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ASTRO Survey of the Galactic Center Region

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The study of galaxy formation is emerging as a dominant theme for astronomy in the 21st Century. ... 3 minutes of ALMA observing time gives 1 mJy sensitivity. ... – PowerPoint PPT presentation

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Title: 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)
6
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)

21
LB Movie
22
LV Movie
23
(No Transcript)
24
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
30
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
36
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
53
CO 4?3 and CO 7?6 in the Galactic Centertheir
ratio varies
C I
C I
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