How GASKAP will change our understanding of galaxy evolution

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How GASKAP will change our understanding of
galaxy evolution
Swinburne University of Technology 22 October 2009

• John Dickey
• University of Tasmania

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ASKAP the Australian Square Kilometre Array
Prototype
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This Focal Plane Array receiver gives ASKAP a
huge effective primary beam (30 sq deg).
Note a single 12m diameter dish has primary
beam area 0.89 sq deg. The point source
sensitivity is the same as Parkes and the ATCA
(total collecting area).
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The Murchison Radio Observatory site is the best
place in the world for a l20-cm telescope. ASKAP
will demonstrate this to the community!
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ASKAP Survey Science Projects EMU - continuum
extragalactic survey WALLABY - HI extragalactic
survey DINGO another HI exgal survey FLASH
exgal HI toward continuum survey POSSUM
rotation measure survey VAST transient
survey CRAFT another transient survey COAST
pulsar timing VLBI - very long baseline
interferometry and... GASKAP
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Supershell and chimneys GSH2770-40
McClure-Griffiths et al. 2000 Astron. J. 119,
2828
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GASKAP the Galactic ASKAP Spectral line survey

• Coordinators - Steering Committee
• Galactic Plane HI and diffuse OH
• John Dickey (U Tas), Steven Gibson (U Western
  Ky),
• Naomi McClure-Griffiths (ATNF)
• Magellanic Clouds, Bridge, and Stream
• Snezana Stanimirovic (U Wisc), Jacco van Loon
  (U Keele)
• OH masers from OH-IR stars (proto-planetary
  nebulae) and
• high mass SF regions
• Jose Gomez (Inst. Astron. Andalucia),
  Hiroshi Imai (U Kagoshima)
• Recombination Lines from Galactic HII regions
• Paul Jones (UNSW)
• Molecular Lines from dense molecular clouds
  regions (and OH 1720)
• Paul Jones (UNSW)

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HI in the Magellanic Bridge Muller et al. 2004
Ap. J. 616, 845.
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• ASKAP has a combination of
• baseline lengths (resolutions)
• spectrometer settings
• sensitivity
• survey speed and
• location
• that make it very well suited for a survey of the
  Milky Way and Magellanic Clouds in the HI OH
  lines simultaneously, observing thermal emission,
  absorption toward continuum sources, maser
  emission, and HI self-absorption simultaneously.

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Galaxy Evolution Begins at Home
GASKAP is a survey of emission and absorption at
l21-cm (the HI line) and maser emission, diffuse
emission, and absorption in the l18-cm lines of
OH. It is an order of magnitude deeper than any
survey at comparable resolution (IGPS), and it
covers an order of magnitude more area at low
latitudes than any survey at comparable
sensitivity (GALFA).
The objective is to get ground truth on the
astrophysics of galaxy evolution by determining
in the Milky Way how processes of accretion and
outflow of gas are driven by star formation in
the disk, and to get the context of star
formation through mapping structures in the warm
neutral interstellar medium, the cool neutral
medium, and the molecular clouds, and to study
the old stellar population of the bulge and halo
using OH masers.
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From the CGPS - an ablating molecular cloud with
atomic tails.
The Dragons Breath Knee et al. 2005 Ap. J.
628, 758.
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GASKAP straw-man survey parameters
Frequency Coverage 1416.4-1424.4 MHz (HI line v
lt 840 km/s) 1610-1614 MHz, 1662-1670 MHz (OH
lines) all with resolution 1.157 kHz 0.25 km/s
observed simultaneously
Approved September 2009
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From the CGPS, an outflow of HI from the disk
into the lower halo of the Galaxy.
The Mushroom Cloud English et al. 2000 Ap. J.
533, 25.
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RMS noise (1 s) in brightness temperature (K)
CE Galactic Centre and Magellanic Clouds AG
Galactic Plane and HII regions BDF
Intermediate and High Latitudes Magellanic
Stream
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Filaments in the Rigel-Crutcher
cloud McClure-Griffiths et al. 2006, Ap. J. 652,
1339.
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• Some specific questions GASKAP will answer
• What is the structure and dynamics of the gas
  in the Magellanic Clouds?
• GASKAP goes more than an order of magnitude
  deeper than any cm-wave survey ever done on the
  LMC and SMC.
• How does the ambient gas in the outer halo
  influence the Magellanic Stream and other gas
  accretion flows ?
• GASKAP will study the multi-phase medium of the
  Stream, phase transitions due to thermal
  instabilities and likely also dynamical
  instabilities due to shock cascades and halo gas
  ram pressure.

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• Where, how, and how much does the Galactic
  Fountain drive gas from the disk into the halo?
• GASKAP will show how the old superbubbles connect
  the hot gas in the disk with the halo, and how
  their shells are ablating and evaporating the
  cool interstellar clouds.
• How does the cool phase of the ISM stay in
  equilibrium with the warm medium even in low
  pressure environments?
• GASKAP will be the first survey to sample HI and
  OH absorption on many thousands of lines of sight
  at low Galactic latitudes.

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• How do molecular clouds form out of cool and
  cold atomic clouds ?
• GASKAP will show HI self-absorption and diffuse
  OH emission that are a better tracers of cold
  atomic and molecular gas in low density clouds
  than CO or other mm-wave lines.
• Where are stars forming now, and where did they
  form in the distant past ?
• GASKAP will discover thousands of OH masers
  associated with red giant stars, and hundreds of
  masers in star formation regions. It will trace
  flow of gas in proto-planetary nebulae. It will
  also trace HII regions through their
  recombination lines.

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3
CGPS
SGPS
2
VGPS
x
1
x
rms noise in TB (K)
Arecibo 0.1 s
0.5
0.3
x
0.2
x
GALFA (10 s)
GASKAP (50 hr)
0.1
x





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1.5
1
30
20
Beamwidth FWHM
20
3
CGPS
SGPS
2
VGPS
x
1
x
rms noise in TB (K)
Arecibo 0.1 s
0.5
HI OH Absorption OH Maser Emission
0.3
x
0.2
x
HI Emission
GALFA (10 s)
0.1
x
diffuse OH Emission





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1.5
1
30
20
Beamwidth FWHM
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An example Emission-Absorption studies of the
ISM temperature distributionDickey et al. 2009
Ap. J. 693, 1250.
Specific Question Where is the outer edge of
the Milky Way disk?
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Interpretation of emission- absorption spectra
together
slope Tcool
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In emission, we measure TEM, i.e. the brightness
temperature, TB
where N is the column density
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In absorption, we measure t, i.e. the optical
depth
where kf is the absorption coefficient
this gives the ratio
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The two-phase approximation spin
temperature Assume just two components of gas
at a given velocity on the line of sight cool
and warm assume the warm phase
temperature (typically 6000 K) is so large that
this phase does not contribute much to the
optical depth then the harmonic mean temperature
becomes simply If we know (or guess) the cool
phase temperature, Tcool, then we can
determine the fraction of gas in the cool phase,
fcool
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Galactic Plane interferometer mosaic plus single
dish surveys
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The outer disk of the Milky Way is dominated by
atomic hydrogen, HI. It shows a strong departure
from a flat plane, the warp, and an increase in
thickness, the flaring, with galactocentric
radius Rg gt 15 kpc.
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21-cm emission longitude- velocity diagram at
latitude 0o (GBTParkes) scale 5K
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LAB survey Kalberla Dedes (2008)
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Longitude Velocity diagrams in emission and
absorption
opacity, k, binned in l and v
density, n, measured at the same locations and
binned in the same way as for k.
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The radial dependence of the density, nH.
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The radial dependence of the opacity, k.
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The radial dependence of the spin temperature, T.
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The azimuthal dependence of the density and
opacity
density, nH
opacity, k
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The vertical dependence of the density, nH
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The vertical dependence of the opacity, k
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In the inner Galaxy, the cooler the clouds, the
more tightly they are confined to the midplane.
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GASKAP will give ten times as many absorption
spectra per square degree, over an area 50 times
larger than the Galactic plane surveys combined.

• This project will reveal the structure of the
  diffuse interstellar medium better than any
  previous study.

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GASKAP will also produce amazing images, as good
or better than those from any other SSP (survey
project). So it will have a strong impact on the
astronomical community beyond just radio people.
It should help to motivate the SKA, and serve as
a reference for extragalactic theory and models
of galaxy evolution.
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