The James Clerk Maxwell Telescope Nearby Galaxies Legacy Survey. I. Star-Forming Molecular Gas in Virgo Cluster Spiral Galaxies

Abstract

We present large-area maps of the CO J=3-2 emission obtained at the James Clerk Maxwell Telescope for four spiral galaxies in the Virgo Cluster. We combine these data with published CO J=1-0, 24 µm, and Hα images to measure the CO line ratios, molecular gas masses, and instantaneous gas depletion times. For three galaxies in our sample (NGC 4254, NGC4321, and NGC 4569), we obtain molecular gas masses of 7 × 10 8 − 3 × 10 9 M ⊙ and disk-averaged instantaneous gas depletion times of 1.1-1.7 Gyr. We argue that the CO J=3-2 line is a better tracer of the dense star forming molecular gas than the CO J=1-0 line, as it shows a better correlation with the star formation rate surface density both within and between galaxies. NGC 4254 appears to have a larger star formation efficiency(smaller gas depletion time), perhaps because it is on its first passage through the Virgo Cluster. NGC 4569 shows a large-scale gradient in the gas properties traced by the CO J=3-2/J=1-0 line ratio, which suggests that its interaction with the intracluster medium is affecting the dense star-forming portion of the interstellar medium directly. The fourth galaxy in our sample, NGC 4579, has weak CO J=3-2 emission despite having bright 24 µm emission; however, much of the central luminosity in this galaxy may be due to the presence of a central AGN.

High-resolution mapping of the physical conditions in two nearby active galaxies based on12CO(1–0), (2–1), and (3–2) lines

“…CO(3-2) single dish observations complemented lower transitions' observations and yielded a wealth of information on the excitation of the molecular gas in galaxy centers (see e.g. Papadopoulos & Seaquist 1999;Papadopoulos & Allen 2000;Dumke et al 2001;Curran et al 2001;Hafok & Stutzki 2003;Yao et al 2003;Weiß et al 2005;Bayet et al 2006;Muraoka et al 2007;Kohno et al 2008;Papadopoulos et al 2008;Wilson et al 2009;Israel 2009) including that of our own Galaxy (Oka et al 2007). These observations showed that the CO(3-2)/(1-0) line ratio generally increases toward the center, where it can reach values greater than unity (in temperature units) in starburst and AGN hosts as well as in the Milky Way (vs. a typical value of 0.4 in galaxy disks).…”

“…We estimated the flux missed by the SMA due to the short spacings inherently missing in interferometric observations by comparing to the single dish data published by Wilson et al (2009) for NGC 4569 and by Israel (2009) for NGC 4826. We compared the total flux within an aperture of ∼36 in diameter after correcting the SMA data for the primary beam attenuation and degrading to the resolution of the single dish data (14.5 ).…”

Variation in GMC Association Properties across the Bars, Spiral Arms, Inter-arms, and Circumnuclear Region of M100 (NGC 4321) Extracted from ALMA Observations

“…For those GMAs with HII regions, SFR ranges between 2.1 × 10 −5 -0.3 M ⊙ yr −1 , with a median value 1.8 × 10 −3 M ⊙ yr −1 . The total SFR in the 165 GMAs is ∼ 2 M ⊙ yr −1 , accounting for ∼ 75% of the global SFR derived from the Hα + 24 µm data (Wilson et al 2009). …”

Mapping the interstellar medium in galaxies withHerschel/SPIRE

“…Bigiel et al (2008) found that if one only considers the surface-density of molecular gas the value of N is 1. Wilson et al (2009) used CO 3-2 observations to map the ISM in the same galaxies we have considered in this paper and found little variation in the gas depletion timescale across the disks, which implies N 1. Our values of N are higher than this, whichever method we use for mapping the ISM.…”

“…We have used the maps of the star-formation rate in these galaxies produced by Wilson et al (2009), who combined 24-μm Spitzer images and Hα images to correct for the effect of dust obscuration, following the prescription of Calzetti et al (2007). We have produced maps of the gas in each galaxy by combining the CO 1-0 maps of Kuno et al (2007) and the HI maps from the VIVA survey (Chung et al 2009).…”