Imaging Molecular Gas in a Nearby Starburst Galaxy

1. Imaging Molecular Gas in a Nearby Starburst Galaxy Science Goal:Measure the distribution and kinematics of molecular gas in a nearby starburst galaxy.The starbursts in luminous and ultraluminous infrared galaxies tend to be small (~1 kpc) and even the nearest of these galaxies are still quite distant in absolute terms (10s of Mpc). Exploring the distribution, kinematics, and physical conditions of the gas and dust in these systems is a natural application for sub-mm interferometers. ALMA'sEarly Science capabilities are well suited to study the nearest such galaxies. In this example, we replicate an SMA study of CO emission from NGC 3256 by Sakamoto et al. (2006). Receivers:Band 6(230 GHz).Molecular gas in starbursts tends to be fairly excited, making either CO(3-2) in Band 7 or CO(2-1) in Band 6 fine tools to probe the gas. The whole source is ~5 kpc in size or ~30” at 35 Mpc, which is perfectly matched to the field of view in Band 6. Angular Resolution:~1”.We want to resolve the central disk of molecular gas hosting the starburst. These disks are often ~1 kpc in size, and this is the case NGC 3256. At 35 Mpc, this is only ~5”. To clearly resolve the starburst, we target a resolution of ~1”. Spectral Resolution:~5 km s-1.We want to be able to measure the dispersion along individual lines of sight. Sakamoto et al. measure velocity dispersions (1σ) of 10-60 km s-1 and we target 5 km s-1 to ensureat least 5 resolution elements across the line at each position. Channel (Line) Sensitivity:0.02 K at 20 km s-1.Sakamoto et al. (2006) report peak 12CO(2-1) intensities of 2-10 K in 10 km s-1channels at ~1.5” resolution. If we cared only about this line we might target a 1σsensitivity of 0.2 K per 5 km s-1channel. However, we would also like to observe less optically thick13CO line, which is often 5-10 times fainter than 12CO. We design our observations assuming a line ratio of 10 and so target 0.02 K. If 13CO is much weaker (as Sakamoto et al. actually found), we canimprove the SNR by degrading the resolution. The 12CO data meets our kinematic science goals, so we target a lower spectral resolution of 20 km s-1 (i.e., 1 channel across the line) for 13CO. This 0.02 K at 20 km s-1 drives our time request. Continuum Sensitivity: N/A. Continuum could easily be a goal of this project, especially if we had chosen Band 7, but we will not focus on it in this example except to say that any unused correlator resources can be put to good use observing the 1mm (Band 6) dust continuum. NGC 3256, a nearby luminous infrared galaxy, as imaged by the SMA. (Left) Integrated CO(2-1) intensity at 2”x1” resolution, (Middle) peak CO(2-1) intensity along each line of sight, (Right) mean velocity along each line of sight derived from the CO line. During Early Science, ALMA should be able make comparable observations of nearby starburst galaxies in only a few hours on source.

2. Observing Time:6-8 hours. For our target, 0.02 K per 1” beam in a 20 km s-1channel at 230 GHz, the sensitivity calculator suggests that we need ~3 hours on source. Making conservative estimates for efficiencies and overheads (always a good idea in the early days of a telescope), we might request 6-8 hours. The fainter, extended emission around the main starburst shows complex structure. To accurately reconstruct this requires good coverage of theu-vplane, so we would not want to try this experiment in less time anyways. In the future: More antennas and large baselines will make it possible to observe more distant systems at similar spatial resolution. Larger baselines also allow improved resolution to study the structure of nearby starbursts like NGC 3256 in more detail. Both of these applications will come at the cost of increased observing time. Small mosaics will allow a chance to probe the extended, fainter gas surrounding the main starburst and short spacing observations from the ACA and total power antennas would be useful for this kind of application. These will be especially important for any study of the dust continuum. Extended CO emission benefits from the velocity gradient in the galaxy, so that the extent of CO emission at a particular velocity is not large, but this does not apply to the continuum. Place holder for real OT screen capture.