NSF 3 May 2004

AST/RO Science 2005-2007

Two main research initiatives:

1. Support for prototype Terahertz instruments (SPIFI, TREND…)

2. Measurements for thermodynamic modeling of molecular gas in southern Spitzer satellite galactic plane legacy fields (~8°□ in mid-J CO and C I lines)

NSF 3 May 2004

Why AST/RO?• AST/RO is the only telescope operating routinely at

Terehertz frequencies.– (there is one other, but it only operates a few days each year)– Many expensive new telescopes are coming, but not for a few

years yet…

• AST/RO has observed, published, and released on the internet a large fraction (perhaps a majority) of all spectral line astronomical data in the frequency octave between 450 and 900 GHz (over 200,000 spectra so far).

• AST/RO is the only telescope capable of determining the density and temperature of molecular gas in large areas of the Milky Way

• AST/RO is relatively inexpensive—AST/RO’s operating budget is significantly less than that of any other full-time millimeter or submillimeter telescope.

NSF 3 May 2004

• Tony Stark (SAO)– PI

• Adair Lane (SAO)– Project Manager

• Alicia Goodman (SAO)– Co-I, Spitzer liaison

• 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 (SAO)– Current Winterover

• Wilfred Walsh (UNSW)– Winterover in 2002

• Kecheng Xiao (SAO)– Winterover in 2002

• Bhaswati Mookerjea (U. Köln)– [C I] analysis

• Sungung Kim (Korea)– 2001 data analysis

• Youngung Lee (Korea)– Data analysis

AST/RO Survey Collaborators

NSF 3 May 2004

AST/RO submm-wave Telescope

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

NSF 3 May 2004

• 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.

AST/RO

NSF 3 May 2004

AST/RO on the roof

AST/RO submm-wave TelescopeFull suite of submm-wave receivers

NSF 3 May 2004

AST/RO Instruments

• Receivers

– 230 GHz (CO 21)

– Wanda

• 460-495 GHz (CO 43, CI 3P13P0)

• 800-810 GHz (CO 76, CI 3P23P1)

– Polestar, 2x2 array Rx @ 800-810 GHz (CO 76,CI 3P23P1)

– TREND, 1.5 THz Rx (NII, CO 11-10)

• Fourier Transform Spectrometers: 1 MHz and 60 KHz wide

NSF 3 May 2004

NSF 3 May 2004

Galactic Center SurveyC. Martin, W. Walsh, K. Xiao, A. Lane, C. Walker, and A. Stark

astro-ph/0211025 ApJS Jan 2004

• -1.3°< l <2.0°, -0.3°< b <0.2°, 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

NSF 3 May 2004

Amount of Data

• The AST/RO Galactic Center Survey is a large fraction, perhaps a majority, of all astronomical data ever obtained in the submillimeter-wave band: 450 to 900 GHz

• These data are published in the Jan 2004 Astrophysical Journal Supplement

• Released on internet since May 2003, including descriptive paper

NSF 3 May 2004

Data Cubes– Galactic Center

• CI (3P1-3P0)

• CO (4-3)

• CO (7-6)

SgrB SgrA

NSF 3 May 2004

AST/RO Observations of CO J = 76 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

NSF 3 May 2004

CO 43 and CO 76 in the Galactic Center—their ratio varies

C I

C I

NSF 3 May 2004

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

NSF 3 May 2004

NSF 3 May 2004

LB Movie

NSF 3 May 2004

LV Movie

NSF 3 May 2004

AST/RO Data Release

• These data are feely-available on Internet see

www.tonystark.orgwww.tonystark.org

• FITS data cubes of three species

NSF 3 May 2004

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 (J=1→0)

to 12CO (J=4→3) is

T1→013/T4→3

12

NSF 3 May 2004

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→0

12 ≈ 1 almost everywhere in the Galaxy, and therefore carries little information about ordinary molecular clouds

NSF 3 May 2004

In the Galactic Center, excitation temperature is similar for all CO states up to J = 4

NSF 3 May 2004

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 wavelengths—that’s where AST/RO comes in.

NSF 3 May 2004

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

NSF 3 May 2004

LVG Estimate of Density and Temperature

temperature (K)

log(density)

T1→013/ T1→0

12 is a measure of optical depth.

T7→612/ T4→3

12 is a measure of excitation.

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.

NSF 3 May 2004

LVG model of Galactic Center CO Emission

• Large Velocity Gradient

• Models are “robust”

• Inputs to our model:12CO/13CO abundance = 2512CO/H2 abundance = 10–4

v ~ 4 km s–1 pc –1

• Model results can be inverted to estimate temperature and density of emitting gas.

Sgr B

x2 orbits x1 orbits

Sgr A

NSF 3 May 2004

LVG Movie

NSF 3 May 2004

Measuring Molecular Gas

• In millimeter-wave astronomy it is often assumed that the brightness of 12CO J=1→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

NSF 3 May 2004

Integrate density over velocity to get column density

This does not look like 12CO J=1→0!

LVG-derived column density T1→0

12

2(H

)dlo

gT

(CO

)d

nv

v

NSF 3 May 2004

Progress in Observations of Star-forming regions

• LVG analyses can be cross-checked– Add data from more CO transitions– 13CO J=6→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

NSF 3 May 2004

x1 and x2 closed orbits in a bar-like potential

x1 orbit (solid)

x2 orbit (dashed)

NSF 3 May 2004

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

NSF 3 May 2004

x1 and x2 orbits in the Milky Way

NSF 3 May 2004

Gas Flow and Stability

• Gas flows down potential well of inner galaxy– Jacobi Integral is conserved, energy-like quantity

• Gas will accumulate in the outer x2 orbits in a ring

• This radius is coincident with the Inner Lindblad resonance. – Accretion stalls because the net torque on material in that

orbit from the rotating bar is zero at that radius.

• If x2 orbits are populated at low density, the material will be stable and form a ring.

• If the gas is in a smooth ring, there is no dynamical friction.

NSF 3 May 2004

Toomre Instability of gas on x2 Orbits

Elmegreen (1994, ApJL 425:L73) showed that this ring is stable until ongoing accretion makes

ρ > ρcrit ≈ 0.6 κ2/G ≈ 7 × 103 mH cm-3

κ is the epicyclic frequency

κ ≈ 2 Vrotation / R for solid-body rotation curve

NSF 3 May 2004

LVG Movie7 × 103 mH cm-3

NSF 3 May 2004

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 ρcrit is reached.• Then it will coagulate into a few giant

clouds.• These clouds will cause a starburst, and fall

into the central black hole due to dynamical friction.

NSF 3 May 2004

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.

NSF 3 May 2004

THE ENDhttp://www.tonystark.org

NSF 3 May 2004

http://www.tonystark.org

(also available: 13CO survey of entire 1st quadrantH I survey of entire sky north of δ > -40° )

NSF 3 May 2004