Molecular and atomic gas in the Local Group galaxy M 33

By P. Gratier, J. Braine, N. Rodríguez-Fernández, K. Schuster, C. Kramer, E. Xilouris, F. Tabatabaei, C. Henkel, E. Corbelli, F. Israel and 15 more. 2010

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We present high-resolution large-scale observations of the molecular and atomic gas in the Local Group galaxy M 33. The observations were carried out using the HEterodyne Receiver Array (HERA) at the 30 m IRAM telescope in the CO(2-1) line, achieving a resolution of 12 '' x 2.6 km s(-1), enabling individual giant molecular clouds (GMCs) to be resolved. The observed region is 650 square arcminutes mainly along the major axis and out to a radius of 8.5 kpc, and covers entirely the 2' x 40' radial strip observed with the HIFI and PACS Spectrometers as part of the HERM33ES Herschel key program. The achieved sensitivity in main-beam temperature is 20-50 mK at 2.6 km s(-1) velocity resolution. The CO(2-1) luminosity of the observed region is 1.7 +/- 0.1 x 10(7) K km s(-1) pc(2) and is estimated to be 2.8 +/- 0.3 x 10(7) K km s(-1) pc(2) for the entire galaxy, corresponding to H-2 masses of 1.9 x 10(8) M-circle dot and 3.3 x 10(8) M-circle dot respectively (including He), calculated with N(H-2)/ICO(1-0) twice the Galactic value due to the half-solar metallicity of M 33. The HI 21 cm VLA archive observations were reduced, and the mosaic was imaged and cleaned using the multi-scale task in the CASA software package, yielding a series of datacubes with resolutions ranging from 5 '' to 25 ''. The HI mass within a radius of 8.5 kpc is estimated to be 1.4 x 10(9) M-circle dot. The azimuthally averaged CO surface brightness decreases exponentially with a scale length of 1.9 +/- 0.1 kpc whereas the atomic gas surface density is constant at Sigma(HI) = 6 +/- 2 M-circle dot pc(-2) deprojected to face-on. For an N(H-2)/ICO(1-0) conversion factor twice that of the Milky Way, the central kiloparsec H-2 surface density is Sigma(H2) = 8.5 +/- 0.2 M-circle dot pc(-2). The star formation rate per unit molecular gas (SF efficiency, the rate of transformation of molecular gas into stars), as traced by the ratio of CO to H-alpha and FIR brightness, is constant with radius. The SFE, with a N(H-2)/ICO(1-0) factor twice galactic, appears 2-4 times greater than for large spiral galaxies. A morphological comparison of molecular and atomic gas with tracers of star formation is presented showing good agreement between these maps both in terms of peaks and holes. A few exceptions are noted. Several spectra, including those of a molecular cloud situated more than 8 kpc from the galaxy center, are presented.

Subfields: Astronomy and Astrophysics, Instrumentation

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THE IMPACT OF INTERACTIONS, BARS, BULGES, AND ACTIVE GALACTIC NUCLEI ON STAR FORMATION EFFICIENCY IN LOCAL MASSIVE GALAXIES (2012, background): “…low mass, low µ∗ local galaxies where high resolution measurements of both gas and star formation are possible, for example M33 (Gardan et al. 2007; Gratier et al. 2010), IC10 (Leroy et al. 2006), and NGC6822 (Gratier et al. 2010) (see however the counter argument in the case of the SMC in…”
LOW CO LUMINOSITIES IN DWARF GALAXIES (2012, methodology): “In the handful of studies that attempt to ac- count for αCO variations and measure H2/SFR in low-metallicity galaxies (e.g., Gratier et al. 2010a,b; Bolatto et al. 2011), there are suggestions that H2/SFR is up to 2− 5 times lower in local dwarfs at Z/Z⊙ ∼ 1/5.”
Turbulent molecular clouds (2012, background): “Individual GMCs (size ∼ 50 pc) are now resolved in nearby external galaxies (e.g. Gratier et al. 2010, Egusa et al. 2011, Koda et al. 2009).”
Star Formation in the Milky Way and Nearby Galaxies (2012, background): “…of the star formation law derived in this way have been carried out by numerous authors (e.g., Kennicutt 1989; Martin & Kennicutt 2001; Wong & Blitz 2002; Boissier et al. 2003; Heyer et al. 2004; Komugi et al. 2005; Schuster et al. 2007; Leroy et al. 2008; Schruba et al. 2011; Gratier et al. 2010).”
The lifecycle of molecular clouds in nearby star-forming disc galaxies (2019, methodology): “For the conversion factor from CO(2-1) to total molecular gas mass, we adopt a fixed ratio CO(2-1)/CO(1-0) = 0.7 (e.g. Gratier et al. 2010; Leroy et al. 2011; T. Saito et al. in prep.) and use the αCO factors provided by Sandstrom et al. (2013) when available (i.e. for NGC628, NGC3351, NGC3627,…”
COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies - II. The non-universality of the molecular gas depletion time-scale (2011, background): “One way to address these issues is through resolved studies of nearby galaxy discs, on sub-kpc scales (e.g. Leroy et al. 2008) or even down to the scale of GMCs (e.g. Gratier et al. 2010; Schinnerer et al. 2010).”
A MOLECULAR STAR FORMATION LAW IN THE ATOMIC-GAS-DOMINATED REGIME IN NEARBY GALAXIES (2011, background): “The efficiency of star formation from H2 gas may be genuinely higher in these systems, a view advocated by Gardan et al. (2007) and Gratier et al. (2010).”
Cloud-scale Molecular Gas Properties in 15 Nearby Galaxies (2018, methodology): “For M33, we use the IRAM 30 m CO(2–1) survey by Gratier et al. (2010) and Druard et al. (2014).”