ASTRO Survey of the Galactic Center Region

1
The Case for a 30 meter Sub-mm Telescope on the
Antarctic Plateau
Antony A. Stark Smithsonian Astrophysical
Observatory
2
Search for Protogalaxies

• The study of galaxy formation is emerging as a
  dominant theme for astronomy in the 21st Century.
• One of the fundamental problems in this field is
  finding the protogalaxies.
• Many instruments and surveys dedicated to finding
  protogalaxies over regions of a few square
  degrees (Maps of the Cosmos)
• Once found, the suite of existing or planned
  telescopes can study them
• ALMA, SMA
• Keck, Gemini, Magellan, OWL, etc.

3
Size of Protogalaxies
In q0 ½ Universe, essentially all protogalaxies
are at an angular diameter distance of 109
parsecs.
This means that a 20 kpc region appears to be 4?
in diameter
4
Protogalaxy Spectra
Telescope curves and points give sensitivity
limits in one hour with one detector element
30 m at SP
5
Fields of View of Sub-mm Telescopes
(SPT will actually have a field of view about
2000 sq. arcmin)
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ALMA cant find them all

• An ALMA map is small there are 50,000 ALMA maps
  per square degree.
• (thats maps, not beams)
• At 450µm wavelength, 3 minutes of ALMA observing
  time gives 1 mJy sensitivity.
• (thats really fast, but)
• 100 days of observing time to survey one square
  degree.
• So if ALMA were dedicated to a protogalaxy survey
  for 10 years, it would cover 10-3 of the sky

7
30 m Telescope can find them all

• 3.5? beam at 350µm
• well-coupled to entire protogalaxy
• Also requires about 3 minutes to achieve 1
  mJy/beam sensitivity
• smaller collecting area but bigger bandwidth
• A 10,000 bolometer detector array will cover 1
  square degree in about an hour
• about 20 array centers per square degree

8
Speed of Radio Mapping

• Searching for point-like sources with a
  radiotelescope at fixed flux level
• survey speed ? nAB Tsys-2
• n total number of detectors
• A total collecting area
• B pre-detection bandwidth
• Tsys total, atmosphere-corrected
  system temperature

9
Comparison between telescopes for protogalaxy
survey speed
10
Telescope Design

• IRAM 30 m diameter mm-wave telescope operates at
  2 mm wavelength with passive optics
• Need factor of 5 boost in surface accuracy
• Achievable with modest active surface technology
  (simplified Keck design)
• On-axis design is OK.
• Cost about 120M

11
THE END

• http//www.tonystark.org

12
The AST/RO Survey of the Galactic Center Region
Antony A. Stark Smithsonian Astrophysical
Observatory
13
AST/RO Survey Collaborators

• Tony Stark (SAO)
• PI
• Adair Lane (SAO)
• Project Manager
• Richard Chamberlin (CSO)
• Co-I
• Jacob Kooi (Caltech)
• Co-I
• Chris Walker (Steward Obs.)
• Co-I
• Jürgen Stutzki (U. Köln)
• Co-I

• Chris Martin (SAO)
• Winterover First Author
• Karina Leppik
• Current Winterover
• Wilfred Walsh
• Winterover in 2002
• Kecheng Xiao
• Winterover in 2002
• Nicholas Tothill
• Winterover in 2004
• Sunguen Kim (U. Mass)
• PostDoc

14

• AST/RO submm-wave Telescope
• Full suite of submm-wave receivers
• First telescope to operate through the Austral
  Winter

15
AST/RO

• Submm telescope operations year-round on
  Antarctic Plateau
• AST/RO has more usable submillimeter-wave
  observing weather than any other observatory
• Comprehensive characterization of South Pole
  submillimeter sky
• Open to proposals from worldwide astronomical
  community
• Receivers operating at 230 GHz, 460-500 GHz, 810
  GHz, 1.4 THz
• Large-scale maps of CO 2-1, 4-3, 7-6, and
  fine-structure lines of C I dominant cooling
  lines of molecular gas
• LVG modeling of density and temperature
• Molecular cloud structure as function of
  metallicity and spiral arm phase
• AST/RO observes the Milky Way and Magellanic
  Clouds as ALMA will observe other galaxies.

16
AST/RO on the roof

• AST/RO submm-wave Telescope
• Full suite of submm-wave receivers

17
AST/RO Instruments

• Receivers
• 230 GHz (CO 2?1)
• Wanda
• 460-495 GHz (CO 4?3, CI 3P1?3P0)
• 800-810 GHz (CO 7?6, CI 3P2?3P1)
• Polestar, 2x2 array Rx _at_ 800-810 GHz (CO 7?6,CI
  3P2?3P1)
• TREND, 1.5 THz Rx (NII, CO 11-10)
• Fourier Transform Spectrometers 1 MHz and 60 KHz
  wide

18
Galactic Center SurveyC. Martin, W. Walsh, K.
Xiao, A. Lane, C. Walker, and A.
Starkastro-ph/0211025

• -1.3ltllt2.0, -0.3ltblt0.2, with 0.5' spacing
• 3 transitions
• CO 7-6 (807 GHz) beamsize 1'
• CO 4-3 (461 GHz) beamsize 2'
• C I 3P1-3P0 (492 GHz) beamsize 2'
• 24,000 spectra per transition
• 108 pixels in three data cubes

19
AST/RO Data Release

• These data are feely-available on Internet see
• www.tonystark.org
• FITS data cubes of three species

20
Data Cubes

• Galactic Center
• CI (3P1-3P0)
• CO (4-3)
• CO (7-6)

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LB Movie
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LV Movie
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(No Transcript)
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Notation for line ratios

• The antenna temperature of the J n?m line of
  the k molecular weight isotope is
• Tn?mk
• So the ratio of antenna temperatures of 13CO
  (J1?0)
• to 12CO (J4?3) is
• T1?013/T4?312

25

• Some line ratios are more useful than others.
• The excitation states of the CO molecule tend to
  be in approximate Local Thermodynamic
  Equilibrium in almost all molecular gas almost
  everywhere.
• This means that transitions between the low-J
  states of CO have approximately the same antenna
  temperature. Furthermore, that temperature
  cannot in general be determined, because the beam
  filling factor is some unknown value less than
  unity.
• T2?112/T1?012 1 almost everywhere in the
  Galaxy, and therefore carries little information
  about ordinary molecular clouds

26

• To determine the excitation temperature, observe
  CO transitions from mid-J states. At some
  energy, the population of the states will drop
  out of Local Thermodynamic Equilibrium. This
  effect is sensitive to excitation temperature.
• The transitions between mid-J states are at
  submillimeter wavelengthsthats where AST/RO
  comes in.

27
Line ratio maps

• Black and white areas indicate no data
• Some areas show remarkably uniform color
• Foreground regions show distinctly different
  color from galactic center material

28
LVG Estimate of Density and Temperature
T1?013/ T1?012 is a measure of optical depth.
temperature (K)
T7?612/ T4?312 is a measure of excitation.
log(density)
LVG modeling with these lines gives an estimate
of temperature and density, over some range of
validity.
This is a significant advance in studying the
properties of molecular gas on the large scale.
29
LVG model of Galactic Center CO Emission

• Large Velocity Gradient
• Models are robust
• Inputs to our model
• 12CO/13CO abundance 25
• 12CO/H2 abundance 104
• ?v 4 km s1 pc 1
• Model results can be inverted to estimate
  temperature and density of emitting gas.

Sgr B
x2 orbits
Sgr A
x1 orbits
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LVG Movie
31
Measuring Molecular Gas

• In millimeter-wave astronomy it is often assumed
  that the brightness of 12CO J1?0 is a measure of
  total molecular mass.
• Such estimates are central to all observations of
  star-forming gas.
• Adding submillimeter-wave data, and analyzing
  using LVG method gives significantly different
  results.
• Variations in excitation
• Variations in column density

32
Integrate density over velocity to get column
density
This does not look like 12CO J1?0!
LVG-derived column density/T1?012
33
Progress in Observations of Star-forming regions

• LVG analyses can be cross-checked
• Add data from more CO transitions
• 13CO J6?5 at 660 GHz is particularly important
  measure of optical depth
• Fully-constrain or over-constrain CO radiative
  transfer models
• Determine velocity gradient
• Determine abundances

34
x1 and x2 closed orbits in a bar-like
potential
x1 orbit (solid)
x2 orbit (dashed)
35
x1 and x2 closed orbits an explanation for gas
velocities in the galactic center

• Bar-like potential in the inner 4 kpc of the
  Galaxy
• Our line-of-sight is about 15 from end-on
• x1 orbits form parallelogram
• x2 orbits lie on diagonal line

line-of-sight from Earth
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x1 and x2 orbits in the Milky Way
37
Gas Flow and Stability in x2 Orbits

• Gas flows down potential well of inner galaxy
• Gas will accumulate in the outer x2 orbits in a
  ring
• Elmegreen (1994, ApJL 425L73) showed that this
  ring is stable until
• ? gt ?crit 0.6 ?2/G 7 103 mH cm-3
• in the Milky Way, where ? is the epicyclic
  frequency

38
LVG Movie
7 103 mH cm-3
39
Starburst Mechanism in our Galactic Center

• Gas flows down potential well in the bar of our
  Galaxy until it reaches the x2 orbit which
  coincides with the innermost x1 orbit.
• There it accumulates until ?crit is reached.
• Then it will coagulate into a few giant clouds.
• These clouds will cause a starburst.

40
Summary

• Large submillimeter-wave line survey of the
  Galactic center is available.
• Modeling of line ratios yields estimate of
  density and temperature of molecular gas.
• Resulting density is suggestive of a starburst
  mechanism for the Milky Way.

41
THE END

• http//www.tonystark.org

42
LVG model of Galactic Center CO Emission

• Large Velocity Gradient
• Models are robust
• Inputs to our model
• 12CO/13CO abundance 25
• 12CO/H2 abundance 104
• ?v 4 km s1 pc 1

43
LVG model of Galactic Center CO Emission

• Large Velocity Gradient
• Models are robust
• Inputs to our model
• 12CO/13CO abundance 25
• 12CO/H2 abundance 104
• ?v 4 km s1 pc 1
• Model results can be inverted to estimate
  temperature and density of emitting gas.

44
LVG model of Galactic Center CO Emission

• Large Velocity Gradient
• Models are robust
• Inputs to our model
• 12CO/13CO abundance 25
• 12CO/H2 abundance 104
• ?v 4 km s1 pc 1
• Model results can be inverted to estimate
  temperature and density of emitting gas.

Sgr A
45
LVG model of Galactic Center CO Emission

• Large Velocity Gradient
• Models are robust
• Inputs to our model
• 12CO/13CO abundance 25
• 12CO/H2 abundance 104
• ?v 4 km s1 pc 1
• Model results can be inverted to estimate
  temperature and density of emitting gas.

Sgr B
Sgr A
46
LVG model of Galactic Center CO Emission

• Large Velocity Gradient
• Models are robust
• Inputs to our model
• 12CO/13CO abundance 25
• 12CO/H2 abundance 104
• ?v 4 km s1 pc 1
• Model results can be inverted to estimate
  temperature and density of emitting gas.

Sgr B
Sgr A
x2 orbits
47
LVG model of Galactic Center CO Emission

• Large Velocity Gradient
• Models are robust
• Inputs to our model
• 12CO/13CO abundance 25
• 12CO/H2 abundance 104
• ?v 4 km s1 pc 1
• Model results can be inverted to estimate
  temperature and density of emitting gas.

Sgr B
Sgr A
x2 orbits
x1 orbits
48
AST/RO Observations of CO J 7?6 and J 4 ?
3 in the Galactic Center Region

• The 300 pc ring density is just below a critical
  threshold for coagulation into a GMC like Sgr B,
  which will spiral into the center and cause a
  starburst
• Foreground spiral arms appear in absorption
• Most of galactic center region has CO excitation
  temperature near 35 K
• Sgr A and Sgr B are denser and more highly
  excited than GC as a whole

49
AST/RO Observations of CO 4?3 and CO 7 ? 6 in the
Galactic Center Region

• Sunguen Kim et al. 2002
• Longitude-velocity strip at b 0
• CO 4?3 is similar to CO 1?0
• Note absorption caused by
• Spiral arm near the Sun at vLSR ? 0 km s1
• 3 Kpc arm at vLSR ? 50 km s1
• CO 7?6 emission arises primarily in Sgr A and Sgr
  B clouds
• C I emission occurs in LSB

50
Bell Labs 7m Antenna Observations of Galactic
Center Gas at b 0º

• Note foreground absorption by local spiral arm
  and 3 kpc arm in CO map.
• There are some localized features with extremely
  broad linewidths, for example Clump 2 at l 3
• CS emission from dense material

51
AST/RO Data Release

• Open to proposals worldwide
• cfa-www.harvard.edu/adair/AST?RO
• Receivers at 230 GHz, 460 GHz, 490 GHz, 806 GHz
• Spectrometer resolution 1 MHz and 60 KHz
• PoleSTAR receiver (4 ? 806 GHz array receiver and
  array AOS) is operational (C. Walker, U.
  Arizona)

52
In the Galactic Center, excitation temperature is
similar for all CO states up to J 4
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CO 4?3 and CO 7?6 in the Galactic Centertheir
ratio varies
C I
C I