A. Luna

The Discovery of Spiral Arms in the Starburst Galaxy M82

We report the discovery of two symmetric spiral arms in the near-infrared (NIR) images of the starburst galaxy M82. The spiral arms are recovered when an axisymmetric exponential disk is subtracted from the NIR images. The arms emerge from the ends of the NIR bar and can be traced up to 3 disk scale lengths. The winding of the arms is consistent with an m = 2 logarithmic spiral mode of pitch angle 14°. The arms are bluer than the disk in spite of their detection on the NIR images. If the northern side of the galaxy is nearer to us, as is normally assumed, the observed sense of rotation implies trailing arms. The nearly edge-on orientation, high disk surface brightness, and presence of a complex network of dusty filaments in the optical images are responsible for the lack of detection of the arms in previous studies.

HST ACS Imaging of M82: A Comparison of Mass and Size Distribution Functions of the Younger Nuclear and Older Disk Clusters

We present the results obtained from an objective search for stellar clusters, both in the currently active nuclear starburst region, and in the poststarburst disk of M82. Images obtained with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) in F435W (B), F555W (V), and F814W (I) filters were used in the search for the clusters. We detected 653 clusters, of which 393 are located outside the central 450 pc in the poststarburst disk of M82. The luminosity function of the detected clusters shows an apparent turnover at -->B = 22 mag ( -->MB = ? 5.8), which we interpret from Monte Carlo simulations as due to incompleteness in the detection of faint clusters, rather than an intrinsic lognormal distribution. We derived a photometric mass of every detected cluster from models of simple stellar populations assuming a mean age of either 8 (nuclear clusters) or 100 (disk clusters) million years old. The mass functions of the disk (older) and the nuclear (younger) clusters follow power laws, the former being marginally flatter ( -->? = 1.5 ? 0.1) than the latter ( -->? = 1.8 ? 0.1). The distribution of sizes (FWHM) of clusters brighter than the apparent turnover magnitude (mass -->2 ? 104 M?) can be described by a lognormal function. This function peaks at 10 pc for clusters more massive than 105 M?, whereas for lower masses, the peak is marginally shifted to larger values for the younger and smaller values for the older clusters. The observed trend toward flattening of the mass function with age, together with an overabundance of older compact clusters, imply that cluster disruption in M82 is both dependent on the mass and size of the clusters.

THE DISCOVERY OF A MOLECULAR CAVITY IN THE NORMA NEAR ARM ASSOCIATED WITH H.E.S.S γ-RAY SOURCE LOCATED IN THE DIRECTION OF WESTERLUND 1

We report on the discovery of a molecular cavity in the Norma near arm in the general direction of Westerlund 1 (Wd1), but not associated with it. The cavity has a mean radial velocity of ?91.5?km?s?1, which differs by as much as ~40?km?s?1 from the mean radial velocity of the Wd1 stars. The cavity is surrounded by a fragmented molecular shell of an outer diameter of about 100?pc and 106 M ?, which is expanding at velocities of 6 to 8?km?s?1. The amount of kinetic energy involved in the expanding shell is ~1051 erg. Inside this cavity, the atomic H I gas surface density is also the lowest. Structure of the extended Very High Energetic ?-ray emission, recently reported by the H.E.S.S. collaboration, coincides with the cavity. The observed morphology suggests that the inner wall of the molecular shell is the zone of the ?-ray emission, and not the dense gas surrounding massive stars of Wd1 as had been speculated by the H.E.S.S. collaboration. A likely candidate responsible for creating the observed cavity and the ?-ray emission is the pulsar PSR J1648 ? 4611.

The Star-formation Law in Galactic High-mass Star-forming Molecular Clouds

We study the star formation (SF) law in 12 Galactic molecular clouds with ongoing high-mass star formation (HMSF) activity, as traced by the presence of a bright IRAS source and other HMSF tracers. We define the molecular cloud (MC) associated to each IRAS source using 13CO line emission, and count the young stellar objects (YSOs) within these clouds using GLIMPSE and MIPSGAL 24 micron Spitzer databases.The masses for high luminosity YSOs (Lbol>10~Lsun) are determined individually using Pre Main Sequence evolutionary tracks and the evolutionary stages of the sources, whereas a mean mass of 0.5 Msun was adopted to determine the masses in the low luminosity YSO population. The star formation rate surface density (sigsfr) corresponding to a gas surface density (siggas) in each MC is obtained by counting the number of the YSOs within successive contours of 13CO line emission. We find a break in the relation between sigsfr and siggas, with the relation being power-law (sigsfr ~ siggas^N) with the index N varying between 1.4 and 3.6 above the break. The siggas at the break is between 150-360 Msun/pc^2 for the sample clouds, which compares well with the threshold gas density found in recent studies of Galactic star-forming regions. Our clouds treated as a whole lie between the Kennicutt (1998) relation and the linear relation for Galactic and extra-galactic dense star-forming regions. We find a tendency for the high-mass YSOs to be found preferentially in dense regions at densities higher than 1200 Msun/pc^2 (~0.25 g/cm^2).

The molecular cloud and embedded young stellar population associated with IRAS 18236–1205

We test a membership method to select embedded young stellar objects (YSOs) from a Galactic molecular cloud with ongoing massive star formation using multiband analysis. We select and discuss the embedded stellar population in the molecular cloud associated with IRAS 18235−1205, a small, geometrically well-defined Galactic molecular cloud. The IRAS source has infrared fluxes characteristic of an UCHii region, CS(J = 2 − 1) emission, and methanol and water maser emission, suggesting that this region is a good candidate for studies of young, massive star formation. The selection method of embedded stellar populations is based on the spatial distribution of CO(J = 1 − 0) and Spitzer/MIPS 24 μm point sources. Photometric analysis using near/mid-infrared images are used to test our selection criteria. Three objects are associated with the IRAS source; two have a characteristic spectral-energy distribution (SED) of a Class I/0 object (protostar) and the third has an SED of Class II.