The James Clerk Maxwell Telescope Nearby Galaxies Legacy Survey. Ii. Warm Molecular Gas and Star Formation in Three Field Spiral Galaxies

Abstract

We present the results of large-area 12 CO J = 3-2 emission mapping of three nearby field galaxies, NGC 628, NGC 3521, and NGC 3627, completed at the James Clerk Maxwell Telescope as part of the Nearby Galaxies Legacy Survey. These galaxies all have moderate to strong 12 CO J = 3-2 detections over large areas of the fields observed by the survey, showing resolved structure and dynamics in their warm/dense molecular gas disks. All three galaxies were part of the Spitzer Infrared Nearby Galaxies Survey sample, and as such have excellent published multiwavelength ancillary data. These data sets allow us to examine the star formation properties, gas content, and dynamics of these galaxies on sub-kiloparsec scales. We find that the global gas depletion time for dense/warm molecular gas in these galaxies is consistent with other results for nearby spiral galaxies, indicating this may be independent of galaxy properties such as structures, gas compositions, and environments. Similar to the results from The H i Nearby Galaxy Survey, we do not see a correlation of the star formation efficiency with the gas surface density consistent with the Schmidt-Kennicutt law. Finally, we find that the star formation efficiency of the dense molecular gas traced by 12 CO J = 3-2 is potentially flat or slightly declining as a function of molecular gas density, the 12 CO J = 3-2/J = 1-0 ratio (in contrast to the correlation found in a previous study into the starburst galaxy M83), and the fraction of total gas in molecular form.

The JCMT Nearby Galaxies Legacy Survey

“…The CO (3−2)/(1−0) ratios for the integrated flux of each peak lie in the range (0.2−0.8) with a mean value of 0.43 ± 0.18, which is consistent with previous results from the JCMT NGLS Warren et al 2010;Bendo et al 2010;Irwin et al 2010;Tan et al, in prep. ).…”

“…The results, shown in Table 2, confirm that all our detections are around 3σ or better. The noise levels calculated are in good agreement with the global values measured in other galaxies of the NGLS Warren et al 2010;Bendo et al 2010). Table 2 also includes the integrated flux ( T MB dv) and the FWHM for each spectral line derived from a Gaussian fit.…”

“…The observations were taken in raster map scanning mode during three nights in February 2008 using the 16 receptors heterodyne array receiver (HARP) with the auto-correlation spectrometer imaging system (Smith et al 2003) as backend Warren et al 2010) The spectrometer was configured to achieve a bandwidth of 1 GHz with a resolution of 0.488 MHz (0.43 km s −1 for the studied 12 CO J = 3−2 transition). The JCMT beam at this wavelength is 14.5 arcsec FWHM and the signal was sampled using 7.276 arcsec pixels to satisfy the Nyquist criterion.…”

“…Following the criteria used for the whole NGLS ), the galaxy was sampled until the target noise parameters (T * A = 19 mK at a resolution of 20 km s −1 ) were reached. The data reduction process is explained in detail in Wilson et al (2009) and in Appendix A of Warren et al (2010), so only a brief summary of the given steps is presented here. After flagging bad baselines and spikes, the individual scans were combined using a sinc(πx)sinc(kπx) weighting function.…”

Molecular gas and dust in spiral galaxies

“…This analysis implies an instantaneous molecular gas depletion time (given by 1/SF E) of 2 Gyr. Similar results for the gas depletion time are found from the JCMT Nearby Galaxies Legacy Survey (NGLS) (Wilson et al 2009, Warren et al 2010) and the CARMA STING survey (Rahman et al 2012).…”