Galactic Distribution of Southern 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, Universitatzu 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 (>105 cm-3), cold (<25 K), and to have very high column densities (>1023-1025 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 Way’s 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 R=8 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 CloudsMichael J. Butler, Jonathan C. Tan, Audra K. Hernandez IRDC Sample  Map (g cm-2)

3. A B C D E F G H I 3 2 1 6 5 4 7 8 9 b l 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. Spitzer IRAC 8m • Clump mass function Initial conditions of IRDC fragmentation • 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 Collaborators:Jongsoo 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

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 COME SEE OUR POSTER FOR THE ANSWERS!