The threshold-crossing velocity as a function of
longitude and latitude
13
(aside)
Sunset over the ocean ? No, its just the
terminal velocity vs. latitude and longitude...
14
At each longitude, we look at the threshold
crossings at for every latitude, and see how they
are distributed in velocity.
We look at the distribution, find the median and
percentiles.
15
The Milky Way Rotation Curve Percentiles
(20, 50, and 80) of latitude slices within b
lt 10 crossing a 25 K threshold.
16
Northern () and Southern (.) Hemisphere data on
the terminal velocity vs. sin(l) RG / Rsun .
17
Terminal velocity data translated into
circular rotation velocity (the rotation curve)
18
HI brightness temperature (density) at the
terminal velocity...
19
Measured at the terminal velocity (i.e. at the
sub- central point)
21-cm Brightness Temperature
Profiles in latitude look like
Galactic Latitude
Galactic Longitude
20
HI Disk Midpoint (past studies)
From Malhotra (1995) using published Parkes data
(boxes) and Dame et al (1987) CO data (line)
21
HI Disk Midpoint SGPS
Norma spiral arm
Scutum-Crux arm
3 kpc arm
22
HI Disk Scale Height (earlier studies)
From Malhotra (1995) using published Parkes data
23
HI Disk Scale Height (SGPS)
Scutum-Crux arm
3 kpc arm
Norma arm
flare?
24
The H I Disk Scale Height

• The scale height increases at spiral arms
• 3 kpc arm, Norma arm, Scutum-Crux arm
• What causes the increase?
• Increased turbulent pressure in areas of active
  star formation?
• Spiral density wave?
• The flaring of the disk for Rg lt 7 kpc is not
  strong, we see only an increase in scale height
  of 40 pc over 4 kpc radial range.

25
On scales of a few hundred parsecs there are
different kinds of structure in the ISM. The H I
is everywhere, so the 21-cm line is good for
tracing the really BIG things. One very
common kind of structure is giant shells and
chimneys, presumably filled with hot, ionized gas
(HIM), perhaps blown by the collective action
of stellar winds and supernova explosions of many
stars.
26
A nearby interstellar shell similar to the local
bubble from the SGPS (Southern Galactic Plane
Survey)
Dickey 2001, ASP Conf. Ser. 231, p. 318.
27
GSH 2770036
McClure-Griffiths et al. 2003, Ap. J. submitted.
McClure-Griffiths, et al. 2000 A.J. 119,
2828. Parkes plus ATCA (1020 field) mosaic.
An HI bubble that blows out of the disk into the
halo above and below.
28
Parkes data only, velocities from 10 to 70 km/s.
29
Schematic face-on view
longitude-velocity (like a top view)
30
(No Transcript)
31
(No Transcript)
32
Some structures in the shell walls are easy
to understand, like these Rayleigh-Taylor fingers.
Others are more problematic, like these narrow
ridges that seem to join at the bottom.
33
GSH 2770036 is large, but not atypical of
supershells and chimneys in the outer Milky Way.
It appears to break out of the disk both above
and below.
34
On even smaller scales (tens of parsecs and
smaller) the structure of the ISM seems to be
stochastic (random). How are we to understand
this ?
35
(No Transcript)
36
(No Transcript)
37
Even noise has a spectrum. Here we compute the
spatial power spectrum of the emission.
Region 1. ATCA plus Parkes
Dickey, McClure-Griffiths, Stanimirovic,
Gaensler, Green 2001 Ap. J. 561, 264
38
Region 2. ATCA only (but mosaicked !)
39
The spatial power spectrum is the Fourier
Transform of the Structure Function
sky brightness distribution
fringe visibility function
Fourier conjugates
magnitude squared
autocorrelation
spatial power spectrum
structure function
Fourier conjugates
40
The Structure Function
In the ionized gas (the WIM) we see a turbulence
spectrum over many orders of magnitude in scale
size.
Figure from Armstrong, Rickett, and Spangler
1995, Ap. J. 443, 209.
41
Region 1. Same data, but now using channel
width 20 km/s instead of 0.82 km/s.
42
The slope of the power law changes with velocity
width !
43
Turbulence theory predicts that the slope should
steepen by one unit when we go from a thin slice
to a thick slice of the medium (Lazarian and
Pogosyan 2000 Ap. J. 537, 720).
44
The spatial power spectrum of a spectral line
tracer gives us the ability to trace the small
scale structure of the ISM dynamically. The
slope change with velocity width is direct
confirmation that the power law structure
function is actually tracing turbulence, rather
than some other random process with power law
statistics.
45
Next up Using absorption and emission
in the 21-cm line together to study the cool
phase vs. warm phase of the neutral atomic
hydrogen.
46
density n, temperature T
the optical depth
the equivalent width (velocity integral of the
optical depth)
EW
47
Wolfire, M.G., Hollenbach, D., McKee, C.F.,
Tielens, A.G.G.M., and Bakes, E.L.O., 1995, Ap.
J. 443, 152.
48
The shape of the cooling curve determines the
heating-cooling equilibrium values of density
and temperature.
Dalgarno and McCray, 1972 ARAA 10, 375.
49
Wolfire, M.G., Hollenbach, D., McKee, C.F.,
Tielens, A.G.G.M., and Bakes, E.L.O., 1995, Ap.
J. 443, 152.
pressure
density
50
McKee and Ostriker 1977, Ap. J. 218, 148.
Result the ISM pressure is always bouncing
around.
51
To study the cool phase gas, we look at 21-cm
absorption toward continuum sources.
52
high contrast
low contrast
SGPS mosaic of the 21-cm continuum emission in
the fourth Galactic quadrant.
53
There are many continuum sources in the SGPS
region. We filter the data to remove the
emission in the direction of the continuum, then
we measure the absorption and emission spectrum
pairs, and we get ...
54
(No Transcript)
55
What do we do with these ? 1. Study
the distribution of CNM in the Galaxy (for
this we use only the absorption spectra) 2.
Study the temperature of the CNM (for this
we combine the emission and absorption)
Talk subtitle Separating the warm and cool
phases of the interstellar medium.
56
We compute the opacity, lt k gt from the optical
depth integral divided by the corresponding path
length
EW is the equivalent width
lt k gt is the line-of-sight averaged opacity
57
The galactic rotation curve tells us the velocity
as a function of distance along the line of
sight, v(r).
The velocity gradient, dv/dr, tells us the
path length corresponding to a given
bandwidth (e.g. one channel).
58
The velocity field due to Galactic rotation
sets v(r) , the radial velocity as a function of
distance along the line of sight.
r
Dr
v
DV
59
(No Transcript)
60
The opacity is larger in the inner Galaxy than it
is at the solar circle. It may be modulated by
the spiral arms.
61
The higher opacity in the inner Galaxy suggests
that either there is more cool phase gas
(relative to warm) or the median cool
phase temperature (Tcool ) is colder in the inner
Milky Way than at the solar circle, or some of
each.
This argues against most of the H I being
recently photo-dissociated H2.
e.g. Allen 2001
62
What is the temperature of the cool HI (CNM) ?
We can measure this by combining the emission and
absorption spectra channel by channel. But we
must somehow separate the emission from the WNM,
that shows no absorption because it is too warm
63
If all the gas at a given velocity were at the
same temperature, we could measure the
excitation temperature (spin temperature)
directly
But in each velocity channel there is overlap of
several regions with different temperatures
64
We make the two-phase assumption, that
the absorption comes from the CNM only.
b
f
density n, temperature Tcool
The emission comes from the CNM, plus the WNM
(with brightness temp., Tw, proportional to
its column density) in front (f) and behind (b)
the cool gas.
65
What we just had was
Next subtract the continuum to get the line
brightness temp
Define x ( 1 - e-t )
The warm gas has much broader linewidths than the
CNM, so over the velocity range covered by one
absorption line we can approximate the WNM
emission by a linear function of v
66
Tw,b
Now define e to be the background fraction of Tw
, i.e. e
Tw,b Tw,f
We measure both x and TB for all velocities,
v, across each absorption line. We assume a
value for e and then least-squares fit the values
of co , c1 , and c2 separately for each
absorption line.
This is a linear least-squares fit. No first
guess, no local minima the solution is unique.
67
Example Fitting two absorption lines toward a
continuum source at intermediate latitudes (data
from Heiles and Troland, 2003, Ap. J. in press,
from Arecibo).
68
(No Transcript)
69
HISA HI self-absorption
Whenever the coefficient c2 is less than zero,
then the absorption line decreases the brightness
temp, i.e. this is HISA. Two out of three
absorption lines show c2 lt 0 !
70
In the end, we get a distribution of CNM
temperatures among the different absorption
lines, depending on the assumed value of e.
The median value is 65 K, but note the tail
toward low temperatures (lt 30 K).
71
What is the Distribution of HI Temperatures ?
The Old Picture
Log HI Column Density
1 2 3
4
Log Temperature (K)
72
What is the Distribution of HI Temperatures ?
WIM
The New Picture
Molecular clouds
WNM
PDRs
Log HI Column Density
Diffuse clouds
1 2 3
4
Log Temperature (K)
73
Different galaxies have different peak brightness
temperatures and different relative abundances
of warm and cool phase H I. This suggests
variations in some or all of Tcool , lt k gt,
nw , and our vantage point (face-on vs. edge-on).
In the SMC 21-cm absorption is rare, the ratio of
absorption to emission is much less than in the
Milky Way.
74
Dickey, J.M., Mebold, U., Stanimirovic, S., and
Staveley-Smith, L., 2000, Ap. J. 536, 756
75
Wolfire, M.G., Hollenbach, D., McKee, C.F.,
Tielens, A.G.G.M., and Bakes, E.L.O., 1995, Ap.
J. 443, 152.
Heating-cooling equilibrium is a strong function
of metallicity, z, and dust to gas ratio, D/G.
76
Conclusions

• From the SGPS data, we find
• the rotation curve of the H I
• the disk thickness, corrugations, warp and
  flare
• the giant shells through which processed
  material is returned to the cool phase
• the spatial power spectrum slope change
• the radial variation of the CNM abundance
• the distribution of CNM temperatures

77
The Cycle of Galactic Evolution
SN explosions
winds
nucleosynthesis
Star formation
78
l21-cm Galactic Plane Surveys trace...
SN explosions
winds
Turbulence
HI emission mapping
nucleosynthesis
HI absorption
Star formation
79
(No Transcript)
80
(No Transcript)
81

• Velocity integrals (over one channel, the entire
• spectrum, or any velocity range in between) give
• the column density, N

• for the emission spectrum (for low optical depth,
  t),
• the equivalent width, EW, for the absorption
  spectrum

Combining the two gives the density weighted
excitation temperature, Tspin . Tspin is
generally equal to the kinetic temperature in
the neutral gas.
82
Note that ltTspgt is not a physical temperature,
but a blend of warm and cool phases along
the line of sight. If we know the mean cool
phase temperature, Tcool , then ltTspgt tells us
the fraction of gas in the cool phase, fc
fc
Ncool
Tcool

Nwarm Ncool
ltTspgt
The two-phase model comes from heating-cooling
equilibrium (Field, Goldsmith, and Habing, 1969,
Ap. J. Lett. 155, L149).
83
(No Transcript)
84
(No Transcript)