The Structure of the Milky Way Disk

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The Structure of the Milky Way Disk in the
Fourth Galactic Quadrant
IGPS Conference 2003
Naomi McClure-Griffiths, ATNF John Dickey,
University of Minnesota Anne Green, Sydney
University Bryan Gaensler, Harvard
University Markijke Haverkorn, Harvard University
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• 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.

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The Longitude-Velocity Diagram
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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)
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A deeper version of the l-v diagram ...
The terminal velocity edge is quite sharp.
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Consecutive l-v diagrams averaged over 0.1 degree
steps in latitude from b 0.5o to b -0.5o.
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threshold in TB
l325.5 deg
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The threshold-crossing velocity as a function of
longitude and latitude
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(aside)
Sunset over the ocean ? No, its just the
terminal velocity vs. latitude and longitude...
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At each longitude, we look at the threshold
crossings at every latitude, and see how they are
distributed in velocity.
We look at the distribution, find the median and
percentiles.
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The Milky Way Rotation Curve Percentiles
(20, 50, and 80) of latitude slices within b
lt 10 crossing a 25 K threshold.
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How does the brightness temperature (HI column
density) drop off beyond the terminal velocity ?
s v 7.6 km/s
An average of all spectra with 290 lt l lt 340 b
lt 12 arc min (18,000 beam areas) shifted
to align the terminal velocities at v0.
s v 16.8 km/s
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Northern () and Southern (.) Hemisphere data on
the terminal velocity vs. sin(l) RG / Rsun .
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The Rotation Curve Depends on Qsun
km s -1
Rgal / Rsun
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Terminal velocity data translated into
circular rotation velocity (the rotation curve)
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Terminal velocity data translated into
circular rotation velocity (the rotation curve)
Brand and Blitz (1993)
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Terminal velocity data translated into
circular rotation velocity (the rotation curve)
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Terminal velocity data translated into
circular rotation velocity (the rotation curve)
Clemens 1985
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Terminal velocity data translated into
circular rotation velocity (the rotation curve)
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Terminal velocity data translated into
circular rotation velocity (the rotation curve)
Best fit Brandt Curve
V max 228 km s -1, R max 8.5 kpc, n 0.7
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Terminal velocity data translated into
circular rotation velocity (the rotation curve)
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Scutum-Crux
Norma
Sgr-Carina
Scutum-Crux
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The Cordes and Lazio model (and the Taylor and
Cordes model) shows an arbitrary pitch angle
change in the Sgr-Car arm just inside the solar
circle. Could this explain the offset ?
Sagittarius - Carina Arm Pitch angle
variation ?
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The density-wave model predicts that the gas
should slow down its rotational motion as it
approaches a spiral arm (from the concave side,
inside corotation) because the acceleration is
mainly radial, causing the gas to slow down to
conserve angular momentum. This is exactly what
we see !
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HI brightness temperature (density) at the
terminal velocity...
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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
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HI Disk Midpoint (past studies)
From Malhotra (1995) using published Parkes data
(boxes) and Dame et al (1987) CO data (line)
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HI Disk Midpoint SGPS
First moment Parkes
First moment SGPS
Gaussian Fit center position
Scutum-Crux arm
Sgr-Carina arm
Norma spiral arm
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HI Disk Scale Height (earlier studies)
From Malhotra (1995) using published Parkes data
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HI Disk Scale Height (SGPS)
Second moment SGPS
Second moment Parkes
flare?
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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 60 pc over 4 kpc radial range.

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Conclusions

• A smooth, Brandt curve can fit the Milky Way
• rotation curve in the radius range 2 to 8.5
  kpc.
• Departures from this smooth curve are aligned
• with the spiral arms in both quadrants IV and
  I.
• The sense of the departures agrees with
  density-
• wave theory prediction.
• The scale height increases weakly with radius.
• The scale height flares in the spiral arms.
• The midplane is strongly corrugated, apparently
• due to some unexplained effect of the spiral
  arms.

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Terminal velocity data translated into
circular rotation velocity (the rotation curve)
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HI Disk Midpoint SGPS
Norma spiral arm
Scutum-Crux arm
3 kpc arm
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HI Disk Scale Height (SGPS)
Scutum-Crux arm
3 kpc arm
Norma arm
flare?