Speaker:	Hsi-An Pan & Alex Pettitt (Hokkaido U)
Title:	(Hsi-An Pan)What is a Giant Molecular Cloud? Are Observers and Simulators Discussing the Same Star-forming Clouds? (Alex Pettitt)Armed and/or dangerous: the sensitivity of galactic spiral generation in tidal encounters
Date (JST):	Thu, Jun 11, 2015, 13:30 - 14:30
Place:	Seminar Room A
Related File:	1462.pdf
Abstract:	(Hsi-An Pan)Observations and simulations have now reached the point where the giant molecular cloud (GMCs) populations can be studied over a whole galaxy. This is immensely helpful for understanding star formation, since the cloud properties set the conditions for new star birth. Yet, are these two groups really comparing the same objects? While simulators work in position-position-position (PPP) space, observers see projected properties along the line of sight, identifying clouds in position-position-velocity (PPV) space. If these methods do not identify the same objects, then the interpretation and comparisons between the data sets may be highly misleading. In this research we generated PPV and PPP data for a high-resolution simulated galaxy and compared the identified cloud properties in both data sets. Results show that the physical properties of molecular clouds in the individual galactic environments (bar, spiral, and outer disk) are highly similar among the two data structures. About 70% of clouds have single counterpart in each dataset, and their cloud properties scatter mostly within a factor of two. Therefore, comparing the simulated and observed GMCs are practical, and it will be the trend in the ALMA era. (Alex Pettitt)The nature of the spiral structure of disc galaxies is still somewhat of an open question. A number of different mechanism have been suggested to underpin spiral features; density waves, swing amplified transient spirals, bar driven arms and tidal encounters. The regimes where each of these mechanisms becomes dominant is an unknown, when does an interaction between a galaxy and a companion become strong enough to drown out the structure formed in isolation? We present the results of an investigation into the various morphologies created in the interaction between a disc galaxy and minor companion using numerous numerical simulations with stars, dark matter and gas. The primary goal of which is to discern the sensitivity of disc galaxies to tidal spiral structure. For what orbital paths, mass and velocity limits does the galaxy cease to feel the companion, and therefore any spiral generation is left to the galaxy itself? This also includes comparisons to transient spiral arm structures, those generated in simulations of isolated discs, and how the longevity, morphology and gas structure differs between the two mechanisms. By understanding the mechanisms driving different spiral morphologies we can better assess the origin of structure in external galaxies and the Milky Way.

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