Friday, March 23, 2007 Adam J. Mott 1Journal Club Presentation on “The BIMA Survey of Nearby Galaxies. I. The Radial Distribution of CO Emission in Spiral Galaxies” by Regan et al.ApJ, 561:218-237, 2001 Nov 1Friday, March 23, 2007 Adam J. Mott 2Fun With Acronyms•BIMA–Berkely–Illinois–Maryland–Association•SONG–Survey–Of–Nearby–GalaxiesFriday, March 23, 2007 Adam J. Mott 3BIMA millimeter interferometer•10 antennas•Was located near Hat Creek, California•Now relocated to Cedar Flat in California, along with the Owens Valley Radio Observatory (OVRO) to form the new Combined Array for Research in Millimeter-wave AstronomyFriday, March 23, 2007 Adam J. Mott 4The New CARMA ArrayFriday, March 23, 2007 Adam J. Mott 5Why study molecular gas?•Molecular gas fuels star formation in the Milky Way and in other galaxies. Important to studies of:–Triggered star formation in spiral arms–Nuclear starbursts–Increased star formation seen at ends of bars•Also, inflow of molecular gas to nuclear region can affect galaxy evolution; may:–Change in the central mass concentration–Cause a bar to be destroyedFriday, March 23, 2007 Adam J. Mott 6CO as a tracer of H2•Most of our knowledge about molecular gas in galaxies comes from studies of carbon monoxide (CO) emission–Unlike H2, CO has a permanent dipole moment•Cannot detect H2 directly in molecular clouds; instead use CO as a tracer–CO emits photons with wavelength 2.6 mm when it undergoes the rotational transition J = 1-0•Frequency ~ 115 GHz•One photon says to another, “Hey man, what’s ‘new’?”Friday, March 23, 2007 Adam J. Mott 7Previous studies of CO in galaxies•FCRAO Extragalactic CO Survey–Young et al. 1995–Spectra from 1412 positions in 300 galaxies–Showed how molecular content of galaxies depends on Hubble type, luminosity•Small scale distribution and physical conditions of molecular gas in galaxies still poorly understood–Most previous extragalactic observations have been carried out using single-dish telescopes with poor angular resolution•Linear resolution ~ many kpc–This is 100X larger than a typical giant molecular cloudFriday, March 23, 2007 Adam J. Mott 8Previous studies of CO in galaxies•Need to use an interferometer in order to get the sub-kpc resolution necessary to study the detailed distribution of molecular gas in galaxies outside of the local group.–Sakamoto et al. 1999•Mapped the central (within 30” from center) CO distribution in 20 nearby spiral galaxies•Focus on how distribution of molecular gas differs for galaxies with and without strong nuclear activityFriday, March 23, 2007 Adam J. Mott 9Motivation for this paper•Previous interferometer-only studies have suffered from limited field of view and possible “missing flux”.–Interferometers are very good at picking up the small scale variations in flux, but can have problems measuring the overall flux level.•Smallest baseline (shortest antenna separation) determines largest angular scale you are sensitive to.•Imagine trying to build a function f(x) as a sum of sines and cosines of various wavelengths, but not knowing the constant term very well.–This makes it difficult to quantitatively compare the molecular (from radio observations of CO) and stellar distributions (from optical or near infrared observations).•Single-dish only studies are useful for measuring the absolute CO flux, but suffer from poor resolution.•To get a more complete understanding of the distribution of molecular gas on sub-kpc scales in galaxies, it is necessary to combine both single-dish and interferometer data, and this is the goal of the Regan et al. paper.Friday, March 23, 2007 Adam J. Mott 10BIMA SONG•This survey produced spatial-velocity data cubes of the centers and inner disks of 44 nearby spiral galaxies.–Angular resolution ~ 6”–Spatial resolution ~ few hundred parsecs–Velocity (spectral) resolution ~ 10 km/s–Field of view ~ 10 kpc (~190”)•Over half of the 44 galaxies were also observed with a single-dish radio telescope (NRAO 12 m).–This sub-sample does not suffer from the “missing flux” problem.Friday, March 23, 2007 Adam J. Mott 11NRAO 12 m (now ARO 12 m)Kitt Peak, ArizonaFriday, March 23, 2007 Adam J. Mott 12BIMA SONG•How does this survey differ from previous high-res CO surveys?–Sample selection•Not based on CO brightness; instead, includes all nearby bright spirals with acceptable declinations and inclinations.–Field of view•Observed a much larger area, covering a significant portion of the optical disk.–Uniform observation and data reduction procedures–Incorporates single-dish data•Over half the galaxy maps include single-dish data so that all the CO flux can be observed for these galaxies.Friday, March 23, 2007 Adam J. Mott 13Main research question•“What is the radial surface brightness distribution of molecular gas and how does it compare to that of the stars?”–Expect the radial distributions of stellar light and molecular gas to be similar because stars form from molecular clouds.•This has been studied before, but with conflicting results:–Young & Scoville (1982) found similar scale lengths for both single-dish CO and optical B-band.–Sakamoto et al. (1999) found that scale length of CO (using interferometer-only data) was only ~500 pc, a small fraction of the scale length for stellar light.–What’s going on here?Friday, March 23, 2007 Adam J. Mott 14Comparison to Optical, NIR•This paper focuses on a sub-sample of 15 galaxies for which Regan et al. had:–Near-infrared or optical images–Incorporated single-dish data in radio mapsFriday, March 23, 2007 Adam J. Mott 15Sample selection criteria•Not explicitly based on the CO luminosity of the galaxies.•Criteria–Hubble types between Sa and Sd–Heliocentric recessional velocity < 2000 km/s•Hubble distances < ~27 Mpc (based on 75 km/s/Mpc)–Inclination < 70 degrees–Declination > -20 degrees•Observable from Hat Creek Radio Observatory–Apparent blue magnitude brighter than 11.0•Sample–45 galaxies found to meet these criteria•Used NASA/IPAC Extragalactic Database (NED)–44 of the 45 galaxies observed (excluded M33)–Average distance ~11 Mpc•6” beam corresponds to ~330 pc at this distanceFriday, March 23, 2007 Adam J. Mott 16Sub-sample of 15 galaxies•Paper focuses on 15 galaxies for which the authors have obtained both single-dish CO observations and optical