Dynamical Properties of Infrared Dark Clouds

1

Galactic Distribution of Southern Infrared Dark
Clouds
J. M. Jackson1, S. Finn1, J. Rathborne2, R.
Simon3, E. Chambers1
1Institute for Astrophysical Research, Boston
University, 2Harvard-Smithsonian Center for
Astrophysics, 3I.Physikal. Institut, Universitat
zu Koln, Germany
Abstract
Infrared Dark Clouds (IRDCs) are a new class of
interstellar clouds seen as dark extinction
features against the bright Galactic background
at mid-infrared (mid-IR) wavelengths. Studies
thus far have shown these IRDCs to be dense (gt105
cm-3), cold (lt25 K), and to have very high column
densities (gt1023-1025 cm-2 e.g., Egan et al.
1998 Carey et al. 1998, 2000). The
characteristic high column densities and low
temperatures of IRDCs suggest that they host the
earliest stages of star formation (e.g.,
Rathborne et al. 2006). Mapping the Galactic
distribution of IRDCs will enhance knowledge of
Galactic structure and the global distribution of
star formation in the Milky Way.
GOAL To measure the distances to IRDCs in order
to understand their Galactic distribution.
TECHNIQUE We measure the radial velocities of
IRDC molecular lines and convert these to
kinematic distances. Because the rotation of the
Milky Way is approximately known (e.g., Clemens
1985), each longitude-velocity pair corresponds
to a unique Galactocentric radius. The
distribution of Southern IRDCs can then be
compared to Northern IRDCs.
OBSERVATIONS We observed the CS 2-1 line toward
a large sample of IRDCs with the Mopra 22-m
Telescope near Coonabarabran, Australia. Because
CS requires high densities for excitation, it
uniquely traces the dense gas found in IRDCs
(Figs 1 and 2).
13CO
CS
Figure 1 A CS 2-1 map of a typical IRDC. CS 2-1
integrated intensity contours are overlaid on a
Spitzer/GLIMPSE three-color image (3.6 µm in
blue, 4.5 µm in green, and 8.0 µm in red). The
CS emission corresponds very well with the mid-IR
extinction.
RESULTS CS velocity (and therefore kinematic
distance) measurements were made for 210 southern
IRDCs (identified by Simon et al. 2006a). The
Galactocentric radial distribution differs in the
northern and southern Milky Way (Figs. 3 and 4).
In the north, the IRDC distribution peaks at a
Galactocentric radius of 5 kpc, and in the south
at 6 kpc.
Figure 2 (Left) 13CO 1-0 and (right) CS 2-1
spectra toward an IRDC. Although the 13CO
typically shows multiple velocity components, the
CS shows only one. Thus, CS uniquely traces
IRDCs, and a single CS spectrum can be used to
find their velocities and kinematic distances.
Northern
Northern
Southern
Southern
Figure 4 (Left) A face on plot of the Galactic
IRDCs Northern 13CO Simon et al. 2006b
Southern, this work. The positions of the Sun
and Galactic Center are marked. Comparing this
to a two-armed model of the Milky Ways spiral
arms (right, courtesy of B. Benjamin), it can be
seen that the IRDC distribution matches the
location of the Scutum-Centaurus spiral arm,
which comes closer to the sun in the southern
Milky Way.
Figure 3 Histograms comparing the Galactocentric
radial distributions of the northern IRDCs (top
Simon et al. 2006b) with the southern IRDCs
(bottom, this work). A peak in the north can be
seen at 5 kpc (the peak at R8 kpc is an
artifact). Surprisingly, the southern
distribution shows a peak at a different radius
of 6 kpc.
References Carey et al. 1998, ApJ, 508,72
Carey et al. 2000, ApJ, 543, L157 Clemens
1985, ApJ, 295, 422 Egan et al. 1998, ApJ,
494, L199 Rathborne et al. 2006, 641, 389
Simon et al. 2006a, ApJ, 639, 227 Simon et al.
2006b, ApJ, 653, 1325

• CONCLUSIONS
• IRDCs are confined to a distinct,
  non-axisymmetric Galactic feature that matches
  the so-called Scutum-Centarus arm in two-armed
  models of the Milky Way.
• Since they are found primarily in spiral arms,
  IRDCs probably form during compression caused by
  the passage of a spiral density wave.

We gratefully acknowledge funding support from
grants NSF AST-0507657 and NASA NNG04GGC92G.
2
Extinction Mapping of Infrared Dark Clouds
Michael J. Butler, Jonathan C. Tan, Audra K.
Hernandez
? Map (g cm-2)
IRDC Sample
3
Dynamical Properties of Infrared Dark Clouds
Audra K. Hernandez, Jonathan C. Tan, Michael J.
Butler Dept. of Astronomy, University of Florida
Virial Masses
The GRS Survey and Kinematic Distances
I
B
G
H
F
C
A
D
E
Log Mv/Mext
Table1
Log Mv/Mext
Log Mext/Msun
Log Mext/Msun
Log Mv,p/Mext
Log Mv,p/Mext
Log Mext/Msun
Log Mext/Msun
4
Initial conditions of IRDC fragmentation

• Clump mass function

Spitzer IRAC 8?m

• Clump Mass function in IRDCs
• consistent with CO clump surveys of local clouds
• inconsistent with dust emission studies

S. Ragan
5
J. Greissl
6
The Properties of Clumps and Cores in Molecular
Clouds Sami Dib CollaboratorsJongsoo Kim,
Andreas Burkert, Roland Jesseit, Thomas Henning,
Enrique Vazquez-Semadeni, Mohsen Shadmehri
molecular cloud models Isothermal, magnetized,
self-gravitating and turbulent
7
Barnard 59 Inside The Dark SpotCarlos
Román-Zúñiga, Charles Lada, Joao Alves, August
Muench Jill RathborneHarvard Smithsonian
Center for Astrophysics
8
(No Transcript)
9
FLAMINGOS Near-IR Survey of Serpens Molecular
Cloud understanding protostellar zoo and a
starformation history in the cloud
Nadya Gorlova, E. Lada , C. Roman-Zuniga , A.
Stolte,A. Steinhauer, J. Levine, B. Ferreira, C.
Gomez, N. Rashkind
10
09
FLAMINGOS SPECTROSCOPY OF LOW MASS STARS AND
BROWN DWARFS IN NGC 1977Noah H. Rashkind1,
Joanna L. Levine1, August A. Muench2, Elizabeth
A. Lada11Department of Astronomy, University of
Florida, Gainesville, FL 32611,
USA2Harvard-Smithsonian Center for Astrophysics,
Cambridge, MA 02138, USA
OBJECTIVE ? Collect NGC 1977 spectra, FLAMINGOS
KPNO 4-m telescope ? Classify spectra sample
to determine effective temperatures ? Combine J,
H, and K-band photometry to determine bolometric
luminosities ? Place objects on H-R diagram,
compare to evolutionary models ? Estimate an age
and brown dwarf fraction for NGC 1977 ?
Investigate dependence of brown dwarf fraction on
environment RESULTS COME SEE OUR POSTER FOR
THE ANSWERS!
11
A Multi-wavelength Study of NGC1333 Brown Dwarfs
Low-Mass Stars Gómez Martín, C.1, Lada, E.A.1,
Levine, J.L.1, Bayo Arán, A.2, Barrado y
Navascués, D.2, Morales Calderón, M.2 1
University of Florida, 2 Laboratorio de
Astrofísica Espacial y Física Fundamental
OBJECTIVE Collect NGC 1333 spectra, FLAMINGOS
KPNO 4-m telescope Classify spectra sample
to determine effective temperatures Combine J,
H, and K-band photometry to determine bolometric
luminosities Place objects on H-R diagram,
compare to evolutionary models Estimate an
age NGC 1333 Investigate dependence of brown
dwarf fraction on environment Combine
FLAMINGOS data with archival data (USNOB, 2MASS,
NOMAD SPITZER) to produce SEDs. RESULTS CO
ME SEE OUR POSTER FOR THE ANSWERS!